PSC

Poster Session C
Display poster from Wednesday 31, 8:00 am to Friday 2, 4:00 pm

Poster Session C
Display poster from Wednesday 31, 8:00 am to Friday 2, 4:00 pm

Poster sessions:
Wednesday 17.00 > 18.45
Thursday at 16.00 > 18.15

08:00 - 18:15 #6442 - IM01-127 Electron tomography analysis of Pt/CeO2 catalyst powders synthesized by solution combustion.

IM01-127 Electron tomography analysis of Pt/CeO2 catalyst powders synthesized by solution combustion.

Transmission Electron Microscopy (TEM) or Scanning Transmission Electron Microscopy (STEM) can provide qualitative information on the distribution of metal nanoparticles over a support, as in the example of Fig 1 showing a STEM image of a Pt/CeO₂ catalyst. Nevertheless, as the obtained electron microscopy images are simply 2D projections of the object, they do not provide information on the 3D distribution of the particles which is an essential parameter controlling the catalytic activity. In this particular system, nanopores exist at the surface and a 3D imaging technique such as electron tomography is necessary in order to determine whether metal nanoparticles are anchored on the support surface or, e.g., trapped inside the oxide nanopores.

A 1 wt% Pt-CeO₂ catalyst was prepared by one-step solution combustion synthesis (SCS) [1], a fast and simple method favoring metal-support interaction. The sample was analyzed by electron STEM-HAADF tomography in a FEI environmental TEM (Fig. 2). The analysis was performed at two complementary scales: (i) at a relatively low resolution to gain insight into the ceria pore size and shape distributions, and (ii) at higher resolution to determine the Pt particle size distribution and location with respect to the support [2].

It is found that ceria has a bimodal pore size distribution, with small pores about 10 nm in size between the crystallites and bigger pores around 150 nm formed by winding up of ceria layers. It is highly probable that the synthesis route promotes this peculiar and usnusal microstructure. Noticeably, the present nano-tomography analysis further demonstrates that only ca. 50% of the 6 nm-sized Pt particles are located at the surface of the ceria, the other 50% being embedded in the support. This implies that only half the particles contribute to catalytic activity, which may have important consequences on the interpretation of catalytic data.

[1] F. Morfin, T.S. Nguyen, J. L. Rousset, L. Piccolo, Appl. Catal. B 2016, in press, doi: 10.1016/j.apcatb.2016.01.056.

[2] L. Roiban, S. Koneti, T.S. Nguyen, M. Aouine, F. Morfin, T. Epicier, L. Piccolo, in preparation.

Acknowledgements

Thanks are due to CLYM (Consortium Lyon - St-Etienne de Microscopie, www.clym.fr) for the access to the microscope funded by the Region Rhône-Alpes, the CNRS and the 'GrandLyon'.

Acknowledgments are also due to BQR project SEE3D granted by Insa-Lyon, ANR project 3DClean, Labex iMUST and IFP Energies Nouvelles for the financial support.

Lucian ROIBAN (MATEIS / INSA, Lyon), Siddardha KONETI, Thierry EPICIER, Thanh-Son NGUYEN, Mimoun AOUINE, Franck MORFIN, Laurent PICCOLO

08:00 - 18:15 #6467 - IM01-129 Multivariate-aided mapping of solute partitioning in a rare-earth magnesium alloy.

IM01-129 Multivariate-aided mapping of solute partitioning in a rare-earth magnesium alloy.

The addition of trace rare-earth alloying elements to wrought magnesium products can dramatically improve the formability of the base metal, leading to the development of the commercial Mg-Zn-Nd-Zr alloy ZEK100 for lightweight vehicular components [1]. Of particular importance in rare-earth alloy design is a greater understanding of the role of solute species to which the favourable texture and ductility are attributed [2,3]. Here we use an analytical transmission electron microscope, combined with electron tomography and multivariate statistical analysis (MSA), to study the microstructure and partitioning of the trace alloying elements in alloy ZEK100.

We observe three distinct precipitate populations in the matrix; large spherical neodymium rich precipitates decorate the microstructure at the micrometer length-scale (Fig. 1a), and at the nanometer length scale, a fine dispersion of smaller round zinc-rich and rod-shaped zirconium rich intermetallic precipitates populations are present, as revealed by energy dispersive x-ray analysis and electron energy loss spectroscopy (EELS). An electron tomographic reconstruction of a precipitate-rich region (b) enabled the distinction between round and rod-shaped precipitates and elucidation of the rod precipitate orientation distribution and preferred habit plane via principal component analysis (iii). By utilizing the high sensitivity of MSA when applied to EELS spectrum images, we interpret a weak component in the spectral dataset to represent the presence of zinc and neodymium rich shells, just a few monolayers thick, encapsulating the zirconium rich intermetallic precipitates (i,ii). This interpretation was supported by subsequent targeted analysis. An individual elongated precipitate was identified as a Zn₂Zr₃ structure by lattice-resolved HAADF-STEM imaging (Fig. 2a, i), and a few monolayer thick shell is observed at the precipitate/matrix interface (ii). The partitioning of neodymium and zinc at a precipitate interface was also observed in a needle specimen of the same alloy by atom probe tomography (b), providing strong evidence in support of the MSA zinc and neodymium rich shell component interpretation.

The combination of EDX, EELS, electron tomography and MSA techniques enabled an efficient and targeted analysis of the complex microstructure in alloy ZEK100. In particular, the use of MSA enabled the detection of a subtle, few monolayer thick solute rich shell around the small rod-shaped precipitates, which may have otherwise gone unnoticed using conventional data analysis techniques.  The tendency of the rare-earth solute Nd to encapsulate precipitates may affect its role as a texture-modifying element, and could therefore be of great significance in optimizing the chemistry and processing of rare-earth magnesium alloy systems.

Acknowledgements

GAB is grateful for funding from NSERC under a Collaborative Research and Development Grant.

References

[1] Robson, J. D. Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 2014, 45, 3205–3212.

[2] Hantzsche, K.; Bohlen, J.; Wendt, J.; Kainer, K. U.; Yi, S. B.; Letzig, D. Scr. Mater. 2010, 63, 725–730.

[3] Al-Samman, T.; Li, X. Mater. Sci. Eng. A 2011, 528, 3809–3822.

David ROSSOUW (Dundas, Canada), Brian LANGELIER, Andrew SCULLION, Mohsen DANAIE, Gianluigi BOTTON

08:00 - 18:15 #6487 - IM01-131 Combined multidimensional microscopy as a histopathology imaging tool for the holistic evaluation of the hepatic microvascular and associated tissue cells.

IM01-131 Combined multidimensional microscopy as a histopathology imaging tool for the holistic evaluation of the hepatic microvascular and associated tissue cells.

          Background: Histological variations in the microanatomical organisation of the functional liver units or hepatic lobules adversely affect general body function. Typically, assessment of liver pathology is based on the examination of paraffin-embedded histological slides counterstained with dyes using bright-field light microscopy (BFLM). In some instances, further clinical electron microscopy is recommended to assess the extent of ultrastructural damage to the microvasculature, to determine the nature of hepatic inclusions, or to stage fibrosis, hepatitis, and malignancy in more detail.

          Methods: Unfortunately, the above histopathology practice lacks the ability to cross-correlate observations on the same tissue sample and within a large tissue volume. With the recent advent of smart tissue preparation methodologies and three-dimensional (3-D) imaging practices, this can be circumvented in a relatively swift manner, allowing the same tissue to be examined across different microscopy platforms. Herein, we outline the combined application of X-ray micro-computed tomography (Micro-CT), BFLM and 3-D backscattered electron microscopy (BSEM) on liver tissue as an alternative good practice multimodal imaging approach.

          Results: A workflow is presented facilitating the collection of combined structure-function 3-D data (i.e., X, Y & Z) on liver architecture from the micron down to the nanometre scale using the same tissue preparation protocol. We illustrated the strength of this combined microscopy methodology to characterise various aspects of the hepatic vasculature, ranging from such large vessels as branches of the hepatic portal vein and hepatic artery, down to the smallest sinusoidal capillaries. Moreover, we were able to further characterise the subcellular features of a range of hepatic sinusoidal cells including, liver sinusoidal endothelial cells, liver-associated natural liver cells and Kupffer cells. Above all, we demonstrate the capabilities of this specimen manipulation and microscopy workflow to generate quality microscopic detail and subsequently extract relevant quantitative 3-D information.

          Conclusions: This contribution illustrates the capability that advanced histology imaging can bring to the gastroenterologist and/or pathologist in the fine structure-function assessment of the liver unit, its associated microvasculature, and the individual make-up of parenchymal and non-parenchymal tissue cells. This good practice further allows the instant generation of combined quantitative data about size, shape and volume changes of key microanatomical structures across multiple length scales. 

Gerald SHAMI (Sydney, Australia), Delfine CHENG, Filip BRAET

08:00 - 18:15 #6529 - IM01-133 New approach for low dose electron diffraction tomography.

IM01-133 New approach for low dose electron diffraction tomography.

Due to strong interaction of electrons with matter, 3D electron diffraction tomography has been proven to be a reliable method to solve structures of very small crystals, compared to X-ray diffraction; even for small protein crystals [1].

Several procedures for automated acquisition of tomographic diffraction data sets have been described [2, 3], using precession technique or discrete beam tilt perpendicular to the goniometer tilt axis. However, those mentioned methods are mainly used for less beam sensitive materials and are not suitable for low dose applications.

Shi et al. [4-7] refined the collection of 3D data sets under low dose condition (less than 10 e^-/Ų). Here, the camera system is continuously acquiring diffraction data during a continuous tilt of the goniometer. This allows to completely scan the Fourier space. It turned out that the stability of the goniometer tilt speed and the limited flexibility of the tilt acquisition parameters are problematic.

 

Here, we present a newly developed automatic data acquisition system, combining real-time direct control of the TEM-deflection systems, the goniometer tilt and the acquisition of high-resolution diffraction patterns with a synchronized CMOS camera. This can be realized by the TVIPS Universal Scan Generator (USG), controlling eight TEM deflection coils, i. e. four beam deflection coils and four image deflection coils. For a static goniometer alpha, the total beam tilt range (±3° to ±5°, depending on the TEM) can be fragmented by a defined beam tilt range, e.g. 0.5° (see Fig. 1). During the camera exposure time the beam is continuously tilted for this defined range (e.g. 0.5°). The continuous beam sweep during exposure time ensures the complete sampling of the Fourier space.

Several crystal structures (Carbamazepin: C₁₅H₁₂N₂O, bismuth oxychloride: BiOCl, Mayenite: Ca₁₂Al₁₄O₃₃) have been successfully solved by this method. The total acquisition time for 190 high resolution (1Å resolution) diffraction patterns is about 15 minutes, with a total electron dose of about 10 e^-/Ų. The collected 3D electron diffraction data sets were processed using the EDT-PROCESS software [8].

 

References

1.       J. A. Rodriguez, M. I. Ivanova, M. R. Sawaya, D. Cascio, F. E. Reyes, D. Shi,S. Sangwan, E. L. Guenther, L. M. Johnson, M. Zhang, L. Jiang, M. A. Arbing, B. Nannenga, J. Hattne, J. Whitelegge, A. S. Brewster, M. Messerschmidt, S. Boutet, N. K. Sauter, T.Gonen and D. Eisenberg: Structure of the toxic core of α-synuclein from invisible crystals, Nature (2015);  525 (7570).

2.       U. Kolb, T. Gorelik, C. Kübel, M.T. Otten and D. Hubert: Towards automated diffraction Tomography: part I- data acquisition, Ultramicroscopy (2007) ; 107(6-7):507-13.

3.       D. Zhang, P. Oleynikov, S. Hovmoller and X. Zou:Collecting 3D electron diffraction data by the rotation method, Z. Kristallogr (2010); 225 94–102.

4.       D. Shi, B.L. Nannenga, M.G. Iadanza and T. Gonen: Three-dimensional electron crystallography of protein microcrystals, eLife (2013); 2:e01345.

5.       B.L. Nannenga, D. Shi, j. Hattne, F.E. Reyes and T. Gonen: Structure of catalase determined by MicroED, eLife (2014); 3:e03600.

6.       B.L. Nannenga and T. Gonen:Protein structure determination by MicroED, Current Opinion in Structural Biology (2014); Volume 27:24–31.

7.       B.L. Nannenga, D. Shi, A.G.W. Leslie and T. Gonen: High-resolution structure determination by continuous-rotation data collection in MicroED, Nature Methods 11(2014); 927–930.

8.       M. Gemmi, P. Oleynikov: Scanning reciprocal space for solving unknown structures: energy filtered diffraction tomography and rotation diffraction tomography methods. Z. für Krist. 228 (2013) 51-58.

Hans TIETZ, Peter SPARLINEK, Peter OLYENIKOV, Reza GHADIMI (Gauting, Germany)

08:00 - 18:15 #6595 - IM01-135 Direct estimation of 3D atom positions of simulated Au nanoparticles in HAADF STEM.

IM01-135 Direct estimation of 3D atom positions of simulated Au nanoparticles in HAADF STEM.

The most commonly used algorithms to reconstruct HAADF STEM data such as Filtered Back Projection (FBP) and iterative reconstruction algorithms such as ART, SART and SIRT assume a linear image formation model. However, the linearity assumption is only a crude approximation of the non-linear behaviour of the real image formation model. Moreover, the limited angular range results in smearing in the reconstruction along the missing wedge and the typically small amount of projections creates a largely undersampled problem. The reconstruction can therefore be regarded as a limited data problem for which a regular SIRT reconstruction was not designed. One way to solve this problem is to reduce the number of unknowns by adding prior knowledge to the reconstruction algorithm. We present a simulation study for the use of a 3D Gaussian atomic model as a prior in the iterative reconstruction method developed in [1] using the ASTRA toolbox [2]. Our algorithm starts from an initial SIRT reconstruction that typically does not reveal the crystal structure but does agree relatively well with the projection data. In the subsequent iterations of our algorithm we combine this initial reconstruction with a gradually refined estimation - both in image space and projection space - of the 3D atom grid. For a quantitative validation we simulated 26 projection images (pixel size 16 pm) for two Au nanoparticles consisting of 1415 and 6525 atoms respectively with a frozen phonon approach using the MULTEM software [3]. The angular range was limited to 100 degrees, resulting in a large missing wedge and Poisson noise was added to obtain a signal to noise ratio of 10. The average distance between atom positions in the phantom and the reconstruction was found to be less than 6 pm in the direction of the missing wedge for both particles and 3 to 5 pm in the other dimensions for the 1415 atom particle (Fig. 1) and the 6525 atom particle (Fig. 2) respectively, resulting in subpixel accuracy for the recovered atom positions (Fig. 3). The recovery of the 6525 atom positions took approximately 20 minutes.

 

References

1. B. Goris, J. De Beenhouwer, A. De Backer, D. Zanaga, K. J. Batenburg, A. Sánchez-Iglesias, L. M. Liz-Marzán, S. Van Aert, S. Bals, J. Sijbers, et al., "Measuring Lattice Strain in Three Dimensions through Electron Microscopy", Nano Letters 2015, 15 (10), pp6996-7001. 

2. W. Van Aarle, W J. Palenstijn, J. De Beenhouwer, T. Altantzis, S. Bals, K. J. Batenburg, and J. Sijbers, "The ASTRA Toolbox: a platform for advanced algorithm development in electron tomography", Ultramicroscopy, vol. 157, pp. 35–47, 2015.

3. I. Lobato, D. Van Dyck, MULTEM: a new multislice program to perform accu-

rate and fast electron diffraction and imaging simulations using Graphics

Processing Units with CUDA, Ultramicroscopy (2015) 9–17.

Jan DE BEENHOUWER (Antwerp, Belgium), Ivan LOBATO, Dirk VAN DYCK, Sandra VAN AERT, Jan SIJBERS

08:00 - 18:15 #6608 - IM01-137 Chemical tomography of nested-network nanoporous gold.

IM01-137 Chemical tomography of nested-network nanoporous gold.

Nanoporous gold (NPG) is the paradigm of the novel class of nanostructured metals consisting of a randomly interconnected solid and void structure. Due to the high surface area of these materials, there are numerous promising fields of application in catalysis, sensors or electronics. The most common synthesis method of NPG is electrochemical dealloying of Ag-Au alloys [1].

To enlarge the surface area of NPG even further, a two-step dealloying process on a Pt-doped Ag-Au master alloy has been developed [2]. In the first step of the dealloying process, a fair amount of silver is removed by electrochemical corrosion, leading to a porous structure with very small ligaments of about 20 nm diameter. After gentle annealing, resulting in a porous structure with a feature size of about 100 nm (NPG), a second dealloying process is performed. The residual silver is removed, generating a porous structure (ligament size smaller than 10 nm) inside the initial formed ligaments of the porous structure, forming a nested-network nanoporous gold (N³PG) (Figure 1).

The retention of silver in the first dealloying step is presumed to be the result of a passivation of the master-alloy surface during dealloying by successive accumulation of platinum on the electrolyte-solid interface during the electrochemical removal of silver. During the subsequent annealing, the platinum diffuses from the surface into the Ag/Au Ligaments, which is consistent with thermodynamic calculations [3] and spectroscopic measurements [4], exposing a fresh surface of Ag/Au for the second dealloying step, resulting in the hierarchical structure of N³PG. The proposed accumulation of Pt is plausible because of the lower mobility of platinum compared to silver and gold and has been proven by cyclic voltammetry [5]. However, cyclic voltammetry and other spectroscopic methods only give an averaged analysis of the surface. To further investigate this theory, localized information of the surface composition of the NPG ligaments is needed.

In this work, we present the electron microscopic examination of NPG by the means of electron tomography in combination with EDX. The complex material structure with a high void fraction, high element numbers, and low concentration of minor constituents (Ag, Pt), however, aggravates the specimen preparation and data collection. To obtain reasonably thin, artifact-free specimen of NPG, the voids have to be filled with epoxy and then cut by ultramicrotomy. The image and elemental maps (Figure 2) collection is performed on a modern TEM optimized for electron tomography, equipped with four EDX-Detectors covering a large solid angle and a high brightness gun for high probe currents. For data analysis HyperSpy [6] is used.

[1] Z. Qi , J. Weissmüller, ACS Nano 2013, 7, 5948.

[2] Z. Qi, U. Vainio, A. Kornowski, M. Ritter, H. Weller, H. Jin, J. Weissmüller,
Advanced Functional Materials 03/2015; 25(17).

[3] P. A. Dowben, A. H. Miller, R. W. Vook, Gold Bull., 1987, 20, 3.

[4] J. A. Schwarz, R. S. Polizzotti, J. J. Burton, J. Vac. Sci. Technol. 1977, 14, 457.

[5] A. A. Vega , R. C. Newman , J. Electrochem. Soc. 2014, 161, C1.

[6] HyperSpy Home Page. www.hyperspy.org (accessed Mar 2016)

Tobias KREKELER (Hamburg, Germany), Ke WANG, Lida WANG, Martin RITTER

08:00 - 18:15 #6657 - IM01-139 3D elemental analysis of the rod-shape alloy by using EDS tomography.

IM01-139 3D elemental analysis of the rod-shape alloy by using EDS tomography.

Energy dispersive X-ray spectroscopy (EDS) is widely used to obtain an elemental map of a sample. EDS tomography reconstructs a three-dimensional (3D) elemental map from a tilt series of two-dimensional (2D) EDS elemental maps by using the back projection theorem. The back projection theorem is applied to tilt series of transmission images. An EDS map is not transmission image but X-ray emission image. Therefore, the tilt series of EDS maps should not be applied to normal tomography in principle.

The basic signal types for reconstruction by tomography are categorized into two, that is, absorbance and emission. In the case of normal transmission image (BF-TEM or BF-STEM), we calculate the mass thickness from the absorbance ( -log[I/I₀] ), where I: detected electron intensity after transmission,　I₀: incident intensity of electron, according to Lambert-Beer’s law. While, in case of the emission type, which includes ADF-STEM, X-ray fluorescence and etc., the emitted signal is proportional to mass thickness or number of atoms in a irradiated probe diameter. Thus, for X-ray elemental maps, we are able to reconstruct a 3D map of each element by measuring X-ray intensity of a certain element.

The X-ray intensity does not represent the number of atoms for the certain element, when the generated X-ray is absorbed in the sample itself. Therefore, only if the X-ray absorption is small enough to be ignored, we can apply the standard calculation procedure to EDS tomography. In practical calculation, the detection efficiency depending on the sample tiling angle is also considered. The efficiencies on sample tilt angles were measured using a known sample beforehand. This study reports how to obtain the 3D elemental maps in high magnification condition and to improve the accuracy of the EDS tomography using a sample of an alloy.

Figures 1(a) shows a DF-STEM image of an alloy composed of Mn, Ga and Ni. The instrumentation we use for this experiment was a field emission microscope (JEOL, JEM-2800) equipped with two SDD detectors whose sensor area is 100 mm² each. A tilt series of EDS elemental maps for the region of sample shown in Fig. 1(a) was collected at tilt angles ranged  ±80 degree. And the tilt steps were 4 degree, resulting in 41 collected maps for an element. The number of pixels for each map was 256 x 256. Figures 1(b)-1(e) show a 3D DF-STEM and elemental maps of the alloy, reconstructed by simultaneous iterative reconstruction technique (SIRT), which can reconstruct a 3D tomogram faster from fewer number of images than one in conventional method. As a result, we found that the Ni and Ga made solid solution, but Mn was segregated into small particles. The 58 Mn particles were visible in the Mn map, and the average diameter of these particles was estimated to be 10.7 nm.

Next, we measured the X-ray self-absorption effect for improvement of the accuracy of 3D elemental maps. Figure 2(a) shows the DF-STEM image from the rod-shape NAND flash memory made by FIB.  The tilt series of EDS maps was obtained from the field of view shown in Fig. 2(a) by an electron microscope (JEOL, JEM-2100) with the single EDS detector. Then, we measured the X-ray counts from the small gold colloidal particle indicated by the arrow in Fig. 2(a). The X-ray counts are plotted on the stage tile angles (Fig. 2(e)). Since the X-ray from the particle was absorbed by the sample itself, X-ray counts were not constant. In order to analyze the 3D structure quantitatively, it is necessary to correct this effect. The effect of the absorption was estimated to be about 0.8 at maximum.

In conclusion, to make an accurate elemental map by EDS tomography, it is necessary to consider the effect of X-ray absorption, sample shape and sample thickness. It is complicated but we can correct these in principle.

Yoshitaka AOYAMA (Tokyo, Japan), Hideo NISHIOKA, Yukihito KONDO

08:00 - 18:15 #6663 - IM01-141 Cryo-Electron Tomography of Protein Nanocrystals.

IM01-141 Cryo-Electron Tomography of Protein Nanocrystals.

    Knowing the structure of biological molecules plays a key role in understanding their function. To date there is no single structure determination method that would always work for every biological molecule. Instead there exists a plethora of complementary methods each with their specific advantages and disadvantages. Small but difficult to crystallize proteins have proven to be a particularly hard target for many of the conventional methods. However, proteins that do not crystallize to large well-diffracting crystals, could still form tiny nanocrystals in many cases, which could be completely invisible under regular light microscope or mistaken for amorphous precipitate when inspecting a crystallization drop.

    Using cryo-electron tomography we demonstrate in this work that real space imaging can be successfully used for structural studies of proteins that have been crystallized to nanocrystals. In this proof of principle research we have used cryo-electron tomography to image lysozyme (molecular weight 14 kDa) nanocrystals in the size range of 100 nm. We show that by exploiting crystallographic symmetry inherent in the tomogram and taking advantage of other possibilities provided by real space imaging we can achieve resolution in the range of 15Å.

    To achieve this result we have used in-house software for 3D reconstruction and subsequent refinement of the reconstruction using maximum entropy regularization algorithm together with contrast transfer function deconvolution. For processing crystalline data we have developed a toolkit for necessary 3D image processing tasks, such as peak detection, crystallographic lattice parameter search and refinement, and 3D image averaging and space group symmetrization.

    Nano-crystal tomography could become an established method in structural biology complementing the existing methods by firstly being applicable on small and difficult to crystallize proteins, but secondly on any general 3D reconstruction that includes crystallographically related parts. On top of studying one single averaged molecule, this method can also be used to analyze the nano-crystal as a whole and study the distribution of disorder within the crystal.

Maert TOOTS (Onna-son, Japan), Ulf SKOGLUND

08:00 - 18:15 #6677 - IM01-143 Three-dimensional electron imaging of dislocations from a single sample tilt.

IM01-143 Three-dimensional electron imaging of dislocations from a single sample tilt.

Linear crystal defects called dislocations are one of the most fascinating concepts in materials science that govern mechanical and optoelectronic properties of many materials across a broad range of application [1-3]. Three-dimensional (3-D) study of dislocation network is in principle accessible by conventional tomographic and stereoscopic techniques in Transmission Electron Microscopy (TEM) [4,5]. In these techniques in general the need to tilt the specimen for acquiring image series over large tilt ranges remain however an intricate problem, in particular when diffraction contrast or sensitivity to electron beam are involved [6].

Here, a novel method in scanning TEM (STEM) is presented that provides a reliable and fast assessment of the 3-D configuration of dislocations using data acquired from just one sample tilt. This technique acquires a stereoscopic pair of images by selecting different ray paths of a convergent illumination in STEM mode. The resulting images are then treated with a dedicated stereovision reconstruction algorithm, yielding a full 3-D reconstruction of dislocations arrangement. The success of this method is demonstrated by measurement of dislocation arrangements in two experimental cases.

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References

1- Mott, N. F. Dislocations and the Theory of Solids. Nature 171, 234-237 (1953).

2- Mohammad, S. N. & Morkoc, H. Progress and prospects of group-III nitride semiconductors. Progress in Quantum Electronics 20, 361-525 (1996).

3- Hua, G. C. et al. Microstructure study of a degraded pseudomorphic separate confinement heterostructure blue-green laser diode. Applied Physics Letters 65, 1331-1333 (1994).

4- Midgley, P. A. & Dunin-Borkowski, R. E. Electron tomography and holography in materials science. Nature Materials 8, 271-280 (2009).

5- Agudo Jácome, L., Eggeler, G. & Dlouhý, A. Advanced scanning transmission stereo electron microscopy of structural and functional engineering materials. Ultramicroscopy 122, 48-59, d (2012).

6- Barnard, J. S., Eggeman, A. S., Sharp, J., White, T. A. & Midgley, P. A. Dislocation electron tomography and precession electron diffraction - Minimising the effects of dynamical interactions in real and reciprocal space. Philosophical Magazine 90, 4711-4730 (2010).

Emad OVEISI (Lausanne, Switzerland), Letouzey ANTOINE, Fua PASCAL, Hebert CECILE

08:00 - 18:15 #6737 - IM01-145 3D structure and chemical composition reconstructed simultaneously from HAADF-STEM images and EDS-STEM maps.

IM01-145 3D structure and chemical composition reconstructed simultaneously from HAADF-STEM images and EDS-STEM maps.

Electron tomography (ET) is nowadays commonly used in materials science to obtain a three dimensional (3D) structural characterization of nanomaterials. Typically it is based on tomographic reconstruction from high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images yielding Z-contrast in final reconstructions. However, when investigating heteronanostructures with small differences in Z, spectroscopic techniques such as Electron Energy Loss Spectroscopy (EELS) and Energy Dispersive X-ray Spectroscopy (EDS) should be used. Here, we focus on EDS-STEM spectroscopic imaging. Recently, EDS in combination with electron tomography was demonstrated [1,2], but the quality of these reconstructions is limited because of the small signal-to-noise ratio (SNR) in the acquired elemental maps compared to HAADF-STEM projection images.
In this study, we propose to combine HAADF-STEM tomography with EDS-STEM tomography instead of processing both signals independently. This combination has not yet been completely explored except for using HAADF-STEM images for aligning EDS-STEM maps [3] and for estimating density to correct X-ray absorption [4]. Here, we introduce the concept of multi-modal tomography to ET by proposing a novel HAADF-EDS bimodal tomographic reconstruction technique. The technique is based on the physical model that both types of projection images are linearly related to the projections of chemical compositions. Based on this assumption, HAADF-STEM images can be approximated as a linear combination of EDS-STEM maps. By estimating the linear relation, we can scale EDS-STEM maps to the same physical unit as HAADF-STEM images. The two types of images are related and used together as the input data for one single reconstruction process. As a result, we are able to reconstruct 3D elemental distributions with reduced noise levels compared to conventional EDS-STEM tomography.
To evaluate the technique, it has been applied to two Au-Ag nanoparticles. The samples were imaged by an electron microscope (Tecnai Osiris, FEI) equipped with four silicon drift detectors (SuperX system, FEI). The first sample was tilted from -75° to 75° (from -70° to 70° for the second sample) with a step of 5°. At each tilt, a Z-contrast image was recorded by HAADF-STEM and two elemental maps for Au and Ag were generated from X-rays spectrum images acquired by EDS-STEM. Both the HAADF-STEM and the elemental maps were aligned using cross correlation algorithms.
The first sample that is investigated consists of a Ag nanoparticle with an embedded Au octahedral core. As indicated in Figure 1, Au and Ag are well separated in this sample. Consequently, a segmention based on the Z-contrast of a HAADF-STEM reconstruction can be considered as ground truth for the distributions of the chemical elements (Figure 1 (a)). As illustrated in Figure 1 (c), we are able to determine the 3D elemental distributions using our novel HAADF-EDS bimodal reconstruction technique. This figure indicates a significant improvement in comparison to conventional EDX tomography (Figure 1 (b)) where the raw elemental maps are used as input for a tomographic reconstruction.
The second sample that is investigated is an alloy of Au and Ag. Since no clear boundaries exist between the two compositions, we are not able to segment their 3D distributions based on the HAADF-STEM tomographic reconstructions. Although 3D compositional distributions can be reconstructed from EDS-STEM maps, the 3D image is very noisy and difficult to interpret (Figure 2 (b)). In comparison, the 3D compositional distributions reconstructed by HAADF-EDS bimodal tomography (Figure 2 (c)) provides more information on the concentration of the different elements while the outer shape of the nanoparticle agrees well with the 3D shape reconstructed from HAADF-STEM images (Figure 2 (a)).

References

[1] Z. Saghi, Applied Physics Letters, 91 (2007) p.251906.

[2] K. Lepinay et al, Micron, 47 (2013) p.43.

[3] B. Goris et al, Nano Letters, 14 (2014) p.3220.

[4] P. Burdet et al, Ultramicroscopy, 160 (2016) p.118. 

Zhichao ZHONG (Amsterdam, The Netherlands), Bart GORIS, Remco SCHOENMAKERS, Sara BALS, K. Joost BATENBURG

08:00 - 18:15 #6754 - IM01-147 Evaluation of feature-based registration algorithms for the improvement of tilt-series alignment in electron tomography.

IM01-147 Evaluation of feature-based registration algorithms for the improvement of tilt-series alignment in electron tomography.

Despite the high performance achieved in goniometers-control for tomographic tilt series recording, image alignment is mandatory to obtain accurate reconstructions. In addition, due to the high resolution currently expected from tomograms, the existing alignment methods (based on cross-correlation, fiducial-markers and landmarks) needs to be improved. Among the alternative methods presently proposed for general purpose image registration, Scale Invariant Feature Transform (SIFT) [1] has demonstrate its performance for the detection of common features occurring in several images. SIFT have been successfully used for panoramic stitching [2], object detection [1] and proposed as a method for image alignment in electron tomography [3].

We have implemented SIFT and two other feature-based registration algorithms porposed since the publication of SIFT (ORB [4], BRISK [5]) in TomoJ [6] and evaluated their respective performances on tilt series data from different transmission electron microscopy (TEM) acquisition methods (TEM, EFTEM, STEM, and Cryo-TEM). We will present here the results obtained validating the use of feature-based registration algorithms on tilt series from the different acquisition modes.

References:

[1] Lowe D.G. Distinctive Image Features from Scale-Invariant Keypoints. Int. J. Comput. Vision. 2004. 60:91-110

[2] Brown M. and Lowe D.G. Automatic Panoramic Image Stitching using Invariant Features. Int. J. Comput. Vision. 2007. 74:59-73

[3] Han R, Zhang F, Wan X, Fernández JJ, Sun F, Liu Z. A marker-free automatic alignment method based on scale-invariant features. J Struct Biol. 2014. 186:167-80.

[4] Leutenegger S, Chli M, Siegwart RY. BRISK: Binary robust invariant scalable keypoints. Computer Vision (ICCV), 2011 IEEE International Conference on, 2011, 2548-2555.

[5] Rublee E, Rabaud V, Konolige K, Bradski G. ORB: an efficient alternative to SIFT or SURF. Computer Vision (ICCV), 2011 IEEE International Conference on.  2011, p. 2564-2571.

[6] http://u759.curie.fr/fr/download/softwares/TomoJ

Amandine VERGUET (ORSAY CEDEX), Sylvain TREPOUT, Sergio MARCO, Cédric MESSAOUDI

08:00 - 18:15 #6842 - IM01-149 3D Reconstruction of Crack Formation in Austempered Ductile Cast Iron after an In-situ Tensile Test.

IM01-149 3D Reconstruction of Crack Formation in Austempered Ductile Cast Iron after an In-situ Tensile Test.

A 3D reconstruction has been performed on a flat specimen after an interrupted in-situ tensile tests. The sample consisted of austempered ductile cast iron (ADI) alloyed with 2.75% nickel. The analysis was performed by using a dual beam system (FIB-SEM).

The procedure of this analysis consisted in studying the elastic-plastic behaviour along with the nucleation and the growing of cracks in ADI at room temperature. For this purpose, a field emission scanning electron microscope (FE-SEM) equipped with a tensile stage was used.

A systematic observation at the same location on the sample surface was used to clarify the tensile curves which are in agreement with deformation mechanisms. During the testing, decohesion of graphite nodules from the matrix was observed. The cracks initiated mainly at the interface of graphite nodule and the austenitic matrix. The elastic-plastic region is connected with the plastic deformation of the matrix, growth and linking of microscopic cracks which are connected and finally leading to fracture of the sample.

Hana TESAŘOVÁ (Brno, Czech Republic), Jiří DLUHOŠ, Martin PETRENEC, Jaroslav POLÁK, Karel KRAHULA

08:00 - 18:15 #6879 - IM01-151 Ionic Liquids for biological SEM and FIB/SEM.

IM01-151 Ionic Liquids for biological SEM and FIB/SEM.

Ionic liquids are low-melting salts which are fluid at room temperature. They show a very low vapor pressure and are therefore persisting as liquids under the high vacuum conditions in an electron microscope.  They are electrical conductive as well. The ionic liquids tested are mixable with water, thus solutions can be easily diluted to suitable viscosities respectively concentrations.

In electron microscopy ionic liquids are used to coat surfaces and cavities to make samples conductive for SEM. This conductive treatment works on biological samples as well. In example, cell cultures or biofilms can be treated with ionic liquids. Preparation steps as heavy metal staining, dehydration and drying might be skipped. Cells get fixed for stopping life dynamics and structural stabilization and then directly treated with i.e. 10% Hitachi IL1000 before going into SEM without further coating.

Cells look adequate in SEM at lower magnification. At higher magnification structural collapse effects are visible. Occasionally charging phenomena can be observed. Additional block stain of the fixed sample with OsO₄ before ionic liquid treatment might attenuate collapse phenomena. Additional sputter coating after ionic liquid treatment prevents residual charging.

In an FIB/SEM cells treated with ionic liquids show after exposure to the ion beam no charging effects anymore; furthermore the backscatter electron signal is strongly enhanced. Cross sections of ionic liquid treated cells show a dense solid cytoplasm, which enables also consecutive sectioning for FIB/SEM-tomography.  While structural details are at best foreshadowed after ionic liquid treatment alone, substructures are visible after block staining with heavy metals before the treatment.

To find the relevant region of interest, correlation with light microscopic data is an option. Ionic liquid treatment doesn't interfere with fluorescence detection in light microscopy. But also direct evaluation of characteristic topographic features of the sample surface can help to find the spot of interest for a FIB/SEM-tomogram.

Ionic liquid treatment makes biological samples vacuum resistant and conductive. The biofilm-substrate interface stays intact. These features are offered also by freeze-drying / critical-point-drying or thin-layer-plastification as well. Compared with other preparation techniques, ionic liquid treatment of biological specimen combines several advantages: preparation time is short, cell topography is clearly visible, cross sections show compact internal structures, fluorescence is preserved. But there are drawbacks as well: compared to freeze-drying or critical-point-drying, the surface is less well preserved; compared to thin-layer-plastification the internal structural is less clear exposed.

For FIB/SEM-studies, ionic liquid treatment is a compromise between critical-point-drying (excellent surface feature preservation, spongy aggregated structures inside the cells) and thin-layer-plastification (excellent internal structure preservation, poor topographic feature exposure). It is a quick SEM-preparation approach and the interface between cells and the substrate stays preserved.

Anne Greet BITTERMANN (Zurich, Switzerland), Simona RODIGHIERO, Roger WEPF

08:00 - 18:15 #6914 - IM01-153 Three-dimensional characterization of Ni-Sm0.2Ce0.8O2-δ cermet for SOFC anodes by high-resolution FIB-SEM Tomography.

IM01-153 Three-dimensional characterization of Ni-Sm0.2Ce0.8O2-δ cermet for SOFC anodes by high-resolution FIB-SEM Tomography.

A solid oxide fuel cells (SOFC) technology is one of the most promising energy conversion device due to its high conversion efficiency, low environmental pollution and high flexibility to various fuel types (1). To overcome all challenges concerning materials’ long term stability at operating temperatures, current research efforts are aimed at intermediate temperature SOFCs (IT-SOFCs) (2). From this point of view samaria-doped ceria (SDC) is a promising ceramic material, with superb ionic conductivity, which can be used as an anode material when combined with Ni in Ni-SDC cermet (2). To ensure electronic and ionic conductivity as well as gas permeability, anode cermet should exhibit carefully tailored microstructure where metallic Ni, ceramic SDC and pores form continuous phases. In an operating cell fuel gas is electrochemically oxidized at the Ni/SDC/fuel interface, called the triple phase boundary (TPB) region. Therefore, both the activity of Ni-SDC and their stability are strongly influenced by the cermet’s morphology and microstructures such as volume fractions, grain connectivity, grain size, pore size, pore distribution and TPB length (3).

For this purpose, exact and accurate microstructural determination is crucial in predicting material's performances in an operating cell. In such a cermet composite critical topological features such as connectivity and the tortuosity of transport pathways in the pores can only be established based on 3D microstructural information (4). In this work we present quantitative characterization of Ni-Sm_0.2Ce_0.8O_2-δ cermets, sintered at 1400°C, using high-resolution FIB-SEM tomography. Initially, a layer of platinum was deposited ontop the region of interest to protect the surface and prevent rounding of top edges of cross section during milling. The volume of interest was separated using an optimised U-pattern pre-milling procedure to prevent material redeposition and shadowing of the signals used for imaging and microanalysis. The sample was then serial sectioned using an automated slicing procedure with drift correction algorithms to obtain a series of 2D images with narrow and reproducible spacing between the individual image planes (5). Experimental milling and imaging parameters have been optimised in order to obtain a high quality 3D reconstruction with phase contrast information. Individual phases were identified from EDXS elemental maps and further segmented according to their grey level. 3D reconstructed volume is a base for determination of: volume fractions of individual phases, grain connectivity, porosity, tortuosity and TPB length.

The presented analytical method will serve as a tool for quantitative characterization of primary microstructural parameters and complex topological features, during microstructure evolution of Ni-Sm_0.2Ce_0.8O_2-δ anode cermets using various sintering procedure.

 

Literature:

[1] S.C. Singhal, K. Kendall, High Temperature Solid Oxide Fuel Cells, Elsevier, 2002

[2] Liu Q, Dong X, Yang C, Ma S, Chen F, J Power Sources 2010; 195: 1543-1550

[3] S.D. Kim, H. Moon, S.H. Hyun, J. Moon, J. Kim, H.W. Lee, J Power Sources, 163 (2006) 392-397

[4] Brus, G., Miyawaki, K., Iwai, H., Saito, M., & Yoshida, H., Solid State Ionics, 265(2014), 13–21

[5] M. Schaffer, J. Wagner, B. Schaffer and M. Schimed, Ultramicroscopy, 107, pp. 587-597 (2007)

Gregor KAPUN (Ljubljana, Slovenia), Sašo ŠTURM, Marjan MARINŠEK, Miran GABERŠČEK

08:00 - 18:15 #6973 - IM01-155 Image deconvolution for fast Tomography in Environmental Transmission Electron Microscopy.

IM01-155 Image deconvolution for fast Tomography in Environmental Transmission Electron Microscopy.

The development of environmental transmission electron microscopes (ETEM) as well as the environmental cells opens the way towards operando electron microscopy. In this respect, following the genesis of a catalyst or a chemical reaction in real time under gaseous environment and at high temperature becomes a possibly achievable goal. Performing such experiments in 2D has already been successful but reaching the 3D information in environmental mode is a challenge as it needs to acquire 2D projections fast enough in order to neglect the morphological evolution of the object during the chemical reaction, a required condition to reconstruct a 'correct' 3D object. Under environmental conditions, the motion and evolution of the objects might produce images corrupted by motion which will then exhibit possibly significant blurred features. These defaults might even be enhanced if one intends to acquire the image series rapidly. This contribution will address the possibility of using deconvolution methods to un-blur such environmental microscopy images. Here we mainly focus on the motion blur correction of corrupted images: g(x,y)=(f*h)(x,y)+n(x,y)

where * denotes the convolution product, f is the ground truth image, g is the observed corrupted image, h is the degradation function of the acquisition system and n is the noise.

The Minimum Square Error (MSE) and the Constrained Least Square (CLS) filtering are used as image restauration processes as shown in figure 1. Both of them require the degradation function h as input and the parameter k related to the noise level which has to be optimized. Denoting G the Fourier transform of the corruptedimage, the Fourier transform of the reconstructed image is then given by one of the following relations:

    F_MSE(u,v)= G(u,v).H(u,v)^c / (|H(u,v)|²+k) and   F_CLS(u,v)= G(u,v).H(u,v)^c / (|H(u,v)|²+k.|P(u,v)|²),

where H is the Fourier transform of the convolution filter h, H^c is its complex conjugate, P is the Fourier transform of the Laplacian filter and |.| denotes the complex modulus. However, the degradation function is not known in practice and has to be estimated from the experimental images. The estimation of a parametric degradation function h is performed by image comparison: so far we manually determine the unknown parameters by comparing the blurred image with a similar one not affected by the motion blur. We will present perspectives of this work in terms of running an automatic estimation of the convolution kernel for in-situ image processing of experimental micrographs.

 

Acknowledgements

Thanks are due to CLYM (Consortium Lyon - St-Etienne de Microscopie, www.clym.fr) for the access to the microscope funded by the Region Rhône-Alpes, the CNRS and the 'GrandLyon'. This work was supported by the BQR SPEE3D granted by INSA Lyon, ANR project 3D-CLEAN, Labex iMUST and IFP Energies nouvelles.

Yue-Meng FENG (VILLEURBANNE CEDEX), Khanh TRAN, Siddardha KONETI, Lucian ROIBAN, Anne-Sophie GAY, Cyril LANGLOIS, Thierry EPICIER, Thomas GRENIER, Voichita MAXIM

08:00 - 18:15 #7044 - IM01-157 3D investigation of a PS/ABS polymer using different microscopy techniques.

IM01-157 3D investigation of a PS/ABS polymer using different microscopy techniques.

This study is developed in the framework of RéCaMiA, a French regional (Rhône-Alpes / Auvergne) microscopy network, which offers facilities through a panel of microscopes such as SEM, TEM, FIB, AFM, Confocal.

The objective of the study is to compare results obtained with different microscopy techniques currently employed in three-dimensional imaging, but it focuses more specifically on polymers [1-3]. For the purpose a blend of polystyrene / acrylonitrile butadiene styrene (PS/ABS) is chosen. This material allows to obtain good contrast in SEM or TEM; microstructural features exist at the submicronic and nano scales; and its glass transition temperature T_g is above 50°C. Two samples are stained by contact with a 4% osmium tetraoxide (OsO₄) water solution to reveal contrasts. 2 FIB–SEM (FEI Helios600, Zeiss NVisio40 dual beam), 2 TEM (JEOL JEM 1400, JEOL 2100F), 2 SEM (FEI ESEM XL30, FEI Quanta250) are used to study the 3D structure of the polymer. This study develops a multiscale approach. The tools used require 2 different procedures to get a 3D image: (a) to acquire a stack of individual imaging planes and to transform it into the corresponding 3D data volume or (b) to perform electron tomography. In the first case individual images are obtained through a slice and view approach in FIB, or through individual observations, in SEM or TEM, of several thin foils prepared by ultramicrotomy method at room temperature (serial sectioning). Backscattered electron images are collected. The 3D reconstructions of the polymer obtained by the different tools are compared each other. Whatever the approach used, the morphology “salami” shape is observed in the structure of the polymer. Structure parameters, such as size distribution of the nodules and the anisotropy coefficient, are extracted and compared. Different test parameters effects are investigated, the preparation of the sample (compression of the microstructure more or less), the voltage, the spacing between the individual imaging planes, etc. The balance between the reconstructed volume size, the voxel resolution, the total time to get the 3D data volume, the advantages and limitations of the methods used in this multiscale approach will also be discussed.

[1] M. Kato et al., Journal of Polymer Science: Part B: Polymer Physics, Vol. 45, 677–683 (2007)
[2] S. Sinha Ray, Polymer 51 (2010) 3966-3970
[3] P. Jornsanoh et al., Ultramicroscopy 111(2011)1247–1254

Acknowledgements            

Thanks are due to CNRS for its financial support. Thanks are due to CLYM (Consortium Lyon - St-Etienne de Microscopie, www.clym.fr) funded by the Region Rhône-Alpes, the CNRS and the 'GrandLyon', for the access to the Zeiss NVisio40 and XL30; and also to Manutech-USD for the access to the FEI Helios600.

X. JAURAND, E. ERRAZURIZ, S. REYNAUD, Th. DOUILLARD, F. DALMAS, S. DESCARTES (VILLEURBANNE), F. SIMONET, M. MONDON, I. ANSELME-BERTRAND

08:00 - 18:15 #6310 - IM02-159 In situ tensile testing of silica glass membranes in the TEM.

IM02-159 In situ tensile testing of silica glass membranes in the TEM.

Increasing research on strength of glasses, which was greatly influenced by Griffith [1], has spawn strengthening strategies such as topological engineering [2]. Pioneering works by Takamori and Tomozawa [3], and Brückner [4] on cooling of glass melts under load and introducing structural anisotropy into the glass structure have been followed by strengthening of glasses by targeted mechanically-induced structural anisotropy [5]. Moderate electron beam (e-beam) irradiation has been exploited to induce enormous ductility and superplasticity into nanoscale silica spheres and wires, and was shown to affect their mechanical response [6-8]. It is, however, not yet known whether e-beam irradiation in combination with tensile loading can lead to anisotropic glasses, and how this affects their mechanical properties.

Recently we have reported that e-beam-assisted quenching under load inside the transmission electron microscope (TEM) alters the mechanical properties of nanoscale silica spheres and attributed this to compression-induced structural anisotropy [9]. Here we transfer this approach to tensile loading of nanoscale silica membranes. Tensile specimens are prepared with the focused ion beam (FIB) from commercially available silica membranes (Plano GmbH) on push-to-pull (PTP) devices (Fig. 1). Raman spectroscopy was performed to investigate the structure of silica membranes and damage induced by FIB (Fig. 2). Raman spectra show that as-received membranes exhibit a structure of vitreous silica [10,11]. After Ga-irradiation in the FIB densification of the membranes occurs, while the membranes still maintain the character of vitreous silica. In situ tensile experiments are carried out with the Hysitron PI95 TEM Picoindenter^TM inside of a Titan³ Themis 300. To achieve mechanical quenching inside the TEM moderate e-beam irradiation is used to mimic temperature, while the e-beam is switched off during elongation of the silica membrane. While the deformation of silica under e-beam irradiation is superplastic [6], the sudden absence of the e-beam during tension (quenching point) translates the deformation from superplastic to elastic (see Fig. 3a)), and finally leads to fracture. The Young’s modulus E = 73 GPa of the membrane drawn at beam-off conditions (Fig. 3b)) almost matches the value known for bulk fused silica [12], while the value of the membrane quenched under load (E = 78 GPa) is slightly increased. The tensile strength is in the range of values known from silica glass fibers with comparable dimensions [13], but clearly exceeds values known for microscale silica glass fibers [5]. Finally, we demonstrate how to directly track structural changes in silica glass during in situ tensile experiments in TEM by in situ electron diffraction. The unique combination of in situ electron diffraction with tensile experiments in TEM enables direct relation of structural changes in silica glass to quantitative nanomechanical data.

[1] A.A. Griffith, Phil. Trans. R. Soc. London 1921, 221, 163.
[2] L. Wondraczek et al., Adv. Mater. 2011, 23, 4578.
[3] T. Takamori, M. Tomozawa, J. Am. Ceram. Soc. 1976, 59, 377.
[4] R. Brückner, Glas. Berichte Glas. Sci. Technol. 1996, 69, 396.
[5] M.D. Lund, Y. Yue, J. Am. Ceram. Soc. 2010, 93, 3236.
[6] K. Zheng et al., Nat. Commun. 2010, 1, 24.
[7] S. Romeis et al., Rev. Sci. Instrum. 2012, 83, 95105.
[8] M. Mačković et al., Acta Mater. 2014, 79, 363.
[9] M. Mačković et al., submitted.
[10] A. Perriot et al., J. Am. Cer. Soc. 2006, 89, 596.
[11] S. Romeis et al., Scr. Mater. 2015, 108, 84.
[12] W.C. Oliver, G.M. Pharr, J. Mater. Res. 1992, 7, 1564.
[13] G. Brambilla, D.N. Payne, Nano Lett. 2009, 9, 831.
Financial support by the Deutsche Forschungsgemeinschaft (DFG) through the SPP1594 “Topological Engineering of Ultra-Strong Glasses”, Cluster of Excellence EXC 315 “Engineering of Advanced Materials” and GRK1896 “In situ microscopy with electrons, X-rays and scanning probes” is gratefully acknowledged.

Mirza MAČKOVIĆ (Erlangen, Germany), Hana STARA, Thomas PRZYBILLA, Christel DIEKER, Florian NIEKIEL, Patrick HERRE, Stefan ROMEIS, Nadine SCHRENKER, Wolfgang PEUKERT, Erdmann SPIECKER

08:00 - 18:15 #6320 - IM02-161 Environmental (S)TEM analysis of Fe nanoparticles under oxygen atmosphere.

IM02-161 Environmental (S)TEM analysis of Fe nanoparticles under oxygen atmosphere.

Fe and Fe-oxide nanoparticles (NPs) have a number of promising potential applications in physical and medical sciences. These include magnetic storage devices, catalysis, sensing, contrast enhancement in magnetic resonance imaging and magnetic hyperthermia [1-3]. Understanding of the metallic Fe NPs formation and the oxidation processes down to atomic scale is paramount for the control of the quality and the optimization of their applications. Although metals oxidation has been studied for decades on bulk and thin film materials [4], there is still a lack of studies on the oxidation of metallic iron at the nanoscale.  A recently modified double aberration corrected JEOL 2200FS (S)TEM [5] has demonstrated the possibility of the analysis of metallic nanoparticles in gas environment at temperature allowing single atom visualisation by HAADF STEM in controlled gas reaction environment [6]. In this study, thin films of Fe were deposited by sputtering ex-situ on C films supported by standard TEM Cu grids. Nanoparticles were produced by annealing in vacuum within the microscope column the pre-sputtered iron thin films (see Figure). Nanoparticle formation and size distribution was monitored in-situ as a function of time and temperature by HAADF STEM imaging. After annealing, nanoparticles were shown to consist of single crystal metallic Fe. The Fe nanoparticles interaction with the Oxygen atmosphere was studied in-situ at 300 °C with an Oxygen partial pressure at the specimen in the range of 2*10^-2 Pa to 2 Pa. The interaction of the nanoparticles with the gas will be discussed in terms of the oxidation mechanism as well as the changes in nanoparticle geometry, composition, size distribution, and crystallinity.

References:

[1] B D Terris and T Thomson, J. Phys. D 38, R199–R222 (2005)

[2] H. Galvis et al. Science 335, 835–838 (2012)

[3] Q A Pankhurst, N T K Thanh, S K Jones, and J Dobson, J. Phys. D 42, 224001 (2009)

[4] N. Cabrera and N. F. Mott, Reports on Progress in Physics, Volume 12, Issue 1, pp. 163-184 (1949)

[5] P L Gai and E D Boyes, Microscopy Research and Technique 72, 153 (2009)

[6] E D Boyes, M R Ward, L Lari, and P L Gai, Annalen der Physik 525, 423 (2013)

Acknowledgement: We thank the EPSRC (UK) for research grants EP/J018058/1 and EP/K03278X/1

Leonardo LARI (York, United Kingdom), Robert CARPENTER, Vlado LAZAROV, Pratibha GAI, Ed BOYES

08:00 - 18:15 #6373 - IM02-163 Depth Dependence of the Spatial Resolution in Scanning Transmission Electron Microscopy Experiments.

IM02-163 Depth Dependence of the Spatial Resolution in Scanning Transmission Electron Microscopy Experiments.

Annular dark field scanning transmission electron microscopy (STEM) is capable of imaging thick specimens. The capability to image thick specimens is relevant, for example, for studying cells embedded in plastic section, polymeric materials in which nanoparticles are embedded, metallic samples containing several phases, and for research on liquid specimens.  However, at sample thicknesses larger than the mean free path length for elastic scattering in the materials under investigation, significant scattering of the beam occurs that leads to beam broadening. This beam broadening results in a reduction of the spatial resolution that becomes more pronounced the deeper the focus of the electron probe is within the sample. In addition, the spatial resolution even of objects focused at the top of the specimen (with respect to a downward traveling electron beam) is reduced by thick materials underneath as the contrast and hence the signal-to-noise ratio decreases in thicker specimen at a given electron dose in STEM experiments. The dependency of the spatial resolution on the specimen thickness was already determined experimentally, via simulation, and in various analytical models [1] for objects at the top and at the bottom of thick specimen. An initial theoretical model was developed about the effective resolution obtained for an object at a certain depth within a scattering matrix [2] but experimental verification is lacking. In this work, we examine the effect of beam broadening on nanoparticles at specific vertical positions within thick samples.

 

For our experiment, we chose to study gold nanoparticles embedded in a solid aluminum film as experimental model system. We deposited multiple layers of aluminum by physical vapor deposition on silicon chips featuring thin (50 nm), electron beam transparent silicon nitride windows in the center, through which the imaging was done. Gold nanospheres of 5-10 nm in diameter were placed between individual layers. By using solid aluminum as support material we benefit from immobilized gold nanoparticles at specific vertical positions in an electrically conducting and stable matrix. Gold nanorods were deposited on top and at the bottom of the aluminum film enabling us to determine the thickness of the aluminum film by tilting the specimen holder. The setup of our experiments is illustrated in Figure 1. The experiments were conducted using a C_S-corrected STEM/TEM (ARM200f, JEOL, Japan) at 200 kV acceleration voltage.

 

Our experiments confirmed that the vertical position of the gold nanoparticles within the aluminum matrix determines the spatial resolution. Particles positioned deeper within the Al matrix were imaged with a lower spatial resolution than those closer to the top surface, where the electron beam entered the specimen. This observation is illustrated in the electron micrographs in Figure 2. The same particles are shown from the different sides of the silicon chip with respect to the direction of the electron beam. In a) the Au nanoparticles are in a depth of 0.48 µm, while in b) they are below only 0.18 µm of Al in. The total thickness of the Al matrix is 0.62 µm. The spatial resolution was determined by analyzing intensity profiles over the particles. Interestingly, the change in the spatial resolution was only reflected by the distance from 25% to 75% of the maximum intensity (d_25-75), but not in the full widths at half maximum of the intensity profiles (see Figure 2c). Thus the scattering mainly led to increased beam tails.

 

[1] H Demers et al., Microsc Microanal 18 (2012), p. 582.

[2] T Schuh and N de Jonge, C R Phys 15 (2014), p. 214.

[3]   We thank E. Arzt for his support through INM. Research in part supported by the Leibniz Competition 2014.

Andreas VERCH (Saarbrücken), Niels DE JONGE

08:00 - 18:15 #6455 - IM02-165 Structural dynamics of copper and nickel substrates during redox reactions studied by in-situ SEM.

IM02-165 Structural dynamics of copper and nickel substrates during redox reactions studied by in-situ SEM.

Many efforts have been made in order to investigate catalyst under relevant working conditions. Indeed, the action of a chemical potential has to be considered when describing the state of a catalyst and recent studies have demonstrated that the catalyst surface evolves dynamically under reaction conditions.^1,2 In order to complement in-situ spectroscopic tools, such as near-ambient X-ray photoelectron spectroscopy (NAP-XPS), with visual information about the active surface structure, we have implemented in-situ scanning electron microscopy (SEM) for the observation of dynamic processes at the µm to nm scale.³

By coupling in-situ SEM with mass spectroscopy measurements we are able to relate structural dynamics at the surface of metal catalysts to changes in the gas phase composition. Under specific conditions, oscillatory behavior of the catalyzed chemical reactions was observed for the case of polycrystalline nickel and copper foils. These self-sustained oscillations can be utilized to assist the identification of kinetic mechanisms⁴ and provide insight in the active and inactive state of the catalyst. Figure 1. demonstrates the dynamic behavior of the Ni foil surface during hydrogen oxidation reaction at 600°C. The active state in the oscillatory reaction is dominated by the presence of metallic Ni, while the formation of NiO coincides with a decrease in the catalytic activity. Overall, it will be outlined how the implementation of complementary in-situ SEM and mass spectroscopy techniques can enrich our understanding of the dynamic behavior of active catalyst and how it complements spatially integrated spectroscopic data that is recorded under similar conditions.

Figure 1. Self-sustained oscillations during hydrogen oxidation over a Ni foil at 0.3 mbar with a hydrogen/oxygen ratio of 7:1. The SEM images correspond to the different points in the oscillation as indicated in the mass spectrum profile. During the active state of an oscillation (A), the Ni foil exhibits a smooth metallic surface. In the low-active state (B), the surface of the Ni foil is covered by an oxide layer.

 

 

References

[1]T. Lunkenbein et al., Angew. Chem. Int. Ed. 2015, 127, 15, 4627-4631

[2] S. Piccinin et al., Phys. Rev. Lett. 2010, 104, 035503

[3] Z. J. Wang et al., ACS Nano., 2015, 9, 1506-1519

[4] V. V. Kaichev et al., Surf. Sci., 2013, 609, 113-118

Jing CAO, Jing CAO (Berlin, Germany), Ali RINALDI, Zhu-Jun WANG, Gisela WEINBERG, Marc WILLINGER, Robert SCHLÖGL

08:00 - 18:15 #6495 - IM02-167 Automatic FIB-SEM Preparation of Straight Pillars for In-Situ Nanoindentation.

IM02-167 Automatic FIB-SEM Preparation of Straight Pillars for In-Situ Nanoindentation.

In-situ indentation tests in FIB-SEMs are a powerful tool to characterize the mechanical deformation properties of matter at the micron scale [1,2]. FIB milling is used to produce micrometer sized – usually cylindrical – pillars from the bulk, while SEM imaging allows to determine the geometry of the pillars prior, during and after the load-displacement data acquisition. In this work, different automatic workflows were tested for the preparation of high aspect-ratio pillars with well-defined geometries, in particular with perfectly perpendicular side walls.

A state-of-the-art FIB-SEM instrument was used to fabricate the pillars. They were machined by milling a series of concentric rings with decreasing FIB currents into the sample. Hereby, the sample was at 54° tilt to ensure normal incidence of the FIB. The last and smallest ring was milled with a 3 nA probe, which yielded a slightly material dependent pillar wall angle of around 2° to the sample normal. After this pre-preparation step, the geometry of the pillars was refined further to achieve perfectly perpendicular pillar side walls using lathe milling [3]. The ideal cylindrical geometry is highly desirable, because it is easier to model for a reliable analysis of the load-displacement measurement.

Two different lathe milling techniques were implemented in this work and compared. They both involve a number of FIB milling steps each performed at different sample rotations to shape the pillar wall along its whole circumference. After each sample rotation the pillar needs to be repositioned accurately by means of SEM and FIB image recognition of fiducial marks on the sample.

The first approach, #1, is similar to the one described in [3]. The walls of the pillar are shaped from the side by FIB milling at zero degree stage tilt as shown in Figure 1(a). For sample repositioning a single fiducial is used which is placed – for symmetry reasons – exactly in the center of the pillar (see Figs. 1(b) and (c)). Approach #1 was automated using the application programming interface (API) of the FIB-SEM instrument. Including lathe milling the total preparation time per typical pillar adds up to about an hour. Because of the space needed for the fiducial mark only pillars with diameters, d>5 µm, can be fabricated automatically in this way.

The need to fabricate smaller pillars with d<5 µm motivated an alternative and new lathe milling workflow, #2 (see Figure 2). Here, the walls of the pillar are shaped from the pillar top (sample at 54° tilt), as it was done in the pre-preparation step, too. By slightly under-tilting the sample a few degrees an edge of the pillar was exposed to the FIB for machining (Fig 2(a)). The sample was then rotated and repositioned for the next milling step. This process was iterated to cover the full circumference of the pillar. In order to reduce the number of iterations the milling was done following the green boomerang type of shape depicted in Figure 2(b). Only eight iterations – as compared to at least 18 with approach #1 – were needed to obtain an almost perfectly circular pillar cross section (see Fig. 2(c)).

In summary, the new lathe milling process can be used to machine very small pillars. It can be combined easily with the pillar pre-preparation step for a fully automatic pillar preparation. Further, because it gets along with less iterations, it is faster than previous approaches.

References:

[1] J.R. Greer et al., Acta Materialia 53 (2005), p. 1821.

[2] D.M. Dimiduk et al, Acta Materialia 53 (2005), p. 4065.

[3] M.D. Uchic and D.M. Dimiduk, Mat Sci. Eng. A 400-401 (2005), p. 268.

Tobias VOLKENANDT (Oberkochen, Germany), Alexandre LAQUERRE, Michal POSTOLSKI, Fabián PÉREZ-WILLARD

08:00 - 18:15 #6523 - IM02-169 In situ liquid-cell transmission electron microscopy of Y-based precursor growth dynamics at elevated temperatures.

IM02-169 In situ liquid-cell transmission electron microscopy of Y-based precursor growth dynamics at elevated temperatures.

Yttrium oxide (Y2O3) nanoparticles (NPs) as a host for heavy rare earth elements (Yb3+, Eu3+) have
shown to be an efficient up-conversion phosphor material with a great potential ranging from therapy
and sensing for drug delivery to photovoltaic applications [1]. In order to achieve desired morphology
and size distribution of Y2O3 NPs the nucleation and growth pathways of Y-based precursors need to be
thoroughly understood. Unfortunately, the mechanism controlling the nucleation and growth of NPs are
often difficult to assess and are conventionally studied by indirect methods. On the contrary, in-situ
transmission electron microscopy (TEM) combined with the specialized liquid cell offers both,
unprecedented experimental and characterization tool for a direct study of nanoparticle’s nucelation and
growth phenomena from solutions.
To perform in-situ TEM experiments Jeol JEM 2100 TEM equipped with Protochips Poseidon 300
liquid flow cell with a heating capability was employed. The synthesis of Y-based precursor NPs was
performed from the solution of urea, yttrium acetate and minor amounts of HNO3 to facilitate efficient
dissolution of yttrium acetate. The solution was sealed between two specially designed chips forming a
close container with the viewing area of 40 x 50 m and the water layer thickness of 150 nm. The urea
precipitation method was selected because [2] it can be well controlled by the temperature of the
solution, triggering the homogenous decomposition of urea throughout the whole chamber volume and
consequently the uniform precipitation of Y-based precursor nuclei, typically Y(OH)(CO3).
To properly evaluate the electron beam effect during the in-situ observation the so prepared solution
was first observed for 30 minutes at room temperature and at dose rate of 5000 e-/nm2*s. No evident
precipitation occurred during that time. This initial experiment served as a confirmation that additional
chemical species that were created during the radiolysis of water (solvated e-, OH-, H0, OH0, H2, H2O2,
H3O+, HO2, …) under the influence of incoming electron beam did not have significant influence on the
nucleation of NPs at the room temperature [3]. The new feature of the in-situ holder setup, which adds
an extremely important thermodynamic variable in the experiment, temperature, allowed us to perform
in-situ heat-triggered nucleation of Y-based precursor NPs. Namely, the abrupt nucleation of NPs was
observed when the temperatures in the cell was raised above 90 °C. Although different morphologies of
nanoparticles could be observed during the nucleation and growth period, in this study we focused only
on NP’s with clear hexagonally shaped faces (Fig. 1). These particles grew with an average growth rate
of a 0.5 nm/s to an average size of 25 nm and remained stable during the whole experimental observation
period (Fig. 2). Selected area electron diffraction (SAED) patterns showed that these NPs were
crystalline already in the early stage of growth period.
The formation of well crystalline nanoparticles by urea precipitation method is unexpected since the
typical products of this reaction result in the formation of Y(OH)(CO3) amorphous precursor. The
formation of crystalline NPs can be explained by the fact that radiolytic decomposition of water provides
additional reactive species in the final solution [3]. One plausible explanation could be that the increase
of [(OH)-] concentration at elevated temperatures, a combined effect of water and urea decomposition,
will promote the precipitation of stable hexagonally shaped Y(OH)3 particles [4].
References:
1. Lojpur, V.M., et al. (2013). Nanoscale Res Lett, 8, 131-137.
2. Qin, H., (2015). Cermic International, 41, 11598-11604.
3. Schneider, N. M., et al. (2014). J. Phys. Chem. 118(38), 22373-22382.
4. Huang, S., et al. (2012). Mater. Chem., 22, 16136-16144.

Bojan AMBROŽIČ (Ljubljana, Slovenia), Nina KOSTEVŠEK, Kristina ŽUŽEK ROZMAN, Marjan BELE, Sašo ŠTURM

08:00 - 18:15 #6531 - IM02-171 Electrical transport-induced transformations in filled carbon nanotubes imaged in situ by scanning-transmission electron microscopy.

IM02-171 Electrical transport-induced transformations in filled carbon nanotubes imaged in situ by scanning-transmission electron microscopy.

Fe-filled CNTs have been proposed as perfect candidates for a large and varied number of applications, ranging from the biological to optoelectronics or memory storage devices ¹. For any of these applications, it seems crucial to understand the interactions, possible phase changes or reactions that can take place as a consequence of exposure to their real working conditions.

Conventional bright-field transmission electron microscopy (BF-TEM) has been the tool traditionally employed to observe in-situ the dynamical effects that take place when individual CNTs are exposed to high electrical currents and Joule heating. Despite being a much more powerful technique, scanning TEM-annular dark field (STEM-ADF) imaging has been scarcely for this purpose ². One of the main advantages of STEM-ADF over BF-TEM is that the intensity in the images is highly dependent on the atomic number of the species which are present and therefore the images show compositional in addition to structural information. This gives a much better understanding of current-induced migration effects, formation of intermediate phases or alloying, or phase separation phenomena, something that using BF-TEM alone would miss. In the STEM-ADF configuration it is also possible to perform complementary analytical spectroscopy simultaneously with imaging and with comparable spatial resolution.

Here we show the significant advantages of combining in-situ experiments with STEM-ADF and related analytical techniques to gain new insights into the electrical transport induced transformations in P, N doped Fe-filled carbon nanotubes. It has been possible to monitor in real time a multistage process (Figure 1) in which the Fe filling reacts with nitrogen to form an intermediate alloy phase, which then decomposes into smaller particles. The presence of N₂ gas within the inner channel of the tube is found to be crucial to this process.

 

1. S. Costa et al.,  Phys. Stat. Sol. B 2007, 244, 4315-4318.
2. Y. Beyer et al., Micron 2012, 43, 428–434.

Juan G LOZANO (Oxford, United Kingdom), Zabeada ASLAM, Rebecca J NICHOLLS, Antal A KOOS, Frank DILLON, Michael SARAHAN, Peter D NELLIST, Nicole GROBERT

08:00 - 18:15 #6535 - IM02-173 Cobalt-cerium coating formation: from ESEM to TEM analyses.

IM02-173 Cobalt-cerium coating formation: from ESEM to TEM analyses.

 

Fuel cells are promising devices for clean energy conversion. In particular, Solid Oxide Fuel Cells (SOFC) convert natural gas and biogas into electricity and cogenerated heat with high efficiency. Their working temperature (700-800°C) may induce performance degradation in the cells and other components of the stack (series connection of cells). Chromium contamination of the cathode (oxygen electrode) is one of these degradation processes in SOFC. This chromium evaporates from the steel bipolar plates which act as cell-to-cell interconnections. To suppress the Cr evaporation, the steel plates must be coated. Understanding the degradation phenomena in the stack and their evolution helps to improve the device lifetime. Post test analyses are not always sufficient but already give good insight. On the other hand, the possibility to reach operating temperature (up to 900°C) and to mimic operating conditions of oxidising and reducing atmospheres (up to 30 Pa of O₂ or H₂) in environmental SEM allow to observe the evolution in near real condition. In this study, Sandvik Material Technology’s cerium-cobalt protective PVD coating on top of the Sandvik SSHT steel (FeCr-base) is analysed. Several movies were recorded during Cr-oxidation growth underneath the coating for 48 hours in an ESEM in oxidising conditions (900°C, 30Pa O₂). Growth observation of the oxide layer allows a better understanding of the behaviour of the steel with this protective layer. Pore stability, elemental diffusion through layers and the probable formation of new phases were observed by SEM and EDX. FIB was used for cross section observations and will be used for TEM lamella preparation in future. TEM observation will allow to analyse the formed crystals by diffraction and establish a correlation with  the observations from the ESEM experiments. 

Acknowledgements

Max Planck Institute – EPFL centre for the funding of this project.
Dr Marco Cantoni(EPFL-CIME) for FIB preparation.

Stéphane POITEL (Lausanne, Switzerland), Jun ZHU, Cécile HÉBERT, Jan VAN HERLE, Marc WILLINGER

08:00 - 18:15 #6537 - IM02-175 Monitoring the Dynamics of Heterogeneous Catalysts by Electron Microscopy.

IM02-175 Monitoring the Dynamics of Heterogeneous Catalysts by Electron Microscopy.

It is known that the shape of metal catalysts adapts to the chemical potential of the surrounding atmosphere and that the active surface evolves dynamically under reaction conditions [1-3].
Different photon-based characterization techniques were improved and implemented to probe the active state of catalysts in situ. However, such techniques lack the spatial resolution as they provide information averaged over a macroscopic scale, which is much larger than the catalytic active nanostructures.

 

In situ Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) have proven to be powerful techniques for the study of atmosphere and temperature induced morphological or compositional changes of catalysts at micrometer- to atomic resolution scale [4,5]. Furthermore, in situ SEM and TEM can be used as complementary methods from the point view of applicable pressure range and the obtainable resolution. Hence, pressure gap impact on the catalyst’s activity can be assessed using these techniques. On the other hand, collective (SEM) and individual (TEM) phenomena on the surface of active catalysts in the micro- to nanometer scale can be compared and correlated to the reactivity.

In this contribution we present in situ TEM and SEM results of the catalytic oxidation of hydrogen on copper. Aside from the fundamental importance of this catalytic system, the investigation of copper redox chemistry is of great importance for decisive future energy related catalytic processes as methanol synthesis or water gas shift reaction.

In situ SEM experiments were conducted in the chamber of a modified commercial environmental scanning electron microscope. The instrument is equipped with a home-built heating stage and attached to a gas feeding system and a mass spectrometer for product analysis.

In situ TEM experiments were carried out using a Protochips, Inc. gas flow holder equipped with an environmental cell (a nanoreactor). The holder is combined with a home built gas feeding and a mass spectrometer for gas analysis. The environmental TEM cell allows monitoring gas-solid interactions under relevant catalytic conditions.

 

Metal catalysts were prepared in situ by decomposing the catalyst precursor in an oxidative atmosphere and subsequent reduction in hydrogen (figure 1). As the resulting metal catalysts were exposed to a reductive gas mixture (H₂ in He) that contains a little amount of molecular oxygen, the catalytic reaction starts to proceed, as proved by the gas analysis of products which shows a concomitant increase of water and decrease of oxygen MS-signals (figure 2). The catalyst at work is very dynamic and shows continuous and erratic morphological changes, provided that the Wulff construction of crystals was preserved most often over time. The interpretation of this dynamic behavior of catalysts under working conditions needs much further elaborated studies that can control or slow down the kinetics of the reactions, and hence make it possible to exploit other TEM analysis techniques as in situ EELS and diffraction. Nevertheless, these results may evoke strong debates about the assumptions that were published in the catalysis literature on the basis of TEM observations made in vacuum.

In this contribution we will outline how the implementation of complementary in situ electron microscopy techniques can enrich our understanding of the dynamic behavior of the active catalysts.

 

References:

[1]   P.L. Hansen, et al., Science, 2002, 295, 5562, 2053-2055

[2]   T. Lunkenbein, et al., Angew. Chem.-Int. Ed. 2015, 54, 4544–4548.

[3]   J. R. Jinschek, Chem. Commun. 2014, 50, 2696–2706.

[4]   Z.-J. Wang, G., et al., ACS Nano 2015, 9, 1506–1519.

[5]   S. B. Vendelbo, et al., Nat. Mater. 2014, 13, 884–890.

Ramzi FARRA (Berlin, Germany), Ali RINALDI, Mark GREINER, Jing CAO, Robert SCHLÖGL, Marc-G WILLINGER

08:00 - 18:15 #6545 - IM02-177 In-situ propagation of Al in germanium nanowires observed by transmission electron microscopy.

IM02-177 In-situ propagation of Al in germanium nanowires observed by transmission electron microscopy.

Semiconductor nanowires (NWs) are promising candidates for many device applications ranging from electronics and optoelectronics to energy conversion and spintronics. However, typical NW devices are fabricated using electron beam lithography and therefore source, drain and channel length still depend on the spatial resolution of the lithography. In this work we show fabrication of NW devices in a transmission electron microscope (TEM) where we can obtain atomic resolution on the channel length using in-situ propagation of a metallic phase in the semiconducting NW. The corresponding channel length is independent on the lithography resolution. We show results on semiconducting NW devices fabricated on two different electron transparent Si₃N₄ membranes: a calibrated heater chip from DENs solution [1] and homemade membranes where the NW-metal contact is locally heated by Joule heating [2]. We demonstrate a real-time observation of the metal diffusion in the semiconducting NW. First we present results on in-situ propagation of aluminum metal in Ge NWs while monitoring the system temperature [3] and by Joule heating while measuring the current through the device. We study the kinetics and rate limiting step by monitoring the position of the reaction front as a function of time. Second we will show characterization of the formed phase at atomic length scales with different (S)TEM techniques (energy dispersive X-ray spectroscopy, HR(S)TEM) to understand how the metal atoms diffuse and incorporate into the Ge NW at the reaction front and how these parameters relate to the electrical properties of the same interface.
Using EDX analysis and comparing with 3D NW model calculations we show that the reacted NW part is pure Al, with a shell of Al₂O₃ with a low Ge content on both sides of the Al₂O₃ shell see Fig.2. EDX analysis show that both Al and Ge are diffusing in opposite directions.

References

[1]  http://denssolutions.com/products/nano-chip

[2] M. Mongillo, P. Spathis, G. Katsaros, P. Gentile, M. Sanquer and S. De Franceschi, ACS Nano, 5, 7117-7123 (2011).

[3] S. Kral, C. Zeiner, M. Stöger-Pollach, E. Bertagnolli, M. I. den Hertog, M. Lopez-Haro, E. Robin, K. El Hajraoui, and A. Lugstein, Nano. Lett, 15, 4783-4787 (2015).

 

 

Acknowledgements

Financial support from the French ANR for the “COSMOS” project is acknowledged. We thank B. Fernadez and T. Fournier for their technical support.

Khalil EL HAJRAOUI (Grenoble), Eric ROBIN, Miguel LOPEZ-HARO, Clemens ZEINER, Florian BRUNBAUER, Sébastian KRAL, Alois LUGSTEIN, Jean-Luc ROUVIÈRE, Martien DEN HERTOG

08:00 - 18:15 #6612 - IM02-179 Graphene-supported microwell liquid cell for in situ studies in TEM and SEM.

IM02-179 Graphene-supported microwell liquid cell for in situ studies in TEM and SEM.

Liquid cell transmission electron microscopy (LCTEM) is used for in situ investigations of dynamic nanoparticulate processes in aqueous and nonaqueous solutions. In contrast to complementary techniques like conventional (S)TEM, cryo-TEM or SAXS, LCTEM ensures real-time high-resolution imaging and is used in various research fields like biology, electrochemistry and materials science [1]. For our approach we make use of a modified microwell liquid cell layout combining the benefits of the microwell liquid cell design used by Dukes et al. [2] and the graphene liquid cell shown by Yuk et al. [3] for in situ (S)TEM and SEM experiments. In this approach, the liquid specimen is confined between amorphous silicon nitride microwells and multilayer graphene (cf. fig. 1). This enables improved imaging conditions compared to other cell designs because the cell profits from the advantages of (i) a robust encasement combined with an ultrathin and electrically conducting low-Z material and (ii) a constant liquid film-thickness defined by the nitride wells. Furthermore, in this design window bulging is largely suppressed due to small microwell diameters on the order of 5 µm. The liquid cells are processed under clean room conditions via conventional semiconductor technology as well as bulk micromachining. In a first step a silicon nitride layer is deposited onto an oxidized thin silicon wafer via LPCVD followed by structuring the front and back-side by photolithography and reactive ion etching. For the simplified device layout no mask alignment is necessary which minimizes the failure probability and improves the yield of the fabrication process. Furthermore, the number of process steps could be considerably reduced compared to the fabrication process of conventional static liquid cells. Bulk micromachining of the silicon wafer is done by an anisotropic potassium hydroxide wet-etching process. The filling and vacuum tight sealing of the cell is conducted in one step by transferring the graphene directly onto a droplet of the specimen solution. When the liquid dries the graphene adheres to the silicon nitride and encloses small amounts of the fluid within the microwells. The compatibility to conventional specimen holders makes the liquid cells feasible for the use with various kinds of TEMs and SEMs.

As first application and test of this optimized liquid cell design electron beam induced growth and degradation phenomena of Au-nanoparticles in HAuCl₄-solution have been studied. Figure 2 shows snapshots of an in situ TEM investigation monitoring the dissolution of a Au-nanoplatelet at a liquid-gas interface and the subsequent growth of smaller particles above (or below) the gas bubble. This redistribution of material is facilitated by the high mobility of gold-atoms in solution as well as by reactive species generated by electron beam irradiation of the aqueous solution. The experiment was carried out in TEM bright-field mode using a Philips CM-30 (S)TEM operated at 300 kV. In order to demonstrate that the cell can be equally used inside an SEM figure 3 shows snapshots of an in situ study of electron beam induced dendrite-growth of Au-nanostructures in a 1 mM HAuCl₄-solution. Here, imaging was carried out in STEM mode using a FEI Helios Nanolab 660 operated at 29 keV primary electron energy.

References:

[1] N. de Jonge et al., Adv. Imag. Elect. Phys. 2014, 186, 1−37

[2] M. J. Dukes et al., Microsc. Microanal.  2014, 20, 338−345

[3] J. M. Yuk et al., Science 2012, 336, 61−64

 

Acknowledgment:

Financial support by the DFG via the Research Training Group GRK1896 "In situ microscopy with electrons, X-rays and scanning probes" is gratefully acknowledged.

Andreas HUTZLER (Erlangen, Germany), Robert BRANSCHEID, Michael P. M. JANK, Lothar FREY, Erdmann SPIECKER

08:00 - 18:15 #6628 - IM02-181 An in situ multiscale approach for the characterization of plasticity and damage in a TA6V alloy.

IM02-181 An in situ multiscale approach for the characterization of plasticity and damage in a TA6V alloy.

Due to its specific strength and  good biocompatibility, Ti-6Al-4V alloy find many applications in  medical and aeronautic industries.

At room temperature, the studied TA6V alloy presents a bi-modal microstructure consisting of hcp alpha phase together with bcc beta phase. When subjected to high cooling rates such as in welding processes, a lamellar hcp alpha’ phase appears, which modifies the plastic and damage behavior of the alloy.  In order to better understand the role of microstructure on failure of the material, a clear connection must be made between microstructural parameters such as interfaces, grain or lath orientation, grain boundaries, morphologies and failure initiation.  

The deformation behavior of both welds and base metals were characterized by in situ EBSD tensile tests.  Slip and twin systems were identified for bimodal and fully martensitic microstructure. Deformation paths were followed upon deformation, and related to original laths or grain orientation, see Figure 1. At higher deformation levels, damage starts and cavity appear equally at grain boundaries, phase boundaries, or inside grains. However, if a large number of information is available from these 2D observations, 3D characterization is required for a quantitative damage study as most of the cavities initiate in the volume and not at the surface. The same samples were then characterized by in situ tensile X Ray Tomography tests conducted at ESRF Grenoble.

For several deformation levels, a quantification of the cavity number and size was performed  and a clear description of the cavity initation, growth, and coalescence was obtained in 3D, as reported on Figure 2.

 

 

 

 

Thanks are due to Aymeric Migliarini, Lea Noirot Coisson and Cyril Lavogiez for their active implication during the project.

Sophie CAZOTTES (VILLEURBANNE CEDEX), Sylvain DANCETTE, Christophe LE BOURLOT, Eric MAIRE, Thomas PARIS

08:00 - 18:15 #6642 - IM02-183 The “Ocean” System: Microfluidic based system for in-situ analysis of liquid processes inside the TEM.

IM02-183 The “Ocean” System: Microfluidic based system for in-situ analysis of liquid processes inside the TEM.

Nanotechnology is driving scientists to better comprehend the real-time dynamics and structure-property relationship of various materials and biological samples under liquid conditions. Such understanding is crucial for a wide range of applications involving, for example, nanoparticle synthesis, self-assembly processes, (bio) molecular interactions, and biological activity in cells. In-situ transmission electron microscopy (TEM) observations in the liquid-phase is expected to lead to better scientific understanding, the discovery of phenomena at the nanoscale in liquid not visible before, and results in novel and innovative applications. Here we present the development of the “Ocean System”, which is an easy-to-use add-on that enables in-situ liquid studies inside the TEM (Figure 1). It consists of an optimized TEM holder that uses a microfluidic chamber as sample carrier, replacing the traditional copper grid. Such device, referred to as Nano-Cell, acts as a multi-functional and micro-sized laboratory that keeps the sample in a fully hydrated state. Furthermore, the system includes an external test station that guarantees the safe loading of the holder into the TEM. Each Nano-Cell consists of two chips (Figure 2) that are sandwiched together to form a sealed microfluidic compartment. Both chips are covered with silicon nitride providing an electron transparent window and ensuring their chemical inertness and biocompatibility. Samples are prepared directly onto the electron transparent windows, which allow for the electron beam to pass through for in-situ imaging. Biological cells can also be directly grown on the chips.

In order to control the liquid thickness to improve imaging resolution, the experiment can be customized by selecting the best-suited spacer based on sample size. Having direct access to the electron transparent windows enables local functionalization of the membrane´s surface, empowering the user to further control the microfluidic environment. The holder tip contains a precision slot with various alignment poles that ensure self-alignment of the top and bottom chips. Similarly, it contains a by-pass structure that prevents overpressures during liquid handling, and that allows rapid liquid exchange in the tubing, since the flow cross-section in this channel is much larger than that of the liquid path between the chips. The tip closure mechanism uses alignment balls, so that the tip correctly closes when screws are tightened independently on the applied force. The mechanism prevents over-compression of the O-rings and ensures that no shear stress will be transferred to the Nano-Cell, as these could damage fragile samples (i.e. biological cell). Additionally, the modular design ensures reliable results with easy replacement of all holder parts, such as tubing, holder tip and the Nano-Cell (Figure 3). This is particularly important, as it prevents cross-contamination between different experiments, and the tubing can be easily replaced by the user if these become clogged. In addition, the tip can be rotated by 180°, so that depending if one wants to use TEM or STEM, the optimal resolution can be achieved for the sample, i.e. TEM achieves the highest resolution for objects below a liquid layer for a downward traveling electron beam, while the opposite is true for STEM.

The Ocean System can be used to study dynamic processes of nanoparticles. E.g. gold nanoparticles can be loosely attached to a SiN membrane. Their detachment during the experiment can be triggered by increasing the induced electron dose. This can provide useful information such as the interaction of nanoobjects (e.g. agglomeration, self-assembly, sintering) in different liquids. Figure 4 shows Au nanoparticles being attached to the SiN membrane. Upon imaging at higher magnifications the nanoparticles start moving along the SiN membrane and start to form agglomerates. Particle tracking was applied to 4 selected Au nanoparticles to study their movement.

Héctor Hugo PÉREZ GARZA (Delft, The Netherlands), Diederik MORSINK, Jeff XU, Justus HERMANNSDÖRFER, Mariya SHOLKINA, Merijn PEN, Sander VAN WEPEREN, Niels DE JONGE

08:00 - 18:15 #6647 - IM02-185 In-situ deformation of Ti6Al4V in electron microscopes.

IM02-185 In-situ deformation of Ti6Al4V in electron microscopes.

Ti6Al4V is the most widely used titanium alloy [1] and comprises two phases, viz. a hexagonally closely packed α phase and a body centred cubic β phase. For the lifing prediction of the alloy components and also for the future alloy development, it is of interest to understand the role of various interfaces such as grain boundaries and inter-phase boundaries in the mechanical response of the alloy. The last ten years have seen the development of nano-mechanical testing within electron microscopes on samples prepared using focused ion beam, which has not only offered the quantitative mechanical properties but also the simultaneous imaging analysis of samples containing specific structural features [2, 3]. In this work, in-situ electron microscopy studies on miniaturised Ti6Al4V samples were used to achieve a better understanding of the plastic deformation micro-mechanisms involved. Micron-sized pillar samples were compressed in an SEM and the mechanical properties of both alpha and beta phases were evaluated, while sub-micron sized pillars were compressed in a TEM to observe the dislocation activities. The nucleation of dislocations, the yielding of the samples and the work hardening observed in the samples with different sizes will be discussed.

References:

1. Lütjering, G. and J. C. Williams (2007). Titanium, Springer.
2. Q. Yu, M. Legros, A.M. Minor, MRS Bulletin, 40.01 (2015): 62-70
3. M.W. Kapp, C. Kirchlechner, R. Pippan, G. Dehm, Journal of Materials Research, 30.6 (2015): 791-797

Xinyu LU, Zhaoran LIU, Yu Lung CHIU (Birmingham, United Kingdom), Ian JONES

08:00 - 18:15 #6660 - IM02-187 Development of an in-situ specimen holder for high-voltage environmental electron microscopy of fuel cells.

IM02-187 Development of an in-situ specimen holder for high-voltage environmental electron microscopy of fuel cells.

Performance of Solid oxide fuel cells (SOFCs) is affected by microstructural changes and electrochemical reactions in interfacial regions between electrode and electrolyte during the operation. SOFCs are operated at high temperatures using a fuel gas and air. Therefore, an environmental electron microscope that allows for observation of specimens in the gas atmospheres is powerful analytical tool for nano-scale interfaces in SOFCs. The redox reaction in the cell has been reported by using in-site environmental electron microscopy and spectroscopy [1, 2]. The cell reaction, however, at interfacial regions between electrode and electrolyte has not completely been understood. In order to observe the interface in the cell reactions, we have developed an in-situ specimen holder, which can heat and apply the external voltage to the specimen. In this paper, we report details of the in-situ specimen holder and preliminary results of observation of a SOFC structure using this holder in a high-voltage environmental electron microscope.

Figure 1(a) shows the operation of SOFCs based on an oxide ion conducting in electrolyte. Supplying external voltage (Fig. 1(b)), we can make same situations as Fig. 1(a) in the cell. The developed in-situ specimen holder shown in Fig. 2(a) was optimized for the cell reaction in Fig. 1(b). The electrode terminal A is for the external voltage applying, and B and C are for the heating. Figure 2 (b) shows the heater with the electrode terminal consists of the nickel-chrome (NiCr) alloy. Between the heater and the electrode terminal is isolated electrically by insulators. We connect both electrodes of the specimen to the electrode terminal A and the center of the heater, respectively (see Fig. 2 (c)).

Figure 3(a) shows the scanning ion microscope (SIM) image of the full cell specimen prepared by a focused ion beam (FIB) instrument (Hitachi FB-2100). The bulk full cell was constructed by a pulsed laser deposition method on the platinum (Pt) substrate. The cell structure was composed of a gadolinium doped ceria (GDC) of 1 μm thick as electrolyte and a Pt layer of 100 nm thick as the electrode. The gold wire connects the electrode terminal A to the tungsten layer which was deposited on the specimen to protect the cell structure in the FIB thinning process. In-situ observation of the cell structure was performed using the reaction science high-voltage electron microscope [3] (JEOL JEM-1000K RS) at an acceleration voltage of 1 MV. The pressure in the specimen chamber was kept to 1 Pa of the oxygen gas.

Figures 3(b) and 3(c) show the cross-sectional annular dark-field scanning transmission electron microscope (ADF-STEM) images in the oxygen atmosphere at the room temperature (R.T.) and ~600 ℃, respectively. In the case of ~600℃, we applied a voltage of +1V to the electrode terminal A. Figure 3(b) clearly shows the cell structure that has the electrodes (Pt) and the electrolyte (GDC) with the protective (W) layers. We cannot, however, distinguish between Pt and W layers at the higher temperature, as shown in Fig.3(c). These results show Pt layer with W layer is unstable in high-temperature oxidizing environments and obstructs to observe the fuel cell reaction between the Pt electrode and the GDC electrolyte.

References

[1] A. H. Tavabi et al., J. Electron Microsc., 60 (2011) 307-314.

[2] A. H. Tavabi et al., Mirosc. Microanal., 20 (2014) 1817-1825.

[3] N. Tanaka et al., J. Electron Microsc., 62 (2103) 205-215.

Acknowledgements

This work was supported by JSPS KAKENHI Grant Number 25246001, and also partially supported by the program “Global Research Center for Environment and Energy based on Nanomaterials Science” of MEXT, Japan

Takafumi ISHIDA (Nagoya, Japan), Takayoshi TANJI, Masahiro TOMITA, Kimitaka HIGUCHI, Koh SAITOH

08:00 - 18:15 #6665 - IM02-189 Monitoring of Dynamic Live Brain Tumor Cells using SICM and its Future Applications.

IM02-189 Monitoring of Dynamic Live Brain Tumor Cells using SICM and its Future Applications.

Monitoring of Dynamic Live Brain Tumor Cells using SICM and its Future Applications

 

Myung-Hoon Choi¹, Goo-Eun Jung¹, Yong-Sung Cho¹,Sang-Joon Cho¹,

 

¹Park Systems, Suwon, Korea

 

 

The high resolution monitoring of live cell membranes in physiological conditions has been the homework for centuries for biologists. Visualization and Understanding of cell membrane activities could give precious insight upon how cells interact with outer environment. Scanning ion conductance microscopy (SICM) can provide the surface morphology of biological soft materials in liquid directly. The SICM uses ionic current as feedback signal by detecting it through the nano-size opening of glass pipette. The dedicated SICM operating mode called approach and retract scanning (ARS) makes SICM imaging stable in liquid environment. The SICM three-dimensional topographical image is generated by composing the recorded height information at each pixels. The in-liquid imaging capability without physical contact allows using SICM for various cell study topics in live status such as cell division, fusion, and other fundamental physiological phenomena.

 

In this study, we examined cell-to-cell interaction and cell’s plasma membrane transformation. By imaging live brain tumor cell with SICM at the connective position between two cells, we successfully acquired the interactive morphological changes of cell’s adhesion molecule at the edge in sequence. From the series of non-invasive cell morphological images, acquired in three dimension, we also calculated and monitored the velocity of membrane transformation and membrane holes volume changes.

Choi MYUNG-HOON, Jung GOO-EUN, Yong-Sung CHO, Sang-Joon CHO (Suwon, Republic of Korea)

08:00 - 18:15 #6666 - IM02-191 Electron beam-induced etching of carbon nanotubes by environmental transmission electron microscope.

IM02-191 Electron beam-induced etching of carbon nanotubes by environmental transmission electron microscope.

   It is crucially needed to develop a novel process to cut and connect nanomaterials artificially for further progress in nanotechnology. Gas-mediated electron beam-induced etching (EBIE) is promising to carve nanomaterials via electron beam-induced chemical reactions between the nanomaterials and precursor gases. In this study, we have investigated the EBIE process of multi-walled carbon nanotubes (CNTs) in oxygen gas using environmental transmission electron microscopy (ETEM).

   Commercially available multi-walled CNTs (NanoIntegris Inc.) were supported on a Cu micro grid with a carbon supporting film of 3 mm in diameter. A grid with multi-walled CNTs was fixed on a specimen holder and transferred to an ETEM (FEI Tecnai F20 equipped with an environmental-cell [1]) operated at 200 kV. As a precursor gas for EBIE, oxygen gas (99.9999%) was introduced into the ETEM. To carry out the EBIE of multi-walled CNTs, a focused electron beam (about 0.2 nm in diameter) was repeatedly scanned across multi-walled CNTs in oxygen gas of 100 Pa at room temperature in scanning TEM (STEM) mode.

   Figure 1 shows the EBIE process of a multi-walled CNT in oxygen gas. A focused electron beam was scanned repeatedly along a white dotted line in Fig. 1(a) with the rate of 103 s^-1. STEM images and TEM images of the multi-walled CNT were recorded after every 62 scans. The electron dose was estimated and shown in each image in Fig. 1. As the electron dose increases, the multi-walled CNT is etched gradually. Figures 1(b), 1(c), 1(f), and 1(g) clearly show that the etching proceeds from the outer walls to inner walls of the multi-walled CNT. Finally, the multi-walled CNT is cut completely along the scanning direction (Fig. 1(d), (h)). The cutting surface remains cylindrical. We confirmed that the areas that do not receive electron dose are not etched. On the other hand, multi-walled CNTs cannot be cut in a vacuum even after receiving the electron dose much higher to cut in oxygen gas. The scanning areas in multi-walled CNTs are amorphized. This means that both electron irradiation and oxygen gas are needed for EBIE of multi-walled CNTs. Oxygen atoms from dissociation of oxygen molecules by electron irradiation [2] react with carbon atoms in multi-walled CNTs to produce CO and/or CO₂ which desorb from the surface of multi-walled CNTs. We have succeeded in cutting of multi-walled CNTs at desired positions at room temperature by EBIE in ETEM.

   In the presentation, we will show the dependence of EBIE of multi-walled CNTs on the pressure of oxygen gas, accelerating voltage of electrons, and gas species. The mechanism of EBIE of multi-walled CNTs will be discussed.

 

References

[1] S. Takeda, Y. Kuwauchi, H. Yoshida, Ultramicroscopy, 151 (2015) 178.

[2] H. Yoshida, H. Omote, S. Takeda, Nanoscale, 6 (2014) 13113.

Yuto TOMITA, Hideto YOSHIDA (Osaka, Japan), Seiji TAKEDA

08:00 - 18:15 #6710 - IM02-193 MEMS-based system for in-situ biasing and heating solutions inside the TEM.

IM02-193 MEMS-based system for in-situ biasing and heating solutions inside the TEM.

Understanding the thermo-electrical properties of different materials demands an in-depth analysis of their structure-property relationship. Therefore, monitoring their dynamic mechanisms in a real-world environment is crucial to better determine how to manipulate and optimize them for various applications. For example, the capability to perform current-voltage measurements while analyzing the corresponding structural changes during resistive switching process of the potential ReRAM materials in real time is crucial for improving the stability and scalability of the most promising next-generation non-volatile memory devices.

Here, we present the development of a system for in-situ biasing and heating manipulations inside the Transmission Electron Microscope (TEM), referred to as the Lightning System. The latter uses the latest Micro Electro Mechanical Systems (MEMS) based technology to scale down the experiment. Consequently, the stability and resolution can be considerably improved. The MEMS devices, known as the Nano-Chips, act as a functional and consumable sample carrier that supplies local stimuli to the sample size required for biasing and/or heating, allowing the users to manipulate and characterize their samples. Figure 1 shows the architecture of the Nano-Chips for simultaneous heating and biasing. As observed, it consists of eight electrical contacts, where half are used for heating and half are used for biasing purposes. As a result, the 4-point probe measurements are used to gain complete control of each parameter and ensure instant, controllable and reproducible responses. This results in high accuracy during the measurements. The unique design of the Nano-Chips ensures reduced specimen drift during heating, as well as a stable and chemically inert environment that enables compatibility with various types of samples (i.e. lamellas, nanowires and 2D materials). Furthermore, it empowers the user to do different types of analysis including I-V measurements as a function of temperature (up to 800 °C) and high electric field studies.

The Nano-Chip is mounted on a functionalized holder, shown in Figure 2, which contains the contact needles to supply the stimuli from the outside world. Such holder can supply up to 100V to the Nano-Chip for the electrical measurements and helps detecting currents in the pA regime. Furthermore, it enables tilting in alpha and beta. The complete “plug and play” system, shown in Figure 3, includes a source measurement unit and a heating control unit. Once the holder is connected to such biasing power supply and the heating controller, the voltage/current can be set and the temperature profile can be programmed for total control during the in-situ experiment.  

The Lightning System can be used to understand the microstructural origins for electric field induced changes in the ferroelectric materials. As a matter of fact, it is also known that the temperature rise of ferroelectric devices during utilization limits its practical application. Therefore, the system can also enable repeating the electric field measurements while working at an elevated temperature environment. Additionally, the Lightning System can be used to study low dimensional materials like nanowires, as their electrical properties and their temperature dependence differs with different growth directions. In-situ heating and biasing experiments of such samples can open a new application opportunity in nanoelectronics.

Héctor Hugo PÉREZ GARZA (Delft, The Netherlands), Kun ZUO, Yevheniy PIVAK, Diederik MORSINK, Marina ZAKHOZHEVA, Merijn PEN, Sander VAN WEPEREN, Qiang XU

08:00 - 18:15 #6739 - IM02-195 Correlating structure and mechanical properties for submicron amorphous silica spheres.

IM02-195 Correlating structure and mechanical properties for submicron amorphous silica spheres.

Spherical amorphous silica particles obtained from the well-known Stöber-Fink-Bohn (SFB) synthesis are frequently used as standards for size or shape and as proxy for the behavior of amorphous silica [1]. So far, several studies have dealt with the peculiarities of the internal structure of these particles (see e.g. references in [2–3]): particles from the SFB synthesis exhibit a highly hydroxylated weakly condensed silica network with a certain microporosity at least for water and ions. Larger particles which are generally obtained from a reseeded multi-step growth protocol show a ring like internal structure. Upon thermal treatment, the particles’ silica network can be condensed - the structure of vitreous silica is approached. Despite the abundant use of the particles, however, detailed investigations which correlate the mechanical properties and the internal structure of (heat-treated) SFB silica are absent. Within this account, the size dependent mechanical and structural properties of SFB silica and thermally derived vitreous silica spheres are assessed for particle diameters of 200 nm to 5 µm. For the mechanical characterization a scanning electron microscope (SEM) supported custom-made indenter [4] and the Hysitron PI95 TEM Picoindenter^TM in a transmission electron microscope (TEM) are used to test a statically representative amount of particles (at least 50). Structural characterization is performed by nitrogen sorption, vibrational spectroscopy, colloid titration and solid-state nuclear magnetic resonance spectroscopy. For SFB spheres with a mean diameter of 500 nm it is shown that hardness, yield strength, and Young’s modulus of the SFB particles are significantly increased after thermal treatments at temperatures exceeding 400°C (Fig. 1). With an increasing treatment temperature the Young’s modulus of bulk fused silica is approached. However, hardness, yield strength and the sustained plastic deformation till catastrophic failure of the spheres occurs clearly exhibit bulk values. The underlying changes of the internal structure are in accordance: a slight shrinkage (~28 vol.%) is accompanied by an overall homogenization, densification, increased cross-linking and dehydroxylation of the particles (Fig. 2) [3]. The size-dependent characterization of the failure modes of SFB particles and corresponding derived vitreous silica spheres provides information on the underlying deformation modes and allows a classification of the different cracking types (Fig. 3 and Fig. 4). Independent of size, the untreated SFB particles exhibit only the formation of (presumably) ductile cracks; full fragmentation does not occur. For the derived vitreous silica spheres, however, a clear brittle-to-ductile transition is observed in the size range of 500 – 800 nm: particles above this size are fragmented into two or more individual parts. The latter behavior is also indicative for bulk fused silica which is known for its brittleness on the mesoscopic scale [5]. In contrast to bulk fused silica, the silica spheres still sustain high plastic deformations (in the order of 40%). By ex situ Raman spectroscopy on the single particle level the observed plasticity can be attributed to local densification of the vitreous silica spheres directly below the contacts of the sphere with the diamond flat punch and the substrate [6]. It is noteworthy that fine details of the contact zones and the role of molecules on the surface might be addressed by non-linear spectroscopy [7].

1              W. Stöber et al., J Colloid Interface Sci 26, 1968, 62–69.

2              J. Paul et al., Powder Technol. 270, 2015, 337–347.

3              S. Romeis et al., Part. Part. Syst. Char. 31, 2014, 664–674.

4              S. Romeis et al., Rev. Sci. Instrum. 83, 2012, 095105.

5              R.F. Cook et al., J. Am. Chem. Soc. 73, 1990, 787–817.

6              S. Romeis, et al., Scripta Mater., 2015, 84–87.

7              C. Meltzer et al., J. Am. Chem. Soc. 136, 2014, 10718–10727.

Financial support by the Deutsche Forschungsgemeinschaft (DFG) through the Cluster of Excellence “EAM” and GRK1896 “In situ microscopy with electrons, X-rays and scanning probes” is gratefully acknowledged.

Stefan ROMEIS (Erlangen, Germany), Patrick HERRE, Mirza MAČKOVIĆ, Jochen SCHMIDT, Jonas PAUL, Dominique DE LIGNY, Erdmann SPIECKER, Wolfgang PEUKERT

08:00 - 18:15 #6782 - IM02-197 Simultaneous Nanoplasmonic Sensing and Transmission Electron Microscopy Characterization.

IM02-197 Simultaneous Nanoplasmonic Sensing and Transmission Electron Microscopy Characterization.

We have developed a platform that allows for real time optical sensing based on localized surface plasmon resonance (LSPR) readout inside a transmission electron microscope (TEM). With the TEM, we obtain insight into the structure and composition of materials by performing imaging and spectroscopy with atomic resolution [1]. However, the probed volume is rather small, and the beam-specimen interaction is often non-negligible and needs to be taken into account, usually by performing additional experiments. Thus, there is an increasing effort towards enabling simultaneous TEM probing and characterization with complementary techniques. Thanks to the strongly enhanced electric fields generated around metallic nanoantennas, LSPR-based sensing is a proven tool to study processes at the nanoscale [2], and an ideal complement to TEM since it probes a much larger volume of the specimen.

 

We have fabricated a TEM specimen holder hosting a miniaturized optical bench [3] that allows for sample illumination and spectroscopic readout. Specimens can be heated up to 1300 ºC, and the holder is compatible with differentially pumped environmental TEM (ETEM), with no prior modification to the microscope required. Comparison between signals obtained simultaneously by TEM and LSPR provides indication of the relevance of electron beam-induced effects. Moreover, we enable for the first time direct correlation of the LSPR response with changes in physical properties of the specimen. We investigate thermally-induced sintering of metal nanoparticles, a crucial process in deactivation of catalysts [4]. We envision the combination of LSPR sensing with probing by ETEM to become a versatile tool to study processes at the nanoscale, especially taking place on (photo)catalysts.

 

References:

 

[1] J.-M. Herrmann Top. Catal., vol. 34, no. 1–4, pp. 49–65, May 2005.

[2] E. M. Larsson, C. Langhammer, I. Zorić, and B. Kasemo, Science, vol. 326, no. 5956, pp. 1091–4, Nov. 2009.

[3] F. Cavalca, A. B. Laursen, B. E. Kardynal, R. E. Dunin-Borkowski, S. Dahl, J. B. Wagner, and T. W. Hansen Nanotechnology, vol. 23, no. 7, 075705, Feb. 2012.

[4] C. H. Bartholomew, Appl. Cat. A, vol.  212, no. 1–2, pp. 17–60, Apr. 2001.

 

Acknowledgments: The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/2007-2013) under REA Grant Agreement No. 609405 (COFUNDPostdocDTU).

Beniamino IANDOLO (Kgs. Lyngby, Denmark), Asger MOSS, Ferry NUGROHO, Christoph LANGHAMMER, Jakob WAGNER

08:00 - 18:15 #6785 - IM02-199 Tuning the surface termination of ceria under gaseous environments in a Cs-corrected Environmental TEM.

IM02-199 Tuning the surface termination of ceria under gaseous environments in a Cs-corrected Environmental TEM.

Surface termination of solids varies according to the environment to which they are submitted. This is very important in catalysis since surface modifications have a strong influence in the way thatadsorption at the surface followed by eventual dissociation, diffusion and recombination of the molecular occur. Gaining information at atomic level on the structural and chemical variations of the surface is thus essential  to understand the mechanisms of catalytic reactions and ultimately obtain a better control on the design of catalysts with specific controlled properties (activity, selectivity, stability). This information can only be obtained (at local level) by high resolution transmission electrom microscopy (HRTEM and thus requires the use of a Cs-corrected (image) Environmental TEM capable of very high resolution observations of the nanometric materials in presence of controlled gaseous environments. In this work we study, in a ETEM, ceria which is a catalytic material used generally in oxidation reactions both as support of active phases or as an active phase itself by taking advantage of its redox properties and oxygen mobility. We focused our interest on the surface atomic termination of ceria nanocubes under different controlled environments (high vacuum, oxygen, carbon dioxide) important for the understanding of CO₂-valorization related reactions.

The studies were performed in the Ly-EtTEM (Lyon Environmental and tomographic Transmission Electron Microscope), a 80-300 kV TITAN objective lens Cs-corrected Environmental TEM from FEI equipped with a GATAN high resolution Imaging Filter (GIF) [1]. The analysis was carried out at room temperature varying the gas pressures from 10^-6 mbar (vacuum) up to 2 mbar.

The HRTEM images in Figure 1 show the termination of a {100} facet of a ceria nanocube under high vacuum (HV) and in presence of molecular oxygen and carbon dioxyde. Under HV conditions (Figure 1 – left image) the incident electron beam induces a partial reduction of ceria [2]. As a consequence cerium surface atoms become highly mobile as it can be readily observed on {100} facets and electron energy-loss spectroscopy (EELS) shows that the characteristic features related with Ce⁴⁺ species rapidly decrease in intensity both on the O-K and Ce-M_4,5 edges. In the presence of molecular oxygen (O₂ pressure in the mbar range; Figure 1 – center image) this beam-induced reduction is “healed” and the surface of ceria NPs is stabilized as it can be confirmed by the absence of mobility of surface cerium atoms on {100} facets as well as by the in situ real-time dynamic monitoring of the O-K and C-M_4,5 edges by EELS that now remain unaltered under irradiation. Finally, under CO₂ pressure (10^-2 - 1 mbar; Figure 1 – right image), the {100} facets become more stable from both structural (oxygen surface termination) and chemical (stable non-reduced state revealed by EELS) points of view.

We notice that the surface terminations of the ceria nanoparticles under vacuum are different from those observed under O₂ or CO₂ (Figure 2).  Under HV the terminal contrasts observed are strong and clearly correspond to the formation of a terminal cerium plane. A different contrast can be noted in the case of the sample exposed to O₂ (Figure 2 – left image) where the presence of terminal oxygen in accordance with the ceria modelis imaged. Exposure to CO₂ (Figure 2 – right image) yields a still different terminal contrast close to the expected position of termina oxygen hinting at the presence of stabilized species at these position of oxygen atoms at the surface termination; the stabilization of surface oxygen atoms is likely related to CO₂ adsorption and the consequent formation of carbonate surface species.

References
[1] The authors thank the CLYM (Centre Lyon-St Etienne de Microscopie) for access to the Ly-EtTEM.

[2] L.A.J. Garvie and P.R. Buseck, J. Phys. Chem. Solids. 60 (1999) 1943-1947 

Francisco José CADETE SANTOS AIRES (VILLEURBANNE CEDEX), Mimoun AOUINE, Amanda K. P. MANN, Zili WU, Steven H. OVERBURY, Thierry EPICIER

08:00 - 18:15 #6847 - IM02-201 In situ environmental HRTEM study of the restructuration under reducing atmosphere of small oxidized silver-indium nanoparticles.

IM02-201 In situ environmental HRTEM study of the restructuration under reducing atmosphere of small oxidized silver-indium nanoparticles.

       In the form of alloyed or segregated structures, bimetallic nanoparticles (NPs) exhibit remarkable catalytic (activity, selectivity, stability) and optical (localized surface plasmon resonance, LSPR) properties, both depending on the morphology and the chemical configuration that they adopt. Moreover, the structure of these nanoparticles may evolve under specific environmental conditions (pressure, temperature, presence of reactants). It follows that structural and optical characterization techniques must be implemented in a controlled environment for probing these restructuring phenomena.

 

Here, we present an environmental TEM (ETEM) study on Ag-In nanoparticles. These particles exhibit a reversible shift of their LSPR, after exposition to oxidizing (air, 25°C) and reducing (H₂+N₂, 500°C) atmospheres [1]. The particles were synthesized by Low Energy Cluster Beam Deposition (at PLYRA in Lyon), which allows an independent control of their size and composition. The microscope used was a FEI Titan ETEM operating at 300 kV, with a Cs corrector of the objective lens. HRTEM images and movies of the bimetallic nanoparticles were recorded at up to 500°C and under 10 mbar H₂ [2].

 

The starting chemical configuration of the particles is a core-shell structure, with an Ag or Ag-In alloyed core of 4-5 nm and an In₂O₃ amorphous shell with a thickness of 1-2 nm. The nanoparticles were exposed in the ETEM to successive (H₂ pressure; temperature) couples, from (1 mbar H₂; 25°C) to (10 mbar H₂; 500°C). These particles were monitored both at the local (single nanoparticle tracking) and global (observations on large assemblies, under low magnification) scales. The structural changes observed at the atomic scale range from the almost complete extraction of the Ag-rich core from the In₂O₃ shell, leading to “Janus” nanoparticles, to the complete reduction of the indium oxide shell. In₂O₃ reductions which occur at higher temperature than the melting point of indium (156°C) induce the competition between two behaviours: i) the melting or evaporation of reduced indium, and thus the decreasing thickness of the shell until its complete disappearance, ii) the diffusion from the shell to the core of reduced indium atoms, leading to a core growth (figure 1). This last point may be closely related to the evolution of the shell thickness during reduction. From these results, we constructed a (pressure, temperature) diagram highlighting the relationship between temperature and H₂ pressure in the reduction activation. These results provide new insights in both physical and chemical processes involved during reduction of oxidized metallic nanoparticles at the atomic scale.

 

The authors thank the technical staff of the CLYM facility for access to the Titan microscope, and the PLYRA cluster facility for the cluster synthesis. ARC Energie (Academic Research Community), Rhônes-Alpes regional council is acknowledged for thesis scholarship.

 

 

[1] E.Cottancin, C.Langlois, J.Lermé, M.Broyer, M.A Lebeault, M.Pellarin, Phys. Chem. Chem. Phys. 2014, 16, 5763

[2] J.R. Jinschek, Chem. Commun. 2014, 50, 2696

Julien RAMADE (VILLEURBANNE), Cyril LANGLOIS, Michel PELLARIN, Laurent PICCOLO, Emmanuel COTTANCIN

08:00 - 18:15 #6851 - IM02-203 In-Situ Formation of Carbon Nanotubes Encapsulated within Boron Nitride Nanotubes via Electron Irradiation.

IM02-203 In-Situ Formation of Carbon Nanotubes Encapsulated within Boron Nitride Nanotubes via Electron Irradiation.

The sensitivity to small changes in the electronic structure of carbon nanotubes (CNTs) from external perturbations limits their successful integration on electronic devices [1]. It is therefore necessary to develop experimental strategies for CNT synthesis that guarantee the formation of crystalline structures of carbon materials while ensuring a protection from the environment without affecting its electronic properties. Under this context, boron nitride nanotubes (BNNTs), due to their uniform electronic properties and their chemical inertness characteristics, is one of the most appropriated nanomaterials for achieving these goals. In fact, BNNTs are large band gap insulators exhibiting a resistivity to oxidation of up to 900 °C [2]. Here we report the synthesis and growth of crystalline carbon nanotubes inside a larger diameter BNNT via in-situ electron irradiation in a TEM [3].

Electron beam irradiation and HRTEM were performed using an imaging-side aberration-corrected FEI Titan-Cube microscope working at 80 kV, equipped with a Cs corrector. Complementary spatially-resolved EELS-STEM studies were also carried out using a FEI Titan Low-Base microscope, working at 80 kV, which is equipped with a Cs probe corrector. In both cases, particular attention was devoted to avoid contamination during acquisition. Single-walled (SW) BNNT were produced by laser vaporization technique [4]. Some of these BNNT can be partially filled by amorphous carbon [4]. Furthermore, density functional theory (DFT) simulations were conducted for determining the structural stability and electronic properties of such a hybrid system.

In Fig. 1, a six-frame HRTEM image sequence showing the evolution towards a nanotube structure of amorphous carbon enclosed within a BNNT. SR-EELS analyses confirm the presence of this amorphous carbon inside the NT, as displayed in Fig. 1 (g) where B-K, C-K and N-K edges are shown. In the HRTEM sequence defined by arrows, which occurs for a total cumulative dose of up to 1.8 10⁷ e^-/Ų and over a period of 380 seconds at room temperature, amorphous carbon is firstly observed in a straight BNNT and evolves over time to a crystalline structure. Simultaneously, a gradual shrinkage of the BNNT is observed. By the end of the process, the BNNT is broken and also disintegrated. We also observed the formation of an atomic-scale bridge between the tip of the carbon tube and the outer BN wall, and the subsequent reparation of the defect on both materials. Initially, a defect at the BNNT surface enhances the C-BN interaction by establishing a connection between both tubes.

These results show that the electron radiation stemming from the microscope supplies the energy required by the amorphous carbonaceous structures to crystallize in a tubular form in a catalyst-free procedure, at room temperature and high vacuum [3]. The structural defects resulting from the interaction of the shapeless carbon with the BN nanotube are corrected in a self-healing process throughout the crystallinization. Structural changes developed during the irradiation process such as defects formation and evolution, shrinkage, and shortness of the BN-NT were in situ monitored. The outer BN wall provides a protective and insulating shell against environmental perturbations to the inner C-NT without affecting their electronic properties, as demonstrated by first-principles calculations, see Fig. 1 (h)-(j).

[1] A. Jorio, G. Dresselhaus, M.S. Dresselhaus, Springer-Verlag: Berlin, 2008.

[2] R. Arenal, X. Blase, A. Loiseau, Advances in Physics 59, 101 (2010).

[3] R. Arenal and A. Lopez-Bezanilla, ACS Nano 8, 8419–8425 (2014).

[4] R. Arenal, O. Stephan, J.L. Cochon, and A. Loiseau, J. Am. Chem. Soc. 129, 16183 (2007).

[5] The research leading to these results has received funding from the EU under Grant Agreements 312483-ESTEEM2 and 604391 Graphene Flagship, from the Spanish Ministerio Economia y Competitividad (FIS2013-46159-C3-3-P) and from the EU under the Marie Curie Grant Agreement 642742 - Enabling Excellence.

 

Raul ARENAL (Zaragoza, Spain), Alejandro LOPEZ-BEZANILLA

08:00 - 18:15 #6852 - IM02-205 In situ TEM analysis of heating of gold nanoparticles on nanocarbon supports and implications for control of ripening.

IM02-205 In situ TEM analysis of heating of gold nanoparticles on nanocarbon supports and implications for control of ripening.

The outstanding structural and physical properties of carbon nanostructures can be extended by decoration with metallic nanoparticles (NPs) to offer a wide range of potential applications. However, such absorbed metal nanoparticles are generally metastable, and the functional properties of such NPs may be strongly affected by their size and shape. It is therefore necessary to obtain a full understanding of the thermal stability of supported nanoparticles, and how it is affected by the structure of the nanocarbon support.

In-situ TEM observations of thermally and electron beam activated transformations of preformed Au nanoparticles absorbed on a range of selected nanocarbon support structures can provide valuable insight into the NP growth mechanisms. Conventional TEM imaging with a high intensity beam can be used to induce ripening of NPs via beam energy only. Conversely, by using an in-situ heating holder and scanning TEM imaging, with the dwell time per pixel set to be less than the thermal equilibrium time, the effect of thermal heating can be studied. Subsequent to heating, HAADF-STEM combined with tilt-series acquisition enables the 3-dimensional distribution of the AuNPs to be determined in relation to the nanocarbon support without inducing further ripening.

A range of nanocarbon supports, providing differing surfaces environments for Au NPs, have been thus studied. Graphitised carbon nanofibres (GNF) offer two differing surfaces for NP growth. Internally, the surface is corrugated, with typical step-edge heights of ~3nm. Externally, the surface is a smooth graphitic layer. Secondly, multi-walled carbon nanotubes (MWNT) on few-layer graphene (FLG) or amorphous carbon support films provides model systems to appraise the relative importance of the effects of electrostatic interaction and structural factors on the nanoscale organisation of metallic NPs. In particular, the intersection between the convex surface of the nanocarbon exterior and the flat surface of the support film delineates a 1D channel along the nanotube growth axis, with the potential to stabilise NP growth as the proportion of the total surface area of the NP in contact with the carbon support is directly related to the nanoparticle size.

The growth mechanism of Au NPs is observed to be unaffected by the nature of the energy source, with beam induced and thermally induced ripening forming similar size distributions. However, different size distributions internally and externally for AuNPs on GNFs show that the structural influences of the carbon support control the growth mechanism, with growth of AuNPs limited on the corrugated internal surface. In addition, differing size distributions are observed between AuNPs on MWNTs on FLG, and AuNPs on MWNTs on amorphous carbon, with the FLG sample showing a highly ordered structure. Hence the growth mechanism is controlled by the structural and electrostatic influences of the carbon support.

Michael FAY (Nottingham, United Kingdom), Alessandro LA TORRE, Maria GIMENEZ-LOPEZ, Carlos HERREROS LUCAS, Paul BROWN, Andrei KHLOBYSTOV

08:00 - 18:15 #6856 - IM02-207 Aberration corrected CVD-TEM for in-situ growth of III-V semiconductors.

IM02-207 Aberration corrected CVD-TEM for in-situ growth of III-V semiconductors.

We will show the first results from a newly designed Environmental TEM with cold FEG emitter, aberration corrector, heating stage and a free choice of up to nine different gaseous reactants to produce heterostructures in-situ during  real-time observation and analysis.

 

Observing growth of semiconductors on an atomic scale, under MOCVD or CVD-like conditions, will bring valuable information and understanding of the possibilities and limitations for making devices, solar cells and LED:s from nanowires. We have constructed a 300 kV Environmental TEM, with cold FEG emitter, heating stage and a free choice of up to nine different gaseous reactants to produce heterostructures in-situ.

The use of an aplanatic  B-COR image corrector provides space in the objective lens polepiece for a heating stage reactor with gas inlets, while still achieving an 86 pm point resolution (in vacuum). The gas inlets are designed to give a total pressure of up to10 Pa, which is sufficient to grow nanowires at a reasonable speed. The gas handling system will allow switching between the different gases at will, or through programmed sequences.

Changes and defects in the crystal structure, and the effect on the outer shape of the nanowires can be followed  by combinations of conventional HREM imaging , STEM-DF and STEM-BF, as well as SE-imaging. Simultaneous analysis by XEDS will provide chemical changes along the growth path.

 

Reine WALLENBERG (Lund, Sweden), Daniel JACOBSSON, Kimberly DICK THELANDER, Joacim GUSTAFSSON, Stas DOGEL

08:00 - 18:15 #6876 - IM02-209 Structural transformations revealed by in-situ HRTEM observations.

IM02-209 Structural transformations revealed by in-situ HRTEM observations.

With the recent advance of TEM, in-situ observations have become a very essential technique to understand the structure - property relationship of materials. Here, a few examples are shown.  1) Metal silicides are widely used in microelectronics as ohmic contact due to their low resistivity and the existence of phases that are thermodynamically stable over a wide range of temperatures. The fundamental mechanisms about the silicide film formation remains unclear. The formation mechanism of Pd silicide is carefully explored by in–situ HRTEM/ STEM observations in a Cs-corrected TEM, and combined with atomic quantitative analysis (Figure 1).  By in-situ STEM imaging observations, it is found that Pd₂Si is initially formed at the tripe junction at the interfaces, and the formation process is advanced by Pd atom intrusion into Si lattice leading to the Si lattice change. Pd₂Si silicide phase transformation process also induces the strain changes. In addition, the Pd silicide phase transformation process is discussed by the thermodynamics analysis. 2) Simultaneous chemistry and structural transitions are also demonstrated in nanocrystalline materials. Bulk Cu-Cr nanostructures subjected to annealing are shown by in-situ tracking microstructural and compositions enabled with EELS analysis. It is found that the destabilization process of nanostructured materials can initiate at a very low temperature [1], and dynamic process can be well revealed once the composition information is in-situ tracked.

[1]. Acknowledgement: Gabriele Moser and Herwig Felber are gratefully acknowledged for their help with sample preparation. Thank are also given to Rostislav Daniel and Christian Mitterer in Montanuniversität Leoben, Austria for preparing nitride films, and to Olivier Thomas (Aix-Marseille Université) for Pd films. The author is also grateful to Gerhard Dehm (Max-Planck-Institut für Eisenforschung) for  helpful discussion.  

Zaoli ZHANG (Leoben, Austria)

08:00 - 18:15 #6890 - IM02-211 Tunable metal-semiconductor junction system deriving from thermal instability of high chalcocite Cu2S elongated nanocrystals.

IM02-211 Tunable metal-semiconductor junction system deriving from thermal instability of high chalcocite Cu2S elongated nanocrystals.

Copper sulfide (Cu_2-xS) nanocrystals (NCs) gained significant attention during the past decade as functional materials for photovoltaics and photothermal applications owing to their high electrical conductivity, low cost and environmental friendliness. Phase transformations of Cu_2-xS NCs upon chemical reactions have been extensively studied [1-2], while their thermal stability, crucial in view of their integration into devices, has not been carefully addressed so far. The main aim of the present study was to monitor thermally-induced transformations within colloidal Cu_2-xS NCs of different aspect ratios, simple and decorated by noble metal NCs. The structural and compositional evolution undergone by NCs was carefully investigated at the nanoscale by transmission electron microscopy (TEM)-related techniques.

The Cu_2-xS NCs were obtained via cation-exchange (Cd²⁺->Cu⁺) on colloidal CdS NCs synthesized according to previous reports [3-5]. This study focuses on two main types of NCs: nanorods (NRs, 20±5 nm diameter, 45±5 nm width) and nanowires (NWs, 20±2 nm width and several μm long). In-situ heating-up experiments were performed by a JEOL heating holder (maximum temperature 800°C) within an image-C_S-corrected JEOL JEM-2200FS TEM. The ongoing processes were monitored via high-resolution TEM (HRTEM) and high-angle annular dark field scanning TEM (HAADF-STEM). The evolution of the chemical composition was followed ex-situ via energy-dispersive X-ray spectroscopy (EDS), carried out in STEM mode. Ex-situ heating experiments were carried out within an ultra-high vacuum furnace.

The set of experiments performed evidence the reproducible and homogeneous formation of crystalline copper domains on both NC systems in the temperature range 350°C-450°C. While in the case of NRs a single domain is formed, NWs exhibit Cu domains at a rather regular spacing along their length. As the initial composition and structure (Cu₂S, high chalcocite) are kept in the remaining region of the NCs, the process seems to be induced by simultaneous oxidation and partial sublimation of sulfur in the NCs and reduction and outward diffusion of excess Cu, probably as an effect of thermal annealing of structural defects (stacking faults, twins) deriving from the CdS synthesis process. This process occurs only in case of relatively high heating rates (~25°C/min). In case of noble metal deposition (Au, Pt) on these Cu₂S NCs, either by sputter-coating or by colloidal growth, the fast diffusion of the noble metal atoms on the NC surface (starting at about 100°C for Au) induces a regular arrangement of domains which acts as a template for the following Cu out-diffusion, making the deriving hetero-structure more robust. The observed reproducible production of metal-semiconductor heterostructures by a simple post-synthesis heating treatment of colloidal Cu₂S NCs opens a path for the production of low cost materials for optoelectronics, as it can simply be extended to a huge range of material combination, for instance achievable by cation-exchange following the heating treatment, which will affect only the Cu₂S segments of the obtained hybrid nanostructures and give rise to application-relevant structures.

[1] Y. Xie, A. Riedinger, M. Prato, A. Casu, A. Genovese, P. Guardia, S. Sottini, C. Sangregorio, K. Miszta, S. Ghosh, T. Pellegrino, L. Manna, J. Am. Chem. Soc. 135, 17630 (2013)

[2] V. Lesnyak, R. Brescia, G. C. Messina, L. Manna, J. Am. Chem. Soc. 137, 9315 (2015)

[3] K. Miszta, G. Gariano, R. Brescia, S. Marras, F. De Donato, S. Ghosh, L. De Trizio, L. Manna, J. Am. Chem. Soc. 137, 12195 (2015)

[4] J. Puthussery, T. H. Kosel, M. Kuno, Small 5, 1112 (2009).

[5] S. Naskar, A. Schlosser, J. F. Miethe, F. Steinbach, A. Feldhoff, N. C. Bigall, Chem. Mater. 27, 3159 (2015).

 

Acknowledgements: The authors acknowledge funding from the European Union under grant agreement n. 614897 (ERC Grant TRANS-NANO).

Muhammad IMRAN, Francesco DE DONATO, Liberato MANNA, Rosaria BRESCIA (Genova, Italy)

08:00 - 18:15 #6902 - IM02-213 10 bar nanoreactors for in situ transmission electron microscopy.

IM02-213 10 bar nanoreactors for in situ transmission electron microscopy.

Environmental transmission electron microscopy (ETEM) is becoming an increasingly important field of study as it is possible to investigate the material-environment interactions on a nanoscale, the scale at which most of these interactions initiate. In a TEM, this can be achieved by one of the following approaches: the opened type, using a differentially pumped vacuum system where the reactive gases are spread around the specimen area of the TEM; and the closed type, using a windowed environmental cell. In the first case the maximum achievable pressure around 1 bar [1] but it is limited to flat samples and for the second type (where two chips are often put on top of each to obtain a closed cell) the maximum pressure rarely exceeds 4 bars [2]. However the last configuration is much closer to what happens in industrial applications.

We present in this paper, a new MEMS nanoreactor fully integrated on a single die. It enables atomic-scale imaging of nanostructured materials under the high pressures and temperatures that are typical for many industrial applications (10 bar and 650°C). The reactor can therefore be used to study the behavior of e.g. catalysts in a transmission electron microscope (TEM). It has a channel of 5 μm (allowing therefore an efficient loading of the samples to study), which is made with surface micromachining techniques and contains pillars that prevent bulging. The channel high can be adjusted by adjusting the right layers in the devices processing. Integrated with the device are 22 very thin windows (20 nm) and a resistive heater. The material chosen for the heater is Molybdenum. It offers a very high stability at 650 C for up to 10 h. The reactor is very transparent and clean (the windows being etched in a such way that contamination/residues are prevented) enabling the imaging of atomic lattice fringes with a spacing down to at least 0.15 nm. The maximum working pressure measured over 50 devices is above 9 bars in 90% of the cases.

 

References

[1] A. K. Erdamar, S. Malladi, F.D. Tichelaar, H.W. Zandbergen,  Controlled Atmosphere Transmission Electron Microscopy, 165-210, Springer International Publishing Switzerland (2016).

[2] T. Yokosawa, T. Alan, G. Pandraud, B. Dam, H. Zandbergen, Ultramicroscopy 112 (1), 47–52, (2012)

Gregory PANDRAUD (delft, The Netherlands), Bruno MORANA, Jia WEI, Casper JUFFERMANS

08:00 - 18:15 #6903 - IM02-215 Surface Dynamics of Cu Oxidation.

IM02-215 Surface Dynamics of Cu Oxidation.

Much is known about oxygen interaction with metal surfaces and about the macroscopic growth of thermodynamically stable oxides. At present, however, the nanoscale stages of oxidation - from nucleation of the metal oxide to formation of the thermodynamically stable oxide - represent a scientifically challenging and technologically important terra incognito. As engineered materials approach the nanometer regime, control of their environmental stability at this scale becomes crucial.  As environmental stability is an essential property of most engineered materials, many oxidation theories exist to explain its mechanisms. However, most classical oxidation theories assume a uniform growing film, where structural changes are not considered due to the lack of traditional experimental procedure to visualize this non-uniform growth under conditions that allow highly controlled surfaces and impurities. Yet, in situ transmission electron microscopy studies reveal that the initial stages of Cu oxidation are due to surface diffusion of oxygen followed by nucleation and growth of oxide islands, and thereby challenge the common assumption of a uniform oxide formation [1]. Understanding this initial oxidation of the metal surface, from the atomic to mesoscale, is the fundamental challenge.

We have previously demonstrated that the formation of epitaxial Cu₂O islands during the transient oxidation of Cu(100), (110) and (111) films bear a striking resemblance to heteroepitaxy, where the initial stages of growth are dominated by oxygen surface diffusion and strain impacts the evolution of the oxide morphologies. We are developing a kinetic Monte Carlo code, Thin Film Oxidation (TFOx), to simulate the nucleation and growth of Cu oxides (see Figure). We are currently investigating oxidation of stepped Cu surfaces as realistic surfaces have many defects such as step edges that can dictate the oxide nucleation and growth dynamics, and result in novel oxide nanostructures.  In situ ETEM studies are complemented with a systematic multiscale theoretical approach.  This approach includes density functional theory (DFT) of (100), (110) and (111) Cu surfaces to illustrate the thermodynamic and kinetic factors of initial oxygen-metal interactions, and molecular dynamics (MD) simulations to validate the DFT predictions and the oxidation process. Our DFT results show that the Ehrlich-Schwöbel (ES) barrier can favor either oxygen ascending or descending diffusion directions, or limited interlayer diffusion depending on the surface step morphology. These simulations are compared to ETEM investigations of stepped Cu surfaces in situ.  The potential and limitations of recent developments in theoretical simulations such as bridging the spatial and temporal gap to in situ ETEM results will be discussed. Correlating the experimental results with theoretical predictions is needed for rational design of oxidation-resistant coating materials and may lead to a paradigm shift in the fundamental understanding of oxidation where surfaces and defects control the early stages of oxidation.

References:

[1]. Q. Zhu, W. A. Saidi, and J. Yang, “ Early and transient stages of Cu oxidation: Atomistic insights from theoretical simulations and in situ experiments”, Surface Science, 2016, doi:10.1016/j.susc.2016.03.003.

Judith YANG (Pittsburgh. PA, USA), Qing ZHU, Lianfeng ZOU, Christopher ANDOLINA, Penghao XIAO, Eric STACH, Graeme HENKELMAN, Guangwen ZHOU, Wissam Abdo SAIDI

08:00 - 18:15 #6947 - IM02-217 In situ electro-chemical liquid TEM experiments to study LiFe0.5Mn0.5PO4 Nanoplatelets.

IM02-217 In situ electro-chemical liquid TEM experiments to study LiFe0.5Mn0.5PO4 Nanoplatelets.

Liquid cell electron microscopy is a developing technique that allows us to apply the powerful capabilities of the electron microscope to image and analyze materials immersed in liquid. We are thus able to examine liquid based processes in materials science and physics that are traditionally inaccessible with conventional TEM. The liquid/bias sealed cell (Protochips Poseidon 510) consists of two designed silicon echips having Si₃N₄ windows and microelectrodes fabricated by lithography technique (figure 1). Electrochemical echips separate the liquid from the microscope vacuum also confining it into a layer that is thin enough for imaging with transmitted electron. The importance of liquid cell TEM in electrochemistry is that liquid cell experiments enable direct imaging of key phenomena during battery operation and relate the structural and compositional changes [1] to electrochemical signature [2,3].

In this work, we carried out in situ liquid TEM studies of LiFe_0.5Mn_0.5PO₄ (LMFP) nanoplatelets synthesized with a colloidal procedure [4], while are very promising nano-objects for high-rate batteries. The synthesized nanoplatelets LMFP were studied using advanced TEM: high angle annular dark field STEM (HAADF-STEM), EDX-STEM mapping, and EELS-STEM mapping to find crystal structure and atomic distributions. The figure 2 shows high-resolution HAADF-STEM image of single LMFP nanocrystal oriented along [010] direction in which Li/(Fe, Mn) anti-site defects is observed (see plot profile). EFTEM and EELS mapping show the coexistence and homogenous distribution of Fe, Mn, P elements in single nanoplatelets. For in situ liquid experiments, this cathode material was deposited onto glassy carbon (on silicon nitride membrane) of the top echip used to encapsulate conventional liquid electrolyte LP30 (LiFP₆/EC/DMC). A gap of 500 nm between both echips and a flow mode introduction were used for the liquid medium. The assembly was followed with TEM/STEM imaging and spectroscopy (EELS, EDX) during electrochemical cycling so as to monitor in real time the delithiation process of LMFP single nanocrystals.

We succeed to follow by TEM imaging in real time the structural and chemical changes taking place on positive electrode materials during the electrochemical cycling. Figure 3 shows electrochemical cyclic voltammetry (25mV/s) corresponding to LFMP in LP30 measured in three-contact mode with silver as reference electrode. Using EELS, it was possible to follow the transition between LFMP and FMP during extraction and insertion of lithium from LiFe_0.5Mn_0.5PO₄ showed by the appearance a peak at 5 eV (in figure 3), also confirmed by EFTEM. The structural changes due to the lithiation/delithiation processes during electrochemical cycling were imaged by high-resolution TEM showing local structural distortion.

[1] A. Demortière et al. Micro&Microanal 2014

[2] Frances M. Ross Science 2015

[3] Megan E. Holtz et al. Nano letters 2014

[4] Andrea Paolella et al. Nano letters 2014

Walid DACHRAOUI, Olesia KARAKULINA, Joke HADERMANN, Arnaud DEMORTIÈRE, Walid DACHRAOUI (Amiens)

08:00 - 18:15 #6948 - IM02-219 CO adsorption on Au(110) and Pd70Au30(110) : an in situ comparative study by environmental STM.

IM02-219 CO adsorption on Au(110) and Pd70Au30(110) : an in situ comparative study by environmental STM.

Surface structure and catalytic properties of metals are often intimately related. Well known surface structures (or reconstructions) in UHV conditions may evolve under gas pressure to yield new configurations. Scanning tunneling microscopy (STM) yields information on surface morphology and structure down to the atomic level. It is thus a well suited technique to follow in situ surface modifications due to gas environments at relatively high pressures [1].

In PdAu alloys, Au has a strong tendency to segregate to the surface at the thermodynamic equilibrium under UHV conditions. It is indeed the case for the Pd₇₀Au₃₀(110) surface for which the outmost layer is essentially formed by Au (85-90% as determined independently by LEIS, variable kinetic energy XPS and grazing incidence surface XRD). We can thus consider to have a Au surface-layer on top of a bimetallic bulk. In such conditions it is interesting to make a comparative study on how CO adsorption affects the surface structure and morphology both of this Au-terminated bimetallic surface and of a Au(110) surface. We have thus used an environmental STM that can be operated from UHV (<10^-9 Torr) to 10³ Torr of controlled environments (in this case CO); the study was donne at room temperature (~ 300K). This STM is an Omicron MicroLH slightly modified (gold plating of copper elements and magnets; coating of the piezoelectric tube) so that it is compatible with elevated pressure and variable temperature operation [2].

Under UHV conditions Au(110) presents a (1x2)-missing-row reconstruction whereas Pd₇₀Au₃₀(110) is unrecontructed with a measured surface parameter closer to Au than to Pd. CO only adsorbs on Au(110) for pressures above 10^-3 Torr whereas CO adsorbs on Pd₇₀Au₃₀(110) at very low partial pressures (<10^-6 Torr) as it was shown by NAP-XPS.

In the range of pressures studied (10^-2 Torr – 5 10² Torr) the unreconstructued flat terraces on Pd₇₀Au₃₀(110) become rough at low pressure  and a “rice grain” morphology is observed with typical domain sizes (oriented in the [1 -1 0] direction) around 4 nm and 0.05 nm corrugation that prevails with no specific variation up to 5 10² Torr (Figure 1). Complementary studies performed by NAP-XPS clearly show the segregation of Pd the surface under CO pressure. So the roughening observed by STM (approximately one third of an atomic step in height) is essentiallydue to the diffusion of Pd atoms to the surface.

In the case of Au(110), the evolution of the surface structure and morphology with increasing CO pressure shows different surface structures [3] (Figure 2): under vacuum conditions, the Au(110) surface exhibits a (1x2) reconstruction which yields aligned terraces in the [1 -1 0] direction at a larger scale. CO chemisorption at 0.01 Torr on this surface induces a slow deconstruction of the (1x2) surface leading to a (1x4) structure under 0.1 Torr of CO.At higher pressure (0.5 to 30 Torr) a dramatic restructuring is observed where the terraces aligned in the [1 -1 0] direction under vacuum evolve to yield monoatomic-high islands. Their size subsequently increases with increasing CO pressure [4]. At 100 Torr of CO the surface exhibits a (1x1) structure prior to the new surface structure observed at 5 10² Torr of CO with ~0.05 nm deep holes arranged in a c(4x4) array. Intensity modulations around these holes were also observed.

CO chemisorption induces a strong restructuring of  both “Au” surfaces as it is evidenced by the high resolution in situ environmental STM images. However while the restructuring is limited to a roughning of the surface (due Pd segregation) for the bimetallic crystal, the structure and morphology of the Au-pure crystal surface evolve (through different configurations) as CO pressure increases showing that we have to take into account the dynamics of the surface and thus the evolution of the active sites during reaction.

References

[1] B.J. McIntyre, M. Salmeron, G.A. Somorjai, Rev. Sci. Instrum. 64 (1993) 687.

[2] F.J. Cadete Santos Aires, C. Deranlot, Proc. EUREM-XII. Volume III, P. Ciampor, L. Frank, P. Tománek, R. Kolarik (Eds), Brno 2000, p.I263 ;;

[3] M.A. Languille et al., Catal. Today 260 (2016) 39.

[4] Y. Jugnet et al., Surf. Sci. 521 (2002) L639.

Marie-Angélique LANGUILLE, Eric EHRET, Francisco José CADETE SANTOS AIRES (VILLEURBANNE CEDEX)

08:00 - 18:15 #6960 - IM02-221 Monitoring morphological evolution of Li-ion cathode secondary particles through In situ FIB/SEM electrochemical experiment.

IM02-221 Monitoring morphological evolution of Li-ion cathode secondary particles through In situ FIB/SEM electrochemical experiment.

This last decade, fundamental studies of electrochemical phenomena have been slowed down by a lack of effective in situ (and operando) experimental setup, which is able to clearly identify structural modifications inside and at the surface of electrode materials. The evolution of microstructures, the appearance of cracks and porosities and the transformation of crystalline phases have to be properly investigated in order to get a better insight into the influence of charge/discharge processes in battery materials and the reaction mechanisms implied in electrochemical storage. Improving our understanding of the microstructural changes and crack formation into Li-ion electrode materials during electrochemical cycling can provide new insight into battery behaviour. In order to monitor microstructural evolution dynamically during electrochemical cycling, we developed a micro-scale battery set-up implemented within a FIB/SEM instrument [1]. Secondary particles (figure 1a) are strongly used by industry for positive electrode fabrication. However, so far just a few works were focused on their morphological evolution during electrochemical cycling. Here, single secondary particles of cathode oxide (NCA and NMC) materials with a size of 5-10 μm are attached to the metal pin of the micromanipulator via a conductive carbon bridge formed using GIS (figure 1d/e). The micromanipulator allows moving the particle in the chamber and immersing into an ionic liquid electrolyte (low vapour pressure), which is deposited on a counter electrode, i.e. lithium metal. Electrochemical measurements are carried out using ultra low current instrument (biologic SP200) with two points connection configuration, external probe tip and SEM metal holder are respectively connected. After immersing into electrolyte, the single particle of active materials is cycled in galvanostatic mode with a steady current around 1 nA, which corresponds to a C-rate of about 1 based on the particle volume and the theoretical capacity [2].

We succeed to carry out in situ experiments inside the FIB/SEM chamber using ionic liquid electrode getting high quality electrochemical measurements in galvatostatic (figure 1f) and impedance modes for single secondary cathode particle. We studied structural modifications of individual particle after each in situ charge/discharge cycle by FIB slicing and SEM imaging. Using specific FIJI plugins and AMIRA software for reconstruction and segmentation steps, we quantified the formation of cracks as a function of cycle number and extracted 2D skeleton and tortuosity (figure 1g/h). Evolution of the discharge capacity was correlated with cracks and porosities appearance inside cathode materials. Impedance measurements suggested an increase of lithium diffusion inside the particle that is relied on the formation of cracks, which induces an enhancement of discharge capacity. On the other hand, the characterization of the 3D structure of these materials is crucial in order to gain a deeper insight into structural configuration and evolution of the discharge capacity. In this purpose, the reconstruction of 3D microstructures by FIB tomography methods was used [3] (figure 1i). The changes of structural parameters such as porosity, grain connectivity and crack propagation that are induced by cycling were extracted from 3D reconstruction and linked with electrochemical properties. Then, 3D structural data was compared to that obtained by 3D Transmission X- ray Microscopy (TXM) tomography [4], which is made in APS synchrotron at Argonne NL. Finally, a mechanical strain mode was used to get a better insight into crack formation and evolution into cathode particle.

References :
[1] Miller, D. J., Proff, C., Wen, J. G., Abraham, D. P., & Bareño, J. (2013). Observation of microstructural evolution in Li battery cathode

oxide particles by in situ electron microscopy. Advanced Energy Materials, 3(8), 1098-1103.
[2] Demortiere, Ando, J., A., Bettge, M., De Andrade, V., Amine, K. , Miller, D. (2016). Study of microstructural evolution in Li-ion battery cathode single particle by in situ FIB-SEM setup and TXM tomography. Advanced Energy Materials (in submission)
[3] Chen-Wiegart, Y. C. K., Liu, Z., Faber, K. T., Barnett, S. A., & Wang, J. (2013). 3D analysis of a LiCoO 2–Li (Ni 1/3 Mn 1/3 Co 1/3) O 2 Li-ion battery positive electrode using x-ray nano-tomography. Electrochemistry Communications, 28, 127-130.
[4] Tariq, F., Yufit, V., Kishimoto, M., Shearing, P. R., Menkin, S., Golodnitsky, D., ... & Brandon, N. P. (2014). Three-dimensional high resolution X-ray imaging and quantification of lithium ion battery mesocarbon microbead anodes. Journal of Power Sources, 248, 1014- 1020.

Arnaud DEMORTIERE (UPJV, Amiens), Jonathan ANDO, Martin BETTGE, Vincent DE ANDRADE, Khalil AMINE, Dean MILLER

08:00 - 18:15 #6967 - IM02-223 Real-time observation of the transformation of silver nanoparticles during carbon gasification.

IM02-223 Real-time observation of the transformation of silver nanoparticles during carbon gasification.

Gasification of carbon can be achieved, in the presence of metal catalysts supported on solid carbon materials, at high temperature in oxidative environments. Indeed, oxygen adsorbs and dissociates on the metal surface then interacts with the carbon at the interface with the metal leading to the formation of carbon dioxide; concurrently carbon is consumed and the metallic nanoparticle maintains the interface with the carbon and thus advances, forming in this way a trench on the surface of the carbon. On structured materials such as graphite or graphene these trenches tend to be rather 2D [1,2] at the surface of the material (hereafter named “pacman effect”). In this work we have studied the gasification of a non-structured material (amorphous carbon) by silver-based nanoparticles (NPs) in presence of oxygen and at variable temperatures. We were particularly interested on the dynamic structural evolution of the silver NPs observed in real-time within an environmental TEM and at atomic resolution as permitted by the imaging Cs-corrector.

The studies were performed in the Ly-EtTEM (Lyon Environmental and tomographic Transmission Electron Microscope), a 80-300 kV TITAN objective lens Cs-corrected Environmental TEM from FEI equipped with a GATAN high resolution Imaging Filter (GIF) [3]. Samples were prepared according to a synthesis method described in [4]; the silver based NPs stand on the supporting carbon film and gasification of this film is observed between 400 and 500°C under variable oxygen partial pressures (between 10^-1 and 5 mbar).

We were able to follow in real-time the dynamics of carbon gasification and catalyst evolution by high resolution imaging (Figure 1) unlike previous studies. In situ EELS yields complementary information necessary to fully interpret the observed phenomena (structural changes, shrinking and coating of the catalyst, …). The mechanism deduced from our in situ study is schematically summarized in Figure 2 : (i) at the beginning of the gasification experiment the NP have surprisingly an hexagonal structure consistent with previously identified hexagonal forms of silver, either metallic or containing diluted oxygen (Figure 2a-c); (ii) at a given moment during gasification the NP transforms to fcc-Ag, gasification slows down, the particle begins to shrink (surface oxide formation that decomposes) while a coating forms around it (Figure 2d); (iii) once the particle is completely coated gasification stops and the NP shrinking stops (Figure 2e).

Gasification of amorphous carbon by Ag NPs was studied in situ (P oxygen, T variable) in an ETEM. Gasification rate slows when the Ag NPs transform from initial hexagonal structure to fcc; concurrently the Ag NPs shrink rapidly. The dynamic real-time high resolution images associated with local EELS measurements allowed to propose a mechanism for the observed phenomena.

References
[1] S.K. Shaikhutdinov and F.J. Cadete Santos Aires, Langmuir 14 (1998) 3501.

[2] T.J. Booth, F. Pizzocchero, H. Andersen, T.W. Hansen and J.B. Wagner, Nano Letters 11 (2011) 2689-2692.

[3] The authors thank the CLYM (Centre Lyon-St Etienne de Microscopie) for access to the Ly-EtTEM.

[4] S. Li, L. Burel, C. Aquino, A. Tuel, F. Morfin, J.L. Rousset and D. Farrusseng, Chem. Comm. 49 (2013) 8507-8509.

Francisco José CADETE SANTOS AIRES (VILLEURBANNE CEDEX), Mimoun AOUINE, Shiwen LI, Alain TUEL, David FARRUSSENG, Thierry EPICIER

08:00 - 18:15 #6188 - IM03-225 Optimizing electron channeling contrast imaging condition in scanning electron microscope.

IM03-225 Optimizing electron channeling contrast imaging condition in scanning electron microscope.

The electron channeling contrast imaging (ECCI) is a technique that makes use of the influence of back scattered electrons (BSE) by the relative orientation of crystalline lattice and incident electron beam [1]. The ECCI in scanning electron microscope (SEM) is especially useful to image the crystallographic defects such as dislocations, stacking faults and twin boundaries [2]. In this contribution the amount of channeling contrast in the total BSE signal is quantified and its dependence on different working conditions is explored.

An example of the ECCI on an epitaxial GaN thin film is shown in Figure 1. The sample is investigated using ZEISS GeminiSEM 500 FE-SEM with a PIN-diode BSE detector under the objective lens. By adjusting the strength of the scanning coil it is possible to move the pivot point of scanning electron down to the surface of the sample, forming the selected area channeling pattern (SACP) [1]. The intensity variation of BSE signal can be extracted by the line profile across the Kikuchi line, shown as the red dotted line in the figure. The relative variation of BSE signal in SACP with respect to average signal can thus be used as a quantification of channeling contrast.

The amount of channeling contrast is influenced by the properties of incident electron beam and the detector. In principle, the energy and convergence angle of the incident electron beam determines the channeling pattern itself, while the position and energy response of the detector influence the amount of channeling contrast. In this study a large parameter space is explored to optimize the channeling contrast, including electron beam energy, working distance, convergence angle and position of detectors. As an example, the influence of electron beam energy to channeling contrast is summarized in Figure 2. The SACPs are shown in the right part of the figure. The position of the lines as well as intensity variations within the pattern changes significantly between different energies. However, quantification of the channeling contrast shows a clear trend of increasing channeling contrast at lower electron beam energy.

Reference:

[1] L. Reimer, Scanning Electron Microscopy, 2nd. Edition (Springer) 359

[2] I. Gutierrez-Urrutia, et. al., Scripta Materialia 61 (2009) 737

Luyang HAN (Oberkochen, Germany), Bjoern GAMM

08:00 - 18:15 #6274 - IM03-227 Faster than 10 ns scintillator material based on YAP:Pr.

IM03-227 Faster than 10 ns scintillator material based on YAP:Pr.

YAP:Pr (praseodymium-doped YAlO₃) is a promising scintillation material with fast response time around 10 ns and a photon yield up to 20 000  photons/MeV (depending on the preparation method) [1], about half of that of widely-used YAG:Ce (which has decay time about 70 ns). Nowadays, there is a growing demand on a scintillation materials with such a fast response times for use in high-speed electron microscopes.

The luminescence spectra of YAP:Pr has two-peak band in the UV range (230-300 nm), and a few more peaks in the green region (500-600 nm) [2]. Pr³⁺ radiative transitions from 5d₁ orbitals are responsible for the fast, former ones. Appropriate colour filter can easily separate the fast and the slow components. Moreover, a modified growth method using co-dopants (patent pending) which we used, led to a decrease of the decay time by one third of the former value. The fastest sample we have obtained up to now has an exponential decay with main decay time 6 ns.

Since the transport of the emitted light can be ensured by a quartz lightguide and the photomultipliers are also available for this wavelength region, the main challenge for the use of YAP:Pr as a scintillator in the electron microscopes for secondary electrons or back-scattered electrons detectors is therefore improvement of light yield of the fast component and inhibition or even full suppression of the afterglow luminescence.

 

 

References

1. T. Yanagida et al. / Nuclear Instruments and Methods in Physics Research A 623 (2010) 1020–1023
2. M. Nikl et al., “Can Pr-doped YAP Scintillator Perform Better?”, IEEE Trans. Nucl. Sci., 41, pp.1168-1174, 2010

Martin POKORNÝ (Turnov, Czech Republic), Petr HORODYSKÝ, Martin NIKL, Jindřich HOUŽVIČKA

08:00 - 18:15 #6304 - IM03-229 EDF Project - FIB-SEM nuclearized for investigations in a hot cell on irradiated materials.

IM03-229 EDF Project - FIB-SEM nuclearized for investigations in a hot cell on irradiated materials.

Context : CEIDRE laboratory (Centre d’Expertise et d’Inspection dans les Domaines de la Réalisation et d’Exploitation) based at EDF Chinon Nuclear Power Plant, is in charge of expertising and monitoring. Results of analytical studies bring to better understand the rate of ageing phenomena on power production installations; in addition it is contributing in improving and making safely reliable the exploitation and maintenance strategies in the electricity production subject. Investigations are performed thanks of various analytical techniques as : Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM) or Castaing microprobe (EPMA). It allows to multi scale images and analyses the damages caused by material ageing during its service (ageing under radiations or under heating, corrosion under load, mechanical breaking). At the ultimate level, the use of a TEM is requesting a specific sample preparation, so called TEM lamella with manual bring to state techniques into hot cell preparation units. The manipulations of such radioactive materials are today outdated by the current regulation regarding safety rules and some new performances of thinning the samples are now needed.

In order to limit these risks, the CEIDRE lab has invested into a new system, a Focused Ion Beam (FIB) dedicated to extract thin TEM lamella from a bulk sample. The FIB is coupled with a SEM column, an Energy Dispersive Spectrometer (EDS) and a Time of Flight Spectrometer (TOF SIMS) for elementary analysis and chemical mappings. Complementary to conventional preparation techniques, this tool is also enlarging the previous SEM capacity with fractography, chemical and crystallography analysis. This modern technique is now installed into a hot activity cell and allows the capabilities to operate on active materials.

Specific conception is mandatory to make operational this tool in such severe conditions; however the system is based from standard components where challenging points must be considered: Sample loading and unloading via tele manipulators arms without damaging the FIB/SEM motorized stage and Remote operations of many (as much as possible) of driving units that cannot sit into the cell.

The entire project has been in trust to ELOÏSE sarl associated with NewTec Scientific companies, a TESCAN LYRA 3 GM. From 2013 to 2015, the system has been rebuilt under NewTec laboratories in Nîmes France, commissioned and finally installed in its final destination into a cell at LIDEC (Laboratoire Intégré d’Expertise du CEIDRE).

Modifications – Features : From the strong environmental conditions (gamma activity, acoustic, thermal and mechanical), the instrument plus its accessories have been totally deconstructed and rebuilt to fits the requested exigencies. Most of the electronics units dedicated to the control of the instrument and its accessories (micromanipulator, EDS and TOF spectrometers) are relocated in the so called front zone, 10 meters away from any radiation and outside the hot cell. Most of the cables have been rebuilt and requalified to over Giga Ohms range. In cell, sensitive components are now fitted with enhanced protection against radiation.

Based on feedback systemic studies have been proceed with various known critical scenarios, then new solutions came from that analysis.  The EDS spectrometer received a tungsten base shielding offering a deep protection against emissive rays coming from the samples. In case of major failure of the instrument, an emergency routine composed by a tele manipulate-able tool and a in chamber toboggan allows to collect the sample holder. Finally a semi-automatic loading station has been totally built around the original instrument. This work has been carefully study regarding the specific ergonomy and way of manipulating the samples entering the hot cell (remoted display and three buttons easy to use panel).

Résults & Perspectives : As far as today, the outside cell units escape is confirmed as the best strategy in retailoring an instrument to operate safely. This includes reconsidering signal to signal treatment (e.g. use of optical fibers for communications). However, and despite the massive modifications done, the system is keeping the original performances for both SEM & FIB capabilities. Beyond this points, the high speed networks available at EDF allows extreme safe and comfortable operations even on critical samples.

This tool is now fully operational, the next challenge is human. Training and transmit to operators the new way of tele-working the SEM/FIB.

Salem MILOUDI, Jean-Claude MENARD (Bois d'Arcy), Bernard JEAN, Antoine CANDEIAS

08:00 - 18:15 #6312 - IM03-231 SEM LEEM – new type of mirror microscope.

IM03-231 SEM LEEM – new type of mirror microscope.

LEEM is a well established method for the imaging of surfaces of materials using the impact of very slow electrons (down to zero energy - mirror microscopy). On the other hand, scanning electron microscopy has not consistently solved the detection of electrons in the mirror mode, i.e. in the incident energy range of 3 to 0 eV. In contrast to LEEM, which forms an integral image of the specimen surface and projects it onto an image screen, the scanning method relies on the detection of signals from individual pixels of the specimen. Since mirror microscopy can be realized only when using immersion lenses (either electrostatic, or combined with magnetic immersion), the detector has to capture the fast electrons with energies in principle of the same value as of the primary beam. Such “signal” electron beam moves back through the optical system very close to its optical axis and standard rotationally symmetric electron optics is in principle not able to provide their detection, and signal electrons return towards the source. The higher magnification is used, the more signal electrons are lost.

One solution is to use a rotationally asymmetric imaging and detection system that provides separate beam lines of primary and signal beams [1]. Any separation of these two beams requires their deviation from the optical axis. Given that the energy of signal electrons is high (close to the energy of the primary electrons) in the mirror microscopy mode, the deflection of signal electrons without influencing the primary beam quality is not a simple problem. One of possible solutions is to use a Wien filter, which does not change the trajectory of the primary beam, but only of the signal beam. However, only relatively small deflection angles of the signal beam are achievable for fast electrons. Larger deflection angles cause non-correctable defects of the primary beam. Another solution is to use magnetic prisms, which are able to compensate for any aberrations of the second order and also for energy dispersion in a symmetrical arrangement [2]. In the asymmetric arrangement, we can reach sufficiently large beam deflection angles (90°) having the energy dispersion on the order of units of micrometers per volt. By proper combination of homogeneous magnetic fields, the beam separator stigmatically transfers the primary beam back to the optical axis, while simultaneously allowing the detection of either energy non dispersed or dispersed signal electrons (for example of secondary or backscattered). Such a through-the-lens detector has zero optical power in the primary beam direction. In the “standard” operation mode with grounded specimen the through-the-lens detector can be (but does not have to be) switched off. The primary beam then passes through the detector rectilinearly. Signal electrons can be collected with any other standard detectors for SEM microscopy.

The first experiments verifying the correctness of the concept were made with the help of an assembly consisting of an (Schottky) electron source equipped with a magnetic gun lens followed by detector unit and electrostatic triode objective lens. The specimen is connected to a variable high voltage supply floating on the cathode potential. This arrangement allows to compensate the instability of the main high voltage supply in such a way that the incident energy remains well defined. Here we present the system with only electrostatic immersion objective lens. Experiments with a combination of electrostatic-magnetic immersion lens are under preparation. The results presented here are demonstrating the ability of the detector to distinguish between BSE and SE electrons for landing energies as low as 0.5 eV. The integral detector is mixing both signals and shows the mirror character of the image.

[1] P. Kruit, in Advances in Optical and Electron Microscopy, vol. 12, ed. by T. Mulvey (Academic, London, 1991), pp. 93-137

[2] V. Kolarik, M. Mankos, L. H. Veneklasen, Optik 87, No. 1 (1991) 1 - 12

Vladimir KOLARIK (Brno, Czech Republic), Pavel JANSKY, Martin MYNAR

08:00 - 18:15 #6328 - IM03-233 Imaging of Electric Fields with the pnCCD (S)TEM Camera.

IM03-233 Imaging of Electric Fields with the pnCCD (S)TEM Camera.

The imaging of electric fields on the nanometer scale is of great interest for modern materials research. Techniques providing a fast, direct and precise measurement of local fields are thus useful for materials science applications. We present microscopic measurements of electric fields with the 4D-STEM technique using the pnCCD (S)TEM camera. In 4D-STEM, a 2D camera image is recorded for each probe position of a 2D scan area, yielding a 4D dataset. With this technique, small shifts of the bright field disc (BFD) due to a deflection of the electron beam through electric and magnetic fields in the sample region can be detected. Hence, the magnitude and direction of the local field at each probe position can be determined. Given the large number of necessary probe positions, this technique requires a fast camera system providing short enough readout times so that instabilities in the microscope and sample drift or radiation damage do not deteriorate the final STEM image. Furthermore, a pixelated detector is required to record and account for the intensity distribution variations caused by interaction of the electron beam with the sample.

 

The pnCCD (S)TEM camera allows fast acquisition of 2D camera images with a direct detecting, radiation hard pnCCD with 264x264 pixels [1]. Routinely, the readout speed is 1000 frames per second (fps) and can be further increased through binning and windowing. For example, with the pnCCD (S)TEM camera a 256x256 STEM image -- where a camera image is recorded at each probe position -- can be recorded in less than 70 s. The 264x264 pixel camera image allows precise determination of the BFD position, yielding information about electric and magnetic fields in the sample. For data analysis, image subsets can be selected freely to obtain virtual diffraction images or perform differential phase contrast (DPC) analysis. A major advantage over conventional segmented DPC detectors is that, with the pnCCD (S)TEM camera, movements of the BFD can be discriminated from intensity variations inside the BFD which is of particular importance for analysis of electromagnetic fields inside specimens. Further 4D-STEM applications benefitting from the pnCCD (S)TEM camera include imaging on the micro- and millisecond timescale [2], strain analysis [3], magnetic domain mapping [1], and electron ptychography [4].

 

A demonstration of electric field mapping in vacuum with the pnCCD (S)TEM camera is shown in Figure 1. A voltage of 50 V was applied to a tungsten needle mounted in an FEI Titan G2 80-200 ChemiSTEM microscope, operated at 80 keV. For each of the 256x256 probe positions, a 2D camera image was recorded (Fig. 1a). From these camera images an incoherent bright field STEM image (Fig. 1b, background) as well as the position in the x- and y-directions of the BFD at each probe position was calculated. A comparison of the position of the BFD with and without an applied voltage yields information about the magnitude and direction of the local gradient of the projected electrostatic potential (Fig. 1b, indicated by coloring and arrows). In addition to this direct mapping of the electric field around a needle with rather well-shaped BFDs, the large number of pixels of the pnCCD (S)TEM camera allows the precise determination of the BFD position, even in cases when the BFD is weak and deformed through the interaction of the electron beam with the sample (Fig. 1c).

 

In conclusion, 4D-STEM techniques like electromagnetic field mapping benefit significantly from the capabilities of the pnCCD (S)TEM camera. The readout speeds of 1000 fps and above allow the fast acquisition of 4D datasets with 2D camera images at each probe position. Through the large number of pixels, position and intensity variations of BFDs can be precisely determined.

 

[1] H. Ryll et al, Journal of Instrumentation, in press.

[2] H. Ryll et al, Microscopy and Microanalysis 21 (Suppl. 3) (2015), p. 1585-1586.

[3] K. Müller et al, Appl. Phys. Lett. 101 (2012), p. 2121101-2121104.

[4] H. Yang et al, Microscopy and Microanalysis 21 (Suppl. 3) (2015), p. 2303-2304.

Robert RITZ, Martin HUTH (Muenchen, Germany), Sebastian IHLE, Martin SIMSON, Heike SOLTAU, Vadim MIGUNOV, Martial DUCHAMP, Rafal E. DUNIN-BORKOWSKI, Henning RYLL, Lothar STRÜDER

08:00 - 18:15 #6335 - IM03-235 Quantification of dopants in nanomaterial by SEM/EDS.

IM03-235 Quantification of dopants in nanomaterial by SEM/EDS.

It is long known that doping is a key element in the development of modern semiconductor technology for applications in electronic, nano-electronics, optoelectronics and photonics. Doping allows modifications in the electrical conductivity of semiconductors that depend on the type, quantity, distribution and activity of added dopants. Characterizing doping is therefore essential for the understanding and improvement of electrical and optical properties of semiconductors in order to produce reliable and performant electronic and optical devices. The increasingly reduced dimensions of semiconducting devices as well as the development of new nanomaterial-based devices require the characterization (dopant type and activity) and quantification (concentration and spatial distribution) of low-levels of dopants at the nano-scale. This represents a real challenge that is not fully achieved today by existing techniques which are globally expensive, time-consuming, difficult to implement in an industrial context (STEM, APT, electron holography), spatially unresolved (SIMS), not very sensitive/accurate as yet (EDS and EELS in TEM) and, for most of them, not fully quantitative.

To date, the state of the art for conventional TEM/EDS (Electron Dispersive Spectrometry in Transmission Electron Microscope) is limited to the detection of a few dopants (As, P) in a few materials (Si, SiGe) with detection limit around 10²⁰ at/cm³ and precision of ±20-30% (Sevanton et al., 2009). We report here the detection and quantification by SEM/EDS (SEM: Scanning Electron Microscope) of dopant concentrations as low as 5 10¹⁸ at cm^-3 with a precision and detection limit around 10¹⁸ at cm^-3. Such a large improvement in detection sensitivity can be achieved at low voltage (< 8 kV) using the experimental Flat Quad 5060F annular detector from Bruker that equips the Ultra55 Zeiss SEM of the Minatec’s PFNC (INAC). This detector belongs to the new generation of silicon drift detectors (SDD) which are composed of four bean-shaped silicon diodes arranged in a ring around a central hole for the electron beam passage (Fig. a). It is positioned a few millimeters above the sample (Fig. b), a geometry which results in a much wider solid angle (up to 1.2 sr) compared to traditional detectors (<0.1 sr), allowing a higher counting rate at any operating conditions (up to 1000 kcps).

The major difficulty for quantifying low level concentrations by EDS (even at low voltage) remains to extract a low intensity signal from a relatively high background signal. This is particularly true for Mg in GaN, the K-line (1.25 keV) of this element being very close to the L-line (1.19 keV) of Ga (Figs. c-d). To overcome this problem, a new method has been developed which is based on two innovations: 1) the use of specific windows that act as X-ray filters, allowing a large enhancement of the signal to noise ratio in the energy range corresponding to the X-ray line of the analyzed dopants, and 2) the development of a new analytical procedure for removing background based on spectrum normalization to pure reference spectrum (Fig. e).

Results obtained on P-doped Ge 2D layers and Mg-, Si-doped GaN 2D layers show good consistency with SIMS analyses, even for the lowest concentrations of dopants (see Fig. f for Mg dopant in GaN). The technique was applied for quantifying low level of dopants (down to 10¹⁹ at/cm³) in various types of Ge, GaN, and AlGaN nanomaterials. Results will be presented and discussed at the conference.

Reference:

G. Servanton, R. Pantel, M. Juhel and F. Bertin (2009) Two-dimensional quantitative mapping of arsenic in nanometer-scale silicon devices using STEM EELS-EDX spectroscopy, micron 40, 543-551

Eric ROBIN (GRENOBLE), Nicolas MOLLARD, Kevin GUILLOY, Nicolas PAUC, Pascal GENTILE, Zhihua FANG, Bruno DAUDIN, Lynda AMICHI, Pierre-Henri JOUNEAU, Catherine BOUGEROL, Michael DELALANDE, Anne-Laure BAVENCOVE

08:00 - 18:15 #6353 - IM03-237 Generation of nondiffracting beams: beam shaping beyond holographic reconstruction.

IM03-237 Generation of nondiffracting beams: beam shaping beyond holographic reconstruction.

The last few years have seen a rapid acceleration in the field of beam manipulation [5].

Central to this expansion is the idea that the purposeful design of the beam's wavefunction can imbue the electron with new properties that potentially enable new measurements previously impossible.

Examples of this are the orbital angular momentum possessed by electron vortex beams [1], or the reduced diffraction of Airy and Bessel beams [2-4].

In these studies, holographic reconstruction remains the most widely used method to generate new wave types due to its generality and flexibility but presents several shortcomings, chiefly the production of multiple diffracted beams [1-3,5].

Here we showcase different approaches to the production of reduced diffraction beams.

We generate Airy waves by a careful tuning of the aberration function of an aberration corrected TEM, and produce a Bessel beam by limiting the angular spread of the electrons with an annular aperture.

We study the beam propagation to prove the limited diffraction, and give experimental proof of extended depth of focus in imaging with Bessel beams.

Acknowledgements:

GG, AB and JV acknowledge funding from the European Research Council under the 7th Framework Program (FP7), ERC Starting Grant No. 278510-VORTEX.

References:

[1] J. Verbeeck et al. Nature 467, 301 (2010).

[2] N. Voloch-Bloch et al. Nature 494, 331 (2013).

[3] V. Grillo et al. Phys. Rev. X 4, 011013 (2014).

[4] R. Shiloh et al. Phys. Rev. Lett. 114, 1 (2015).

[5] G. Guzzinati et al. Ultramicroscopy 151, 85 (2015).

Giulio GUZZINATI (Antwerpen, Belgium), Armand BÉCHÉ, Jo VERBEECK

08:00 - 18:15 #6358 - IM03-239 Surface-sensitive investigation of semiconductor devices with a signal-selective SEM detection system.

IM03-239 Surface-sensitive investigation of semiconductor devices with a signal-selective SEM detection system.

The detection system of a scanning electron microscope (SEM) is just as important for image quality as the design of its electron optics [1]. Recent studies [2, 3] emphasize the importance of angle- and energy-filtering of backscattered electrons signal (BSE) to reveal details not usually observable in the integral BSE signal.

 

Here we introduce a novel detection system tested on a recently developed ultra-high resolution electron optical column [4, 5] that comprises three BSE detectors capable of angle- and energy-filtering. The system can be switched between a “high-signal” mode where no bias is applied to the filtering grid and an “energy-selective” mode where the filtering voltage can be tuned to achieve various degrees of signal separation.

High-pass energy filtering enables detection of BSEs which have undergone minimal energy loss. Their escape depth is limited and the resulting micrograph becomes surface sensitive. This is beneficial for samples with complex surface and sub-surface structure. Clear visualization of the topmost layer of the sample is critical e.g. for failure analysis of 3D semiconductor devices, given the continuously decreasing dimensions of the local structures in such devices.

  

Figure 1 shows a SEM micrograph of a semiconductor memory device cross-section acquired at 3 keV primary beam energy and 265 pA probe current. Contrast of the images decreases continuously with increasing bias of the filtering grid. With no bias applied the material contrast is highest, yet it contains information about both surface and subsurface area of the specimen. With increasing filtering bias only the electrons backscattered from the very surface of the sample are detected. This is in agreement with theoretical simulations for 3 keV primary beam in silicon that show an escape depth of 80 nm for the integral BSE signal and only 2 nm for the low-loss BSE signal.

 

References:

[1] B J Griffin et al, Proceedings of  the 18^th International Microscopy Congress (2014).

[2] I Müllerová et al, Material Transactions 48 (2007), p. 940.

[3] H Jaksch, Proceedings of  EMAS 2011 (2011), p. 255.

[4] J Jiruše et al, Ultramicroscopy 146 (2014), p. 27-32.

[5] J Jiruše et al, Proceedings of Microscopy and Mircoanalysis 2014, (2014).

Jaroslav JIRUŠE (Brno, Czech Republic), Jolana KOLOŠOVÁ, Petr MAREŠ, Rostislav VÁŇA

08:00 - 18:15 #6393 - IM03-241 Depth Resolution and Surface Sensitivity with the Multiple Detection System of a HR-SEM.

IM03-241 Depth Resolution and Surface Sensitivity with the Multiple Detection System of a HR-SEM.

State-of-the-art high-resolution scanning electron microscopes (HR-SEMs) attain a lateral resolution of about 1 nm. To obtain this high resolution even at low landing energies, a deceleration voltage is applied to the sample while the beam has a higher energy inside the electron column to minimize aberrations. The excellent lateral resolution is one key property of HR-SEM images, but the quality of the obtained contrast and its relation to the sample properties have to be considered as well. It is obvious that an electric field above the specimen changes direction and energy of the detected electrons and thus the detected contrast. The purpose of this contribution is to investigate depth resolution and surface sensitivity obtained with the multiple detection system of the Hitachi SU8030 SEM. Fig. 1 illustrates a typical application requiring an excellent surface sensitivity to image the 50 nm thin wall of sheath forming bacteria at high lateral resolution.

As a test specimen to characterize the depth resolution we used glassy carbon, which was structured by an ion beam (FEI Helios NanoLab600). Three 5 µm x 15 µm trenches with a depth of 1 µm were milled and filled with platinum. On top of these structures and the contiguous glassy carbon, Pt-layers with thicknesses of 12, 25 and 50 nm were deposited [Fig. 2]. For the evaluation of the depth information the following detectors were used: The standard inlens detector (upper), a second inlens detector (top) that is positioned above the upper detector, and a retractable photo diode backscattered electron detector (PDBSE). All the detectors were used both, in the normal imaging mode (specimen grounded) and in the deceleration mode (specimen applied with variable potential). The traditional chamber detector has been left out, because there is no possibility to control the ratio of the SE- and BSE-signal with the Hitachi SU8030.

In normal mode, the upper detector receives a pure signal of secondary electrons [Fig. 2a]. The information depth depends on the accelerating voltage only. However, it is complicated to interpret the resulting contrast because topography, edge effects, charging, and potential differences affect the result. The top detector in normal mode [Fig. 2b,c] gives a high angle backscattered electron signal. Again, the information depth depends on the energy of the primary beam. As the amorphous test specimen did not produce channeling effects, the arising contrast is solely a result of the penetration depth of the primary electrons. If the annular control electrode located inside the objective lens is set to negative bias, the SE signal is filtered out and the upper detector also detects BSE. Because these are low angle BSE, the resulting contrast shows a mixture of topographic and depth information [Fig. 2d]. The PDBSE detector operated in the composition mode with the sum signal of four symmetric segments gives a pure BSE signal. Only the acceleration voltage is responsible for the information depth [Fig. 3a-d].

In the deceleration mode, the PDBSE and the upper detector deliver a proper depth information. However, the SE leaving the sample are accelerated towards the detector as well, and thus the result is a mixed SE-BSE signal, and the depth information is superimposed by topographic, edge effect, potential, and material contrast. The top detector in the deceleration mode provides a pure SE signal with an excellent surface sensitivity, but no depth information at all.

Therefore, the most suitable way to get surface and depth information is using the PDBSE detector in composition mode with the sample set to ground potential. At 1 kV acceleration voltage there is no difference in signal intensity between the Pt-area with 1 µm in thickness and the region with the 10 nm Pt-layer on carbon. This means that the information depth is not more than about 10 nm [Fig. 3a]. At 3 kV the 50 nm Pt-layer on carbon has the same contrast compared to bulk Pt, therefore the information depth is about 50 nm [Fig. 3b]. When using 4 and 5 kV there is a contrast visible even between the 1 µm thickness area of platinum and the 50 nm Pt-coating. In this case, the depth of the volume of emitted BSE exceeds the 50 nm Pt-layer [Fig. 3c,d].

In conclusion, it is recommended to use a low angle backscatter detector in composition mode and a grounded stage since other sample properties, like topography, edge effect, charging and potential do not affect the contrast and a pure and tunable depth information is obtained.

Ulrich GERNERT (Berlin, Germany), Dirk BERGER

08:00 - 18:15 #6394 - IM03-243 A magnetic electron energy analyser for use in an SEM.

IM03-243 A magnetic electron energy analyser for use in an SEM.

A new concept for an electron energy analyser is presented whereby a magnetic field is used to disperse the electrons on to an imaging detector. Although using magnetic fields to disperse electrons is not a new idea [1], we show that some of the previous difficulties with using magnetic fields as an electron energy analyser in a SEM can be overcome. Figure 1 shows a schematic of the basic principle. Electrons emitted from the sample pass through a narrow slit placed very close to the sample. Electrons are then dispersed on to an imaging detector. The principle is very close to the well known 180 degree magnetic spectrograph. However, in this case electrons are detected out of the dispersion plane. Using this very simple approach, one can perform Auger electron spectroscopy (AES) over a large energy range and a large angular range in parallel. The small red lines on the detector (see Fig. 1) show how electrons of the same energy but different azimuthal angle will land on the detector. Since the electrons on the red curves are spread quite far, this will create a poor energy resolution since different electrons of different energies and take off angles can land at the same location on the detector. We show that by increasing the angle of rotation (to greater than 180 degrees) that the electron undertake before striking the detector , we can improve the energy resolution and maintain a high transmission while retaining the parallel acquisition capability.

The major advantage of the analyser is that it can acquire electron energy spectra in parallel and over a large angular range with a much greater transmission than another parallel acquisition analyser the Hyperbolic Field Analyser (HFA). 

An Active Pixel Sensor (APS) was used to detect the electrons directly. No use of microchannel plates was made to detect the electron energy spectrum. Since the APS is not very sensitive to low energy electrons, this resulted in rather poor statistics. However, considerable improvement would be expected if a microchannel plate (MCP) were to be used. Figure 2 shows a schematic of the actual realisation of the analyser. Due to space constraints (largely due to the size of PCB board and the narrow space between the Helmholtz coils), the APS had to be placed below the sample as indicated.

Figure 3 shows an image acquired on the Active Pixel Sensor. The primary electrons were selected to have 900 eV electrons and the curved line shows elastic peak electrons acquired for many different polar take off angles. This is indicated by the line A. The image was integrated to form a spectrum. Lines B and C were used as the limits of integration.

Figure 4 shows the resultant electron energy loss spectrum (EELS) after the process of integration. The energy resolution can be seen to be about 4eV.

Further details of the analyser will be presented such as estimates of the energy reolution, an Auger electron spectrum and calculations of the Field of View.

Advantages of the analyser are:

(a) Acquisition of electron spectra over a large energy range and large angular range in parallel.

(b) The detector and plate containing the sperture have a small size and could be fitted between the sample and objective lens

(c) Simple construction

Disadvantages of the analyser are:

(a) Helmholtz coils needed to produce the magnetic field are quite bulky.

(b) The spectrum is sensitive to the distance between the location of the electron source (i.e. where the primary beam strikes the sample) and the aperture slit.

(c) The magnetic field of the Helmholtz coils changes the electron current needed to focus the objective lens (but does not affect the spatial resolution).

(d) Current detectors are too large to fit between sample and objective lens. A bespoke electron sensor is required.

References
1. J. K. Danycz, Le Radium 1913, 10, 4-6; E. Rutherford, H. Robinson, Phil. Mag. 1913, 26, 717-729.

Christopher WALKER, Mohamed EL GOMATI (York, United Kingdom), Xiaoping ZHA

08:00 - 18:15 #6445 - IM03-245 High Resolution and Variable Pressure Imaging of Pyramidal Nano-antennas in a SEM.

IM03-245 High Resolution and Variable Pressure Imaging of Pyramidal Nano-antennas in a SEM.

The photovoltaic (PV) market nowadays is dominated by 1^st generation solar cells made of crystalline silicon. But silicon solar cells generate power from only a small portion of the electromagnetic spectrum and demonstrate efficiencies of about 25% in laboratory [1]. 2^nd generation solar cells based on thin films have an efficiency theoretical upper limit of about 30% [2].

Nano-antennas coupled to rectifying diodes (known as rectennas) are developped for 3rd generation PV solar cells. They directly convert the electromagnetic waves into electricity from far infrared to visible wavelengths. With a theoretical conversion efficiency of 85% [3] and a technology compatible with low costs fabrication techniques on flexible substrates, rectennas appear to be promising candidates for next generation PV solar cells.

In the present study, pyramidal structures were nano-imprinted on a flexible substrate (polyethylene terephthalate: PE) and then covered by a thin metallic layer. We have investigated the morphology, structure and chemistry at each step of the fabrication process via SEM techniques, in order to highlight structural and/or chemical defects at nanometer scale which can affect the overall physical and electrical properties of the device. The results were obtained with a Zeiss GeminiSEM 500 ultra-high resolution FESEM. This system is equipped with In-lens SE and the Energy Selective Backscattered detector (EsB), as well as variable pressure (VP) SE and VPBSE detectors to observe non-conductive samples. For the chemical and crystallographic analyses, EDS, WDS, and EBSD accessories are additionally installed on this microscope achieving the full imaging linked to the analytical capabilities. The low voltage acquisition was suitable for surface studies since it increases topographic contrast and reduces specimen charging. The VP mode was also employed, which features not only a very restricted skirt effect under high gas pressures, but also enables the detection of pure secondary electron (SE) signal using all in-lens detectors [4].

Figure 1 shows a cross-section image of a pyramidal nano-antenna array, taken with the VP SE detector at 140 Pa and 5 kV. The charging effect on the PE polymer is completely eliminated with the use of the VP mode. The morphology and wetting of the silver metallic layer could be clearly observed. Figure 2 displays a top view of one nanopyramid. The image was taken with the in-lens SE detector at 1 kV and at a very short working distance (0.4 mm), which greatly improves the resolution. Low kV imaging allows the observation of the silver layer structure on the pyramid facets. Crystal grains and defects such as grain boundaries and twins are acutely visible.

This work highlights the use of SEM high resolution imaging, at low kV and even in variable pressure mode, to improve the understanding of structural and chemical properties of hybrid nanomaterials as plastic/metal pyramidal nano-antennas.

References

[1] M.A. Green, K. Emery, Y. Hishikawa, W. Warta and E. D. Dunlop, Prog. Photovolt: Res. Appl. 20:12–20 (2012)

[2] W. Shockley and H. J. Queisser, Journal of Applied Physics, Volume 32:. 510-519 (1961)

[3] D. K. Kotter, S. D. Novack, S. D. Slafer, P. Pinhero, Proceedings of the 2^nd International Conference on Energy Sustainability, ES 2008, 2, 409 (2009)

[4] L. Han, C. Berger, M. Boese, A. Thesen, F. Zhou, S. Meyer, Erik Essers, Microsc. Microanal. 21, Suppl 3 (2015)

Acknowledgments

This study is part of "Energy Storage by Direct Conversion of Electromagnetic Radiation Captured within Nano-Antennas" (ETNA) project funded by the Amidex Fundation (PR2I projects)

Andrea P. C. CAMPOS (Marseille), Christian DOMINICI, Claude ALFONSO, Loic PATOUT, David DUCHÉ, Cécile GOURGON, Ludovic ESCOUBAS, Ahmed CHARAÏ

08:00 - 18:15 #6491 - IM03-247 Ultra-High Resolution SEM for Materials Analysis.

IM03-247 Ultra-High Resolution SEM for Materials Analysis.

The need for increased SEM resolution and the simultaneous demand for enhanced analytical capability have led to the development of increasingly sophisticated instruments. Here we describe the design of a novel SEM column where recently-developed high-resolution optics [1] is brought together with traditional analytical capabilities. Combined with FIB it greatly enlarges the capability for 3D tomography inspection of a specimen especially when used in combination with other analytical techniques like EDS or EBSD.

 

The SEM column comprises a triple objective lens design. The first objective lens is optimized to yield an image resolution of less than 1.1 nm at 1 kV. The design retains a single-pole lens [2] which creates a strong magnetic field around the sample, dramatically decreasing optical aberrations. However, the leaking magnetic field distorts ion trajectories of the FIB and causes beam splitting of ion-isotopes. To overcome this, FIB processing is performed in a magnetic-field-free mode, where the second objective lens of a conventional type with a resolution of 2.5 nm is used. The third objective lens enables a large field of view. A combination of all three objective lenses allows for multiple display modes, e.g. for enlarged field of view, greater depth of field or optimizing resolution at high probe currents. To prevent thermal instability due to changes of lens excitation when switching between the imaging, analytical and other modes, the column works in a regime where constant thermal power dissipation is maintained independent of lens excitation. It significantly reduces image drift and enables stable, long-term, automatic 3D analysis.

 

3D BSE tomography of an SERS-active structure of gold-coated, partially-etched polystyrene spheres (Figure 1) was performed maintaining stable operation over a 13 hour period [3]. The sample was sliced using FIB and each cross-section was automatically imaged at 2 kV using one of the three dedicated back-scattered electrons (BSE) detectors to obtain the gold distribution on the polystyrene surface. InBeam BSE detector placed in the column acquires high-angle BSE signal, whereas the two other detectors collect BSE with lower angles. The BSE detector triplet thus enables angle filtration. Furthermore, energy-filtering of the BSE signal enhances material contrast, (Figure 2), where low-loss 1.95-2 keV BSEs reveal details not observable in the integral BSE signal.

The redesigned electron gun further enhances the analytical capabilities of the column. It allows probe currents as high as 400 nA for structural analysis and ten times faster beam energy alternation compared to the previous generation. The sharp conus of the objective pole-piece enables FIB processing of large tilted samples, e.g. 8 inch wafers. The new SEM column will be used in the Mark II generation of the TESCAN MAIA electron microscope and the dual FIB-SEM instruments GAIA and XEIA [4].

 

References:

[1] J Jiruse et al, Microsc. Microanal. 19 (Suppl 2), 2013, p. 1302.
[2] Z Shao, PSD Lin, Rev. Sci. Instrum. 60(11), 1989, p. 3434.
[3] L Stolcova et al, Imaging & Microscopy, Issue 4, 2015, p. 34.
[4] The authors acknowledge support from the EU Initial Training Network STEEP (Grant no. 316560).

Jaroslav JIRUŠE (Brno, Czech Republic), Miloslav HAVELKA, Jan POLSTER

08:00 - 18:15 #6556 - IM03-249 Quasi in-situ catalytic studies using a TEM grid microreactor.

IM03-249 Quasi in-situ catalytic studies using a TEM grid microreactor.

Conventionally, catalysts are analyzed in the transmission electron microscope (TEM) under static conditions in the vacuum. However, catalysts surfaces dynamically response to the reaction conditions. In addition, comparing TEM images before and after the catalytic reaction may give conclusions that are difficult to verify, because of the inhomogeneous conditions along the catalyst in normal fixed bed reactors [1]. For obtaining relevant information it is advisable to study the material under catalytic relevant conditions (gas composition, pressure, temperature) which also includes the monitoring of conversion. Therefore, in-situ TEM approaches, such as environmental TEM (ETEM) or microelectromechanical systems (MEMS)-based gas cell TEM holders, were developed. They allow the direct observation of the dynamic changes of a catalyst during the prevailing time of a reaction [2]. However, drawbacks regarding the reaction pressure and fast dynamics give rise to pressure gaps and may limit the resolution, respectively.

To overcome these limitations we have developed a TEM grid microreactor that allows a decoupling of the catalytic reaction and the imaging process. This quasi in-situ method combines relevant and well-controlled conditions for all catalyst particles at ambient pressure with high resolution imaging of exactly the same particles. In order to avoid the exposure to ambient air, a secure transfer of the catalyst to and from the TEM can be guaranteed via the glovebox and vacuum transfer holders. Furthermore, the catalytic conversion can be followed by using an ultra-sensitive proton-transfer reaction mass spectrometer (PTR-MS).

Here, we present the utilization of this microreactor in the CO oxidation over metal catalysts. For instance, identical location imaging of active Pt nanoparticles (proven by PTR-MS measurements) reveals structural changes, which are induced by the catalytic process (Figure 1).

In conclusion, the microreactor complements the state of the art in-situ imaging, where one can study the structural dynamics, by delivering catalytic relevant kinetic information, which can be coupled to changes on the atomic scale.

 

References[1]

1.         Korup, O., et al., Catalytic partial oxidation of methane on platinum investigated by spatial reactor profiles, spatially resolved spectroscopy, and microkinetic modeling. Journal of Catalysis, 2013. 297: p. 1-16.

2.         Vendelbo, S.B., et al., Visualization of oscillatory behaviour of Pt nanoparticles catalysing CO oxidation. Nat Mater, 2014. 13(9): p. 884-890.

 

Liudmyla MASLIUK (Berlin, Germany), Marc G. WILLINGER, Darren DUNPHY, Robert SCHLÖGL, Thomas LUNKENBEIN

08:00 - 18:15 #6577 - IM03-251 A versatile high-vacuum cryo transfer system for cryo microscopy and analytics.

IM03-251 A versatile high-vacuum cryo transfer system for cryo microscopy and analytics.

The conservation of the native state during sample preparation is mandatory for a correct interpretation of any micrograph. Particularly for EM, the preservation of the pristine architecture is challenging. If not imaged in situ (1), two different strategies can be followed to prepare hydrated samples for electron microscopy: the conventional and the cryogenic routine. Conventional preparation protocols typically rely on the chemical fixation and staining of the sample material, whereby both steps are known to induce artefacts (2). The second preparation routine, introduced by Moor (3) and Dubochet (4), circumvents chemical artefacts. The basic principle of every cryogenic (cryo) preparation protocol is the physical immobilization of the sample in a frozen-hydrated solid state, by vitrifying the sample within milliseconds. Due to the excellent structural preservation cryo imaging techniques have gained increasing popularity (5, 6).

However, the handling of a vitrified sample becomes rather delicate. Two prerequisites must be fulfilled during the entire handling in order to maintain the artefact free conservation of the specimen: 1) the sample must be kept well below the de-vitrification temperature of water (approximately -137°C) in order to avoid structural rearrangements due to ice crystal growth (5). As a consequence of the permanent cooling, the sample material acts as a cold trap and is therefore prone to contamination. 2) Thus, the sample must be transferred within an anhydrous environment all the time. In the case of the latest preparation protocols or imaging strategies this has been proven to be particularly challenging, since these methods include several transfer steps, either due to their extensive post-processing or complex workflow (7).

In the past, several cryo-transfer concepts were introduced mainly for cryo SEM, with bulk add-ons and cryo-stages inside the microscope to allow e.g. high-vacuum cryo-transfer (8), which is not feasible for “in-lens” systems like S/TEM’s. Standard cryo-transfer systems for cryo-TEM, however, work under ambient pressure from liquid nitrogen direct into the load lock of cryo-TEM.

Here, we present a high-vacuum cryo-transfer system that overcomes the limitations of existing systems. The system includes four parts: 1) sample cartridge (Fig 1a), 2) storage device (Fig 1b), 3) high-vacuum cryo shuttle (Fig 1c) and 4) a side-entry TEM cryo-stage. Moreover, our solution offers connectivity between different kind of instruments not limited to post-lens systems (8), enabling new types of multimodal imaging approaches by transferring cryo-samples between different imaging and manipulation devices (Fig 1d). The performance of the developed system is demonstrated on an “in-lens” cryo-STEM. In order to determine the quality of the transfer process, the temperature and pressure level were recorded during the entire transfer. Moreover, prior and subsequent to the cryo-transfer, the mass of the TMV as well as the thickness of a carbon film were measured and compared. Here, any possible contamination would falsify the scattering characteristics of the sample material, and consequently cause an apparent increase in mass or thickness, see also (9).

REFERENCES

(1) M. J. Dukes et al, Microsc. Microanal. 20 (2014) p.338.

(2) M. Pilhofer et al, Environ. Microbiol.16 (2014) p.417.

(3) H. Moor, and K. Mühlethaler, J. Cell Biol. 17 (1963) p.609.

(4) J. Dubochet, and A.W. McDowall, J. Microsc. 124 (1983) p.3.

(5) A. Al-Amoudi et al, EMBO J., 23 (2004) p. 3583.

(6) W. Kühlbrandt, Science 6178 (2014) p. 1443.

(7) A. Rigort and J. M. Plitzko, Arch. Biochem. Biophys. 581 (2015), p. 122.

(8) M. Ritter et al, Microsc. Microanal. 5 Suppl. 2 (1999) p. 424.

(9) S.Tacke et al, Biophys. J. 110 (2016), p. 758.

(10) Rudolf Reichelt initiated this project. Unfortunately, he passed away on 2^nd October 2010, too early to see the final results. This research was supported by the DFG Grant RE 782/11. V. Krzyzanek acknowledges the support by the grant 14-20012S (GACR).

Sebastian TACKE (Zürich, Switzerland), Vladislav KRZYZANEK, Harald NÜSSE, Alexander ROSENTHAL, Jürgen KLINGAUF, Roger Albert WEPF, Rudolf REICHELT

08:00 - 18:15 #6597 - IM03-253 Random scanning mode for the spectroscopy of sensitive materials.

IM03-253 Random scanning mode for the spectroscopy of sensitive materials.

Electron Energy Loss Spectroscopy (EELS) and Cathodoluminescence (CL) in a STEM microscope allow to characterize chemical composition and optical properties down to a subatomic scale. Nevertheless, a high electron dose is conveyed onto limited regions of the specimen and generates strong radiation damages in fragile materials. Spectroscopy of individual molecular systems remains therefore still highly challenging. Here we demonstrate how this traditional limit can be largely overcome by acquiring hyperspectral images through a non-standard random scanning mode. This is achieved by considering the pixels in the scanning array as a list that is shuffled and loaded into a dedicated scan generator. At the end of spectrum acquisition a signal from the detector triggers the next beam position.

The luminescence of high efficiency molecular dyes has been investigated through an original CL set up integrated within a STEM microscope, which allows a nanometric spatial resolution. Illumination damages have been reduced thanks to a liquid nitrogen cooling system at the sample stage and a 60 keV electron beam.

The dyes have been deposited at a very low concentration on hexagonal boron nitride (h-BN) flakes. This one represents the ideal substrate for luminescence, being chemically inert and displaying a wide energy band gap. Furthermore, it can actively participate in the luminescence processes: electron induced excitations in the h-BN substrate may recombine at the molecular sites. The luminescence of the molecules should appear therefore in CL-hyperspectral images as isotropic emission spots some tens of nanometres wide, corresponding to the excitation diffusion length in h-BN.

We show that a traditional sequential line-by-line scan leads to a rapid bleaching of the molecules, whose luminescence appears only in a short line of pixels. On the contrary, by using a random scan routine we were able to obtain the expected isotropic emission spots. Unlikely a sequential scan, in a random survey the energy deposited in an illuminated area can be evacuated before an adjacent pixel is illuminated. Even when the bleaching of the molecule cannot be avoided, this acquisition mode allows to collect a signal from a higher number of pixels with respect to the sequential scan.

Beside CL, EELS investigations could in principle derive great advantage from this random acquisition. More precisely, core EELS studies of fragile materials could benefit from a higher spatial resolution of the hyperspectra. Finally, the random scan opens also new perspectives in terms of compressed sensing techniques.

Anna TARARAN (Orsay), Marcel TENCÉ, Nathalie BRUN, Mathieu KOCIAK, Alberto ZOBELLI

08:00 - 18:15 #6661 - IM03-255 Proposal for an electron orbital angular momentum spectrometer.

IM03-255 Proposal for an electron orbital angular momentum spectrometer.

  Measurement of magnetization at atomic resolution via dichroic electron energy loss spectroscopy (EELS) with vortex beams remains a hotly-discussed and as-yet unproven goal for electron microscopy [1-5]. A satisfactory explanation for the lack of successful demonstrations of this technique has not yet emerged. However, a number of recent predictions [2,5,6] for dichroism include one common oversight: the efficiency of the dichroic measurement strongly depends on whether the outgoing phase distribution--or, equivalently, the outgoing orbital angular momentum (OAM) distribution--is measured.

  Dichroic absorption spectroscopy experiments with circularly polarized light are much simpler; the photon is absorbed, and necessarily transfers all angular momentum to the specimen. Electrons in a transmission electron microscope are not absorbed, and can carry non-zero linear and orbital angular momentum away from an interaction with a  specimen. Incident electron vortices with mħ OAM have a significant probability to scatter to an outgoing state with unchanged mħ OAM. Dichroic electron energy loss spectroscopy, then, can be made far more efficient by post-selecting for the component of the final state which has nonzero Δm. There exist qualitative methods to post-select for more of this component of the final state [7]. There are also some semi-quantitative interferometric methods [8] that do not work for the inelastically scattered final states that one must measure in a dichroic EELS experiment, as they cannot be coherently interfered with an incident reference wave. In order to accomplish practical, highly efficient dichroic EELS, a quantitative non-interferometric method to spatially separate OAM will be necessary.

  We propose a magnetic field-based mechanism for on-axis separation of electron OAM modes. We show that the vector potential of any cylindrically symmetric current distribution that flows azimuthally--a standard magnetic round lens--can produce an orbital angular momentum-dependent focal length, where the focal length is linear with the electron orbital angular momentum (see figure 1 for a cartoon visualization). Figure 2 shows multislice simulation [9] of the phase distribution acquired by a 20ħ electron vortex passed through a lens constructed to produce exaggerated OAM dispersion stronger than the normal lensing effect. Figure 3 shows the 20ħ and a -20ħ passed through the same strongly dispersive lens and propagated 10 millimeters. This shift in focal length can be used, in conjunction with a spectrometer entrance aperture to block out defocused modes, to produce a more efficient magnetic dichroism spectrum.

  As one might expect, this shift in focal length is very small in a standard magnetic round lens. We have explored the physical design and optical configurations that might allow for a practically realizable orbital angular momentum spectrometer. In particular, there are two stackable lens combinations that produce an OAM-dependent magnification. If successfully implemented, this OAM magnification system would allow for parallel, quantitative measurement of the full orbital angular momentum distribution of an electron beam without a coherent reference beam.

[1] J. Verbeeck et al., Nature 467 (2010) p.301.
[2] S. Lloyd et al., Phys. Rev. Lett. 108 (2012) p.074802.
[3] P. Schattschneider et al., Phys. Rev. Lett. 110 (2013) p.189501 .
[4] P. Schattschneider et al., Ultramicroscopy 136 (2014) p.81–85.
[5] J. Rusz and S. Bhowmick, Phys. Rev. Lett. 111 (2013) p.105504.
[6] J. Rusz et al., Phys. Rev. Lett. 113 (2014) p.145501.
[7] K. Saitoh et al., Phys. Rev. Lett. 111 (2013) p.074801.
[8] L. Clark et al., Phys. Rev. A 89 (2014) p.053818.
[9] V. Grillo et al., New Journal Phys 15 (2013) 093026.

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award # DE-SC0010466.

Tyler HARVEY (Eugene, USA), Vincenzo GRILLO, Benjamin MCMORRAN

08:00 - 18:15 #6674 - IM03-257 Transmission Kikuchi Diffraction: effective nano-scale analysis using conventional EBSD hardware.

IM03-257 Transmission Kikuchi Diffraction: effective nano-scale analysis using conventional EBSD hardware.

SEM-based Transmission Kikuchi Diffraction (SEM-TKD) [1,2] is an extension to the conventional  technique of reflection EBSD (using bulk samples) to transmision EBSD using thin samples.  The main advantage of SEM-TKD over the conventional method is a large improvement in spatial resolution of point-analysis from small particles and of mapping data from thin films.

 

This improvement is primarily a consequence of examining thin, electron-transparent TEM samples in transmission, which reduces the effective diffraction and escape volume, as well as the use of zero- to low sample tilts which reduces anisotropic beam-spreading effects.  The technique is used to produce 2D datasets from flat, thin sample areas.  This data is then used for microstructural analysis including crystallographic orientation, grain size, phase distribution, and grain boundary character and distribution.  It is especially useful in characterizing highly-strained materials and materials with grain sizes under 50nm.

 

A significant attraction of TKD is that these improvements can be realised using conventional EBSD hardware, in its conventional position on the SEM, without any modification.  However, for TKD the sample position, pattern centre (high up or above the detector screen) and sample tilt (close to horizontal) are very different to those required for conventional EBSD (just above the screen centre line, and 70° respectively).  Thus the TKD-mode imposes a more extreme case of the gnomonic projection which is already inherent to the capture of conventional electron backscatter patterns.

 

The most notable effects of the TKD projection geometry are that horizontal bands near to the bottom of the screen are imaged wider than normal, and that the non-symmetric intensity across these bands is highlighted (as illustrated in Figure 1).

 

Here we discuss how these distortions affect analysis using a conventional EBSP solving engine and some new technology developed to improve performance.

 

References:

[1] R.R. Keller, R.H. Geiss, Transmission EBSD from 10nm domains in a scanning electron microscope, Journal of Microsopy 245 (2012) 245-251

[2] Trimby, P.W., Orientation mapping of nanostructured materials using transmission Kikuchi diffraction in the scanning electron microscope, Ultramicrosopy 120 (2012) 16-24

Jenny GOULDEN, Haithem MANSOUR (Gometz la ville), Angus BEWICK

08:00 - 18:15 #6681 - IM03-259 A New Compound Lens Equipped UHR SEM.

IM03-259 A New Compound Lens Equipped UHR SEM.

The quality of a scanning electron micrograph is determined by both the resolution and the contrast: the size of the features that can be resolved and the type of information that the micrograph gives about them. Recent advances in SEM have improved both, for example using magnetic immersion or electrostatic lenses for ultra-high resolution, or by employing in-lens, angle-sensitive detection for tunable contrast. This paper introduces a further improvement in resolution and in contrast using a new FEI SEM equipped with a compound electrostatic-magnetic final lens.

The compound final lens SEM combines a magnetic final lens in the pole piece, a magnetic immersion final lens and an electrostatic lens formed by the potential at the bottom of the column. This new final lens design provides a resolution equal to 1.0 nm at 1 kV acceleration voltage. Figure 1 demonstrates the ultra-high resolution performance by imaging the SBA-15 sample at 500 V landing energy.

The contrast performance is based on the independent in-lens detection of secondary (SE) and backscattered (BSE) electrons. Secondary electrons are further separated and independently detected by in-lens (T2, higher SE energies) and in-column (T3, lowest SE energies) detectors. Separate collection of the lowest energy SEs delivers extremely surface sensitive imaging. Simultaneous detection of high energy SEs adds more freedom for imaging of non-conductive samples - high energy SEs are less sensitive to sample charging while still clearly showing the topography – see the high energy SE image of a hydroxyapatite nano-sheets grown on bioactive glass fiber [1] in Figure 2.

The backscattered electrons are detected by the T1 detector which is positioned at the very bottom of the pole piece. Thanks to that position it receives a high signal intensity which enables ultra-low beam current BSE imaging. This is important for beam sensitive samples, such as polymers, porous materials or other fragile samples, which require the maximum amount of signal to be acquired in the shortest amount of time with the smallest dose possible. With the T1 detector BSE imaging is possible during TV-rate navigation, so that material contrast is always available. Combined with the new compound final lens, it is possible to further energy filter the backscattered electrons detected on the T1 detector. When high-loss (low energy) BSEs are filtered out, T1 provides strong material contrast images formed by low-loss (high energy) BSEs only – see surface of the Pt activated carbon particle in Figure 3. The acquisition of topographical information is ensured by the SEs simultaneously collected by the T2 and T3 detectors. The T1 detector keeps providing low noise images even at probe currents down to few tens of pA. The low probe current operation together with the energy selective BSE detection enables charge-free material contrast imaging of non-conductive samples.

The SE and BSE filtering combined with excellent low dose performance (low kV and low current) and “smart” scanning strategies allow for high vacuum imaging of uncoated insulators that are otherwise susceptible to charging. The capabilities on these samples are completed with a low vacuum mode with chamber pressure up to 500 Pa, which is essential for analytical measurements where high acceleration voltages and probe currents are required.

In conclusion, the new SEM equipped with the compound final lens and the in-lens and in-column detectors improves both the imaging resolution and contrast performance. It allows researchers to capture the maximum amount of information from conductive as well as insulating samples, with the right detail and with the least amount of compromises.

 

Acknowledgements:

[1] Hydroxyapatite sample courtesy of Devin Wu, FEI China

Petr WANDROL (Brno, Czech Republic), Ernst Jan VESSEUR

08:00 - 18:15 #6690 - IM03-261 Addressing Pseudo-Symmetric Misindexing in EBSD Analysis of gamma -TiAl with High Accuracy Band Detection.

IM03-261 Addressing Pseudo-Symmetric Misindexing in EBSD Analysis of gamma -TiAl with High Accuracy Band Detection.

 Technological developments in EBSD has enabled great improvements in indexing reliability and accuracy [1].   However, some individual phases continue to pose challenges, especially those that present extremely similar Kikuchi patterns to the EBSD camera for different crystallographic orientations.  This phenomenon is called “pseudosymmetry”, as it commonly involves relatively high intensity bands in certain patterns with an apparent higher symmetry than the crystal structure actually possesses.  In many cases, only very slight differences in inter-band angle separate candidate solutions, and only robust and accurate band detection may identify the correct one among them. 

Conventional Hough-based band detection methods are sufficiently accurate for most indexing requirements.  High accuracy Hough transform settings improves band detection accuracy, and is useful in mitigating pseudosymmetric misindexing [2].  However, these settings result in reduced data acquisition speeds, and may not completely eliminate pseudosymmetry problems.

New band detection refinement methods improve EBSD indexing performance for some of the most chronic cases, at reasonable data acquisition speeds.  Higher accuracy band detection is achieved by iteratively comparing the positions of simulated bands with bands in the actual Kikuchi pattern image, using expected versus actual band widths and, accounting for the hyperbolic shape of bands.  In addition to delivering high precision crystallographic orientation data and helping discriminate different phases with similar Kikuchi patterns, this method is sufficiently sensitive to resolve fine differences in inter-band angle to nearly eliminate many cases of pseudosymmetric misindexing. 

An important application is γ-TiAl alloys, which are promising jet engine turbine materials, combining low density with good oxidation and creep resistance.  The high temperature deformation behaviour of these alloys must be better understood before they can widely replace the higher density Ni-base superalloys; For example, an improved knowledge of the fundamentals of crystallographic slip and its interaction with the γ/γ  lamellar variants could be critical.  Pseudosymmetry, however, is a major issue here (Fig. 1a), arising from γ-TiAl’s close tetragonal c:a unit cell parameter ratio of 1.018, giving the Kikuchi patterns it generates a pseudo-cubic configuration, resulting in indexing inaccuracies.  These mistakes show the same misorientations as boundaries between real γ-TiAl lamellae, causing problems in revealing the true microstructure.  Phase discrimination between coexisting γ(TiAl) andα₂(Ti₃Al) phases is also difficult (Fig. 2a).  Application of a new, automated band detection tool and system knowledge of the confronting pseudo-symmetry almost completely eliminate these issues (Fig.’s 1b and 2b) and are used in real-time during data collection, at normal acquisition speeds.

References:

[1] K. Thomsen et al., Royal Microscopy Society EBSD 2014 conference proceedings (2014)

[2] C. Zambaldi et al., J. Appl. Cryst. 42 (2009), p. 1092-1101

Niels-Henrik SCHMIDT, Haithem MANSOUR (Gometz la ville), Jenny GOULDEN, Alberto PALOMARES-GARCÍA, Rocio MUÑOZ-MORENO

08:00 - 18:15 #6725 - IM03-263 Time-resolved cathodoluminescence in a scanning electron microscope.

IM03-263 Time-resolved cathodoluminescence in a scanning electron microscope.

The past decade has witnessed enormous progress in the design and fabrication of nanostructured materials with unique optical properties. These tiny physical structures allow geometrical control over the electromagnetic shape of light well below its own wavelength. A large variety of applications for this field of subwavelength optics known as nanophotonics have already begun to emerge, including solar photovoltaics, chemical sensing, quantum cryptography, and LED lighting. Many fundamental optical interactions are hidden beneath the diffraction limit, however, which makes the retrieval of subwavelength optical behavior impossible with standard microscopy. An alternative approach, known as cathodoluminescence (CL) [1], is to probe nanostructures with an electron beam, which excites optical resonances and transitions and then collects the emitted light. It has recently been used to characterize plasmonic structure [2] or III-N heterostructures [3]. Thanks to a very efficient CL system, in a scanning electron microscope (SEM), working in free space, we can directly probed many key dimensions of light – including intensity, angle, polarization, and frequency - with ~10 nm resolution [4].

In this presentation we will show how we can use a blanker to modulate a continuous electron beam to generate electron pulses with a resolution of ~10 ns. This approach has several advantages compared to alternatives such as a laser driven pulsed electron gun [5]. It can be easily implemented in any SEM, it requires few experimental modifications, it does not require a dedicated microscope and the pulsed electron beam can be controlled at will (pulse duration and repetition rate). We will show how this pulsed electron gun can be used to explore the dynamics of a single photon emitter, such as a rare earth ion or the NV⁰ center of nano-diamond. An HBT interferometer allows recording of the autocorrelation function of the CL signal [6][7]. Combining secondary electron images with measurements of the CL spectra, emission polarizations, lifetimes and second order correlation function, we are able to fully characterize optically active systems at the nanometer scale.

 

[1] L. H. Robins, L. P. Cook, E. N. Farabaugh and A. Feldman. PRB 39 13367 (1989)

[2] E.J.R. Vesseur and A. Polman, Nano Lett. 11, 5524 (2011)

[3] L.F Zagonel et al, Nano Lett. 11, 568 (2011)

[4] C. Osorio, T. Coenen, B.J.M. Brenny, A. Polman, and A.F. Koenderink, ACS Photon. 3, 147 (2016)

[5] D-S. Yang, O. F. Mohammed and A. H. Zewail, PNAS 107, 14993 (2010)

[6] Mark Fox. Quantum Optics: An Introduction. Oxford Master Series in Physics, (2006).

[7] L. H. G. Tizei and M. Kociak, PRL 110  153604 (2013)

Sophie MEURET (CEMES, Toulouse), Hans ZEIJLEMAKER, Elke NEU, Patrick APPEL, Patrick MALETINSKY, Albert POLMAN

08:00 - 18:15 #6744 - IM03-265 ColorSTEM – A Novel STEM Detector for Advanced Materials Analysis in SEM.

IM03-265 ColorSTEM – A Novel STEM Detector for Advanced Materials Analysis in SEM.

Performing scanning transmission electron microscopy (STEM) in a scanning electron microscope (SEM) is a popular technique for laboratories without transmission electron microscopy (TEM) capabilities.

The new versatile STEM detector by TESCAN is optimized for both high-sensitivity orientation contrast as well as materials composition contrast imaging.

Transmitted electrons scattered in different angles represent different types of information that arises within a thin foil in scanning transmission electron microscopy. The new retractable STEM detector design allows maximum manipulation capabilities with the sample, including changes in working distance or eucentric tilt above the detector. Changing the working distance also changes the angular distribution of electrons detected by the detector segments. The sample can be therefore navigated to optimum contrast conditions easily, using changes in the working distance (see Fig. 1).

The redesigned electronics and signal processing apparatus of the detector allows simultaneous acquisition of multiple signals from transmitted and diffracted electrons including bright field (BF), dark field (DF) and high angle dark field (HADF) channels. The newly implemented ColorSTEM concept allows  simultaneous  acquisition of all channels as a live, color-coded image with no information loss in either compositional color coding (Fig. 2) or orientation contrast color mode (Fig. 3).

The STEM analysis can be further supplemented with high-resolution transmission-EDS or transmission-EBSD microanalysis. Higher resolution with these techniques is possible by reducing sample thickness thereby minimizing the interaction volume within the sample [1]. This combination takes advantage of the fact that these analytical techniques are already available in the SEM.

References:

[1] P.W. Trimby, Ultramicroscopy 120 (2012), 16-24

Jiří DLUHOŠ (Brno, Czech Republic), Michal BILÍK, Stanislav PETRÁŠ

08:00 - 18:15 #6758 - IM03-269 A protected inert-gas sample manipulation and transfer environment for cryo electron microscopy and analytics.

IM03-269 A protected inert-gas sample manipulation and transfer environment for cryo electron microscopy and analytics.

The preservation of the native sample state during manipulation and exchange between sample preparation and imaging devices is a prerequisite for artefact and contamination free structure investigation. In particular during cryo-transfer and (re-)mounting of frozen hydrated samples, which often in electron microscopy (EM), have a very large surface area compared to its volume, can be very challenging when samples have to be exchanged or remounted several times. Some of the main challenges are: 1. preventing the sample from warming up to the de-vitrification temperature of water (approximately -137°C), 2. Preventing the sample from being covered with ice or other contaminants condensing from the surrounding atmosphere and 3. Handling these samples without chemical or mechanical alteration of their native state.

Currently the handling and transfer of samples under vacuum and cryo conditions is solved by systems provided from different suppliers. However the transfer of frozen (vitrified) samples under environmental pressure (e.g. from a high-pressure-machine to a low temperature coater or cryo-EM) is still a challenge and a satisfactory practical solution is missing.

Here, we present an ambient pressure inert-gas sample mounting and exchange chamber (Glove-Box) (Fig. 1 &B) that addresses all the needs for structure and composition preservation during the manipulation, mounting and transfer of mainly frozen or hydration/oxygen sensitive samples. Once mounted these samples can be transferred and exchanged between different cryo-preparation and imaging systems (Fig. 1 E) under controlled inert-gas and temperature conditions. This controlled inert-gas “Glove-Box” includes a LN2 immersed sample mounting area with various interfaces to different inert-gas/cryo transfer systems for RT or low temperature TEM or FIB/SEM applications, Furthermore, monitoring of the environmental parameters (Fig. 1 C) during transfer (humidity, oxygen, carbon dioxide), on-screen imaging to support sample manipulation (Fig. 1 D), heating of tools and recording of all experimental parameters for documentation are included into this dedicated LN2 and inert-gas “Glove-Box”. The interface to other sample carrier devices is open and flexible and can be adapted upon individual laboratory needs. Finally such a “Glove-Box” can play an essential role in a dedicated modern cryo-EM & analytical environment as sketched in Fig. 1 E.

Georg Alexander ROSENTHAL (Eisenstadt, Austria), Sebastian TACKE, Falk LUCAS, Roger Albert WEPF

08:00 - 18:15 #6760 - IM03-271 SILICON CARBIDE 3C IDENTIFICATION BY THE NEW DEVELOPED NORDIF EBSD EXTRACTION SOFTWARE.

IM03-271 SILICON CARBIDE 3C IDENTIFICATION BY THE NEW DEVELOPED NORDIF EBSD EXTRACTION SOFTWARE.

    

    

Electron backscatter diffraction (EBSD) is a powerful tool to automatically and quantitatively characterize the orientation of grains and phases of ceramic and composite materials [1].  EBSD ultra-fast pattern acquisition, i.e., patterns streaming to hard disk (HD) and later pattern indexing, becomes an interesting technique to investigate a series of large-scale ceramic, which can quickly provide phase distribution information for further improving ceramic production. However at the same time, it becomes more and more important to handle the huge offline EBSD raw data efficiently and correctly.

    

In this project, we already reported the EBSD characterisation results of the polymorphs of a solid state sintered silicon carbide component based on Densitec 15, a ready to press powder-made by Saint Gobain Ceramic Materials AS [2].  The pre-sintered powder was produced using the Acheson process and would typically consist of the 4H- and 6H-polytypes, with small amounts of 3C and sometimes traces of 15R. The offline EBSD raw data collection was carried out in a Hitachi SU-6600 FESEM and the patterns were acquired by using NORDIF UF-1100 EBSD detector and written directly to HD. The previous results only identified 4H- and 6H-phases in the sample, consistent with the expected transformation of 3C-crystals that takes place at temperatures significantly lower than the sintering temperature of 2110°C. In this abstract further detailed phase identification of silicon carbide 3C by the new developed NORDIF EBSD Extraction Software is carried out.

    

Figure 1 is the overview for the new developed NORDIF EBSD Extraction Software, where the indexed phase map of the present silicon carbide sample shows partly on the right part of the figure. Offline EBSD indexing reveals that the content of the silicon carbide 3C phase is below 0.5% in the sample. By using the point analysis tool (+) in the program, the individual EBSD patterns from the silicon carbide 3C phase can be retrieved from HD with auto contrast and background subtraction corrections. Representative 3C-SiC EBSD patterns shown in Fig. 2 together with the corresponding internal coordinations (x,y), where the pattern (6,59) is exactly the same retrieved pattern shown that in Fig. 1. Further detailed 3C-SiC pattern indexing is performed. Under the system optimized calibration settings, the best confidence indexes (CIs) in Fig. 2 are all below 0.03, and the representative indexing result shows in Fig. 3. As checking back and comparing that in Fig. 1, it can be concluded that the indexed 0.5% 3C-SiC phases are mostly located at the boundaries among different grains, phases and pores, which resulted those EBSD patterns mis-indexed as 3C-SiC phase.

    

Offline EBSD together with its Extraction Software is hence an excellent tool to study not only the crystallographic texture and phase distribution, but also fully confirmed that the 3C-SiC recrystallisation and phase transformations processes taking place during sintering of ceramic materials. It also reveals that the present EBSD Extraction Software is good practice to carefully view the offline EBSD raw data before applying further processing algorithms.

    

     

[1] Koblischka-Veneva M, Muklich F and Koblischka M R  2002  Cryst. Engng. 5 411-418

[2] Yu, Y.D. et al EMAS 2016, 12th Regional Workshop on Electron Probe Microanalysis Today-Practical Aspects

Yingda YU, Torkjell BREIVIK, Paal RUNDE, Jarle HJELEN (Trondheim, Norway)

08:00 - 18:15 #6793 - IM03-273 En route to ion microprobe analysis of soluble compounds at the single cell level: The CryoNanoSIMS.

IM03-273 En route to ion microprobe analysis of soluble compounds at the single cell level: The CryoNanoSIMS.

The NanoSIMS 50L instrument is a magnetic-sector, multi-collecting ion probe. The physical basis for ion micro-probe analysis is the ability to perform mass-spectrometry on secondary ions sputtered from a solid target by the continuous impact of a beam of charged particles. This primary beam sputters ionized atoms and small molecules (as well as many neutral particles) from the upper few nanometers of the sample surface. These secondary ions from the sample are transferred with high transmission to a high mass-resolution, multi-collection mass-spectro­meter, where they are counted one-by-one in electron multiplier detectors, or as currents in Faraday cups.

The unique strength of the NanoSIMS ion microprobe is the ability to focus the primary beam (either Cs⁺ or O^-) onto an extremely small spot on the sample surface, smaller than 100 nm in linear dimension. A controlled raster of this highly focused primary beam across the sample surface allows secondary ion images to be produced with a spatial resolution that can clearly resolve structures larger than a few hundred nanometers in linear dimension. For example, in recent studies we have been able to clearly resolve substructures of the cell nucleus, such as nucleolus and chromatin packages, along with individual clusters of glycogen in both liver and brain cells from mice (1, 2).

The conventional NanoSIMS instrument operates at room temperature and ultra-high vacuum (10^-9-10^-10 Torr). In order to preserve this vacuum, biological samples must be prepared to minimize volatilization or degassing. Classical sample preparation procedures developed for electron-beam imaging techniques (e.g. TEM and SEM) meet such constraints. However, these procedures involve steps such as fixation of the tissue with glutaraldehyde solutions, staining (e.g. with OsO₄ or uranyl-acetate), dehydration in ethanol series, and finally embedding into epoxy resin. These procedures effectively remove soluble compounds originally present in the tissue. Left behind in the sample are macromolecular structures, such as proteins, lipids, RNA, and DNA. These macromolecular structures can, on the other hand, be isotopically imaged in great detail with a conventional NanoSIMS instrument at a spatial resolution down to 100-50 nm. This has already created vigorous research programs and important biological insights have been gained across an impressive range of organisms; reviewed in ref. (3).

However, a multitude of fundamental biological processes involve the action of soluble compounds (ions, metabolites, drugs, etc.) that cannot be imaged with the conventional NanoSIMS instrument because they are lost or significantly displaced during classical sample preparation. The only certain way to preserve and observe soluble molecular compounds and ions unperturbed in situ in a biological tissue is to create and maintain highly controlled cryo-conditions throughout the chain of preparative and observational procedures. Our method is based on state-of-the-art cryo-methods for sample preparation (starting with high-pressure freezing, followed by cryo-planing of the tissue in a cryo-ultramicrotome) and subsequent ultra-structural (i.e. sub-cellular) observations with cryo-scanning electron micro­scopy (4). What is missing from the currently existing observational chain is an instrument that can isotopically image cryo-prepared samples with ultra-high spatial resolution, permitting precise correlation with the structural information provided by electron microscopy. Our vision has been to develop a CryoNanoSIMS to achieve this goal. From an instrument development point of view, we have now succeeded in this and we will present our preliminary data.

 

1          Takado, Y. et al (2014). Nanomed Nanotech Biol Med, doi: 10.1016/j.nano.2014.09.007.

2          Takado, Y. et al (2015). J Chem Neuroanat 69, 7-15, doi:10.1016/j.jchemneu.2015.09.003.

3          Hoppe, P. et al (2013). Geostand Geoanal Res 37, 111-154, doi:10.1111/j.1751-908X.2013.00239.x.

4          Walther, P. & Müller, M. (1999). J Microsc 196, 279-287, doi: 10.1046/j.1365-2818.1999.00595.x.

Louise Helene Søgaard JENSEN (Lausanne, Switzerland), Tian CHENG, Florent Olivier Vivien PLANE, Stéphane ESCRIG, Arnaud COMMENT, Ben VAN DEN BRANDT, Bruno Martin HUMBEL, Anders MEIBOM

08:00 - 18:15 #6855 - IM03-275 Simulation of a Bessel box electron energy analyser for analysis of secondary electrons.

IM03-275 Simulation of a Bessel box electron energy analyser for analysis of secondary electrons.

Our understanding of the surface topography, texture, elemental composition etc. have not only improved the reliability of the fabrication procedure of micro circuits but also the working of the final device. Low-voltage scanning electron microscopes (LVSEM) have played a major role in the ever improving quality control of Si devices. ­The smaller interaction volume of the low-energy electrons with the specimen leads to the ability to observe finer details. Lowering the beam energy to a few hundred eV results in higher yield of secondary electrons (SE) relative to the number of backscattered electrons [1]. A conventional SEM uses the widely used Everhart-Thornley (ET) detector for detection of secondary electrons [2], however it acquires secondary electrons over a large angular and energy range. This means that information regarding angularly resolved and energy resolved data is lost. What is needed is a small scale device that is relatively little affected by ambient electric and magnetic fields that can access this information. Furthermore demands of low voltage SEM at short working distances require a device of small dimensions to be placed near to the sample.

In this study we propose the lesser known Bessel box Electron Energy Analyser (EEA) [3] to detect and energy analyse secondary electrons emanating from the specimen. The Bessel box is a very simple EEA with a cylindrical geometry. It comprises a hollow cylindrical electrode coupled with two input/output electrodes. The acceptance angle of the Bessel box is determined by the slit on the input cap electrode. Focusing/defocussing action of the Bessel EEA depends on the voltage difference applied to the cylindrical and i/o cap electrodes. The Bessel EEA is essentially a band pass filter with a retarding field effect on the electrons. We examine the properties of a Bessel box EEA by simulation over a range of take off angles with SIMION 8.1 and COMSOL 5.2 Multiphysics. Figure 1 demonstrates the working of such a Bessel filter, lower energy electrons lose energy as they travel through the Bessel EEA and are repelled back before they reach the output slit (fig. 1a). In contrast, higher energy electrons are not focussed by the Bessel EEA (fig. 1b) and only a narrow band of energies are allowed to pass through (fig. 1c). Its size can be much smaller than other EEAs and so could potentially be inserted into existing SEMs. Its small size also makes it less affected by the disturbing effects of external electric and magnetic fields.

The Bessel box was simulated using the conditions: Cylindrical and output electrodes set to -22.2V and input cap at ground. The transmission of the Bessel EEA for a range of incoming electron energies is shown in fig. 2. We have investigated a range of cylindrical and output cap electrode voltages and determined energy resolution and transmission for several geometries with the aim of optimising the design for use in a SEM. In addition the effects of magnetic fields from the objective lens of the SEM on the characteristics of the Bessel box are studied.

[1] Goldstein et. al. “Scanning Electron Microscopy and X-ray Microanalysis” , Springer Third edition, ISBN 9781461502159 (2003).
[2] Everhart, TE and Thornley, RFM. "Wide-band detector for micro-microampere low-energy electron currents". Journal of Scientific Instruments 37 246–248 (1960).
[3] G. Schiwietz, et al., “The retarding Bessel–Box—An electron-spectrometer designed for pump/probe experiments”, Journal of Electron Spectroscopy and Related Phenomena 203 51–59 (2015).

Ashish SURI (York, United Kingdom), Andrew PRATT, Steve TEAR, Christopher WALKER, Mohamed EL GOMATI

08:00 - 18:15 #6865 - IM03-277 Spatial resolution and compositional contrast in imaging using the low-loss electron signal in SEM.

IM03-277 Spatial resolution and compositional contrast in imaging using the low-loss electron signal in SEM.

Nanotechnology places increasing demands on techniques for sample characterisation on the sub-100 nm length scale. The scanning electron microscope (SEM) is one, widely used, technique for imaging and characterising nanomaterials using the intensity of secondary (SE) or backscattered electron (BSE) emission from a probed region of the nanomaterial to generate spatially resolved contrast in an image. The acquisition of the low-loss electron (LLE) signal [1] in the SEM provides an alternative method which may offer the advantage of improved spatial resolution compositional imaging.

Spatial resolution and contrast in compositional imaging of the LLE signal has been investigated by means of experimental measurements in a scanning electron microscope and Monte Carlo simulations for the case of a semiconductor superlattice structure comprising Si_0.85Ge_0.15 of 11.5 ± 0.4 nm separated by pure Si layers at a periodicity of 69.2 ± 0.2 nm. Both continuous slowing down approximation (CSDA) and discrete-loss based Monte Carlo models were considered (NISTMonte and PENELOPE) and it was found that the calculated contrast values were particularly sensitive to the choice of model in the low-loss regime. Experimental data were obtained using a purpose-built low-energy electron loss detector [2,3] comprising a retarding field analyser with an electron-optical input lens. The detector was attached to an FEI Sirion FEGSEM. Experimental data indicated that improved contrast was obtained as the maximum loss energy was lowered (fig 1a), and this trend was reproduced by the simulations (fig 1b). The results indicate that the LLE technique is a useful alternative to operating at low primary beam energies when performing compositional imaging on samples which have nanoscale compositional structure. Statistical noise considerations that affect the LLE signal are discussed.  In the case of spatial resolution, resolution metrics for compositional imaging are discussed. It was found that the LLE signal shows improved resolution compared with the backscattered electron signal (figs 2 & 3), however CSDA-based simulations predict better resolutions than simulations based on a discrete loss model. It is found that the energy-straggling has the most significant influence on the predicted resolution in the low-loss regime. The simulations suggest that a SEM with a high-quality small-diameter probe is required to fully appreciate the resolution benefits of the LLE signal. Experimental data indicates that certain samples (such as those with sub-surface compositional inhomogeneity or nanoscale topography) benefit from the LLE technique even when the SEM used has a more modest probe diameter.

[1] O C Wells. Appl. Phys. Lett. 19 232 (1971)

[2] I R Barkshire, R H Roberts, M Prutton. Appl. Surf. Sci. 120 129 (1997)

[3] C Bonet, A Pratt, M M El-Gomati, J A D Matthew, S P Tear. Microsc. Microanal. 14 439 (2008)

 

Chris BONET, Steven TEAR, Mohamed EL-GOMATI (York, United Kingdom)

08:00 - 18:15 #6889 - IM03-279 Temporally and spatially resolved local strain tracking microscopy.

IM03-279 Temporally and spatially resolved local strain tracking microscopy.

Abstract

             Cell-stretching is a key method to regulate deformation magnitude, cyclic strain levels, and frequencies, therefore elucidating the biological processes involved in activation of mechanosensitive pathways, cell patterning and morphological changes at physiologically relevant mechanical loads. Although several approaches and methods such as uniaxial or biaxial devices have been demonstrated to deform cells and usually compute ration cross-head displacement to original length of sample as an indicator of percentage stretch, however in-plane strain components may have a non-uniform spatial distribution due to heterogeneous extracellular matrix of connective tissue around cells. Therefore, the average value of the applied cross-head strain is expected to be different than local strain in the vicinity of cells. This limitation has prompted us to develop an alternative approaches.

            Here, we present a novel uniaxial cell-stretching device integrated into inverted fluorescence microscope that provides a high spatial resolution to determine the local strain changes around fluorescent and non-fluorescent cells (RSI, 2016, 87, 023905). Transparent and biocompatible polydimethylsiloxane PDMS elastomer modified with small fluorescent beads are used to deliver uniform strain at the physiologically relevant magnitude and cycles. The design of our device for acquisition of real-time spatiotemporal data and single-particle tracking methods to determine bead positions that was used for computation of strain fields at various sample geometries will be described (Fig. 1). Briefly, trajectory of beads is computed by comparing each registered location in consecutive frames and minimizing the square displacement of centroids. Displacement vector is obtained from displacements and later used to calculate longitudinal normal, traverse normal and shear components of strain with relevant deformation tensor (Fig. 2).  Lastly, we will discuss that HeLa S3 cells adhered to biocompatible and flexible collagen coated surface are stretched to determine simultaneously detection of morphological changes and local strains around the cells by tracking embedded fluorescent beads (Fig. 3). The method enables to measure local strain field and image adherent cells simultaneously, therefore provides accurate and time-resolved correlation between applied mechanical deformation and cell response.

Acknowledgements

This work is financially supported by the Scientific and Technological Research Council of Turkey (TUBITAK) under Grants 112E580 and 112T823. O.B.A. is supported by 2210 national scholarship predoctoral training program. 

 

Halil BAYRAKTAR (Turkey, Turkey), Onur AYDIN, Bekir AKSOY, Ozge Begum AKALIN, B. Erdem ALACA

08:00 - 18:15 #6905 - IM03-281 Imaging photon camera with high spatiotemporal resolution.

IM03-281 Imaging photon camera with high spatiotemporal resolution.

Photon counting detectors with high spatiotemporal resolution levels are key tools of new super-resolution imaging techniques, as required for time resolved single molecule imaging. PHOTONIS Imaging Photon Counter, IPC, is a wide field (18 mm active area) camera able to detect individual incoming photons with 2 dimensional spatial resolution < 40 µm FWHM and < 100 ps timing resolution. The IPC consists of a dual microchannel plate detector equipped with a cross strip anode and high speed electronics that allows global count rates up to 5 MHz. The PHOTONIS IPC comes with a user-friendly plug-and-play GUI for operation and data acquisition. The 1 Gbps Ethernet interface provides the connection to a computer for data storage and analysis.

The first PHOTONIS IPC was installed at the National Institute of Standards and Technology (NIST) in 2015; the IPC is being used in a new single-molecule super-resolution fluorescence lifetime imaging microscope at the Center for Nanoscale Science and Technology at NIST.

PHOTONIS launched in Q1 2016 a new low-noise and high quantum efficiency cathode technology that boosts the detection efficiency and dual resolution of the next IPC generation.

Rene GLAZENBORG, James MARR, Adrian MARTIN, Raquel ORTEGA (sturbridge, USA), Emile SCHYNS, Oswald SIEGMUND, John VALLERGA

08:00 - 18:15 #6916 - IM03-283 Defocus and probe-position coupling in electron ptychography.

IM03-283 Defocus and probe-position coupling in electron ptychography.

Defocus and probe-position coupling in electron ptychography

Shaohong Cao, Peng Li, Andrew M Maiden, John M Rodenburg

 Department of Electronic and Electrical Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK.

One of the most promising applications of electron ptychography is the possibility of converting a conventional SEM, with a relatively poor stability envelope, into a high-resolution TEM. This is simply achieved by placing a transmission specimen in stage and a CCD camera placed below the specimen [1]. SEM ptychography does not need any image-forming lenses: the objective lens is only used to generate a conical illumination. Defocusing this into a large area allows for a large ptychographical step size and a very large field of view. High resolution is obtained by solving the phase problem for the high-angle scattered intensity (lying outside the Ronchigram) via iterative algorithms such as ePIE [2] and DM [3]. See Fig. 2a, where gold atomic fringes in gold nano particles on a carbon support film are clearly visible in a ptychographic reconstruction obtained from a conventional SEM (an FEI Quanta 600 SFEG at 30keV, data from [1]). We also solve for the illumination function, which in the case of conical illumination has a uniform curvature dependent on defocus.

A complication of electron (as opposed to visible light and X-ray) ptychography is that is normally necessary to solve also for the illumination positions [4] because of errors in the scan coils resulting from magnification calibration inaccuracy and hysteresis. However curvature in the illumination and probe shift are coupled. Figure 1 shows a simplified ray diagram. For any two specimen planes where the ratio of the shift distance () and the ratio of the feature size in the object () equals the ratio of the defocus (), the resulting Ronchigrams will behave identically as the probe is shifted. In fact in the presence of wave interference the correct solution is unique, but for probe position-searching algorithms that are necessarily guided by the Fourier error metric, the bright intensity of the Ronchigram dominates. When partial coherence is present (so that Fresnel effects in the Ronchigram are supressed) the problem is exacerbated, generally resulting in a distorted probe position lattice (Fig. 3), a wrong estimate of the defocus, and an inconsistent reconstruction in which lattice fringes are strongly delocalised. Of course, delocalisation renders the reconstruction entirely invalid, like an out-of-focus conventional TEM image.

We explore the coupled position-defocus stagnation problem based on both experimental and simulated data. We report progress on various methods that can resolve this important issue.

Acknowledgment: SC gratefully acknowledges financial support from Phase Focus Ltd and for access to the SEM data.

References:

[1] Humphry MJ, Kraus B, Hurst AC, Maiden AM and Rodenburg JM, Nature Communications, vol. 1733, no. 10.1038, 2012

[2] Maiden AM and Rodenburg JM, Ultramicroscopy, 109, (2009) 1256-1262

[3] Pierre Thibault P, et. al., Ultramicroscopy, 109 (2009) 338-343, 2009

[4] Maiden AM, et. al., Ultramicroscopy, 120 (2012) 64-72

Shaohong CAO (sheffield, United Kingdom), Peng LI, Andrew MAIDEN, John RODENBURG

08:00 - 18:15 #6932 - IM03-285 Chiral electron sieves for electron vortex beam generation.

IM03-285 Chiral electron sieves for electron vortex beam generation.

The recent discovery of vortex electron beams [1-3] has generated many interests and the new understanding may lead to applications in material characterization [4]. The method of creating vortex electron beams using a forked aperture [2] is popular but has a drawback that electron vortex beams with different orders are produced side-by-side in the specimen plane. More recently a method of producing electron vortex using a spiral holographic aperture was demonstrated, with the advantage of being able to select a specific vortex beam in focus on the sample [5,6].  However, the spiral aperture is difficult to reproduce accurately because of many fine features and it is also mechanical fragile and the extra reinforcement bars were added in the mask for mechanical stability[5].

In this study, we propose and experimentally demonstrate a simple and versatile way of generating electron vortex beam carrying orbital angular momentum (OAM), using a chiral electron sieve illuminated by a plane electron wave.  Fig. 1 (a) shows an electron sieve mask as viewed using a scanning transmission electron microscope.  It consists of a number of spirals (5 in this case), each spiral consists of suitablly arranged holes in such a way that they produce a vortex beam approximate at a near field distance [7].  Fig. 1(b) shows the resultant beam intesnity profile as the designated distance.  This shows a characteristic donut shaped beam illumination characteristic of an electron vortex beam.  The is confirmed by direct comparison of the intensity profile with the simulated result using Fresnel diffraction theory (Fig. 1(c)).  The same simulation shows the phase structure at the center of the donut ring does contains a vortex structure of order 5.  

For this particular electron sieve mask, the mask pattern can be compared with the spiral diffractive holographic grating structure produced by interference between the vortex beam and that of a spherical reference waves [5], showing comparable efficiency.  However, it has the practical advantage of being much easier to produce and mechanically more robust than the normal binary diffractive grating.   We will show that this is a practical example of a much rich set of electron sieves that we can generate to shape electron beams, many of which has no counter part in conventional holographic diffractive grating theory [8].

[1]  M. Uchida, A. Tonomura, Nature 464, 737 (2010).

[2]  J. Verbeeck, H. Tian, P. Schattschneider, Nature 467, 301(2010).

[3] B. J. McMorran, et al, Science 331, 192 (2011).

[4] R. A. Herring, Science, 331, 155 (2011).

[5] J. Verbeeck, H.Tian, and A. Béché, Ultramicroscopy 113, 83 (2012).

[6] K. Saitoh1, Y. Hasegawa, N. Tanaka, and M. Uchida, J. Electron Microsc. 61, 171 (2012).

[7] Z. Li, M. Zhang, G. Liang, X. Li, X. Chen, and C. Cheng, Opt. Express 21, 15755 (2013).

[8] Y.J. Yang, G. Thirunavukkarasu, M. Babiker, J. Yuan, paper submitted.

Jun YUAN (York, United Kingdom), Yuanjie YANG, Gnanavel THIRUNAVUKKARASU, Mohamed BABIKER

08:00 - 18:15 #6952 - IM03-287 A micro-combinatorial TEM method for phase mapping of thin two-component films.

IM03-287 A micro-combinatorial TEM method for phase mapping of thin two-component films.

The aim is to introduce a new method to transmission electron microscopy (TEM) for the preparation and investigation of combinatorial samples. TEM is a very efficient analytical technique in materials science and technology in structural characterization of bulk and thin films that are determined by their composition. The equilibrium phase diagrams of the bulk are well explored (Binary Alloy Phase Diagrams), however, phases of thin films may remarkably differ and due to difficulties they are hardly studied. The common procedure to reveal the properties of concentration dependent phases is the preparation of numerous two-component samples, one for each C_A/C_B=1-A composition, and the investigation of these individuals. This is a low efficiency procedure, that costs enormous time of man and machine. For a study of high number of samples combinatorial methods are preferred i.e. instead of carrying out numerous individual experiments samples of varying composition are prepared in a single process. In materials science and especially, electron microscopy, however, due to technical difficulties, combinatorial methods are not widespread.

The first examples for combinatorial TEM investigations are published from the 90-ies. P. Schultz et al [1] and K.E. Roskov [2] These were partly efficient solutions since only the preparation of discrete samples was combinatorial the investigation was not. P. B. Barna, and later G. Radnóczi and F. Misják implemented an experimental arrangement [3] that was already micro-combinatorial, since samples with various compositions were both deposited and investigated in a single TEM grid: Two sources were facing the substrate through an aperture at inclined angles so that the thin film was deposited at two overlapping areas, where two-component film of Ag-Cu was grown. The drawback of that method [3] is that the region of changing concentration is very short (100-150 µm), due to the concentration gradient the phases are accumulated and the sample does not contain the entire (0-100%) composition range.

The idea for developing a really efficient micro-combinatorial method that eliminates the incompleteness of the above solutions means that the preparation and investigation have to be carried out in a single TEM grid within the whole 0-100% composition range (micro-combinatory) so that the formed binary phases and variants may be well separated. The method and device was patented by G. Sáfrán [4]. The present method is typically for the preparation of two-component combinatorial samples with a constant concentration gradient that are deposited on electron transparent film on microgrid suitable for TEM investigations, as well as SEM, EDS and Auger analysis, or nanoindentation. The micro-combinatorial device incorporates a cover plate with a narrow slit that is moved in fine steps above the substrate e.g. TEM grid, meanwhile the fluences of the magnetron sources “A” and “B” are regulated as synchronized to the path of the slit [4]. This new solution provides micro-combinatorial preparation and investigation, so that the length of the concentration transition region, compared to the former solution [3], increases with one order of magnitude to 1500 microns.

For a demonstration MnAl micro-combinatorial samples were DC magnetron sputter deposited on TEM grids covered with amorphous C  and SiOx films at various temperatures. The results of the phase analysis of the combinatorial samples are not discussed here.

A photo of a TEM grid with a deposited Mn-Al micro-combinatorial sample is shown in Fig. 1. Fig 2.(b) represents energy dispersive X-ray spectrometer (EDS) data that, aside from the edges, shows linear concentration distribution along the 1500 micrometer stripe. The gradient of a limited concentration range can be adjusted to arbitrarily low values for an enhanced separation of the formed thin film phases. In addition, the device is suitable for the study of the effects of parameters (residual gas pressure, temperature, etc.) changing with time.

References

[1] P.G. Schultz et al: Science (1995) Vol. 268 no. 5218 pp. 1738-1740

[2] K.E. Roskov: J. Comb. Chem. (2008) 10, 966–973

[3] F. Misják et al: Thin Solid Films 516 (2008) 3931–3934

[4] Hung. Patent No. P 15 00500 (2015)

György SÁFRÁN (Budapest, Hungary)

08:00 - 18:15 #6963 - IM03-289 Novel focused ion beam in-situ methods for stressed and cracked TEM sample preparation.

IM03-289 Novel focused ion beam in-situ methods for stressed and cracked TEM sample preparation.

Conventional Ga⁺ Focus Ion Beam (FIB) dual beam system has proven to be an important and popular tool for Transmission Electron Microscope (TEM) sample preparation [1]. It is especially useful for localized TEM sample preparation. However, it is commonly known that the process can fail due to the internal stress leading to crack. We propose two strategies and their methods are detailed here. One is to release the stress by milling away constraining materials and another is to prethe crack rowthby in-situ filling method [2]. The two strategies can be broadly applied to most stressed/cracked samples.  

In the first example, a chemical vapor deposition (CVD) diamond like carbon film with large internal compressive stress has failed in conventional TEM sample preparations. In order to release the internal stress, a ring is cut surrounding the area of interest, the ‘bump’. The cut could be performed by milling at large current (30-65nA) for efficient removal of the surrounding materials. Subsequent removal of the materials on the ‘bump’ is performed at progressively lower current until a 1um lamella remains for a conventional in-situ lift-out procedure. The results of TEM analysis showed the amorphous structure and columnar growth of the ‘bump’ area (Fig. 1).

To analyze the stress release of the local area using the cutting procedure, digital image correlation (DIC) was employed in this study. A cluster of small dots were deposited on the surface of the ‘bump’ area using carbon gas injection system (GIS) as shown in Fig. 2.  After milling the ring pattern to remove the material around the ‘bump’, secondary electron images were taken by electron column of this dual beam system (FEI Quanta3D), before and after the cutting respectively. These images were then imported to the DIC software (Davis) for calculation of the stress/strain release. It was measured by the displacements of the deposited dots. The result shows the stress around the ‘pump’ is released after material removal around the ‘bump’ which led to successful TEM sample preparation.

The second example is a hot isostatic pressing (HIP) processed Ti-SiC composite with internal thermal mechanical stress on the interface area which is however the area of interest. The broad ion beam preparation and conventional FIB preparation all failed due to stress concentration induced cracking at the interface region (Fig. 3). To increase the stability of the lamella, the sample was wedge polished in the parallel direction of the interface. This resulted in both ends of the lamella being held by the matrix material after rough milling process. An in-situ filling process by e-beam Pt deposition was then carried out to ‘fill’ the crack and strengthen the interface area. Caution was taken with the cut off step for both ends of the lamella. It is recommended to use parallel milling mode of the FIB in order to keep balance on the cutting speed of both ends so that it can better prevent failure during the cut off. E-beam Pt deposition at the free end of the lamella may enhance its stability. TEM results have shown the successfully prepared sample with filled crack at the interface and its deformed grains due to thermal mechanical stress induced by the HIP process.

Reference

[1] N. B. K. Scott, L. A. Giannuzzi, MRS Bulletin, 39(04)  p. 317-325

[2] X. L. Zhong, G. E. Thompson, Z. Liu, P. Skeldon, M. G. Burke (2014), mmc2014 Proceedings, Abstract No. 1062

[3] The authors thank Mr A. Broughton, Prof S. Lyon, and Mr Y. Fan for providing the samples.

Zhong X. L. (Manchester, United Kingdom), Mcdonald S. A., Withers P., Burke M.G.

08:00 - 18:15 #6987 - IM03-291 Device for the transport, storage as well as the chemical or physical treatments of AFM tip series.

IM03-291 Device for the transport, storage as well as the chemical or physical treatments of AFM tip series.

Atomic force microscope - AFM - is a scanning probe microscope used to probe surfaces with a high degree of accuracy. AFM is based on the measurement of forces of interaction between, on one hand, atoms of the surface of a sample and, one the other hand, the last atoms of an AFM tip or probe. AFM is also a tool to measure the local forces allowing access, through the acquisition of force spectra, to the nanomechanical properties of a sample (adhesion, deformation, elasticity, dissipation, etc.). An AFM probe consists of a tip positioned at the end of a lever, which is itself attached to a support ("chip" or "wafer"). An AFM probe is only a few millimeters long and is very fragile and expensive.

When new or unused AFM probes can be stored in plastic transport boxes or more rarely cassettes suitable for packaging and transporting dry these probes without risk of damaging them. The ground of these boxes is covered by a layer of adhesive polymer (gel) designed to maintain AFM probes on the surface of the wafer by adhesion. However, this direct contact with the gel may cause contamination of the tips and the presence of gel avoids the physical or chemical treatment of the probes in this box, as well as their conservation in liquid medium, which is required for any chemically grafted tip.

We introduce here a new generation of AFM device that has been specifically developed to provide both the transport and storage of AFM probes in air or in liquid (water, buffers, alcools,…). It is also well adapted for physical treatments (UV/ozone treatment, plasma, metal sputtering,…) and chemical application (tip chemistry). The device is mainly based on an originally way to hold a large series of AFM probes for all the applications, preventing them from any breakage or damage. [1]

By fixing a series of AFM probes in the box, the successive steps of UV/ozone for tip surface cleaning, piranha activation of SiN tip surface, chemical reactions, bio-functionalization, water and buffer washing and long term storage in buffer can be operated with keeping the AFM probes in their initial position. The liquid reactants, water and buffers are simply injected and removed using a (micro)syringe. Application of the AFM boxes was useful to prepare series of tips using rigorously the same experimental conditions. Antibodies functionalized tips may be prepared in one experiment to further analysis the nanomechanical properties of different substrates by AFM and force spectroscopy and mapping. Tips may also be prepared at a first place (for example, in a lab expert in chemistry of biomolecules) and be sent by mail to another place for analysis (for example, in a lab expert in AFM imaging and force measurements).

The figure 1 shows pictures of the device and an example of application to study the adhesive properties of lactalbumine milk proteins. This device takes the advantage to have internal containers that can be used independently from the outer lager container. This feature is very well adapted for chemical grafting of series of AFM tips according to different protocols.

 

Reference

Gaillard, C. and Sire A., Storage Box for AFM Probes, INRA Patent FR2011/051989, 2010/2011, WIPO Patent Application WO/2012/028822

Cédric GAILLARD (NANTES)

08:00 - 18:15 #7046 - IM03-293 Counter Electrode Design Considerations in Atom Probe Tomography Microscopes.

IM03-293 Counter Electrode Design Considerations in Atom Probe Tomography Microscopes.

In the development of atom probe tomography instruments, a variety of counter electrode designs have been considered.  Each design takes into account a wide variety of design criteria including;  complexity of manufacturing, undesired electron emission, stage motion, cryogenic cooling, vacuum performance, voltage and laser pulse introduction, geometry, serviceability, field enhancement, signal-to-noise, energy spread, field of view, and cost of ownership.  Examples of some designs are shown in Figure 1. 

The local electrode atom probe (LEAP®) puts a premium on a geometry and proximity to the specimen in order to enable faster voltage pulsing, minimization of energy spread, and the maximization of throughput with microtip geometries [1].  Such a design, with a small spot focused laser [2], maximizes field enhancement and minimizes the portion of the flight path exposed to field variations [3]. The design does however require a vibration isolated and flexible cryogenic path and a high precision stage with sophisticated alignment cameras. 

Prior to the proposal of a micro-extraction electrode by Nishikawa in 1993 and until the introduction of the LEAP in 2003 [4,5], atom probe field ion microscope systems used a simple counter electrode that was simply a wire ring or a copper disc with an opening of a few millimeters.  This design is simple to construct, and allows substantial flexibility in sample stage design, including the use of a goniometer stage.  Especially in early atom probe design, where the field of view was as much as 100 times smaller than is achievable today, the ability to rotate and tilt the specimen towards the TOF detector was key.  Alignment of the specimen with respect to the counter electrode is not critical and could be achieved by line of sight and the projection of the data to a phosphor screen or the TOF detector.  The simplicity and flexibility are countered by the limitation to wire geometry specimens, a flexible and lower conductance cryogenic cooling path, low field enhancement, and degradation of mass resolving power due to the ions being exposed to varying electric fields during larger portions of the flight path. 

Several atom probe systems have been proposed and constructed using a flat disk counter electrode with an aperture (or a metal TEM grid) with the specimen moved aligned in close proximity to, or even protruding through the plane [6-8].  Alignment to the electrode requires wire geometry specimens, a precision stage and long range microscopes, but substantial field enhancement is possible, even while using relatively large apertures (~ 1mm) which minimizes the chance of damage to the electrode during specimen fracture events.  For a ~ 1mm aperture in a disc electrode, the field enhancement is ~25% less than a local electrode and simulations show it is insensitive to specimen penetration distance (Figure 2) [9]. 

Achieving the highest data quality and highest throughput in an atom probe substantially complicates design requirements.  A variation of the flat disk counter electrode approach removes the requirements of a precision stage, flexible connection to the cryogenic system and sophisticated alignment microscopes by doing ex-situ alignment of a disk electrode with the specimen.  Although still requiring a wire geometry specimen, such a system could have a directly couple cryogenic system and could take advantage of the substantial field enhancement, wide field of view, and relatively high data quality when compared to previous generation atom probe microscopes.  This work presents our current advances in simplification of electrode geometries for atom probe and the performance associated with such designs.

[1]    T.F. Kelly, D.J. Larson, Mat. Char. 44 (2000), p. 85.
[2]    J. H. Bunton et al., Micro. Microanal. 13 (2007), p. 418.
[3]    D. J. Larson et al., Appl. Surf. Sci. 94/95 (1996) p. 434.
[4]    O. Nishikawa, M. Kimoto, Applied Surface Science 76/77 (1994), p. 424.
[5]    T. F. Kelly et al., Micro. Microanal. 10 (2004), p. 373.
[6]    S. S. Bajikar et al., Appl. Surf. Sci. 94/95 (1996) p. 464.
[7]    M. Huang et al., Ultramicroscopy 89 (2001), p. 163.
[8]    R. Schlesiger, et al., Review of Scientific Instruments, 81 (2010), p. 043703.
[9]    R. Gomer, in “Field Emission and Field Ionization” (AIP, New York) (1993), p. 45.

Acknowledgements

Thanks are due to the entire engineering team at CAMECA for assistance in preparing this abstract.  

Robert ULFIG (Madison, USA), Dan LENZ, Joseph BUNTON, Mike VANDYKE, David LARSON

08:00 - 18:15 #6132 - IM05-295 Including the effects of atomic bonding in TEM and STEM image simulations.

IM05-295 Including the effects of atomic bonding in TEM and STEM image simulations.

Most of the software used to simulate TEM/STEM images completely neglects interatomic interactions in the sample and thus leaves out the effects of charge transfer and chemical bonding. The effect of these interactions on the image is generally small compared to the total charge density, but essential to understanding many important properties of materials. These interactions are especially important for fully quantitative interpretations of TEM/STEM images of certain sample types such as bulk oxides because of the strong ionic bonds, and 2D materials composed of light elements since most of the electrons are involved in bonding. In order to accurately simulate TEM/STEM phase images of these and other such materials, we have employed the CASTEP [1] code to generate accurate projected potentials which take into account both intra- and interatomic electron interactions. CASTEP uses density functional theory (DFT) to calculate electron densities, which are directly related to the electrostatic potential via Poisson's equation. These potentials are then used to calculate the projected potentials for use in multislice algorithms to simulate TEM/STEM images. We have added to the functionality of CASTEP so that these projected potentials can be obtained directly from the software, thus allowing us easy and accurate simulation of images with bonding effects included which enables accurate interpretation of experimental images of materials, such as h-BN. Although a similar approach has been used by Kurasch et al. who obtained projected potentials by post-processing output from the WIEN2k software [2], no one, to our knowledge, has done this with a code which benefits from the use of pseudopotentials such as CASTEP.

[1]  S. J. Clark et al.  First principles methods using CASTEP. Zeitschrift fuer Kristallographie, 220(5-6):567-570, 2005.

[2] Simon Kurasch et al.  Beilstein J. Nanotechnol., 2:394-404, 2011.

[3] The authors gratefully acknowledge funding from the EPSRC under grant number EP/LO15722/1.

Timothy NAGINEY (Oxford, United Kingdom), Peter NELLIST, Rebecca NICHOLLS, Jonathan YATES

08:00 - 18:15 #6139 - IM05-297 Accurate and fast electron microscopy simulations using the open source MULTEM program.

IM05-297 Accurate and fast electron microscopy simulations using the open source MULTEM program.

The most practical method for TEM image simulations is the multislice method, which is known to be an accurate numerical procedure for solving the quantum mechanical electron-specimen interaction. Although most simulation codes treat the scattering process as purely elastic and coherent, inelastic scattering cannot be neglected and it has to be included in realistic simulations. Inelastic phonon scattering is often incorporated by using the frozen phonon model [1, 2] and the electronic excitations by using the density matrix approach [3, 4]. Nowadays, new computer technologies allow us to perform large TEM calculations with inclusion of accurate approximations of the electron-specimen interaction in an acceptable amount of time.

A general overview of the MULTEM program along with a number of examples has been reported in [5]. In this work we present a new version of the open source MULTEM program, which adds key features including a graphical user interface, tapering truncation of the atomic potential, CPU multithreading, single/double precision calculations, STEM simulations using experimental detector sensitivities, ISTEM simulations, EFTEM simulations, STEM-EELS simulations along with other improvements in the algorithms. A screenshot of the user interface is shown in Fig. 1. This figure shows the main available options of the program. In Fig. 2, a simulated HRTEM image and ED pattern of an isolated gold nanoparticle of 21127 atoms are shown. In these simulations, plane-wave illumination is assumed with the following electron microscope setting: acceleration voltage (300 keV), spherical aberration (0.002 mm), defocus (19.8 Å), defocus spread (30 Å) and beam divergence angle (0.1 mrad). A numerical real space grid of 4096x4096 pixels has been used. The frozen atom simulation is performed by using the Einstein model with 200 configurations, slice thickness of 0.5Å and the three-dimensional rms displacements of all the atoms are set to 0.085Å. For the HRTEM simulation, the spatial and temporal incoherences are included by applying the partially coherent microscope transfer function to each exit wave of the frozen atom. A multislice simulation of each frozen atom configuration only took 2.6 s on a Nvidia GeForce GTX TITAN GPU card.

The MULTEM's C++ classes, Matlab mex functions and the GUI are available in the GitHub repository https://github.com/Ivanlh20/MULTEM.

References

1. E.J. Kirkland. Springer, New York and London, (1998).

2. D. Van Dyck. Ultramicroscopy 109, 677(2009).

3. L.J. Allen and T.W. Josefsson. Physical Review B 52, 3184 (1995).

4. J. Verbeeck, P. Schattschneider, and A. Rosenauer. Ultramicroscopy 109, 350, (2009).

5. I. Lobato and D. Van Dyck.  Ultramicroscopy 156, 9 (2015).

 Acknowledgement

The authors acknowledge financial support from the Research Foundation Flanders (FWO,Belgium) through project fundings (G.0374.13N, G.0369.15N and G.0368.15N). The research leading to these results has also received funding from the European Union Seventh Framework Programme [FP7/2007- 2013] under Grant agreement no. 312483 (ESTEEM2).

Ivan LOBATO (Antwerpen, Belgium), Sandra VAN AERT, Johan VERBEECK

08:00 - 18:15 #6164 - IM05-301 Experiment design for quantitative dark field imaging and spectroscopy of catalyst nanoparticles using Scanning Transmission Electron Microscopy (STEM).

IM05-301 Experiment design for quantitative dark field imaging and spectroscopy of catalyst nanoparticles using Scanning Transmission Electron Microscopy (STEM).

The performance of catalyst nanoparticles is generally dependent on their size, shape, strain, composition and support. However, the relationship between these parameters and the catalyst performance is not well understood. In most instances, the catalyst design process involves several design and testing iterations until the desired performance is attained, and identifying the structure-property relationships would enable rational catalyst design. In order to relate the nanoparticle properties to catalytic activity, it is necessary to characterise them at atomic resolution.

With aberration corrected STEM it is now possible to image and perform spectroscopy on nanocatalysts at high resolution to obtain detailed structural and compositional information. Here we show how experiment design including detailed calibration of imaging and spectroscopy parameters can reveal the required information, using Ru, Pt and alloyed Pt-Co catalyst nanoparticles as examples.

As a first example, structural studies of Ru nanoparticles, using methods recently developed^[1]–[3], indicated the formation of thin Ru rafts (Figure 1). The formation of Ru rafts has been controversial since their hypothesis^[4]. These results demonstrate how quantitative ADF imaging can be used to resolve this question.

Using a new detector mapping technique developed at Oxford University, a range of Pt nanoparticles were characterised for their three dimensional structure (Figure 2). The models, created using an energy minimisation approach previously described in ref [3], indicate a Wulff-like structure. We will show how these models can be used as input for density functional theory simulations, thus unlocking the possibility to study the electronic structure of experimental catalyst nanoparticles.

It is important to increase analysis throughput by software automation ^[3] in order to gain statistically meaningful results.  In Figure 3 a rapid particle size measuring algorithm was used to measure the size distribution of Pt and Pt-alloy nanoparticle systems. The size distribution can then be used as a guide for the microscopist to selectively choose nanoparticles which represent the particle ensemble.

Finally, experiment design and preliminary EDX and EELS results at high resolution will be presented. This approach aims to decouple composition and thickness effects in order to obtain structural information of alloyed nanoparticles.

References

[1]         H. E, K.E. MacArthur, T.J. Pennycook, E. Okunishi,  A. J. D’Alfonso, N.R. Lugg, L.J. Allen, P.D. Nellist, Ultramicroscopy 2013, 133, 109.

[2]         S. Van Aert,  A. De Backer, G.T. Martinez, B. Goris, S. Bals, G. Van Tendeloo,  A. Rosenauer, Phys. Rev. B - Condens. Matter Mater. Phys. 2013, 87, 064107.

[3]         L. Jones, K.E. MacArthur, V.T. Fauske, A.T.J. Van Helvoort, P.D. Nellist, Nano Lett. 2014, 14, 6336.

[4]         E.B. Prestridge, G.H. Via, J.H. Sinfelt, J:Catal. 1977, 50, 115.

Aakash VARAMBHIA (Oxford, United Kingdom), Lewys JONES, Annick DE BACKER, Vidar FAUSKE, Sandra VAN AERT, Dogan OZKAYA, Sergio LOZANO-PEREZ, Peter NELLIST

08:00 - 18:15 #6168 - IM05-303 The inverse problem of quantitative emission electron microscopy with account of distribution of the electron energy.

IM05-303 The inverse problem of quantitative emission electron microscopy with account of distribution of the electron energy.

In emission electron microscope (ЕЕМ) energy distribution of emitted electrons can significantly affect the image contrast of microfields. This influence is the more the less energy of emitted electrons in volts differs from the residual of potentials of effective microfields. As usual it is accomplished in threshold photoelectron emission. Analytically this objective was solved for cases of contrast formation in EEM with using aperture or without that one. Determined from the obtained formulas and measurements built-in potential between Ag particles (thickness 30 nm, lateral size 20×20 µm²) and Si(100) substrate is 0.7 eV. Calculation shows that this value is two times higher without accounting of photoelectrons energy distribution (high pressure mercury lamp was used for excitation).

Remarkable that the expression for correction that appeared in energy distribution of electrons emitted from the sample coincides with contrast defocus formula. It means that some initial energy of emitted electrons is equivalent to image defocus and thus can be compensated by some refocusing of images.

Sergej NEPIJKO (Mainz, Germany), Gerd SCHÖNHENSE

08:00 - 18:15 #6217 - IM05-305 Molecular Salad: multi-slice simulation of particles for cryo-TEM.

IM05-305 Molecular Salad: multi-slice simulation of particles for cryo-TEM.

The combination of counting, direct electron detectors, such as the DE-20, Gatan K-2, and FEI Falcon 3 and Bayesian maximum-likelihood reconstruction tools embodied by Relion and Frealign, have revolutionized the technique of single particle analysis in cryo-TEM. Only a handful of years before, 10 Å results from film or scintillator-coupled CCDs represented the state-of-the-art. At present, many structures are being published with resolutions of 3 Å and the current world record stands at 2.2 Å.

However, many of the physics assumptions that are used throughout cryo-TEM are left-over from the previous, low-resolution era. In particular, the Weak-Phase Object (WPO) approximation is widely used, in that all reconstruction tools back-project 2D projections to a 3D map of electrostatic potential using the Central Projection Theorem (CPT). The WPO approximation is much poorer at higher spatial frequencies, in particular when resolution exceeds inter-atomic spacing and/or high electron wavefront curvature is present in the projection system optics. Thus to proceed to further improved resolution with confidence the accuracy of the underlying physics assumptions must be assessed.

Results will be presented from a plug-in for C. Koch’s QSTEM multislice simulation package, called Molecular Salad (MS). MS can take a structure from the Protein DataBank (PDB), randomly orientate it, merge the particle with a vitreous ice matrix, determine the exit-wave function by multi-slice, and then apply an aberration waveplate up to spherical aberration (C3). Many particle groups are generated by a multi-processed approach, which permits generation of thousands of synthetic particles that may be reconstructed by Relion or similar packages for independent verification of techniques. Multislice-simulated particles can also have simulated radiation damage using B-factors in the same code that is typically used for thermal diffuse scattering for phonon calculations. 

In Fig. 1 the Fourier Ring Correlation (FRC) is shown between the projected potential (i.e. WPO) and the multislice Exit Wave Function (EWF) averaged over many random orientations of protein 2QI9. The shaded area represents the range of correlations over different angles. The correlation between the WPO and the EWF rapidly drops, being approximately 0.6 at 2.0 Å and negatively correlated at 1.0 Å resolution. This suggests strongly that the current generation of CPT backprojection algorithms will not suffice for sub-2.0 Å resolution. Examples are shown in Fig. 2 of a 2QI9 particle, and in Fig. 3 the same particle with 1 µm defocus. Especially large defocus values require special treatment of aliasing that occurs when high curvature phase-plates are applied.

The limits to resolution in single particle analysis are also thought to include the uncertainty in defocus. The ensemble average defocus is estimated from entire micrographs, but particles are embedded at different heights in the ice block which can generate a per-particle defocus error of ± 50 nm. With simulated particles this effect can be tested. The introduction of reliable simulation into the field of single particle analysis would be a valuable addition to the field for validation of algorithms.

Robert MCLEOD (Basel, Switzerland)

08:00 - 18:15 #6248 - IM05-307 Optimal detectability combined with picometre range precision to position light atoms from HR STEM images.

IM05-307 Optimal detectability combined with picometre range precision to position light atoms from HR STEM images.

In the past few years a lot of research has been done to improve the imaging power to detect light atoms like oxygen, lithium, and hydrogen, since they play a key-role in interesting industrial applications such as lithium-batteries or hydrogen-storage materials. Since material properties crucially depend on the _exact atomic arrangement, an estimation of the atomic column positions with picometre range precision is needed, which is feasible using HR STEM [1]. It is investigated if a single optimal design can be found to both detect and position light atoms. The principles of statistical detection theory [2] are used to quantify the so-called probability of error P_e, in a binary hypothesis test. P_e can be computed using realistic simulations to describe the experimental images [3] and can be used to optimise the experimental settings for the detection of light atoms from HR STEM images, as shown in [4]. To determine the optimal experiment design to position light atoms, use is made of the concept of Fisher information. The attainable precision with which unknown continuous structure parameters can be estimated is given by the lower bound on the variance with which an unknown parameter can be estimated from a set of observations, which is given by the so-called Cramér-Rao Lower Bound (CRLB) [5]. The optimal statistical experiment design of a HR STEM experiment for positioning light atoms is given by the microscope settings that minimise this CRLB. For both research questions, it will not only be investigated where in the detector plane the most sensitive region is located, but moreover, precise optimal inner and outer STEM detector angles can be derived quantitatively. The ultimate goal is then not to achieve optimal visual interpretability, but to obtain quantitatively the optimal experiment design for which the unknown structure parameters are obtained with the highest possible precision.

To illustrate the concept, the problem of suggesting optimal detector settings to detect and position the oxygen atoms in SrTiO₃ is considered, as well as detecting and positioning the lithium atoms in LiV₂O₄. A 4.66nm thick LiV₂O₄ crystal is therefore simulated for an incoming electron dose of 10⁵e^-/Ų, and a 1.95nm thick SrTiO₃ crystal is simulated, using an incoming electron dose of 10⁴e^-/Ų. For the detection problem, a binary hypothesis test is performed where both hypotheses correspond to either the presence or absence of the oxygen or lithium atoms in the crystal. P_e is computed as a function of the STEM inner and outer detector angles of which results are shown in Fig.1(a) for SrTiO₃ and in Fig.1(b) for LiV₂O₄. For the positioning problem, the CRLB is computed as a function of the STEM inner and outer detector angles of which results are shown in Fig.2(a) for SrTiO₃ and in Fig.2(b) for LiV₂O₄. The same optimal detector angles for the detection and positioning problem are found, which lie in the low angle ADF STEM regime for both applications, for a probe semi-convergence angle of 21mrad. To detect and position oxygen in SrTiO₃, the optimal detector range is 21-100mrad, while for the detection and positioning of Li in LiV₂O₄ the optimal detector settings are 23-26mrad.

In conclusion, it is demonstrated that the experiment design can be optimised in order to detect and position light elements with the highest possible precision. Consistent optimal designs are found for both problems. It can be shown that picometre range precision is feasible for the estimation of the atom positions using an appropriate incoming electron dose at the optimal experimental settings to detect the light atoms.

 

[1] S. Van Aert, et al., Ultramicroscopy 90 (4) (2002) 273–289.

[2] S. M. Kay, Fundamentals of Statistical Signal Processing. Volume II Detection Theory, Prentice-Hall, Inc., New Jersey, (2009).

[3] A. Rosenauer and M. Schowalter, Springer Proc. Phys. 120 (2007) 169–172.

[4] J. Gonnissen, et al., Applied Physics Letters 105:6 (2014).

[5] A. J. den Dekker, et al., Ultramicroscopy 134 (2013) 34–43.

 

The authors gratefully acknowledge financial support from the Research Foundation Flanders (FWO, Belgium) through project fundings (G.0368.15N, G.0369.15N and G.0374.13N) and a post-doc grant to A.D.B. The research leading to these results has received funding from the European Union Seventh Framework Programme [FP7/2007-2013] under Grant agreement no. 312483 (ESTEEM2). 

Julie GONNISSEN, Annick DE BACKER, Arnold Jan DEN DEKKER, Jan SIJBERS, Sandra VAN AERT (Antwerp, Belgium)

08:00 - 18:15 #6260 - IM05-309 Quantitative Stage Mapping of a Zircon grain by WDS on an SEM.

IM05-309 Quantitative Stage Mapping of a Zircon grain by WDS on an SEM.

Because of the effects of Bragg defocusing, accurate quantitative analysis by WDS can only be done with the electron beam in “spot mode” and with samples at the proper analytical working distance. As a result, WDS X-ray mapping in the SEM has only achieved simple raw counts mapping of a single element by rastering the beam over the sample. More complicated WDS X-ray mapping (quantitative or otherwise) has been relegated to the electron microprobe. However, modern SEMs and WDS systems permit quantitative WDS mapping in which a complete WDS quantitative analysis, including background and Φ(ρz) corrections, is done at each pixel. Here, we use WDS quantitative analysis to map the concentration of Hf in a zoned zircon grain. Zircon has emerged as the most critical geochronological tool in the earth sciences. These tiny crystals are truly zircon-halfnon solid solutions and typically contain 10’s of thousands of ppm Hf. Zr/Hf ratios are used as an index of magma evolution and Hf isotopes in zircon are an important recent tool employed to explore magma sources and processes. The ability to map Hf concentrations in detail and to pair this data with geochronology and Hf isotopes will yield important insights into the interpretation of zircon geochronology and the magmas from which zircons crystallize.

Quantitative WDS maps were acquired using a Thermo Scientific™ MagnaRay™ Parallel beam WDS spectrometer and a JEOL JSM-7001F FE-SEM. Data were processed using the Thermo Scientific™ NORAN™ System 7 microanalysis system. The quantitative WDS map was acquired by automatically slewing the SEM stage over a 106×46 grid while keeping the beam at a fixed position. The resulting map is 106×46 pixels with a 2 µm resolution. All measurements were made using a 15 kV beam accelerating voltage and a focused electron beam that was set to 206 nA at the beginning of the run. The beam current was measured at the beginning of each analysis.

Standardization was done using a Hf-bearing zirconia (for Hf Mα and Zr Lα) and quartz (for Si Kα) on a commercially prepared (SPI), carbon coated mineral standards mount. The O concentration for each analysis was calculated by stoichiometry.

The zircon sample was picked from a granite from the Cretaceous Cadiz Valley Batholith in the central Mojave Desert. This grain was mounted in epoxy, polished, and carbon coated. SIMS geochronology and spot trace element analyses were previously conducted on two spots on the grain (Economos, pers. comm.) constraining absolute Hf concentrations and indicating that there are trace or minor concentrations of the REEs, Th, and U, which were not included in these analyses and explain the resulting somewhat low analytical totals.

For unknown and standard analyses, Si Kα and Zr Lα were each counted on-peak for 5 s and off-peak for 2.5 s (at both low and high positions). Hf Mα was counted until the error (background corrected) from counting statistics was better than 2%, which was typically achieved after ~11 s on-peak. Off-peak measurement positions were confirmed to be free of higher order reflections or trace element peaks by inspection of WDS energy scans.

4,876 analyses were acquired, from which Zr, Si, Hf, (Fig. 1) and O quantitative concentrations maps were extracted. The maps reveal that the zircon grain is strongly zoned with respect to the Hf concentration.

The full range of WDS mapping (raw counts, net counts, and quantitative) is now available to the SEM user. Additionally, an EDS spectrum could be concurrently acquired with the WDS measurements, meaning that any arrangement of EDS or WDS standards-based, quantitative stage maps could be extracted. For example, when mapping zircon, Zr and Si could be mapped with EDS and trace elements (e.g., Hf) could be mapped with WDS.

[1] PK Carpenter, SN North, BL Jolliff, and JJ Donovan, Lunar Planet. Sci. Conf. 44 (2013) p. 1827.

Steve SEDDIO, Keith THOMPSON (Madison, WI, USA)

08:00 - 18:15 #6261 - IM05-311 Analysis of Food Packaging Layered Polymers by SEM/EDS and Raman.

IM05-311 Analysis of Food Packaging Layered Polymers by SEM/EDS and Raman.

Food packaging can consist of many layers of materials each engineered for different purposes. The layers may be very thin, in some cases much less than one micron. Principally composed of polymers layers may be composed of inorganics and inorganic particles may be embedded by design or as artifacts. Analysis for quality assurance or failure analysis is difficult due to the complex nature of the sample.

Raman can easily identify the polymers used in these products. Raman mapping has a resolution of one micron or better. SEM imaging easily achieves a resolution of ten nanometers and can distinguish different polymers by their appearance in electron imaging. EDS achieves a spatial resolution of a few tens of nanometers. It can identify all elements present here except hydrogen.

Together these techniques provide complementary information. In addition imaging and elemental mapping with SEM/EDS may be faster than Raman mapping.

For this work a JEOL JSM-7610F FESEM equipped with a Thermo Scientific NS7 EDS analyzer and 60mm2 area Ultradry silicon drift detector were used for the SEM/EDS data. A Thermo Scientific DXRxi Raman spectrometer with optical microscope was used for the Raman spectroscopy.

The SEM conditions were 10 kV acceleration voltage and about 3 nA beam current.

The sample used here is a cross section of a commercially available potato chips bag. The sample was sectioned by a fresh razor blade. For SEM/EDS the sample was carbon coated.

Figure 1 compares the results from both SEM/EDS and Raman. In this view the interior of the package is towards the top of the image. The right image shows a Backscatter Electron (BSE) image of the cross

section overlaid by elemental maps for Al, Ti, Si and O. Twelve layers were identified by inspection ranging from about 25 microns to approximately 175 nm thick. Some are not visible at this low magnification view. An Al layer about 200 nm thick was seen. A 175 nm thick layer containing S and Cl was found (not shown in the figure). Several layers contained Ti rich particles. Some particles of silicon oxide, possibly artifacts, were seen. The live time for this map was 931 seconds.

The image at left in Figure 1 shows the results of the Raman mapping analysis. The identified polymers layers and their thicknesses are labeled. In general the two techniques agree well on the overall composition of the sample. The Raman analysis identifies the polymers making up each of the layers which was not possible by SEM/EDS. It also found the inorganic rutile particles embedded in some of the layers. It did not find any of the sub 200 nm layers seen in the SEM.

John KONOPKA, Keith THOMPSON (Madison, WI, USA)

08:00 - 18:15 #6334 - IM05-313 Characterization of the NiBe nanoprecipitates in CoCr superalloys.

IM05-313 Characterization of the NiBe nanoprecipitates in CoCr superalloys.

Cobalt based superalloys were developed for applications in high temperature, corrosive environments and for bio-implants such as orthodontic devices. CoCrNi alloys have an FCC structure and excellent mechanical properties and their high strength is obtained through a combination of solid-solution and precipitate strengthening. As such, these alloys are primarily Co with 15-20 wt.% Ni and 15-20 wt.% Cr and contain a cocktail of additives composing of Fe, Ti, W, Mo, and C. Some alloys contain a small amount of Be which is thought to improve mechanical properties via solid-solution strengthening mechanisms. In copper based alloys, Be additives are well-known to precipitate into GP zones and improve mechanical propertiesvia precipitate hardening mechanisms. Beryllium having a low solubility in most alloys and an affinity to form intermetallic compound, especially with Ni and Ti [1], and thus secondary precipitation of the Be intermetallics in cobalt superalloys is likely. So the question arises, whether the observed improved mechanical results from GP zone and precipitation hardening rather than the assumed solid-solution solute strengthening.

Using aberration corrected STEM and HR-STEM EDS, we have characterized wrought and overaged alloys and find that Be forms GP zones and Ni-Be B2 intermetallic precipitates. These alloys contain roughly 1 at. % Be, and thus detecting its presence in solid-solution or within precipitates, especially those containing Ni, is challenging. Beryllium cannot be detected with EDS X-ray analysis nor can it be detected with EELS if the precipitates contain large concentrations of Ni as the K edge of Be overlaps with the M edges of Ni. HAADF-STEM images (Fig. 1 A)) are indirect proof and suggest that Be forms intermetallic precipitates with Ni that have a much lower intensity compared to the matrix material, which is primarily Co, Ni and Cr. The EDS maps in Figure 1 confirm that the precipitates primarily contain Ni and thus should have similar Z contrast and intensity as the matrix, though it does not. The precipitates must have a high concentration of lighter Z element, the only one having an affinity to form compounds with Ni and Co is beryllium. Furthermore, we have directly confirmed the presence of beryllium using a special imaging technique, integrated differential phase contrast imaging (IDPC), that is shown in Figure 2 B) [2]. IDPC images are generated using a segmented ADF detector and by integrating the 2 components of the center of mass (COM) of the image given by the momentum transfer of electron probe to the specimen. The resulting image from the integration of the differential COM signals gives a direct map of the associated phase shift due to the interaction of the probe with the sample and thus maps both Be and Ni atoms. Using IDPC is an easy method for directly visualizing Be in the precipitates.

Crystallographic analysis of the precipitate’s orientation using aberration corrected (AC) STEM images (Figure 2 C) ) suggests that they have a Kurdjomov-Sach relationship ({110}BCC/{111}FCC). Though the precipitates differ in orientation relationship with those previously observed in Ni-Be alloys[1], their measured lattice constants (0.262 nm) and ordered B2 structure are the same. The coherency strain can be easily observed in the IDPC image in which the first two lattice planes are compressed by ~1% along the <211> direction. In the wrought alloys, we observe Guinier-Preston (GP) zones with similar orientation relationships as observed in the overaged material. The GP zones generate a large coherency strain along the <211> direction as seen in the ADF-STEM images (Figure 3 B). The GP zones grow along three <111> matrix directions, two that lie in the [110] Z.A. and one on an inclined plane. Using the strain contrast in the ADF images to locate the GP zones in a thin region of the foil, we can observe their size and structure in AC-HAADF STEM images. The GP zones are small being only 1 monolayer thick and are elliptically shaped with the major axis being 20-30 nm. The atomic column intensity of the GP zone is much lower than the matrix and similar to what was observed in the overaged precipitates. Though beryllium cannot be directly detected, it can be inferred that the GP zones contain Be from the analysis of the overaged precipitates. It can be speculated that the nanoprecipates form via the ripening of GP zones, and beryllium being observed in them is  the first direct link that the improved mechanical properties may result from precipitation hardening rather than the perceived solid-solution mechanism.

[1]        Z. Liu, Y. Y. Cao, and D. Feng, "The Microstructure of the Precipitates in a Ni-Be Alloy wiht Atom-Probe FIM," Journal de Physique Colloques, vol. 48, pp. C6-343-C6-348, 1987.

[2]        I. Lazić, E. G. T. Bosch, and S. Lazar, "Phase contrast STEM for thin samples: Integrated differential phase contrast," Ultramicroscopy, vol. 160, pp. 265-280, 1// 2016.

Thomas LAGRANGE (Lausanne, Switzerland), Raffaele COSIMATI, Sorin LAZAR, Daniele MARI

08:00 - 18:15 #6351 - IM05-315 Where does the FIB sputtered matter accumulate in the SEM chamber ?

IM05-315 Where does the FIB sputtered matter accumulate in the SEM chamber ?

Scanning electron microscopes (SEM) equipped with focused ion beam (FIB) column are nowadays dual platform instruments commonly used in various micro sample preparation processes. While the mainstream of FIB usage remains within the semiconductor industry, it has expanded to a wide range of materials in metallurgical research. In particular, FIB milling capabilities are today used in the area of advanced materials for nuclear applications, involving in most cases the manipulation of activated samples. Sputtering of radioactive materials is far from trivial and numerous questions remain still open to guarantee the user safety even if small volumes of matter are generated by FIB milling [1]. For instance, where does the FIB sputtered matter accumulate in the SEM chamber?

Angular distributions of FIB sputtered atoms have been investigated for 0°, 30° and 54° ion beam incidence. FIB patterns have been milled on a nanocrystalline Nickel alloy using a 30 keV Ga⁺ ion bombardment in a scanning electron microscope (Fig 1a). Sputtered matter is collected on a silicon planar collector. The thickness of the deposit on the collector is measured by means of Z-contrast imaging and two dimensional spatially resolved thickness maps are drawn (Fig 1b and Fig 1c). Angular distributions of sputtered atoms, in a plane containing the primary ion beam, are deduced from the maps (Fig 3). Our experimental data show that, at oblique incidence, sputtering matter is emitted in two main emission directions, normal to the target surface and towards the FIB column. For normal incidence, all the sputtered matter is projected towards the FIB column. The microstructure of the deposit, at normal incidence, has been studied by Transmission Electron Microscopy and Atom Probe Tomography (Fig 2).

[1] Contamination analysis of radioactive samples in focused ion beam instruments A.R. Evelan, Richard R. Brey, Health Phys. 104, 2 Suppl 1(2013) S23-30

Emmanuel CADEL (ST ETIENNE ROUVRAY CEDEX), Fabien CUVILLY, Charly VAUDOLON, Auriane ETIENNE

08:00 - 18:15 #6381 - IM05-317 Simulation of elastic and inelastic electron scattering in EF-HRTEM mode by back-propagation of electron beam.

IM05-317 Simulation of elastic and inelastic electron scattering in EF-HRTEM mode by back-propagation of electron beam.

In this work we present a new approach for simulation of energy-filtered HRTEM images by back-propagating the electron beam. Here, the back-propagation means that we consider the fast electrons going in the opposite way through the microscope: from a detector through a specimen, where they undergo an energy-gain process, and then they continue to the source. We show that this time-reversed approach is computationally feasible and can bring consistent results. In this new approach, real collection angle becomes a convergence angle of the simulated probe and the real convergence angle becomes the simulated collection angle, as shown in the scheme below. In this new simulation setup we use small collection angles in order to mimic nearly parallel illumination in HRTEM. The “probe”, on the contrary, has a large convergence angle, because for HRTEM image simulation wide objective apertures are used. The contrast transfer function of the imaging system is inverted by changing the sign of aberrations. This reciprocal approach is well suited to generalize the in-house code developed for energy-filtered STEM simulations using combined Multislice/Bloch-waves approach [1]. We can now switch between different simulation modes, e.g., energy-filtered TEM and STEM, by employing different sets of parameters in the code. It allows to avoid multiple multislice evaluations at each atom position, instead it requires separate calculation for each pixel of the desired EF-HRTEM image, which is often a smaller computational cost. The mathematical proof of the concept will be outlined. Simulated energy-filtered HRTEM images of selected materials, such as STO or LSMO based on this approach will be presented as well.

[1] J. Rusz, S. Bhowmick, M. Eriksson, N. Karlsson, Phys. Rev. B 89, 134428 (2014).

Dmitry TYUTYUNNIKOV (Uppsala, Sweden), Axel LUBK, Jan RUSZ

08:00 - 18:15 #6493 - IM05-319 Correlation of interface morphology and composition in GaInP/GaAs with growth conditions.

IM05-319 Correlation of interface morphology and composition in GaInP/GaAs with growth conditions.

Ternary (GaIn)P materials ststems grown on GaAs have attracted a lot of attention for laser applications, especially due to the low recombination velocities at the interface [1]. The physical properties of the interface are greatly influenced by the interface morphology, which can be controlled by either the growth temperature or growth interruptions or the introduction of interlayers, consisting e.g. of GaP. The interface morphologies of the semiconductor quantum wells can be characterized by the quantitative evaluation of high resolution high angle annular dark field (HAADF) images in scanning transmission electron microscopy (STEM). In the present work, quantitative evaluation of HAADF imaging in STEM is used to correlate the interface morphology and composition in (GaIn)P grown on GaAs with the growth conditions.

 

The (GaIn)P/GaAs QWs were grown with metal organic vapor phase epitaxy (MOVPE) on GaAs (001) substrate at temperatures of 525°C and 625 °C, respectively, with different growth interruption times with or without GaP interlayer. In order to be able to compare different samples, a carefully applied method to gain reliable results from high resolution STEM micrographs was used. Also, to derive the chemical composition maps, the chemical sensitive background intensity is subtracted after image normalization as shown in Figure 1. From the composition maps, the interface features are revealed and then correlated with the growth conditions. The growth interruptions can significantly affect the composition fluctuation and the interface morphology. At higher temperature of 625 °C, with the two GaP monolayers between (GaIn)P and GaAs substrate, shorter growth interruption time leads to intermixing at the interface while a longer growth interruption results in a sharp interface. Also, without the GaP buffer layer, platelet islands can be observed at the interface. At lower temperature of 525 °C the GaP buffer layer has less influence. Hence, the quantitative evaluation of HAADF STEM images can reveal the interface morphologies, which also have important influence on the optoelectronic properties.

Acknowledgement

We gratefully acknowledge financial support of the DFG in the frame work of SFB1083.

References

[1] J.M. Olson, R.K. Ahrenkiel, D.J. Dunlavy, B. Keyes and A.E. Kibbler, Appl. Phys. Letters 55 (1989)

Han HAN (Marburg, Germany), Andreas BEYER, Jürgen BELZ, Alexander KÖNIG, Wolfgang STOLZ, Kerstin VOLZ

08:00 - 18:15 #6600 - IM05-321 Direct imaging of hydrogen atomic columns in hydride phases in titanium grade 2.

IM05-321 Direct imaging of hydrogen atomic columns in hydride phases in titanium grade 2.

Despite diligent work on titanium hydrides the crystal structures of some titanium hydride phases are still not well established. Transmission electron microscopes (TEMs) can support crystallographic data by direct imaging of unknown structures with superior resolution. Direct imaging of hydrogen atomic columns is extremely challenging due to their weak scattering. However, an introduction of TEMs with corrected spherical aberration has made such experiments feasible. In this work, direct imaging of hydrogen atomic columns in titanium hydrides was performed in order to provide additional support to incomplete crystallographic data for titanium hydrides.

 

The commercially pure titanium grade 2 annealed at 900 °C for 1 h was examined in this work. Samples for transmission electron microscopy (TEM) were prepared by mechanical grinding of 3 mm discs and subsequent thinning by twin-jet electropolishing in perchloric acid solution using Struers TenuPol-5. The titanium grade 2 contains 0.24 at.% of hydrogen, which can create hydride phases. Face-centered tetragonal γ-hydride phase was found in a lentil-like shape in the α-Ti matrix and in a shape of long thin lamellae at either the α-Ti matrix/β-Ti phase interface or originating at α-Ti matrix grain boundaries. This phase introduces hydrogen-induced diffracted reflections that do not coincide with the diffracted reflections coming from Ti atoms. For that reason, the presence of the hydrogen can be observed even using the selected area electron diffraction (SAED) technique. The hydrogen-induced reflections translate also to a phase-contrast pattern (PCP). Compared to the titanium columns, the hydrogen columns produce much broader peaks/dips that enable us to differentiate between the positions filled with hydrogen and the vacant columns in the structure.

Annular bright-field scanning transmission electron microscopy (ABF-STEM) and high resolution transmission electron microscopy (HRTEM) techniques were employed. As both techniques rely on phase-contrast imaging, they can produce imaging artifacts that can be easily mistaken for genuine structural features caused by the presence of the hydrogen. To increase robustness of our results we support them with simulations based on theoretical models of the hydrides. Multislice method was used to simulate the dynamic scattering effects. This helped us to find optimal imaging conditions with regard to resolution, maximal contrast for the hydrogen positions and the absence of imaging artifacts for a wide range of thickness and defocus values.

 

Acknowledgements:

The project was supported by GACR GBP108/12/G043.

Kamil DANĚK (Prague, Czech Republic), Viera GÄRTNEROVÁ, Martin NĚMEC

08:00 - 18:15 #6616 - IM05-323 EBSD measurements on the weld seam area of differently extruded ME21 hollow profiles.

IM05-323 EBSD measurements on the weld seam area of differently extruded ME21 hollow profiles.

The production of hollow profiles using porthole dies is a widespread manufacturing process for aluminum alloys. However, the extrusion of magnesium and its alloys into hollow profiles has not yet been established due to several reasons. One thereof is the lack of knowledge about the interdependencies between process parameters, die design and the product quality (microstructure, texture, mechanical properties etc.). Therefore, different extrusion experiments were carried out using the magnesium alloy ME21 (2.1 wt% Mn, 0.7 wt% Cer, Mg balance). For the realization of hollow profiles with varying wall thickness and thus a varying extrusion ratio (ER) a modular porthole die (Fig. 1) has been used. It consists typically of a mandrel part and a die part. The die part defines the outer shape of the extrudates and includes the welding chamber. The mandrel part consists of a mandrel that forms the inner shape of the hollow profile while the mandrel itself is kept in place by three bridges. Through the usage of three differently dimensioned mandrel parts the variation of the wall thickness (and the ER) can be done while the outer dimensions are kept constant for each profile. Extrusion ratios of ER = 8:1, 16:1 and 30:1 were applied.

During the extrusion process the billet material is split up into three separate metal streams which flow around the bridges of the mandrel part and successively weld under solid-state conditions behind the bridges in the so-called welding chamber. These welds are called longitudinal weld seams. It is known that there is an additional amount of straining of the material that forms these weld seams due to the friction. Hence, these weld seams always pose a potential anomaly when compared to the weld free material.

First analysis using polarized light-optical microscopy indicated that both sides of the weld seam (representing each metal stream) have different textures. In order to quantify that observation EBSD is the appropriate technique to gather exact information of the weld line, which displays the separating line between the two visible parts of the weld seam and of the material in the vicinity. The EBSD measurements were performed on a ZEISS DSM 982 with Gemini optic and the EDAX Hikari camera. EDAX also supplied the necessary “OIM Data collection” and “OIM Data analysis” software. The resulting maps have the dimensions of 800 µm x 650 µm with a resolution of 0.5 µm. For better focusing at low magnification and the largest aperture the samples were tilted to 67° instead of 70°. Beforehand the samples were ground with SiC abrasive paper and then polish with 6 µm, 3 µm and 1 µm diamond-based suspension following a chemical polish with CP2 agent. The IPF maps were taken at half wall thickness on a plane parallel to the profile surface.

The first EBSD measurements reveal for all extrusion ratios a pronounced weld line dividing the weld seam in two parts featuring sub-textures whereby the orientation of the {0001}-planes of one metal stream is mirror-inverted to the other. The texture is also significantly different from that of the weld free material.

In future work the focus will be among others on the die design, especially the shape of the bridges, in order to modifiy the local weld seam texture and to optimize the overall properties of the hollow extrudates.

Christoph FAHRENSON (Berlin, Germany), Felix GENSCH, Sven GALL, Dirk BERGER

08:00 - 18:15 #6803 - IM05-325 Im2Cr: An efficient tool for crystallographic indexing of HR(S)TEM images.

IM05-325 Im2Cr: An efficient tool for crystallographic indexing of HR(S)TEM images.

      Transmission Electron Microscopy (TEM) and Scanning TEM (STEM) have been widely used to characterize nanostructured materials with atomic resolution, and significant advances on their experimental setup greatly extended the current pool of analysis possibilities at the nanoscale. The exploration of advanced (S)TEM characterization capabilities and their reproducible application to reach a suitable sampling is often restricted by the extensive data analysis procedures required to reliably interpret experimental results and to extract quantitative information. Even routine tasks such as nanoparticles crystallographic indexing from electron diffraction patterns or from high resolution (S)TEM images are mostly carried out manually by the users, resulting in a reduced TEM characterization yield and significant user bias.

      This work presents Im2Cr, a new software tool to aid the crystallographic indexing of nanostructured materials using high resolution (S)TEM images. Im2Cr implementation aims for a minimal user interaction, supporting the detection of zone-axis oriented particles, and including an efficient peak detection process applied to the images Fourier Transform (FT). With basis on the FT peaks distances and relative angles, crystallographic indexation is carried out autonomously via comparison with a list of candidate structures named by the user, and a ranking of the best matching combinations of crystallographic structures and viewing zone axes is generated.

       Im2Cr was successfully tested for robustness and execution efficiency in a wide range of High Resolution (S)TEM images from crystalline nanomaterials, with domain size ranging from 4 to 100 nm. The autonomous indexation with preset parameters has a very high success rate, and runs in a small fraction of typical (S)TEM images acquisition time by taking advantage of the inherent hardware parallelism. Alternatively, the user can operate Im2Cr in a semi-autonomous mode and control relevant parameters related to the region of interest (ROI) selection on the (S)TEM image and on the FT peaks detection.  Im2Cr promising results point to the possibility of real-time image analysis with reduced user interaction, allowing for an increased (S)TEM characterization yield and also enabling the interpretation of complex images, such as those from nanocrystalline materials imaged in high-order zone axis orientations.

André SILVA (Braga, Portugal), Enrique CARBÓ-ARGIBAY, Alberto PROENÇA, Daniel STROPPA

08:00 - 18:15 #6804 - IM05-327 Ion imaging in a Focused Ion Beam microscope: modeling the channeling contrast to construct EBSD-like orientation maps.

IM05-327 Ion imaging in a Focused Ion Beam microscope: modeling the channeling contrast to construct EBSD-like orientation maps.

Electron backscatter diffraction (EBSD) is routinely employed as a characterization tool to obtain individual grain orientations, local texture and phase identification. Efforts are currently being made to optimize the compromise between speed of indexation, spatial and angular resolution, quality of phase recognition and dimensions of the field-of-view. In this context, any new technique that can lead to a better compromise would be welcome.

With this in mind, we proposed recently the iCHORD method (for ion CHanneling ORientation Determination), aiming at constructing orientation maps based on the well-known channeling contrast phenomenon observed in a rotation series of ionic images (see figure 1) [1]. The proof-of-concept of the iCHORD method was obtained by predicting the intensity loss received by a detector for specific orientations of a crystal thanks to crystallographic calculations. However, the number of parameters to adjust when applying the technique to different materials was quite high. In order to improve the method and make it more versatile, a new modelisation of the channeling contrast is proposed in the present study.

Experimentally and theoretically, an intensity loss is observed for a crystal when the ion beam arrives parallel to some low index crystallographic planes, and particularly when the sample is in a zone axis. Therefore, if the atomic structure of the crystal is projected onto a surface perpendicular to the ion beam, an intensity loss will corresponds to large “free spaces” between the atomic columns (see figure 2). In other words, the intensity loss for a specific crystal orientation is closely related to the sum of the grey levels of the pixels constituting the projection. More quantitatively, to avoid some projection artefacts, it is necessary to adjust the number and disposition of atoms to be considered, as well as their interactions with the ion beam as a function of their depth to obtain an efficient model of the channeling effect. As the atomic positions of atoms are used, only the structure file of the material under consideration has to be provided, without specific calculations for cubic or hexagonal structures for instance. Only the parameters regarding the ponderation function have to be adjusted, which is quite straightforward, hence meeting the versatility required. In this framework, theoretical intensity profiles obtained by varying the orientation of a crystal (see figure 3) fit quite well with experimental profiles obtained by varying the sample orientation in the same way.

This improvement in the modelisation of the channeling contrast has been embedded in the workflow aiming at obtaining orientation maps without the use of the EBSD technique. Together with new developments regarding the sampling of the orientation space and the indexation algorithm, a new version of the iCHORD method is released. The impact on the angular resolution as well as speed of indexing are discussed. The benefits of using the iCHORD method in place of the EBSD technique are also summarized.

 

References:

[1] Crystal Orientation Mapping via ion channeling: an alternative to EBSD

C. Langlois, T. Douillard, H. Yuan, N.P. Blanchard, A. Descamps-Mandine, B. Van de Moortèle, C. Rigotti, T. Epicier, Ultramicroscopy 157 65-72 (2015)

Cyril LANGLOIS (Villeurbanne Cedex), Thierry DOUILLARD, Sébastien DUBAIL

08:00 - 18:15 #6806 - IM05-329 Quantitative image analysis of binary microstructures: Application to the characterisation of dairy systems.

IM05-329 Quantitative image analysis of binary microstructures: Application to the characterisation of dairy systems.

Modern microscopy devices give the possibility to obtain images of high resolution at various scales and of a large variety of samples, organisms and materials. However, the quantitative analysis of these images remains a challenge. The reasons are both the difficulty to automatically identify the structures of interest within the images and the lack of appropriate tools or methods to quantitatively describe such structures. In particular, when the structure of interest does not appear as a set of individual particles but as an interconnected network, conventional shape or size distribution analyses are no longer adapted. Examples are numerous in materials science and can be also found in other areas such as molecular biology and food science. Figure 1 shows images of dairy gels observed by transmission electron microscopy (TEM), and the result of their segmentation into two phases: the protein (dark) and the void (white) phases.

The aim of this contribution was to explore several mathematical tools to describe TEM images that can be represented by binary microstructures, and to show how they can be applied to the characterisation of dairy systems. The application to other domains of application will also be discussed.

A first group of methods is based on intrinsic volumes, which gives access to morphometric features such as boundary density, volume ratio and Euler number density. The covariance function can describe the microstructure using probabilities of inclusion within the set, and can be related to global morphometric features. The computation of granulometric curves based on mathematical morphology provides size distributions that can be interpreted as thickness distribution or as the pore size distribution (Fig. 2). The tortuosity describes the ability to travel between two points within one binary phase, and quantifies long-range morphology (Fig. 3).

In the present work, the combination of different morphometric features could discriminate TEM images of dairy systems obtained from different technological processes. More advanced descriptive features have been explored, involving modelling of aggregated particles and of diffusivity properties in the void phase of the network. 

David LEGLAND (Nantes), Juliana V.c. SILVA, Chantal CAUTY, Kolotueva IRINA, Julianne FLOURY

08:00 - 18:15 #6836 - IM05-331 The interior of the rotavirus capsid.

IM05-331 The interior of the rotavirus capsid.

Rotavirus and other double-stranded RNA viruses confine and protect their genome inside the capsid even after cell entry, hiding viral ds-RNA from the cellular immune system. Thus, Rotavirus must be able to perform transcription and replication of its packed genome within a virus core formed by viral proteins by using a complex composed by the viral ds-RNA and the RNA-dependent RNA polymerase. For transcription, the resulting mRNAs has to exit the virus through channels in the capsid protein shells. For replication, viral mRNAs must enter the newly formed particles to be used by the polymerases as templates for dsRNA synthesis.

The symmetry mismatch between the symmetric capsid and the asymmetric complex formed by the polymerase and the ds-RNA was used to reconstruct in 3D the interior of the rotavirus capsid showing for the first time how the ds-RNA is organized and how it interacts with the polymerase.

Rotavirus has eleven different ds-RNA segments and 11 to 12 polymerases inside each viral particle. Each polymerase is located underneath each one of the 12 icosahedral vertices [ref1].

The 3D reconstruction with icosahedral symmetry imposed obtained from cryo-EM images of rotavirus was used to calculate synthetic 2D projections which were subtracted from the corresponding experimental images. This allowed the in-silico isolation of the asymmetric parts of the structure by signal subtraction (Fig.4). The geometrical relationship between symmetric and asymmetric parts is known (assuming stable relative positioning) and it was is used to convert each one of the views associated to the symmetric 3D reconstruction into 12 sets of 5 views corresponding to the isolated asymmetric parts (5 views per icosahedral vertex). These 5 views were tested by projection matching [ref2] against a low-resoltuion initial model and only the view giving the highest correlation coefficient was used to reconstruct an asymmetric structure by using the sub-images without imposing symmetry.

References:

1. Estrozi, L.F. et al. Location of the dsRNA-dependent polymerase, VP1, in rotavirus particles. J Mol Biol. 425(1):124-32. (2013).

2. Estrozi LF, Navaza J. Fast projection matching for cryo-electron microscopy image reconstruction. J Struct Biol. 162(2):324-34 (2008).

Leandro F. ESTROZI (Grenoble Cedex 9)

08:00 - 18:15 #6845 - IM05-333 High Resolution Scanning Electron Microscopy study of Au nanocrystals on Si nanowire surfaces.

IM05-333 High Resolution Scanning Electron Microscopy study of Au nanocrystals on Si nanowire surfaces.

Nanowires (NWs) are a typical example of nano-objects that can be functionalized in order to obtain multiple applications: transistors, solar cells, lasers, gas sensors ... Moreover, Park et al. [1] showed that silicon NWs coated by gold nanocrystals (NCs) may be used in photothermal therapy in vivo where cancer cells are captured and destroyed. By controlling the size and the density of the superficial gold NCs population one can obtain a sufficient plasmon coupling and thus achieve an even unprecedented efficiency. The development, control and optimization of these functional nanomaterials therefore require advanced characterization tools able to quantify these nano-object populations. In this context, TEM has remained for decades the only imaging technique with sub-nanometer resolution. But the small fraction of the analyzed region with respect to the sample size makes it difficult to obtain representative measurements from a statistical point of view. On the other hand the high resolution achieved by modern SEMs allows them to compete clearly with TEM.

 

In a previous work [2], we showed the first results obtained with a Zeiss Gemini 500 ultra-high resolution FESEM just installed in CP2M at the Aix-Marseille University. By combining low kV In-lens SE and Energy Selective Backscattered (EsB) imaging to EBSD and Annular BF-DF-HAADF-STEM, we demonstrated the possibility of obtaining, in the same instrument, topographical, chemical and structural information from the same NW with a resolution as good as the STEM-HAADF one [3].

 

Figures 1 and 2 show examples of In-Lens SE images obtained at 1kV on a MBE Si NW grown on Si(111) with Au as catalyst. The saw-tooth faceting of one over two faces of the NW is visible with high resolution. Furthermore, one can see that it is also possible to resolve nanometric gold nanocrystals and their non-homogeneous repartition onto the different surfaces.

 

The outstanding quality of this SE image, with a high signal to noise ratio, demonstrates not only the possibility to image these nano-objects but also to quantify them. Indeed, we will show that coupling low kV mode to image processing and analysis allows studying the nanometric object population with a resolution better than one nanometer, i.e. even better than the nominal resolution of this microscope at this acceleration voltage.

 

This methodology can be applied to perform a systematic study of the distribution of gold NCs on the substrate and on different facets of the nanowires, and can be extended to any problem involving nano-objects quantification.

 

 

References:

[1] G-S. Park et al., Full Surface Embedding of Gold Clusters on Silicon Nanowires for Efficient Capture and Photothermal Therapy of Circulating Tumor Cells, Nano Lett. 12, (2012) 1638−1642. DOI: 10.1021/nl2045759

[2] C. Alfonso et al., Low kV high resolution Scanning Electron Microscopy study of silicon nanowires surfaces

Microscopy and Microanalysis, 21 (Suppl.3) (2015) 1261-1262. DOI: 10.1017/S1431927615007096

[3] T. David et al., Gold coverage and faceting of MBE grown silicon nanowires, J. Cryst. Growth 383 (2013) 151-157. DOI: 10.1016/j.jcrysgro.2013.08.023

Claude ALFONSO (Marseille), Andrea P.c. CAMPOS, Christian DOMINICI, Sidnei PACIORNIK, Luc ROUSSEL, Loïc PATOUT, Lyuang HAN, Fang ZHOU, Ahmed CHARAÏ

08:00 - 18:15 #6864 - IM05-335 Nano-scale strain measurements from high-precision ADF STEM.

IM05-335 Nano-scale strain measurements from high-precision ADF STEM.

    High-resolution annular dark-field imaging in the scanning transmission electron microscope (ADF STEM) offers a powerful and readily interpretable mode for materials analysis at the atomic scale. However, like all serial (scanned) imaging techniques subtle disturbances in the instrument’s surroundings can lead to deleterious artefacts in the recorded data. Recent developments in non-rigid registration of multi-frame data allows a route to mitigate these scanning errors [1] and obtain accurate strain information in the STEM [2].

    Here we make use of these new multi-frame techniques, but go further and use experiment design to optimally fractionate an allotted total electron-budget to achieve maximum precision. For the conditions used this occurs at around 20-25 ADF frames. Using these optimised conditions we record data from two specimens as proof-of principle examples; a rod-like AlMgSi precipitate in an Al matrix and a Pt₃Co dealloyed nanoparticle.

    In the case of the Al precipitate, Figure 1, the wide field-of-view allows geometric phase analysis (GPA) to be used to analyse the strain field [3]. At a 0.6nm resolution we achieve a strain precision of 0.3%. The accuracy of these results were confirmed through comparison with DFT simulation.

    For the Pt₃Co nanoparticle GPA is no longer appropriate owing to its limited size and the need for atomic resolution detail. Here a real-space approach was used where atomic-column positions were compared to sites defined by a pair of base vectors. The offsets from these are shown in Figure 2. Here we see the last monolayer and a half exhibit an expanded lattice parameter consistent with a platinum enriched outer shell.

Acknowledgments

    The research leading to these results has received funding from the European Union Seventh Framework Programme under Grant Agreement 312483 - ESTEEM2 (Integrated Infrastructure Initiative–I3) and NORTEM (Grant 197405) within the programme INFRASTRUCTURE of the Research Council of Norway (RCN). NORTEM was co-funded by the RCN and the project partners NTNU, UiO and SINTEF. AV was financially supported by Johnson Matthey. Computational resources were provided by the Notur consortium. The authors acknowledge Eva Mørtsell for providing the Al alloy specimen.

References

[1]         L. Jones, H. Yang, T. J. Pennycook, M. S. J. Marshall, S. Van Aert, N. D. Browning, M. R. Castell, and P. D. Nellist, Adv. Struct. Chem. Imaging 1, 8 (2015).

[2]         L. Jones, S. Wenner, M. Nord, P. H. Ninive, O. M. Løvvik, R. Holmestad, and P. D. Nellist, [in press]. (2016).

[3]         M. J. Hÿtch, E. Snoeck, and R. Kilaas, Ultramicroscopy 74, 131 (1998).

Lewys JONES (Oxford, United Kingdom), Aakash VARAMBHIA, Sigurd WENNER, Magnus NORD, Per Harald NINIVE, Ole Martin LØVVIK, Randi HOLMESTAD, Peter NELLIST

08:00 - 18:15 #6891 - IM05-337 Quantitative STEM Atom Counting in Supported Metal Nanoparticles.

IM05-337 Quantitative STEM Atom Counting in Supported Metal Nanoparticles.

Transmission electron microscopy (TEM) has proven itself an invaluable tool for measuring composition, chemistry, and internal structure at the nanoscale and below. TEM micrographs, however, are inherently two-dimensional projections of the real three-dimensional object; important structural information can be lost and ambiguities introduced. Nanoparticles (NP) of different structures – which can possess dramatically different catalytic activity and effectiveness – can yield similar apparent projected sizes in micrographs [1]. The ability to distinguish between them – and, ideally, recover the 3D atomic structure – is necessary for continued improvement in catalyst design.

Quantitative STEM offers an approach for extracting this information contained in a single high-angle annular dark-field scanning TEM (HAADF-STEM) micrograph. First demonstrated nearly two decades ago by Singhal et al. [1], the recent advancements in aberration-correction for STEM spurred a resurgence in interest in quantitative STEM when it was shown the technique could be performed with atomic resolution by LeBeau et al. [2]. Through careful calibration of the microscope and image, the contrast (scattered intensity) in the micrographs can be explicitly related back to the number of atoms involved in the scattering through comparison with image simulation, yielding information about the 3D structure [3] and composition [4] of the specimens.

The current quantitative STEM work has been performed using microscopes equipped with Schottky field-emission gun (FEG) electron sources. The highest-performance analytical STEMs, however, are equipped with cold FEG (CFEG) sources, since CFEGs offer superior spatial and temporal coherences, yielding higher spatial and energy resolutions for imaging and spectroscopy. This performance gain comes at the cost of reduced stability of the emission current, which, in a CFEG, decays in a continuous, non-linear fashion even image-to-image. This poses a challenge for quantitative STEM, as one of the key calibrations requires knowledge of the incident beam current to normalize image intensities into units of fractional beam current for comparison with image simulations [2,5].  In this presentation we will discuss our method to overcome this issue by adapting the condenser aperture of a Hitachi HD-2700C STEM to act as a beam monitor to measure the incident probe current in real-time concurrent with STEM image acquisition. This method enables a more accurate calibration of intensity to be achieved on microscopes with CFEGs.

We are also adapting quantitative STEM to enable its use on a conventional, non-aberration-corrected TEM/STEM – a JEOL JEM2100F – without the need for any special modifications or attachments, as most university facilities do not possess such aberration-corrected instruments with corresponding expensive service contracts. The lower magnifications (2-4 MX) used in this approach means that a much larger number of nanoparticles can be present in each micrograph, such as in Figure 1a, enabling robust statistics about particle size, shape, and monodispersity to be gathered from hundreds to thousands of nanoparticles. We have developed a program for MATLAB to automatically perform the quantitative STEM analysis on batches of micrographs. For each nanoparticle, the optimal intensity integration cut-off radius is calculated via an iterative process that determines when the scattered intensity from the NP is fully enclosed, Figure 1b [6]. It should be noted that this approach can be paired with analysis of selected NP at atomic-resolution studies, to gain the benefit of both robust statistics and atom-level characterization.

References:

[1] A Singhal, et al., Ultramicroscopy 67 (1997), p. 191-206

[2] JM LeBeau, et al., Physical Review Letters 100 (2008), p. 20601

[3] L Jones, et al., Nano Letters 14 (2014), p. 6336-6341

[4] A Rosenauer, et al., Ultramicroscopy 109 (2009), p. 1171-1182

[5] H E, et al., Ultramicroscopy 133 (2013), p. 109-119

[6] This work was supported by DOE BES through grant DE FG02-03ER15476, and performed using the facilities at the Center for Functional Nanomaterials at Brookhaven National Laboratory, which is supported by DOE BES through contract DE-SC0012704.

Judith YANG (Pittsburgh. PA, USA), Stephen HOUSE, Yuxiang CHEN, Dong SU, Tom SCHAMP, Russell HENRY, Eric STACH, Rongchao JIN

08:00 - 18:15 #6977 - IM05-339 Effect of amorphous surface layers on HRTEM image evaluation.

IM05-339 Effect of amorphous surface layers on HRTEM image evaluation.

The possibility to image atomic columns with high contrast and minimum contrast delocalisation in aberration corrected transmission electron microscopes opened new perspectives to analyse the structural property relationships in crystalline solids. The analysis of composition in strained heterostructures, the structure of ferroelectric domains, or of extended defects are some recent examples. For all these applications the ultimate precision in analysis of atomic positions is required. To measure compositional fluctuations in InGaN quantum wells with a precision of 1% requires a measuring precision of atomic column positionss of 0.5 pm. While the influence of the electron detection system, has attracted attention in the last years and effects like the modulation transfer function and shot noise has been in the focus of some papers, e.g. [1], the effect of amorphous surface layers has attracted less attention. This is the more astonishing since TEM samples are needed to stay in the weak phase object approximation for direct imaging. For such a few nanometer thick samples amorphous surface layers influence the image contrast as they contribute noticeable to the overall phase of the exit wave. We study the influence of amorphous surface layers in detail based on the analysis of image series of thin GaN samples.

Amorphous layers are observable as a fluctuating contrast underneath the high-resolution lattice pattern. The analysis of the unit cell parameters shows that the unit cell parameter in the single image fluctuates by 4.5 pm. In image series we see that the unit cell parameters at a given location fluctuates statistically and gives the impression of 'jumping' atomic columns, i.e. a distortion of the unit cell parameter. The STD drops down to 1.5 pm when averaging about all N = 30 images although a reduction to 0.84 pm is expected for random errors ( figure 1). In simulations with shot noise and MTF the highest STD in single images is already smaller than 1 pm. This indicates that the amorphous layers remains the main source for the measurement error. 

To estimate the origin of the static error in the measurements caused by amorphous layers a systematic analysis was carried out with the aforementioned GaN structure. Using experimental amorphous contrast from the edge of a cross section sample (figure 2) as a model for the noise in image simulation the same reduced improvement in the STD as in the experiment can be observed (figure 1). One could suspect the non vanishing correlation in the amorphous contrast between successive images to be responsible for the static error in the measurement. Therefore uncorrelated patches of amorphous contrast pattern were used in a further image simulation. The result shows again the reduced improvement of the STD, thus we suspected that the source of the static error must lies in the radial distribution function of the amorphous layer. Looking at the frequency distribution in the power spectra of amorphous and crystalline areas it is noticeable that the broader distribution underlying the sharp peaks of the crystalline material is similar to the frequency distribution of the amorphous area. This broad frequency distribution is the result of the nearest neighbour distances in the amorphous network which are in the same range as the inter-atomic distances in crystalline material. But as the direction of the nearest neighbour bonds are irregular in the amorphous network the projected distances have a broader distribution. 

Using a random pseudo amorphous network neglecting nearest neighbour distances in a simulated series the STD of the c-lattice parameter measurement follows the random error rule. By adjusting the amorphous network to inter-atomic distances a reduced drop in the STD with increasing N is obtained again. In conclusion amorphous surface layers are the limiting factor of relative position measurements in HRTEM lattice patterns on local scale because of the similar nearest neighbour distances in amorphous and in crystalline material which lead to a static measurement error. As a consequence of the similar distances filtering in the frequency domain is not useful to improve local measurements. Instead improvements in the reduction of amorphous surface layers in the sample preparation has to be found. On the other hand measurements of longer distances well above nearest neighbour distances as for example in case of superstructures or ordering phenomena should not be affected by the static error caused by amorphous layers as the correlation is limited to nearest and next nearest neighbour distances.

[1] T. Niermann , J.B. Park, M. Lehmann, Ultramicroscopy 111 (2011) 1083–1092

Thilo REMMELE (Berlin, Germany), Tobias SCHULZ, Martin ALBRECHT

08:00 - 18:15 #6141 - IM06-341 Determination of a 3D Displacement Field at a Vicinity of a GeSn/Ge Interface by the Phase Retrieval of Electron Rocking Curves.

IM06-341 Determination of a 3D Displacement Field at a Vicinity of a GeSn/Ge Interface by the Phase Retrieval of Electron Rocking Curves.

Strain in materials is one of the important factors affecting physical properties of the materials such as carrier mobility, dielectric property, magnetism and so on. In semiconductor industry, strain engineering has been playing a primary role for the improvement of the device performance. Measurement of strain has also been very important as a key technique supporting the strain engineering. So far, the strain measurement by diffraction technique has been done mainly by measuring the positions of diffraction peaks and by fitting the experimental peak positions to simulated ones. This method implicitly assumes that strain in the volume contributing to diffraction intensities is uniform. Strain in real materials, however, is not always uniform and varies over the diffraction volume of the specimen. In the present study, we applied convergent-beam electron diffraction (CBED) to determine such non-uniform strain, whose lattice displacement vector varies along the incident direction of the electron beam.

Lattice scattering amplitude of reflection g, φ_g, is given as a sum of scattered waves from each lattice point, which can be expressed by the Fourier transform of the lattice. If the lattice has a displacement field R(r), a phase factor of 2πg·R(r) has to be taken into account for φ_g, where r is a positional vector for a point in the crystal. In the case of convergent-beam electron diffraction, R(r) is unchanged in the direction perpendicular to the incident direction of the beam because the probe diameter is sufficiently small, and thus, R(r) can be written as R(z), where z indicate a coordinate along the incident direction. The Fourier transform of the phase factor of exp(2πig·R(z)) can be written as φ_g(s) with an excitation error s, which is a conjugate variable of z. We applied the Fourier iterative phase retrieval technique to restore the phase part of φ_g, and to determine the phase factor of the lattice displacement 2πg·R(z). In the present study, the modulus of φ_g(s) was measured from a rocking curve profile observed by a CBED pattern.

A Ge_92.9Sn_7.1 layer of 200 nm was deposited on a Ge (001) substrate by the chemical vapor deposition method in a ultra-high vacuum. Cross section samples for electron microscopy were prepared by mechanical polishing and ion-beam thining. Rocking curves were obtained by the CBED technique at an incidence inclined by about 10 degree from the [110] direction. CBED experiment was conducted by using a transmission electron microscope operated at an acceleration voltage of 200 kV. CBED patterns were taken by using Gatan imaging fileter with an energy window of 5 eV to remove inelastic scattering mainly by the plasmon loss.

Figure 1(a) shows a cross-section TEM image of the specimen. CBED patterns were taken from the positions indicated as 1 to 6 in the Ge substrate. Figure 1(b) shows a whole CBED pattern used in the phase retrieval. Figures 2(a), 2(b) and 2(c) show enlarged disks of the -26-8, -553 and -317 reflections and their rocking curve profiles, respectively. Figures 2(d), 2(e) and 2(f) respectively show phase profiles of 2πg·R(z) of the -26-8, -553 and -317 reflections as a function of the z-coordinate determined by the present study. From these phase profiles, the lattice displacements in the [001], [110] and [110] directions are determined as shown in Figures 3(a), 3(b) and 3(c), respectively. It is clearly seen that the displacement field in the [001] direction is of mirror symmetry about the center of the specimen, which is consistent to the elasticity theory. The displacements field determined by the present method is quantitatively compared to the simulated values obtained by the finite element method.

Saitoh KOH (Nagoya, Japan), Miura MASASHI, Tanaka NOBUO, Nakatsuka OSAMU, Zaima SHIGEAKI

08:00 - 18:15 #6283 - IM06-343 Structure resolution of the new phase Ba19Cr12O48.

IM06-343 Structure resolution of the new phase Ba19Cr12O48.

After nearly 30 years of research on the superconducting cuprates, the discovery of high T_c superconductivity in related iron based pnictides in 2008¹ has stimulated the search of new superconducting compounds. In particular, phases presenting 2D squared planes of 3d transition elements with antiferromagnetic interactions, like the famous cuprate YBa₂Cu₃O_7-d, are of great interest. In that sense, the n=1 (Sr₂CrO₄ (Sr214)) and n=2 (Sr₃Cr₂O₇ (Sr327)) members of the Sr_n+1Cr_nO_3n+1 Ruddlesden-Popper (RP) series, synthesized by J.A. Kafalas and J.M. Longo under high pressure - high temperature for the first time in 1972 are interesting². Only recently this Sr‑Cr‑O system has been revisited by Baikie et al. and E. Castillo-Martinez & M.A. Alario‑Franco^3,4.

To explore new Cr-based systems where superconductivity could potentially be induced by changing doping and Cr-Cr interactions, we have synthesized new chromates by replacing Sr with other alkaline earth elements.

The replacement of Sr by a smaller Ca alkaline earth has given rise to a new Ca-based chromate Ca₃Cr₂O₇ (Ca327) RP phase synthesized at 4 GPa and 1000°C⁵. When a bigger alkaline earth element, such as Ba, was used, a new unknown phase was isolated instead of the traditional RP-type phase in this conditions of synthesis; the low pressure Ba₂CrO₄ orthorhombic phase was first obtained from the solid state reaction of BaCO₃ and Cr₂O₃ at 1000°C under Ar flow, then this Ba-based precursor was treated at 1000°C under 6 GPa for 30 minutes.

Since it was impossible to solve ab initio the crystallographic structure of this new phase from powder X-ray diffraction, electron diffraction (ED) appeared to be the most suitable method. First of all the selected area electron diffraction highlighted a big cubic cell with the cell parameter a = 13.6 Å, as illustrated by the two [110] and [111] zone axis electron diffraction patterns on figure 1. The extinction conditions are consistent with a body-centered lattice. Then an ED tomography was performed manually by executing a rotation of the sample holder from +50° to -50° and recording an ED pattern each 1° step, with the application of a 1° precession. The reflections picking, 3D reciprocal space reconstruction, cell determination and reflection intensities extraction were realized with the software PETS⁶, and the structure model was obtained using charge flipping on JANA2006⁷. All the cations were properly determined as well as most of the oxygens, and the remaining oxygens were identified by fast Fourier transform difference. Then the structure was successfully refined from powder X-ray diffraction data using Rietveld method. On the final structure presented on figure 2, all Cr are situated in face-sharing octahedra.

 

1 Y. Kamihara, T. Watanabe, M. Hirano, and Hideo Hosono, J. Am. Chem. Soc. 130, 3296 (2008).

2 J.A. Kafalas and J.M. Longo, J. Sol. St. Chem. 4, 55-59 (1972).

3 T. Baikie et al. Journal of Solid State Chemistry 180, 1538-1546 (2007).

4 E. Castillo-Martinez, M.A. Alario-Franco, Solid State Sciences 9, 564-573 (2007).

5 J. Jeanneau, P.Toulemonde, M. Nunez-Reguerio et al., unpublished.

6 L. Palatinus, PETS-program for analysis of electron diffraction data; Institute of Physics of the AS CR:Prague, Czechia, (2011)

7 V. Petricek, M. Dusek, L. Palatinus, Jana2006 : Structure Determination Software Programs; Institute of Physics :Prague, Czechi Republic, (2006)

Christophe LEPOITTEVIN (Grenoble), Justin JEANNEAU, Pierre TOULEMONDE, Manuel NUNEZ-RUGUEIRO

08:00 - 18:15 #6284 - IM06-345 Pixelated STEM detectors: opportunities and challenges.

IM06-345 Pixelated STEM detectors: opportunities and challenges.

Conventionally, imaging in Scanning Transmission Electron Microscopy (STEM) has been performed using annular detectors that integrate up large fractions of the scattered electrons into a single value for each position in a scan, leading to a loss of information. Recently, advances in counting detection have enabled the development of fast 2-D pixelated detectors, such as the Medipix-3 detector used in this work.  These can be used to collect a large fraction of the scattered electrons in the back focal plane yielding a STEM diffraction pattern (Fig. 1a) for every scan position. The end result is a 4-D dataset, with two spatial sample positions and two reciprocal detector positions. This diffraction pattern contains a wealth of information, and being able acquire them at Ångström spatial resolutions enables many exciting applications, however there are many challenges in how to use and analyse these large datasets. In this presentation we focus on recent progress at University of Glasgow on pixelated STEM imaging, and how analysing different aspects of the diffraction images can yield information about the material properties. 

In standard STEM imaging, one usually gets information about the spatial dimensions orthogonal to the electron beam. By using a pixelated STEM detector, the lattice parameter parallel to the electron beam can also be extracted. This is achieved by looking using the higher order Laue zone rings (arrow in Fig. 1a). When this is combined with conventional atomic resolution STEM images, information of 3-D crystallography can be determined from just one projection. Examples will be given on how the 3-D structure of perovskite oxides has been determined.

The magnetic induction of a sample can be imaged in Lorentz mode, where the objective lens is usually turned off. In STEM mode, magnetic induction in the sample causes the electron beam to deflect through a typically small angle, 1-100μrads, which can be seen as a shift in the bright field disc. This has conventionally been mapped using the Differential Phase Contrast Technique with a split detector (e.g. into quadrants), but this suffers from additional contrast due to diffraction effects which affect intensity distribution within the bright field disc.  Pixelated detection allows an improved methodology accurate disc-shift measurements using edge-detection of the disc, which separates these disc shifts from diffraction contrast more robustly. The resulting imaged magnetic induction in a patterned FeAl film is shown in Fig. 1b, which visualizes ferromagnetic domains in a nanostructure created using focused ion beam nanopatterning.

Since the 4-D datasets contain the full diffraction patterns, it is possible to create virtual apertures in post-processing. This allows the construction of arbitrary shaped “detectors”. Making it possible to get HAADF, MAADF, LAADF, ABF and BF from the same dataset. Such a virtual ADF-aperture is shown in Fig. 1c, for gold deposited on a carbon film.

We will also describe how this detector can be used to determine ordering in amorphous materials using a fluctuation electron microscopy based method, and Figure 1d shows one diffraction pattern from a series of ~ 2000 diffraction patterns taken on a thin ~5 nm film of amorphous MoSi_x for use in a superconducting nanowire single-photon detector (SNSPD).  The use of the Medipix detector has significant advantages over earlier CCD detectors, due to the absence of electronic detection noise, meaning that the statistics are much cleaner and more interpretable at lower beam doses, and thus higher acquisition rates.  The resulting variance plots and conclusions about short- and medium-range ordering in the material will be briefly summarised.

In conclusion, we will demonstrate a range of new and interesting applications for pixelated detectors in STEM, which allow new or improved imaging modes and the improved extraction of information relevant to the understanding of the nanoscale or atomic scale structure of materials, nanostructures and devices.

Acknowledgements

This research has been supported by the EPSRC through the provision of a research grant (Fast Pixel Detectors: a paradigm shift in STEM imaging, EP/M009963/1) and the award of a CDT studentship to AD.  RHH acknowledges support via EPSRC grants EP/I036273/1, EP/L024020/1 and EP/M01326Z/1, and a European Research Council Consolidator Grant

Ian MACLAREN (Glasgow, United Kingdom), Magnus NORD, Andrew ROSS, Matus KRAJNAK, Martin HART, Alastair DOYE, Damien MCGROUTHER, Rantej BALI, Archan BANERJEE, Robert HADFIELD

08:00 - 18:15 #6369 - IM06-347 Reflection profile and angular resolution with Precession Electron Diffraction.

IM06-347 Reflection profile and angular resolution with Precession Electron Diffraction.

Template matching has proved to be an efficient numerical approach to identify orientation and/or  phase signatures in electron diffraction patterns [1]. With this technic, all patterns for all orientations and all phases considered are pre-calculated and compared to the experimental data by cross-correlation. The capacity of the approach to recognize the diffracting signal is substantially improved with precession electron diffraction (PED). This is because dynamic effects are partly swept out of the experimental signal. Of interest is the fact that, up to now, templates were always computed without including precession. In particular the specific reflection profile resulting from the beam rotation was never considered properly. The present work is an attempt to enhance the degree of matching by adapting the templates to precession in particular for large precession angles.

 

First, the Bragg spot profile is determined for Si single crystal by tilting progressively the sample. The pattern collection procedure is similar to the fast diffraction tomography setup of Gemmi et al [2]. The precession angle was monitored with the Nanomegas Digistar P1000 attachment to a JEOL 2100F TEM and set up to 0.3, 0.6 and 1.2°. The crystal orientation evolution is followed with the ASTAR system through template matching and compared to the quasi linear trend expected from the constant angular speed of 1°/min imposed by the TEM goniometer.

 

The change in reflection profile with increasing precession angle is illustrated in figure 1. The profile exhibits two maximums separated by twice the precession angle. This  shape, systematically recorded,  is modeled and introduced in the template generation routine. The improvement in angular resolution is characterized by comparing the misorientation measurements with respect to the expected linear trend (Fig 2).

 

The templates computed with the recorded PED profile, differ significantly from the standard ones (Fig 3). The resulting set of templates being closer to the diffraction patterns acquired with precession, the crystal orientation is determined with an increased accuracy (Fig. 2.b), at least at large precession angles. The standard deviation obtained without and with precession adaptation is nearly identical and equal to 0.17° for precession angles lower or equal to 0.6°.  By contrast, this value degrades down to 0.5° at the largest precession angle if no precession correction are included (Fig. 2) but remains equal to 0.17° with adapted templates.

Besides, the orientation resolution may be further refined by using the interpolating algorithm presented elsewhere [3]. With adapted templates and interpolation the  standard deviation decreases down to 0.13°, for the largest precession angle. A non-intuitive conclusion of the present work is that the angular resolution of the orientation determination is not bounded by the precession angle.   

 

Acknowledgements

 

The authors acknowledge TEM facilities of the CMTC characterization platform of Grenoble INP supported by the Centre of Excellence of Multifunctional Architectured Materials "CEMAM" n°AN-10-LABX-44-01 funded by the "Investments for the Future" Program.

 

[1] E.F. Rauch, M. Véron, Mater. Charact. 98 (2014) 1–9.

[2] M. Gemmi, M.G.I. LaPlaca, A.S. Galanis, E.F. Rauch, S. Nicolopoulos, J.Appl.Cryst. 48(2015)1-10.

[3] E. F. Rauch and M. Véron, Microscopy and Microanalysis 01/2010; 16:770-771.

Edgar RAUCH, Gilles RENOU (Grenoble), Muriel VERON

08:00 - 18:15 #6526 - IM06-349 3D characterization using transmission electron diffraction, neural network optimization, and density functional theory.

IM06-349 3D characterization using transmission electron diffraction, neural network optimization, and density functional theory.

Three-dimensional characterization using the transmission electron microscope (TEM) can reveal complex nanoscale structural and chemical properties. Because the TEM generates two-dimensional images and diffraction patterns, an inversion algorithm is necessary to retrieve the three-dimensional specimen. An inversion algorithm we have developed includes multiple scattering, and thus can provide three-dimensional nanoscale characterization of crystals from TEM diffraction data using artificial-neural-network optimization tools and GPU-accelerated matrix exponentials - we have previously reported retrieving strain and ferroelectric polarization on simulated data [1-2]. Some algorithms reconstruct individual atomic positions [3]; our algorithm retrieves crystal properties and is suitable for larger specimens and structures. Mapping ferroelectric polarization domains and strain state variations in arbitrary geometries as a function of specimen depth with nanometer-scale resolution can enable novel nanoscale analytical insights for a wide range of crystalline materials, including quantifying 3D structures and understanding surface-induced artifacts.

However, our algorithm requires accurately modeling a layered crystalline specimen for Bloch-wave-type calculations. The conventional approach - using isolated-atom scattering factors (IASF) - is fast, but neglects chemical bonding, while directly fitting the Ug structure-factors might result in accurate pattern replication, but with drawbacks for this three-dimensional application - direct-Ug-fitting greatly increases the number of free parameters, and thus likely decreases the precision of each individual parameter while simultaneously complicating analysis [4]. The specifics of this problem enable a third method - density functional theory (DFT) - which provides self-consistent ab-initio structure factors with chemical bonding effects, and has been previously used to precompute key low-order structure factors, comparing well with experiment [4-5]. Here, we use DFT to generate all the structure factors, because the GPAW DFT code, which we have used for mean inner potential calculations, can provide the all-electron density, which can then be processed to yield Ug structure factors [6-7]. With modern hardware, thousands of small-scale DFT simulations can be performed in a reasonable time, enabling DFT integration into our iterative inversion algorithm, which has been improved to be multi-CPU+multi-GPU parallelized.

Figures 1 and 2 show the results of combining DFT-computed self-consistent ab-initio structure factors with our depth-direction parameter retrieval algorithm on simulated data. Simulated SrTiO3 is our test material for simultaneous retrieval of ferroelectric atomic displacements (single-atom property) and oxygen octahedral rotation (multi-atom property); DFT is used both to generate the test data and during the retrieval routine. For perovskites, both of these specific parameters can be of interest for different systems; for other materials, the combination of single-atom and multi-atom parameters could be useful. For this noise-free data, the results cease improving because the DFT simulations use a user-selectable grid spacing; a finer grid can be used, at the cost of computational time.

In this work, we accurately retrieve ferroelectric atomic displacements and perovskite-style octahedral oxygen rotation for SrTiO3 from simulated composite-CBED-type data using ab-initio DFT structure factors [2]. Experimental applications of this technique to both 2D and 3D data will be discussed.

References
[1] R. S. Pennington, W. Van den Broek, and C. T. Koch, Phys. Rev. B 89, 205409 (2014).
[2] R. S. Pennington and C. T. Koch, Ultramicroscopy 155, 42 (2015).
[3] W. Van den Broek and C. T. Koch, Physical Review Letters 109, 245502 (2012).
[4] J. M. Zuo, M. Kim, M. O'Keeffe, and J. C. H. Spence, Nature 401, 49 (1999).
[5] A. Rosenauer, M. Schowalter, F. Glas, and D. Lamoen, Phys. Rev. B 72, 085326 (2005).
[6] R. S. Pennington, C. B. Boothroyd, and R. E. Dunin-Borkowski, Ultramicroscopy 159, 34 (2015).
[7] J. Enkovaara, C. Rostgaard, et al., J. Phys.: Cond. Mat 22, 253202 (2010).

Acknowledgements
The authors thank the German Research Foundation (DFG) for financial support via grants SFB 951 and PE2500/1-1 (PolaRIS-3D).

Robert S. PENNINGTON (Berlin, Germany), Christoph T. KOCH

08:00 - 18:15 #6751 - IM06-351 Clustering for scanning transmission electron diffraction data.

IM06-351 Clustering for scanning transmission electron diffraction data.

Modern scanning transmission electron microscopes (STEMs) routinely produce very large datasets with a variety of signals, ranging from conventional integrated scattering (annular bright- or dark-field), to X-ray spectroscopy, electron energy loss spectroscopy, and, of particular interest in this study, localised diffraction. While the signal efficiency can be very high in STEM, the available information can be lost or neglected when using traditional data analysis techniques. However, the ever-increasing interest in (and availability of) “big data” and data mining technologies has led to a wealth of techniques suitable for processing STEM data in more intelligent and meaningful ways.

Data clustering represents one such technique. Clusters are groups of data points which exhibit similar features, such as scan pixels which have similar diffraction patterns. However, clustering is not straightforwardly applicable to diffraction data, which typically has a very large number of features, meaning that the “distance” metrics used in most algorithms work poorly.¹ Moreover, standard clustering methods are only suitable where data are well-distinguished, which is not the case for diffraction data which often exhibit considerable overlap.

The first problem is most readily solved by taking advantage of recent work focusing on applying dimensionality reduction methods, such as principal component analysis (PCA) and non-negative matrix factorisation (NMF), to diffraction data. Alone, these methods are are capable of extracting relevant features from STEM data, but only in fairly ideal cases.² However, these methods do preserve the essential structure of the data, allowing clustering to find those features which are actually well-related. The results can then be reprojected into higher dimensions for interpretable results. The second problem can be solved using fuzzy clustering methods, which allow data points to belong to several clusters simultaneously, under algorithm-dependent constraints.³

In this study, clustering has been applied to a number of real experimental datasets, proving to be capable of (a) accurately extracting the spatial location of unique sample orientations/phases, and(b) separating the unique diffraction signals from those phases.

(a) is achieved via “direct” clustering – the diffraction patterns at each scan pixel are compared, and similar patterns brought together. Figure 1 shows an example of this from a part of a NiFe sample that contains a number of different superstructures of the conventional cubic Ni structure. On the left are component “diffraction patterns” derived from NMF alone. On the right are patterns determined from clustering. The latter do not exhibit the incomplete summation artefacts typical of the NMF patterns, such as sub-background intensity or “doughnut” profiles, and are therefore significantly easier to interpret and associate physical quantities – note, for example, the clear presence of superlattice peaks in cluster 0. This may have useful consequences in, for example, pattern matching, or automatic separation of constituent phases.

(b) can be thought of as “inverted” clustering. Each pixel in diffraction space is associated with some real-space signal, and using clustering to group these together distinguishes diffraction spots which produce unique signals, as well as finding unique regions of the sample. Figure 2 shows the result of this method applied to a dataset acquired from a GaAs nanowire containing twin defects. Clusters 0 and 5 represent diffracted beams corresponding uniquely to each twin, and cluster 4 represents diffraction spots that are the same in either orientation. This method also overcomes the effect of local bending and thickness effects in the dataset that made automatic identification of the twin phases difficult. Clusters 1, 2, and 3 represent variation in the direct beam and background intensity.

These techniques are relatively straightforward to implement, rapid, and scale well with the size of the datasets. Ongoing work is focused on new experimental data, as well as algorithmic work to reduce the computational overhead associated with decomposing the data.

¹Kailing, K., Kriegel, H., & Kröger, P. (2004). Density-connected subspace clustering for high-dimensional data. Proc. SDM. Retrieved from http://epubs.siam.org/doi/abs/10.1137/1.9781611972740.23

²Lee, D. D., & Seung, H. S. (1999). Learning the parts of objects by non-negative matrix factorization. Nature, 401(6755), 788–91. doi:10.1038/44565

³Bezdek, J. C., Ehrlich, R., & Full, W. (1984). FCM: The fuzzy c-means clustering algorithm. Computers & Geosciences, 10(2-3), 191–203. http://doi.org/10.1016/0098-3004(84)90020-7

Ben MARTINEAU (Cambridge, United Kingdom), Alexander EGGEMAN

08:00 - 18:15 #6788 - IM06-353 Cations distribution in synthetic (MgFe2O4 and FeAl2O4) spinels by precession electron diffraction tomography.

IM06-353 Cations distribution in synthetic (MgFe2O4 and FeAl2O4) spinels by precession electron diffraction tomography.

Since recently, a method using precession electron diffraction tomography (PEDT) and dynamical calculations of the diffracted intensities has been developed, allowing the structure determination and refinement at the nanoscale in a TEM [1, 2]. We present the application of this method to the refinement of (MgFe₂)O₄ and (FeAl₂)O₄ spinels in order to determine the Fe, Mg and Al cations distributions on specific sites of the structure. This determination is essential for understanding the electrical or magnetic properties of spinels, their chemical reactivity or to retrieve their thermal history in the field of geosciences.

Studied samples were obtained by the flux growth method and for different chemical compositions [3]. Some samples have also been heat-treated (24 h at 1000°C followed by a rapid quench) in order to intentionally induce some structural disorder associated with Fe, Al and Mg occupancy variations on the tetrahedral and octahedral sites of the structures. Single crystals were large enough to be also studied and refined by X-ray diffraction. The structures thus deduced serve as model for comparison with the much more local results obtained using PEDT. Thin sections for TEM observations are extracted from the crystals previously studied by XRD via two methods: Focused Ion Beam (FIB) thinning and simple mechanical grinding.

The full refinement method is precisely described in [1]. It is based on the acquisition of a serie of precession (angle varying from 1 to 2°) diffraction patterns continuously acquired for various tilt angles of the sample (+/- 45 to 60°, by step of 1°) as currently done in tomography. The 3D reciprocal space of the structure is then reconstructed (Fig. 1) and experimental I_hkl intensities are integrated using the softwares PETS and JANA2006. From the I_hkl data set, the structure is solved, using conventional X-ray methods based on the kinematical approximation implemented in JANA2006, in order to obtain a first reliable structure model (Fig.2). The model is finally accurately refined (atomic position and occupancies) using least-squares methods based on the comparison of experimental intensities with calculated ones using the multi-beam dynamical theory, taking into account interactions between diffracted beams for a given thickness and orientation of the sample.

In this work, we will describe and discuss the influence of the various experimental and computational parameters on the accuracy and precision of the PEDT refinement results. These parameters are: i) thinning method (FIB / grinding), ii) precession angle, iii) refinement procedure (including thickness and/or orientation refinement). We will also discuss the need of constraining or not the chemical composition of the samples during the refinement procedure in order to obtain the most reliable results.

 

1. Palatinus et al. (2015). Acta Cryst. A, 71(2), 1-10.

2. Palatinus et al. (2015) Acta Cryst. B, 71(6), 740-751.

3. Andreozzi et al. (2001). Per. Mineral. 70(2), 193-204.

Ngassa Tankeu YVAN GEORGES (Villeneuve d'Ascq), Jacob DAMIEN, Roussel PASCAL, Roskosz MATHIEU, Andreozzi GIOVANNI B.

08:00 - 18:15 #6951 - IM06-355 Applying of Electron Backscatter Diffraction (EBSD) for Studying Structural and Phase Composition of Multilayer CrN/MoN Coatings Fabricated by Arc-PVD.

IM06-355 Applying of Electron Backscatter Diffraction (EBSD) for Studying Structural and Phase Composition of Multilayer CrN/MoN Coatings Fabricated by Arc-PVD.

Main proposes of produced CrN/MoN coatings in this research are protection of various tools, machines and materials. For this aim they need to have predicted properties and characteristics in hardness, elasticity and plasticity. Structural and phase composition of films play an important role in this case.

Electron Backscatter Diffraction (EBSD) is a powerful quantitative technique which has been significantly developed and spread over the last few decades. Nowadays it’s used both in R&D sector and industry. Usually in labs EBSD occurs as an additional option of scanning electron microscope (SEM): another detector embedded into the same chamber, as it is probably more common for energy-dispersive X-ray spectroscopy (EDS) devices. Also it is clear that limits of maximum potential for this technique haven’t been achieved yet.

The present paper is dedicated to investigation of structural and phase composition of multilayer metal nitride coatings by EBSD. Studied coatings are multilayer films based on nitrides of Cr and Mo metals. CrN/MoN coatings were deposited on steel substrate using Arc-PVD deposition. Total thickness of films is in range 8-13 µm. Samples have various numbers of layers in coatings: 11÷354, and, hence, they have different bilayers thickness from 100 nm up to 2 µm.

Coatings were studied by observing polished cross-sections of samples as well as side of top surface. The films thickness and bilayer thickness were measured using scanning electron microscope JEOL JSM-7001F and FEI Quanta 400FEG with EDAX EBSD Forward Scatter Detector System and high resolution DigiView III camera. This unit also was used for main part of reported research – EBSD studying of structure and phase composition. Crosschecking analysis of coatings structure was performed by X-ray Diffraction (XRD) using Panalytical X'Pert diffractometer.

Figure 1 shows the sample of CrN/MoN cross-section coating with layer thickness 300nm and 22 bilayers in total. Good interfaces and contrast between layers are present in films. Using EBSD and corresponding software Uniqe Grain Color Maps for samples were designed. Columnar structure and grains growth was observed (see Figure 2). It is should be noted that only CrN layers gave a good diffraction pattern, which probably corresponds to soft material of MoN layers, surface deformation or present of noncrystalline compounds.

Crystals orientation in films was studied and visualized by pole figures (Figure 3). Results of phase and orientation analysis were confirmed by cross-checking XRD measurements.

The relation between bilayer thickness and grain size of films was found. Figure 4 demonstrates grainsize distribution in films with different bilayer thickness. As thinner layers in coating as smaller grain size in proposed samples serial.

Decreasing of grain size in films may cause different mechanical properties due to higher amount of interfaces. It is an important characteristic for hardness, elasticity of material and in its turn wear resistance and other protective features.

Analysis of mechanical properties of considered coatings is a next step in subsequent research.

Bogdan POSTOLNYI (Sumy, Ukraine), João Pedro ARAÚJO, Alexander POGREBNJAK

08:00 - 18:15 #6959 - IM06-357 Virtual dark-field and Virtual high angle angular dark field images reconstructed from electron diffraction patterns.

IM06-357 Virtual dark-field and Virtual high angle angular dark field images reconstructed from electron diffraction patterns.

Dark-field images are routinely produced in transmission electron microscopy (TEM) by selecting a specific diffracted beam with the objective aperture while the incident beam illuminates a large area of the thin specimen. Similar pictures are produced in scanning-transmission electron microscopy (STEM) mode by scanning the focused beam over the region of interest and reconstructing the microstructure thanks to the signal collected with an annular detector. When using high angle annular dark field (HAADF) or low camera length, the signal is then sensitive to chemical composition, and the resulting image illustrates phase composition.

 

In both cases TEM or STEM, the information carried by the diffracted electron beam is filtered thanks to a given technical component that provides a limited range of possible settings. Typically, few apertures, which differ by their diameter, are available in conventional TEM. In STEM mode, the camera length is the only practical parameter that may be adapted for sorting the signals. In some cases it could be of interest to extend these capabilities to non-standard situations. For example, small precipitates that promote faint diffracting beams could be highlighted by collecting the intensities of not only one but several of them.

 

To that respect, acquiring the entire diffracted signal with a spatially resolved detector - e.g.: a CCD camera - and sorting numerically the information out of this complete set of signal may reveal refined features and produce uncommon views of the microstructural features. ACOM-TEM technique [1] allows such image treatment. It is this approach that is used to construct so-called virtual bright field (VBF) or dark field (VDF) images. The construction of such images has been described in [2].

The set of diffraction patterns (DP’s) collected during a scan is a digital data that may be post-processed in non-restricted way for sorting the relevant information about material’s microstructure. An example of the filtering capability of the present approach concerns materials in which phases were not easily recognise by template matching technique, because of their diffraction patterns being too similar. Such a case is illustrated with sintered Diamond/Colbalt material (fig 1 a). Indeed due to dynamical diffraction effects, diffraction patterns from Diamond (crystal structure fd3m, a=0.3566nm) and Cobalt (crystal structure fm3m, a=0.3544nm) are geometrically similar, and cannot be interpreted in terms of two different crystal structure. The resulting crystal orientation map is correct, but the Phase Map is irrelevant (fig 1. b). Limitation of template matching technique (TM) used by ACOM-TEM technique is illustrated figure 1.c) and d), when a Diamond’s DP can be better recognize using Cobalt templates (green) with index quality (IQ) of 1723 than diamond templates (red, IQ=1700). By contrast, the background of the diffraction patterns for both materials are different, depending essentially of the atomic number Z of the compound. Therefore, creating, as post data treatment, VDF using information’s contained in the background of DP will help to discriminate chemical composition (fig 2 a) and b)), and generate correct phase map (fig2.c)), without help of further complexed chemical analysis.

Acknowledgements

The authors acknowledge TEM facilities of the CMTC characterization platform of Grenoble INP supported by the Centre of Excellence of Multifunctional Architectured Materials "CEMAM" n°AN-10-LABX-44-01 funded by the "Investments for the Future" Program.

 

[1] E.F. Rauch, M. Véron, Mater. Charact. 98 (2014) 1–9.

[2] E.F Rauch, M. Véron, SFµ Nantes, (2013)

 

Muriel VERON (Saint Martin d'Hères), Johan WESTRAADT, Edgar RAUCH

08:00 - 18:15 #6276 - IM07-359 Magnetic imaging of skyrmions in FeGe using off-axis electron holography.

IM07-359 Magnetic imaging of skyrmions in FeGe using off-axis electron holography.

Magnetic skyrmions are topologically protected spin structures that have recently attracted considerable interest as a result of their physical properties and potential applications in energy-efficient spintronic devices for information technology [1]. Magnetic skyrmions were first observed in B20 compounds, whose non-centrosymmetric crystal structure gives rise to strong spin-orbit coupling. In these materials, the Dzaloshinskii-Moriya interaction results in the formation of a particle-like chiral spin structure in a regular hexagonal lattice. Transmission electron microscopy (TEM) offers a variety of methods for imaging the magnetic structure of skyrmions, including the Fresnel mode of Lorentz TEM combined with phase retrieval based on the transport of intensity equation, scanning TEM combined with differential phase contrast imaging and off-axis electron holography (EH). Here, we discuss recent advances in EH-based methods and related techniques for imaging skyrmion and helical spin structures in B20 FeGe single crystals as a function of temperature and applied magnetic field.

 Focused ion beam (FIB) milling was used to prepare TEM specimens of FeGe with a homogenous thickness of ~100 nm and a large surface area of ~50 µm². FIB-induced damage was reduced by using low energy (<1 kV) ion milling. In order to form helical and skyrmion spin structures [2], the FeGe specimens were cooled below 280 K using a Gatan 636 liquid nitrogen cooling holder. Fresnel images and off-axis electron holograms were recorded using an FEI Titan 60-300 TEM operated at 300 kV in magnetic field free conditions (<0.5 mT) in aberration-corrected mode. The microscope was equipped with a conventional Gatan Ultrascan 2k x 2k charge-coupled device (CCD) camera and two biprisms, which were located in the first and second selected area aperture planes.

 Figure 1 shows experimental magnetic phase images and corresponding magnetic induction maps of skyrmion and helical spin structures in FeGe recorded using EH. The mean inner potential and magnetic contributions to the total phase shift were separated by taking differences between measurements recorded at low temperature and at room temperature (when the FeGe is non-magnetic). The phase resolution of the EH experiments was optimised by acquiring multiple series of electron holograms and combining them after cross-correlation, as well as by recording electron holograms with a direct electron detection (Gatan K2-IS) camera, which offers an improved detective quantum efficiency and modulation transfer function when compared with standard CCD cameras [3]. Skyrmions were studied as a function of both temperature and magnetic field, which was applied parallel to the electron beam direction using the objective lens of the microscope (in free lens control mode). The twin construction of the objective lens used allowed the strength and polarity of the magnetic field to be changed continuously, in order to study the magnetization reversal dynamics of the skyrmions in situ in the TEM. The recorded magnetic phase images were also used to calculate the projected in-plane magnetization distribution in the sample using a model-based iterative reconstruction technique. As the inverse problem of reconstructing the magnetization distribution is ill-posed, regularisation parameters were used to constrain the solution. Examples of the resulting magnetisation maps are shown in Fig. 2 for the helical and skyrmion structures.

 Acknowledgements. We are grateful to K. Shibata, Y. Tokura for providing the FeGe samples and for valuable discussions, as well as to the European Commission for an Advanced Grant.

 

[1] N.S. Kiselev, A.N. Bogdanov, R. Schäfer and U.K. Rössler. J. Phys. D: Appl. Phys. 44 (2011) 392001.

[2] X. Z. Yu et al. Nature Materials 10 (2011) 106.

[3] S. L. Y. Chang et al. Ultramicroscopy 161 (2016) 90.

András KOVÁCS (Juelich, Germany), Zi-An LI, Jan CARON, Rafal DUNIN-BORKOWSKI

08:00 - 18:15 #6291 - IM07-361 New design of Möllenstedt electrostatic biprism setup for off-axis electron holography.

IM07-361 New design of Möllenstedt electrostatic biprism setup for off-axis electron holography.

Many different forms of electron holography have been explored or imagined. The off-axis configuration that uses a post-specimen electrostatic Möllenstedt biprism (BP) is the most widespread. It can be easily set up in a modern transmission electron microscope (TEM) thanks to the general introduction of field emission guns. The In situ Interferometry Transmission Electron Microscope (I2TEM), is a microscope designed to easily performed electron holography experiment [1,2,3]. The microscope is equipped with a 300kV cold field emission source, one biprism (BP) installed before the three condenser lenses, and three biprisms placed between the intermediate lenses.

Regarding the high numbers of biprisms used in this microscope, their quality, their stability, and their easy servicing are one of the major concerns to maintain the performance of the machine. A wire surrounded with a ground anode basically constitutes commercial biprisms. The wire is, in most of case, produced by coating ultrasmall quartz fibers with noble metals. The resulting biprisms, although they are quite small by most fabrication standards (approximately 700 nm in diameter), can have various mechanical, electrical, structural ... properties. Furthermore, their preparation methods are, in general, not very reproducible.

We have developed a new method to produce biprism in a more reproducible way, maintaining the best mechanical and electrical performance of the wire. Regarding the H-bar shape of these new wires, due to the FIB preparation, the electric field that appears between the wire and the ground electrode, could be strongly affected by the system geometry. Finite element models using Comsol Multiphysics [4], have been used to better understand this effect which can, combining with the biprism holder shape, strongly influence the final hologram performance. We have then proposed a new biprism holder, which overcome all these drawbacks, and test these news holders, with new FIB prepared wires inside the I2TEM microscope.  

 

[1]  T Denneulin et al, Ultramicroscopy  160 (2016), 98–109.

[2]  F Röder et al, Ultramicroscopy 161 (2016), 23–40.

[3]  F Houdellier et al, Ultramicroscopy 159, Part 1 (2015), 59–66.

[4] https://www.comsol.fr

Robin COURS (CEMES, Toulouse), Cécile MARCELOT, Florent HOUDELLIER

08:00 - 18:15 #6298 - IM07-363 Mapping electrostatic potentials across the p-n junction in GaAs nanowires by off-axis electron holography.

IM07-363 Mapping electrostatic potentials across the p-n junction in GaAs nanowires by off-axis electron holography.

The development of III−V materials on Si platforms, with the aim of reducing production costs while achieving high conversion efficiency, has been a continuing area of photovoltaic research in the last decades [1,2]. This process is challenging due to large lattice mismatches, the polar non-polar interfaces and the differences in thermal expansion coefficients. The use of III–V nanowires (NWs) provides a novel method of integrating III-V materials with Si, which avoids dislocations [3]. However the control of other parameters, such as vertical yield in a patterned array, crystal phase, dopant concentrations and electrostatic potential distribution, become challenging.

The electrical performance of a semiconductor device relies strongly on how precisely the electrostatic potentials are distributed across the active region. An accurate measurement of this potential distribution is of vital interest to the semiconductor industry. The technique of off-axis electron holography in the transmission electron microscope (TEM) is a powerful tool for fulfilling the required accuracy in mapping electrostatic potentials [4]. Here, we present electron holography measurements from single GaAs core-shell nanowires with a p-n junction, grown on a Si (1 1 1) substrate.

The Ga-assisted vapor–liquid–solid (VLS) growth mechanism on a silicon substrate was used for the formation of a patterned array of radial p-i-n GaAs NWs encapsulated in AlInP passivation. A cross-sectional specimen for off-axis electron holography was prepared perpendicular to the growth direction of the NW using focused ion beam milling (FIB) and the in-situ lift-out technique in an FEI Helios Dualbeam FIB/SEM, equipped with a micromanipulator. Holograms were acquired at 120 kV using an FEI Titan 80-300ST TEM, equipped with a rotatable Möllenstedt biprism. The thickness of the specimen was measured to be around 280 nm by convergent beam electron diffraction (CBED).

Fig. 1 shows the reconstructed phase and amplitude from the hologram of the cross-sectional specimen. A core-shell structure is observed, with the core being p-type and the shell being n-type. The phase shift across the p-n junction is close to 1 radian, corresponding to a built-in potential of 0.4 V, as shown in Fig.2. The potential variation measured by holography is used to quantify the actual doping densities in the n-type layer and p-type layer of the NW. This holography measurement indicates that the active dopant concentrations are lower than nominal values, causing a low built-in potential. A greater control on the dopant concentration and distribution is required in order to achieve a higher efficiency of the NW solar cells.

 

 

[1] Bolkhovityanov, Y. B. and Pchelyakov, O.P., Physics-Uspekhi (2008), 51, 437-456.

 

[2] Jain N., and Hudait M. K., Energy Harvesting Syst. (2014), 1, 121–145.

 

[3] Kavanagh, K. L. Semicond. Sci. Techn. (2010), 25(2), 024006-024013.

 

[4] Yazdi, S., Berg, A., Borgström, M. T., Kasama, T., Beleggia, M., Samuelson, L., & Wagner, J. B. (2015) Small, 11(22), 2687-2695.

Elisabetta Maria FIORDALISO (Hedehusene, Denmark), Zoltan Imre BALOGH, Takeshi KASAMA, Ray LAPIERRE, Martin AAGESEN

08:00 - 18:15 #6362 - IM07-365 Wave front reconstruction vi the transport of intensity equation: Introduction of non-convex constraints.

IM07-365 Wave front reconstruction vi the transport of intensity equation: Introduction of non-convex constraints.

The transport of intensity equation (TIE) is an elliptical, second order partial differential equation which relates the intensity variation along the optical axis to Laplacian-like expression involving of the unknown phase. Due to its simple mathematical formulation and the straight forward procedure to acquire the experimental data, the TIE has attracted enormous attentions from various research communities such as transmission electron microscopy, X-ray microscopy, neutron imaging, etc.. The FFT approach to the TIE due to its deterministic nature and computational speed is widely used. However, periodic boundary condition inherent to the FFT approach results in low frequency artifacts in case of non-periodic objects. Alternatively, one can impose an additional constraint on the solution in order to overcome the problem of only weakly encoded low spatial frequency phase information and unknown boundary conditions. For example, by total variation minimization approach employs gradient based optimization techniques to minimize the convex l₁-norm of the derivative of the phase is minimized, leading a solution which is preferentially flat. However, the TV-minimization approach should only be considered for piece-wise constant objects.
Here, we report on an iterative algorithm namely, the gradient flipping algorithm (GFA) [1,2], which imposes non-convex constraint on phase. The GFA assumes that the wavefront to be recovered is sparse in its gradient basis and therefore, combines the reciprocal space solution of the TIE with the principle of the charge flipping algorithm [3] by flipping the sign of phase gradients below a determined threshold. This leads to the sparsest solution in the gradient domain. In an iterative manner, the boundary conditions are updated in such a way that consistency of the recovered wavefront with the experimental data is assured and at the same time the solution is sparse. The algorithm iterates until the convergence criterion is fulfilled.
Figure 1(a) depicts the variation of the image intensity along the optical axis of images recorded of HeLa cells which was estimated from a focal series comprising 20 images with defocus step of 1 µm and laser illumination at a wavelength of λ = 520 nm. Figures 1(b), 1(c) and 1(d) show the phase reconstructed by a Tikhonov-regularized FFT-based solution of the TIE (q^-2 -> (q²+α)^-1, with q being the reciprocal space coordinate) for α =0.001 µm^-2,α=0.01 µm^-2 and α=0.1 µm^-2 where α is the regularization parameter. The phase retrieved by employing the TVAL3 package [4], (convex TV-minimization) is shown in Fig. 1(e). Finally, Fig. 1(f) presents the phase map reconstructed by the proposed approach. This GFA-reconstruction provides a physically very reasonable solution while that of TV-minimization suffers from missing low frequency information, and the Tikhonov regularized FFT-based reconstruction suffers from low frequency artifacts when ααα is small and is not capable of recovering low frequency information when the regularization parameter is increased. 

References

1. A. Parvizi, W. V. den Broek, and C.T.Koch, “Recovering low spatial frequencies in wavefront sensing based on
intensity measurements,” Adv. Struct. Chem. Imag., DOI 10.1186/s40679-016-0017-y
2. A. Parvizi, W. Van den Broek, and C. T. Koch, “The gradient flipping algorithm: introducing non-convex con-
straints in wavefront reconstructions with the transport of intensity equation ,” Opt. Express accepted (2016).
3. G. Oszlányi and A. Süto, “The charge flipping algorithm,” Acta Crystallogr. Sect. A 64, 23–134 (2007).

4. C. Li, An efficient algorithm for total variation regularization with applications to the single pixel camera and

compressive sensing (Rice University, 2009).

Amin PARVIZI (Berlin, Germany), Wouter VAN DEN BROEK, Katharina BLESSING, Christoph T. KOCH

08:00 - 18:15 #6383 - IM07-367 Mapping electrostatic potentials and deformation in semiconductor devices by off-axis electron holography and other techniques.

IM07-367 Mapping electrostatic potentials and deformation in semiconductor devices by off-axis electron holography and other techniques.

Off-axis electron holography can be used to measure the electrostatic and magnetic potentials in semiconductor devices with high-sensitivity and nm-scale resolution [1]. In this presentation we will show experimental results that have been obtained using combinations of electron holography, precession diffraction and differential phase contrast (DPC) on a range of different semiconductor devices.

Deformation maps have been acquired using dark field electron holography on a variety of different device structures and these results have been compared to those obtained by precession electron diffraction (NPED). Figure 1 shows STEM images and results obtained by dark holography and NPED on a Si specimen containing 10-nm-thick SiGe layers with different Ge concentrations and also a recessed source and drain SiGe device. The deformation maps obtained by dark holography and shown here have a spatial resolution of 6 nm and a precision of 0.05 %. The maps obtained by precession electron diffraction have a spatial resolution of 2 nm and a precision of 0.02 %. In this presentation we will show the advantages and difficulties associated with the use of the different techniques [2].

We will also compare electron holography and DPC for dopant profiling on fully processed and electrically tested devices. Figure 2(a) and (b) shows STEM images that have been acquired from two different pMOS devices with different spacer widths. The spacers are used to prevent dopants from diffusing under the gate during the activation anneals. Figure 2(c) shows how the specimen is rotated to remove the top metal layers in the device, backside milling is then used to provide a high quality TEM specimen. Figures 2(d) and (e) show maps of the electrostatic potential in the devices that have been acquired using off-axis electron holography. The spatial resolution in these maps is 5 nm and the difference in the potential distribution under the gate can be clearly seen. Potential profiles have been obtained from across the device and the parameters such as the electrical gate width can be measured. From the analysis of these real devices, the advantages and problems that are associated with electron holography and DPC can be discussed.

Acknowledgements : This work has been funded by the ERC Starting Grant 306365 « Holoview ».

References
[1] A. Tonomura, Reviews of Modern Physics, 59 (1987) 1
[2] D. Cooper et al., Micron, 80 (2016) 145
[3] D. Cooper et al., Semicond. Sci. Tech., 28 (2013) 215013

Victor BOUREAU (Toulouse), David COOPER, Nicolas BERNIER, Jean-Luc ROUVIERE

08:00 - 18:15 #6470 - IM07-369 Quantitative measurements of nanoscale electrostatic and mean inner potentials in crystals by electron beam refraction using CBED and DPC.

IM07-369 Quantitative measurements of nanoscale electrostatic and mean inner potentials in crystals by electron beam refraction using CBED and DPC.

Probing nanometer scale electrostatic and mean inner potential (MIP) and establishing structure–properties relationship at this length scale in advanced functional materials are not only of fundamental interest but also of technological relevance and importance, especially for materials with application in the ever miniaturizing electronics. As pure phase object, these potentials can only be “seen” by phase of the wave of probing radiation/particle; and electron microscopy (EM) based phase contrast methods are the most suitable, if not the only, tool for this purpose. Here, MIP is, by definition, the local volume (or unit cell) average of the Coulomb potential of the sample, which can be theoretically calculated or accurately measured (cf. Ref [1]). While the measurable, the local scattering potential being probed by the high-energy electrons, can be considered (assuming homogeneous potential in projection) superposition of MIP and electrostatic potential, which is the deviation of local Coulomb potential from the MIP of bulk from the perspective of measurement.

Differential phase contrast (DPC) using electrons based on scanning transmission EM (STEM) has seen a renaissance of interest mainly due to the recent demonstration of probing electrostatic potential in real space at atomic resolution ^[3], beside its demonstrated robustness in studying magnetic properties in the past decades, e.g. Ref. [2]. The DPC-STEM signal composes the difference of intensity from opposite quadrants from a segmented annual bright field detector. In principle, a DPC-STEM experiment effectively records, under kinematic approximation, a two dimensional vector of electron beam deflection (i.e., phase gradient) and an absorption/amplitude contrast signal (i.e., sum intensity of all quadrants) simultaneously at each probing position/pixel which raster over the specimen at desired field of view (FOV). Therefore, DPC-STEM is expected to show advantages of 1) direct interpretability, 2) sharp features in focus, 3) flexible FOV and 4) simultaneous phase and amplitude contrast, compared to interference- (e.g. electron holography) and propagation- (e.g. Fresnel contrast) based phase contrast methods. Despite these simple descriptions, there are, however, very limited applications of DPC-STEM in the quantitative and systematic study of electrostatic potential and MIP in crystals. Moreover, as STEM based method using convergent illumination, convergent beam electron diffraction (CBED) patterns under identical condition are indispensable to evaluate the DPC-STEM results quantitatively. 

In this contribution, we focus on experimental studies of quantitative measurements of MIP from crystal wedges by compiling the results from DPC-STEM and CBED raster arrays under identical diffraction conditions. The experiments were performed on a Titan Themis³ TEM equipped with Cs correctors, working at 200 kV in μ-probe STEM mode. The camera length and probe convergence angle are carefully calibrated and chosen to balance resolution (to about 1 nm) and detection sensitivity. Figure 1 shows the representative results of measuring MIP from a cleaved 90° Si wedge. Under quasi-kinematic condition (cf. Fig. 1a), remarkable beam refraction is observed when the probe is moved from vacuum to inside sample and the refraction angle is constant to a considerablely large sample thickness. Meanwhile the total intensity decreases homogeneously within the beam disk and exponentially as a function of the local thickness, as expected. The refraction angle measured from a CBED raster array is quantified to sub-pixel accuracy, which corresponds to a MIP of Si to be 12.52±0.21 V. The calibrated DPC-STEM signals deliever very close mean value of the magnitude of refraction, but with much greater variance, due to the orders-of-magnitude shorter dwell time and thus noisier signal (Fig. 1g,h). The same measurements have been carried out with a 90° GaAs wedge, from which we derived the MIP of GaAs to be 14.10±0.33 V from CBED measurement and a very close mean value from DPC-STEM data. The MIP values agrees very well with previous measurements based on electron holography and theoretical calculations ^[1].

Further examples on the application of the method for mapping electrostatic potentials in semiconductor nanostructures, as well as attempts to map piezo-electric potentials will be presented at the conference. 

Acknowledgements: Financial support by the German Research Foundation (DFG) via research training group GRK 1896 “In-situ microscopy with electrons, X-rays and scanning probes” and cluster of excellence EXC 315 “Engineering of advanced materials” is gratefully acknowledged.

References:

[1] P. Kruse, et al., Ultramicroscopy 106, 105 (2006). 

[2] J. Chapman, J. Phys. D, 17, 623 (1984); T. Uhlig & J. Zweck, Phys. Rev. Lett. 93, 047203 (2004). 

[3] N. Shibata, et al., Nat. Phys. 8, 1 (2012); K. Müller, et al., Nat. Comm. 5 5653 (2014)

Mingjian WU (Erlangen, Germany), Erdmann SPIECKER

08:00 - 18:15 #6625 - IM07-371 Imaging by Zernike phase plates in the TEM.

IM07-371 Imaging by Zernike phase plates in the TEM.

Abbe lens theory can be used to calculate the images produced by a system made up of a simple phase object, a round lens and a Zernike phase plate.  When all these have rotational symmetry, 1D Fourier-Bessel transforms are suitable. 

 

Given a disc object producing a uniform phase shift over its diameter and a phase plate providing a simple step in phase by α at a defined spatial frequency q₀, the resulting image intensity can be defined for any size of object phase change.  The distortion of the image by the central hole in the Zernike plate depends on a parameter B which is proportional to the product of the diameter of the object and q₀ and also depends on the lens focal length; when B is less than 1, the distortion is small (Fig. 1).  The range of object phase that produces a monotonic variation of intensity is found to depend strongly on the phase change α introduced by the phase plate, being greatest when α is between π and 2π  (Fig. 2). 

 

For this object and plate, the weak phase approximation (WPA) is useful in giving an indication of the radial distribution of intensity.  It does not predict well the range of monotonic variation of intensity with object phase.

 

In the search to reduce the ‘ringing’ in the image produced by large objects with a stepped plate, we analysed also the behaviour when the phase change at the plate increases to its maximum value α over a range of radius (instead of a step).   When the phase change is proportional to radius for q < q₀, we find that as object diameter increases, the intensity is reduced gracefully when α ~ π/2, but that contrast reversal can occur when α ~ 3π/2. 

 

The intensity has also been calculated in WPA for an object phase distribution typical of a spherical object.  With this object, the image intensity varies continuously with object radius, as expected, and is reduced as B increases above 1.  The behaviour when the phase profile of the plate is ramped is similar to that for the disc object with the same phase profile for the plate (Fig. 3).

 

The advantages for different kinds of objects offered by plates with phase shifts α either less than or greater than π suggest that in practical use it will be desirable for phase plate holders to be fitted with plates of both thicknesses.

Chris EDGCOMBE (Cambridge, United Kingdom)

08:00 - 18:15 #6634 - IM07-373 Effects of dose and image registration on exit wave reconstruction of low-dose focal series.

IM07-373 Effects of dose and image registration on exit wave reconstruction of low-dose focal series.

Dose effects are an important topic in electron microscopy (EM) due to the close connection to radiation damage and hence to quantitative image analysis [1]. Two main problems restricting the application of exit wave reconstruction to radiation-sensitive materials are the high noise level in low-dose images and the contrast reversals close to zero defocus. Both problems cause difficulty in correctly registering a focal series of images and a well-aligned image series is a prerequisite for valid wave restoration [2].

 

In this work, a simulation-assisted cross-correlation function (SA-XCF) registration scheme is proposed and tested with five focal series of the same cerium oxide (CeO₂) nanoparticle taken under identical imaging conditions except for varying electron dose between different focal series. The registration results demonstrate the superiority of the new registration scheme over a simple neighboring-reference cross-correlation function (NR-XCF) registration (Figure 1). The impact of registration quality on exit wave reconstruction is explored by the comparing the IQ factor, an image quality measurement calculated from the power spectrum of an image of the phase restored from the same focal series aligned by two different registration methods. Better phase restoration results are obtained with the focal series registered by SA-XCF in comparison to NR-XCF.

 

With improved image alignment exit waves reconstructed from focal series data at variable dose were compared. The comparison result confirms the natural hypothesis that higher electron dose series give improved restored exit waves than lower dose series. However, this improvement with increased dose appears to plateau beyond a certain dose threshold. Exit wave reconstruction from focal series at very low dose tends to preserve more noise from the images and leads to worse IQ factor values (Figure 2).

 

It has also been observed that the restored exit waves from focal series at very low dose can be unreliable for quantitative interpretation (Figure 3) where the phase shift is noticeably attenuated. This leads to the question of how a critical dose can be determined in order to obtain a quantitatively interpretable exit wave subject to the most efficient use of an allowed electron dose budget. 

  

 

Reference

[1] M. Pan, Micron, Vol. 27, No. 3-4, pp. 219-238, 1996.

[2] W. O. Saxton, Journal of Microscopy, 174(2):61–68, 1994. 

Chen HUANG (Oxford, United Kingdom), Hidetaka SAWADA, Angus KIRKLAND

08:00 - 18:15 #6723 - IM07-375 Direct determination of calibration factors for quantitative DPC measurements.

IM07-375 Direct determination of calibration factors for quantitative DPC measurements.

Differential phase contrast microscopy (DPC) is a measurement technique which is utilized in a scanning transmission electron microscope (STEM) equipped with a special direction sensitive detector [1,2]. It is based on the deflection of the electron beam by an angle α due to either Lorentz or Coulomb force, when the electron probe is scanned over an area with intrinsic magnetic or electric fields. This deflection causes a shift of the diffraction disk (DD) in the detector plane of the microscope, which is proportional to strength and direction of the field within the specimen. By measuring this shift with a direction sensitive segmented ring detector one obtains information about the intrinsic fields.

In this work we present an approach to determine calibration factors relating the qualitative DPC signals to absolute magnetic and electric field strengths or deflection angles just by measuring the size of the DD on the DPC detector. For this we derived a formula for the calibration factor by considering geometrical properties of the annular DPC detector and the DD. With this we can for example describe the absolute electric field strength E(x,y) at a certain specimen position (x,y) by:

                                             E(x,y)[V/m] = ( S_DPC(x,y) / S_sum(x,y)  ) · ( κ_el[V]  / t(x,y)[m] )     (1)

S_DPC(x,y) being the DPC signal normalized with the sum signal S_sum(x,y) of all four detector segments, the specimen thickness t(x,y) and the calibration factor κ_el for electric fields. The latter can be described by:

                                             κ_el = [ (R² - r²) / (R · C) ] · [ (m_rel · v_rel²) / e ]                             (2)

with R and r being the radii of the DD in the detector plane and the detector hole (see fig. 1) and C the used camera length of the microscope. The second term describes the energy of the accelerated electrons with their relativistic mass m_rel, velocity v_rel and elemental charge e.
Equation 2 shows us two important things. Firstly that the calibration factor is highly dependent on the radius R of the DD (see fig. 2 and 3). This means that even a small change of the DD radius due to beam broadening caused by the specimen can lead to significant different field values when they are quantified with a calibration factor determined for the same set of microscope parameters but with a slightly smaller disk radius.
The other one is, that it is possible to calculate κ_el just by the measurement of R, because all the other variables in eq. 2 are well known parameters of the DPC measurement. The disk size itself can be easily determined e.g. with a CCD camera. This is a convenient way to calibrate a DPC system for quantitative measurements. Further it is possible to obtain individual calibration factors for each DPC measurement performed. The latter allows to minimize the error of the quantification due to disk broadening.

In addition to the theoretical approach we will also present our first experimental results showing the validity of the statements made above.

[1] Rose H., Phase Contrast in Scanning Transmission Electron Microscopy, Optik 39, 4, 416-436, (1974)
[2] Chapman J.N., The Investigation of Magnetic Domain-structures In Thin Foils By Electron-microscopy, J. Phys. D: Appl. Phys. 17, 623-647 (1984)

Felix SCHWARZHUBER (Regensburg, Germany), Johannes WILD, Josef ZWECK

08:00 - 18:15 #6816 - IM07-377 Direct comparison of differential phase contrast and off-axis electron holography for the measurement of electric potentials by the examination of reverse biased Si p-n junctions and III-V samples.

IM07-377 Direct comparison of differential phase contrast and off-axis electron holography for the measurement of electric potentials by the examination of reverse biased Si p-n junctions and III-V samples.

In this presentation we will compare differential phase contrast (DPC) [1] and off-axis electron holography [2] for the measurement of electrostatic potentials in semiconductor devices. DPC uses the lateral shifts of a convergent electron beam to determine the field in the sample whereas for electron holography the changes in potential is encoded in interference fringes that are formed using a biprism [1]. To fairly assess the relative sensitivity of the different techniques on the same specimen, a symmetrically doped p-n junction with a dopant concentration of 1 x 10¹⁹ cm^-3 has been measured as a function of reverse bias applied in situ in the TEM.  Figure 1(a) and (b) shows maps acquired by DPC of the p-n junction with 0V and 4V reverse bias applied. At 0V, it is difficult to see the presence of the junction whereas for a reverse bias of 4 V the space charge region is now visible. Profiles acquired from across the junction for various reverse bias voltages can be seen in Figure 1(c).  Electron holograms were acquired and Figure 1(d) and (e) show reconstructed phase images of the junction at zero bias and 4 V reverse bias. Even though a low magnification has been used to obtain a large field of view at the expense of sensitivity, the junction is clearly visible in both of the phase images. Corresponding electric field profiles that have been calculated from the potential maps are shown in Figure 1(f).  These results show that off-axis electron holography has a significantly better sensitivity than DPC. However  the advantages of using DPC is that a large field of view has been obtained and it is not necessary to examine a region close to vacuum.

These techniques have also been applied to an InGaN/GaN system. Figure 2(a) shows a HAADF STEM image of the specimen. Figure 2(b) shows a potential map and (c) profile acquired by off-axis electron holography with a spatial resolution of 5 nm.  When using DPC, sub-nanometer spatial resolution is expected and maps of the electric field in the InGaN layers can be observed in Figures 2(d) and (e). Here the specimen has been tilted onto and away from a zone axis and a large variation in the measured signal is observed which is visible in Figure 2(f). In this presentation will discuss the effects of diffracted beams on the measured DPC signal. We will also discuss the advantages and disadvantages of using DPC and electron holography for the measurement of electrostatic fields in a range of different doped and III/V semiconductor specimens and show improvements that have been applied.

 

Acknowledgements : This work has been funded by the ERC Starting Grant 306365 « Holoview ». The experiments have been performed on the platform nanocharacterisation at Minatec.

References

[1] N.H. Dekkers, Optik, 30 (1974) 452

[2] A. Tonomura, Reviews of Modern Physics, 59 (1987) 1

Benedikt HAAS (GRENOBLE CEDEX 9), David COOPER, Jean-Luc ROUVIERE

08:00 - 18:15 #6835 - IM07-379 Exotic Electron topologies - Knitting with electron vortices.

IM07-379 Exotic Electron topologies - Knitting with electron vortices.

Electron vortex studies have proliferated in the last few years, with many examples of their
production, measurement and some examples of application.
However, these studies focus on the more standard cylindrically-symmetric ideal electron vortex
beam. We have recently noted an abundance of exotic vortex behaviours, outwith cylindrical
symmetry [1]. Vortex-antivortex loops were formed and annihilated at many different positions
within the beam, with varying sizes. Other studies have shown how a vortex core can wend its way
through a crystal lattice, with the vortex always remaining conserved [2].
In the sister field of optical vortices, it has recently been discovered that it is possible, with
simultaneous manipulation of holography and careful limiting apertures, to produce 'knots' with the
vortex cores [3]. These knots are not knots as known to the layman, but specific forms of multiply
interlinked loops. These knots are unusual, and robust topological structures, and take the form of
knotted dark threads within the beam [4].
Here, we investigate the feasibility of producing these exotic structures in the rather more confined
space of a modern TEM, with finite aperture positioning and strict paraxial limitations. Adjusting
the phase hologram technique of Leach et al [3], we have produced a design for a TEM phase mask
(see figure 1 and 2). Such a design is more detailed and technologically challenging than those
produced so far in the electron vortex research field [5], but may just be within reach to enable the
study of these unusual topological electron structures for the first time.

References
[1] Clark, L., et al. "Symmetry-constrained electron vortex propagation." Under review, arXiv preprint
arXiv:1603.00687 (2016).
[2] Lubk, Axel, et al. "Topological analysis of paraxially scattered electron vortex beams." Physical Review A 87.3
(2013): 033834.
[3] Leach, J., et al. "Vortex knots in light." New Journal of Physics 7.1 (2005): 55.
[4] Leach, Jonathan, et al. "Laser beams: knotted threads of darkness." Nature 432.7014 (2004): 165.
[5] Shiloh, Roy, et al. "Sculpturing the electron wave function using nanoscale phase masks." Ultramicroscopy 144
(2014): 26-31.

Acknowledgements
LC and JV acknowledge funding from the European Research Council under the 7th Framework Program (FP7), ERC
Starting Grant No. 278510-VORTEX. JV acknowledges financial support from the European Union under the 7th
Framework Program (FP7) under a contract for an Integrated Infrastructure Initiative (Reference No. 312483
ESTEEM2).

Laura CLARK (Antwerp, Belgium), Jo VERBEECK

08:00 - 18:15 #6848 - IM07-381 Remnant states and magnetic coupling in Co/Cu multilayered nanowires observed by electron holography.

IM07-381 Remnant states and magnetic coupling in Co/Cu multilayered nanowires observed by electron holography.

Magnetic nanowires (NWs) are of great interest due to their potential applications in technological devices and fundamental analysis in the spintronic field[1]–[3]. Among the wide diversity of NWs, the multilayered ones appear as good candidates for studying the spin torque phenomenon, and increasing the radio frequency (RF) output power by designing nano-oscilators connected in series[4], [5]. For this propose the fundamental study of the magnetic states and coupling in cylindrical multilayered NWs is required. The resulting remanent states in these one-dimensional multilayered systems will be the result of the competition between the shape anisotropy, crystal anisotropy, exchange interactions and the dipolar coupling that occurs due to the interlayer character.

 

In this work we have used electron holography (EH) technique to reveal the remnant states of Co/Cu multilayered NWs after applying a saturation magnetic field perpendicular (PP) and parallel (PL) to the wire axis. Cylindrical Co/Cu multilayered NWs were prepared by electrodeposition technique using the single bath method, and polycarbonate membrane to allow the wires growth. Local chemical analysis has been performed by energy-filtered TEM (EFTEM) to distinguish and measure the thickness of the Co(Cu) layers founding 42 nm (46 nm) as its average value. Figure 1 c shows a EFTEM map with the positions of cobalt and copper layers for a wire with 80nm of diameter. For elucidating the different local remnant states along the wire, magnetic phase shift images retrieved by EH experiments were compared with those calculated from micromagnetic simulations (See Figs 1 a and b). We found that the resulting remnant states present a strong dependence with the local morphology of the layers (Co and Cu thicknesses, NW diameter, Co/Cu interface tilting) and the magnetic coupling between consecutives Co layers. Surprisingly only a weak effect due to the applied magnetic field direction was observed. For the studied nanowires, the ferromagnetic layers can present either a monodomain state perpendicularly to the NW axis, or a vortex state where the core orientation is determined by the layer morphology and the magnetic interaction with neighbors Co layers. This latter state is the most observed magnetic state. Thus, the remnant magnetic state of the multilayered NW is formed by a mixture of monodomain and vortex states. Figure 1 d shows the 3D representation of the simulation, in which there are two vortices with different chirality and polarization, the cores of the vortices are tilted respect to the Z axis.

 

References

[1] S. Fukami, T. Iwabuchi, H. Sato, and H. Ohno, Jpn. J. Appl. Phys., vol. 55, no. 4S, p. 04EN01, Apr. 2016.

[2] C. Bran, E. Berganza, E. M. Palmero, J. A. Fernandez-Roldan, R. P. Del Real, L. Aballe, M. Foerster, A. Asenjo, A. Fraile Rodríguez, and M. Vazquez, J Mater Chem C, vol. 4, no. 5, pp. 978–984, 2016.

[3] A. Mourachkine, O. V. Yazyev, C. Ducati, and J.-P. Ansermet, Nano Lett., vol. 8, no. 11, pp. 3683–3687, Nov. 2008.

[4] J.-V. Kim, “Spin-Torque Oscillators,” in Solid State Physics, vol. 63, Elsevier, 2012, pp. 217–294.

[5] M. D. Stiles and J. Miltat, Springer, 2006, pp. 225–308.

 

Acknowledgments

This work was performed using HPC resources from CALMIP (Grant 2015-1428), the ANR EMMA 12-BS10-0013 and NASSICS 12-JS10-008 01 projects, and the French microscopy network METSA.

David REYES (Toulouse), Luis Alfredo RODRÍGUEZ, Bénédicte WAROT-FONROSE, Nicolas BIZIERE, Travis WADE, Christophe GATEL

08:00 - 18:15 #6854 - IM07-383 Inside a FeRh layer during the ferromagnetic/antiferromagnetic transition: a quantitative study by off-axis electron holography.

IM07-383 Inside a FeRh layer during the ferromagnetic/antiferromagnetic transition: a quantitative study by off-axis electron holography.

The ordered FeRh alloy presents very intriguing magnetic properties, among which a remarkable magnetic phase transition from an antiferromagnetic (AF) state at low temperature to a ferromagnetic (F) state just above room temperature accompanied by a 1% volume expansion upon entering the FM state [1-2]. In recent years, this alloy has encountered a huge regain of interest for its strong potential for future applications: its properties can be usefully exploited in new devices for microelectronics, heat-assisted magnetic recording [3-5] or magnetic random access memories based on AFM spintronics [6].

Many studies are devoted to the understanding of the ferromagnetic/antiferromagnetic transition mechanism in FeRh and several models have been proposed. In most of these studies, magnetic properties are measured through macroscopic measurements on the whole film. A local magnetic study could provide new insights regarding the transition process, by clarifying phenomena happening at a nanometer scale.

We propose to bring new details on the F-AF transition in a FeRh layer by acquiring quantitative magnetic mapping at the nanoscale on a cross section. The experimental technique that combines high sensitivity to the electromagnetic field up to nanometer resolution on a cross sectional sample with an in situ temperature control is the electron holography (EH) in a TEM. We have then achieved to get magnetic mapping of a 50 nm FeRh layer grown on a MgO substrate through the F-AF transition (Fig. 1a).

The evolution of the induction as a function of temperature has been recorded at a local scale and shows similar features than the one obtained by macroscopic measurements (Fig. 1b). However we observed heterogeneity of the transition in the film thickness: near the interfaces, the magnetic transition from the AF state to the F state starts much earlier and is spread over a wider range of temperature than in the middle of the layer (Fig. 2). The interfaces not only lower the transition temperature, but make this transition more difficult to achieve over a long distance. The presence of structural defects (dislocations, ...) at the interface with the substrate but also the breaking of symmetry significantly locally modifies the transition F / AF.

Various schemes of the transition have also been evidenced (Fig. 3). For instance, in the heating process (AF->F), "homogeneous" transition to the F state at interfaces starts first, following by a F domain nucleation in layer that begins even if the transition at interfaces is not completed, growth of the F domains within the AF matrix and then coalescence until the complete disappearance of the AF state. One of the most remarkable results during the F domain growth is the constant period of about 100 nm reflecting the regular alternation of the areas F and AF (Fig. 3). Note that the value of this period is comparable to the distance between dislocations to get a complete relaxation of a FeRh layer on MgO (80 to 100 nm) and could explain the nucleation F domain pinned by structural defects such as dislocations.

 

[1] M. Fallot, Ann. Phys. (Paris) 10, 291 (1938); M. Fallot and R. Hocart, Rev. Sci. 77, 498 (1939)

[2] M. R. Ibarra and P. A. Algarabel, Phys. Rev. B 50, 4196 (1994)

[3] J. U. Thiele, S. Maat, E.E. Fullerton, Appl. Phys. Lett. 82, 2859 (2003),

[4] J. U. Thiele, S. Maat, J. L. Robertson, E.E. Fullerton, IEEE Trans Mag.,40, 2537(2004)

[5] R. O. Cherifi et al., Nature Materials 13, 345 (2014).

[6] X. Marti et al., Nature Materials 13, 367 (2014). 

Christophe GATEL (CEMES, Toulouse), Bénédicte WAROT-FONROSE, Luis-Alfredo RODRIGUEZ, David REYES, Nicolas BIZIERE, Robin COURS, Marie-José CASANOVE

08:00 - 18:15 #6857 - IM07-385 DPC measurements on annealed cobalt thin films.

IM07-385 DPC measurements on annealed cobalt thin films.

In this work we present the results of differential phase contrast (DPC) [1,2] measurements of micro magnetic field distributions in annealed cobalt thin films. The polycrystalline specimens with a thickness of about 50 nm were produced by thermal boat evaporation under high vacuum conditions. After annealing the average size of individual cobalt crystals is increased and their magnetic properties are changed as shown by G. Herzer in [3] and [4]. Our goal was to determine, if DPC is a suitable technique to investigate how these changes in the crystallographic structure effect the magnetic properties of our specimen. Therefore we performed single DPC measurements as well as DPC tilting series to investigate if and how the magnetic behaviour of the annealed cobalt films is changed.

We investigated 5 different samples. One being the untempered thin film and the others were annealed over one hour with increasing temperatures ranging from 520 K to 820 K. To investigate the magnetic properties we performed a DPC tilt series for each sample. By tilting the specimen relative to the magnetic field of the objective lens we change the effective external in plane field on our samples. This leads to a change of the in plane induction in our cobalt thin films. We started each tilting series at large angles of α=20°, to be sure that all magnetic moments in our specimen are aligned in one direction. Figure 1 shows a DPC measurement of such a saturated state. The homogeneous green colour of the measurement confirms that the magnetic induction is saturated along the direction indicated by the colourwheel. Figures 2-4 show an excerpt of DPC measurements performed during the tilt series of the specimen annealed at 850 K. It can be seen, that the magnetic structure changes with decreasing α and it seems, that one of the magnetic ripples is pinned to the highlighted crystallites in the center of figures 2 and 3.

By calculating the average direction and strength of the beam deflection for each individual DPC measurement we get information about the dependency between the magnetic induction and the tilting angle respectively the external magnetic field applied. We will present the results of our measurements on the different annealed specimen showing hysteretic behaviour of the magnetic induction and discuss the differences between them.

During our experiments especially the tilt series, which take about 4 hours to obtain, we encountered some difficulties with the DPC technique. In this work we will present these shortcomings and show solutions to some of the problems occured.

[1] J. N. Chapman, The Investigation of Magnetic Domain-structures In Thin Foils By Electron-microscopy, J. Phys. D: Appl. Phys. 17, 623-647 (1984)
[2] H. Rose, Phase Contrast in Scanning Transmission Electron Microscopy, Optik 39, 4, 416-436, (1974)
[3] G. Herzer et al., Grain structure and magnetism of nanocrystalline ferromagnets,IEEE Transactions on Magnetics 25, 3327-3329, (1989)
[4] G. Herzer et al., Grain size dependence of coercivity and permeability in nanocrystalline ferromagnets,IEEE Transactions on Magnetics 26, 1397-1402, (1990)

Thomas BEER, Felix SCHWARZHUBER (Regensburg, Germany), Josef ZWECK

08:00 - 18:15 #6939 - IM07-387 Transport of Intensity Equation (TIE) without filtering and TIE videos.

IM07-387 Transport of Intensity Equation (TIE) without filtering and TIE videos.

In the transmission electron microscope (TEM) almost all specimens are very thin and create mainly phase shifts of the electron beam, hardly any absorption. The phase shift gives a lot of information about the specimen like e.g. thickness modulations, electric fields or magnetic fields. Measuring the phase by conventional imaging techniques is quite difficult as in general only changes in intensity are detected.

One way to get the phase difference between object wave and a reference wave is the transport of intensity equation (TIE) which was initially developed for light microscopes [1]. The most common use of the TIE is to input a linear approximation for the intensity changes which occur when the image is taken at different values of defocus along the z direction and then to solve the equation for the phase. Thus it is possible to calculate the phase in good approximation. From the phase it is e.g. possible to evaluate direction and strength of intrinsic magnetic and electric fields of the specimen.

Recent publications use mainly a commercially available software to perform TIE calculations [2-4]. This software uses different filter algorithms which are not all known to the user. Therefore it turns out to be difficult to interpret the TIE reconstructions as the the filtering may create measurement artifacts.

Here we present a home written TIE Matlab code to calculate the phase. Our aim was to create a transparent code where the user is aware of all parameters. We use as little filtering as possible to minimize the room for misinterpretations and clearly show where modifications have been made to the original data. We are even able to calculate useful TIE images without any filtering as shown in figure 1.

Our software is capable of calculating magnetic or electric field maps from focal series (in-focus, over-focus, under-focus). Further, it is possible to create automated multiple TIE images for videos to visualize e.g. the temporal development of a magnetic structure. We show one example of a fluctuating skyrmion lattice in Cu₂OSeO₃.

We will provide the code as a Matlab applet for everybody interested for free.

 

References

[1] Michael Reed Teague, Deterministic phase retrieval: a Green’s function solution, J. Opt. Soc. Am. 73, 1434-1441 (1983).

[2] Kazuo Ishizuka, and Brendan Allman, Phase Measurement in Electron Microscopy Using the Transport of Intensity Equation, Microscopy Today 13, 22-24 (2005).

[3] Yu, X. Z. et al. Real-space observation of a two-dimensional skyrmion crystal. Nature 465, 901–904 (2010).

[4] X. Z. Yu, N. Kanazawa, Y. Onose, K. Kimoto, W. Z. Zhang, S. Ishiwata, Y. Matsui, Y. Tokura, Near room-temperature formation of a skyrmion crystal in thin-films of the helimagnet FeGe, Nature Materials 10, 106–109 (2011).

Johannes WILD (Regensburg, Germany), Michael VOGEL, Felix SCHWARZHUBER, Christian BACK, Josef ZWECK

08:00 - 18:15 #6971 - IM07-389 Low dose electron holography using direct-electron detection camera.

IM07-389 Low dose electron holography using direct-electron detection camera.

The advent of commercially-available direct detection cameras (DDCs) for transmission electron microscopy (TEM) offers the opportunity to reduce noise in images and diffraction patterns as well as providing fast frame rates for image recording. For sufficiently low dose rates, their design can enable significant improvements in detective quantum efficiency (DQE) and modulation transfer function (MTF) when compared to conventional charge-coupled device (CCD) cameras. Existing literature on DDCs is focused predominantly on structural biological applications, where they provide clear advantages under low dose conditions, e.g., typically < 10 e⁻Å⁻² .  Whereas the characteristics of DDCs at dose rates and spatial resolutions that are applicable to biological materials are already well established, in many other areas of TEM the dose rate can exceed 1000 e⁻Å⁻², while the spatial resolution can vary from nanometers to better than 1 Å. In these contexts, the benefit of DDCs is less clear.

Here, we examine this question in the context of high-resolution phase contrast imaging and off-axis electron holography and demonstrate that the improved MTF and DQE of a DDC result in clear benefits over conventional CCD cameras. For electron holography, we find a significant improvement in the holographic interference fringe visibility and a reduction in statistical error in the phase of the reconstructed electron wavefunction.  In addition,  we show that at least three-fold improvement in optimum phase resolution using the counting mode provided by DDC with four time less dose rate compared that of a conventional CCD camera (with a fringe spacing of 83pm in this case). Further improvement in SNR could be obtained by correlation and averaging over a series of holograms. As a result of the low camera noise, the correlation of individual hologram is robust even at low dose rates, and the averaging leads to an improvement in SNR that is close to the ideal root-N behavior (N being the number of images). 

Using BiFeO₃ on DyScO₃ substrate as an example, we demonstrate that both specimen and birpism fringe drift can be successfully correlated over 100 frames of hologram (total exposure of 20 sec at a dose rate of 10 e⁻ per pixel per sec), as shown in Fig. 2.  Our results show that DDCs are highly beneficial for electron holography (and similarly to high-resolution TEM ) at low dose rates, thereby minimising potential specimen damage while maintaining an adequate SNR for analysis.

Shery CHANG (Tempe, USA), Lei JIN, Juri BARTHEL, Rafal DUNIN-BORKOWSKI, Christian DWYER

08:00 - 18:15 #6302 - IM08-391 Model calculations for low-loss EEL spectra of 2D multilayer systems.

IM08-391 Model calculations for low-loss EEL spectra of 2D multilayer systems.

Collective electronic excitations (plasmons) in single-layer and few-layer graphene have been studied extensively in the past few years. In particular, the dispersion and nature of the π plasmon peak in free-standing single-layer graphene was investigated by means of momentum-resolved electron energy-loss spectroscopy (MREELS) [1-3]. Besides, it was also studied how the transition from mono- to multilayer graphene changes the shape of EEL spectra. Within a layered electron-gas (LEG) model, this transition can be modeled very precisely [4,5].

For other 2D materials such as transition metal dichalcogenides (TMDs), however, this approach may be highly inaccurate. We therefore evaluate more precise model calculations for multilayer systems. Our calculations are based on time-dependent DFT calculations for the individual monolayers. The plane-wave pseudopotential DFT code abinit [6] is used to simulate the ground state within local density approximation (LDA). The linear response is then calculated within random-phase approximation (RPA) using the dp-code [7]. Compared to the LEG model where the monolayers are assumed to be perfectly 2-dimensional and homogeneous, we preserve the layers' microscopic structure in our calculations.

We demonstrate that only by taking the finite thickness of the constituent monolayers into account, the spectra of multilayer MoS₂ can be modeled correctly (see the figure). Our calculations can be also applied to arbitrary van-der-Waals heterostructures. Our results are directly compared to MREEL spectra recorded with a Zeiss Libra 200 based TEM prototype (“SALVE I”, [8,2]) equipped with a monochromator and an Ω type in-column energy filter. This allows for the verification of our calculations over a large range of energy losses (0-40 eV) and different momentum transfers within the Brillouin zone. [9]

[1] Kinyanjui et al., EPL 97(5), 2012

[2] Wachsmuth et al., Phys. Rev. B 88, 2013

[3] Liou et al., Phys. Rev. B 91, 2015

[4] Jovanović et al., Phys. Rev. B 84, 2011

[5] Wachsmuth et al., Phys. Rev. B 90, 2014

[6] Gonze et al., Comput. Phys. Commun. 180(12), 2009

[7] Olevano et al., www.dp-code.org

[8] Kaiser et al., Ultramicroscopy 111(8), 2011

[9] We acknowledge financial support by the German Research Foundation (DFG) and the Ministry of Science, Research and Arts (MWK) of the state Baden-Württemberg within the Sub-Angstrom Low-Voltage Electron Microscopy project (SALVE).

Michael MOHN (Ulm, Germany), Ralf HAMBACH, Philipp WACHSMUTH, Ute KAISER

08:00 - 18:15 #6346 - IM08-393 Synchrotron Infrared Microspectroscopy, an innovative approach to investigate tissue chemical changes in mouse model of Pompe of disease (glycogenosis type II) and to assess efficiency in gene therapy.

IM08-393 Synchrotron Infrared Microspectroscopy, an innovative approach to investigate tissue chemical changes in mouse model of Pompe of disease (glycogenosis type II) and to assess efficiency in gene therapy.

Pompe disease is an autosomal recessive disorder caused by the deficiency of the lysosomal enzyme acid alpha-glucosidase (GAA). The disease is characterized by lysosomal glycogen storage in heart and muscles, and manifests as a fatal cardiomyopathy in infantile form. Cardiac correction by enzyme replacement therapy (ERT) has recently prolonged the lifespan of these patients, revealing a new natural history. The emergent neurologic phenotype and the poor correction of skeletal muscles in survivors are currently partly attributed to central nervous system (CNS) glycogen storage, uncorrected by ERT. A gene therapy strategy using AAV vectors delivered to cerebrospinal fluid has been set up to restore GAA activity into the CNS. We demonstrate the use of Infrared Micro spectroscopy with synchrotron light as an innovative tool to map glycogen at the subcellular level in motor neurons and cardiac fibers. Principal Component Analysis (PCA) of infrared spectral data from motor neurons and cardiac fibers show that both treated and wild-type animals are merged in the same cluster whereas infrared spectra obtained from untreated Pompe mice are characterized by increase of the bands assigned to the carbohydrates of glycogen.

This new analytical approach that allows an highly sensitive and resolutive direct probing of tissue glycogen is required to explore early biochemical change at a subcellular level and therefore to assess therapeutic efficiency for Pompe disease.

Acknowledgments : We thank the vector core of the Atlantic Gene Therapies Institute (AGT) in Nantes for the preparation of the rAAV vectors, Véronique Blouin and Philippe Moullier (INSERM UMR1089) for vector production and the technical staff of Oniris rodent facility for animal care. We acknowledge assistance from SOLEIL SMIS beamline staff for his help. This work was supported by a  grant from “Investissement d'Avenir - ANR-11-INBS-0011”  - NeurATRIS :  A Translational Research Infrastructure for Biotherapies in Neurosciences

Laurence DUBREIL (INRA/ONIRIS), Juliette HORDEAUX, Johan DENIAUD, Lydie LAGALICE, Karim BEY, Christophe SANDT, Frederic JAMME, Marie-Anne COLLE

08:00 - 18:15 #6347 - IM08-395 Towards atomic magnetic measurements with single electron vortex beams on FePt nanocubes.

IM08-395 Towards atomic magnetic measurements with single electron vortex beams on FePt nanocubes.

X-ray magnetic circular dichroism is a well-established method to study element specific magnetic properties of a material, while electron energy-loss magnetic chiral dichroism (EMCD), which is the electron wave analogue to XMCD, is scarcely used today. Recently discovered electron vortex beams, which carry quantized orbital angular momenta (OAM) L, promise to also reveal magnetic signals [1]. Since electron beams can be easily focused down to sub-nanometer diameters, this novel technique provides the possibility to quantitatively determine local magnetic properties with unrivalled lateral resolution. In order to generate the spiralling wave front of an electron vortex beam with an azimuthally growing phase shift of up to 2p and a phase singularity in its axial centre, specially designed apertures are needed [2,3]. Dichroic signals on the L₂ and L₃ edge are expected to be of the order of 5% [4,5].

 

The generation of EVBs in the double aberration-corrected FEI Titan₃ 80-300 transmission electron microscope (TEM) is achieved by the implementation of a dislocation-type apertures into the condenser lens system. The setup allows for scanning TEM investigations (STEM) with vortex beams, whose OAM is selected by means of an additional discriminator aperture. New FIB cutting strategies facilitate the production for 50 µm wide and 1 µm thick high quality vortex apertures (see fig. 1a).  However, in the case of a fork-type aperture, the EVB are dispersed in the x-y plane resulting in mixed probe that interacts with the magnetic sample.

 

We have recently devised an escape route to this problem by blocking any partial beams that carry other but the desired OAM prior to the interaction of the beam with the ferromagnetic sample. This is achieved by using a special condensor aperture in combination with a fork-type aperture to select a single partial beam with the chosen OAM (s. fig 1b). This approach allows to generate atom-sized EVB with angstrom-sized probes and a well-defined OAM by which atomic resolution HR-STEM is achieved (see. fig 2). Although this discretization results in an increased signal-to-noise ratio, this novel technique is capable of atomic resolution EELS measurements which is the prerequisite for atomic EMCD measurements. First experiments using this new optical setup show very promising EMCD results on ferromagnetic FePt nanoparticles.

 

[1] J. Verbeeck et al., Nature 467 (2010), p. 301-304.

[2] J. Verbeeck et al., Ultramicroscopy 113 (2012), p. 83-87.

[3] D. Pohl et al., Ultramicroscopy 150 (2015), 16-22.

[4] P. Schattschneider et al., Ultramicroscopy 136 (2014), p. 81-85.

[5] J. Rusz and S. Bhowmick, Phys. Rev. Lett. 111 (2013), 105504.

Darius POHL (Dresden, Germany), Jan RUSZ, Jakob SPIEGELBERG, Sebastian SCHNEIDER, Peter TIEMEIJER, Kornelius NIELSCH, Bernd RELLINGHAUS

08:00 - 18:15 #6391 - IM08-397 Measurement of energy-loss anisotropy along [001] in monoclinic hafnia and comparison with ab-initio simulations.

IM08-397 Measurement of energy-loss anisotropy along [001] in monoclinic hafnia and comparison with ab-initio simulations.

Abstract- Monochromated aberration-corrected STEM-VEELS (Valence Electron Energy-Loss Spectroscopy) measurements are performed on monoclinic hafnia. The maximum energy-loss anisotropy is observed for the [001] orientation, in agreement with ab-initio TDDFT simulations. From the fine structure of the energy loss spectra, the hypothesis of a possible Fano effect on the Hf 5p edge may be investigated.

1. Introduction

Nanoscopy inside a transmission electron microscope opens a large field of applications, in particular in the semiconductor industry. VEELS has been developed to map the opto-electronic properties at a subnanometer scale inside a HR(S)TEM. A dielectric anisotropy in m-HfO₂ has recently been observed¹ by energy-filtered TEM. The agreement with ab initio simulations is nearly quantitative² but the anisotropy along [001] and the finest structures of the spectra have not been published yet. The motivation of this presentation is to compare STEM-VEELS spectra recently recorded at high energy resolution with their ab initio simulations.

1. Methodology 

STEM measurements are performed with the FEI Titan Ultimate aberration-corrected STEM in monochromated mode with an energy resolution of 0.17 eV. The convergence and collection angles are 15.6 and 20 mrad respectively. The 25 nm-thick samples are prepared with a dedicated³ FIB, following a procedure optimized for VEELS measurements. The time-dependent density-functional theory (TDDFT) simulations are generated with the DP⁴ code using the random phase approximation including local field effects.

1. Results and discussion

The structure of monoclinic hafnia is usually complex because of the presence of small overlapping nanocrystals separated by intermixed defective interfaces. Therefore it is rather difficult to find an isolated HfO₂ grain with the well-defined [001] orientation, as checked by HRTEM focal series and associated JEMS simulations (fig. 1). The agreement between experimental and simulated diffraction patterns is also nearly perfect. The highest anisotropy in EELS spectra, is observed for the [001] orientation around the plasmon peak position (~ 16 eV, see fig. 2), when compared to random orientation spectra and with the reference spectrum of Couillard et al.⁵[A1]  . The agreement with ab-initio simulations is also remarkable (fig. 3), especially in terms of predicted peak positions, although measurements realized on a perfect single crystal of m-HfO₂ would improve the comparison. With the high energy resolution of our instrument, the Fano effect suggested to be responsible for the spectral dip associated with the excitation of the Hf 5p electrons in m-HfO₂ can be more finely investigated. The associated asymmetric line shape can be identified around 42.3 eV, with a resonance width of 0.8 eV and a coupling factor of 0.6.  The optical absorption spectra deduced from a Kramers-Krönig analysis also confirm the occurrence of such an effect in m-HfO₂.

1. CONCLUSIONS

Monochromated STEM experiments demonstrate a clear anisotropy between [001] and other crystal orientations in m-HfO₂. This anisotropy is mostly visible around the plasmon energy (16 eV).  The statistical analysis of thousands of EELS spectra confirms the existence of a Fano effect around 42.3 eV to be associated with the excitation of the Hf 5p electrons.

Acknowledgments: This work has been carried out in the nanocharacterisation platform (PFNC) of MINATEC.

[1] C. Guedj et al., Appl. Phys. Lett. 105, 222904 (2014)

[2] L. Hung et al., submitted to Phys. Rev. B (2016) 

[3] C. Guedj et al., submitted to IPFA conference (2016)

[4] http:\\www.dp-code.org

[5] M. Couillard et al., Phys. Rev. B 76, 165131 (2007)

Cyril GUEDJ (GRENOBLE CEDEX 9), Nicolas BERNIER, Christian COLLIEX, Valerio OLEVANO

08:00 - 18:15 #6396 - IM08-399 Some applications of analytical electron microscopy and high-resolution spectroscopy in the study of functional materials.

IM08-399 Some applications of analytical electron microscopy and high-resolution spectroscopy in the study of functional materials.

Electron microscopy has always played an important role in the development of new materials and for understanding properties of complex functional materials. The recent developments in instrumentation have significantly improved the insight that such techniques can provide, particularly for nanoscale materials and for fundamental studies related to bonding and electronic structure. In the area of functional materials, namely energy storage and conversion materials, plasmonic structures, and quantum materials, detailed microscopy is needed to optimize material properties and to understand their electronic properties. Here we highlight recent examples of work related to the study of functional materials, illustrating the crucial role of imaging and spectroscopy for the characterization and understanding of these materials.

 

Using an aberration-corrected TEM equipped with electron energy loss spectroscopy (EELS), we have studied the mechanism of cluster formation following atomic layer deposition on graphene nanosheets. We have also shown, with electron energy loss near-edge structures (ELNES), that it is possible to detect the presence of N dopant atoms at different atomic sites [1]. With high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and EELS, we have studied the evolution of alloy catalysts following in-situ and ex-situ annealing procedures. Starting with a disordered PtFe nanoparticle, we captured the ordering transformation, showing evidence of the formation of ordered Pt and Fe rich planes, and evidence of both Pt and Fe-rich shells over an ordered core (Figure 1) [2]. We also showed that the Pt surface segregation induces local strain and atomic displacements [2] (Figure 2) that can be further correlated to the enhanced activity of the material [3,4]. Using in-situ heating, it has also been possible to study the alloying phenomena of AuPt nanoparticles showing evidence of full miscibility starting at 200ºC (Figure 3), well below the thermodynamically expected temperature. At high-temperature, we have also detected the formation of unexpected ordered structures (Figure 4). Furthermore, we found that the annealing leads to mostly phase separation and monolayer surface segregation [5]. In a related catalyst system, we have been able to study the evolution of catalysts and hybrid supports, visualizing the presence of single atom dissolution of catalysts [6].

 

Similar approaches have been used to study the structure of LiNi_xMn_yCo_1-x-yO₂ (known as “NMC”) and (Li rich) NMC compounds. In this work, using a combination of HAADF-STEM and EELS, together with multiple-linear least squares fitting, we have demonstrated the mechanisms of charge compensation, following electrochemical cycling and the presence of monolayer-like surface changes in the valence of transition metal ions. STEM imaging and ELNES demonstrate the presence of local heterogeneities in the Li and transition metal distribution and in the local carriers distribution. The same techniques are used to probe the localization of charges in a variety of high-temperature superconductors [7,8]. Finally, examples of plasmonic imaging of hybridization phenomena in metallic nanostructures, together with rigorous simulations of the optical response, will be shown [9]. These examples highlight the power and versatility of analytical techniques in the TEM to solve important materials science and fundamental physics problems.

 

[1] S. Stambula et al., Journal of Physical Chemistry C, 118, 3890-3900, (2014)

[2] S. Prabhudev et al., ChemCatChem, 7, 3655-3664, (2015)

[3] S. Prabhudev et al., ACS Nano 7, 6103-6110,  (2013)

[4] M.C.Y. Chan et al., Nanoscale, 4 (22), 7273-7279, (2012)

[5] S. Prabhudev, C. Chiang, M. Chatzidakis et al., In Communication

[6] L. Chinchilla-Reyes, D. Rossouw et al. In Preparation

[7] N. Gauquelin et al., Nature Communications 5,  4275.  (2014)

[8] M. Bugnet et al., Science Advances, 2, (3), e1501652, (2016)

[9] E.P. Bellido et al., ACS Photonics, 3 (3), 428–433, (2016)

Sagar PRABHUDEV, Samantha STAMBULA, Lidia CHINCILLA, Hanshuo LIU, Edson BELLIDO, Isobel Claire BICKET, Alexandre POFELSKI, Steffi Y WOO, Matthieu BUGNET, Stefan LOEFFLER, David ROSSOUW, Christian WIKTOR, Gianluigi A BOTTON (Hamilton, Canada)

08:00 - 18:15 #6426 - IM08-401 Great advantages of using low voltage HR-SEM in spatial resolution and sensitivity for low energy X-ray analyses.

IM08-401 Great advantages of using low voltage HR-SEM in spatial resolution and sensitivity for low energy X-ray analyses.

Understanding surface fine features, such as topological and compositional information, is essential for controlling synthesis conditions of these materials and for designing novel materials and utilizing their functions. The materials are becoming complicated composites composed of Nano crystals in order to have functions with better performance.  Therefore, higher spatial resolution, sensitivity and capability are required for characterization techniques are required now. The recent developments in compound type objective lens, electron beam deceleration method, high solid angle multi EDS system and Soft X-ray Emission Spectrometry (SXES) of high-resolution scanning electron microscopy (HR-SEM) show great advances for the study of structures, chemical information, and electron state of Nano structured materials (1). Here, we will discuss newly developed spectroscopy approaches in EDS and SXES, and show some of their examples using low voltage (LV) HR-SEM.

The electron beam deceleration method brings a lot benefits. Because it is improving size of electron beam diameter due to smaller aberration even at LV condition, high probe current condition and long working distance (2).  Fig. 1 shows high spatial resolution EDS map from Au@TiO₂ Yolk-shell type sample where a gold nanoparticle with 15 nm in diameter is encapsulated in a TiO₂ hollow sphere. The gold nanoparticles are well resolved within 3 min even at low electron landing-energy (see observation conditions in the Fig caption).

The low energy SXES spectra obtained from Al-B nano composites are shown in Fig. 2. SXES technique requires high probe current to acquire enough X-ray signals as it is based on wavelength dispersive spectrometry (WDS). The spectra from small area of aluminum alloy (a) and aluminum boride (b) show different peak top energy and shape for aluminum L-line at around 70 eV. In addition, the boron peak was also detected at around 67 eV. The spectra at this energy range, which corresponds to K-line from light elements including Lithium, was hard to observe by EDS and WDS before. So the SXES detector has high-energy resolution and can also detect extremely low energy X-ray. 

The electron beam deceleration method has advantages in SXES analysis.  We demonstrated SXES line profiles in between metal Aluminum and Aluminum boride in Fig. 3. Actually, there is deference between with beam deceleration method or not even at 5 keV with 30 nA. Compares to Line (b) and (c), Electron beam deceleration method of line (c) shows better sharpness and details of spectrum than line (b). Probably it is due to smaller probe size was created by electron beam deceleration method. Here we have also tired low voltage condition that is 1 keV with -5 keV sample bias in line (a).  The line (a) shows much higher spatial resolution line profile due to smaller interaction volume in sold than 5 keV.  

Reference

1) M. Terauchi, H. Yamamoto and M. tanaka, Journal of Electron Microscopy, 50, 101, (2001)

2) S. Asahina, M. Suga, H. Takahashi, H. Y. Jeong, C. Galeano, F. Schuth, and O. Terasaki,. APL Materials 2, 113317 (2014); doi: 10.1063/1.4902435

Asahina SHUNSUKE (Tokyo, Japan), Takahashi HIDEYUKI, Takakura MASARU, Ferdi SCHÜTH, Terasaki OSAMU

08:00 - 18:15 #6437 - IM08-403 Probing the directionality of local electronic states in SrTiO3 by momentum-selected STEM-EELS.

IM08-403 Probing the directionality of local electronic states in SrTiO3 by momentum-selected STEM-EELS.

  Electron energy-loss spectroscopy in the scanning transmission electron microscope (STEM-EELS) enable to investigate the local electronic states at the specific atomic column selected by an incident electron probe. Even though the same elements, crystallographically non-equivalent atoms give the different energy-loss near-edge structure (ELNES) [1]. Although SrTiO₃ has a single oxygen atom in the unit cell, the chemical bonds between Ti and O have directionality as shown in Fig. 1(a), depicting the ligand field orbital with e_g symmetry formed by Ti-d_x²−y² and O-p orbital. If the oxygen K-edge ELNES is acquired with an off-axis collection aperture to select a specific direction of momentum transfer, the intensity related to the transition to the unoccupied e_g state should vary with O1 and O2 sites as predicted previously [2]. In this study, we show that the directionality of local electronic states at different oxygen sites can be detected by momentum-selected STEM-EELS.

  STEM-EELS measurements were performed using Cs-corrected STEM (JEM-9980TKP1 equipped with a cold-FEG and an omega filter) operated at 200 kV. Fig. 1(b) shows the setup of experiment. An electron probe with a convergence semi-angle (α) of 23 mrad incidents along the c-axis of SrTiO₃ and EEL spectra are recorded with the spectrum collection aperture (β = 10 mrad) placed at the edge of the transmitted beam disc along θ_y direction. In order to simulate the experimental results, the electronic structure of SrTiO₃ was calculated by WIEN2k [3] based on the density functional theory, and the O-K edge ELNESs were calculated by TELNES3 code incorporated in WIEN2k.

  Fig. 2(b) shows the experimental O-K edge ELNES obtained by scanning electrons along two different O-TiO-O lines shown in Fig. 2(a). It is found that the intensity of peak b which can be assigned to the transition to the e_g state is different between the two spectra. Since these spectra are acquired with a collection aperture shifted in the θ_y direction, the partial density of state of O-p_y hybridized with Ti-d_x²-y² state should predominantly contribute to the spectrum. Actually, the spectrum acquiring from the oxygen atoms aligned in the y-direction has an enhanced peak b, which is indicated that the present method can detect the directionality of Ti-O bonding at a specific oxygen site.

  Fig. 3(b) shows the O-K edge ELNES calculated by adding the contribution from the two oxygen sites shown in Fig. 3(a). The features in the calculated spectra are in good agreement with that of the experimental one, which verifies the reliability of the experimental finding.

References

[1] M. Haruta, H. Kurata, H. Komatsu, Y. Shimakawa and S. Isoda, Phys. Rev. B, 80, 165123 (2009).

[2] T. Mizoguchi, J. P. Buban, K. Matsunaga, T. Yamamoto and Y. Ikuhara, Ultramicroscopy, 106, 92 (2006).

[3] P. Blaha, K. Schwarz, G. Madsen, D. Kvasnicka and J. Luitz, WIEN2k, An Augmented Plane Wave Plus Local Orbital Program for Calculating Crystal Properties, edited by K. Schwarz (Vienna University of Technology, Austria, 2001).

Atsushi YAMAGUCHI (Uji, Kyoto, Japan), Mitsutaka HARUTA, Takashi NEMOTO, Hiroki KURATA

08:00 - 18:15 #6447 - IM08-405 Combined stem-eels and stem-cl analysis of plasmonic coupling between chemically grown silver nanocubes.

IM08-405 Combined stem-eels and stem-cl analysis of plasmonic coupling between chemically grown silver nanocubes.

These recent years, study of surface plasmon resonances in transmission electron microscope has undergone an increasing interest. Indeed, low-loss EELS has proven its remarkable efficiency to probe plasmonic resonances at the nanometer scale while recent developments in cathodoluminescence (CL) spectroscopy brought a new insight into the coupling of plasmons and far field [1]. Several types of particles have already been numerically and experimentally studied ranging from nano-disk to nano-prism which plasmonic behaviors are now fairly understood [2,3]. However, despite this systematic effort to characterize the widest variety of plasmonic nano-particles, some structures still remain challenging e.g. the nano-cube. In the latest case, the difficulties encountered are plural. First, this structure being highly symmetric, plasmonic modes exhibit a significant number of natural degeneracies leading to superposition or hybridization of modes, which dramatically harden their understanding. Second, cube's plasmonic modes turned out to be very sensitive to the geometry of the underlying nano-structure (e.g. edge rounding [4]) and to be strongly affected by the presence of substrate [5]. Third, particularly because of the degeneracies mentioned above, the coupling between two nanocubes brings unexpected difficulties, which are enhanced when the inter-particle gap goes below 1nm. Although this coupling has been widely tackled recently, no definitive theory has been given and this question remains controversial [6,7].

In the present work, we experimentally and numerically investigated plasmonic silver nanocubes and their coupling using STEM-EEL and STEM-CL spectroscopies (see figures 1 and 2) aiming at giving a clear and complete understanding of these modes. The cube samples have been prepared by chemical growth, which provides them a high degree of crystallinity. This property dramatically enhance the CL signal and thus enable us to obtain remarkably relevant CL-maps (see figure 2). For studying the coupling, in order to get rid of the quantum charge transfer problem arising at very small gap regime (<0.5nm), we restricted ourselves to inter-particle separations larger than 5nm. Complementary BEM simulations [8] have been carried out to understand on a deeper level the observed plasmonic modes by computing the corresponding charge distributions. We observed a good agreement between computations and experiments, which strengthen our conclusion. In addition to be consistent with the earlier studies, our work bring an overall understanding of the nanocubes coupling.

References

[1] Kociak, Stephan, Chemical Society Reviews 43, 3865-3883, 2014.

[2] Schmidt and al, Nano Lett. 12 (11), 5780–5783, 2012.

[3] Nelayah and al, Nano Lett. 10 (3), 902–907, 2010.

[4] Grillet and al, ACS Nano 5 (12), 9450–9462, 2011.

[5] Mazzuco and al, Nano Lett. 12 (3), 1288–1294, 2012.

[6] Tan and al, Science 243 (6178), 1496-1499, 2014.

[7] Knebl and al, Phys. Rev. B. 93 (8), 081405(R), 2016.

[8] Hohenester, Trügler, Comput. Phys. Commun 183, 370, 2012.

Hugo LOURENÇO MARTINS (Palaiseau), Yih Hong LEE, Yejing LIU, Hiang Kwee LEE, Mathieu KOCIAK, Xing Yi LING

08:00 - 18:15 #6486 - IM08-407 New version of the EELS database: eelsdb.eu.

IM08-407 New version of the EELS database: eelsdb.eu.

     Since its creation at the end of the 1990’s, the EELS and X-ray Absorption Spectroscopy (XAS) database has gathered more than 220 spectra covering 37 elements of the periodic table, becoming the largest open-access electronic repository of spectra from EELS and XAS experiments. The EELS database is now a common tool used by spectroscopists, theoreticians, students and private firms as a reference catalog for fine structures and data-treatment analyzes^2-4 and has been referenced by more than 30 papers. Much of this success is due to the open-access nature of the database. The database depends on voluntary user contributions; to encourage these contributions, we have performed a major update of the website.

 

     The EELS and XAS database has been completely rewritten, with an improved design, user interface and a number of new tools. The database is accessible at https://eelsdb.eu/ (Fig. 1) and can now be used without registration. The submission process has been streamlined to encourage spectrum submissions (Fig. 2) and the new design gives greater emphasis on contributors’ original work by highlighting their papers. With numerous new filters and a powerful search function, it is now simple to explore the database of several hundred of EELS and XAS spectra. Interactive plots allow spectra to be overlaid, facilitating online comparison. An application-programming interface has been created, allowing external tools and software to easily access the information held within the database. In addition to the database itself, users can post and manage job adverts and read the latest news and events regarding the EELS and XAS communities.  In accordance with the ongoing drive towards open access data increasingly demanded by funding bodies, the database will facilitate open access data sharing of EELS and XAS spectra.⁵

 

Acknowledgement: The authors would like to thank the IMN and CEMES laboratories, the European microscopy network ESTEEM 2, the French microscopy network METSA and the French microscopy society Sfµ, for the funding. The authors warmly acknowledge everyone who has contributed to the database.

 

1. T. Sikora and V. Serin, EMC 2008 14th European Microscopy Congress, pp-439-440, Springer-Verlag Berlin (2008)

2. N. Bernier et al., Materials Characterization, 86, pp-116-126 (2013)

3. L. Zhang et al., Physical Review B, 81, 035102 (2010)

4. R. Núñez-González et al., Computational Materials Science, 49,  pp-15-20 (2010)

5. P. Ewels, T. Sikora, V. Serin, C.P. Ewels and L. Lajaunie, Microscopy and Microanalysis, In Press. DOI: 10.1017/S1431927616000179

Philip EWELS, Thierry SIKORA, Virginie SERIN, Chris P. EWELS, Luc LAJAUNIE (Zaragoza, Spain)

08:00 - 18:15 #6512 - IM08-409 Spectroscopic and chemical characterization at sub-nanometer scale using EELS.

IM08-409 Spectroscopic and chemical characterization at sub-nanometer scale using EELS.

Advanced materials require control of the structural properties at the sub-nanometer or atomic scale. This is mandatory in the case of interfaces for electronic devices, or in nanostructures, where the surface/interface has a fundamental role in view of their applications (e.g. nanoparticles for catalysis, functionalized nanoparticles, etc…). To fully characterize these systems, the chemical information at high spatial resolution (even at atomic scale) has to be achieved. This is possible by using electron energy loss spectroscopy (EELS) in a scanning transmission microscope (STEM), taking advantage of probe aberration correction and high resolution spectrometers. In this contribution we will show some examples of the use of this technique for resolving the oxidation state (valence) of atoms and their electronic configuration in different nanosystems. The first case is a CeO₂/Pt epitaxial heterostructure, in which EELS from the Ce-M4,5 ionization edge reveals a one atomic thick layer of reduced Ce3+ atoms at the interface [1] due to the charge transfer between CeO₂ and Pt, as predicted by theory (see Figure 1a). The second case focuses on magnetic FeCoO_x nanoparticles (NPs), in which EELS mapping of the oxidation state of the metal atoms can be used to resolve a core/shell structure between wustite and magnetite, and its evolution in the oxidation process. (Figure 1b). Finally, recent results are shown on mapping the extension of the plasmons and excitons states in a coupled system as Au functionalized ZnO nanostructures (Figure 2). After careful fitting of the spectra, a signal resembling the ZnO exciton (NBE) is seen to extend in the metal nanoparticle [2,3].

ACKNOWLEDGEMENTS

[1] Luches, P. et al. Atomic Scale Structure and Reduction of Cerium Oxide at the Interface with Platinum. Adv. Mater. Interfaces, 2. doi: 10.1002/admi.201500375 (2015)

[2] Bertoni, G. et al. Nanoscale mapping of plasmon and exciton in ZnO tetrapods coupled with Au nanoparticles. Sci. Rep. 6, 19168. doi: 10.1038/srep19168 (2016)

[3] European Union FP7 Grant Agreement n. 265073 ITN-Nanowiring, and FP7 Grant Agreement n. 312483 ESTEEM2 for Integrated Infrastructure Initiative – I3

Giovanni BERTONI (Parma, Italy), Stuart TURNER, Giancarlo SALVIATI

08:00 - 18:15 #6541 - IM08-411 Revealing the Morphology of Organic Bulk Heterojunction Solar Cells Using EFTEM and Low-Energy STEM.

IM08-411 Revealing the Morphology of Organic Bulk Heterojunction Solar Cells Using EFTEM and Low-Energy STEM.

The morphology of organic bulk heterojunction (BHJ) solar cells decisively influences the device performance and efficiency and therefore is an important factor that needs to be investigated to gain a better understanding and improvement of the devices. Especially the nanoscale morphology of the active layer plays an important role as it determines the charge separation at the interfaces and the percolation pathways to the electrodes. This nanoscale morphology depends not only on the involved materials but also on their molecular weight and their treatment like thermal annealing and solvent vapor annealing.

Transmission electron microscopy (TEM) is an approved technique to study the morphology of organic solar cells. Due to the similarity of the organic materials involved in the BHJ active layer regarding the chemical composition and the formation of homogeneously thin films, the contrast in TEM images is often uniform and no significant structures can be seen. Thus conventional imaging techniques are often not sufficient to identify and distinguish the polymers and the fullerene derivatives. Here we will demonstrate that energy-filtered TEM (EFTEM) is a powerful technique to visualize the material distribution in organic BHJ active layers. We present three concepts using different information gained in EFTEM investigations: i) the elemental information, ii) the plasmonic information, and iii) pre-carbon imaging. We demonstrate that the results of these three concepts are in good agreement and that the morphology can be reliably and consistently determined using EFTEM. To corroborate the reliability of these three concepts we present different material systems.

Figure 1 shows the results of the EFTEM investigation for a P3HT:PCBM BHJ film. Due to the different plasmon energies of the materials (P_P3HT = 21.9 eV, P_PCBM = 24.5 eV) the respective plasmonic energy regions represent P3HT and PCBM in the blend. The elemental maps of sulfur and carbon are used to represent P3HT and PCBM, respectively, due to the different elemental compositions (S_P3HT = 4.0 at%, S_PCBM = 0.0 at%, C_P3HT = 40.0 at%, C_PCBM = 81.8 at%). Additionally, the pre-carbon image represents P3HT as the carbon signal is suppressed and the sulfur signal enhanced. These EFTEM investigations clearly elucidate the morphology of this blend exhibiting P3HT fibers with diameters of 10 nm. Comparing the results of the three concepts clearly shows the good agreement of the determined morphology.

Furthermore, we demonstrate the capabilities of low-energy scanning transmission electron microscopy (STEM). STEM at low electron energies exhibits enhanced material contrast and can therefore be used to visualize the material distribution of polymers and fullerene derivatives in a BHJ film. Figure 2 shows a STEM BF image of the same P3HT:PCBM BHJ film at a high tension of 15 kV. The P3HT fibers are clearly visible and consistent with the EFTEM investigation. The origin of material contrast will be discussed. Low-energy STEM is a highly promising alternative for determination of the morphology of organic BHJ solar cells as it features a high throughput SEM based technique.

Using EFTEM and low-energy STEM the morphology of various organic BHJ solar cells can be elucidated, leading to a better understanding and improvement of the device performance.

Acknowledgements:

Financial support by the German Science Foundation (DFG) within the frameworks of the SFB 953 “Synthetic Carbon Allotropes”, the Cluster of Excellence EXC 315 “Engineering of Advanced Materials”, the Research Training Group GRK1896 as well as by the Marie Curie Initial Training Network (ITN) within the European Union’s Seventh Framework Programme (Grant agreement no. 607585, OSNIRO) is gratefully acknowledged.

Stefanie FLADISCHER (Erlangen, Germany), Peter SCHWEIZER, Tayebeh AMERI, Christoph BRABEC, Erdmann SPIECKER

08:00 - 18:15 #6559 - IM08-413 High Resolution Chemical Imaging on the Helium Ion Microscope.

IM08-413 High Resolution Chemical Imaging on the Helium Ion Microscope.

The ORION NanoFab helium ion microscope (HIM) has become an ideal tool for imaging and nano-patterning tool owing to its high lateral resolution [1]. Helium ions lead to resolutions of 0.5 nm for SE based imaging, while structures with sub 20 nm feature sizes may be rapidly patterned using Ne. Despite these advantages, the analysis capability of the machine is currently limited. At beam energies of 35 kV helium ions do not lead to the emission of characteristic X-rays from a sample. While some compositional information can be obtained from back scattered helium [2], identifying elemental information is more difficult. Secondary Ion Mass Spectrometry (SIMS) is a powerful ion beam based technique for analyzing surfaces with high sensitivity and high mass resolution. SIMS is based on the generation and identification of characteristic secondary ions by irradiation with a primary ion beam (in this case helium or neon). The typical interaction volume for SIMS is around 10 nm in the lateral direction. As the probe size in the HIM is substantially smaller (both for He and Ne) the lateral resolution is limited only by fundamental considerations and not, as is currently the case on commercial SIMS instruments, the probe size [3-4]. The prospect of adding SIMS to the HIM yields not just a powerful analytical capability, but opens the way for in-situ correlative imaging combining high resolution SE images with elemental maps from SIMS [5].

We have developed a prototype SIMS spectrometer specifically adapted to the HIM. Notably the instrument is capable of producing elemental SIMS maps with lateral resolution limited only by the fundamental interaction between the primary beam and the sample. All elements/isotopes and small clusters with masses up to 500 amu are detectable with a mass resolution M/ΔM greater than 400 and parallel detection of 4 mass channels.

In this presentation we will introduce the HIM-SIMS technique and present the latest application results in the field of materials science.

 

References

[1] L. Scipioni et al, J. Vac. Sci. Technol. B 27 (2009) 3250

[2] G. Hlawacek, V. Veligura, R. Van Gastel , B. Poelsema, J. Vac. Sci. Technol. B 32 (2014) 020801

[3] T. Wirtz, N. Vanhove, L. Pillatsch, D. Dowsett, S. Sijbrandij, J. Notte, Appl. Phys. Lett. 101 (2012) 041601

[4] L. Pillatsch, N. Vanhove, D. Dowsett, S. Sijbrandij, J. Notte, T. Wirtz, Appl. Surf. Sci. 282 (2013) 908

[5] T. Wirtz, P. Philipp, J.-N. Audinot, D. Dowsett, S. Eswara, Nanotechnology 26 (2015) 434001

David DOWSETT, Jean-Nicolas AUDINOT, Florian VOLLNHALS, Santhana ESWARA (Esch-sur-Alzette, Luxembourg), Tom WIRTZ

08:00 - 18:15 #6583 - IM08-415 Quantitative energy dispersive X-ray spectroscopy on thin SiGe layers.

IM08-415 Quantitative energy dispersive X-ray spectroscopy on thin SiGe layers.

The precise measurement of Ge content is of utmost importance in SiGe technology. Analytical methods like X-ray diffraction (XRD) or secondary ion mass spectrometry (SIMS) allow the SiGe stoichiometry measurement in structures larger than 100 µm. However, for SiGe heterojunction bipolar transistors (HBT), Ge profiles in areas of typical transistor dimensions of about 100 nm are of interest.  (Scanning) transmission electron microscopy ((S)TEM) in combination with energy dispersive X-Ray spectroscopy (EDX) or electron energy loss spectroscopy (EELS) is one of the very few suitable methods for this purpose.

Here, we present an approach for measuring Ge profiles in small areas with a lateral resolution of about 5 nm using the Cliff-Lorimer method for quantification of EDX data of thin TEM samples and a calibration of the Cliff-Lorimer factors to Ge profiles measured by XRD in large areas. We have investigated thin SiGe layers with thicknesses of about 20 nm and Ge content of about 30 at%. The Ge content was proofed by XRD as well. For EDX analysis, we have used a TEM FEI Tecnai Osiris operated at 200 kV in STEM mode. The EDX quantification was performed with Cliff-Lorimer method using Esprit-Software from Bruker.  Measurements were taken on TEM sample with different thicknesses. The sample thicknesses were evaluated by EELS log-ratio method in silicon area close to the SiGe-layer [1]. Using this method, the specimen thickness is given in inelastic mean free path (mfp) units. The Cliff-Lorimer method is widely used for quantification of EDX data of thin TEM samples. However, there is an uncertainty in the Cliff-Lorimer factors for standard free measurements and the Cliff-Lorimer method neglects any absorption effects which may become important for thicker TEM samples. Therefore it is necessary to proof the accuracy of quantification by using calibration samples with known Ge concentration.   

Figure 1 shows bright field STEM images of a Si_1-xGe_x layer with x=0.305 (a) and a SiGe HBT (b). Figure 2 shows Ge line profiles of SiGe layer from figure 1a quantified using the K edges of Si and Ge measured at different TEM sample thicknesses. The obtained Ge concentration does not directly depend on sample thickness. However for very thick samples with a thickness of 2.73 mfp or 4.17 mfp, the apparent Ge concentration is clearly reduced. It is possible to explain the lowered concentration of very thick samples by stray radiation of surrounding Si and limited lateral resolution. Figure 3a shows the lateral resolution which was determined from line profiles using the method suggested by Williams and Carter in [2]. Points with concentration of 10 % and 90 % of the maximum concentration were measured and the distance between these points is multiplied by a factor of 1.8. The results from figure 3a and figure 2 clearly show that the sample thicknesses in the range 0.5 mfp to 1.0 mfp are a good compromise between lateral resolution and adequate signal-to-noise ratio. However, even for usual sample thicknesses of 0.45 mfp or 0.77 mfp, we have obtained a Ge concentration above 30.5 at% for the quantification with Ge K edge using a Cliff-Lorimer factor of 2.386 and a slightly lower concentration for the quantification with Ge L edge.  This suggests that an adjustment of Cliff-Lorimer factors of Ge K edge and L edge for accurate quantified results of Ge concentration in SiGe samples is necessary. By using an adjusted Cliff-Lorimer factor of 2.2 for Ge K edge and sample thickness below 1.0 mfp an error below ±10 % and a resolution of about 5 nm is achieved. Figure 3b shows a Ge line profile measured on SiGe HBT from figure 1b using EDX and quantified with adjusted Cliff-Lorimer factor.

 

1. T. Malis, S.C. Cheng, and R.F. Egerton, J. Electron. Microsc. Tech. 8, 193-200, 1988

2. D.B. Williams and C.B. Carter, Plenum Press 1996, page 626

Markus Andreas SCHUBERT (Frankfurt (Oder), Germany), Peter ZAUMSEIL, Ioan COSTINA, Holger RÜCKER

08:00 - 18:15 #6587 - IM08-417 Detection of magnetic circular dichroism in amorphous materials utilizing a single-crystalline overlayer.

IM08-417 Detection of magnetic circular dichroism in amorphous materials utilizing a single-crystalline overlayer.

   Electron energy-loss magnetic chiral dichroism (EMCD) in a transmission electron microscope allows the quantification of the magnetic structure of crystalline materials down to the nanometer scale ^[1-3]. However, restricted by a confined diffraction geometry applied in EMCD experiments ^[4], no experiments or theories have yet been performed to obtain EMCD signals for amorphous materials, due to their lack of long range ordering.

  In this work, we for the first time demonstrate it is possible to detect element-specific magnetic signals in amorphous materials utilizing a single-crystalline overlayer as an EMCD beam splitter. The approach is applied to a bilayer sample where a very thin amorphous magnetic FeO_x layer is grown on a single-crystalline Yttrium-stabilized ZrO₂ substrate. We found that both experimental results and theoretical calculations lead to unprecedented EMCD signals. The quantitative orbital to spin magnetic moment ratio of Fe in amorphous FeO_x layer has been acheived.

  Our approach allows us to break through the constraint of crystal formats in EMCD spectra measurements, providing new prospects of detecting EMCD signals from amorphous and ultrathin materials at the nanometer scale. This approach might also be extended to the magnetic quantitative analysis of other heterogeneous materials at high spatial resolution.

  This work may open a door to meet the challenge of exploring magnetic states and behaviors of amorphous films, and have important consequences for revealing the magnetic structures of magnetic materials in various crystal forms at the nanoscale using transmission electron microscopy.

References

[1] Schattschneider, P., et al, Nature 441, 486 (2006).

[2] Rusz, J., et al, Phys. Rev. B 75, 214425 (2007).

[3] Wang, Z.Q., Zhong, X. Y., et al, Nat. Commun. 4, 1395 (2013).

[4] Lidbaum, H., et al, Phys. Rev. Lett. 102, 037201 (2009).

Acknowledgements

This work was financially supported by National 973 Project of China (2015CB921700, 2015CB654902), National Natural Science Foundation of China (51471096, 11374174, 51390471 and 51322101), Tsinghua University Initiative Scientific Research Program and National High Technology Research and Development Program of China (2014AA032904). This work made use of the resources of the National Center for Electron Microscopy in Beijing and Tsinghua National Laboratory for Information Science and Technology. J.R. acknowledges financial support of Swedish Research Council, STINT and Göran Gustafsson's Foundation. H.L. Xin. acknowledges support from the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. We are grateful to Prof. R. Yu, Dr. Y. Shao, Mrs. Z.Y. Cheng, Mr. D.S. Song for beneficial discussions, and Prof. P. Yu, Dr. Z.P. Li and Mr. Z.Y. Liao for providing the substrate and preparing the specimen.

Xiaoyan ZHONG (Beijing, China), Jie LIN, Song CHENG, Jan RUSZ, Huolin XIN, Bing CUI, Kocevski VANCHO, Lili HAN, Jing ZHU

08:00 - 18:15 #6599 - IM08-419 IR Nano-spectro-imaging: Characterization of polymeric nanoparticles and in vitro study of their interaction with cells.

IM08-419 IR Nano-spectro-imaging: Characterization of polymeric nanoparticles and in vitro study of their interaction with cells.

Recently near-field techniques play a fundamental role in Nanoscience microscopy. Two different ways exist to make infrared studies with near-field techniques: optical techniques measuring the transmitted signal coming from the nano-object or photothermal approachs using thermometer to link temperature to absorption measurements. Considering the limitations of the optical techniques, we have developed in our team an innovative photothermal technique called AFM-IR¹. The AFM-IR technique is a user-friendly benchtop technique that enables infrared spectroscopy with a spatial resolution well below conventional optical diffraction limits. It acquires IR absorption imaging spectrally resolved with lateral resolution of tens of nanometer².

First we will present the experimental set-up and some of the critical technical sides and then illustrate technological advances. Then we will show the result obtained on biodegradable polymeric nanoparticles. This work has been done in collaboration with the Institut of molecular science (ISMO). We have used this technique to characterize the composition of the nanoparticles (drug encapsulation, presence of lignad) and to localize them inside fixed macrophages at the subcellular scale.

[1] A.Dazzi, R.Prazeres, F.Glotin, J.M.Ortega, Opt. Lett., Vol. 30, Issue 18, 2388-2390 (2005).

[2] F. Lu, M. Jin, M.A. Belkin, Nat. Photon. 8, 307–312 (2014).

Jérémie MATHURIN (Orsay), Elisabetta PANCANI, Ariane DENISET-BESSEAU, Ruxandra GREF, Alexandre DAZZI

08:00 - 18:15 #6614 - IM08-421 Probing the radiative and full electromagnetic local densities of states with electron energy loss spectroscopy and cathodoluminescence spectroscopy.

IM08-421 Probing the radiative and full electromagnetic local densities of states with electron energy loss spectroscopy and cathodoluminescence spectroscopy.

Electron Energy Loss Spectroscopy (EELS) and CathodoLuminescence spectroscopy (CL) map the surface plasmon modes of metallic nanoparticles at the nanometer scale [1]. Although they yield similar results, EELS and CL differ by nature. EELS measures the energy lost by fast electrons interacting with a sample whereas CL measures the energy of the light emitted from this interaction [2]. However, the consequence of this different nature has remained unexplored because combining EELS and CL on single nanoobjects could not be achieved.

We combined EELS and CL on single nanoobjects using a Scanning Transmission Electron Microscope (STEM) [3]. In a STEM, an electron beam is focused onto a nanometric probe that is scanned over the sample. At each point of the sample, the transmitted electrons are collected for EELS, while the emitted light is collected for CL; both a full EELS spectrum and a full CL spectrum are recorded. We studied small gold triangular nanoprisms as a model system (Fig. 1a). The combination of EELS and CL allows us to map the same surface plasmon modes using both techniques (Fig. 1b and c).

We showed that CL only probes the dipolar mode, contrary to EELS that probes both the dipolar and quadrupolar modes. Furthermore, the dipolar mode resonates at shifted resonances in EELS and CL for prisms with edge lengths larger than 100 nm relying on carbon substrate (Fig. 2a). The magnitude of the shift relates to the mode damping.

Having demonstrated that EELS and CL signals differ, we showed that the full and radiative ElectroMagnetic Local Densities Of States (EMLDOS) differ similarly (Fig. 2b) [4]. Whereas EELS measures a quantity close to the full EMLDOS projected along the electron direction [5], CL measures a quantity close to the radiative EMLDOS projected along the electron direction [4]. Contrary to EELS, CL probes only the radiative modes, which are not necessarily dipolar for object sizes beyond the quasistatic limit. The ratio of the CL resonance to the EELS resonance quantifies the radiative weight of the mode damping. If the modes are damped and induce interfering radiation, CL resonant line shapes are shifted and asymmetric compared to EELS.

This work demonstrates the great interest of combining EELS and CL for plasmonics [3, 4].

[1] M. Kociak and O. Stéphan, Chem. Soc. Rev. 43, 3865 (2014)

[2] F. J. García de Abajo, Rev. Mod. Phys. 82, 209 (2010)

[3] A. Losquin et al., Nano Lett. 15, 1229 (2015)

[4] A. Losquin and M. Kociak, ACS Photonics 2, 1619 (2015)

[5] F. J. García de Abajo and M. Kociak, Phys. Rev. Lett. 100, 106804 (2008)

The research leading to these results has received funding from the European Union Seventh Framework Programme [No. FP7/2007-2013] under Grant Agreement No. n312483 (ESTEEM2). We gratefully aknowledge the following collaborators who significantly contributed to this work : Luiz F. Zagonel, Viktor Myroshnychenko, Benito Rodríguez-González, Marcel Tencé, Leonardo Scarabelli, Jens Förstner, Luis M. Liz-Marzán, F. Javier García de Abajo and Odile Stéphan.

Arthur LOSQUIN (Lund, Sweden), Mathieu KOCIAK

08:00 - 18:15 #6645 - IM08-423 High energy-resolution EELS of ferroelectric and paraelectric BaTiO3 phases.

IM08-423 High energy-resolution EELS of ferroelectric and paraelectric BaTiO3 phases.

BaTiO₃ (BTO) is a widely studied material with several potential applications as a result of its intrinsic ferroelectricity. It undergoes multiple structural phase transitions across a range of accessible temperatures, which have an effect on its ferroelectric properties. While BTO is ferroelectric in its low-temperature phases—rhombohedral below ∼183 K, orthorhombic in the range ∼183–273 K, and tetragonal in the range ∼273–393 K;  it becomes paraelectric above ∼393 K [1]. The ferroelectricity of BTO is directly related to the deviation of the TiO₆ octahedra from perfectly undistorted units, which is linked to the off-centering of the Ti⁴⁺ cation within a octahedron constituted of six O atoms. Nevertheless, the phase transition mechanisms are still widely discussed, and the exact structure of the paraelectric phase remains unclear. Probing the structural distortion within TiO₆ octahedra of the different BTO phases is therefore of particular interest, especially at the nanoscale for BTO thin films and nanostructures. Recently, it was shown that the O-K energy-loss near-edge structures (ELNES) permitted the probing of this subtle structural distortion [2]. The broadening of the ELNES at lower energy is directly related to the Ti⁴⁺ off-centering. The O-site symmetry affects the core-hole potential created during excitation, which then induces the broadening in the ELNES.

In this contribution, the structural distortion of BaTiO₃ (BTO) is studied in its ferroelectric (rhombohedral and tetragonal), and paraelectric phases from the O-K and Ti-L₂₃ near-edge structures in electron energy-loss spectroscopy [3]. The high energy-resolution O-K and Ti L₂₃ ELNES of ferroelectric and paraelectric BTO are recorded in a monochromated scanning transmission electron microscope (STEM), using cooling and heating stages to reach the phase transitions in a single crystal thin foil. Modifications of the electronic structure are detected in the lowest energy fine structure of the O-K edge in the ferroelectric phases (Fig. 1a), and are interpreted by core-hole valence-electron screening geometry (Fig. 1c). The broader and more asymmetric lowest energy fine structure at low temperature, suggest that the magnitude of the Ti⁴⁺ off-centering along ⟨111⟩ increases in lower-temperature phases. Interestingly, the lowest energy fine structure of the paraelectric phase is comparable to the one obtained at room temperature, hence supporting reports in the literature that the paraelectric phase is actually not cubic [4]. First principles calculations support these experimental evidences: they confirm that the lowest energy fine structure of the O-K edge is broader for a lower O-site symmetry, but do not reproduce the asymmetry and the overall shape of this fine structure (Fig. 1b). These discrepancies are ascribed to the approximations inherent to the static core-hole used within the DFT framework.

Furthermore, while the Ti-L₂₃ ELNES is commonly used to probe and interpret the structural distortions in titanates, we show that they are only as sensitive as the O-K ELNES to the structural distortions in BTO (Fig. 2). This finding indicates that the O-K edge can be used instead of, or complementary to, the Ti-L₂₃ edge to probe the structural distortion, and therefore the ferroelectricity, in BTO. The sensitivity of the O-K edge to subtle structural distortions in BTO shows a new way to probe and better understand the ferroelectricity at the nanoscale, on defective or strained BTO thin films for example [5].

[1] G. H. Kwei et al., J. Phys. Chem. C 97, 2368 (1993).

[2] M. Bugnet et al., Physical Review B 88, 201107(R) (2013).

[3] M. Bugnet et al., Physical Review B 93, 020102(R) (2016).

[4]  R. Comes et al., Solid State Commun. 6, 715 (1968); B. Zalar et al., Phys. Rev. B 71, 064107 (2005).

[5] The EELS work was carried out at the Canadian Centre for Electron Microscopy, a National Facility supported by NSERC and McMaster University and the Canada Foundation for Innovation under MSI funding. G.R. acknowledges the Institut de Physique (INP) of Centre National de la Recherche Scientifique (CNRS) for financial support through an International Program for Scientific Cooperation (PICS).

Matthieu BUGNET (Ontario, Canada), Guillaume RADTKE, Steffi Y WOO, Guo-Zhen ZHU, Gianluigi A BOTTON

08:00 - 18:15 #6704 - IM08-425 Nanoscale phase separation and electronic structure of metastable bcc Cu-Cr alloy thin films.

IM08-425 Nanoscale phase separation and electronic structure of metastable bcc Cu-Cr alloy thin films.

The excellent electrical and thermal transport properties of copper (Cu) make it an attractive material for electronic applications. However, its poor intrinsic mechanical properties and oxidation resistance are strongly limiting the field of application. Alloying Cu with chromium (Cr) is an effective way of increasing the oxidation resistance of such alloys. However, under equilibrium conditions the solubility limit of Cr in Cu is far below 1 at.%. Supersaturated Cu-Cr alloys can be synthesized by non-equilibrium thin film deposition techniques. The high cooling rates bring another positive effect of grain refinement that leads to a strong increase of the mechanical properties. In a thin film with nominal composition of Cu₆₇Cr₃₃ (at.%) nanoscale phase separation is characterized by energy dispersive X-ray spectroscopy (EDS) in an aberration-corrected scanning transmission electron microscope (STEM). The electronic structure of Cu in body centered cubic (bcc) crystal structure is analyzed by localized electron energy loss spectroscopy (EELS) and experimental spectra are contrasted to spectra calculated by density functional theory (DFT).

 

Co-evaporation using molecular beam epitaxy (MBE) obtains metastable Cu-Cr alloy thin films with nominal thickness of 300 nm and composition of Cu₆₇Cr₃₃ (at.%). Selected area electron diffraction confirms the bcc crystal structure of the thin films with columnar grains of ~50 nm in diameter. Aberration-corrected STEM in combination with EDS establishes compositional fluctuations within the grains on the order of 1 – 5 nm. The domains adopt the bcc crystal structure shown in the HAADF(High Angle Annular Dark-Field)-STEM image of Fig. 1. The chemical phase separation in Cu- and Cr-rich domains with composition of Cu₈₅Cr₁₅ (at.%) and Cu₄₂Cr₅₈ (at.%) is illustrated in Fig. 2. The alignment of the interface between the Cu- and Cr-rich domains shows a preference for {110}-type habit plane. The electronic structure of the Cu-Cr thin films is investigated by EELS and is contrasted to an fcc-Cu reference sample given in Fig. 3. The main differences between bcc- and fcc-Cu are related to differences in van Hove singularities in the electron density of states. In Cu-Cr solid solutions with bcc crystal structure a single peak after the L₃-edge, corresponding to a van Hove singularity at the N-point of the first Brillouin zone is observed. Spectra computed for pure bcc-Cu and random Cu-Cr solid solutions with 10 at.% Cr confirm the experimental observations. Changes in electronic structure of supersaturated solid solutions of Cu by alloying with Cr are discussed in detail.

Christian H. LIEBSCHER (Düsseldorf, Germany), Christoph FREYSOLDT, Teresa DENNENWALDT, Tristan HARZER, Gerhard DEHM

08:00 - 18:15 #6705 - IM08-427 Characterization of calcifications in human kidney by spectromicroscopy at the nanometer scale.

IM08-427 Characterization of calcifications in human kidney by spectromicroscopy at the nanometer scale.

Randall's plaques are calcium phosphate deposits, at the origin of kidney stones. To date, little is known about the mechanisms involved in their formation. µFTIR (µFourier Transformed InfraRed spectroscopy) on samples from different kidneys indicate the presence of carbo-apatite, whitlockite and their co-existence with amorphous calcium phosphate phases. In the present study, our aim was to localize Randall's plaques at the early stages of their formation and to characterize their composition and crystallinity as a function of their localization, at the nanometer scale.  Small pieces of papilla tip from healthy papillae of human kidneys were chemically fixed and embedded in epoxy resin.  Ultrathin sections were analyzed by Transmission Electron Microscopy and Electron Energy-Loss Spectroscopy (EELS).  Nano-calcifications were identified within vesicles, in many cases in close contact with collagen bundles (figure 1). These vesicles whose role and nature is to be determine could be the first step toward plaque formation. Selected Area Electron Diffraction (SAED) evidence that microvesicles contained a few nanocrystals whose diffraction pattern is compatible with the presence of crystalline apatite or whitlockite. Basic maps of light elements of biological interest (Ca, P, N, O) confirm that dense deposits are mainly composed of CaP (figure 1). To go further, data were decomposed using the Hyperspy open source software for principal components analysis (PCA). The fine structure of the different EELS characteristic signals allows to investigate the composition of the nano-calcifications in order to try to discriminate between the various phases identified by µFTIR (carbo-apatite, whitlockite, amorphous calcium phosphate).

Marta DE FRUTOS (LPS, Orsay), Alexandre GLOTER, Dominique BAZIN, Marie-Christine VERPONT, Michel DAUDON, Emmanuel LETAVERNIER, Odile STEPHAN

08:00 - 18:15 #6714 - IM08-429 Momentum-resolved energy loss spectra from ultra-thin metal oxide layers obtained at high spatial resolution.

IM08-429 Momentum-resolved energy loss spectra from ultra-thin metal oxide layers obtained at high spatial resolution.

Momentum-resolved electron energy loss spectroscopy (MREELS) probes the momentum (q) dependence (dispersion) of energy losses from characteristic excitations such as excitons, plasmon, and interband excitations thus probing bandstructures, indirect excitations, and dipole-forbidden excitations.¹ However, the ability to obtain momentum-resolved spectra both at high spatial and momentum resolutions has remained difficult. As such, layer-resolved momentum spectra from ultra-thin films, heterostructures as well as from nanostructures has not been widely reported.

We present an experimental approach that enables the acquisition of momentum resolved spectra at  high spatial resolution down to 2 nm using a nano-beam electron diffraction approach.² Through this approach we have obtained momentum resolved spectra from individual, differently-oriented nano-domains in an ultra thin (12 nm) PrNiO₃ layer as well as spectra from different positions in a LaNiO₃ thin film (70 nm).

Figure 1(a) displays a Z-contrast image of the LaNiO₃ thin layer grown on a LaSrAlO₄ substrate. A nano-beam electron diffraction pattern from the LaNiO₃ layer is shown in Figure 1(b). A slit is used to select the 200, -200 set of diffraction spots (shown by the red rectangle) allowing electrons that have been scattered to certain scattering angles into the spectrometer. This corresponds to the Г-X direction of the Brillouin zone in the pseudo-cubic symmetry of LaNiO₃. The resulting image (ω-q map) shown in figure 1(c) displays energy losses as function of momentum transfer along the Г-X (q// [100] ) direction in the reciprocal space. The presented approach will enable the acquisition of momentum resolved spectra from nano-structured materials, thin films, interfaces, surfaces, and heterostructures at high spatial, energy, and momentum resolutions.

References & acknowledgements:

^1. R. D. Bringans, and W. Y. Liang, J. Phys. C: Solid State Phys. 14, 1065 (1981). 

^2. M. K. Kinyanjui, G. Benner, G. Pavia, F. Boucher, H.-U. Habermeier, B. Keimer, and U. Kaiser, Appl. Phys. Lett. 106, 203102 (2015) 

^3. We gratefully acknowledge financial support by the German Research Foundation (DFG) and the Ministry of Science, Research and the Arts (MWK) of the state Baden-Württemberg within the DFG: KA 1295/17-1 project.

Michael KINYANJUI (Ulm, Germany), Gerd BENNER, Ute KAISER

08:00 - 18:15 #6733 - IM08-431 Experimental Determination of the Solid Angle of EDXS Detectors.

IM08-431 Experimental Determination of the Solid Angle of EDXS Detectors.

A limiting parameter in energy-dispersive X-ray spectrometry (EDXS) in a transmission electron microscope (TEM) is the magnitude of the solid angle of the EDXS detector: Characteristic X-rays are emitted equally distributed into the whole space, but only a small part within the detector solid angle is detected. A larger solid angle results in a larger collection efficiency of X-rays and a higher sensitivity of the detector.
A geometrical calculation of the solid angle is difficult since it is based on the knowledge of the detector geometry [1,2], which is not very well known. An experimental procedure was described by Egerton et. al.  [3], using NiO where density and composition are known, measuring the number of characteristic X-rays of the Ni-K line and applying values for the ionization cross section and fluorescence yield. This measurement was made using the Ni-K line only. In addition reliable values for ionization cross sections and fluorescence yields are hard to find and typically exhibit severe uncertainties. Therefore we did an extensive literature research to obtain the most accurate, state-of-the-art values.
 
We use the following experimental approach to determine the solid angle: The intensity of an X-ray line depends on several parameters which are related to the specimen, the measurement setup and to the detector including the solid angle (Fig.1). The latter can be determined via obtaining values for all these variables either through searching for databases and literature or experimentally. A literature research reveals the databases NIST 164 [4] for ionization cross sections, EADL [5] for fluorescence yields and relative X-rays line intensity ratios from Scofield [6] improved by values from Aßmann et. al. [7] and Wendt [8] as reliable sources. We use X-ray lines of six elements (Al, Si, Ti, Ga, As, Sr) via EDXS for the solid angle determination. In addition the measurement of the specimen thickness is necessary. Therefore we work with a special sample configuration using the focused ion beam instrument to produce a lamella with a rather uniform specimen thickness for the actual measurement and a conical, circular symmetric rod of each specimen. The latter is used for the experimental determination of the inelastic mean free path λ [9]. Due to the circular symmetric shape of the rod the mean free path can be measured by directly linking it to the absolute thickness/diameter of the rod (Fig. 2) and thus the thickness of the lamella can be calculated accurately using the t/λ method.

In our case we work with a FEI Titan³ 60‑300 equipped with the ChemiSTEM technology. Hence we have four windowless SDD detectors symmetrically placed around the optical axis (Fig. 3) and we are using the high visibility holder from FEI.

 

With the profound knowledge about the reliability of ionization cross sections, fluorescence yields and relative X-ray line intensity ratios were are able to determine the solid angle of our EDXS system properly. We show that the solid angle for each detector is between 0.15 to 0.17 sr, which roughly corresponds to the manufacturer value of 0.175 sr per detector.

 

 

[1]          Zaluzec, Microsc. Microanal. 2014, 20, 1318–1326, .

[2]          Conway, Nucl. Instrum. Methods 2010, 614, 17–27.

[3]          Egerton et al., Ultramicroscopy 1994, 55, 43–54.

[4]          Llovet et al., J. Phys. Chem. Ref. Data 2014, 43, 13-112.

[5]          Perkins et al., LLNL Report 1991, UCRL-50400, vol. 30.

[6]          Scofield, "Radiative Transitions" in: Ionization and transition probabilities, 1975, Acad. Press, New York.

[7]          Aßmann et al., Spectrochim Acta B 2003, 58, 711-716.

[8]          Wendt, Microchim Acta 2002, 139, 195-200.

[9]          Kothleitner et al., Microsc. Microanal. 2014, 20, 678-686 .

 

 

The authors acknowledge the Austrian Research Promotion Agency FFG (project 850220) for funding.

Judith LAMMER (Graz, Austria), Johanna KRAXNER, Werner GROGGER

08:00 - 18:15 #6746 - IM08-433 SIev: Implementation of an anisotropic binning strategy to optimize the chemical analysis of heterogeneous interfaces.

IM08-433 SIev: Implementation of an anisotropic binning strategy to optimize the chemical analysis of heterogeneous interfaces.

Outstanding properties emerge at the interfaces of heterogeneous materials, so that their engineering offers promising prospects for achieving novel functional structures. The design and realization of highly-controlled interfaces require reliable characterization techniques with high spatial resolution and chemical sensitivity, and X-rays Energy Dispersive Spectroscopy Spectrum Imaging (XEDS-SI) has been used qualitatively for this purpose with ample success. However, the extraction of quantitative features from XEDS-SI by the overall signal integration around the X-rays peaks location tends to be inaccurate for high spatial resolution analysis due to their typically reduced signal to noise ratio. 

This work presents a strategy for an improved and quantitative chemical analysis at the interface of heterostructures based on the processing of XEDS-SI datasets obtained by aberration-corrected Scanning Transmission Electron Microscopy (STEM). The successive XEDS-SI dataset breakdown with decreasing binning sizes is implemented in the SIev software tool, and its application results in the improved detection of X-rays peaks and estimation of local noise levels. This approach supports the actual chemical signal extraction from XEDS-SI with the maximum spatial resolution with respect to signal to noise ratio (SNR) significance limit.

Results of anisotropic XEDS-SI dataset breakdown obtained with aid of SIev software indicate that sub-nm precision, considering a 2σ confidence level, can be routinely attained on the determination of a projected intermixing layer at the interface of heterogeneous materials. Given that the SIev data processing is significantly faster than a high-SNR XEDS-SI dataset acquisition by the use of currently available high efficiency X-rays detectors, the perspective of an accurate and real-time profile across the interfaces of heterogeneous materials via chemical mapping is foreseen.

Carlos F. AFONSO (Braga, Portugal), Enrique CARBÓ-ARGIBAY, Marcel S. CLARO, Daniel G. STROPPA

08:00 - 18:15 #6792 - IM08-435 STEM EELS plasmon imaging (SEPI) for mixed phase silicon / silicon-oxides systems.

IM08-435 STEM EELS plasmon imaging (SEPI) for mixed phase silicon / silicon-oxides systems.

Plasmon imaging using energy filtered transmission electron microscopy (EFTEM) has been a well-established technique for investigating mixed phase silicon systems for more than a decade [1]. To image the silicon distribution typically an energy window of 4 eV centered at 17 eV is used. With this approach the signal contains significant contributions of the silicon monoxide and dioxide plasmons which deteriorates contrast and prohibits quantitative imaging.

As alternative approach we developed SEPI. A method, based on the combination of scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS). SEPI allows to separate the contributions of Si, SiO and SiO₂ (see Fig. 1(a)) and, thereby, to map them individually. This was achieved by a spectrum-by-spectrum evaluation routine of EELS data cubes including the extraction of the single scattering distribution (SSD) and the subsequent fitting with reference spectra of silicon monoxide and dioxide and the analytical Drude model for silicon.

In this work we present the successful application of SEPI for investigations of three silicon / silicon oxides systems. All measurements were performed on FEI Tecnai Osiris microscope equipped with a FS-1 electron energy spectrometer.

In Fig. 1(b) the silicon nanostructure of a nanocrystalline hydrogenated SiO_x (nc-SiO_x:H) layer deposited on a silicon wafer using PECVD is shown. During the last years nc-SiO_x:H was under investigation for several applications in silicon thin film solar cells [2], as it offers high electronic conductivity compared to other silicon alloys. The electrical properties are commonly explained by the presence of silicon nanofilaments [3], which can be seen in Fig. 1(b).

In Fig. 2, a line scan across the interface of a 14.5 nm thick silicon oxide layer on a silicon wafer is shown. The oxide layer was thermally grown at 900 °C partially using HCl atmosphere. The distribution of the three components Si, SiO, and SiO₂ demonstrate that between the silicon substrate and the SiO₂ layer an intermediate SiO layer exists.

Fig. 3 (a) shows the oxygen distribution measured using energy dispersive X-ray spectroscopy (EDX) of an oxygen precipitate in silicon. Oxygen precipitates in silicon are generated by precipitation of the supersaturated interstitial oxygen during thermal processing. Their stoichiometry was under discussion for decades and the compositions found were ranging from SiO₂ to SiO. Recently, it was shown that the center of the precipitate consists of SiO₂ [4]. The SiO plasmon ratio distribution of the oxygen precipitate, shown in Fig. 3 (b), demonstrates that the oxygen precipitate is also surrounded by an SiO layer similar to the oxide layer.

 

1. G. Nicotra, S. Lombardo, C. Spinella, G. Ammendola, C. Gerardi and C. Demuro, Appl. Surf. Sci. 205, 304 (2003).

2. L.V. Mercaldo, I. Usatii and P.D. Veneri, Energies 9(3), 218 (2016).

3. M. Klingsporn, S. Kirner, C. Villringer, D. Abou-Ras, I. Costina, M. Lehmann and B. Stannowski, submitted (2016).

4. D. Kot, G. Kissinger, M. A. Schubert, M. Klingsporn, A. Huber, and A. Sattler, Phys. Status Solidi RRL 9, 405 (2015).

Max Johann KLINGSPORN, Markus Andreas SCHUBERT (Frankfurt (Oder), Germany), Simon KIRNER, Dawid KOT, Daniel ABOU-RAS, Bernd STANNOWSKI, Gudrun KISSINGER

08:00 - 18:15 #6820 - IM08-439 Plasmonic Resonance in Metallic Nanoparticles.

IM08-439 Plasmonic Resonance in Metallic Nanoparticles.

The optical properties of noble metal nanoparticles are dominated by the surface plasmons (SP), which are collective oscillations of the free electrons confined at the surface. Surface plasmons in noble-metal nanoparticles have received considerable attention for the wide range of applications, ranging from surface-enhanced Raman spectroscopy (SERS), biomolecule sensing, labeling of biomolecules, cancer therapy, plasmonic absorption enhancement in solar cells to nanophotonics.

Influence of particle size on the surface plasmons is of great interest. For large particles (> 10 nm), pure classical effects related to confinement of classical electromagnetic waves at the surface of the objects are well-known. For smaller sizes, the major effects are related to quantum physics. Optical techniques have been employed to study the optical properties of sub-10 nm nanoparticles¹. However in these techniques the measurements are made for a set of particles. This implies an inhomogeneous broadening of the surface plasmon resonance which largely prevents observing the influence of quantum phenomena. In order to overcome these challenges, single particles measurements have been made in the past years by electron energy loss spectroscopy (EELS) in combination with a scanning transmission electron microscope (STEM). STEM-EELS measurements allow the study of individual particles on the atomic scale and with high spectral resolution².    

Here we investigate the plasmonic response of individual silver and gold nanoparticles, ranging from 2 to 10 nm in diameter. We will first discuss the instrumental progresses made to improve signal to noise ratio allowing detection of surface plasmons in ultrasmall nanoparticles and data treatment to improve energy resolution. We will then illustrate how bulk and surface plasmons evolve as a function of particle size. Figure 1 shows plasmons resonances well defined for a 5 nm silver particle and weak plasmons resonances (but yet visibles after deconvolution treatment) in a 2 nm silver particle. Furthermore in this work different configurations of substrate-matrix will be shown in order to understand their influence on the plasmonic response.  

 

References:

(1) Jonathan A. Scholl et al. Quantum plasmon resonances of individual metallic nanoparticles. Nature, 483:421-427, 2012

(2) Soren Raza et al.Multipole plasmons and their disappearance in few-nanometre silver nanoparticles. NATURE COMMUNICATIONS | 6:8788 | DOI: 10.1038/ncomms9788 

Alfredo CAMPOS (Orsay), Troc NICOLAS, Hans-Christian WEISSKER, Odile STÉPHAN, Matthias HILLENKAMP, Mathieu KOCIAK

08:00 - 18:15 #6877 - IM08-443 Advances in FIB EDX-Nanotomography.

IM08-443 Advances in FIB EDX-Nanotomography.

FIB-tomography is used in materials science for 3D-analysis of nanostructured materials [1]and in life science for the analysis of complex structures like brain tissue [2]. This presentation summarizes recent technological improvements, which include advancements in detector technology for electron imaging and elemental analysis, scan generator technology for high throughput imaging, and automated drift correction for reliable 3D reconstruction. New in-column detectors have a higher sensitivity for low energy electrons, which is the basis for a very high resolution down to a few nm voxel size. The low kV imaging can be combined with energy filtering in order to detect a pure signal of backscattered electrons (BSE), which improves the reliability of phase segmentation and quantitative analysis. The quality of the 3D reconstructions can also be improved with refined procedures for drift correction based on reference marks. In addition, with the new scan generators image acquisition and ion milling can be performed synchronously. In this way the acquisition speed increases further. Finally, spectral and elemental mapping (XEDS) based on Silicon Drift Detectors (SDD) with higher collection solid angles provides higher X-ray count rates. These increased count rates open new possibilities in chemical analysis that provide larger data cubes with higher representativeness. As EDX Analysis requires higher beam energies in order to ionize the elements of interest the interaction volume increases and the resolution decreases. The latest development of acquisition software allows to switch high tensions in order to acquire data sets with small voxel sizes and high resolution using SE and BSE detectors with high efficiency at low voltages and to acquire a second data set under conditions that match the requirements for EDX Analysis (higher voltages and larger voxel sizes). The new possibilities of FIB EDX-Tomography are illustrated with the following examples:

1. High throughput elemental analysis is performed of a NiTi stainless steel with a complicated multi-phase microstructure [3]. The examples document the recent advancements in resolution, contrast, stability and throughput, which are necessary for reliable and representative 3D-analysis. The segmentation of the different phases was done using the EDX-maps and further refined with the SE-images. Fig.1

2. Representation of the complex chemistry of a diamond-anvil cell laser heated spot. The conditions used (50GPa, 4000K) resemble conditions at the earth core and reveal a complex microstructure with phases that are only formed under theses extreme conditions. Fig.2

References

1. L. Holzer, M. Cantoni, in Nanofabrication Using Focused Ion and Electron Beams—Principles and Applications, I. Utke, S. Moshkalev, P. Russell, Eds. (Oxford University Press, New York, 2012), pp. 410–435.

2. M. Cantoni, C. Genoud, C. Hébert and Graham Knott, Microsc. & Anal. 24(4): 13-16 (2010)2010.

3. P. Burdet, J. Vannod, A. Hessler-Wyser, M. Rappaz, M. Cantoni, Acta Mater. 61 (8), 3090 (2013).

Marco CANTONI (Lausanne, Switzerland), Farhang NABIEI, Pierre BURDET

08:00 - 18:15 #6887 - IM08-445 Separating Magnetic and Non-magnetic Signals at the Fe/MgO Interface.

IM08-445 Separating Magnetic and Non-magnetic Signals at the Fe/MgO Interface.

Despite various efforts, it remains an experimentally challenging task to access magnetic properties at
(sub) nanoscale. One route towards a direct measurement of magnetism is the measurement of electron
magnetic circular dichroism (EMCD) [1]. Being based on the measurement of electron energy loss
(EEL) spectra, EMCD can in principle be measured at atomic resolution and can open the door to study
exciting new area of physics such as magnetism in the vicinity of defects or interfaces. However, in
addition to general concerns of low signal to noise ratios of EMCD spectra measured at high
resolutions calling for a statistical data treatment, there might be other, non-magnetic contributions to
the signal which cause a change in the white line ratio of the L3/L2 edge peak of the magnetic species.
These white line changes might be related to the occurrence of a different chemical species of the same
element, e.g., due to in situ oxidation of the sample, or also to position dependent changes of the
electron wavefunction if the EMCD experiment is carried out at atomic resolution [2]. Such effects
may render the correct interpretation of EMCD signals impossible if they can not be clearly separated
from the true magnetic signal. The issue calls out for a statistical tool to separate the components.

We demonstrate how a canonical polyadic decomposition (CPD) [3],[4],[5] can be used to separate
magnetic and non-magnetic signals measured at the Fe/MgO interface. The system has recently
received a lot of attention as a candidate for magnetic tunnel junctions due to its large tunneling
magnetoresistance (e.g. [6],[7]), its magnetic properties, epecially at the interface are thus of interest.
Through the additional explanatory power of CPD, insight is gained on a perceived increase of orbital
to spin moment ratio at the interface [8]. Besides the spectral components and their spatial maps
(Fig.1), CPD also returns a vector containing the weight of the respective component in the data sets
measured with an aperture position such that the sign of the EMCD signal is positive, negative and
such that the magnetic component vanishes (Fig.2). The components shown below indicate a
significant non-magnetic white line branching towards the Fe/MgO interface.

CPD can not only be used as a technique to extract EMCD from noisy data and separate it from
potential non-magnetic signals, targeting both the aforementioned problems, but it can be generalized
to any problem of identifying different signal contributions in experiments where multiple data sets are
measured on the same sample area, such as momentum resolved EELS. It possesses desirable features
such as uniqueness while not constraining the components along either of the modes and comparatively
low computational costs. The assumptions on the tensor's structure match the physical model and thus
lead to directly interpretable components. Hence, CPD is a useful addition to the set of statistical tools
for the analysis of microscopy data.

References:

[1] P. Schattschneider et al, Nature 441 (2006) 486.
[2] A. Gulec et al., Appl. Phys. Lett. 107 (2015) 143111.
[3] J. Carroll, J.-J. Chang, Psychometrika 35 (1970) 3.
[4] A. Cichocki et al, IEEE Signal Processing Magazine 145 (2015).
[5] J. Spiegelberg et al, submitted.
[6] S. Gautam et al, J. Appl. Phys. 115 (2014) 17C109.
[7] V. Serin et al, Phys. Rev. B 79 (2009) 144413.
[8] T. Thersleff et al, Manuscript.

Jakob SPIEGELBERG (Uppsala, Sweden), Thomas THERSLEFF, Ján RUSZ

08:00 - 18:15 #6910 - IM08-447 Nanometer Scale Time of Flight Back Scattering Spectrometry in the Helium Ion Microscope.

IM08-447 Nanometer Scale Time of Flight Back Scattering Spectrometry in the Helium Ion Microscope.

Helium Ion Microscopy (HIM) (Hlawacek et al., 2014)^⁠ is well known for its high resolution imaging and nano fabrication capabilities. However, in terms of analytic capabilities it lags behind comparable techniques such scanning electron microscopy (SEM) or transmission electron microscopy (TEM). Although several primary and secondary particles are available to date none of them has been exploited in practical way to obtain analytic information. While  electrons are used for imaging they are of limited use due to matrix effects which complicate the elemental analysis. The existing models are based on empirical data and can deliver useful results only for a selected number of materials (Ramachandra et al., 2009)⁠. Photons have been exploited in the past to obtain information on the sample composition but turned out to be of limited practical use due to the high sensitivity for damage induced by the ion beam (Veligura et al., 2015)⁠. In the past backscattered ions have been used to obtain materials contrast in a qualitative way. Recently, the successful use of sputtered particles for analytic purposes has been demonstrated by adding a sophisticated secondary ion mass spectrometer to the HIM (Wirtz et al., 2015)⁠.

Here, we present the first successful attempt to use t ime of flight ion backscatteringspectrometry (TOF-BS) for materials characterization in a HIM (Klingner et al., 2015)⁠. The start signal for the TOF measurements is created by chopping the primary beam of the ion microscope using the built—in blanker and a custom made electronics that allows pulse lengths of 10 ns to 250 ns. The stop signal is given by the arrival of the backscattered particles at a micro channel plate. The setup has the advantage of providing a high lateral resolution, a good energy resolution and at the same time is minimal invasive to the microscope and therefore not deteriorating the high resolution capabilities of the device when the BS setup is not in use.

TOF-BS spectra of HfO₂ on Si are presented in fig. 1. The time resolution is limited by the physical length of the microscope blanker to approximately 17 ns or 5.4%. This value can be decreased to 2.7% by using a longer flight path. Thanks to a home built scan system to control the beam TOF-BS data can be recorded also in imaging mode. This allows an efficient post acquisition analysis by applying energy filters to extract the elemental distribution. An example is presented in fig. 2. The corresponding bulk BS spectra (fig. 3) are color coded to the extraction areas in the secondary electron (left in fig. 2) and total BS image (right in fig. 2). A lateral resolution of 54 nm has been determined. Although this is not comparable to the native resolution of the tool, the value is close to the physical limit and can be overcome by using correlative approaches in connection with the high resolution SE data available in the HIM. Modifying the sample holder slightly one can also perform TOF-SIMS. The sputtered particles are accelerated towards the stop detector of the TOF setup by means of a high voltage applied to the sample and a grounded grid. TOF-SIMS spectra obtained from the same sample shown in fig. 2 are presented in fig. 4. The usefulness of the combined TOF-SIMS and TOF-BS setup becomes evident by noting that the TOF-SIMS setup fails to detect the gold due to the low sputter and ionization yield of gold. This inherent weakness of SIMS is overcome in this combined setup.

References:

Hlawacek, G., Veligura, V., van Gastel, R. & Poelsema, B. (2014). Helium ion microscopy. Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures 32, 020801.

Klingner, N., Heller, R., Hlawacek, G., Borany, J. von, Notte, J. A., Huang, J. & Facsko, S. (2015). Nanometer scale elemental analysis in the helium ion microscope using time of flight spectrometry. Ultramicroscopy accepted.

Ramachandra, R., Griffin, B. J. & Joy, D. C. (2009). A model of secondary electron imaging in the helium ion scanning microscope. Ultramicroscopy 109, 748–757.

Veligura, V., Hlawacek, G., van Gastel, R., Zandvliet, H. J. W. & Poelsema, B. (2015). Investigation of ionoluminescence of semiconductor materials using helium ion microscopy. Journal of Luminescence 157, 321–326.

Wirtz, T., Philipp, P., Audinot, J.-N., Dowsett, D. & Eswara, S. (2015). High-resolution high-sensitivity elemental imaging by secondary ion mass spectrometry: from traditional 2D and 3D imaging to correlative microscopy. Nanotechnology 26, 434001.

Nico KLINGNER, Gregor HLAWACEK (Dresden, Germany), Rene HELLER, Johannes VON BORANY, Stefan FACSKO

08:00 - 18:15 #6926 - IM08-449 The analysis of Mo5SiB2 in the SEM with the use of EDS and WDS.

IM08-449 The analysis of Mo5SiB2 in the SEM with the use of EDS and WDS.

Refractory metals and their alloys show potential for high temperature applications due to their increased melting point and creep resistance. Mo-Si-B ternary alloys consisting of the phases Mo_ss (molybdenum-based solid solution)-Mo₃Si (A15)-Mo₅SiB₂ (T2), with melting points over 2000 °C, are particularly favorable for new high-temperature materials. However these alloys show a lack of oxidation resistance in the intermediate temperature range, 650-750 °C, and possess a relatively high density (9.6 g/cm³) compared to Nickel-Based Superalloys.

 

The characterization of the Mo₅SiB₂ phase with an SEM with X-ray microanalysis analytical capabilities presents a real challenge. The B-Kα X-ray has an energy of 183.3 eV, but might shift slightly due to its bonding with Mo. This X-ray line energy is very close to the Mo-Mζ line which is at 192.6 eV. In addition, the K-shell absorption edge of boron is at 192 eV, just below the Mo-Mζ line. Finally the absorption coefficient of B-Kα in Mo is very high, and is even more extreme in Si.

As a result, EDS microanalysis proved to be insufficient. The EDS spectrum of the Mo₅SiB₂ phase shows a very small peak at the B-Kα position, but without any separation from the Mo-Mζ line. The elemental distribution maps even showed a strong artifact: due to the higher Mo content in the Mo_ss phase and the absence of a boron absorption edge the Mo-Mζ peak is considerably higher in that phase, resulting in an incorrect increase of intensity of boron in the Mo_ss phase. See figure 1.

 

Alternatively, WDS spectrometers have a much better energy resolution and are capable of separating to a large extend the B-Kα and Mo-Mζ peaks. Modern parallel-beam WDS spectromers are also very sensitive to the low-energy part of the spectrum, and can detect small amounts of boron with very high efficiency. A careful energy scan over the B-Kα and Mo-Mζ peaks can be seen in figure 2. And of course creating an element distribution image for boron now indeed showed the correct boron distribution, as can be seen in figure 3.

 

The final challenge lies with the quantitative analysis: what exactly is the weight percentage of boron in the supposed Mo₅SiB₂ phase? The B-Kα and Mo-Mζ peaks still partially overlap even with WDS, and one has to be very careful in the selection of the background support points to correctly subtract the background X-ray intensity.  Using a stoichiometric Mo₅SiB₂ standard this can still be done, and an accurate quantification can be performed.

 

This case shows how vital it can be for certain applications to widen the range of available microanalysis tools with a parallel-beam WDS spectrometer to perform analyses beyond the performance limit of EDS.

Hans DIJKSTRA (Eindhoven, The Netherlands), Peter KELLNER, Werner REICHSTEIN, Uwe GLATZEL

08:00 - 18:15 #6934 - IM08-451 High Spatial Resolution Spectrum Imaging in the FEG-SEM at Low Voltages: A New Option for Materials Characterisation.

IM08-451 High Spatial Resolution Spectrum Imaging in the FEG-SEM at Low Voltages: A New Option for Materials Characterisation.

Analytical field emission gun scanning electron microscopes (FEG-SEMs) provide essential morphological and compositional data for the analysis of a broad range of materials.  However, the characterisation of certain materials can be problematic under “conventional” (i.e., > 10 kV) FEG-SEM imaging conditions.   In particular, complex multiphase materials, oxides and polymers can exhibit pronounced charging effects.   However, low voltage operating conditions (< 5 kV) precluded the generation of X-ray energy dispersive spectroscopy (XEDS) data for microchemical analysis.  The need to perform XEDS analysis at higher operating voltages made it difficult to acquire structural and compositional data from the same feature for materials that charge or degrade under the electron beam.  The ability to perform both high resolution imaging and XEDS spectrum imaging at low voltage in the FEG-SEM provides new opportunities for the evaluation of many materials.  In this study, we demonstrate how low voltage XED spectrum imaging in the FEG-SEM can rapidly provide data on two-layer oxides that form in the cracks present in an austenitic stainless steel corrosion specimen.

The Oxford Instruments X-Max^N 150 silicon drift detector (SDD) and an X-Max Extreme windowless SDD with the Oxford Instruments AZtecEnergy acquisition and XEDS analysis software were used for the XED spectrum imaging experiments in a Zeiss Merlin FEG-SEM with a Gemini-II column.  All SEM images were obtained with a parallel on-axis in-lens secondary electron (SE) detector with primary electron beam energy of 1.5 kV, 3 kV, 5 kV and 15 kV, respectively.  The SEM imaging conditions were fixed with the expection of the accelerating voltage and the working distance. All XED spectrum images were acquired with a spectrum image resolution of 512 x 512 pixel and a pixel dwell time of 1000 µs.   The Oxford Instruments TruMap processing software was used for peak deconvolution and background subtraction.  

Analyses were performed on a small oxide-filled crack in the austenitic stainless steel sample, which had been metallographically polished to a 1 μm diamond finish.  In-lens secondary electron images and corresponding XED spectrum images revealed the effect of accelerating voltage on the oxide microstructure.  Fe XED spectrum images acquired at 1.5 kV and 3 kV indicated the presence of a 2-layer oxide structure: an Fe-poor oxide adjacent to the oxide/metal interface, whereas these details are absent for the 15 kV spectrum image. Complementary Cr XED spectrum images confirmed that the oxide adjacent to the metal was Cr-enriched.  TEM and selected area electron diffraction confirmed the presence of a Cr₂O₃ innermost layer and an Fe-rich M₃O₄ in the centre of the oxide-filled crack.  The ability to generate viable XEDS datasets at low voltages provides significant improvements in spatial resolution of the analysis due to the significantly reduced depth of x-ray generation in the sample.  Thus, low voltage XEDS is providing new insights into materials analysis and new options for microstructural characterization.

References:

[1]. We thank James Holland for valuable technical discussions and suggestions.

Arne JANSSEN (Manchester, United Kingdom), M.g. BURKE, Simon BURGESS

08:00 - 18:15 #6953 - IM08-453 Is the electronic structure of few layer transition metal dichalcogenides always two dimensional ?

IM08-453 Is the electronic structure of few layer transition metal dichalcogenides always two dimensional ?

The transition metal dichalcogenides (MoS₂ etc.) are a new class of layered materials that can be prepared in variable layer thickness down to single molecular layer.  Compared with the more well-known graphene, the monolayer version of graphite, the transition metal dichalcogenides are semiconductors and hence can be more useful in applications such as light emission or photovoltaics where an energy gap is essential.  Ultra thin transition metal dichalcogenides also show interesting layer thickness dependent physical properties.  For example, the semiconductor gap was found to change from indirect to direct when the layer thickness is reduced to monolayer, making monolayer MoS₂ an efficient light emitter.

Here we present investigation of the dimensionality of the joint density of states involved in the interband transition in MoS₂ using angle resolved as well as angle integrated electron energy loss spectroscopy [1].  To aid the analysis, we have extended the theory of joint density of states from three-dimensional semiconductors to low-dimensional semiconductors.

Our result and analysis shows not only that the character of the interband transition changes from indirect to direct, as the layer thickness is reduced down to monolayer, as expected, but the indirect band gap retains a three dimensional character down to the monolayer limit.  This is compared with the two dimensional character found for the direct bandgap transition in the monolayer MoSe and presumably also in MoS_2.  This raised a question about the condition for observing a true two dimensional electronic structrue even when the atomic structure is reduced to single molecular thickness.  We will discuss physical factors might affecting the dimensionality of the electronic density-of-states.

Our result has practical implication.  For example, it is consistent with the assumption made by Castellanos-Gomez et al. [2] when interpreting the electrostatic screening effect observed in monolayer MoS₂, which is very different from that of graphene.  Our result also can explain the sensitivity of the indirect interband transition as a function of the layer thickness and the bandgap cross-over of few layer MoS₂ from indirect to direct.    

 

[1] J. H. Hong, K Li, C. H. Jin, X. X. Zhang, Z Zhang and J. Yuan (2016) “Layer-dependent anisotropic electronic structures of freestanding quasi-two-dimensional MoS₂”, Phys. Rev. B93, 075440

[2] A. Castellanos-Gomez et al. (2013), "Electric field screening in atomically thin layers of MoS₂: the role of interlayer coupling" Adv. Mater. 25, 899-903

Jun YUAN (York, United Kingdom), Jinghua HONG, Chuanhong JIN

08:00 - 18:15 #6986 - IM08-455 Raman imaging of biology and soft matter samples: a few examples.

IM08-455 Raman imaging of biology and soft matter samples: a few examples.

Electron microscopy (SEM, TEM) is widely used to characterize the internal structures with micrometer spatial resolution. AFM is also commonly employed for analyzing the surface topography with a superior spatial resolution. However, confocal Raman microspectrometry is a unique way to complete such structural investigation by knowledge on chemical information. Raman imaging combines the spatial resolution of optical microscopy with the molecular analysis capabilities of Raman scattering. Then, it is very well adapted to provide direct information about conformation, structure and behavior of lipids and biopolymers (proteins, polysaccharides) from micronsized zones of interest.

Also, the mainly structure-sensitive bands in protein and lipid spectra makes them suitable for multivariate analysis techniques such as principal component analysis (PCA) to deduce structural relationships.

Raman microspectrometry is then a method of choice for the compositional investigation with a high spectral resolution of hydrated biological systems of food interest along with the structural investigation performed conjointly with laser scanning confocal microscopy, AFM or electron microscopy with a superior spatial resolution.

We aim here to review few examples of Raman imaging performed on different biological systems (algae, plant and animal tissues) and on soft-matter based systems (highly diluted gels, dispersions) relevant in food industry, see Figures 1 and 2. We will focus on the complementarity with the other types of microscopy, and on the practical details concerning the sample preparation (choice of objectives, choice and influence of supports, smoothing or not of the surfaces,…).

 

 

REFERENCES

G. Philippe, C. Gaillard, J. Petit, N. Geneix, M. Dalgalarrondo, R. Franke, C. Rothan, L. Schreiber, D. Marion, B. Bakan, Ester-crosslink Profiling of the Cutin Polymer of Wild Type and Cutin Synthase Tomato (Solanum lycopersicum L.) Mutants Highlights Different Mechanisms of Polymerization, Plant Physiology

M. Gayral, C. Gaillard, B. Bakan, M. Dalgalarrondo, K. Elmorjani, C. Delluc, S. Brunet, L. Linossier, M.H. Morel, D. Marion, Transition from vitreous to floury endosperm in maize (Zea mays L.) kernels is related to protein and starch gradients, Journal of Cereal Science, 2016, accepté

Covis R., Vives T., Gaillard C., Maud Benoit, Benvegnu T., Interactions and hybrid complex formation of anionic algal polysaccharides with a green cationic glycine betaine derived surfactant, Accepted in Carbohydrate Polymers, 2015, May 5;121:436-48.

Cédric GAILLARD (NANTES)

08:00 - 18:15 #6989 - IM08-457 TEM analysis of multilayered nanostructures formed in the rapid thermal annealed silicon rich silicon oxide film.

IM08-457 TEM analysis of multilayered nanostructures formed in the rapid thermal annealed silicon rich silicon oxide film.

Silicon (Si) nanoparticles (NPs) embedded in an ultrathin silicon rich silicon oxide (SRSO) film through the thermal annealing process has emerged as a highly absorbing layer for third-generation solar cells ¹. The concept of using Si NPs is to achieve a band gap tunable absorber layer by controlling the size and structure of Si NPs because of the quantum confinement effect ². In our study, a multilayer stack of silicon oxide with 35 periods of alternating layers of 1-nm thick near-stoichiometric and 3-nm thick Si-rich hydrogenated silicon oxide were deposited on fused quartz substrate by plasma-enhanced chemical vapor deposition (PECVD) method. Two samples were annealed using a rapid thermal annealing (RTA) furnace in forming gas atmosphere (90% N₂ + 10% H₂) for 210s and 270s respectively. From the Raman spectroscopy, a reduction in crystallinity of Si has been discovered from 210s annealed sample to 270s annealed sample (shown in Figure 2). The goal of transmission electron microscopy (TEM) analysis is to investigate the nanostructural change of Si in these two annealed samples and try to correlate the TEM observations to the Raman spectroscopy results. 

As the dimension of the Si nanostructures formed in SRSO films is in nanometer-scale, the energy-filtered TEM (EFTEM) tomography technique using the low-loss signals in electron energy-loss spectroscopy (EELS) has been applied as a powerful technique to correlate the precipitated Si nanostructures to the phase transformation mechanisms in the thermally annealed SRSO films ³. In this case, EFTEM spectrum-imaging (SI) technique was applied to characterize the Si nanostructures formed in SRSO films by different annealing times. The EFTEM SI dataset was acquired from -4eV to 40eV using a 2eV energy slit and the reconstructed zero loss peak (ZLP) was used to calibrate the spectra shift. Si plasmon images were extracted by fitting a Gaussian into the low-loss region with a peak position at 16.7 eV ⁴ and FWHM of 4.5 eV. In order to analyze the multilayer structures at different annealing durations, the TEM samples were prepared in cross sectional geometry using the conventional polishing and ion milling methods.

Figure 1 shows the EFTEM images extracted from the Si plasmon peak, in these images Si appears as bright contrasts. For shorter annealing time, an alternating bright and dark contrast can be observed which indicates that the multilayer structure still remains whereas for longer annealing time, Si shows nanoparticles like contrast. The continuous layer like contrasts shown in Figure 1(a) indicates the overlapping of the contrasts generated by small Si crystallites in a very high density. After longer annealing time (Figure 1(b)), the small Si crystallites grow in size but may take overall less volume fraction due to the Ostwald ripening process. Therefore, it explains the reduction in crystallinity of Si discovered from 210s annealed sample to 270s annealed sample by Raman. However, such a reduction in Si crystallinity was not observed in nitrogen annealed SRSO films, this indicates that samples annealed in the forming gas environment follow a different crystallization mechanism and hydrogen must play a decisive role during the Si crystallization at the initial stage.

1.                 Conibeer, G. et al. Silicon quantum dot nanostructures for tandem photovoltaic cells. Thin Solid Films 516, 6748–6756 (2008).

2.                 Green, M. a. Third generation photovoltaics: Ultra-high conversion efficiency at low cost. Prog. Photovoltaics Res. Appl. 9, 123–135 (2001).

3.                 Friedrich, D. et al. Sponge-like Si-SiO2 nanocomposite-Morphology studies of spinodally decomposed silicon-rich oxide (vol 103, 131911, 2013). Appl. Phys. Lett. 103, (2013).

4.                    Egerton, R. F. Electron energy-loss spectroscopy in the electron microscope. Electron energy-loss Spectrosc. electron Microsc. xii+410 pp–xii+410 pp (1986). 

Hasan ALI (Uppsala, Sweden), Ling XIE, Martijn Van SEBILLE, Adele FUSI, Ren´e A C M M VAN SWAAIJ, Miro ZEMAN, Klaus LEIFER

08:00 - 18:15 #7043 - IM08-459 The highest characterization potentialities of sub-20 meV spatially resolved STEM-EELS.

IM08-459 The highest characterization potentialities of sub-20 meV spatially resolved STEM-EELS.

Recent developments in monochromator technologies have demonstrated electron microscope resolution of infrared features, by electron energy loss spectroscopy (EELS) carried out with an energy resolution of ~10 meV [1]. In this contribution, we will present two experiments in nanoplasmonics and phonon spectroscopies performed on the NION HERMES scanning transmission electron microscope (STEM) at Arizona State University with a spectral resolution better than 16 meV. EELS in a STEM is an invaluable technique for mapping optical excitations with a nanometer spatial resolution, with a clear interest in mapping surface plasmons [2] now down to plasmon energies as small as 170 meV thanks to new advances in monochromators [3]. We will go to show on the case of long plasmonic bimetallic nanorods (Fig 1.) the interest of increased spectral resolution such as the one provided by the HERMES for plasmonic studies. For the same reason, recording EEL spectra showing vibrational features of hydrogen-containing biological samples is now possible, resolving the characteristic C–H, N–H and NH₂ vibrational signatures, with no observable radiation damage (Fig 2.) [4]. These two examples will illustrate the HERMES performances to show how such increases in performance may unveil new physical effects.

References:

[1]          Krivanek, O. L. et al. Nature 514, 209–212 (2015).
[2]          Kociak, M. & Stéphan, O. Chem. Soc. Rev. 43, 3865–3883 (2014).
[3]          Rossouw, D. & Botton, G. A. Phys. Rev. Lett. 110, 066801 (2013).
[4]          Rez, P. et al. Nature Communications 7, 10945 (2016).

 

Acknowledgements: This work has received support from the National Agency for Research under the program of future investment TEMPOS-CHROMATEM with the Reference No. ANR-10-EQPX-50. We gratefully acknowledge the use of facilities within the LeRoy Eyring Center for Solid State Science at Arizona State University. 

Katia MARCH (Orsay), Leonardo SCARABELLI, Luis M. LIZ-MARZÁN, Toshihiro AOKI, Peter REZ, Hagai COHEN, Peter A. CROZIER, Ondrej L. KRIVANEK, Odile STÉPHAN, Mathieu KOCIAK

08:00 - 18:15 #6190 - MS00-461 Characterization of CoO@MnFe2O4 Magnetic Hollow Nanoparticles.

MS00-461 Characterization of CoO@MnFe2O4 Magnetic Hollow Nanoparticles.

Magnetic core@shell nanoparticles are interesting candidates for applications in areas such as drug delivery, medical imaging, hyperthermia, sensing and biodetection. In many of these applications the availability of particles, being at the same time hollow and magnetic is a great technological advantage ahead of a realistic implementation. In this work we show the characterization of CoO@MnFe2O4 magnetic hollow nanoparticles obtained using octahedron-shaped CoO nanoparticles as templates.  A 3 nm magnetite (Fe₃O₄) layer was first deposited over the CoO octahedrons. In a second steep, a solid-state reaction transforms the Fe₃O₄ into MnFe₂O₄, and the former solid octahedrons into CoO@MnFe₂O₄ hollow and pierced nanoparticles. Characterization showed the octahedron shape was maintained along the process, electron diffraction and FT also confirmed an almost perfect epitaxial growth of the shell. Finally, EELS showed a higher Fe content in the shell.

J. Benito RODRÍGUEZ-GONZÁLEZ (VIGO, Spain), Rosalía MARIÑO-FERNÁNDEZ, Miguel A. RAMOS-DOCAMPO, Verónica SALGUEIRIÑO

08:00 - 18:15 #6226 - MS00-463 Development of nanostructures in hydrothermally grown TiO2.

MS00-463 Development of nanostructures in hydrothermally grown TiO2.

Development of nanostructure as a function of production temperature in hydrothermal processing of TiO₂ is reported here. Nanostructured TiO₂ was grown in autoclave at 3 different temperatures of 130, 170 and 200 °C. The samples were referred to as T‑130, T‑170 and T‑200 here. Powder of TiO₂ with the particle size of 100-200 nm was mixed with NaOH and closed in an autoclave for 24 h at the respective temperatures. The resulting material was cooled to room temperature, washed in distilled water several times, followed by washing in diluted hydrochloric acid (pH=1.6) and again in distilled water until pH=7 was restored. The final product was dried a 400°C.

The structure of the as-received powder samples was first examined by X-ray diffraction (XRD). XRD suggested that all specimens are mixture of anatase and a monoclinic titanate phase, and the ratio of the two phases varied with changing the processing temperature. The analysis of the dependence of the width of the diffraction peaks as a function of the length of the scattering vector gave the average values of the crystallite size in the two phases, however provided no clue either about the shape of these crystallites or the separation of the phases between different morphological entities. Therefore, a detailed transmission electron microscopy (TEM) analysis was performed on the three samples.

Morphology and phase distribution were studied by two transmission electron microscopes after dispersing the “powder” sample in distilled water or alcohol and dropped on “Quantifoil” holey carbon support films. Bright field (BF) and dark field (DF) images together with selected area electron diffraction (SAED) patterns were recorded on imaging plates (IP) in a Philips CM-20 TEM, operated at 200 kV and equipped with a Bruker X-ray detector (EDS). Higher resolution images and SAED patterns were also recorded on a GATAN Orius CCD in a JEOL 3010 TEM operated at 300 kV. The EDS showed that the samples contained a few atomic percent Na. It was proved that the samples contain nanofibers and equiaxed nanoparticles. The SAED patterns were processed with the “ProcessDiffraction” program [1]. The measured 2D patterns were converted into an XRD-like 1D intensity distribution by averaging over ellipses (that correct for the minor elliptical distortion caused by the lenses of the TEM). Additionally, maximal intensity values over the ellipses were also rendered to detect faint features, caused by the presence of a small number of diffraction spots, not forming a complete ring. The SAED patterns recorded from a collection of large number of nano-components gave diffraction peaks at the same positions as XRD.

Individual fibers (Fig. 1) were examined by SAED (Fig.2) whenever they protruded from the bunch of fibers. For sample T‑130 these nanofibers look like nanotubes (Fig. 1). However the number of layers at the “wall” is occasionally different at the two sides of the same “tube”, which indicate that it might also be a rolled-up sheet. All diffraction patterns contained well-defined reciprocal lattice planes with less characteristic spots within the planes. None of them contained the diffraction spots characteristic of anatase (although the patterns from the bunch did contain them). DF images recorded by the anatase lines showed that this phase is always present in the small equiaxed nanoparticles (Fig. 3). High resolution (HRTEM) images recorded from sample T‑200 showed filled nanofibers in contrast to nanotubes (Fig. 4). They looked as covered with small particles. Sample T‑170 was between the previous two extremes, as expected.

[1] J.L. Lábár, M. Adamik, B.P. Barna, Zs. Czigány, Zs. Fogarassy, Z.E. Horváth, O. Geszti, F. Misják, J. Morgiel, G. Radnóczi, G. Sáfrán, L. Székely, and T. Szüts, Microsc. Microanal. 18, 406–420, 2012

[2] S. Cravanzola, L. Muscuso, F. Cesano, G. Agostini, A. Damin, D. Scarano, and A. Zecchina, Langmuir 2015, 31, 5469−547

János LÁBÁR (Budapest, Hungary), Mohammed EZZELDIEN, Khaled EBNALWALED, Regina NÉMETH, Jenő GUBICZA

08:00 - 18:15 #6245 - MS00-465 Dispersion, dose and stability of semiconductor quantum dot biomarkers.

MS00-465 Dispersion, dose and stability of semiconductor quantum dot biomarkers.

Photoluminescent semiconductor nanoparticles or quantum dots have significant potential for medical imaging. For optimum performance however, the dispersion of the nanoparticulate material when suspended in delivery or incubation media, any transformation of the particles in the media, plus the nature and degree of uptake of the nanoparticles by a particular cell or organism all need to be understood.  Analytical electron microscopy can play a vital role in assessing this complex inter-relationship, and we discuss here specific methods developed for this type of analysis.

First, we will review the in vitro cellular uptake of commercially available CdSe/ZnS quantum dot nanoparticles with a coating specifically targeted for endocytic uptake (Invitrogen QTracker 705), dispersed in cell culture media and exposed to human osteosarcoma (U-2 OS) cells. We have examined these nanoparticles as-dispersed in cell culture media (t = 0 h), after 1 hour exposure to cells and after a round of cell division (t = 24 h). Transmission electron microscopy (TEM) has been used to assess the dispersion state of the nanoparticles in media after rapidly freezing suspensions to avoid drying artefacts [1], and in exposed cells which have been fixed and resin embedded [2]. The resin-embedded cells have been further examined using serial block face scanning electron microscopy (SBF-SEM), which enables quantification of nanoparticle loaded organelles in whole cell volumes for quantitative correlation to imaging flow cell cytometry [2].  From this we have measured probability densities for the number of quantum dots per agglomerate when in cell culture media and following uptake by cells in vitro [3, 4 and Figure 1 a-d]. Thus, we will discuss the agglomeration processes that occur both in suspension and during endocytosis.

Second and looking forward, most commercially available semiconductor quantum dots currently contain cadmium although its health and environmental risks may limit exploitation.  Thus, copper indium sulfide (CIS) quantum dots have been investigated as a potential replacement [5]. Aberration corrected STEM-EELS has identified some elemental separation of Cu and In within individual quantum dots [Figure 1 e-h], which may be the origin of an In-Cu anti-site defect state known to act as a donor in the radiative recombination pathway for chalcopyrite CIS quantum dots. We will report here on further analysis using a FEI Titan cubed Themis 300 G2 S/TEM to assess elemental distribution by STEM-EDX.  Such analysis will enable additional characterisation of core-shell coatings (e.g. CIS/ZnS/ZnS:Al) designed to improve photo luminescent quantum yield while enhancing environmental stability of the particles.

 

[1] N. Hondow, R. Brydson, P. Wang, M.D. Holton, M.R. Brown, P. Rees, H.D. Summers and A. Brown (2012) Quantitative characterization of nanoparticle agglomeration within biological media. J. Nanopart. Res., 14, 977.

[2] H.D. Summers, M.R. Brown, M.D. Holton, J.A. Tonkin, N. Hondow, A.P. Brown, R. Brydson and P. Rees (2013) Quantification of nanoparticle dose and vesicular inheritance in proliferating cells. ACS Nano, 7, 6129-6137.

[3] M.R. Brown, N. Hondow, R. Brydson, P. Rees, A.P. Brown, H.D. Summers (2015) Statistical prediction of nanoparticle delivery: From culture media to cell. Nanotechnology, 26.

[4] N. Hondow, M.R. Brown, T. Starborg, A.G. Monteith, R. Brydson, H.D. Summers, P. Rees, A. Brown (2016) Quantifying the cellular uptake of semiconductor quantum dot nanoparticles by analytical electron microscopy. Journal of Microscopy, 261, pp.167-176.

[5] M. Booth, A.P. Brown, S.D. Evans, K. Critchley (2012) Determining the concentration of CuInS₂ quantum dots from the size-dependent molar extinction coefficient. Chemistry of Materials, 24, pp.2064-2070.

Andy BROWN (Leeds, United Kingdom), Andrew HARVIE, Kevin CRITCHLEY, Ruth CHANTRY, Demie KEPAPTSOGLOU, Quentin RAMASSE, Paul REES, M Rowan BROWN, Huw SUMMERS, Rik BRYDSON, Nicole HONDOW

08:00 - 18:15 #6321 - MS00-467 Characterization and Preparation of Carvacrol Loaded Solid Lipid Nanoparticles and Imaging with Scanning Electron Microscopy (SEM).

MS00-467 Characterization and Preparation of Carvacrol Loaded Solid Lipid Nanoparticles and Imaging with Scanning Electron Microscopy (SEM).

Characterization and Preparation of Carvacrol Loaded Solid Lipid Nanoparticles and Imaging with Scanning Electron Microscopy (SEM)

Gökhan DIKMEN^1*, Ilknur DAG², Bukay YENICE GURSU³

^1,2,3Eskisehir Osmangazi University, Central Research Laboratory Application and Research Center (ARUM), Odunpazarı, 26480, Eskisehir-Turkey

gokhandikmen1@gmail.com

Backgrounds: Drug carrier systems such as nano lipid particles, polymeric micelles, dendrimers and solid lipid nanoparticles (SLNs) provide controlled release of the drug both in the desired time and in specific region. A lot of anti cancerogen and anti microbial drugs such as carvacrol exhibit various side effects due to especially high dosage. These side effects can be eliminated using drug delivery systems such as SLNs and NLPs.

Material and methods: SLN formulations were prepared using hot homogenization method after then these formulations were characterized by Zeta Sizer, FT-IR, NMR and SEM. In order to image of SLN formulations, formulations were coated with Au-Pd complex and than imaged by SEM.

Results: Carvacrol loaded SLNs formulations were proven good stability (-34.4 mV) and small size (almost 190 nm). SLN formulations were compared with the freshly prepared formulations of carvacrol and tween 80 by FT-IR spectroscopy. In FT-IR spectra, any chemical shift or deformation in the bands and any stability problems were not observed. Moreover, SEM images were taken and measured the particle size of these formulations.

Conclusions: Particle size of Carvacrol was decreased using solid lipid nanoparticles and this formulation may be suitable as a nano drug carrier system for cancer and microbial treatment.

Keywords: Carvacrol, SLNs, SEM, Caharcterization, Spectroscopy

Acknowledgment: This work was supported by a grant from Eskisehir Osmangazi Universty (Project Number: 2015-910).

Gokhan DIKMEN (Eskısehir, Turkey), Ilknur DAG, Bukay YENICE GURSU

08:00 - 18:15 #6330 - MS00-469 Energy dispersive x-ray analysis of platinum-nickel nanoparticles embedded in hollow graphitic spheres used as fuel-cell catalysts.

MS00-469 Energy dispersive x-ray analysis of platinum-nickel nanoparticles embedded in hollow graphitic spheres used as fuel-cell catalysts.

Introduction

Proton Exchange Membrane Fuel Cells (PEM-FCs) are known as potential energy conversion devices in transportation systems. For the efficiency of these fuel cells a highly efficient and stable catalyst is needed. Platinum-Nickel bimetallic nanoparticles embedded in hollow graphitic spheres (HGS) have been proven as highly active catalyst for the oxygen-reduction reaction which plays a key role in the efficiency of PEM-FCs.¹

These catalysts are synthesized via the confined-space alloying approach. The synthesis of PtNi@HGS starts with the impregnation of HGS with the metal precursors. After high temperature annealing treatments metallic nanoparticles with a small size (3.5 nm) and a narrow particle size distribution encapsulated in the pores of the HGS can be obtained.²

 

 

Methods

In this study the PtNi@HGS synthesized via confined-space alloying were analyzed using different electron microscopy methods like HAADF-STEM, EDX line scan and elemental mapping. Especially the electrochemically degraded samples were of great interest questioning the distribution of the two metals in the metallic nanoparticles.

To confirm the experimental results and to explain the structure of the nanoparticles after degradation theoretical EDX line scan profiles were calculated using Monte-Carlo-simulations³.

 

 

Results

In HR-STEM images the structure of this catalyst system can be seen clearly: The crystalline metallic nanoparticles are encapsulated in pores of the hollow graphitic spheres (fig. 1). The EDX elemental mapping shows that the metal nanoparticles contain Pt and Ni. EDX line scans with a high spatial resolution clearly evidence a 0.5-1 nm thick Pt-rich outer shell and Pt-Ni core after electrochemical degradation of the catalyst.

To confirm the experimental results and to explain the structure of the nanoparticles theoretical EDX line scan profiles were calculated using Monte-Carlo simulations. Simulated line scans of PtNi@Pt particles (0.5 nm shell and 3 nm core) show profiles similar to the experimental data as shown in figure 2. This excellent agreement supports the formation of core shell particles during the electrochemical degradation of the catalyst. 

 

 

References

1)    Mezzavilla, S., Baldizzone, C., et al, in preparation

2)    Baldizzone C., Mezzavilla S., et al, Angew. Chem. Int. Ed. 53, 14250-14254 (2014)

3)    Drouin, D., Couture, A. R., et al, Scanning, 29, 92–101 (2007)

Ann-Christin SWERTZ (Muelheim an der Ruhr, Germany), Stefano MEZZAVILLA, Norbert PFÄNDER, Ferdi SCHÜTH, Christian W. LEHMANN

08:00 - 18:15 #6433 - MS00-471 (S)TEM Study of the Influence of Synthesis conditions on the Nanostructure and Performance of Au/CeO2 Model Catalysts.

MS00-471 (S)TEM Study of the Influence of Synthesis conditions on the Nanostructure and Performance of Au/CeO2 Model Catalysts.

Ceria nanoparticles exhibiting controlled morphologies have been studied as model supports of gold nanoparticles to establish correlations between surface crystallography and catalytic performance [1]. It is generally assumed that the contacts between Au nanoparticles and support involve the expected ones, i.e. Au//CeO₂ {100} in the case of nanocubes, Au//CeO₂ {110} in nanorods and Au//CeO₂ {111} for nano-octahedra. Nevertheless, it is important to recall at this respect that: (1) model crystallites involve more than one type of surface plane [2]; (2) previous reports have evidenced that gold nanoparticles anchor preferentially at sites allowing to maximize contact with {111} facets [3]; (3) the surface structure of the model support crystallites may strongly depends on the activation treatments performed on the catalyst prior to the catalytic assay. Therefore, as illustrated in this contribution, a combined 2D and 3D (S)TEM investigation is necessary to reveal the actual nature of the Au//ceria interfaces on these model supports.

CeO₂ nanocubes (CeO₂NC) were prepared using a hydrothermal method. This support was then treated in an O₂(5%)/He atmosphere at 600^oC for 1 hour to simulate the influence of activation at high temperatures (CeO₂NC600). Gold was deposited over the two supports by Deposition-Precipitation method with the objective to anchor 1.5 wt. % of Au on each of these materials. The final gold loadings on those samples were 0.4% and 1.0% respectively (0.4% Au/CeO₂NC and 1.0% Au/CeO₂NC600 samples), which clearly indicated a large influence of the activation treatment on the capability of the model support crystallites to anchor the metallic phase on its surface.

HREM and HAADF-STEM tomography confirmed that the CeO₂NC surface was a mixture of {100}, {110} and {111} facets, the latter two resulting from truncations of the cubes at edges and corners. In the CeO₂NC600 support, the {110} surfaces on the edges of the nanocubes, which appeared flat in the CeO₂NC sample, were fully transformed into a system of {111} nanofacets. Additionally, those edges grew in extent after the calcination process, from less than 1% up to 25% after the thermal treatment. These results suggest that surface nanofaceting significantly promotes the deposition of gold.

In contrast to expectations, STEM studies revealed that Au nanoparticles were always preferentially anchored on {111} surfaces: on the vertices of 0.4% Au/CeO₂NC (Figures 1a,2a) and on the {111}-nanofaceted edges in the 1.0% Au/CeO₂NC600 (Figures 1b, 2b). This result does not only explain the larger metal loading observed on the calcined support but also warns about simplifications about the nanostructure of these systems as well as on the importance of their detailed characterization by STEM.   

 

References:

[1] Wu, Z. et al., J. Catal. 285 (2012), p. 61-73.

[2] Florea, I.et al., Cryst. Growth Des. 13 (2013), p. 1110-1121.

[3] González, J. C. et al., Angew. Chem. Int. Ed. 48 (2009), p. 5313-5315.

Miguel TINOCO, Susana FERNANDEZ-GARCIA, Miguel LOPEZ-HARO (Puerto Real, Spain), Ana Belen HUNGRIA, Xiaowei CHEN, Ginesa BLANCO, Jose Antonio PEREZ-OMIL, Sebastian COLLINS, Hanako OKUNO, José Juan CALVINO

08:00 - 18:15 #6468 - MS00-473 Structural Transition of PtAg nanoalloys: annealing effect on atomic ordering and segregation.

MS00-473 Structural Transition of PtAg nanoalloys: annealing effect on atomic ordering and segregation.

Metallic nanoparticles made of more than one element (i.e. nanoalloys) are developed because they can present synergetic effects which enhance a wide range of properties in many fields of science such as reactivity, magnetic storage or medical imaging. Taking advantage of both alloying and size effects, the so-called nanoalloys have considerably widened the technological potential of nanoparticles due to their tunability by size, shape and composition [1-4].

The structural landscape of bi-metallic nanoparticles is very rich: the structure and the state of mixing or segregation depend on multiple parameters leading to different structures (crystalline, non-crystalline, alloyed, segregated, core shell, onion like, Janus…). Obtaining a complete and precise description of a single particle, and checking its representativeness, remain challenging and the latest developments in electron microscopy could contribute to address this key issue.

The present study was done in an attempt to investigate the effects of composition, post –annealing treatment on the size-dependent atomic arrangement in Pt-Ag nanoparticles to study the phase diagram of this system at the nano-scale. The availability of sub-nanometer electron probes in a STEM and aberration corrected (Cs-corrected) high resolution transmission electron microscopy (HRTEM), ensure great capabilities for the investigation of size, shape, structure and composition.

PtAg nanoparticles were prepared by electron beam deposition of Pt and Ag on an amorphous carbon layer kept at room temperature during the deposition. The deposition rate was adjusted to reach an average composition of Ag-52at%. Post deposition annealing treatment were carried out under ultrahigh vacuum at 673K.

Figure 1 shows TEM and Cs-corrected HRTEM images of the PtAg nanoparticles prepared at room temperature and after a post annealing treatment reaching 673K. After the room temperature deposition the particles are crystalline and ramified. This is due to static coaslescence effect: the consequence of high particle density on the carbon support. The post annealing treatment induces mobility of the atoms and particles and consequently Oswald ripening, dynamic coalescence and restructuration mechanisms leading to particles with a quasi-spherical shape. EDX analysis performed on single particles shows an evolution of the particles composition after the annealing process. Moreover, after the post annealing treatment some particles present an alternating contrast on HRTEM images. This is confirmed on HAADF STEM images as shown on figure 2. The combination of HAADF images and EDX analysis on single particles have shown that structural and chemical configurations depend on the composition showing alloyed or core-shell particles. Moreover a structural transformation from an alloyed disordered PtAg phase to an ordered L1₁ PtAg phase with the post annealing treatment was observed. This L1₁ phase appears for a very narrow interval of composition and presents some internal strains.

 

References:

[1] Nanoalloys: Synthesis, Structure and Properties edited by D. Alloyeau, C. Mottet, C. Ricolleau, London, Springer-Verlag, 2012

[2] Nanoalloys: from fundamentals to emergent applications edited by F. Calvo, Elsevier, 2013

[3] R. Ferrando, J. Jellinek, R.L. Johnson, Chem. Rev. 108 -3, 845 (2008)

[4] P. Andreazza, V. Pierron Bohnes, F. Tournus, C. Andreazza-Vignolle, V. Dupuis, Surf. Sc. Rep. 70, 188-258 (2015)

 

Acknowledgement:

The authors acknowledge financial support from the CNRS-CEA “METSA” French network (FR CNRS 3507) for the HRTEM experiments conducted on the MPQ – Paris Diderot platform

Caroline ANDREAZZA-VIGNOLLE (ICMN, Orléans), Pascal ANDREAZZA, Jérome PIRART, Asseline LEMOINE

08:00 - 18:15 #6488 - MS00-475 Quantitative Electron Tomography Study of Metal Catalysts Supported on Heavy Oxides Combining Image De-noising and Compressed Sensing Techniques.

MS00-475 Quantitative Electron Tomography Study of Metal Catalysts Supported on Heavy Oxides Combining Image De-noising and Compressed Sensing Techniques.

3D quantitative characterization of metal catalysts supported on heavy oxides by High Angle Annular Dark Field (HAADF) STEM electron tomography (ET) is a very challenging task. Noble metal nanoparticles (Au, Ru) supported on ceria or ceria mixed oxides illustrates the case. The difference, very small in some cases, between the atomic number (Z) of metal and support (Z_Ru=44, Z_Au=79, Z_Ce=58) complicates the discrimination of the small particles.  This makes that each step, acquisition, alignment, reconstruction and segmentation, has to be carefully optimized not only to improve the visualization of the nanoparticles but also to extract relevant quantitative results, e.g. metal loading, specific surface area, or particle size distribution. In this work, we have combined advanced image processing algorithms, based on undecimated wavelets transform (UWT) [1], to improve the contrast and denoising the tilt series projections, with new reconstruction algorithms, based on compressed sensing (CS) [2], to study a series of catalysts with a high potential in different processes related to the production of hydrogen, as CeO₂ Nanorods, Au/CeO₂ Nanocubes or Ru/Ce₂Z₂O₈ . The results obtained by ET have been compared to those determined by macroscopic characterization techniques as Inductively Coupled Plasma (ICP) or Brunauer, Emmett and Teller (BET) isotherm method.

HAADF-STEM ET has been carried out using a FEI TITAN³ THEMIS 60-300 operated at 200kV recently installed at Cadiz University. Data collections were obtained by tilting the specimen about a single axis perpendicular to the electron beam. Series of projections were acquired between -70º and +70° either every 2° or 5º. The images series were aligned using Inspect3D and TomoJ and reconstructed using the ASTRA Toolbox implemented in Matlab [3]. In the particular case of CS, Total Variation Minimization (TVM) was carried by using the TVAL3 solver [2]. 

Figure 1 shows the 3D-rendered voxel of a catalyst consisting of Au nanoparticles supported on CeO₂ Nanocubes after reconstructing the raw tilt series by SIRT (Figure 1a) and TVM (Figure 1b) and after denoising the tilt series projections by UWT and reconstructed by SIRT (Figure 1c) and TVM (Figure1d). Note how in the case of UWT-TVM reconstruction an important improvement in terms of morphology of the support and visualization of nanoparticles is obtained. The 3D rendered surface of a similar sample and the 3D quantifications of relevant properties as metal loading, specific surface area and average particle size are shown in Figure 2. It is important to point out how the values are quite similar to those determined by macroscopic characterization techniques. These results indicate that this combination of techniques allows determining nanostructural features representative of the catalysts at macroscopic level.

References:

[1] T. Printemps et al. Ultramicroscopy 160 (2016) 23-24

[2] B. Goris et al. Ultramicroscopy 113 (2012) 120-130

[3] W. van Aarle et al. Ultramicroscopy 157 (2015) 35-47

[4] Authors acknowledge funding from MINECO/FEDER (MAT2013-40823R and CSD09-00013). Financial resources from the European Union Seventh Framework Programme under Grant Agreement 312483 – ESTEEM2 (Integrated Infrastructure Initiative – I3) is also acknowledged.

Miguel LOPEZ-HARO (Puerto Real, Spain), Miguel TINOCO, Ana Belen HUNGRIA, Jose Juan CALVINO

08:00 - 18:15 #6499 - MS00-477 Electron Tomography of entrapped iron nanoparticles in silicalite-1 (Fischer-Tropsch catalyst).

MS00-477 Electron Tomography of entrapped iron nanoparticles in silicalite-1 (Fischer-Tropsch catalyst).

It is known that Fischer-Tropsch synthesis [1] is one of the best processes to convert syngas emitted from combusted of fossil fuels like coal and natural gas into liquid gas/ liquid fuels and other petro chemicals. Moreover, iron based catalysts are known to be efficient and active components of Fischer-Tropsch catalyst. Encapsulating iron nanoparticles into mesosporous silica allows to provide a highly selective, active and stable Fe@silicalite-1 catalyst [2]. Its architecture prevents the particles from sintering under reaction conditions, it does not show any activity to water-gas-shift reaction during FTS and it does not produce any CO₂ which is another advantage of this catalyst. These attributes made the Fe@silicalite-1 catalyst extremely selective.

 The 2D observation and analysis of iron nanoparticles on Silicate-1 gives only partial information regarding the distribution and position of the particles making necessary electron nano-tomography analyses. To understand the 3D position and size distribution of the iron nanoparticles in silica cages, high resolution electron tomography in bright field mode was performed in a FEI-Titan ETEM using a Fischione tomographic holder. We worked in a low dose mode to avoid as much as possible any beam damage of silica. After the segmentation of reconstructed volumes, the quantification of the 3D models was made (Fig 1). It was found that a large majority of iron nanoparticles are encapsulated in the walls of silica. The Fe particles average size is of 3.5 nm (Fig 2) [3].

[1] A. de Klerk, Fischer–Tropsch Process, Kirk-Othmer Encyclopedia of Chemical Technology,  (2013), 1–20.

[2] S. W. Li, L. Burel, C. Aquino, A. Tuel, F. Morfin, J. L. Rousset and D. Farrusseng, Chem. Commun., 49, 8507 (2013).

[3] Thanks to CLYM (Consortium Lyon - St-Etienne de Microscopie, www.clym.fr) for the access to the microscope funded by the Region Rhône-Alpes, the CNRS and the 'GrandLyon'.

Siddardha KONETI (VILLEURBANNE CEDEX), Lucian ROIBAN, David FARRUSSENG, Joffrey HUVE, Thierry EPICIER

08:00 - 18:15 #6548 - MS00-479 Microstructural characterization of Delaminated 2D Cobaltites.

MS00-479 Microstructural characterization of Delaminated 2D Cobaltites.

The lamellar cobaltites Na_xCoO₂ have awakened the interest of scientific community because of their superconductive phases [1], thermoelectric [2] and photocatalytic properties [3], and their possible use as cathodes in sodium batteries [4]. Their basic structure can be described on the basis of infinite [CoO₆] octahedral edge-sharing layers with Na cations at the interlayer space. Depending on the Na content different polymorphs have been described [5]. The synthesis of these species at the nanoscale could be a key factor for their technological exploitation. Furthermore, recent researches have proved that the inclusion of certain transition metals as dopants in the original structure could improve some of these properties [6].

 

Here we present an electron microscopy study of delaminated 2D cobaltites in the Na_yCo_1-xM_xO₂, (M = Mn, Ni, Cu) system obtained through a new synthesis strategy based on a co-precipitation reaction in a basic medium. After purification by centrifugation, a soft thermal treatment is performed before a selective sedimentation process in order to select the smallest flakes. Transmission electron microscopy proves the success of the delamination process. Low magnification TEM (figure 1a) shows the presence of two remarkable features: i) an apparently very thin matrix and ii) elongated particles in between 5 and 30 nm. The corresponding HREM (figure 1b) study reveals that both matrix and particles are crystalline with periodicities that can be interpreted in terms of [001] and [100] projections of the monoclinic cell of Na_0.6CoO₂ [5]. To get information about the atoms distribution and oxidation states a study in a probe aberration corrected microscope JEOL JEM ARM200 cFEG has been performed. Preliminary HAADF and ABF (figure 2) studies evidenced a lamellar structure similar to the Na_0.6CoO₂ with the alkali metal interlaying the Co_1-xM_xO₂ sheets. EELS study confirms the presence of all Na, Co, Ni and O (figure 2c) suggesting an intermediate oxidation state of cobalt cations between 3 and 4 in good agreement with the sodium content stablished by ICP.   

 

References:

 

[1] K. Takada et al., Nature, 422 (2003) 53-55.

[2] M. Lee et al., Nature Materials, 5 (2006) 537-540.

[3] L.  Liao et al., Nature Nanotechnology, 9 (2014) 69-73.

[4] B. L Ellis et al., Current Opinion in Solid State and Materials Science, 16 (2012) 168-177.

[5] L. Viciu et al., Physical Review B, 73 (2006) 174104.

[6] E. Levi et al, Solid State Ionics, 264 (2014) 54-68.

 

Alberto AZOR LAFARGA (Madrid, Spain), María Luisa RUIZ, David PORTEHAULT, Clemént SANCHEZ, José María GONZÁLEZ-CALBET

08:00 - 18:15 #6554 - MS00-481 Topotactic reduction in SrMnO3-δ nanoparticles followed by atomically-resolved microscopy.

MS00-481 Topotactic reduction in SrMnO3-δ nanoparticles followed by atomically-resolved microscopy.

The current technological demand of new devices requires the continuous stabilization of new compounds with nanometric scale for novel applications. The huge diversity of functional properties of the perovskite related mixed oxides make them ideal candidates to study the effect of the particle size reduction [1]. Particular attention has been paid to Mn related perovskites since Mn can be stabilized in different oxidation states leading to a series of functional materials. Here, we present the study of SrMnO₃ nanoparticles obtained from a novel [SrMn(edta)(H₂O)₅]·3/2H₂O heterometallic precursor and we evidence the different reactivity of the SrMnO₃ material in nanometric size [2].

Atomically-resolved high angle annular dark field (HAADF) images, acquired in a JEOL JEMARM200cF microscope, reveals 40-60 nm size SrMnO₃ nanoparticles with 4H hexagonal related-perovskite structure (Figure 1). Oxygen deficient samples were obtained from SrMnO_3.0 nanoparticles by reducing the oxygen content in a controlled way. The 4H-related structure is kept in the SrMnO_2.82 sample confirming a topotactic reduction of the nanoparticles when the oxygen content decreases down to δ=0.2. This fact makes a difference with respect to the bulk material, where the stabilization of a cubic-related perovskite structure is observed for the same oxygen content [3]

The structural study performed on SrMnO_3.0 reveals the presence of dislocations at the origin of the structural disorder in the 4H-perovskite structure, while more complex arrangement of cubic and hexagonal layers seems to be responsible for structural defects in SrMnO_2.82 sample (Figure 2a). Additionally, electron energy loss spectra (EELS) were acquired to access the local oxidation state of manganese. The coexistence of Mn³⁺ and Mn⁴⁺ is observed in SrMnO_2.82 but differences in the Mn-L_2,3 line position suggests a higher presence of Mn³⁺ associated to the structural defect areas (Figure 2b).

References

[1] A. Querejeta et al. Chem. Mater.21, (2009), pp.1898

[2] I.N. González-Jiménez et al. Chem. Mater. 26, (2014), pp. 2256

[3] A. Varela et al. J. Am. Chem. Soc., 131 (24), 2009, pp. 8660

 

Almudena TORRES-PARDO (Madrid, Spain), Irma Noemí GONZÁLEZ-JIMÉNEZ, Aurea VARELA, Marina PARRAS, Jose M. GONZÁLEZ-CALBET

08:00 - 18:15 #6592 - MS00-483 Heterogeneity in nanoparticle size determination using several biophysical and microscopy methods.

MS00-483 Heterogeneity in nanoparticle size determination using several biophysical and microscopy methods.

Nanoparticles belong to the class of nanomaterial that are natural, incidental, or manufactured materials containing elementary particles where at least one external dimension is in the range of 1 nm to 100 nm for 50% or more of the particles in the number size distribution. Therefore, size criteria is a key control parameter for defining nanoparticles. Moreover, nanoparticles have the property of assembling, agglomerating, or aggregating elementary particles into larger entities. But, the definition clearly refers to elementary particle and thus fine methodology must be used to obtain the critical size of nanoparticles. Besides, nanotoxicology which is the field of study of toxicological effect of nanoparticle in health aims to relate putative toxicological effect to the size of nanoparticles as the current dogma in the field suggest a greatest impact/reactivity with smaller particles than larger ones. Thus, obtaining accurate size measurements of nanoparticles is of great scientific significance.

In this work, we have combined several biophysical and microscopic methods to characterize the size of several nanoparticles of main interest in toxicological studies. Although we have worked on more than 50 different metallic nanoparticles, we only focused our interest on two families: Silver (Ag) and titane oxide (TiO₂). We have used atomic force microscopy, wet scanning transmission electron microscopy, dynamic light scattering, small-angle X-ray scattering. First, controls on well-behaved nanosize samples were performed and maximum dispersion between low-bound and high-bound sizes was about 30%. The striking results of this work is the relatively large dispersion of results obtained on Ag and TiO₂ (several hundreds of percent) depending on the method used. Several hypotheses can explain these results: nanoparticle solutions are highly heterogeneous and some methods may capture different forms, measurement methods are biased toward a certain size, methodologies used to extract sizes are inaccurate. All these hypotheses are plausible and will be discussed in this work.

Jean-Luc PELLEQUER (GRENOBLE CEDEX 9), Julien CAMBEDOUZOU, Adèle GERDIL, Christian GODON, Aurélie HABERT, Nathalie HERLIN-BOIME, Renaud PODOR, Johann RAVAUX, Jean-Marie TEULON

08:00 - 18:15 #6605 - MS00-485 Impact of water and oxidation states in the galvanic replacement formation of hollow oxide nanoparticles.

MS00-485 Impact of water and oxidation states in the galvanic replacement formation of hollow oxide nanoparticles.

Metal-oxide hollow nanoparticles are appealing structures from the applied and fundamental viewpoints. The synthesis of bi-phase metal-oxide hollow nanoparticles has been reported based on galvanic replacement using an organic-based seeded-growth approach, but with the presence of H2O. Here we report on a novel route to synthesize hollow core-shell MnOx/FeOx nanoparticles by galvanic replacement without the use of H2O. We demonstrate that the role of H2O in the MnOx/FeOx galvanic replacement synthesis is to oxidize the MnO/Mn3O4 seeds into pure Mn3O4 in order to obtain the suitable oxidation state so that the Mn3+→Mn2+ reduction by the Fe2+ ions can occur. Thus, if no H2O is added, onion-like MnO/Mn3O4/Fe3O4 nanoparticles are obtained, while whenMn3O4 seeds are used, hollow core-shell MnOx/FeOx are achieved. Thus, a critical step for galvanic replacement is the existence of proper oxidation states in the seeds so that the chemical reduction by the shell ions is thermodynamically favoured.

Pau TORRUELLA BESA (Barcelona, Spain), Alberto LÓPEZ-ORTEGA, Alejandro ROCA, Michelle PETRECCA, Sónia ESTRADÉ, Francesca PEIRÓ, Victor PUNTES, Josep NOGUÉS

08:00 - 18:15 #6617 - MS00-487 Thermodynamics of Pd(-Au) nanoparticle-titania interface studied by aberration-corrected TEM.

MS00-487 Thermodynamics of Pd(-Au) nanoparticle-titania interface studied by aberration-corrected TEM.

Over the last decade, supported metallic nanoparticles (NPs) have garnered continuous interests across many fields of research due to their novel physico-chemical properties which are, among others, shape-dependent. Though many synthesis schemes are being developed to generate a variety of NP shapes, understanding why a NP adopts a given shape is still challenging due to the intricate influences of thermodynamic, kinetic and energetic factors. In this contribution, we first report on the structural properties of Au-Pd nanoalloys supported on rutile titania, which can be considered as model catalysts. Then, using a recently proposed scheme that combines TEM imaging of single nanoparticles and a generalized Wulff-Kaishew theorem [1], the interface and triple-line energies of the Au-Pd NP-titania system are determined experimentally and studied as a function of particle composition and epitaxial relationship.

Bimetallic Au-Pd nanoalloys with well-controlled composition were grown on well-defined rutile titania nanorods by pulsed laser deposition. Titania with rod-like shape and narrow size distribution was prepared using a two-step hydrothermal procedure developed by Li and Afanasiev [2]. The nanorods preferentially expose (110) facets. Bimetallic Au-Pd nanoalloys with well-controlled composition were grown on these nanorods by alternate ablation of two monometallic Au and Pd targets in a UHV chamber. During particle nucleation and growth, the rods were kept at a temperature of 300°C, the nominal thickness of deposited metal was 1 nm. For ultra-high resolution TEM imaging and X-ray spectroscopy, a JEOL ARM 200F microscope was used. This microscope combines a cold field emission gun and a CEOS hexapole spherical aberration corrector (CEOS GmbH) to compensate for the spherical aberration of the objective lens. The microscope was operated at 80 kV to limit beam damage.

Bimetallic nanoparticles with Au, Pd, Au₃₈Pd₆₂ and Au₅₇Pd₄₃ stoichiometries were synthesized. Their composition was precisely determined by EDX analyses of assemblies of particles. Single-particle imaging of the as-synthesized samples showed the formation of well-separated NPs with size range 2-8 nm. As a result of the poor epitaxy between the metallic NPs and their support, most NPs displayed droplet-like morphology with ill-defined crystalline structure. Wherever a higher degree of epitaxy prevailed, Au-Pd NPs in the shape of truncated octahedra and having a completely disordered fcc structure (random alloy) were observed. Various epitaxial relationships were identified between the nanoparticles and the titania support, with the two dominant and previously unreported relationships being Au-Pd(111)//Rutile(110)[1-1-1] and Au-Pd(100)//Rutile(110)[1-10] (Figure 1).

With the precise equilibrium morphology of the NPs known, the interface and triple-line energies of the metal/oxide systems were determined by combining particle size measurements in atomically-resolved projected TEM images acquired parallel to the metal-oxide interface and a generalized Wulff-Kaishew theorem derived from Sivaramakrishnan et al. [4] (Figure 2). This theorem takes into account the influence of triple-line energy on nanoparticle equilibrium shape. Interface and triple-line energies were investigated as a function of particle composition and epitaxy. For any given epitaxial relationship, the relative amplitude of the NP truncation at the interface is found to increase linearly with particle size, i.e. the bigger the NP, the more it wets the oxide surface. On the rutile support, analysis of Pd, Au₃₈Pd₆₂ and Au₅₇Pd₄₃ NPs in epitaxial relationship Au-Pd(111)//Rutile(110)[1-1-1] shows clearly that the interface and triple-line energies are strongly influenced by particle composition and epitaxy. The value of the interface energy of the bimetallic Au-Pd NPs γ_i,Au-Pd is about 1 J m^-2, which is about two  times that of the monometallic Pd NPs, respectively (γ_i,Pd = 0.5 ± 0.1 J m^-2 ). As for the triple-line energy, it is 0.8 ± 0.2 J m^-2 for the monometallic Pd nanoparticles. This value is about four times the average triple-line energy measured in Au-Pd NPs.

[1]        S. Sivaramakrishnan et al, Phys. Rev. B 82, 195421-195431 (2010)

[2]       C H. Li and P. Afanasiev, Mater. Res. Bull. 46, 2506–2514 (2011)

Nhat Tai NGUYEN, Jaysen NELAYAH (LMPQ, Paris), Laurent PICCOLO, Pavel AFANASIEV, Damien ALLOYEAU, Guillaume WANG, Christian RICOLLEAU

08:00 - 18:15 #6637 - MS00-489 Lanthanide distribution in NaLuF4:Gd,Yb,Er upconversion nanocrystals by EFTEM and EELS.

MS00-489 Lanthanide distribution in NaLuF4:Gd,Yb,Er upconversion nanocrystals by EFTEM and EELS.

Lanthanide-doped nanoparticles (NPs) have gained interest within the last decade due to their photon upconversion properties. Upconversion is a multi-photon process in which two or more lower energy photons are converted to a higher energy photon by step-wise energy transfer between an absorber ion and an emitter ion (1). These upconversion nanoparticles (UCNPs) are a promising alternative to traditional organic fluorphores and quantum dots in the area of bioimaging because their excitation wavelength (980 nm) lies within an optical window where there is the least absorption and scattering by biomolecules (lower background signals), and they do not exhibit photobleaching or photoblinking.

One of the main challenges limiting their application is a trade-off between luminescence intensity and size of the NP. Smaller NPs exhibit less toxicity as they can be excreted through the urinary system, and they are better for intracellular imaging as the smaller size prevents interference with molecular trafficking within the cell, pharmacokinetics, and protein function; however, upconversion luminescence intensity decreases with size due to, in part, surface quenching from the presence of surface defects (2).

Few reports exist on the synthesis of sub-10 nm UCNPs, and even fewer exist on the synthesis of sub-5 nm UCNPs that show visible upconversion emission (3-6). We have recently synthesized two sizes of NaLuF₄:Gd, Yb, Er UCNPs (ca. 4 nm and ca. 12 nm), through a facile one-pot method, that both show bright upconversion luminescence upon excitation by a 980 nm laser.

In order to better understand their luminescent properties, we have used energy filtered transmission electron microscopy (EFTEM) and electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy (STEM) to investigate the distribution of the different lanthanides within the nanocrystal. Measurements were performed on an HF-3300 instrument operating at 300 kV.

The results suggest that Lu is enriched in the outer (shell) region of the nanocrystal, while Gd is enriched in the inner (core) region of the nanocrystal (Figure 1). Based on these results, we propose that the Lu shell protects the upconverting core from surface quenching, thereby allowing even 4 nm NPs to show upconversion luminescence. We also propose that the formation of the core-shell structure is mediated by the way the individual lanthanides nucleate in solution – Gd nucleates first to form the core, while Lu nucleates later to form the shell. This proposal is also in accordance with previous reports on the nucleation of lanthanide NPs (7).  

EFTEM and EELS in STEM are shown to be valuable tools to characterize the structure of lanthanide-doped UCNPs and to determine structure-property relationships, and can aid in the further development of these materials for various applications.  

(1)  Haase, M.; Schafer, H. Angew. Chem. Int. Ed., 2011, 50, 5808-5829.

(2)  Kobayashi, H.; Ogawa, M.; Alford, R.; Choyke, P. L.; Urano, Y. Chem. Rev., 2010, 110, 2620-2640.

(3)  Liu, Q.; Sun, Y.; Yang, TS.; Feng , W.; Li, CG.; Li, FY. J. Am. Chem. Soc., 2011, 133, 17122-17125.

(4)  Ostrowski, A. D.; Chan, E.C.; Gargas, D. J.; Katz, E. M.; Han, G.; Schuck, P. J.; Milliron, D. J.; Cohen, B. E. ACS Nano, 2012, 6, 2686-2692.

(5)  Gargas, D. J.; Chan, E. M.; Ostrowski, A. D.; Aloni, S.; Virginia, M.; Alteo, P.; Barnard, E. S.; Sanii, B.; Urban, J. J.; Milliron, D.; Cohen, B. E. Nature Nanotechnology, 2014, 9, 300-305.

(6)  Rinkel, T.; Nordmann, J.; Raj, A. N.; Haase, M. Nanoscale, 2014, 6, 14523-14530.

(7) Mai, H. X.; Zhang, Y. W.; Yan, Z. G.; Sun, L. D.; You, L. P.; Yan, C. H. J. Am. Chem. Soc., 2006, 128, 6426-6436.

Elsa LU (Thornhill, Canada), Jothirmayanantham PICHAANDI, Lemuel TONG, M. A. WINNIK

08:00 - 18:15 #6669 - MS00-491 Multiscale investigation of USPIO nanoparticles in atherosclerotic plaques and their catabolism and storage in vivo.

MS00-491 Multiscale investigation of USPIO nanoparticles in atherosclerotic plaques and their catabolism and storage in vivo.

Since applications of nanotechnologies for life and health sciences are booming, magnetic nanoparticles (NP) have undergone considerable development. They combine physical, chemical and magnetic properties that make them appropriate as contrast agents  for diagnosis using medical imaging techniques such as Magnetic Resonance Imaging (MRI) or ultrasonic imaging. Nanometric contrast agents are made of metallic rare earth elements (Gd) or transition metal (Mn, Fe) oxide cores.

      The storage and catabolism of Ultrasmall Super Paramagnetic Iron Oxide (USPIO) nanoparticles was analyzed through a multiscale approach combining Two Photon Laser Scanning Microscopy (TPLSM) and High-Resolution Transmission Electron Microscopy (HRTEM) at different times after intravenous injection (iv) in an atherosclerotic ApoE^-/- mouse model. The atherotic plaque features and the USPIOs heterogeneous biodistribution were revealed from organ’s scale down to subcellular level. The biotransformation of the nanoparticle iron oxide (maghemite) core into ferritin, the non-toxic form of iron storage, was demonstrated for the first time ex vivo in atherosclerotic plaques as well as in spleen, the iron storage organ. These results rely on an innovative spatial and structural investigation of USPIO’s catabolism in cellular phagolysosomes. This study showed that these nanoparticles were stored as non-toxic iron compounds: maghemite oxide or ferritin, which is promising for MRI detection of atherosclerotic plaques in clinic using these USPIOs [1].

 

 

References

 

[1] VA Maraloiu, F Appaix, A Broisat et al., Nanomedicine: Nanotechnology, Biology and Medicine 12 (1), 191-200 (2016)

 

 

Acknowledgement

 

 The two-photon microscopy studies were performed on the National Platform of Intravital Microscopy in Grenoble (France Life Imaging). This platform was partly funded by the French program “Investissement d’Avenir” run by the “Agence Nationale pour la Recherche”; grant “Infrastructure d’avenir en Biologie Santé - ANR11-INBS-0006”. Researches were partially granted by the French National Agency for Research (ANR) in the frame of the INFLAM project and partially supported by a grant of the Romanian National Authority for Scientific Research, CNCS – UEFISCDI, project number PN-II-RU-PD-2011-3-0067.

Valentin-Adrian MARALOIU (Magurele, Romania), Florence APPAIX, Alexis BROISAT, Dominique LE GUELLEC, Valentin Serban TEODORESCU, Catherine GHEZZI, Boudewijn VAN DER SANDEN, Marie Genevieve BLANCHIN

08:00 - 18:15 #6695 - MS00-493 Influence of gamma-irradiation on the photocatalytic activity of undoped and Cu doped titania nanoparticles.

MS00-493 Influence of gamma-irradiation on the photocatalytic activity of undoped and Cu doped titania nanoparticles.

Copper doped and undoped TiO₂ powders have been prepared by sol-gel technique and annealed at 400 °C. The powders were gamma irradiated at ambient temperature and doses between 14 and 60 KGy. The objective of the proposed work is to study the effect of gamma irradiation on the physical properties of undoped and Cu doped TiO₂ photocatalyst. The structural, morphological and optical properties of initial undoped and doped TiO₂ powders have been investigated. The performance of such a system strikingly depends on the insertion of impurities in the TiO₂ structure sublattice and of the disruption brought about. Consequently, this work is mainly focused on a nanoscale electron energy loss spectroscopy (EELS) study, which has been performed on samples with various Cu to Ti molar ratios (0-12at.%). This study is an attempt to identify at the atomic level, the nature of the dopant insertion in the structure. It also allows evaluation of the composition of the studied TiO₂ powders throughout individual crystallites. In Cu doped TiO₂ sol gel powders, Cu²⁺ ion has been found to substitute to Ti⁴⁺ ions. Gamma irradiated samples present a crystalline core and a disordered shell structures (see Figure 2) as a result of the formation of oxygen vacancies. Such oxygen vacancies at the surface of TiO₂ nanocrystals lead to a remarkable enhancement of the photocatalytic activity of Cu doped TiO₂ nanocrystals (see Figure 3). This preparation method by gamma treatement could be potentially used for large-scale production of high-surface-area anatase titania nanoparticles with trapped electrons on oxygen vacancies. Such catalysts show remarkable enhancement in the visible light absorption and photocatalytic activity. The doping and gamma irradiation doses impacts on the structural, optical and photocatalytic efficiencies will be thus highlighted and correlated.

 

Lolwa SAMET (Tunis, Tunisia), Katia MARCH, Odile STEPHAN, Radhouane CHTOUROU

08:00 - 18:15 #6720 - MS00-495 Synthesis of ultra-small iron-oxide and cobalt ferrite nanoparticles by a simple thermal decomposition approach.

MS00-495 Synthesis of ultra-small iron-oxide and cobalt ferrite nanoparticles by a simple thermal decomposition approach.

 

Ferrite nanoparticles are promising candidates for a range of bio-medical applications due to their interesting physical and magnetic properties.  In order for their nano-scale properties to be utilised however, synthesis methods which enable fine control over size, shape and composition are required.

In this work we explore the size-controlled synthesis of iron-oxide and cobalt ferrite nanoparticles by thermal decomposition and their characterisation by advanced microscopy techniques. The tailored synthesis of the nanoparticles was addressed by exploring the parameters in the reduction of both an iron precursor (Fe(III)(acac)₃) and a cobalt precursor (Co(II)(acac)₃) in oleylamine. In this thermal decomposition approach, oleylamine acts as the sole surfactant, reducing agent, and solvent. By controlling the surfactant-to-precursor ratio, we have produced ultra-small nanoparticles with a narrow size distribution. The size, shape and atomic structure of the resulting particles were examined by aberration-corrected annular dark-field scanning-transmission electron microscopy; and the results show that their average size is <3 nm and they are highly crystalline. Electron energy loss spectroscopy was used to analyse the composition of the resulting nanoparticles and the oxidation state of the metals, confirming that they consist of Fe₃O₄ and CoFe₂O₄.

Our results serve to form a solid support for future studies into the size-dependent interaction of iron-oxide and cobalt ferrite nanoparticles with cells.

 

The authors thank the EPSRC for financial support for this work.

 

[1] K. Mandel, F. Dillon, A. A. Koos, Z. Aslam, F. Cullen, H. Bishop, A. Crossley, and N. Grobert, ‘Customised transition metal oxide nanoparticles for the controlled production of carbon nanostructures’, RSC Adv., vol. 2, no. 9, pp. 3748–3752, Apr. 2012.

[2] S. Amiri and H. Shokrollahi, ‘The role of cobalt ferrite magnetic nanoparticles in medical science’, Materials Science and Engineering: C, vol. 33, no. 1, pp. 1–8, Jan. 2013.

[3] Y. Yu, W. Yang, X. Sun, W. Zhu, X.-Z. Li, D. J. Sellmyer, and S. Sun, ‘Monodisperse MPt (M = Fe, Co, Ni, Cu, Zn) Nanoparticles Prepared from a Facile Oleylamine Reduction of Metal Salts’, Nano Lett., vol. 14, no. 5, pp. 2778–2782, May 2014.

[4] V. Georgiadou, C. Kokotidou, B. L. Droumaguet, B. Carbonnier, T. Choli-Papadopoulou, and C. Dendrinou-Samara, ‘Oleylamine as a beneficial agent for the synthesis of CoFe2O4 nanoparticles with potential biomedical uses’, Dalton Trans., vol. 43, no. 17, pp. 6377–6388, Apr. 2014.

Dominique PICHÉ (OXFORD, United Kingdom), Juan G LOZANO, Frank DILLON, Nicole GROBERT

08:00 - 18:15 #6724 - MS00-497 Atomic resolution electron microscopy of cobalt ferrite nanoparticles.

MS00-497 Atomic resolution electron microscopy of cobalt ferrite nanoparticles.

Cobalt ferrite (CoFe₂O₄) nanoparticles have recently emerged as a potential candidate for a range of bio-medical applications due to their interesting physical and magnetic properties. The high magneto-crystalline anisotropy of CoFe₂O₄ nanoparticles in particular offers improved efficiency over iron-oxide nanoparticles, allowing for smaller particles to be used. Synthesis of cobalt ferrite nanoparticles is conventionally achieved using thermal decomposition in oleic acid and oleylamine. Recent methods using oleylamine alone demonstrate greater suitability for biomedical applications, as oleylamine facilitates the phase-transfer process required to make the nanoparticles water-soluble. However changing the surfactant is known to have a significant effect on the crystal structure and morphology of metal-oxide nanoparticles, and the crystal structure of oleylamine-capped cobalt ferrite nanoparticles has not been studied in detail before. Here we demonstrate the inverse spinel structure of cobalt ferrite nanoparticles synthesised with oleylamine as the sole surfactant, by aberration-corrected annular dark-field scanning-transmission electron microscopy. The crystal structure is resolved with atomic-level detail which has not been demonstrated with cobalt ferrite nanoparticles previously. Furthermore the distribution of cobalt and iron atoms is shown by atomic-resolution EELS spectroscopy mapping. This data serves to form a solid support for future studies into the size-dependent interaction of cobalt ferrite nanoparticles with cells.

 

Acknowledgements

The authors thank the EPSRC for financial support for this work through grant numbers EP/M010708/1 and EP/K040375/1, for the South of England Analytical Electron Microscope. The research leading to these results has also received funding from the European Union Seventh Framework Programme under Grant Agreement 312483 - ESTEEM2 (Integrated Infrastructure Initiative–I3).

 

[1] K. Mandel, F. Dillon, A. A. Koos, Z. Aslam, F. Cullen, H. Bishop, A. Crossley, and N. Grobert, ‘Customised transition metal oxide nanoparticles for the controlled production of carbon nanostructures’, RSC Adv., vol. 2, no. 9, pp. 3748–3752, Apr. 2012.

[2] L. Jones, H. Yang, T. J. Pennycook, M. S. J. Marshall, S. V. Aert, N. D. Browning, M. R. Castell, and P. D. Nellist, ‘Smart Align—a new tool for robust non-rigid registration of scanning microscope data’, Adv Struct Chem Imag, vol. 1, no. 1, pp. 1–16, Jul. 2015.

[3] Y. Yu, W. Yang, X. Sun, W. Zhu, X.-Z. Li, D. J. Sellmyer, and S. Sun, ‘Monodisperse MPt (M = Fe, Co, Ni, Cu, Zn) Nanoparticles Prepared from a Facile Oleylamine Reduction of Metal Salts’, Nano Lett., vol. 14, no. 5, pp. 2778–2782, May 2014.

[4] V. Georgiadou, C. Kokotidou, B. L. Droumaguet, B. Carbonnier, T. Choli-Papadopoulou, and C. Dendrinou-Samara, ‘Oleylamine as a beneficial agent for the synthesis of CoFe2O4 nanoparticles with potential biomedical uses’, Dalton Trans., vol. 43, no. 17, pp. 6377–6388, Apr. 2014.

Dominique PICHÉ (OXFORD, United Kingdom), Juan G LOZANO, Aakash VARAMBHIA, Frank DILLON, Lewys JONES, Peter D NELLIST, Nicole GROBERT

08:00 - 18:15 #6745 - MS00-499 XTEM observations revealing high diffusivity and Ge segregation in UV laser pulse annealed SiGeO and GeTiO amorphous films.

MS00-499 XTEM observations revealing high diffusivity and Ge segregation in UV laser pulse annealed SiGeO and GeTiO amorphous films.

XTEM observations revealing high diffusivity and Ge segregation in UV laser pulse annealed SiGeO and GeTiO amorphous films  

 

V.S. Teodorescu¹, A.V. Maraloiu¹, A. Kuncser¹, C. Ghica¹, M.L. Ciurea¹, A.M. Lepadatu¹,

I. Stavarache¹, D.N. Scarisoreanu², M. Dinescu², M-G. Blanchin³

 

 

¹ National Institute of Materials Physics, 077125, Bucharest-Magurele, Romania

² National Institute of Lasers Plasma and Radiation Physics, 077125 Bucharest-Magurele, Romania

³ ILM- Université Claude Bernard Lyon1, 69622 Villeurbanne Cedex, France

 

Ge nanoparticles embedded in dielectric oxide matrix are very interesting for application in memory devices. Amorphous SiGeO and GeTiO films with different Ge/oxide ratio where produced by magnetron sputtering deposition on Si wafers. In these systems, the Ge nanoparticles are formed during RTA annealing by segregation of the Ge atoms via a nucleation and growth processes. The Ge nanoparticles can remain in amorphous state or become crystallized. The SiO₂ matrix remains amorphous after annealing, but the TiO₂ matrix crystallize in the same time with the Ge nanocrystals formation in the case of RTA annealing or remain amorphous in the case of laser pulse annealing [1]. The SiGeO and the GeTiO oxide films were laser pulse irradiated, using the forth harmonics radiation of the Nd-YAG laser (l = 266 nm) and a low laser pulse fluences between 15 and 100 mJ/cm². These fluence values are less than the melting threshold of the amorphous oxide films. 

 

The nanoscale films structure investigation was performed with the JEM ARM200F electron microscope using specimens prepared by cross section method (XTEM).  The structural modifications induced by the laser pulse irradiation are present under the film surface in a depth of about 60 to 80 nm, which is more than the expected absorption depth of the laser radiation. The XTEM structural study is the only method for the study of the structural modifications induced by the laser pulse irradiation in a nanometric layer under the film surface. Figure 1 shows the Ge segregation under surface of the irradiated GeTiO amorphous film. The Ge nanoparticle and the titanium oxide matrix remain amorphous after the laser annealing. Figure 2 shows a similar situation in the case of the SiGeO amorphous film. In this case, the estimated temperature of the film surface is close to the Ge melting point in a lapse of time of the order of magnitude of 10ns.

 In all cases, the oxide matrix remains in solid phase during the laser pulse action. Formation of the Ge nanoparticles during laser pulse action shows the presence of a very high diffusivity of the Ge atoms in the laser pulse field, similar to the diffusivity taking place in the liquid state.

 

 This work was supported by  PNII- ID –project number 289/2011.

 

[1].  Nanostructuring of GeTiO amorphous films by pulsed laser irradiation , V.S. Teodorescu, C. Ghica, A.V. Maraloiu, M. Vlaicu ,A. Kuncser, M.L.  Ciurea,  I. Stavarache,  A. M Lepadatu, N.D.  Scarisoreanu, A. Andrei, V. Ion, M. Dinescu,  Beilstein Journal of Nanotechnology, 04/2015; 6(1):893-900.

Valentin Serban TEODORESCU, Valentin Serban TEODORESCU (Bucharest-Magurele, Romania), Adriaan Valentin MARALOIU, Andrei KUNCSER, Corneliu GHICA, Magdalena Lidia CIUREA, Ana-Maria LEPADATU, Ionel STAVARACHE, Doinel Nicolae SCARISOREANU, Marie-Genevieve BLANCHIN, Ana-Maria DINESCU

08:00 - 18:15 #6755 - MS00-501 Deformation behavior of micron-sized polycrystalline gold particles studied by in situ compression experiments and frictional finite element simulation.

MS00-501 Deformation behavior of micron-sized polycrystalline gold particles studied by in situ compression experiments and frictional finite element simulation.

Gold particles in the micro- and nanometer size regime find wide spread technical applications. Especially for micro- and nanoelectromechanical applications the mechanical properties can determine device reliability and overall performance. Whereas studies on the compression of single crystal gold particles are quite abundant [1–3], the detailed deformation behavior of polycrystalline gold or other fcc metal particles is far less understood [4–5]. It has been shown for that the hardness of gold particles varies with strain and depends on the particle size: whereas hardness decreases for submicron particles (geometric softening) an increase in hardness is commonly observed for millimeter sized samples (work hardening). Geometric softening is driven by an ongoing geometric shape change during stressing and occurs for work hardened samples. Also an increase of yield strength has been observed: this effect is related to the particles’ small dimensions and restricted dislocation activities. Particularly for metal particles it is known that the initial deformations are concentrated in proximity to the contact areas; only with increasing strain the deformed regions progresses deeper into the particles. Friction at the contact interfaces thus affects the plastic deformation inside the particles. In general frictional processes at the contact interfaces are difficult to access - one possibility is the finite element method (FEM). So far mainly full stick (infinite coefficient of friction) or perfect slip (frictionless contacts) conditions have been modeled and the effect of friction on the deformation behavior of gold particles has not seen significant attention.

Within this contribution a combined experimental and finite element study on the deformation behavior of micron-sized polycrystalline gold particles is presented [6]. In situ uniaxial compression experiments of single spherical polycrystalline gold particles in the size range of 1 µm were performed with an in situ scanning electron microscope supported custom built manipulation device [7]: stress-strain data and information on particle morphology are thus accessible. A well reproducible stress-strain behavior without plastic creep is observed. From the FEM modelling a detailed insight into the deformation and the influence of friction is obtained. The stress-strain behavior and the observed geometric shape of the stressed particles can be modeled by an elastic-perfectly plastic finite element model which accounts for frictional effects at the contact interfaces. Coefficients of friction are experimentally assessed by atomic force microscopy. A comparison to a frictionless finite element model reveals the necessity of considering the effects of friction: at small strains the particles appear to be softer due to a reduced dissipation of plastic energy, whilst at large strains the resistance to deformation is increased. The latter effect is found to be mainly due to the dissipation of frictional energy at the contact interfaces.

1 W.M. Mook, C. Niederberger, M. Bechelany, L. Philippe, J. Michler, Nanotechnology 21, 2010, 055701.

2 Z.J. Wang, Z.W. Shan, J. Li, J. Sun, E. Ma, Acta Mater. 60, 2012, 1368–1377.

3 D. Mordehai, S.-W. Lee, B. Backes, D.J. Srolovitz, W.D. Nix, E. Rabkin, Acta Mater. 59, 2011, 5202–5215.

4 N. Gane, Proc. Roy. Soc. Lond. A. 317, 1970, 367–391.

5 M.M. Chaudhri, I.M. Hutchings, P.L. Makin, Philos. Mag. A, 1984, 493–503.

6 J. Paul, S. Romeis, P. Herre, W. Peukert, Powder Technol. 286, 2015, 706–715.

7 S. Romeis, J. Paul, M. Ziener, W. Peukert, Rev. Sci. Instrum. 83, 2012, 095105.

Financial support by the Deutsche Forschungsgemeinschaft (DFG) through the Cluster of Excellence “Engineering of Advanced Materials” and GRK1896 “In situ microscopy with electrons, X-rays and scanning probes” is gratefully acknowledged.

Stefan ROMEIS (Erlangen, Germany), Jonas PAUL, Patrick HERRE, Wolfgang PEUKERT

08:00 - 18:15 #6759 - MS00-503 High Resolution Study of Epoxy Resin with Silicon Dioxide Nanoparticles in Sputter Coated and Natural state in ESEM.

MS00-503 High Resolution Study of Epoxy Resin with Silicon Dioxide Nanoparticles in Sputter Coated and Natural state in ESEM.

A number of sites have studied properties of nano-composites, such as epoxy resin with nano-particles or micro-particles. The studies have shown that the already very good electrical insulation properties of epoxy resin, used for example in protective coats of transformers, can be substantially improved by addition of nano-particles. Enrichment of epoxy resin with silica nano-particles increases inner resistance and significantly reduces the loss agent [1]. Improved properties of nano-composites are beneficial for all industries from military and cosmic to electric energy, electronics and cosmetics.

Due to the high carbon and hydrogen levels epoxy resin shows lower signal electron emission coefficient. The material electric conductivity is poor. When observing in the classical SEM the resin specimen must therefore be coated with a thick electrically conductive layer or observed under primary electron beam very low energy conditions, in the order of single keV units. Due to the high sensitivity of the sample to damage by radiation it is further necessary to significantly reduce the beam current and observe the specimen at higher scanning speeds. All of the abovementioned factors significantly complicate observation of these samples in the classical scanning electron microscope and due to the low detected signal to noise ratio make achievement of high resolution impossible. As a consequence of interactions of the signal electrons with the gas positive ions are generated in the environmental scanning electron microscope (ESEM), causing compensation of the emerging charge on the non-conductive samples. This allows for observation of non-conductive samples under conditions of higher energy of the electron beam without metal plating. The positive ions hitting the sample surface also remove contamination from the sample surface which facilitates escape of the low-energy secondary electrons from the sample and their getting to the detector. 

Although under the high gas pressure conditions in ESEM the charge is compensated with positive ions and the sample is in addition covered with a 5 nm thick layer of carbon, charging can be observed on the sample peaks and edges, see fig. 1A. The charging is caused by the higher beam current chosen to compensate for the insufficient intensity of signal of the secondary electrons penetrating through the carbon layer and in the effort to achieve higher resolution of the nano-particles. Despite all this the image shown in fig. 1 is blurred and does not show visible details. This may be addressed by the sample coating with a thicker conductive layer, which results in a substantially negative effect, as shown by the results of our experiments. The specimen surface is modified, the nano-structure is made invisible and artefacts are produced. This may be resolved by reduced beam current, optimisation of the detection ability of the detector by correct selection and setting of the microscope and especially by removal of the conductive layer form the sample surface. Figs. 1B and 1C show clearly visible nano-particles with a substantially higher resolution. In the case of figs. 1B and 1C the specimen is in its natural condition without use of any conductive layer, observed in the ESEM environment with 150 Pa vapour pressure. This work was supported by the project [4].

References:

[1] Hudec J, et al., Fine Mechanics and Optics 60 (9) (2015), p. 268.

[2] Neděla V, et al., Nucl. Instrum. Methods in Physics A, 645 (1) (2011), p. 79.

[3] Maxa J, et al., Advances in Military Technology 7 (2) (2012), p. 39.

[4] The European Commission (ALISI No. CZ.1.05/2.1.00/01.0017)

Jiří HUDEC (Brno, Czech Republic), Vilém NEDĚLA

08:00 - 18:15 #6766 - MS00-505 Improvement of Soft Matter transmission Electron Microscopy; Application and Interpretation Pitfalls.

MS00-505 Improvement of Soft Matter transmission Electron Microscopy; Application and Interpretation Pitfalls.

Transmission Electron microscopy (TEM) is an increasingly popular tool for the characterization of soft, supramolecular materials. TEM can give direct structural information and can be used to complement indirect techniques like dynamic light scattering, NMR and spectroscopic methods. Although the most common TEM preparation techniques are well documented, many are erroneously used or the images misinterpreted. A survey of around 200 recent and highly cited publications indicates that artefacts are a wide-spread problem in the field of soft matter electron microscopy.

Subjecting doxorubicin loaded liposomes, “Doxil^®”, to the four most common TEM techniques, drying, drying followed by staining, negative staining and cryo-electron microscopy,  reveals the possibilities and impossibilities of each of them. Upon drying (figure 1a,b), the structure of the liposomes is completely altered. Because of the lack of heavy elements, without staining the remaining structures are only visible at extreme defocus values (Figure 1a). Staining after drying improves the contrast, but the structure of the liposomes was lost (Figure 1b). Negative staining preserves the structure of the liposomes but only the outside is visible (figure 1c). Cryo-electron microscopy reveals both the liposome and the doxorubicin crystal inside the liposomes. Furthermore, the bilayer of the phospholipid vesicle is clearly visible and its dimensions can be measured (figure 1d).

Unaware of the lack of structural preservation, many studies use drying of their soft materials without staining to prove self-assembly into vesicles or micelles. Most supramolecular self-assembled structures are composed of light elements which hardly scatter electrons, nevertheless high contrast images are presented. Most likely these are the result of the drying process where also dissolved materials will form particles. A better solution to image soft materials is negative staining, where the structures are surrounded by a “glassy” dried heavy-metal stain that preserves the structures and gives the objects a high contrast halo. Although better than just drying, negative staining suffers from limited resolution by the grain size of the stain, and the obtained information originates from the surface of the object as the stain obscures the internal structure.  However,  various mistakes are made including positive contrast after negative staining and measuring the stain layer as proof of bilayer thickness. The best method is cryo-electron microscopy where the sample is vitrified in a thin layer of its own solvent. The structure is well preserved and a near atomic resolution can be achieved. The lack of heavy metal stain is compensated by a very good phase contrast upon defocusing. Expertise is needed to distinguish between sample and artefact. Lack thereof results in ice contamination interpreted as part of the sample.

A systematic search on “self-assembly” or “vesicle” in combination with “electron microscopy” in the recent, highly cited, literature brings to light that almost half of the evaluated papers contains erroneous or non-interpretable electron microscopy pictures or images that were misinterpreted. In most cases soft, supra molecular, materials were dried without any staining on the grid and present dense round objects. Apparently peer review fails at this point.  Lack of TEM expertise amongst reviewers and previously published incorrect literature seem to be the main cause. Regarding the number of mistakes in application and interpretation, there is room for improvement in both the reviewing process and the application of different TEM techniques and their corresponding interpretation.

Linda FRANKEN (Groningen, The Netherlands), Egbert BOEKEMA, Marc STUART

08:00 - 18:15 #6790 - MS00-507 Combined macroscopic, nanoscopic and atomic-scale characterization of highly dispersed bimetallic particles supported on ceria-zirconia mixed oxide catalysts.

MS00-507 Combined macroscopic, nanoscopic and atomic-scale characterization of highly dispersed bimetallic particles supported on ceria-zirconia mixed oxide catalysts.

Highly dispersed gold nanoparticles appear as promising catalysts in the field of fine chemistry. However, the relatively low resistance to sintering of gold particles under reaction conditions often leads to a significant loss of catalytic activity. One strategy to overcome this limitation is combining gold with another metal to form a bimetallic system with enhanced stability, activity and/or selectivity. [1]

 

The catalytic behavior of bimetallic particles is determined by their structure, size, morphology and chemical composition. Undoubtedly, to design efficient and high-quality catalyst requires controlling all these features during the synthesis as well as a careful characterization of the synthesized catalysts at the finest scales. Such a characterization poses demanding challenges to STEM techniques if we want it to be representative of the actual macroscopic state of the surface of the catalyst.

In this work, the capabilities and limitations of both the macroscopic (XPS, XRD, ICP-AES, and chemisorption) and atomic scale (STEM-XEDS) techniques in the characterization of highly dispersed bimetallic particles, AuRu and AuPd, supported on a ceria-zirconia mixed oxide (CZ) have been evaluated. In particular, the effects of the catalyst activation pretreatment on nanostructure and catalytic performance for the selective oxidation of glycerol to glyceric acid of both bimetallic systems are investigated in comparative terms.

Following the approach described in [2], the relationship between composition (Au at.%), as determined by quantitative STEM-XEDS analysis, and size, as determined by HREM and HAADF, of a large ensemble of individual nanoparticles on the two types of catalysts was established as a function of calcination temperature, Figures 1 and 2. According to size-composition maps shown there, Figures 1(a) and 2(a), the monometallic gold particles exhibit a wider larger of sizes and a larger average size, whereas monometallic Ru or Pd particles are smaller in average. Bimetallic entities are found in the intermediate size range. Raising the temperature up to 700ºC, Figures 1 and 2 (b), induces a compositional homogenization in the case the AuPd catalysts which is not observed in the case of the AuRu bimetallics. From this point of view both catalysts behave in a quite different manner, in spite of Ru and Pd being elements very close in the Periodic Table.

Comparison of data obtained on the corresponding monometallic reference catalysts clearly indicates that the second metal, Ru or Pd, moderates the sintering behavior of Au, i.e. the stability of this type of catalysts. Moreover, a significant improvement in the catalytic activity takes place in the bimetallic catalysts, which is related to the presence of bimetallic entities in both catalysts. STEM data suggest that these bimetallic nanoparticles are formed by decoration of the surface of Au nanoparticles with 3D, nanosized, domains of the second metal, instead of forming actual Au-Ru or Au-Pd alloys. [3,4]

The limitations of STEM studies to reach a precise representation of the composition on both bimetallic systems determined by macroscopic techniques (like XPS or ICP) will also be addressed, Figure 1and 2 (c). The likely origin of such discrepancies as well as strategies to overcome this limitation will also be discussed.

[1] Ch. W. Han et al, Nano Lett.,2015, 15, 8141–8147.

[2] L. Bednarova, C. E. Lyman, E. Rytter, A. Holmen, J. Catal. 2002, 211, 335-346.

[3] L. Chinchilla et al, Catal. Today 2015, 253,178-189.

[4] C. Olmos et al, Appl. Catal. B-Environ. 2016 (In press).

Lidia CHINCHILLA, Carol OLMOS, Xiaowei CHEN, Ana Belen HUNGRIA, José Juan CALVINO (Cadiz, Spain)

08:00 - 18:15 #6791 - MS00-509 Observation of the graphitization process of hollow graphitic spheres.

MS00-509 Observation of the graphitization process of hollow graphitic spheres.

Introduction

Hollow graphitic spheres (HGS) are interesting support materials for catalytic active metal nanoparticles. HGS can be widely used as tailored mesoporous carbon support with several advantages over other carbon materials.¹ The spheres have a high surface area and a pore system with narrow pore size distribution which makes this material interesting for the impregnation of metal nanoparticles and techniques like the confined-space alloying of bimetallic nanoparticles.² Additionally HGS have a high degree of graphitization what makes the material stable under common catalytic reaction conditions.  A typical synthesis route¹ for HGS covers 3 major steps starting with silica spheres coated with a mesoporous silica shell:

-          Impregnation of the SiO₂ template with iron nitrate and polymer-precursor

-          Carbonization/graphitization of the polymer shell

-          Leaching of the silica to obtain hollow graphitic spheres

 

Methods

The polymer-coated SiO₂ spheres were dry-prepared  and heated up to 1000°C with a heating rate of 5 °C min^-1. At certain points the temperature was maintained to minimize the thermal drift for the image acquisition.

 

Results

Starting the thermal treatment at first the activation of iron particles could be observed. Beginning at about 250°C the finely dispersed iron started to agglomerate and formed metallic nanoparticles. By further heating the iron particles became mobile and the graphitization started at about 900°C.

 

References

1)      C. Galeano et al., J. Am. Chem. Soc., 134, 20457-20465 (2012)

2)      C. Baldizzone, S. Mezzavilla et al, Angew. Chem. Int. Ed. 53, 14250-14254 (2014)

Norbert PFÄNDER (Mülheim an der Ruhr, Germany), Ann-Christin SWERTZ, Christian W. LEHMANN, Robert SCHLÖGL

08:00 - 18:15 #6801 - MS00-511 Characterization of bimetallic PdAg nanoparticle arrays by the diblock copolymer micelle approach.

MS00-511 Characterization of bimetallic PdAg nanoparticle arrays by the diblock copolymer micelle approach.

Bimetallic nanoparticles (NPs) display unique properties drastically different from those of the corresponding single-component particles. These properties are assumed to result from both the electronic and structural effects of the bimetallic NP. As these properties depend also on the preparation conditions, the synthesis of bimetallic NPs with accurately controlled structures and compositions is essential to obtaining advanced materials for electronic, magnetic, optic and catalytic properties. In the present study, 2D ordered arrays of bimetallic PdAg NPs were successfully synthesized via the copolymer micelle approach and characterized by various spectroscopic and microscopic characterization methods. A special focus was laid on the influence of the type of reduction treatments on the chemical nature and the stability of the PdAg NPs. A comparison with the synthesis of single metal (Pd and Ag) NPs obtained by the same method was made.

A series of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymers of various compositions and molecular weights was synthesized by nitroxide mediated radical polymerization using an alkoxyamine unimolecular initiator (Styryl-SG1). Intermolecular interactions between the PdCl2 and/or AgNO3 salts and P4VP micellar core and formation of nanoparticles from the micellar complex were investigated by various spectroscopic (UV-vis, XPS) and microscopic (AFM, HRTEM/EDX) characterization methods.

Once the base solutions were made, three ways to produce the final arrays of bimetallic NPs were investigated (Figure 1). One consisted in using an oxygen-plasma treatment after dip-coating of the SiO2 surface (Figure 1a); this method uses the large amount of electrons generated by the plasma to reduce the metallic cations and lead to the formation of the NPs that eventually oxidize under the oxygen-plasma. A second method consisted in introducing a reducing agent (hydrazine) to the base solution (Figure 1b-path1). The third method consisted on flowing a vapour of the reducing agent (hydrazine) over the film after dip-coating of the SiO2 surface (Figure 1b-path2). Spectroscopic and microscopic characterization of the resulting films before and after reduction showed that the two first methods consistently lead to the formation of rather regular arrays of NPs. In the case of the films before reduction we observe that, for the three methods, the metallic loaded micelles preferentially arrange to form a quasi-hexagonal pattern on the carbon coated copper grid; a close observation of the P4VP cores revealed a fine grain substructure inside every micelle corresponding to ultrasmall (bi)metallic NPs (Figure 2e). After reduction either by oxygen-plasma or by hydrazine in the solution (Figure 2) we obtain rather organized arrays of bimetallic NPs. The NPs have uniform sizes (Figure 2d-insert) and compositions. Unlike the NPs obtained by oxygen plasma, those obtained by hydrazine in the solution are not oxidized. A full characterization of the physicochemical properties of the NPs was enabled by the use of different methods (ICP, AFM, HRTEM/EDX, XPS, UV-Vis). 

The copolymer micelle approach is an excellent method to obtain ordered arrays of bimetallic nanoparticles supported on flat surfaces with controlled sizes, spacing and compositions [1]. These collections of NPs can thus be used as model catalysts (for COV abatement in the case of PdAg) where important parameters that influence their catalytic behaviour can be finely monitored and modulated.[2]

References
[1] E. Ehret, E. Beyou, G.V. Mamontov, T.A. Bugrova, S. Prakash, A. Aouine, B. Domenichini and F.J. Cadete Santos Aires, Nanoscale 7 (2015) 13239-13248.

[2] E. E. and F. J. C. S. A. thank CNRS for grant n° 98087 within the Franco-Siberian Center; G. V. M. and T. A. B. acknowledgethe Tomsk State University Academic D. I. Mendeleev Fund Program.The authors thank the CLYM (Centre Lyon-St Etienne de Microscopie) for access to the Ly-EtTEM.

Eric EHRET, Emmanuel BEYOU, Grigory V MAMONTOV, Tatiana A. BUGROVA, Bruno DOMENICHINI, Swamy PRAKASH, Mimoun AOUINE, Francisco José CADETE SANTOS AIRES (VILLEURBANNE CEDEX)

08:00 - 18:15 #6814 - MS00-513 Nanometer WOx species evidenced by HRTEM/EDX in WO3/SiO2 catalysts for the metathesis reaction.

MS00-513 Nanometer WOx species evidenced by HRTEM/EDX in WO3/SiO2 catalysts for the metathesis reaction.

The demand for propylene is growing rapidly in the chemical industry since it is used for the production of commercial products such as polypropylene, acrylonitrile, alcohol, acrylic acid, and other petrochemical products. The metathesis reaction (using feeds of ethylene and butenes) is among other methods (naphtha steam crackers, fluid catalytic crackers, propane dehydrogenation, Fischer–Tropsch reactions) a way of producing propylene. Rather recently, metathesis reaction over WO₃/SiO₂ catalysts has received considerable attention by the industry to produce propylene [1] even though WO₃/SiO₂ catalysts have long been considered and used in the metathesis of linear olefins [2]. Despite thorough characterization of the WO₃/SiO₂ catalysts only a few works make use of transmission elec-tron microscopy to study such catalysts [3]. In those studies particular interest has been taken to the morphology of the WO₃ rather large particles as well as to coking issues. Indeed TEM observations show the presence of large WO₃ crystallites (tens or hundreds of nm). It is generally admitted that two tungsten phases co-exist : crystalline WO₃ and a surface phase in strong inter-action with the support. The latter is supposed to be at the origin of the active sites but no TEM characterization of it has yet been performed to our knowledge. In this work we report HRTEM/EDX evidence of the co-existence of highly divided WOx nanoparticles (< 2 nm) with WO₃ large crystallites.

Catalysts with different tungsten loadings (4, 6, 8, 10 wt%) were prepared by incipient wetness impregnation of an aqueous solution of ammonium metatungstate hydrate (Aldrich, 99.9%) over silica gel. Electron microscopy (HRTEM, EDX) was performed with a JEOL JEM 2010 TEM equipped with a LaB₆ gun, UHR pole-piece and a Pentafet LinK-Isis EDX spectrometer (Oxford Insts.).

In all samples irregular shaped polycrystalline (Figure 1a) and singlecrystal (Figure 1b-d) large particles (10–50 nm) were observed. For both cases they result from the agglomeration of smaller crystallites. Together with these large particles, numerous smaller particles exist in every sample (Figure 2a). These nanopar-ticles are generally smaller that 2 nm and even in the subnanometer range (Figure 2a-c). The former can be isolated (Figure 2d) or tend to agglomerated and rearrange in larger structures after sometime under the electron beam (Figure 2b-bottom, Figure 2e). The latter are rather difficult to detect and only the EDX spectra confirm the presence of  W; diffraction contrast (Figure  2c) only appears after sometime under the electron beam. It is rather difficult to estimate the oxygen quantitative concentration on the WO_x nanostructures since we are dealing with only a few atoms and above all the support is an oxide yielding a large contribution to the oxygen signal. The smaller nanoparticles (≤ 1 nm) present internal diffraction contrasts that can be attributed to a pseudo-structure. However this is only a short-range order and no finite periodicity was devised. For larger nanoparticles (1–2 nm) high resolution images (Figure 2d,e) show interplanary distances and angles between planes that can reasonably be attributed to a cfc structure. We cannot rule out however hcp structures in some cases. This is not inconsistent with the more classic orthorhombic or tetragonal structures for WO₃ since the values of the structural parameters a, b and c are very close to each other and since we expect these structures to be rather WO_x (x<3) than WO₃. These very small species may thus form the so-called surface phase thought to be at the origine of the active sites for the metathesis reaction.[4]

References
[1] J.C. Mol, J. Mol. Catal. A: Chem. 213 (2004) 39.

[2] A.J. Moffat et al., J. Catal. 18 (1970) 345; R.C. Luckner, G.B. Wills, J. Catal. 28 (1973) 83; R. Westhoff, J.A. Moulijn, J. Catal. 46 (1977) 414;. F.P.J.M. Kerkhoff, J.A. Moulijn, J. Electron Spectrosc. Related Phenom. 14 (1978) 453.

[3] A. Spamer et al., Appl. Catal. A: Gen. 255 (2003) 133; A. Spamer et al., Appl. Catal. A: Gen. 255 (2003) 153; D.J. Moodley  et al., Appl. Catal. A: Gen. 318 (2007) 155.

[4] The Thailand Research Fund and SCG Chemicals Co. Ltd. are gratefully acknowledged for financial support.

Narongrat POOVARAWAM, Kongkiat SURIYE, Joongjai PANPRANOT, Piyasan PRASERTHDAM, Francisco José CADETE SANTOS AIRES (VILLEURBANNE CEDEX)

08:00 - 18:15 #6860 - MS00-515 Structure characteristic of Ni-poly-p-xylylene nanocomposites synthesized by VDP.

MS00-515 Structure characteristic of Ni-poly-p-xylylene nanocomposites synthesized by VDP.

The simplicity of synthesis of polymer nanocomposites together with elasticity and plasticity, make these materials attractive in the aspect of possible applications for different application, including electronics and sensors, like different rectifiers and switching devices based on their high nonlinear VAC (volt-ampere characteristics) and tendency to form Schottky diodes, transistors, nanowires. Recent progress in spintronic and application of nanocomposites in that area have been shown in the work of Y. Liu, et al. [1,2]. Distinct attention in than and other investigations was attracted to the spin-valves, which use the effect of giant or tunneling magnetoresistance. Typically these devices consist on three-layer system: nonmagnetic layer is sandwiched between two ferromagnetic (hard and soft) of those. Based on these systems scanning heads of magnetic plates and magnetoresistance memory elements were produced. Usually the polymer materials were used as interlayer material between two ferromagnetic layers (hard and soft), while the adaptation of ferromagnetic polymer nanocomposites with embedded magnetic atoms (Ni, Co, Fe) will allow to construct spin-valve structures based on the homogeneous material, introducing a number of technological advantages. For the fabrication of these heterostructures the study of physical properties, like magnetoresistive effects, including anomalous Hall effect demonstrating the evidence of spin polarization of charge carriers have to be performed together with the correlation between these effects and the structure of material. The coercive force can be changed by pinning of the ferromagnetic stage with antiferromagnetic inclusions.

In this work we present the results of electron microscopy and microanalysis study of Ni-poly-p-xylylene nanocomposites stuffed with Ni nanoparticles in 5 - 30 vol. % range of concentration. The magnetic and electrophysical properties for these samples are slightly above the percolation threshold and strongly above it.

 

1. Liu, Yaohua; Watson, Shannon M.; Lee, Taegweon; Gorham, Justin M.; Katz, Howard E.; Borchers, Julie A.; Fairbrother, Howard D.; Reich, Daniel H., Physical Review B, 2009. V. 79. 075312 

2. «Structure and optical properties of thin poly(p-xylylene) – silver nanocomposite films prepared by low–temperature vapor deposition polymerization» Dmitry R. Streltsov, Karen A. Mailyan, Alexey V. Gusev, Ilya A. Ryzhikov, Yury I. Kiryukhin, Anton S. Orekhov, Alexander L. Vasiliev, Natalia A. Erina, Andrey V. Pebalk, Yaroslav I. Odarchenko, Dmitri A. Ivanov, and Sergei N. Chvalun // POLYMER 71:60-69 · AUGUST 2015

Anton OREKHOV (Moscow, Russia), Alexsander VASILIEV, Sergey OZERIN, Artem VDOVICHENKO, Sergey CHVALUN

08:00 - 18:15 #6893 - MS00-517 Identification of core-shell structures in high active Pt-alloy catalysts for oxygen reduction by electron spectroscopy.

MS00-517 Identification of core-shell structures in high active Pt-alloy catalysts for oxygen reduction by electron spectroscopy.

A potential clean alternative to traditional power sources, in particular for the automotive industry, is represented by low temperature fuel cells [1].

The major limitation of this promising technology lies in the high loading of the mostly used catalyst Pt necessary to drive the chemical reactions at the electrodes of the cells, impacting significantly on costs.

Generally, the performance is limited by the sluggishness of the Oxygen reduction reaction (ORR) that occurs at the metal catalyst surface present at cathode. Therefore, a great effort is put in finding catalysts with higher efficiency for the ORR [2][3].

A promising strategy for enhancing the ORR activity is to alloy Pt with other metals [2]. However, commercially available Pt alloys with late transition metals are mostly unstable under the harsh conditions in a fuel cell cathode [4].

Pt-Gd has been identified as a promising material among a new class of Pt-lanthanide alloys as highly performing catalyst for the ORR in both activity and stability, surpassing the previously reported Pt-Y [5-7].

The experimental results have shown that Pt_xGd nanoparticles exhibit an outstanding ORR activity of 3.6 A (mg Pt)^-1 at 0.9 V with respect to a reversible hydrogen electrode [8].

To fully understand the enhanced performance of these catalysts, the knowledge of the detailed structure and elemental distribution at the nanoparticle level is of extreme importance. Here we present an advanced Scanning Transmission Electron Microscopy (STEM) study of Pt_xGd nanoparticle catalyst.

High resolution HAADF-STEM imaging and STEM-EDX spectrum imaging has been performed in a double C_s-corrected FEI Titan Themis 60-300 at CIME laboratories, equipped with a monochromated high-brightness FEG, an EDS SUPER X system with a 0.7 srad of solid angle and a Gatan GIF Quantum ERS high energy resolution EELS spectrometer and energy filter.

Pt_xGd nanoparticles have been prepared through a gas aggregation technique in a multi-chamber ultrahigh vacuum system (Omicron Multiscan Lab) at DTU CINF laboratories.

Figure 1 shows a high resolution STEM micrograph and STEM-EDX elemental maps of Pt_xGd nanoparticles after exposure to ORR conditions during electrochemical tests. From the micrographs the sample presents a polycrystalline structure with a Pt rich shell and a Pt-Gd core. The formation of the Pt shell is assigned to the initial dissolution of Gd when exposed to the acidic solution of the ORR test. 

References

 

[1] M. K. Debe, Nature, 486 (2012), 43-51.

[2] I. E. L. Stephens et al, Energy & Environmental Science, 5 (2012), 6744.

[3] P. Strasser et al, Nature chemistry, 6 (2010), 454–60.

[4] S. Chen et al, J. Electrochem. Soc. (2010), 1571, A82.

[5] J. Greeley et al, Nature chemistry, 7 (2010), 552–6.

[6] M. Escudero-Escribano, et al, J. Am. Chem. Soc. (2012), 130, 16476.

[7] P. Hernandez-Fernandez et al. Nature Chemistry 6, 732–738 (2014).

[8] A. Velázquez-Palenzuela et al. Journal of Catalysis 328, 297–307 (2015).

 

Acknowledgment

The Danish National Research Foundation’s Center for Individual Nanoparticle Functionality is supported by the Danish National Research Foundation. 

Davide DEIANA (Lausanne, Switzerland), Amado Andres VELAZQUEZ-PALENZUELA, Maria ESCUDERO-ESCRIBANO, Federico MASINI, Ifan STEPHENS, Ib CHORKENDORFF, Cécile HEBERT

08:00 - 18:15 #6909 - MS00-519 Structure of CoAg nanoparticles embedded in a matrix.

MS00-519 Structure of CoAg nanoparticles embedded in a matrix.

Since CoAg nanoparticles are composed of two immiscible metals, their theoretical structure is predicted to be Co@Ag due to the weaker surface energy of Ag [1]. In the literature these particles are synthesized by chemistry techniques [2-3], the structure obtained is Co(core)Ag(shell) and has been confirmed by the optical response in UV-Vis spectroscopy [2]. The localized surface plasmon resonance (LSPR) of silver shell and the magnetic anisotropy of cobalt core open the access to magneto-plasmonic studies. That is why we have decided to elaborate those particles in a different approach using physical method under ultra high vacuum (UHV).

We have elaborated by low energy cluster beam deposition (LECBD [4]) technique Co₅₀Ag₅₀ nanoparticles of different diameter (approximatively 1 to 10 nm) in UHV which are mass selected by a quadrupole ion deflector. These clusters are embedded in different matrices (metal oxides, amorphous carbon) to protect them from oxidization. The clusters stoichiometry has been checked by EDX measurements.

The structure of Co₅₀Ag₅₀ clusters in carbon matrix is polycrystalline fcc. On high resolution transmission electron microscopy (HRTEM) images we can distinguish two different phases between the top and the bottom of the particle (figure 1.a) by using the intensity as a discrimination factor. Then we have determined by Fourier transforms analysis that the silver is in fcc structure and we measured approximatively the same lattice parameter than the silver bulk. Nevertheless we could not identify expected bcc, hcp or fcc [5] crystalline cobalt structure. High angle annular dark field electron microscopy (HAADF) measurements provided us informations about the contrast between cobalt and silver (figure 1.b), which depends of their respective atomic number. Inhomogeneous Janus-like structures have been identified, with cobalt assumed to be in dark and silver in bright (figure 1.b), this segregation is partially in agreement with theoretical predictions [1] which expect a core/shell segregated structure.

In Al₂O₃ matrix another phenomena occurs, an amorphous shell appears on each particle. As we can see on TEM image (figure 1.c and 1.d), we can distinguish a core in the particles composed of a dark part and a brighter one and an amorphous bright shell, assumed to be a reaction with the matrix. SQUID magnetic measurements confirmed us that the clusters are still ferromagnetic, an antiferromagnetic behaviour is expected for CoO, which means that the Co cannot be completely oxidized. As in carbon matrix, we can distinguish an inhomogeneous structure and an intensity contrast between the topand the bottom of the particle (figure 1.d). Then the silver is also in fcc structure and the lattice parameter obtained by Fourier analysis measurements matches with the bulk one. Moreover the same problem encountered in carbon matrix appears when we try to identify the Co crystalline structure.

[1]: F. Dorfbauer, T. Schrefl, M. Kirschner, G. Hrkac, D. Suess et al J. Appl. Phys. 99, 08G706 (2006)

[2]: P. Saravanan et al Journal of Alloys and Compounds 509, 3880–3885 (2011)

[3]: A.J. Garcia-Bastida et al Science and Technology of Advanced Materials 6, 411–419 (2005)

[4]: V. Dupuis et al Phys. Chem. Chem. Phys. 17, 27996-28004 (2015)

[5]: M. Pellarin et al Chemical Physics Letters, volume 2 17, 4 (1994)

Ophelliam LOISELET (Villeurbanne), Florent G TOURNUS, Katia MARCH

08:00 - 18:15 #6945 - MS00-521 Exchange-coupled spinel oxides: micro-Raman and in field Mossbauer spectroscopies correlated to HRTEM.

MS00-521 Exchange-coupled spinel oxides: micro-Raman and in field Mossbauer spectroscopies correlated to HRTEM.

In this work, we investigate on spinel oxides exchange-coupled on Co_xFe_3-xO₄ (x=0, 0.2, 0.4, 0.6, 0.8 and 1). The samples were synthesized using a solvothermal chemical route. X-ray (XRD) and electron diffraction (ED), associated with High Resolution Transmission Electron Microscopy (HRTEM), Magnetic investigations, micro-Raman spectroscopy and in-field ⁵⁷Fe Mossbauer Spectroscopy were used to study the Co_xFe_3-xO₄structural and physical properties.

XRD (fig.1), ED and HRTEM (fig.2) analyzes exhibit the presence of two phases, the hematite and the magnetite, for x=0. However, typical spinel structures for samples with cobalt concentrations x>0 have been found. Low temperature magnetic measurements demonstrate the presence of a jump in M (H) curves for x>0 (fig.3). This result has been reported to the presence of two exchange-coupled magnetic phases. TEM images have shown the presence of two shapes and sizes till x=0.6. The largest rectangular particles (~250 nm for x=0), investigated by ED, index in the spinel space group (Fd-3m, magnetite) for x=0. Referring to M (H) curves, these two shapes may have different chemical compositions. ED analyses done on the other compositions didn’t exhibit any structure difference between the nanoparticles. The presence of the second phase (magnetite) has been deduced by a heat treatments correlated to micro-Raman spectroscopy. In-field Mossbauer (fig.4 up) correlated to HRTEM (fig.4 down) investigations evidence the exchange-coupling of the magnetite and the cobalt ferrite. The jump like behavior has been found to disappear for x=1 (single phase).

Mohamed SAIDANI (Tunis, Tunisia), Wajdi BELKACEM, Jean François BARDEAU, Adrian BEZERGHIANU, Loic PATOUT, Jena Marc GRENECHE, Najeh MLIKI

08:00 - 18:15 #6950 - MS00-523 STEM investigation of titania supported gold nanoparticles stabilized by ceria.

MS00-523 STEM investigation of titania supported gold nanoparticles stabilized by ceria.

Gold nanoparticles show an excellent catalytic performance in energy production related processes such as low temperature CO oxidation, PROX and WGS reactions [1-2]. The effect of the support, like titania or ceria, and/or the presence of different kinds of promoters on the catalytic performance of gold have also been investigated, and a general agreement about the advantages of using reducible oxides is deduced from a revision of the most recent literature. Gold catalysts have also proven to be active in environmental reactions as those occurring in the catalytic converters (TWC) for depuration of exhaust gases in vehicles. The use of gold as a component of the current TWC formulations, substituting the more critical Pt and Pd, would only be feasible if the excellent catalytic properties of this metal could be maintained after exposure at high temperature operating conditions. However due to its relative low melting point gold suffers a severe sintering process starting at temperatures of around 473 K [3].

In order to investigate the stabilization of gold nanoparticles a reference system, 1.5 wt% Au/TiO2World Gold Council catalyst, was modified by depositing on its surface a monolayer of CeO2by incipient wetness impregnation to a final ceria molar loading of 5.4%.Scanning Transmission Electron Microscopy (STEM) studies show that the dispersion of the gold nanoparticles remained unchanged after the impregnation in a value about 36%. Afterwards, both catalysts were tested in consecutive CO oxidation reaction loops at increasing final temperatures. In these cycles, the ceria-modified catalyst showed not only a higher activity but, more importantly, a largely enhanced stability against deactivation.

Scanning Transmission Electron Microscopy has provided key information to rationalize the origin of the stabilization effect provided by ceria. In particular, STEM-HAADF images haveclearly revealed the presence of nanometer-sized ceria rafts, less than 1 nm thick, on the surface of the fresh CeO2(5.4%)/Au(1.5%)/TiO2catalysts. After the CO oxidation test at the highest temperature, 1223 K, the conventional WGC catalyst suffered from a very severe Au nanoparticle sintering, Figure 1(a),whereas Au nanoparticle growth was very much limited in the ceria-modified catalyst after the same aging test, Fire 1(b).

Cs-corrected STEM results reveal that a major fraction of the Au nanoparticles (75%), comprising all the smaller ones (< 5 nm), was contacting the ceria nanolayers. This evidences an important stabilizing effect of the proposed surface modification. Moreover, these results open up possibilities for gold catalysts in applications where high temperatures are reached under working conditions.

(1) G.J. Hutchings, Catalysis by gold, Catal. Today. 100 (2005) 55–61. doi:10.1016/j.cattod.2004.12.016.

(2) M. López-Haro, J.J. Delgado, J.M. Cies, E. Del Rio, S. Bernal, R. Burch, et al., Bridging the gap between CO adsorption studies on gold model surfaces and supported nanoparticles, Angew. Chemie - Int. Ed. 49 (2010) 1981–1985. doi:10.1002/anie.200903403

(3) H. Zhu, Z. Ma, S.H. Overbury, S. Dai, Rational design of gold catalysts with enhanced thermal stability: post modification of Au/TiO2 by amorphous SiO2 decoration, Catal. Letters. 116 (2007) 128–135. doi:10.1007/s10562-007-9144-3.

Ana HUNGRÍA, Miguel TINOCO, Eloy DEL RÍO, Ramón MANZORRO, Miguel A. CAUQUI, Jose Juan CALVINO, José A. PEREZ-OMIL (Puerto Real, Spain)

08:00 - 18:15 #6969 - MS00-525 AEM Characterization of the products yielding from Chlorination Process to obtain TiO2 from natural Ilmenite.

MS00-525 AEM Characterization of the products yielding from Chlorination Process to obtain TiO2 from natural Ilmenite.

In the titanium industry, chlorination of ilmenite (FeOTiO₂) at elevated temperatures in the presence of a reducing agent promotes formation of TiCl₄ in gaseous form, which can be subsequently selective condensed and then reduced to titanium metal or oxidized to producing TiO₂ [1]. Alternatively, in the absence of the reducing agent, the process becomes selective, favoring the formation of iron chlorides and promoting enrichment in content of TiO₂ obtained in the solid product of the reaction [2].  Therefore, the present study reports the observations by Analytical Electron microscopy (AEM) of principals characteristics of the TiO₂ polymorphs, anatase and rutile, obtained by chlorination with and without reducing agent respectively.  The experimental analysis involved the as-received material, the intermediate and final products, all sequentially characterized by means of X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and Transmission Electron Microscopy (TEM).

Fig. 1a., shows a low mag SEM image of the as-received material constituted essentially by Ilmenita particles rich on Ti and Fe.  Fig. 1b and 1c., are secondary electrons (SE) SEM images of the products after chlorination process for 60 minutes.  While Fig 1b., corresponds to direct chlorination product shows faceted TiO₂ crystals, Fig 1b., corresponds to the product resulting from chlorination in the presence of carbon showing spheroidal TiO₂ anatase aggregates.  The XR-Difractograms shown in Fig 2., corroborated the crystal identification on each mentioned products.  Fig 3.a and 3b., is TEM bright field (BF) -dark field (DF) pair of a robust single crystal of TiO₂ rutile and corresponding diffraction patron pointing in red the diffracted bean use for the DF image.  Fig 4a., and 4b, is also BF and DF pair however of an anatase aggregates and the correspond diffraction patron in the Fig 4c., showing, namely in the DF imagen shaped nanocrystals about 10 nm in sizes.  Interested to note in this aggregated nucleation rate of nanocrystals in a still shapeless aggregated.  The same anatase product has been captured in the BF-DF pair of Fig 5a and 5b and corresponding diffraction patron of Fig. 5c. These images correspond to a later state of nucleation and growth processes, configurating well-defined spherical nanocrystalline aggregates of TiO₂ anatase in order to minimized of surface energy.  After calcination of this product under N₂ atmosphere, it can seen the TiO₂ nanoparticles of about 10 nm in size have been more homogeneity distributed, mostly spheroidal and some exhibiting facets thereby implying in a thermodynamics states closer to the equilibrium.[2]

[1]. MOODLEY S, et.al. Chlorination of titania feedstocks, 3^rd International Symposium on High-Temperature Metallurgical Processing, 2012.

[2]. L.M. Cáceres, I.G. Solorzano, E. Brocchi.  Electron Microscopy observations over the processes yielding to obtaining TiO₂ from Natural Ilmenite Mineral.

 

The authors are grateful to LaBNano /CBPF for the access of TEM and XRD, the financial support of CAPES, CNPq and FAPERJ (Brazil).

Ludy CACERES MONTERO (Rio de Janeiro, Brazil), Guillermo SOLORZANO, Eduardo A. BROCCHI

08:00 - 18:15 #7041 - MS00-527 Surface effects in nano-cobalt ferrites.

MS00-527 Surface effects in nano-cobalt ferrites.

Magnetic nanoparticles have been the issue of continuous and growing interest, from both fundamental and technological points of view, in the last decades. Their unique physical objects with remarkable magnetic properties differ significantly from their parent massive materials. These properties are due to finite size effects of the magnetic core, related to the reduced number of spins cooperatively linked within the particle, and to surface and interface effects, related to the lack of coordination for the surface ions, inducing broken exchange bonds that can result in frustration and spin disorder. A description of ferrite magnetic properties requires a through characterization of the crystallographic structure down to atomic scale. Indeed, synthesis processes significantly alter the chemistry of the involved compounds, and then their physical properties.

In this work, experimental investigations on cobalt ferrite nanoparticles CoxFe3-xO4 (1 < x < 1.8) are reported, providing a comprehensive description of different surface and interface effects. Surface and exchange anisotropy in ferrite nanoparticles are investigated by means of several experimental techniques such as X ray diffraction, Transmission Electron Microscopy and magnetization measurements. A strong correlation between the structural and the magnetic properties has been revealed. It is shown that the proposed synthesis technique leads to the formation of a spinel nanoparticles well dispersed. Many of them are faceted having cub-octahedral polygonal growth forms with exclusively the {1 1 1} and {1 0 0} type faces. Rather than core-shell structures, a number of ordered defects were observed.

Mohamed SAIDANI, Wajdi BELKACEM, Loïc PATOUT, Ahmed CHARAI, Najeh MLIKI (Tunis, Tunisia)

08:00 - 18:15 #6145 - MS00-527b Quantitative compositional characterisation of fuel-cell catalysts using EDX ionisation cross sections.

MS00-527b Quantitative compositional characterisation of fuel-cell catalysts using EDX ionisation cross sections.

Hydrogen fuel-cells are a ‘zero emission’ technology because they only release H₂O, making them a very attractive source of clean electric power. Unfortunately, fuel-cells such as the polymer exchange membrane (PEM) fuel-cell are heavily reliant on a platinum catalyst at both the anode and cathode in order for the reaction to proceed at the low operating temperatures. It is largely the cost of this platinum metal which limits their more wide-scale manufacture and use.

In light of this, the search is on for more active catalysts with lower platinum content. A great deal of success has been seen in the research of bimetallic alloy catalysts where the expensive platinum metal is combined with a much less expensive metal such as nickel or cobalt. Such catalysts not only provide a reduction in the amount of platinum used but also an increase in the activity, which may be due to the compressive strain that the alloying element introduces.

In order to develop these catalysts further and make them a viable alternative to platinum, we need to understand what is happening at the nanometre and even sub-nanometre scales. For this we need to be able to characterise our catalysts at high resolution, understanding both the composition and structure. A new method for quantitative energy dispersive x-ray (EDX) [1] analysis in the scanning transmission electron microscope (STEM) has been developed with this in mind.

In the same way that the scattering cross section, σ, can be calculated from ADF image intensity and for ionisation edges in EELS, it is possible to calculate an EDX partial cross section using an approach that demonstrates similarities with the ζ-factor method. Rather than the ratio approach provided by the traditional k-factor method, this is a direct measurement which yields the number of atoms of each element after quantification, as such thickness can easily be extracted as well. This quantification method was applied to PtCo alloy nanoparticles that have been acid-leached to provide platinum enrichment (or rather cobalt depletion) at the particle surface [2]. It is possible to quantify the levels of cobalt depletion in the first few atomic layers of the particle, showing that the leaching produces a localised surface depletion that can only be determined by this high resolution EDX quantification.

[1]          K. E. MacArthur, T. J. A. Slater et al. Microsc. & Microanal.  22 (1) (2016) 71-81

[2]         K. E. MacArthur, T. J. A. Slater et al. Mater. Sci. Technol. In press

Katherine MACARTHUR (Juelich, Germany), Thomas SLATER, Sarah HAIGH, Dogan OZKAYA, Marc HEGGEN, Peter NELLIST, Sergio LOZANO-PEREZ

08:00 - 18:15 #6080 - MS01-529 Identification of occupation site of Al doped in Y2Ti2O7 based by ab initio calculation and statistical high-angular resolution electron channeling X-ray spectroscopy.

MS01-529 Identification of occupation site of Al doped in Y2Ti2O7 based by ab initio calculation and statistical high-angular resolution electron channeling X-ray spectroscopy.

     SiC_f/SiC_m is expected to be a turbine material of next generation aircraft engine, though the environment barrier coating (EBC) is indispensable to shield oxygen and reflect the radiant heat from the outer environment. It was reported that the periodic layers of Al₂O₃ and Y₂Ti₂O₇ (YT) that have different refraction indices worked very effectively as EBC.^*1 However, a problem was found that Al effusion from Al₂O₃ to Y₂Ti₂O₇ gave rise to collapse of the layer structure and the property of EBC was significantly deteriorated. In order to avoid this problem, it was also found that doping a small amount of Al into YT stabilized the layer structure and improved the oxygen permeation property. It is thus necessary to identify which atomic site and how much fraction Al occupies in Al doped Y₂Ti₂O₇ (AYT) to clarify the mechanism of this property improvement. In this study, we applied a statistical ALCHEMI technique, electron energy loss spectroscopy (EELS) and first principles theoretical calculation for the above purpose by comparing with the obtained macroscopic analysis.

     ALCHEMI is a technique for quantitatively identifying the occupancies of substitutional impurities in a crystalline material. The method takes advantage of the electron channeling effect, which is a phenomenon that electron probability densities change with the incident electron beam direction with respect to the crystalline orientation. However, there is a possibility that we may obtain a wrong result using this method because of delocalization effects of electron that provides systematic errors from difference in the electron orbital spread of different atoms. The statistical ALCHEMI technique acquires a large amount of data points as an incoherent channeling pattern (ICP), followed by the statistical analysis, so that the site occupancies are very precisely determined, insusceptible to the delocalization effects.^*2

     The samples used for the ICP measurement are three kinds of AYT (nominal Al concentration was approximately 1 cation%) annealed under different oxygen partial pressures (P_O2=10^-10, 10^-1, 10⁵ Pa) at 1300 °C for 50 hours after vacuum sintering at 1500 °C for 5 hours. We assumed that Al can only substitute for the Y and Ti sites in the crystals.

     Fig.1 shows the measured ICP of these samples. The statistical ALCHEMI analysis showed that Al occupied both the Y and Ti sites at a ratio of approximately 1:1, and at the lower oxygen partial pressure the Al occupancy was slightly biased to the Ti site. The concentration of Al was estimated to be 1.6 cat%. It is considered that the concentration of oxygen vacancies in AYT should increase under the lower oxygen partial pressures, where AYT tends to be positive charged, which can be compensated for by preferential substation of Al³⁺ for Ti⁴⁺ to maintain the electrical neutrality.

     We also measured the Al-K and L_2,3 ELNES and compared them with those obtained by ab initio calculation to confirm if the Al substitution sites in AYT were consistent with the statistical ALCHEMI results. For the theoretical ELNES simulation we created the AYT conventional cell (a, b, c = 10.45 Å, α, β, γ = 90°) by substituting an Al atom for a Y or Ti atom in the YT conventional cell and carried out the geometry optimization (Fig.2). The concentration of Al atoms is expected to be very small and the calculating cell must be large enough to ignore the interaction between two Al atoms in the neighboring cells. We used CASTEP code for the geometry optimization and the FEFF code based on a multiple scattering method for the ELNES calculation.

     Fig.3 shows the theoretical (upper) and experimental (lower) Al-K ELNES. ‘Al_Y’ and ‘Al_Ti‘ in the upper figure are calculated spectra which respectively substitute Al for the Y or Ti site. ‘Sum’ is the summed spectra of both with the Gaussian function convolved, to compare with the experimental spectrum with 1:1 occupation deduced by the ALCHEMI method above. The first peak of ‘Sum’ was aligned to match the peak position of the experimental spectrum. The ELNES result seems to be consistent with the ALCHEMI result.

     In summary, it is considered that Al atoms substitute the Y and Ti atoms at a rate of 1:1. The more detailed discussion is presented in the poster.

^*1 M. Tanaka et al, j. soc. mater. sci. jpn, 2015, Vol. 64, No. 6, 431-437

^*2 C. J. Rossouw et al, Phil, Mag, Lett, 1989, Vol. 60, No. 5, 225-232

Yoshihiro OBATA (Nagoya, Japan), Kenji ODA, Masahiro OHTSUKA, Shunsuke MUTO, Makoto TANAKA, Satoshi KITAOKA

08:00 - 18:15 #6084 - MS01-531 Interfaces and defects in directionally solidified oxide-oxide eutectics.

MS01-531 Interfaces and defects in directionally solidified oxide-oxide eutectics.

Oxide-oxide eutectic ceramic materials prepared by unidirectional solidification from the melt seem to be promising candidates for thermo-mechanical applications at high temperatures such as for gas turbine parts applications. They are prepared from Al₂O₃ - RE₂O₃ - ZrO₂ (RE = Y, Er, Sm) systems and associate 2 or 3 phases among: Al₂O₃ (corundum structure), RE₃Al₅O₁₂ (garnet structure), REAlO₃ (perovskite structure) and ZrO₂ (fluorite structure) phases. The microstructure consists of a three-dimensional interpenetrated network of single-crystal phases, free of grain boundary (fig. 1) and preferred orientation relationships occur between constituent phases¹. Consequently, they exhibit a noticeable thermal stability of the microstructure and good mechanical properties ^2,3.

Compressive tests were carried out at 1450°C and 1550°C under 100 MPa and 200 MPa. Low strain rates obtained confirm good creep resistance at high temperature of these materials. Deformation mechanisms were studied by TEM with conventional two-beam analysis. During creep tests, the specimens deform via micro-twinning in alumina and dislocation activity (climb in most cases) in all phases (fig. 2).

The interface nature of directionally solidified eutectic ceramics is the origin of their good properties in comparison with sintered equivalent ceramics. Thus, the different types of interfaces were studied at atomic scale by HRTEM (fig. 3). In a first approach, the chosen observation direction axis was parallel to the growth direction. Investigations were made on as-grown materials and after creep tests in order to:

     - understand interface structure and defects at the atomic scale in as-grown materials ;

     - analyse modification of interfacial structure after creep deformation ;

     - correlate microstructure and deformation mechanisms.

References:

1. Mazerolles, L. et al. New microstructures in ceramic materials from the melt for high temperature applications. Aerosp. Sci. Technol. 12, 499–505 (2008).

2. Waku, Y. et al. High-temperature strength and thermal stability of a unidirectionally solidified Al2O3/YAG eutectic composite. J. Mater. Sci. 33, 1217–1225 (1998).

3. Mazerolles, L., Perriere, L., Lartigue-Korinek, S., Piquet, N. & Parlier, M. Microstructures, crystallography of interfaces, and creep behavior of melt-growth composites. J. Eur. Ceram. Soc. 28, 2301–2308 (2008).

Laura LONDAITZBEHERE (Thiais), Sylvie LARTIGUE-KORINEK, Leo MAZEROLLES

08:00 - 18:15 #6108 - MS01-533 Evaluation of local atomic arrangements and lattice distortions in layered Ge-Sb-Te crystal structures.

MS01-533 Evaluation of local atomic arrangements and lattice distortions in layered Ge-Sb-Te crystal structures.

Ge-Sb-Te (GST) compounds are of high interest due to their outstanding optical and electronic properties. Thin films of GST alloys are widely used as phase change materials (PCMs) in optical and electronic data storage devices [1]. The operating principle of conventional PCMs is based on ultrafast, reversible transformation between the amorphous and metastable (cubic) crystalline phases.  Recently, a new type of phase change memory device, so called ‘interfacial phase change memory’ (iPCM) was proposed. iPCM consists of GeTe-Sb₂Te₃ superlattices. In the case of iPCMs, the phase transitions occur between two crystalline structures, thus allowing for a drastic reduction in energy consumption in memory devices. However, it has been experimentally demonstrated that the structure of iPCM corresponds to van der Waals bonded layers of Sb₂Te₃ and various layered GST crystal structures [2]. The switching mechanism of iPCM and the electronic properties of these materials are determined by the local atomic arrangement of Ge and Sb atoms. Consequently, knowledge on the proper local atomic arrangement in layered GST crystal structures is of paramount importance. The aim of this work is to study the local atomic arrangements and lattice distortions in GST thin films consisting of layered Ge₂Sb₂Te₅ (GST225), Ge₁Sb₂Te₄ and Ge₃Sb₂Te₆ crystal structures using a combination of atomic-resolution Cs-corrected HAADF-STEM imaging and theoretical image simulation. 

In this work, large crystallites of trigonal GST were prepared by ex-situ heating of amorphous GST225 thin films [3].  Fig. 1 shows the microstructures of GST thin films heated at different temperatures. The thin films consist of various building blocks with 7, 9 and 11 layers, indicating pronounced chemical disorder along the c-axis. The averaged composition of the thin films was verified to be 20 at.% of Ge, 24 at.% of Sb and 56 at.% of Te. Consequently, the disorder is attributed to deviations in local chemical composition of GST thin films which appears to be typical for layered GST compounds.

Fig. 2 gives a HAADF image of a single 9-layer GST225 building block. The block consists of alternating cation (GeSb) and anion (Te) layers. There are four different stacking sequences proposed in the literature, which differ in site occupancy of the distinct cation layers and in thermal displacement parameters (B). Due to the sensitivity of image intensities to B factors, the parameters can be used for the distinguishing between various stacking sequences. Fig. 2(b) and Figs. 2(c)-(f) show the comparisons between experimental and theoretical averaged intensity maxima for specific lattice sites in a GST225 lattice, respectively. The results reveal that the Ge and Sb atomic species tend to form intermixed cation layers with different ratios of Ge to Sb between distinct cation layers. Moreover, the Ge and Sb atoms in the studied structures are off-centre displaced from the centre of (GeSb)Te6 octahedrons. However, the distortions in Ge₁Sb₂Te₄ and Ge₃Sb₂Te₆ lattices are found to be larger than in the literature reported structures and strongly depended on the annealing temperature. Thus, the crystal structure of a single GST building block is conceptually similar to the local structure of the cubic GST [4].

In conclusion,the outcomes of this work shed new insights into the local structure of layered GST compounds, which may assist theoretical modelling of the switching mechanism of iPCM.

[1] S. Raoux, W. Wełnic, D. Ielmini, Chem. Rev. 110, 240 (2010).

[2] J. Momand, R. Wang et al., Nanoscale 7, 19136 (2015).

[3] U. Ross, A. Lotnyk et al., Appl. Phys. Lett. 104, 121904 (2014).

[4] A. Lotnyk, S. Bernütz et al., Acta Mater. 105, 1 (2016).

Andriy LOTNYK (Leipzig, Germany), Ulrich ROSS, Sabine BERNÜTZ, Erik THELANDER, Bernd RAUSCHENBACH

08:00 - 18:15 #6117 - MS01-535 In situ TEM electrical biasing studies on defect based crystal-amorphous transformation in GeTe nanowire devices.

MS01-535 In situ TEM electrical biasing studies on defect based crystal-amorphous transformation in GeTe nanowire devices.

Germanium telluride (GeTe), a phase-change material, switches rapidly and reversibly between crystalline and amorphous phase. The crystalline to amorphous transformation pathway is based on melting the crystalline phase and quenching it to amorphize in nanoseconds timescale. However, this is inefficient and energy consuming, and it is necessary to look for alternate pathways to carry out this transformation.

Using in situ TEM electrical biasing experiments on GeTe nanowires, we discovered a defect-assited low energy pathway for crystal-amorphous transformation. GeTe is both ferroelectric (FE) and inherently defective (Ge vacancies) metallic material, an unusual combination of properties in any material system. We show through dark-field TEM movies that interaction between extended defects of different origins (ferroelectric and metallic) can result in a pathway towards amorphization. In the “forming stage”, voltage pulses applied on metallic GeTe act as heat shocks creating anti-phase boundaries (APB) by coalescing and quenching Ge vacancies in a Ge plane. These APBs interact with the pre-existing 71^o ferroelectric domain boundaries converting them into 109^o boundaries. The partial dislocations surrounding the APBs migrate in the direction of the electrical wind-force, only to be impeded by the 109^o boundaries. Eventually, the mobility of the partial dislocations becomes zero, analogous to a scenario of traffic jam on a highway. Accumulating more extended defects at the jam, eventually collapses the crystallinity ‘nucleating’ an amorphous phase with medium range order, in a very local region that cuts across the cross-section of the nanowire. The amorphous mark can be recrystallized via another electrical pulse which heat it beyond the crystallization temperature. 

Pavan NUKALA (Massy-Palaiseau), Ritesh AGARWAL

08:00 - 18:15 #6152 - MS01-537 He-filled bi-dimensional defects in Ti3SiC2 MAX Phases.

MS01-537 He-filled bi-dimensional defects in Ti3SiC2 MAX Phases.

Ti₃SiC₂ is a member of the family of layered ternary compounds well known as the M_n+1AX_n phases (n = 1 – 3, or ‘MAX phases’) where M is a transition-metal, A is an A-group element and X is either carbon or nitrogen. These compounds have attracted increasing attention since they possess a unique combination of metallic and ceramic properties. They are elastically stiff, electrically and thermally conductive, relatively soft, damage tolerant and resistant to thermal shock. Therefore, Ti₃SiC₂ have been proposed to be used in the generation IV nuclear reactors and several works were devoted to studying radiation damage by high energy heavy ions. Ti₃SiC₂ is mainly synthesised by powder metallurgy. Nevertheless, it was recently shown that monocrystalline Ti₃SiC₂ thin films onto 4H-SiC can be formed by magnetron sputtering using Al and Ti co-deposition on SiC at room temperature following by an annealing [1]. Ti₃SiC₂ formation is based on interdiffusion processes between the substrate and the deposited layer. A layered hexagonal structure is obtained which consists of alternate near-close-packed layers of Ti₆C octahedral interleafed with layers of Si atoms. Thin-film of Ti₃SiC₂ could become suitable for many applications such as electrical contacts, wear protective coatings and to produce the new 2D materials labelled MXène.  It is then crucial to progress in the knowledge of the physical properties of Ti₃SiC₂ thin-film especially their behaviour under ion implantation. Light ion implantation (He, H) is commonly used to produce two dimensional defects in Si which are considered to be the precursors of cracks that are particularly interesting for the thin layer transfer technique well-known as the smart-cut process. The purpose of this work is to study the defects induced by light ion implantation (He, H) in Ti₃SiC₂ thin-film in order to create He/H-platelets. The microstructural analysis of the implant Ti₃SiC₂ thin films and their evolution under subsequent annealing will be performed by XRD and XTEM.  First XTEM results (Fig. 1) show a high density of He-platelets after low energy He implantation at medium fluence and subsequent annealing.  Works are in progress for studying in more detailed the MAX phase and the He-platelets. Results will be compared to a theoretical work who predicts that He atoms prefers staying near Si atoms [2].

The authors wish to acknowledge P. Guerin for his help during the film deposition and M. Marteau for He implantation experiments.

 

[1] A. Drevin-Bazin, J. F. Barbot, M. Alkazaz, T. Cabioch, and M. F. Beaufort, Appl. Phys. Lett. 101, 021606 (2012)

[2] L.X. Jia, Y.X. Wang, X.D. Ou, L.Q. Shi, W. Ding, Mater. Lett. 83, 23 (2012)

Julien NICOLAI (Buxerolles), Marie-France BEAUFORT, Jean-François BARBOT, Bénédicte WAROT-FONROSE

08:00 - 18:15 #6162 - MS01-539 Dislocation and microstructure analysis of tungsten.

MS01-539 Dislocation and microstructure analysis of tungsten.

The demand for neutron scattering and imaging techniques for material characterization under controlled conditions is continuously growing over the last couple of years. For such measurements large scale neutron facilities are needed and the European Spallation Source (ESS) will soon be the world’s most powerful neutron source, build in Lund, Sweden [1]. Here, neutrons are produced through a spallation process in tungsten, which due to its high atomic number has a high neutron production density and localized neutron production. The use of tungsten avoids issues related to corrosion effects related to water cooling and it is relatively environmentally friendly compared to other target materials. Its disadvantages, however, are its low ductility and a high ductile-to-brittle transition temperature. Tungsten is the most critical non-structural material and its integrity during operation is essential for maintaining helium circulation and retaining of transmutation elements, therefore it must operate reliably and predictably for the planned lifetime of the target. In order to estimate the target life, reliable data is needed on the mechanical properties of tungsten, both in unirradiated and irradiated conditions. Dedicated irradiation programs were established to examine the behavior of tungsten under representative operational conditions, which studies are performed in PSI.

In this current study, in order to evaluate the properties of irradiated tungsten, initially as a comparison we must observe the microstructure and mechanical properties of unirradiated tungsten. Evaluating the microstructure of the unirradiated tungsten material normally would not be complicated; such transmission electron microscopy (TEM) samples can be created relatively easily by cutting 3 mm discs and then thinning them by jet polishing. However, in this case we developed samples in a more complex way, with the use of focused ion beam (FIB) and then flash electrochemical polishing the lamella in order to establish a method where in the future the irradiated tungsten samples can also be examined safely. Thanks to FIB, with the reduction of the sample size the activity of the sample also gets reduced dramatically, ensuring safe handling. The sample is lifted out internally inside the FIB by a micromanipulator and a 8 x 8 μm² and 200 nm thick lamella is created. Then, by flash polishing (in 0.5% NaOH, 2 °C) this lamella is more thinned down to the thickness of approx. 60 nm, which is necessary to remove the FIB induced damages on the surface of the lamella. Electron microscopy observation is performed with a JEOL 2010 type TEM operated at 200 keV and equipped with EDX. Bright field (BF and weak-beam dark field (WBDF) imaging conditions were used at (g, 4g) or (g, 5g), while g=110. In order to obtain quantitative information of the dislocations, they are counted on several low magnification pictures on different areas of the sample.

Reference:

[1] K. Andersen et al., ESS Conceptual Design Report, ISBN 978-91-980173-0-4, edited by S. Peggs

Barbara HORVATH (Villigen, Switzerland), Yong DAI, Yongjoong LEE

08:00 - 18:15 #6192 - MS01-541 Characterization of the microstructure of composite Ti6Al4V-AA2519 obtained by explosive welding.

MS01-541 Characterization of the microstructure of composite Ti6Al4V-AA2519 obtained by explosive welding.

A composite which combines good thermal resistance and high mechanical properties of titanium alloys with good plasticity and low density of aluminium alloys seems to be a promising materials for airspace applications. However, welding of titanium and aluminium is difficult because they are extremely chemically reactive with oxygen and nitrogen at high temperatures. In this concept, the explosive welding (EXW) is one of most promising solid-state welding methods because it gives an opportunity to form the bonding on over the entire junction surface. In order to understand the phenomena taking place at joined surfaces, the detailed microstructural analysis is required, which was a prime goal of the present work.

Ti6Al4V and AA2519 plates 5 mm in thickness were joined via EXW with a detonation speed of 2000 m/s. The samples for scanning electron microscopy (SEM) observations were prepared by using HITACHI IM-4000 ion milling system, which is a damage-less polishing process and allows to see the structure on  SEM through the channelling contrast. SEM observation was carried out in a HITACHI SU-70 field emission Schottky scanning electron microscope at 10 kV accelerating voltage with the in-lens back-scatter electron (BSE) mode at 7mm working distance to get as much as possible surface contrast and resolution. Careful observations of selected areas were carried out on CS-corrected dedicated scanning transmission electron microscope (STEM) Hitachi HD-2700 operating at the accelerating voltage of 200 kV. The STEM thin foilswere prepared via in-situ lift-out in Hitachi NB 5000 focused ion beam. Precise EDX analysis were carried out on FEI Tecnai Osiris  200 kV microscope using Super-X EDX detectors.

Figure 1a shows an overview of a Ti-Al interface. EXW caused a high grain refinement in aluminium plate. The interface zone can be divided into two sections. The first one is  5 µm thickness zone consisted of nano-sized grains of three intermetallic phases: TiAl₃, TiAl₂ and TiAl which were identified by X-ray analysis and electron diffraction. The second zone consist of a small aluminium grains and a white (BSE) Cu rich net around them (Figure 1b). These intermetallic inclusions were often accompanied by nano-cracks (Figure 1b). Figure 2a  shows the magnified structure of this interfacial area in High-Angle Annular Dark-Field (HAADF)-STEM mode. The high resolution imaging (Figure 2b) shows that the copper net region have a crystalline structure. EDX mapping presented on Figure 3 shows the distribution of Ti, Al and Cu in the matrix and a gradient distribution of these alloying compounds in the structure. Copper net phenomenon was explained by the breaking or dissolving of big rich in copper equilibrium particles during explosive welding. Supersaturated solid solution of aluminium cannot contain all dissolved copper, so its excess was secondarily separated in the grain boundaries. 

Piotr BAZARNIK (Warsaw, Poland), Marco CANTONI, Lucian ŚNIEŻEK, Małgorzata LEWANDOWSKA

08:00 - 18:15 #6214 - MS01-543 Nanostructure of Mo2BC thin films and the effect on mechanical properties.

MS01-543 Nanostructure of Mo2BC thin films and the effect on mechanical properties.

The demand for materials in structural and functional applications requires nowadays more and more combined properties like high stiffness and simultaneously good ductility. The investigation and understanding of these unique properties is very important regarding a continuing optimizing of the materials and finally taking benefit from the outstanding properties in the future. Mo₂BC is a promising candidate in this field as it combines properties preventing crack formation and also ensuring long lifetime. Density functional theory calculations predicated a high stiffness and a moderate ductility for Mo₂BC by determining the electronic structure and the mechanical properties.[1, 2] A possible application of this material is the use as a hard coating for cutting tools protection.

In our work, we investigated Mo₂BC thin films on silicon substrates which were synthesized by high-power pulsed magnetron sputtering deposition technique.[3] Transmission electron microscopy (TEM) methods (bright-field, high-resolution TEM, selected area diffraction, electron energy loss spectroscopy) were used to compare the nanostructure of several Mo₂BC films which were deposited at different substrate temperatures ranging from 380 °C to 630 °C. TEM samples were prepared conventionally using grinding and Ar⁺ ion beam milling as well as by focused ion beam fabrication of thin lamellae extracted from the Mo₂BC films. TEM investigations were performed at 300 kV using a FEI Titan Themis 60-300. In Figure 1, TEM cross-sectional micrographs of the Mo₂BC film synthesized at the highest substrate temperature are shown exemplarily. The coating is fully crystalline and exhibits nanocrystalline columnar grains with a diameter of 10-20 nm.[4]

Furthermore, X-ray diffraction experiments and micromechanical tests were performed in order to correlate the mechanical properties of the films with their nanostructure. We detected an increasing crystallinity and hardness but a decreasing fracture toughness with increasing substrate temperature.

 

 

 

References

[1] J. Emmerlich et al., Journal of Physics D-Applied Physics 42 (2009), 1-6.

[2] H. Bolvardi et al., Journal of Physics-Condensed Matter 25 (2013), 1-6.

[3] H. Bolvardi et al., Thin Solid Films 542 (2013), 5-7.

[4] S. Djaziri et al., Surface & Coatings Technology 289 (2016), 213-218.

Stephan GLEICH (Düsseldorf, Germany), Soundes DJAZIRI, Hamid BOLVARDI, Jochen M. SCHNEIDER, Gerhard DEHM, Christina SCHEU

08:00 - 18:15 #6241 - MS01-545 Microstructural investigation of Fe-Mn-B-Nb glassy ribbons.

MS01-545 Microstructural investigation of Fe-Mn-B-Nb glassy ribbons.

The microstructure of Fe_79.7-xMn_xB₂₀Nb_0.3 glassy ribbons has been investigated using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX), for manganese concentrations of x=12, 16 and 20. For each manganese concentration the ribbon is found to consist of nanocrystalline inclusions in an amorphous matrix. Through detailed analysis of electron diffraction patterns and in conjunction with JEMS simulation, the crystalline phase is shown to be manganese diboride (MnB₂), and high resolution TEM investigation confirms this. Using dark-field TEM, the mean size of the nanocrystalline inclusions as well as their phase fraction are measured and found to increase with manganese concentration. EDX mapping shows the tendency for manganese to form nano-scale clusters in agreement with the dark-field investigation.

This research was supported by the European Commission FP7-REGPOT-2012-2013-1, Grant Agreement no. 316194 NANOSENS.

Lawrence WHITMORE (Iasi, Romania), Gabriel ABABEI, Luiza BUDEANU, Nicoleta LUPU, Horia CHIRIAC

08:00 - 18:15 #6243 - MS01-547 InAs/GaSb interface investigation by high resolution HAADF-STEM.

MS01-547 InAs/GaSb interface investigation by high resolution HAADF-STEM.

Heterojunctions with broken band alignment are of significant interest for tunnel field effect transistors (TFETs) because they decrease the tunneling distance for band-to-band tunneling and thereby increase the tunnel current [1]. InAs and GaSb crystallize in the familiar zincblende structure which consists of two interleaved face-centred cubic sublattices. The lattice constant of these two materials is about 0.61nm so that lattice-matched heterostructures can be grown [2]. At such a heterojunction between two semiconductors that do not share a common species, two distinct bond configurations are possible with either In–Sb or Ga–As bonds. Which kind of bond is formed depends on the details of the growth process and presents different characteristics in terms of defects and stress.

Broken gap InAs/GaSb heterojunctions are grown by molecular beam epitaxy. In order to characterize the nature of the interface two cross-section transmission electron microscopy (TEM) samples are prepared for observation along the [110] and [-110] zone axes respectively. The TEM specimens are prepared by FIB lift-out. HAADF-STEM is performed using a double corrected TEM (FEI Titan³ 60-300) at 300 kV.

In Figure 1, a schematic of the two possible bonds (In-Sb type in Figure 1 a,b and Ga-As type in Figure 1 c,d) interface configurations is shown along the [110] (a,c) and [-110] (b,d) zone axes. For a perfectly flat interface, adjacent intensity profiles across the interface along the [001] growth direction will show:

-          in Figure 1a vs b, and in 1c vs d, a reversal of the brightness of the dumbbells in the bulk of the InAs and GaSb for the 2 zone axes. 

-          that the brightness reversal is present at the interface for all traces in case of Figure 1b (low-high high-low) and 1c (high-low low-high) while, in Figure 1a and 1d, alternating traces show either reversed brightness or dumbbells with nearly equal brightness at the interface (high-high on Figure 1a or low-low on Figure 1d).

Based on these models a unique interpretation of the nature of the interface can be made if the interface is free of roughness over the thickness of the TEM specimen. In Figure 2, HAADF-STEM micrographs along the [110] (a) and [-110] (b) zone axes are shown. Three adjacent intensity profiles across the interfaces, corresponding to traces 1-3 in the micrographs, are shown in Figures 2c and d. In Figure 2c, traces 1 and 3 show unequal intensities - high-low dumbbells - whereas trace 2 reveals an “equal” intensity dumbbells at the interface consistently with the case in Figure 1a. The brightness of the “equal” dumbbells is not as high as expected for an In-Sb bond. This is probably caused by stress at the interface which also results in the dark band contrast as can be seen on Figure 2a,b. Figure 2d shows intensity profiles with unequal intensity dumbbells (low-high) which for all traces invert at the interface. This is consistent with Figure 1b. The observations along both [110] and [-110] zone axes indicate that the interface is In-Sb type, which is consistent with the RHEED patterns acquired during the MBE growth of the stack.

[1] CMOS and Beyond: Logic Switches for Terascale Integrated Circuits edited by Tsu-Jae King Liu, Kelin Kuhn, p. 125 (2014).

[2] H. Kroemer. The 6.1A family (InAs, GaSb, AlSb) and its heterostructures: a selective review. Physica E, 20:196–202, 2003.

Paola FAVIA (Leuven, Belgium), Olivier RICHARD, Salim EL KAZZI, Patricia VAN MARCKE, Hugo BENDER

08:00 - 18:15 #6337 - MS01-549 Studies of the structure of spray pyrolysed bioactive glasses using electron diffraction and DFT simulations.

MS01-549 Studies of the structure of spray pyrolysed bioactive glasses using electron diffraction and DFT simulations.

Bioactive glasses have received considerable attention during the past few decades. Recently, Shih et al. demonstrated that Si-Ca-P based glasses prepared by spray pyrolysis can have better bioactivity than glasses prepared by other methods [1]. To understand the reason behind these improved properties we have studied the structure of several Si-Ca-P glasses with different compositions.

In this study structural information from bioactive glass samples was obtained using electron diffraction. Compared to X-ray and neutron diffraction electrons can be easily focused on specific nano volumes and used to probe nanoscale variations in structure. Using reduced density function (RDF) analysis local structural parameters of the materials can be extracted from experimental diffraction patterns with high precision [2]. However, experimental diffraction data alone is not sufficient to build reliable atomic models. Therefore, we used DFT molecular dynamics simulations of liquid-quench to obtain atomic models of the materials, which serve as initial models for further structure refinements.

All experimental data presented here was collected using a JEOL JEM2100 transmission electron microscope (TEM) operating at 200kV. Electron diffraction patterns were recorded on a Gatan Orius CCD that eliminates charge overflow to the neighbouring pixels, even when saturated. Using a camera length of ~180mm and the central beam positioned at the edge of the detector, the usable range of scattering vectors, q extends to 18Å^-1, comparable with X-ray experiments.  Figure 1 (a-d) shows typical TEM images and diffraction patterns of two specimens, with the corresponding RDF curves shown in Figure 1 (e).

DFT simulations were carried out on ARC supercomputer facilities at the University of Oxford. The CASTEP software [3] was used to perform molecular dynamics simulations of liquid quench from 3000K to 300K, with cooling rate of 2·10¹⁴K/s. Energy optimizations with a 300eV pseudo potential cut-off were performed after quenching. Figure 2 (a) shows a model of 60%SiO₂-35%CaO-5%P₂O₅ glass simulated by Reverse Monte Carlo using only experimental diffraction data and the model after DFT simulation is presented in Figure 2 (b), indicating considerable structural changes. This model already demonstrates a good agreement with the experimental RDF curve (Figure 2 (c) and has been used for further RMC refinements.

We will discuss correlations between bioactivity and structural data in these materials.

Financial support from the European Union under the Seventh Framework Program under a contract for an Integrated Infrastructure Initiative (Ref 312483-ESTEEM2) is gratefully acknowledged.

[1] S.J. Shih, Y.J. Chou, I.C. Chien, One-step synthesis of bioactive glass by spray pyrolysis, J. Nanopart. Res., 14 (2012) 1-8.

[2] D.J.H. Cockayne, D.R. McKenzie, Electron diffraction analysis of polycrystalline and amorphous thin films, Acta Crystallogr. A, 44 (1988) 870-878.

[3] S.J. Clark, M.D. Segall, C.J. Pickard, P.J. Hasnip, M.I. Probert, K. Refson, M.C. Payne, First principles methods using CASTEP, Zeitschrift für Kristallographie-Crystalline Materials, 220 (2005) 567-570.

Yu-Jen CHOU (Oxford, United Kingdom), Konstantin BORISENKO, Shao-Ju SHIH, Angus KIRKLAND

08:00 - 18:15 #6359 - MS01-551 Atomic structure and segregation phenomena at copper grain boundaries.

MS01-551 Atomic structure and segregation phenomena at copper grain boundaries.

Abstract

The segregation of impurity atoms to grain boundaries can have significant influence on the cohesive properties, atomic arrangements and the mechanical properties of such interfaces. Therefore, it strongly impacts the macroscopic behavior of materials and understanding the atomic structure and related segregation behavior at grain boundaries is crucial to tailor materials with optimized physical properties ^[1]. Copper (Cu) is an attractive material for electronic applications, because of its good electrical and thermal conductivity. The effect of Sulphur on grain boundaries in Cu is of particular interest since it is improving the electromigration resistance but can also lead to grain boundary embrittlement ^[2].

In this study the atomic structure and chemistry of grain boundaries in polycrystalline Cu with variable Sulphur content (7 – 4000 ppm) are analyzed. In order to determine the distribution, size and orientation of the grains, Electron Backscatter Diffraction (EBSD) measurements were performed, as shown in Figure 1. The measurement reveals a grain size of 1-4 mm and an orientation of the grain normal close to the [001] direction. Transmission electron microscopy (TEM) specimens are prepared conventionally by grinding, electro polishing and Ar⁺ ion beam milling at specific regions from the EBSD scan in order to select special grain boundaries. This is shown exemplarily by the black circle in Figure 1.

TEM methods (bright-field imaging, selected area diffraction and high resolution TEM) are used to investigate the structure of selected grain boundaries. A preliminary example represented in Figure 2 reveals a high angle grain boundary. The specimen is tilted so that the upper grain is oriented in [001] zone axis while the resulting orientation of the lower grain is close to the [013] zone axis. The tilt component between both grains was determined to  by measuring the misorientation between the (200) reflexes of both grains which is in good agreement with the EBSD measurement. The twist component of this random grain boundary is not yet fully determined and is part of current investigations.

The atomic structure and segregation of Sulphur are characterized by aberration-corrected scanning transmission electron microscopy (STEM) in combination with analytical techniques including energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). Hence, information on the atomic arrangement of impurity atoms (interstitial and substitutional), the respective bonding and possible grain boundary precipitates will be obtained with sub-nanometer resolution.

References:
[1] M. Rajagopalan, M. A. Tschopp, and K.N. Solanki, JOM1, 1(2014)
[2] A.Wimmer, M. Smolka, W.Heinz, T. Detzel, W.Robl, C. Motz, V.Eyert, E. Wimmer, F. Jahnel, R. Treichler, G. Dehm; Mater. Sci. Eng. A, 618(2014), pp. 398-405

Thorsten MEINERS (DUSSELDORF, Germany), Christian H. LIEBSCHER, Gerhard DEHM

08:00 - 18:15 #6371 - MS01-553 Grain boundary structure and its interaction with dislocations in copper.

MS01-553 Grain boundary structure and its interaction with dislocations in copper.

Most of the nowadays used structural materials are of polycrystalline nature since single crystal fabrication is very cost intensive and sometimes not even desired. For instance, increasing the internal interface fraction – like grain boundaries – is known to strengthen metals significantly according to the Hall-Petch relation, thereby reducing the amount of load bearing material needed and hence the overall cost. Unfortunately, internal interfaces are often found to be weakest links within a failed material, contradicting the benefit from increased interface fractions. Consequently, understanding the dislocation interaction with distinct grain boundaries has the potential to tune a structural material’s performance. However, profound knowledge does not only include tracking of dislocation movement but it is crucial to understand grain boundaries from the atomistic structural and chemical level including aspects like faceting, grain boundary phase transformations and/or segregation of impurities. Thus, the present study aims on comprehensively understanding the interplay between atomistic grain boundary structure and its mechanical behavior (i.e. interaction with dislocations) using a ∑5 36.9° {310} tilt grain boundary in copper as a model system.

 

Macroscopic copper bicrystals were grown using the Bridgeman method. The seed crystals were arranged to form a symmetric ∑5 36.9° {310} tilt grain boundary. Grain boundary orientation and structure where studied by aberration-corrected high-resolution STEM as well as conventional TEM methods and compared to findings from SEM-EBSD measurements. For mechanical testing, tailored square-shaped nanocompression pillars were FIB machined on top of TEM lamellae to ensure a defined testing geometry and stress state. Defects introduced into the compression samples upon FIB machining were annihilated during an annealing step. Finally, in situ nanomechanical testing was performed using conventional TEM as well as STEM mode to study the dislocation behavior at the boundary. In addition, in situ microcompression experiments were performed inside the SEM to establish the mechanical size effect of the chosen boundary system.

 

It is shown by EBSD that the desired ∑5 grain boundary was successfully grown within the entire macroscopic bicrystal. However, structural investigations at the atomic level reveal a range of deviations from the ideal symmetrical case along the course of the boundary throughout the bicrystal. The observed grain boundary structures include symmetrical and asymmetrical segments as well as segments showing grain boundary facets (Figure 1). The overall 36.9° misorientation is found to remain within the deviation predicted by Brandon’s criterion in all of the structures. In situ SEM deformation along the [100]-compression direction reveals activation of multiple major slip systems in single crystals that were not found in the ∑5 bicrystal compression samples (Figure 2). Preliminary TEM compression experiments indicate a cross slip mechanism as the prevalent dislocation-boundary interaction rather than the formation of pile ups. This observation is also supported by the measured stress vs. strain curves as boundary containing pillars do not show increased hardening behavior relative to single crystalline reference samples.

Nicolas J PETER (Düsseldorf, Germany), Christian H LIEBSCHER, Christoph KIRCHLECHNER, Gerhard DEHM

08:00 - 18:15 #6377 - MS01-555 Determination of the platelet structure in natural diamond by ADF-STEM.

MS01-555 Determination of the platelet structure in natural diamond by ADF-STEM.

Electron microscopy is well known for its capability of determining the structure of many materials down to the atomic scale. A heavily studied material is natural diamond, but its rather small interatomic distance has made it a challenging material for TEM investigation. In this work we will demonstrate how novel TEM techniques can give fresh insights to this heavily studied material. Imperfections in natural diamond range from point lattice defects to dislocations and platelets. Platelet defects were first observed by means of TEM in the 1960s [1, 2] and nitrogen was detected making use of EELS in the 1980s [3]. Nevertheless, the exact crystalline structure of the platelets in natural diamond remains, perhaps surprisingly, still unclear.

In this work, we demonstrate our approach to determine the structure of platelet defects making use of annular dark field scanning transmission electron microscopy (ADF-STEM) combined with advanced image processing. Combining experimental data with image simulations we are able to offer a consistent crystallographic model of the defect. Furthermore, using monochromated energy loss spectroscopy technique we suggest the type of nitrogen embedding into the defect plane.

TEM samples were prepared by a focused ion beam lift out procedure from natural type Ia diamond. ADF-STEM and EELS experiments were carried out on FEI Titan microscope equipped with probe aberration corrector. We studied more than 50 platelets from different parts of the sample. In order to reduce scan distortions and damage to the defect plane a series of images with fast dwell time was obtained. The images were treated in the Smart Align software [4] to reduce the scan noise, correct the sample drift and a final summed image was obtained. Improved signal to noise ratio in this image allowed us to estimate the atomic positions and create a model of the platelet. Based on this model STEM image simulations were carried out. They are found to be in the good agreement with the experimental data. Using STEM-EELS we prove the presence of nitrogen exactly in the platelet plane. Comparing DFT calculations of different nitrogen centres in diamond and the experimental spectra extracted from the defect plane we suggest the model for the nitrogen coordination in the platelet. Using this detailed information on the platelet defect, we will discuss the possible mechanisms of platelet formation.

[1] Evans T., Phaal C. (1962) Proc. R. Soc., A270, 535-552.

[2] Lang A. R., (1964) Proc. phys. Soc. Lond. 84, 871-876.

[3] Berger S.D., Pennycook S.J. (August 1982), Nature Vol. 298, 635-637, 12.

[4] Pennycook T. J., Jones L., Pettersson H., Coelho J., Canavan M., Mendoza-Sanchez B., Nicolosi V., Nellist P.D., Scientific Reports, 4, 7555.

[5] S.K. and J.V. aknowledge the FWO-Vlaanderen for financial support under contract G.0044.13N ‘Charge ordering’.

Svetlana KORNEYCHUK (Antwerp, Belgium), Stuart TURNER, Artem ABAKUMOV, Johan VERBEECK

08:00 - 18:15 #6500 - MS01-557 Electron microscopic characterization of thermally modified surface layers generated by electro-discharge machining.

MS01-557 Electron microscopic characterization of thermally modified surface layers generated by electro-discharge machining.

Functional properties of construction components like wear resistance or fatigue strength heavily depend on their surface modifications like grain size, residual stresses, hardness, etc. Since these modifications are generated during machining, knowing about the resulting modifications beforehand is in high demand in industry. So far many time-consuming iterative procedures are necessary to find the machining parameters that provide the desired surface integrity. Brinksmeier et al. [1, 2] introduced a new energy based approach called “Process Signatures” that is supposed to solve this problem. Process Signatures describe the relationship between internal material loads – like strain, temperature and their gradients – and the resulting material modifications for instance residual stresses, hardness and microstructure. Hence production processes are characterized by their mechanical, thermal and chemical loads or a combination of those instead of their machining parameters. The present work focuses on electro discharge machining (EDM), which is mainly characterized by thermal effects due to the high temperatures and temperature gradients arising from the generated spark plasma [3, 4].

 

In the present studies, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), electron probe microanalysis (EPMA) and transmission electron microscopy (TEM) have been used to analyze the surface modification of 42CrMo4 steels processed by EDM. According to the very high surface temperatures (above melting temperature) and high temperature gradients applied by EDM, the surface zone contains martensite and even dendrites (figure 1). Investigations on the carbon content in the surface zone reveal about 8 times higher carbon contents than in the matrix due to carbonization (figure 2). Carbonization occurs because of decomposition of the dielectric fluid and carbon diffusion results due to the high applied temperatures. The increase in carbon content also stabilizes the fcc crystal structure, as the EBSD phase map shows (figure 3). These surface modifications reveal information about the mechanisms and applied internal loads, since the internal loads, in particular temperature and temperature gradients, influence diffusion of carbon, generation of dendrites and martensitic transformation. Together with temperature measurements during processing and simulation of temperature evolution in the material, Process Signatures for EDM can be developed.

 

Acknowledgement:

The authors gratefully acknowledge the financial support of the German Research Foundation (DFG) within the Collaborative Research Center SFB TRR136 „Process Signatures“ (Bremen, Aachen, Oklahoma), subprojects C02 and F02.

 

Literature:

[1] E. Brinksmeier et al., Process Signatures- an Alternative Approach to Predicting Functional Workpiece Properties, Procedia Engineering 19 (2011), p. 44–52

[2] E. Brinksmeier et al., Process Signatures - a new approach to solve the inverse surface integrity problem in machining processes, Procedia CIRP 13 (2014), p. 429–434

[3] A. Klink et al., Surface Integrity Evolution of Powder Metallurgical Tool Steel by Main Cut and Finishing Trim Cuts in Wire-EDM. Procedia Engineering 19 (2011), p. 178-183

[4] F. Klocke et al., Investigations on Surface Integrity of 42CrMo4 (AISI 4140) with Varied Heat Treatment Conditions Processed by Sinking EDM, Procedia CIRP 42 (2016), p. 580-585

Lisa EHLE (Aachen, Germany), Sebastian SCHNEIDER, Alexander SCHWEDT, Silvia RICHTER, Joachim MAYER

08:00 - 18:15 #6503 - MS01-559 A precession electron diffraction study of ordered-disordered phases in Ni-Cr based alloys.

MS01-559 A precession electron diffraction study of ordered-disordered phases in Ni-Cr based alloys.

The alloy 690 is a nickel-based alloy (60% Ni, 30% Cr, 10% Fe) used in nuclear Pressurized Water Reactors for different components (steam generator tubes, reactor vessel clevis etc.). These components are subjected to long thermal ageing for 40 to 60 years at 325 °C.
For these kind of alloys, thermal ageing can produce an order-disorder transformation due to the formation of the Ni₂Cr ordered phase by a nucleation growth mechanism. This transformation strongly modifies mechanical properties (strength increasing and ductility decreasing) as well as electrical properties when the orthorhombic ordered domains grow in the cubic disordered matrix.
In order to correlate the ordering state of these alloys to the evolution of their macroscopic properties, TEM studies are conducted. Ordered domains are generally very small (a few nanometers) so that imaging is not always feasible. Hence, ordering state is investigated using electron diffraction where we want to measure the intensity ratio between superlattice reflexions, related to the ordered domains, to the fundamental ones, related to both ordered domains and the disordered matrix (see figure 1). Indeed, in the kinematical approximation, the square of the order parameter s is proportional to Is/If, where Is and If are the intensities of the superlattice and fundamental reflections respectively. We want to appreciate the validity of this approach using Precession Electron Diffraction (PED) coupled to dynamical calculations of the intensities¹. For this kind of quantitative study, PED is advantageous since it strongly reduces sensitivity to experimental parameters such as sample thickness and exact orientation, which renders comparison with intensity calculations much more reliable. Reflections of the <011> cubic matrix zone axis pattern are targeted. In this orientation, superlattice reflections come from only one of the six possible orientation variants of the ordered phase related to the disordered one (see figure 2). This particular type of zone axis thus gives the richest information about the ordered phase.
In the present study, model alloys with iron content of 0 to 3 wt. % Fe are aged between 24 and 20 000 h in the range of 325 to 500 °C. The precipitation of Ni₂Cr ordered phase is detected by an increasing Vickers hardness and ThermoElectric Power (TEP) at the macroscopic scale.
We first need to validate our dynamical calculations on the fully ordered orthorhombic domains. In this attempt, specifically aged alloys are also studied, where ordered domains are well extended (a few tens of nanometers). Microdiffraction patterns are then collected on single domains and a systematic study of the intensity ratio Is/If as a function of the sample thickness is conducted in order to compare experimental and calculated data. Generalization of the procedure to the mixed alloys containing both ordered and disordered domains in various proportions will then be proposed.

 

Acknowledgements: The authors gratefully acknowledge L. Legras², J. Stodolna² and D. Loisnard² of for useful advice for thin foil preparation and observation; Y. Fontaine and his team² for all the machining work; C. Vincent² for the thermal treatments and P. Stadelmann³ for helpful discussions about JEMS software.

 

¹ Dynamical diffraction are simulated with JEMS software
² Matériaux et Mécanique des Composants, EDF Lab les Renardières, Ecuelles FRANCE
³ Centre Interdisciplinaire de Microscopie Electronique, Ecole Polytechnique Fédérale de Lausanne, Lausanne, SUISSE

Baptiste STEPHAN (Juvisy-sur-Orge), Damien JACOB, Frederic DELABROUILLE

08:00 - 18:15 #6540 - MS01-561 ANALYTICAL ATOMIC-RESOLUTION MICROSCOPY OF OXYGEN DEFICIENT Ca2Mn3O8-d.

MS01-561 ANALYTICAL ATOMIC-RESOLUTION MICROSCOPY OF OXYGEN DEFICIENT Ca2Mn3O8-d.

Manganese related perovskites (AMnO₃, A=alkaline earth) present a wide range of fascinating functional properties due to the ability of Mn to adopt several oxidation states and different coordination polyhedra. Regarding their catalytic behaviour, CaMnO_3-δ selectively oxidizes, at least on a laboratory scale, propene to benzene and 2-methyl propene ^[1]. Moreover, the Ca-Mn-O system presents a great variety of oxides with different Ca/Mn ratio and crystalline structure and a particular behaviour: their reduction process leads to a rock-salt type structure which, in most cases, can be again oxidized to the starting material ^[2].

Here we show the combination of oxygen engineering performed under adequate controlled atmosphere with local characterization techniques like atomic resolution electron microscopy associated to Energy Electron Loss Spectroscopy (EELS) to provide a complete characterization of other member of the Ca-Mn-O system. In particular, Ca₂Mn₃O₈ crystallizes in a monoclinic layered structure ^[3] with sheets of Mn^IV in octahedral coordination held together by Ca²⁺ cations alternately stacked along a axis (Fig. 1). The total reduction process of this material leads to Ca₂Mn₃^IIO₅ with rock-salt type structure.

By means of partial reduction, different samples have been stabilized in the Ca₂Mn₃O_8-d system. Among them, Ca₂Mn₃O_6.5, a new layered calcium manganese oxide with only Mn³⁺, has been stabilized. The different samples obtained in the Ca₂Mn₃O_8-δ system have been characterized by using High Angle Annular Dark Field (HAADF) and Annular Bright Field (ABF) imaging associated to EELS and Energy Dispersive Spectroscopic (EDS) in an aberration-corrected JEOL JEMARM200cF electron microscope. The structural evolution and the local variation of the Mn oxidation state in different phases with different anionic composition will be discussed.

 

[1] A. Reller et al, Proc. R. Soc. Lond. A, 394 (1984), 223-241.

[2] A. Varela et al, J. Am. Chem. Soc., 131 (2009), 8660-8668.

[3] G. B. Ansell et al, Acta Crystallogr. B, 38 (1982) 1795-1797.

Ángel MAZARÍO-FERNÁNDEZ (MADRID, Spain), Almudena TORRES-PARDO, Raquel CORTÉS-GIL, Aurea VARELA, Marina PARRAS, Maria HERNANDO, Jose María GONZÁLEZ-CALBET

08:00 - 18:15 #6553 - MS01-563 Towards imaging of defects in diamond by high-resolution TEM.

MS01-563 Towards imaging of defects in diamond by high-resolution TEM.

Nitrogen-Vacancy (NV), silicon-vacancy (SiV), germanium-vacancy (GeV) and other colour centres in diamond have been of rising interest in recent years due to their potential applications such as quantum information processing [1] and fluorescent labelling in biology. For such applications, it is necessary to incorporate the colour centres in diamond thin films or nanodiamond particles in a controllable way. Since the colour centres are formed by single atomic vacancy and single substitution, one fundamental step towards controlled incorporation is to characterize the structure of the colour centres with high spatial resolution. Although structural characterisation of NV-centres has been accomplished by optical and magnetic resonance methods, atomic structures of other colour centres often remain elusive.

Aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM) has proven to be a powerful technique for structural analysis with atomic resolution. However, the imaging conditions necessary to directly image the structure of an SiV, GeV or other colour centres remain unclear. Here we explore the optimum imaging conditions for the detection of SiV and GeV centres by using HRTEM image simulation. In order to maximize the image contrast from the SiV and GeV centres, we carried out a series of HRTEM simulations under various imaging conditions such as crystal orientation, acceleration voltage, spherical aberration coefficient (C_s) and defocus. The HRTEM image simulation was implemented using a multislice algorithm in the QSTEM software package [4].

Our results show that it should be possible to directly image both SiV and GeV centres in [120] projection at an acceleration voltage of 300 kV (see Figures 1 and 2, respectively). Under negative C_s imaging conditions (C_s = -15 µm and slight overfocus), both SiV and GeV centres can be clearly detected. Due to the higher atomic number of Germanium, GeV centres provide slightly higher contrast as compared to SiV centres.

[1] F. Jelezko and J. Wachtrup, phys. stat. sol. (a) 203, 13, 3207–3225 (2006)

[2] J. Tisler et al., ACS Nano, 3(7), 1959-1965 (2009)

[3] I. Vlasov et al., Nature Nanotechnology 9, 54–58 (2014)

[4] C. Koch, Determination of core structure periodicity and point defect density along dislocations, PhD. thesis, 2002.

Robert LEITER (Ulm, Germany), Haoyuan QI, Johannes BISKUPEK, Boris NAYDENOV, Fedor JELEZKO, Ute KAISER

08:00 - 18:15 #6613 - MS01-565 Order/disorder mechanisms in complex (CaFe2O4)(FeO)n ferrites (n=1,3).

MS01-565 Order/disorder mechanisms in complex (CaFe2O4)(FeO)n ferrites (n=1,3).

Numerous studies are always devoted to mixed valence state iron oxides in materials research due to their complex magneto transport properties like the famous Verwey transition [1].  Among these oxides, a special attention is focused on the orthoferrites LnFeO₃ (Ln= rare earth) related to the distorted GdFeO₃-type perovskite structure which can exhibit some possible spin reorientation transitions versus temperature and the nature of Ln [2]. Recently, iron based oxides like LnFe₂O₄ have also focused a large attention due to their ability to exhibit some multiferroic properties [3]. In these systems, both kinds of Fe species (Fe²⁺ and Fe³⁺) localize magnetic moments leading to a ferrimagnetic ordering associated to ferroelectric properties. An exciting challenge is to evidence similar properties in other iron based systems. The Ca-Fe-O system offers several interesting candidates like the CaFe₅O₇ and CaFe₃O₅ phases in regard to the richness of its phase diagram.

CaFe₅O₇ oxide exhibits a complex structure which can be described as an intergrowth between one CaFe₂O₄ unit and n=3 slices of FeO Wustite-type structures [4]. A recent structural study performed by transmission electron microscopy (TEM) observations has revealed a supercell with a lower monoclinic symmetry [5]. From the intensity extraction and hkl conditions deduced from the precession electron diffraction (PED) study, a structural model considering to this supercell and the centrosymmetric P2₁/m setting can be proposed. The fine structural analysis combining Rietveld refinements from neutron and X-ray data evidence six independent iron sites and two specific oxygen environments with coordination 6 and 5+1 respectively. According to the chemical formula CaFe₅O₇, the iron species average state valence is +2.4 and implies the coexistence of Fe⁺³ and Fe²⁺. The magnetic dependence versus temperature has been studied and susceptibility measurements have revealed discontinuity around 360K [6]. The structural evolution of CaFe₅O₇ depending on temperature has been also tuned from diffraction techniques. A clear reversible transition (monoclinic to orthorhombic) has been detected in the same temperature range with the disappearing of the supercell [6]. A complementary STEM-HAADF study has allowed to highlight the impact of this superstructure at atomic scale (Fig.1) : ordered contrasts at the level of calcium rows can be observed. A second ferrite, CaFe₃O₅ related to the n=1 member of the generic (CaFe₂O₄)(FeO)n series, has also been analysed by TEM techniques. Thus a superstructure is revealed but the first STEM-HAADF highlight a complex nanostructural feature related to the coexistence of two polymorphs (Fig.2)

 

[1] E J W Verwey, Nature 144, 327 (1939)

[2] R. Bozorth & al  Phys. Rev. Lett., 1, 3, (1958)

[3] M. Hervieu & al, Nature Materiels, 13 (2014)

[4] O. Evrard & al,  JSSC 35, 112 (1980)

[5] C. Delacotte & al Key Engineering Materials (2014)

[6] C. Delacotte & al Inorg. Chem. (2014)

Charléne DELACOTTE, Laurine MONNIER, Yohann BRÉARD, Sylvie HÉBERT, Denis PELLOQUIN (Caen)

08:00 - 18:15 #6620 - MS01-567 Dislocation mobility in GaN nanowire arrays by in-situ heating in the TEM.

MS01-567 Dislocation mobility in GaN nanowire arrays by in-situ heating in the TEM.

Wide bandgap semiconductors are a current area of interest for a new generation of high-temperature, high-voltage and high-power semiconductor-based electronics. Gallium nitride (GaN) is a promising candidate and is already extensively used in blue and UV LEDs[1,2]. Thin films are grown on substrates such as silicon and sapphireas to produce transistors with high electron mobility. However, these films suffer from large dislocation densities (10⁸-10¹⁰cm^-2)[3,4] due to the intrinsic strain of the substrate, resulting in devices with reduced reliability because of increased current leakage, disrupted electric field distribution and a premature breakdown of microplasma[5].

Therefore, novel methods of GaN device manufacturing using nanostructured systems such as nanowires and nanorods are a promising solution to minimise the presence of structural defects. Understanding how growth conditions affect the nanowire's structure is important for future device fabrication, in particular the effect of annealing conditions on the dislocation density.

Here, we investigate unannealed GaN nanowiresgrown by metalorganic vapour phase epitaxy on Si substrates[6]. The nanowires are doped with varying levels of Si, subjected either an N₂ or N₂+NH₃ atmosphere upon growth.

The annealing process is seen to reduce dislocation density[7]. Ex-situ annealing during growth shows a slight reduction in dislocation density. Figure 1 shows a comparison of dislocation density between an unannealed sample and a sample annealed at 900^oC. Dislocation densities are measured to be 1.6x10⁹ cm^-2 and 1x10⁹ cm^-2 for unannealed and annealed nanowires respectively. The drop in dislocation density is, however, lower than expected for annealing at this temperature. In-situ heating experiments allow the direct observation of dislocation mobility during annealing, giving a valuable insight into dislocation bending.

[1] Y Huang et al, Science 294 (2001) 1313-1317.

[2] F. Qian et al, Nano Lett. 4 (2004) 1975.

[3] S Nakamura, J. Appl. Phys. 30 (1991) 1705-1707.

[4] H Amano, N Sawaki and I Akasaki, Appl. Phys. Lett. 48 (1986) 353.

[5] E Cicek et al, Appl. Phys. Lett. 96 (2010).

[6] B Alloing et al, Appl. Phys. Lett. 98 (2011).

[7] R Collazo et al, J. Cryst. Growth, 287(2), 586–590 (2006).

 

Mathew MCLAREN (Belfast, United Kingdom), Vitaly ZUBIALEVICH, Peter PARBROOK, John SHEN, Miryam ARREDONDO

08:00 - 18:15 #6621 - MS01-569 Contribution of transmission electron microscopy to the elaboration of transparent (glass)ceramics.

MS01-569 Contribution of transmission electron microscopy to the elaboration of transparent (glass)ceramics.

Transparent polycrystalline (glass)ceramics present significant economical and functional advantages over single crystal materials for optical, communication, and laser technologies. To date, transparency in (glass)ceramics is ensured either by an optical isotropy (i.e. cubic symmetry) or a nanometric crystallite size. Polycrystalline ceramics offer several advantages, particularly in the fabrication of complex shapes and large-scale industrial production, and enable greater and more homogenous doping of optically active ions than is possible in single crystals. Our recent work shows the possibility to obtain transparent polycrystalline ceramics by full and congruent crystallization from glass. Transparency in these materials is observed despite micrometer scale crystals and a non cubic symmetry (no structural isotropy) of the crystalline phase. Transmission electron microscopy used in TEM – STEM – High Resolution modes and coupled with EDX map or line scan, enables to complete the nanostructure characterization which explains the transparence properties and to highlight the crystallization processes occurring during the heat treatments. Three cases where transmission electron microscopy has played an important role to develop transparent (glass-)ceramics will be presented.

Highly transparent ZnGa₂O₄ glass-ceramic materials are elaborated via a simple heat treatment of a 55SiO₂–5Na₂O–17ZnO–23Ga₂O₃ parent glass composition, which presents nanoscale spinodal phase separation. This optimized glass-ceramic exhibits 50 wt% of ZnGa₂O₄ nanocrystals showing a homogeneous and tuneable size. To describe the crystallization process, the glass and glass-ceramic nanostructures are studied by high resolution scanning transmission electron microscopy analysis coupled with in situ high temperature X-ray diffraction and optical measurements. From these results, an original mechanism is proposed to explain the crystallization process occurring in a spinodal phase separated glass. Remarkably, red long-lasting luminescence arising from the entire sample volume is observed in the Cr³⁺ doped transparent glass-ceramics, opening the route to a wider range of performing applications for this famous zinc gallate persistent phosphor.^[1]

A series of biphasic (100 − z)BaAl₄O₇−zBaAl₂O₄ (0 < z ≤ 45) transparent polycrystalline ceramics have been synthesized by full crystallization from glass process. Despite being composed of two birefringent crystalline phases, these new materials exhibit improved transparency compared to the pure BaAl₄O₇ ceramic recently reported to show remarkable scintillation properties. Multiscale structural characterizations demonstrate that this transparency enhancement can be explained by the presence of nanometer scale BaAl₂O₄ crystals which crystallize coherently with the BaAl₄O₇ matrix. We show that the BaAl₂O₄ nanostructuration limits the BaAl₄O₇ growth via an original Zener pinning effect, such decreasing light scattering due to the material birefringence. Interestingly, the BaAl₄O₇ scintillation properties can be retained in these two-phase transparent ceramics.^[2]

The full and congruent crystallization of Sr_1+x/2Al_2+xSi_2-xO₈ (0 < x ≤ 0.4) leads to new transparent polycrystalline ceramics with hexagonal symmetry. We use a controlled degree of chemical disorder in the structure to obtain optical isotropy at the micrometer length scale. The structure and the chemical disorder were characterized by X ray Diffraction, solid state NMR and STEM-HAADF imaging. These materials reach the theoretical limit in transmittance that is to say 90%.^[3]

1. S. Chenu, E. Veron, C. Genevois, A. Garcia, G. Matzen, M. Allix, Long-lasting luminescent ZnGa₂O₄:Cr³⁺ transparent glass-ceramics, J. Mater. Chem.C, 2 10002-10010 (2014).

2. M. Boyer, S. Alahrache, C. Genevois, M. Licheron, F-X Lefevre, C. Castro, G. Bonnefont, G. Patton, F. Moretti, C. Dujardin, G. Matzen, M. Allix, Enhanced transparency through second phase crystallization in BaAl₄O₇ scintillating ceramics, Crystal Growth and Design, Vols 16, Issue 1 (2016) 386-395.

3. K. Al Saghir, S. Chenu, E. Veron, F. Fayon, M. Duchomel, C. Genevois, F. Porcher, G. Matzen, D. Massiot, M. Allix, Transparency through structural disorder: a new concept for innovative transparent ceramics, Chemistry of Materials, Vols 27, Issue 2 (2015) 508-514.

Cécile GENEVOIS (Orleans Cedex 2), Marina BOYER, Kholoud AL-SAGHIR, Sébastien CHENU, Emmanuel VÉRON, Franck FAYON, Guy MATZEN, Mathieu ALLIX

08:00 - 18:15 #6626 - MS01-571 In-Situ SEM Study of Mechanical Properties of Aluminide Bond Coating at Elevated Temperature.

MS01-571 In-Situ SEM Study of Mechanical Properties of Aluminide Bond Coating at Elevated Temperature.

Diffusion aluminide bond coats are compositionally and microstructurally graded material with significant variation in engineered mechanical properties across the cross-section. Bond coating exhibits three-layered microstructure: (a) outer layer contains intermetallic PtAl2 and Cr-rich fine precipitates, (b) intermediate layer contains B2-(Ni,Pt)Al and (c) inner layer is interdiffusion zone containing coarse precipitates in B2-NiAl matrix. This study focuses on understanding deformation mechanisms at elevated temperature and the variation in mechanical properties as a function of temperature. In-situ nanomechanical instrument, PI 87xR SEM PicoIndenter (Hysitron, Inc., Minneapolis, USA) with an integrated high-temperature stage and an active tip heating was used to conduct uniaxial compression of pillar samples. In-situ mechanical testing allows precise alignment of the tip with the sample as well as direct, real-time observation of the deformation processes. A side benefit of performing these tests in the SEM is that the high vacuum environment limits the oxidation of the sample, especially at high temperatures, enabling the measurement of the true mechanical properties of the bond coating and superalloy. Water circulation through cooling blocks at the sample heater and transducer minimizes thermal drift of the system.

Micropillars of dimensions 8 μm x 8 μm in cross-section and 25 μm in height were prepared from the outer layer of PtNiAl coating and Ni-base superalloy region by focused ion beam (FIB). In-situ quasistatic uniaxial compression experiments were conducted with a 20 μm flat punch diamond probe. Using the displacement-controlled feedback mode of the system, the pillars were compressed to 5-12% strain at a strain rate of 10-3 s-1. Compression tests were conducted at room temperature (RT) as well as several elevated temperatures up to 800°C. Heating was achieved through closed-loop resistive heating of both the probe and sample. Tilt and rotation in sample motion allows flexibility in pillar alignment with respect to flat punch probe.

The microstructure of the pillar surface after uniaxial compression of bond coating and superalloy are shown in fig. 1. Fig. 2 displays stress-strain curves which are calculated from load-displacement data. The stress-strain curves of bond coating indicate that plasticity is characterized by major strain hardening after yielding at RT and limited strain hardening at higher temperature. The surface of the bond coating pillars shows grain boundary sliding at higher temperature. Elastic moduli of the bond coating remain nearly constant up to 800oC whereas yield stress of the coating decreases to ~50%. Transgranular fracture appears on the pillar surface at room temperature whereas intergranular fracture dominates deformation at higher temperature.

Stress-strain curves plotted in figure 2b show larger load drops in superalloy samples. Figure 1c and d indicate that load drops are associated with the formation of slip bands. A large number slip bands with bigger step size can be observed on the pillar surface at 600oC. At 600°C the yield strength of superalloy sample decreased by ~20% compared to room temperature. In summary, uniform plastic deformation without slip bands was observed in Pt-rich layer of bond coating. Grain boundary siding predominantly controlled plastic deformation. In contrast, multiple slip bands were observed in Ni-base superalloy. The density and severity of the slip banding increased with temperature.

Sanjit BHOWMICK, Douglas STAUFFER (Minneapolis, USA), S.a. Syed ASIF

08:00 - 18:15 #6632 - MS01-573 Simulation of STEM-HAADF image contrast of Ruddlesden Popper faulted LaNiO3 thin films.

MS01-573 Simulation of STEM-HAADF image contrast of Ruddlesden Popper faulted LaNiO3 thin films.

Perovskite nickelates are interesting candidates to be used as electrode materials or in the research towards artificial superconductors. Among them, LaNiO₃ (LNO) is of high interest because of its exceptional transport properties, with a resistivity lower than 100 µΩ·cm, and the potential tuning of electrical and magnetic interactions through appropriated strain engineering. A precise control of the stoichiometry, structural phases, lattice distortions and the presence/absence of dislocations/defects is critical to make LNO electrodes and related heterostructures suitable for electronic applications^1–3.

Frequent defects present in perovskite structures are Ruddlesden Popper (RP) faults which consist on the relative displacement of two perfect defect free perovskite ABO₃ blocks a distance of half- unit cell along the [111] direction. The earlier observations of RP faults were based on diffraction results, but they have also been imaged using advanced microscopy tools^4,5. When observed along the [001] zone axis, this defect appears as a zig-zag arrangement of the A cations with a BO₂ plane lost at the defect boundary.

In the present work we focus our attention on the characterization of RP faults observed in LNO thin films 14 nm and 35 nm thick, grown by pulsed laser deposition on (001) oriented LaAlO₃ (LAO) single crystal substrates at an oxygen pressure P = 0.15 mbar and at a temperature T = 700°C. The preliminary characterization of the layers by high resolution transmission electron microscopy (HREM) and electron diffraction, confirmed the good pseudo-cube-on-cube epitaxial growth with atomic-sharp interfaces and the expected [010]LNO(001)//[010]LAO(001) epitaxial relationship, despite the compressive strain driven by the 1% mismatch between LNO (3.838 Å) and LAO (3.795 Å). Nevertheless, defects identified as potential RP faults oriented in both [100] and [010] directions were also observed.

Detailed high angle annular dark field (HAADF) imaging of these defects enabled a better identification of the defects as Ruddlesden Popper type through the appropriated correlation of the contrast of the atomic columns with cationic La(A) and Ni(B) positions as shown in the figure 1.

We will systematically address the interpretation of the contrast of these HAADF images through STEM-HAADF simulations calculated through the multislice procedure⁶ from atomistic models based on different arrangements of defect free perovskite blocks with octaetrahedral Ni³⁺ coordination (Fig. 2).  Gradual variation of Z-contrast is interpreted in terms of the average of the atomic numbers of the La and Ni in A and B sites in RP displaced overlapping perovskite crystals⁷. The good agreement between the experimental images and the simulated ones (Fig. 3) validates the proposed geometrical configurations of the RP faulted crystals.

References:

1.           Scherwitzl, R., Zubko, P., Lichtensteiger, C. & Triscone, J. M. Electric-field tuning of the metal-insulator transition in ultrathin films of LaNiO₃. Appl. Phys. Lett. 95, 10–13 (2009).

2.           Zhu, M. et al. Effect of composition and strain on the electrical properties of LaNiO₃ thin films. Appl. Phys. Lett. 103, 141902 (2013).

3.           Detemple, E. et al. Polarity-driven nickel oxide precipitation in LaNiO₃-LaAlO₃ superlattices. Appl. Phys. Lett. 99, (2011).

4.           Beznosikov, B. V & Aleksandrov, K. S. Perovskite-Like Crystals of the Ruddlesden – Popper Series 1. Crystallogr. Reports 45, 864–870 (2000).

5.           Ruddlesden, S. N. & Popper, P. New compounds of the K₂NiF₄ type. Acta Crystallographica 10, 538–539 (1957).

6.           Kirkland, E. J. Computation of Transmission Electron Micrographs. Plenum New York 129, (1998).

7.           Detemple, E. et al. Ruddlesden-Popper faults in LaNiO₃/LaAlO₃ superlattices. J. Appl. Phys. 112, 1–6 (2012).

 

Catalina COLL (Barcelona, Spain), Lluís LÓPEZ-CONESA, Cesar MAGÉN, Florencio SANCHEZ, Josep FONTCUBERTA, Sònia ESTRADÉ, Francesca PEIRÓ

08:00 - 18:15 #6650 - MS01-575 Size and orientation effects on the mechanical response of a Ni-Ti-Al-Nb-Hf alloy.

MS01-575 Size and orientation effects on the mechanical response of a Ni-Ti-Al-Nb-Hf alloy.

NiTi-Al based alloy is considered as a potential high temperature structural material [1]. The addition of Nb is able to improve the oxidation resistance of NiTi-Al based alloys at ~ 800°C [2] and also their strengths at room and elevated temperatures [3]. A Ni-43Ti-4Al-2Nb-2Hf alloy has been demonstrated to be a promising lightweight high-temperature structural material [4]. It is thus of significance to understand the effect of crystal orientations on its mechanical behaviours. In the past, mechanical testing of single crystal metals was performed on large crystal. However, the preparation of single crystals of this class of alloys is very difficult. Meanwhile, understanding material behaviour with decreasing sample size is necessary for exploring the potential application in nano-technologies.  In this study, electron backscattered diffraction (EBSD) was used to identify the grain orientations (e.g. close to [001], [011] and [111] directions) of a directional solidified Ni-43Ti-4Al-2Nb-2Hf alloy. Pillars with diameters ranged of 0.5 mm ~ 4 mm were machined using a focussed ion beam (FIB) microscope. Compression tests were performed using a Hysitron PI85 picoindenter. Specimens for transmission electron microscopy (TEM) were prepared along longitudinal direction of the pillars by FIB, and then examined in an FEI Tecnai F20 microscope.

The results show that its strength depends on the crystal orientations and the size of pillars (Fig. 1). The pillars were plastically deformed via dislocation gliding (Fig. 2) and twinning.

References:

1. Koizumi Y, Ro Y, Nakazawa S, Harada H. Mater Sci Eng A 1997; 223:36–41.
2. Zhao XQ, Xu J, Tang L, Gong SK. Intermetallics 2007; 15; 1105-15.
3. Meng LJ, Li Y, Zhao XQ, Xu HB. Intermetallic 2007; 15: 814-8.
4. Pan LW, Zheng LJ, Han WJ, Zhou L, Hu ZL, Zhang H. Mater. & Design 2012, 39: 192-9.

Rengen DING (Birmingham, United Kingdom), Lijing ZHENG, Yulung CHIU

08:00 - 18:15 #6685 - MS01-577 Effect of Alloying Content on the Defect Structure Formation and Evolution in the Ta-W system.

MS01-577 Effect of Alloying Content on the Defect Structure Formation and Evolution in the Ta-W system.

I INTRODUCTION
Nowadays tantalum coating of tungsten targets used in spallation sources (e.g. ISIS, LANCE) is considered as a promising route to improve the target integrity, neutron production, operational reliability, and hopefully reduce the need of handling radioactive materials at the end of the target lifetime [1]. Tantalum offers attractive corrosion and mechanical resistance properties, and its neutronic performance is relatively similar to that of tungsten. However, the difference in the thermal expansion coefficient of tantalum and tungsten may lead to significant geometry variations between the coating and the substrate that could cause the degradation and failure of the Ta/W component at elevated temperatures. This suggests that doping Ta with controlled amounts of W (up to 10wt.%) would improve the thermal stability of the compound target.
Supplementary to this another question which is apparently open now is that fatigue failure of tantalum cladding will be the limiting factor of target lifetime. Tensile pre-stress and radiation embrittlement can make the fatigue situation worse and irradiation creep and stress relaxation may reduce the average stress. And in this case the addition of tungsten to tantalum can potentially increase the yield strength and the rate of work hardening of the final material.

II EXPERIMENTAL

A Non-radioactive materials

In this work pure tantalum and tungsten samples, together with selected binary alloy compositions (Ta2.5wt.%W; Ta5wt.%W and Ta10wt.%W) have been analyzed before study of the proton irradiation. Four alloys were annealed at temperatures close to the onset of recrystallization. Changes in a grain size and grains orientation have been observed together with the presence and nature of nano-scale defects such as dislocation lines, nets and tangles Tungsten has been reported to influence the dislocation density and dynamics in non-irradiated Ta-W alloys during mechanical testing [3]. However, W contents about 10wt.% would induce the formation of a secondary brittle phase in the structure, according to the Ta-W phase diagram.
Using analytical electron microscopy techniques different structure of dislocations has been detected with increasing concentration of tungsten in tantalum in non-irradiated samples. In alloys with highest tungsten concentration of 10% dislocations form nets of intersected lines with increasing its number density (Fig.1) which directly affects modifications in hardening behaviour of these kinds of materials. Detailed examination of thin foils indicates that dislocations presented in this alloy are screw dislocations with ½ a [111] type Burgers vector.

B Proton irradiation of Tantalum-Tungsten alloys
Two proton irradiation experiments have been implemented using a 5MV Tandem Pelletron ion accelerator (NEC model 15SDH-4) and high current TORVIS source in order to go up to 3 MeV 1H+ at 350C and up to 1.6 dpa level in Dalton Cumbrian Facility (University of Manchester) for the aim of investigating tungsten doping impact on the irradiation-induced defects in microstructure and changes in mechanical behaviour such as radiation-induced embrittlement in Ta-W alloys.
Pure Tantalum and two Tantalum-Tungsten alloys with different tungsten concentration have been loaded together on a specially designed target stage, pumped with high level of vacuum (7.44 *10-7Torr) and equipped with ceramic heater and cooling system allowing not overheating the stage.

III STUDY OF IRRADIATED SAMPLES
A Bragg peak position. Hardness test
Hardness of bulk UHP Ta, Ta5wt.%W and Ta10wt.%W irradiated with single proton beam of 3 MeV during 36 hours at 350oC, as described above is assessed by nanoindentation.
Nanoindentation tests (Fig.2) have been carried out from the cross-section area of each sample exposed under irradiation in order to observe changes in hardness depending on different dpa level and defect density. This experiment also has been performed in order to compare the data with SRIM calculations of Bragg peak position (Fig.3). As it is showed on the graph above Bragg peak position for all alloys is varied between 25-40 μm which corresponds to the SRIM calculations. 

B Scanning electron imaging
The determination of the Bragg peak can be straight forward in Scanning electron microscope as different contrast may arise with back scattered electrons detector (BSE) since the irradiation damage mutes more the crystal structure. In the Fig. 4 presented below, the BSE detector revealed (at high voltage of 30 kV) a line of brighter contrast, perfectly parallel to the irradiated surface and all along the sample. It crosses several grains of different orientation without interruption, as clearly showed by the different BSE contrast. Also nano indenters after hardness test are showed in order to prove matches of hardness data. It should be noticed that the higher contrast line crosses the point with the highest hardness.

C Current work on irradiated materials

Irradiation-induced dislocation loops formation are reported to occur in pure tantalum at a damage dose of ≤ 0.3 dpa at a relatively high temperatures of 700°C [2]. However, radiation-induced hardening in pure Ta as well as in Ta-W alloys seems to take place already at a dose of ≤ 0.3 dpa and temperatures up to 350°C, based on mechanical testing data of irradiated samples [4-6]. Nevertheless, the correlation of the hardening phenomenon with the characteristics of the irradiated structures and mechanism of dislocation loops formation still remains unknown. Irradiated sample preparation of irradiated Ta and Ta-W alloys is carrying out for advanced Transmission microscopy analysis in order to observe irradiation-induced defects such as vacancy clusters and dislocation loops and influence of dose on the nature and forming of these defects . This detailed study constitutes the stepping stone in understanding the effect of the alloying content as well as radiation dose on the defect formation and dynamics of these materials under mechanical deformation and irradiation conditions.

ACKNOWLEDGMENT
First of all the author acknowledges the financial support of the Dalton Cumbrian Facility and Professor Simon Pimblott for the funding throughout the project. Also I would like to thank Material Science Centre in the University of Manchester for the equipment provided.

Iuliia IPATOVA (Carlisle, United Kingdom), Enrique JIMENEZ-MELERO

08:00 - 18:15 #6691 - MS01-579 Fivefold symmetries in silicon thin films induced by multiple twinning.

MS01-579 Fivefold symmetries in silicon thin films induced by multiple twinning.

Fivefold symmetry, like any kind of n-fold rotational symmetry, can be identifiable when rotating a crystalline configuration 5 times (or n times) around a certain axis and realizing that the structure is transformed into a configuration that is equivalent to the initial one. The occurrence of this specific symmetry, forbidden by the conventional periodic crystallography, was attributed in the literature to the presence of a new state of matter “the quasicrystals” [1] [2] or simply to an effect of multiple twinning. Particularly, the tendency of multiply twinning in a fivefold symmetry has been widely reported in small particles having a special morphology like the decahedral [3] or icosahedral [4] structures, usually called multiply twinned particles. In this study, we will highlight on the fivefold symmetry observed in the electron diffraction patterns of two types of materials elaborated in different growth conditions, originating from multiple twinning and not from the presence of multiply twinned particles.

The first case concerns the fivefold symmetry on p-type doped silicon thin films containing a non-negligible amount of carbon and oxygen. These films were deposited in a plasma enhanced chemical vapor deposition reactor (PECVD) at 0.2 W/cm² using silane, hydrogen, diborane and hexamethydisiloxane (C₆H₁₈OSi_2, HMDSO) diluted in argon. Since all the diffraction patterns recorded on different regions of these films exhibit a fivefold symmetry along [0-11] zone axis (Figure 1), it is clear that this symmetry is real and characteristic of our films. Further diffraction measurements reveal that there is a relation of epitaxy with the (100) crystalline silicon substrate. This is also confirmed by high resolution TEM images, where {111} planes are continuing from the substrate to the film across the interface. Moreover, energy filtered TEM images were correlated with SIMS measurements to provide elemental mapping of silicon, carbon and oxygen with absolute values.

The second case illustrates a quasi-fivefold symmetry recorded on intrinsic silicon thin films deposited by PECVD using silicon tetrafluoride, hydrogen and argon chemistry at a purposely high power density of 0.3 W/cm². After few hundred nanometers of epitaxial growth, a high density of defects appears, followed by a multiply twinned part (as shown in Figure 2a). Fourier Transforms recorded on the first part reveal a monocrystalline structure (Figure 2c), and on the second part a fivefold symmetry (Figure 2b), which is, in this case also, linked to an epitaxial growth.

It has been proved in some references [5] [6] that a high power density is responsible for a high ion energy impinging on the substrate and causing some surface or even bulk damage. Thus, the twin defects present in our films are most probably caused by the application of a high power density. However, to obtain a fivefold symmetry, it is necessary to have at least three orders of twinning that contribute to 10 spots in the diffraction pattern, i.e, if there only exist two orders of twinning, some additional diffraction spots appear without giving rise to a fivefold symmetry as it is the case of Figure 3. Detailed investigation of the multiple twinning in a fivefold symmetry fashion will be presented.

 

1.      Shechtman, D., et al., Metallic Phase with Long-Range Orientational Order and No Translational Symmetry. Physical Review Letters, 1984. 53(20): p. 1951-1953.

2.      Pauling, L., Apparent Icosahedral Symmetry Is Due to Directed Multiple Twinning of Cubic-Crystals. Nature, 1985. 317(6037): p. 512-514.

3.      Iijima, S., Fine Particles of Silicon. II. Decahedral Multiply-Twinned Particles. Japanese Journal of Applied Physics, 1987. 26(3R): p. 365.

4.      Yang, C.Y., Crystallography of decahedral and icosahedral particles: I. Geometry of twinning. Journal of Crystal Growth, 1979. 47(2): p. 274-282.

5.      Rosenblad, C., et al., Silicon epitaxy by low-energy plasma enhanced chemical vapor deposition. Journal of Vacuum Science & Technology A, 1998. 16(5): p. 2785-2790.

6.      Ohmi, T., et al., Study on further reducing the epitaxial silicon temperature down to 250 °C in low‐energy bias sputtering. Journal of Applied Physics, 1991. 69(4): p. 2062-2071.

Farah HADDAD (Palaiseau), Prabal GOYAL, Ronan LÉAL, Junegie HONG, Erik JOHNSON, Pere ROCA I CABARROCAS, Jean-Luc MAURICE

08:00 - 18:15 #6696 - MS01-581 Optimisation of the FIB induced damage in TEM diamond samples.

MS01-581 Optimisation of the FIB induced damage in TEM diamond samples.

Due to diamond extreme properties, it is very hard to prepare a TEM sample from diamond using traditional methods of preparation including mechanical thinning, ion milling or chemical etching. At the same time diamond can be relatively easily micro-machined using focused ion beam (FIB) technique. Using this technique a cross-sectional TEM sample of diamond can be prepared in a few hours. Also, combination of ion implantation and FIB milling allows device fabrication in diamond at micro and nano scales levels. In the last decade FIB milling became essential tool for TEM sample preparation as well as for nanofabrication in diamond. However, Ga FIB milling has an unavoidable result in formation of the damage layers which can significantly reduce the device working areas and limit the applications of the FIB technique for nanofabrication of diamond. The damage layers in the FIB prepared TEM diamond samples can also significantly aggravate the quality of high-resolution imaging. So, the knowledge of the extent of damage induced in diamond during FIB milling is critical for nanofabrication as well as for TEM imaging. In this work the damage layers after FIB milling of the synthetic single crystal diamond at different ion beam energies were studied using high-resolution and analytical electron microscopy.

TEM image of cross-section of TEM lamella prepared using 30 keV Ga FIB milling is shown in Fig. 1.  Amorphous layer with thickness ~ 16 nm are clearly visible on both sides of TEM lamella. EELS measurements of the carbon K-edge in the amorphous region shows a prominent feature at 285eV, the p^* peak associated with the presence of sp² bonding. This indicates the conversion of diamond sp³ bonds to sp² in the amorphous damage area. Electron energy loss spectrum image was taken in STEM mode from central part of cross-section of TEM lamella shown in Fig.1. Using Gatan Digital Micrograph software the chemical maps for sp² and sp³ bonded carbon were obtained. Fig.2 shows maps for features of carbon K-edge at 285 eV (sp²) and 290 eV (sp³ bonding). It is visible from Fig. 1, 2 that TEM lamella prepared from diamond using 30 keV FIB milling contained ~ 20 % of amorphous sp² bonded carbon. In case of thinner lamellas (60 nm) prepared using 30 keV FIB milling the fraction of amorphous sp² bonded carbon increases to ~50%.

The thickness of damage layers with sp² bonded carbon in TEM diamond samples can be reduced by using low voltage FIB milling. Fig.3 and Fig. 4 show the HREM images of damage layers in diamond after 30 keV and 2 keV FIB milling. The thickness of amorphous damage layers reduced from 16 to less than 2 nm. Thus, very thin TEM diamond samples with low fraction of amorphous sp² bonded carbon could be prepared by using 2 keV FIB milling at final stage of TEM lamella preparation.

 

Acknowledgments

 This work was supported by the Australian Research Council under Linkage Infrastructure, Equipment and Facilities program (LE 130100090).

Sergey RUBANOV (Melbourne, Australia)

08:00 - 18:15 #6697 - MS01-583 Analytical STEM study of sintered polycrystalline c-BN materials for cutting tool applications.

MS01-583 Analytical STEM study of sintered polycrystalline c-BN materials for cutting tool applications.

Cubic boron nitride (c-BN) is the second hardest material next to diamond with high thermal conductivity but better chemical stability than diamond, therefore, it is widely used in the form of sintered polycrystalline cubic boron nitride (PCBN) for cutting tools in machining of hardened steel, cast irons and super alloys showing high chemical stability when abraded by ferrous materials at high temperature. Commercial PCBN cutting tools are classified into two types, firstly  low c-BN content composites containing  typically 40 -70 vol%  c-BN with ceramic binders such as TiN and TiC, and secondly high c-BN content PCBN containing about 75–95 vol% of c-BN with Al and other metallic binders in addition. PCBN materials designed for mild to medium interrupted hard part turning (HPT) applications have been produced by using different raw materials and sintering conditions. These PCBN materials have been investigated by SEM and XRD for microstructure and phase information and XRF for the overall chemical composition.  It was observed that, for some of the PCBN material, there was a discrepancy between the elements detected as well as their measured quantity by XRF and the phases identified by XRD. Advanced analytical STEM was applied  for an in depth characterization of the complex microstructures formed in those PCBN materials with grain size ranging from sub-micrometer down to a few nanometers. The aim of this study was to increase the understanding of the relationship between raw material selection, processing conditions, and machining performance. The raw material selection, milling and sintering operations play a crucial role in the solid state phase diffusion processes which governs what phases are formed and how these constituents bond to each other and, thus, the mechanical behaviour. To be able to capture the light and heavy elements present in the microstructure, elemental maps were acquired in a C_S corrected Titan³ 60-300 kV (FEI Company) equipped with ChemiSTEM and DualEELS capabilities. Figure 1 depicts elemental maps of two PCBN materials produced by different methods of Al processing showing greatly differing microstructures.  In Fig. 1a, the Al appears to have flowed well and coated the c-BN interphases forming a continuous binder phase, mainly consisting of AlN.  It appears that almost all of the AlN phase is connected to c-BN.  Whereas in Fig. 1b, the Al phases are much less continuous, but still coat most of the c-BN.  Most of the Al appears to be in discrete aluminium oxide phase. To be able to determine all reaction products and phases present advanced TEM and STEM techniques had to be applied due to the small and overlapping grains. The results presented are of importance to further improve machining performance by tailoring the microstructure by carefully selecting the initial raw material and processing conditions.

Jacob PALMER, Martina LATTEMANN (Stockholm, Sweden), Ernesto CORONEL, Arno MEINGAST, Larry DUES, Rachel SHAO, Gerold WEINL

08:00 - 18:15 #6716 - MS01-585 HR(S)TEM, EELS and Raman spectroscopy analysis of a LCMO and PCMO films.

MS01-585 HR(S)TEM, EELS and Raman spectroscopy analysis of a LCMO and PCMO films.

Interplay between charge, orbital and magnetic orderings in perovskitemanganitesR_1-xA_xMnO₃ (R – rare earth, A – alkaline element) is of fundamental interest to describe electronic correlations in transition metal oxides. In particular, the overdoped systems with x > 0.5 are a part of the phase diagram that is less explored as compared to the underdoped manganites (x~0.3), where colossal magnetoresistance is observed. Until now, the overdoped manganites were studied manily as the polycrystalline pellets or powders. Here we report on the study of highly doped heteroepitaxial La_0.25Ca_0.75MnO₃/MgO films grown by metalorganic aerosol deposition (MAD) technique on MgO(100) substrate. Cross-sectional and plan-view TEM specimens were prepared by mechanical polishing followed by Ar⁺ ion milling.

TEM studies reveal that the La_0.25Ca_0.75MnO₃ film consisted of domains with three different orientations (Fig. 1) described by epitaxial relationships with respect to the MgO substrate as: 1) [001]_MgO||[010]_LCMO and [100]_MgO||[10-1]_LCMO; 2) [001]_MgO||[-101]_LCMO and [100]_MgO||[010]_LCMO; 3) [001]_MgO||[-101]_LCMO and [100]_MgO||[101]_LCMO. The domain interfaces are CaO layers as revealed by atomic resolution STEM imaging and EELS measurements (Fig. 2). These defects cause the appearance of streaks visible in the ED patterns.

Upon the cooling down to the temperature below 140K the material demonstrates a structural transition manifested by the elongation of diffraction spots (marked in Fig. 3 with arrows)  and appearance of crosses along [10-1]* and [010]* directions in the selected area diffraction patterns taken from plan-view specimens (see, for example, selected spot in Fig.3). These changes evidence the formation of charge-ordered (CO) domains, however their size is rather small due to the presence of numerous defects and compositional variations. The CO transition is also visible in the Raman spectra, which were recorded in four different polarizationconfigurations as shown in Fig.3. Three additional lines pop up at 521cm^-1, 646 cm^-1 and 707 cm^-1 and the P2₁/m structure can be identified as a new emerging structure when the sample undergoes CO-transition.

As a reference system we have also studied heteroepitaxial thin films of Pr_0.65Ca_0.35MnO₃ deposited on single crystalline SrTiO₃ substrates by sputtering deposition technique. Upon cooling this model system undergoes a phase transition to the CO state, which has been extensively studied, e.g. [1,2]. This CO state is revealed by the appearance of 2a and 2c superstructure reflexes in the diffraction pattern, and two emerging peaks at 650 cm^-1 and 709 cm^-1 which can be seen (Fig. 4) when the temperature falls below CO-transition temperature.

Thus, besides the electron diffraction experiments Raman spectroscopy can also be used as a powerful tool to visualize CO-transitions that are often accommodated by structural transitions.

 

We would like to thank M. Abrashev for very helpful discussions regarding the measured Raman intensity and twinning of the Pnma structure. F. F. and V.M. acknowledge funding from DFG Project DR 228/36. We also are thankful for funding from DFG Sonderforschungsbereich 1073 (TP B02, B04).

 

 

[1] Jooss et al., Proc. Natl. Acad. Sci. U. S. A.2007,104, 13597–13602

[2] Wu et. al., Phys. Rev. B 2007, 76, 174210

Vladimir RODDATIS (Göttingen, Germany), Florian FISCHGRABE, Sebastian MERTEN, Benedikt IFLAND, Vasily MOSCHNYAGA, Christian JOOSS

08:00 - 18:15 #6810 - MS01-587 Transmission electron microscopy study of defects generated during chemical vapor deposition diamond lateral growth.

MS01-587 Transmission electron microscopy study of defects generated during chemical vapor deposition diamond lateral growth.

Synthetic diamond is one of the most promising materials for high power devices due to its extraordinary physical properties, such as high thermal conductivity (22 W/cm K, 4 times that of Cu), electric breakdown field (>10 MV/cm), and carrier mobility (m_n =1000 cm²/V_s, m_p = 2000 cm²/ V_s). Moreover, 3D architectures, i.e. lateral growth, allowed the use of vertical geometries for the design of such devices, in addition to other advantages such as higher miniaturization, distribution of the electric field and reduction of technology steps and costs. In fact, in the quest for power electronic devices sustaining ever higher reverse blocking voltages and forward currents, buried heavily boron doped (p⁺) diamond layers have been shown recently [1] to reduce markedly the on-state resistance (R_on) of pseudo-vertical crystal diamond Schottky diodes. Such advanced designs rely on an improved control of selective 3D overgrowth of dry etched mesa and trenches [2].

Here, MPCVD diamond overgrowth on patterned-etched diamond substrate is demonstrated to be highly selective depending on the methane concentration. This can be very useful for the design of 3D engineered semiconducting devices such as p-n junctions. However, some growing conditions are shown to generate defects (dislocations, planar defects…). In addition, boron doping is also shown to induce the generation by a proximity strain related mechanism [3] of another type of defects. Mesa structures were fabricated by reactive-ion etching (RIE) on masked substrates. Overgrowth was performed by microwave induced plasma chemical vapor deposition (MPCVD). A stratigraphic approach of heavily boron doped layers and undoped ones allows to follow the “history” of the growth, in the vicinity of mesa patterns [4], thanks to further cross section TEM observations. The latter identify and distinguish between extended defects generated by: (i) the boron inclusion, (ii) the strain related to the mesa-step and (iii) the growth conditions.

Defects are studied using dark field (DF) and weak beam DF (WB) in diffraction contrast modes on focused ion beam lamellas. From the invisibility criterion,  and  families of Burger vectors have been identified. Based on the position of this defects respect the MESA structure, their origin is identified to be: (i) edge and threading dislocations with  type of Burger vector are favorably generated by the boron doping while (ii) planar defects with  type of Burger vector are highly influenced by the strain accumulated in the corner of the step generated by the mask before the overgrowth.

In addition, multilayer doping allows identifying the regions of different growth orientation in the mentioned stratigraphic approach. Susceptibility of dislocation generation by boron proximity effects respect to the surface growth orientation is well revealed in such growth geometrical design where several growth orientations have to coexist at the same time. Higher density of type (i) of defects where obtained in closer planes as (111). Finally, the role of the methane concentration in the generation of extended defects will be discussed.

 

 

[1]            A. Traoré, P. Muret, A. Fiori, D. Eon, E. Gheeraert, and J. Pernot, Appl. Phys. Lett. 104, 052105 (2014).

[2]             K. Sato, T. Iwasaki, Y. Hoshino, H. Kato, T. Makino, M. Ogura, S. Yamasaki, S. Nakamura, K. Ichikawa, A. Sawabe, and M. Hatano, Jap. J. Appl. Phys. 53, 05FP01 (2014).

[3]             M.P. Alegre, D. Araújo, A. Fiori, J.C. Pinero, F. Lloret, M.P. Villar, P. Achatz, G. Chicot, E. Bustarret, and F. Jomard, Appl. Phys. Lett. 105, 173103 (2014).

[4]            F. Lloret, A. Fiori, D. Araujo, E. Eon, M.P. Villar, and E. Bustarret, Appl. Phys. Lett. , to be published.

Fernando LLORET (Puerto Real, Spain), Daniel ARAUJO, David EON, M. Pilar VILLAR, Etienne BUSTARRET

08:00 - 18:15 #6811 - MS01-589 STEM-EELS investigation of planar defects in olivine.

MS01-589 STEM-EELS investigation of planar defects in olivine.

Iron is an abundant element in meteorites, where it is contained in various forms such as metal, oxides, silicates or sulfides. In these materials, investigation of the oxidation state of transition metal elements and Fe in particular is an indicator of the environmental conditions during the meteorite formation and evolution. The detailed quantification of the element distribution, concentration variation and the structure of defects are important for understanding the formation and transformation stages. Therefore the correlative investigation of defects structure and oxidation state variation within a meteoritic material at the atomic level can be essential to the understanding of the distinct formation mechanisms.

In this context, the current study focusses on the detailed investigation by STEM-EELS coupled to HAADF imaging of planar defects in Fe-rich olivine (Mg,Fe)₂SiO₄, present in the Allende meteorite [1]. The nature of the planar defects in olivine is still controversial in the literature: it can be associated to a deformation mechanism [2] or can be due to an aqueous alteration episode [3]. HAADF images of two types of planar defects, present in the Allende meteorite are given in Fig. 1 (a) and (b). Both planar defects are parallel to (100) planes of the olivine. The thicker ones, Fig. 1(a), are Cr-rich as seen on the EELS spectra given in Fig. 2. STEM-EELS mapping reveals an enrichment of Fe at the interface between the thick Cr-rich defects and olivine. The Fe-L_3,2 ELNES structures do not seem to indicate any change of the oxidation state in this Cr-rich defect. The thinner (100) defects, Fig. 1(b), are associated to considerable Fe enrichment and Mg depletion over 2 to 4 atomic planes. Even though these thinner defects are often linked to the Cr-rich defects, they do not contain Cr. The analysis of the atomically resolved Fe-L_3,2 fine structure reveals the appearance in certain cases of a change of oxidation state of iron and partial Fe³⁺ state within the thin Fe-rich planar defects. This study gives an important insight on the structure of planar defects in the olivine and the associated change of the Fe oxidation state. Based on this fine structure analysis, a possible formation mechanism constraining the evolution of the environment redox condition will be proposed.  

  

References

[1] H. Palme, B. J. Fegley, Earth and Planetary Science Letters (1990), vol. 101, p. 180–195.

[2] L. P. Keller, Meteoritics and Planetary Science (1998), Vol. 33, p. A83.

[3] Khisina, R. Wirth, S. Matsyuk, M. Koch-Müller, Eur. J. Mineral. (2008) vol. 20, p. 1067.

Acknowledgement

The CEA-METSA network is acknowledged for financial support. The TEM national facility in Lille (France) is supported by the Conseil Regional du Nord-Pas de Calais, the European Regional Development Fund (ERDF), and the Institut National des Sciences de l’Univers (INSU, CNRS).

Maya MARINOVA (Villeneuve d'Ascq), Priscille CUVILLIER, Alexandre GLOTER, Damien JACOB, Hugues LEROUX

08:00 - 18:15 #6821 - MS01-591 The microstructure of ZnSnO and its correlation to electrical and optical properties.

MS01-591 The microstructure of ZnSnO and its correlation to electrical and optical properties.

Over the last years, the interest in the field of transparent conductive oxides (TCOs) has grown dramatically due to their wide applicability and improved properties that may be reached when incorporating these materials into devices. TCOs are mainly used in the industry of low-emissivity windows, flat panel displays, light emitting diodes and photovoltaics [1]. For photovoltaic applications, the main purpose of TCOs is to let light enter into the solar cell and to extract the electric charges allowing them to be drifted towards the electric contacts. Therefore, it is necessary for these materials to be as transparent and as conductive as possible [2]. Ideally, TCOs should be indium-free, as indium is scarce and hence expensive [3]. The goal is therefore to optimize a material that is earth-abundant, low-cost and with good electrical and optical properties. As many steps in photovoltaic device fabrication require a high temperature, a crucial requisite for TCOs is also thermal stability.

Based on these criteria, an amorphous compound of Zn-Sn-O (ZTO) deposited by sputtering was selected for the present study [4]. The microstructure of ZTO is known to strongly influence its electrical and optical properties, as well as its thermal stability. In that regard, transmission electron microscopy (TEM), in situ X-ray diffraction (XRD) experiments and conventional electrical and optical characterization were performed to assess the links between annealing treatments, ZTO microstructure and optical and electrical properties. 

First, samples were annealed in air, in an oven up to 150 and 500 °C and then investigated by transmission electron microscopy. While electrical and optical properties were measured to change significantly upon annealing, no major microstructural change was observed in TEM images. In situ theta-2theta XRD experiments were then performed by increasing the temperature up to 1000-1200°C in air and vacuum. Substrates resistant to these temperatures were employed, namely fused silica and sapphire. Different heating rates were used, ranging from 3°C/min up to 10°C/min. The XRD results (Fig.1) demonstrate that the amorphous phase is stable up to >500 °C when annealed in air and > 900 °C when annealed in 10^-4 mbar, hence highlighting a strong influence of the annealing atmosphere on the crystallisation temperature. Rutile SnO₂ is the first phase to crystallize and remains the main crystal structure observed throughout the whole process, with Al₂ZnO₄ forming at higher temperatures as a result of an interaction between the TCO layer and the sapphire substrate. Electrical properties were measured to decrease after annealing, with TEM measurements demonstrating that Zn migration at high temperature leads to the formation of a defective crystalline structure (Fig.2). This effect is more severe when annealing in air when compared to vacuum conditions. Indeed, the presence of oxygen in the surrounding atmosphere facilitates the formation of crystalline SnO₂, a process that repeals  Zn atoms to grain boundaries and surfaces of the TCO layer (Fig.3). On the other hand, the formation of crystalline SnO₂ and the release of zinc are both delayed when annealing in vacuum. In general, crystallisation and Zn evaporation are observed to be detrimental to the electrical properties as it leads to the formation of voids in the structure. On a technological level, the high thermal stability of the defect-free amorphous ZTO microstructure in oxygen-poor atmospheres may enable its application in high efficiency photovoltaic architectures. 

References:

[1] R.G. Gordon, "Criteria for Choosing Transparent Conductors," MRS Bulletin 25, 52 (2000)

[2] A.J. Leenheer, J.D. Perkins, M. Van Hest, J.J. Berry, R.P. O’hayre and D.S. Ginley, “General mobility and carrier concentration relationship in transparent amorphous indium zinc oxide films”, Physical Review B 77, 115215 (2008)

[3] European Commission, “Report on critical raw materials for the EU” (2014)

[4] M Morales-Masis, F Dauzou, Q Jeangros et al., “An Indium-Free Anode for Large-Area Flexible OLEDs: Defect-Free Transparent Conductive Zinc Tin Oxide” Advanced Functional Materials 26 (2016)

Federica LANDUCCI (Lausanne, Switzerland), Quentin JEANGROS, Esteban RUCAVADO, Carole SPORI, Monica MORALES-MASIS, Christophe BALLIF, Cécile HÉBERT, Aïcha HESSLER-WYSER

08:00 - 18:15 #6838 - MS01-593 Finding pinholes in carrier selective polycrystalline Si / crystalline Si contacts: like a needle in a haystack.

MS01-593 Finding pinholes in carrier selective polycrystalline Si / crystalline Si contacts: like a needle in a haystack.

Reducing surface recombination is a key factor in the race for high efficient silicon-based solar cells. One solution is the use of carrier selective contacts, which have been investigated extensively by several groups in the last years [1]-[5]. Even though polycrystalline (poly-)Si/SiO_x/crystalline (c‑)Si junctions were subject of research decades ago for use in bipolar transistors [6]-[7] there are still different point of views about the physical principles of current transport in these junctions [8],[2]. Besides tunneling, current transport through pinholes is discussed [2]. It was already shown that high fractions of oxide disruptions lead to inferior electrical properties [5]. However, as stated in the model [2], only pinhole densities as low as 8 × 10⁵ to 3 × 10⁹ cm^-² are needed to describe the current flow / the junction resistance correctly.

Analyzing such hole densities in poly-Si/SiO_x/c‑Si junctions is very difficult. If we assume, that the existence of holes leads to an increased diffusion of dopants from the poly-Si into the c-Si we can indirectly prove the existence by measuring the local dopant concentration. However, methods like electrochemical capacitance-voltage profiling or secondary ion mass spectrometry show the averaged distribution of dopants in a larger area and can only be used to compare different samples qualitatively. Conductive AFM is possible in general but it is not suitable for our samples due to the high transverse conductivity of the highly doped poly-Si of about 150 nm thickness. We have to remove or at least drastically reduce the thickness of the poly-Si, but therefore an etch process with a high selectivity ratio to SiO₂ with a thickness well below 2 nm is needed. Even when all these issues are solved, we don’t get detailed information about hole diameter (expected to be around 5 nm [2]) and structure of the oxide around these holes. These questions can be answered by TEM, but the probability of finding holes with densities that low is very small. However, proving the existence of holes in the oxide by TEM for samples with good electrical results would be the first step of proving that current through poly-Si/SiO_x/c‑Si junctions is not only related to tunneling.

In this paper we investigate the influence of annealing temperature on the structural properties of in situ boron doped p+-poly-Si/SiO_x/c-Si interfaces after annealing at temperatures between 800 °C and 1050 °C. We analyze the evolution of holes for 1.7 nm thin wet-chemically grown SiO_x in comparison to thermally grown oxide using high resolution TEM and compare our findings to electrical results.

We confirm that a massive break-up of the oxide leads to poor electrical properties. Additionally, we prove the existence of pinholes in a sample with wet-chemically grown oxide annealed at 800 °C showing good electrical results with an emitter current density J_0e as low as 41 fA/cm². These results indicate a certain area fraction of pinholes is not in contradiction with good passivation properties of  poly‑Si/SiO_x/c‑Si junctions. The flow of charge carriers through these pinholes possibly poses an important current transport mechanism besides the superposed tunneling in the region in which the interfacial oxide is still intact.

 

References

[1]   U. Römer et al., IEEE Journal of Photovoltaics 5 (2015) 507-514.

[2]   R. Peibst et al., IEEE Journal of Photovoltaics 4 (2014) 841-850.

[3]   S.W. Glunz et al., 31^st European Photovoltaic Solar Energy Conf. and Exhibit., Hamburg, Germany, 2015.

[4]   A. Cuevas et al., 42^nd IEEE Photovoltaic Specialist Conference (2015), 1-6.

[5]   A. Moldovan et al., Solar Energy Materials and Solar Cells 142 (2015) 123-127.

[6]   H. C. de Graaff and J. G. de Groot, IEEE Transactions on Electron Devices 26 (1979) 1771-1776.

[7]   A. A. Eltoukhy and D. J. Roulston, IEEE Transactions on Electron Devices 29 (1982) 1862-1869.

[8]   H. Steinkemper et al., IEEE Journal of Photovoltaics 5 (2014) 1348-1356.

Dominic TETZLAFF (Hannover, Germany), Jan KRÜGENER, Yevgeniya LARIONOVA, Sina REITER, Mircea TURCU, Nils FOLCHERT, Robby PEIBST, Uwe HÖHNE, Jan-Dirk KÄHLER, Tobias WIETLER

08:00 - 18:15 #6849 - MS01-595 A Study of Heavy Ion Irradiation-Induced Segregation at Grain Boundaries in Alloy 800H.

MS01-595 A Study of Heavy Ion Irradiation-Induced Segregation at Grain Boundaries in Alloy 800H.

Environmentally-Assisted Cracking (EAC) of  structural components in a nuclear reactor core of light water reactors (LWRs) and water-cooling systems in fusion reactors is of concern in that it can directly influence on the safety of a nuclear reactor.  Previous studies showed that one of the many factors that governing the EAC of a structural component is the changes of the chemical composition of the grain boundaries of the material used after exposed to neutron radiation [1].  This phenomenon is also known as irradiation-assisted stress corrosion cracking (IASCC). The changes of the chemistry of grain boundaries induced by irradiation, i.e. radiation-induced segregation (RIS), and the increase in irradiation-induced defects can promote hardening of the material, thereby affecting the resultant mechanical and environment-sensitive behaviour.  With long-term (~80 years) reactor operation under consideration for power generation, Alloy 800H is one of the candidate alloy systems that may be used in the next generation advance light water nuclear energy system [2].  One of the reasons for this choice is that Alloy 800H is code-certified for high temperature (up to 760 ᵒC) use in nuclear power systems [3].  Also, the high Ni and Cr concentrations in the alloy provide good resistance to void swelling and corrosion in light water environments [4].

 

In this study, RIS in ion-irradiated Alloy 800H was investigated.  A commercial grade Alloy 800H was subjected to simultaneous Fe⁺⁺ and He⁺⁺ ion-beam irradiation to simulate the effect of neutron irradiation-induced segregation.  The alloy was irradiated to a dose of 16.6 dpa at temperature of 440ᵒC using the facility at University of Michigan Ion Beam Laboratory, Ann Arbour, Michigan.  The microstructural characterisation was performed using a spherical aberration-corrected FEI Titan G2 80-200 with Super X EDX (ChemiSTEM™) operated at 200kV and equipped with a GIF Quantum 965 EELS to provide independent compositional analyses of the intergranular segregation induced during ion irradiation.  Scanning transmission electron microscopy (STEM) – Electron Energy Loss Spectroscopy (EELS) analyses show that the grain boundaries in Alloy 800H are enriched with Ni, Si and Ti after irradiation, as shown in Figure 1. Fe and Cr depletion was also detected at the same grain boundary, as shown in Figures 2 and 3. Apart from that, a denuded zone with no chemical-related irradiation-induced defects decorating along the grain boundary was observed.  However, diffraction contrast imaging shows that a high number density of the irradiation-induced dislocation loops was observed along the grain boundary. This observation and its implications for the alloy will be further discussed.

 

References & Acknowledgement:

[1] K. Fukuya et. al., Role of Radiation-Induced Grain Boundary Segregation in Irradiation Assisted Stress Corrosion Cracking, Journal of Nuclear Science and Technology, (2004), Vol, 41, No. 5,

[2] U.S. DOE Nuclear Energy Research Advisory Committee and the Generation IV International Forum, A Technology Roadmap for Generation IV Nuclear Energy Systems, 2002.

[3] K. Natesan et. al., 2003, “Materials Behaviour in HTGR Environments,” ANL-02/37 and NUREG/CR-6824.

[4] F.A. Garner and A.S. Kumar, in: Radiation-Induced Changes in Microstructure: 13th International Symposium (part I), in: F.A. Garner, N.H. Packan, A.S. Kumar (Eds.), ASTM STP, 955, ASTM, Philadelphia, 1987, p. 289.

[5] ACKNOWLEDGE EPSRC grant and G. Was for providing the ion-irradiated specimens.

Joven Jun Hua LIM (Manchester, United Kingdom), M Grace BURKE

08:00 - 18:15 #6866 - MS01-597 Study the microstructure of three and four component phases in Al-Ni-Fe-La alloys.

MS01-597 Study the microstructure of three and four component phases in Al-Ni-Fe-La alloys.

Aluminium alloys play a key role in modern engineering since they are the most used non-ferrous material. They have been widely used in automotive, aerospace, and construction engineering due to their good corrosion resistance, superior mechanical properties along with good machinability, weldability, and relatively low cost. The progress in practical application has been determined by intensive research and development works on the Al alloys. A new class of Al–REM–TM aluminum alloys (REM indicates rare earth metal and TM is transition metal) was revealed in the end of the last century. These alloys differ from conventional ones by their extraordinary ability to form metal glasses and nanoscale composites in a wide range of compositions. Having low density, these alloys possess unique mechanical characteristics and corrosion resistance. Two as received alloys, namely Al₈₅Ni₉Fe₂La₄ and Al₈₅Ni₇Fe₄La₄ were obtained in the form of ingots from melts of corresponding compositions upon cooling in air were studied by scanning/transmission electron microscopy (STEM), energy dispersive X-ray (EDX) microanalysis and X-ray diffraction (XRD). The microstructural analyses were performed in a aberration corrected TITAN 80-300 TEM/STEM (FEI, USA) attached with EDX spectrometer with ultrathin window (EDAX, USA). The specimens for transmission electron microscopy (TEM) were prepared by an electrochemical or ion etching. It was found that the received alloys exhibits along with fcc Al and Al4La (Al11La3) particles, these alloys contain a ternary phase Al3Ni1 ⎯ хFeх isostructural to the Al3Ni phase and a quaternary phase Al8Fe2 ⎯ хNiхLa isostructural to the Al8Fe2Eu phase and monoclinic phase Al₉(Fe,Ni)₂ isostructural to the Al₉Co₂. The study by HRSTEM together with a new atomic resolution energy dispersive X-ray microanalysis method demonstrated that Fe and Ni atoms substituted one another in the Al8Fe2-хNiхLa quaternary compound. Besides, several types of defects were determined: first ones have a form of a δ -layers and they are Al3.2Fe1-хNix ternary compound with a Al3.2Fe structure type. Second ones were point defects, which are La vacancies.

The experimental part of this work was partially done on the equipment of the Resource Center of Probe and Electron Microscopy (Kurchatov Complex of NBICS- Technologies, NRC "Kurchatov Institute")

Natalia KOLOBYLINA (Moscow, Russia), Alexander VASILIEV, Sergey LOPATIN, Mikhail PRESNIAKOV, Natalia BAKHTEEVA, Anna IVANOVA, Elena TODOROVA

08:00 - 18:15 #6921 - MS01-599 TEM of microstructures and lattice bending formed in thin Sb film with thickness gradient.

MS01-599 TEM of microstructures and lattice bending formed in thin Sb film with thickness gradient.

Thin-film Sb-based phase-change materials (PCMs) are widely used for memory devices that utilize amorphous-crystalline transitions in local areas. Microstructures of Sb thin films are of special interest since Sb films are known for fast crystallization rates [1], including “explosive” crystallization (more known and studied for Si and Ge films). In this paper we study the role of film thickness and use bend-contour technique [2] for crystal misorientation studies [3] (supported by local thickness estimates). It was out of focus for several dozens of papers devoted to the Sb crystallized thin films.

Sb thin films were evaporated in vacuum on mica substrates covered by thin evaporated carbon film to provide initial amorphous structure. Since thin-film crystallization strongly depends upon thickness we use masks to obtain strong thickness gradients (about 40 nm per film length 1 mm). The film separated from the substrate and placed on TEM grid are studied using TEM, SAD, STEM, SEM, EDX in JEM-2100 (80Kv and 200 Kv). Bend contours are indexed with the help of bright and dark fields and indexed electron diffraction patterns. Measured distances between bend contours and ZAPs are used for estimates of lattice bending, while measurements of fine structure of some strong contours in dark fields add estimates of film thickness.

The final microstructures give indications of initial amorphous structure. The crystallization starts at the thicker area and stops at the thinner areas, Fig. 1 a, where labyrinthine and islands microstructures are observed. SADs of these areas demonstrate superposition of amorphous hallo and single crystalline orientation. The spot pattern usually does not change while moving aperture several microns around. The amorphous hallo outside crystallization front does not include any crystal spots. Going from thinner area to the thicker one, Fig. 1 c-d-e-f, along the thickness gradient one can trace in TEM the increase of density of amorphous islands finally resulting in labyrinthine microstructure. In parallel the spread of island sizes decreases from 0-20nm to around 40 nm and their density increases twice from ~250/µm².

More or less prominent bend contours (Fig. 1 b) or similar weak contrast (Fig. 1 a) are observed everywhere in crystalized areas. Similar but not analyzed bend contour patterns were published earlier in some TEM studies of Sb films (see e.g. [1, 4]). Going from thinner area to the thicker one in the areas of entire film along the thickness gradient, Fig. 1 g-h-i-j, we calculate essential decrease of lattice planes bending (around axes lying in the film plane) in the range from most strong values, 120 degrees per µm, to ~ 10 degrees per µm. We suppose that this lattice bending can be attributed to the “transrotational” structure revealed and proved earlier for other thin-film crystals of different chemical nature and preparation conditions [3]. Both maximal values and film thickness dependence of transrotation correspond to those studied earlier for some other substances [5]. The microstructure texture reminding parquet drawing is also observed (in the regions of intermediate thickness) with subgrain sizes: width 0,1 - 1 µm, length 2-10 µm and above. The lattice orientation texture is also revealed with preferred orientations [0 0 1], [-1 1 1], [1 2 -1]. There are other details of lattice orientation texture observed along the crystal grains and thickness gradient. 

The nature of transrotation, unusual phenomenon for crystal growth in amorphous films, is discussed.

References

[1] J. Solis; C.N. Afonso, Appl. Phys., 2003, A76, 331-338

[2] I.E. Bolotov, V.Yu. Kolosov, Phys. Stat. Sol., 1982, 69a, 85-96.

[3] V.Yu. Kolosov, A.R. Thӧlen,. Acta Mater., 2000, 48, 1829–1840.

[4] H. Müller, Phys. Stat. Sol. 1982, 70a, 249-255.

[5] V.Yu. Kolosov et al., Semiconductors, 2005, 39, 955-959.

Supported by RF Ministry Education & Sci. (No. 1362), Program 211 of RF Government (No. 02.A03.21.0006).

Vladimir KOLOSOV (Ekaterinburg, Russia), Anton YUSHKOV, Lev VERETENNIKOV, Ilya POLOGOV

08:00 - 18:15 #6923 - MS01-601 Electron microscopy characterization of the wear of textured α-Al2O3 and κ-Al2O3-TiN multilayer coatings for cutting tool applications.

MS01-601 Electron microscopy characterization of the wear of textured α-Al2O3 and κ-Al2O3-TiN multilayer coatings for cutting tool applications.

In metal cutting applications, coatings are often applied onto cemented carbide inserts as wear and heat resistant protection layers extending the tool life. A commonly used coating on inserts is Al₂O₃ which provides chemical stability, thermal and wear protection for the cemented carbide insert. The crater wear and plastic deformation behavior in certain applications can be improved by altering the phase and/or texture of the wear protective coating.  Worn (0001)-textured α-Al₂O₃ and κ-Al₂O₃-TiN multilayer coatings on inserts which had been used in cutting applications have been investigated by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS), XRD, electron backscatter diffraction (EBSD) in the SEM and (scanning) transmission electron microscopy ((S)TEM) techniques. The microstructure of the α-Al₂O₃ coating can be seen in the SE-SEM image and its (0001)-texture is shown in the EBSD orientation map, see Fig. 1a and b. The specimens for (S)TEM characterization were prepared in a Helios NanoLab 650 instrument (FEI Company) equipped with an EBSD detector (NordlysMax², Oxford Instruments), EDXS detector (X-Max^N, Oxford Instruments) and AZtec software package (Oxford Instruments). (S)TEM characterization was performed using a Titan³ 60-300 equipped with ChemiSTEM (FEI Companay). (S)TEM characterization of the worn α-Al₂O₃ coating revealed a modified region and EDXS elemental maps showed that this region contains Mg, Ca and Si beside the Al and O, see Fig. 2. Also, small grains of Fe were observed below this modified region.  For comparison, (S)TEM characterization was performed on the worn κ-Al₂O₃-TiN multilayer coating. By determining the elemental distribution and formed phases the chemical wear mechanisms can be described.

Ernesto CORONEL (Stockholm, Sweden), Arno MEINGAST, Babak RABIEI, Jeanette PERSSON, Martina LATTEMANN

08:00 - 18:15 #6938 - MS01-603 Structural and chemical analysis of topologically close packed (TCP) phases in high generation Ni-based superalloy.

MS01-603 Structural and chemical analysis of topologically close packed (TCP) phases in high generation Ni-based superalloy.

The investigated material belongs to the new generation single crystal Ni-base superalloy family. These kind of alloys exhibit excellent high-temperature creep strength as well as oxidation and corrosion resistance [1,2]. However, due to high content of refractory elements the microstructural stability can be strongly affected by precipitation of topologically close packed (TCP) phases. [3,4] The morphology, structure type as well as chemical composition of TCP phases can significantly vary and depend on alloy’s composition and applied heat treatment. It is expected that TCP phases growth from gamma matrix, which is mainly composed of Ni, Re, Co, Ru and Cr in high generation Ni-based superalloys. For this reason the tested material was subjected to long term aging at high temperature and investigated at different states of TCP phase evolution. The aim of the study is to understand the process of TCP phase precipitation including identification of nucleation sites, the chemical composition and structure type determination at different stages of heat treatment.

     The microstructural imaging was performed using transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and scanning electron microscopy (SEM). The chemical analysis was accomplished by energy dispersive X-ray (EDX), and the TEM lamella were cut by focused ion beam (FIB) technique.

     The results were obtained from two samples which come from the parent material. The material was subjected to standard heat treatment procedure consisting of homogenization and a two step aging treatment followed by long term exposure at high temperature. Sample 1 was derived from the initial state of long term exposure, while sample 2 was obtained from the equilibrium state of TCP phase precipitation.

     Analysis of the microstructure of the 2 samples shows distinct differences between the states. The microstructure of the initial stage of long term exposure exhibits only few TCP precipitates in the early stage of precipitation, while in sample 2 the extensive formation of TCPs with plate-like morphology was observed. To fully understand the process of TCP precipitation the determination of nucleation sites is essential. Figure 1 presents a high magnification bright field STEM image taken from sample 1. In this figure it is clearly seen, that TCP starts to nucleate within a gamma channel. After long-term exposure, gamma, gamma prime as well as TCP phases strongly evolved, what is shown in Figure 2. This figure presents the equilibrium stage of TCP precipitation. It was also shown, that gamma prime phase changed its morphology from regular cubic (beginning of exposure) to rafted irregular shape (after long term exposure). In case of TCP precipitates not only the number, but also their size strongly increased. Consequently, it is expected that a change of creep and fatigue strength will occur with a changing microstructure morphology.

     The selected area diffraction analysis performed in TEM allowed to identify the structure type of individual phases as well as determination of lattice parameter change during thermal exposure. Moreover the EDX measurements showed tendency to strong segregation of Re and Ru elements into TCP phase at the expense of gamma matrix. All microstructural investigations combined with EDX chemical analysis resulted in evaluation of TCP precipitation process at the structural and chemical aspects.

 

    

References:

[ 1] A.F. Giamei and D.L. Anton, “Rhenium Additions to a Ni- base Superalloy: Effects on Microstructure”, Metal1 Tram 16A, (1985), 1997-2005.

[ 2] K. Harris, G.L. Erickson, S.L. Sikkenga, W. Brentnall, J.M. Aurrecoechea and K.G. Kubarych, “Development of the rhenium-containing superalloys CMSX-4 & CM186LC for single crystal blade and directionally solidified vane applications in advanced turbine engines”, Suuerallovs, ed. S.D. Antolovitch et al., (Warrendale, PA, USA, 1992), 297-306.

[ 3] A.K. Sinha, “Topologically Close-Packed Structures of Transition Metal Alloys”, Progress in Material Science, (1972), 79-185.

[ 4] C.M.F. Rae, M.S.A. Karunaratne, C.J. Small, R.W. Broomfield, C.N. Jones, and R. Reed, “Topologically Close Packed Phases in an Experimental Rhenium–Containing Single Crystal Superalloy,” Superalloys 2000, ed. T.M. Pollock et al., (Warrendale, Pa: TMS, 2000), 767-776.

Roman SOWA (Gniewczyna, Poland), Magdalena PARLINSKA-WOJTAN

08:00 - 18:15 #6954 - MS01-605 Examination of semiconducting properties of oxides in the vicinity of metal-oxide interfaces for selected alloys.

MS01-605 Examination of semiconducting properties of oxides in the vicinity of metal-oxide interfaces for selected alloys.

Radovan Vanta¹, Sousan Abolhassani¹, Shiv Ashish Kumar², Massoud Dadras², Adrienn Baris¹, Guillaume Boetsch³, Harry Brandenberger⁴, Andreas Rummel⁵

¹ Laboratory for Materials Behaviour, Paul Scherrer Institut, 5232 Villigen-PSI, Switzerland, ² Service of Microscopy and Nanoscopy, IMT, University of Neuchâtel, Neuchâtel, Switzerland, ³ Imina Technologies SA, EPFL Innovation Park, Bâtiment E, 1015 Lausanne, Switzerland, ⁴ Gloor Instruments AG,  Schaffhauserstrasse 121, 8301 Kloten, Switzerland, ⁵ Kleindiek Nanotechnik GmbH, Aspenhaustr. 25, 72770 Reutlingen, Germany,

This paper provides a brief overview of the studies performed on semi-conducting properties of oxides and the change of these properties, for specific materials. A direct method is developed to measure the properties of the oxide by means of micromanipulators, in the SEM.

The interest of this method is to evaluate the role of such change of properties in the vicinity of interface of metal-oxides and to correlate the properties to the oxidation behavior and also to the hydrogen uptake of the alloy.

In a previous study the properties of a material has been measured by means of micromanipulators, outside of the microscope. This study has been performed in the SEM. The present study reports the properties of two families of alloys. It shows that an alloy with a sub-stoichiometric oxide in the vicinity of the interface has a lower resistivity.  The method consists of using a fine micromanipulator installed in the SEM; a surface is created by FIB micromachining and the measurements are made by means of the micromanipulators inside the SEM, in this manner an accurate positioning is possible.

As it can be observed in Figure 1, the oxide in alloy Zr2.5%Nb is sub-stoichiometric near the metal oxide interface [1]. Figure 2 presents the results of measurements of the Zr2.5%Nb and low-tin Zircaloy-4. The results of measurements and the comparison of the two alloys show that the second alloy having a stoichiometric oxide does not show such variation in resistivity in the vicinity of the metal-oxide interface. The results will be discussed to confirm the role of the different properties of the oxide, on the oxidation behavior.

Acknowledgements: Mr. Andrej Bullemer (AHL) and Dr. Elisabeth Müller (EMF) are acknowledged for the assistance for sample preparation.

Reference: [1] Abolhassani, S., Bart, G., and Jakob, A., “Examination of the Chemical Composition of Irradiated Zirconium Based Fuel Claddings at the Metal/Oxide Interface by TEM,” J. Nucl. Mater., Vol. 399, 2010, pp. 1–12.

Radovan VANTA, Sousan ABOLHASSANI, M DADRAS (Neuchâtel, Switzerland)

08:00 - 18:15 #6962 - MS01-607 Microstructural and chemical characterization of the system CaO–Al2O3 using environmental scanning electron microscopy (ESEM).

MS01-607 Microstructural and chemical characterization of the system CaO–Al2O3 using environmental scanning electron microscopy (ESEM).

The calcium and alumina oxide is a highly interesting functional materials. In the binary compound 12CaO·7Al₂O₃ has a unique crystal structure as a nano-sized cage with free oxygen anions randomly distributed inside the cages. C12A7 has gained much attention for potential applications in various fields, such as ion conducting solid electrolyte, field and ion emitters, oxidizing catalyst and as a transparent conductive oxide (TCO) in flat panel displays, solar cells and energy conservation (smart windows) devices [1, 2]. The functional properties of electrides are strongly depend on the microstructure. There is in the literature some information about the influence of specific surface area and impurities of alumina on the sintering behaviour of an alumina material. But in this study, the objective is to determine if the various processing techniques have an influence on the microstructure evolution of materials made from alumina and calcium oxide.

Post-fabrication C12A7 samples were subjected to heat treatment using different processing methods in order to achieve desired crystal structure. The final products were prepared by sintering (1) and melted in an electric furnace (2). Crystalline and amorphous phases were obtained.

The aim of the present study was to evaluate the microstructure of CA ceramics under different melting conditions. The microstructure characteristics were analyzed by means of optical microscopy (OM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDXS) and environmental scanning electron microscopy (ESEM).

For scanning electron microscopy (SEM) examination, sintered specimens were cut and polished. ESEM allows the direct examination of specimens without a conductive metallic coating. The application of ESEM could have advantages in the study of ceramic samples due to a reduction in preparation time. The microstructures of bulk materials were observed before and after sintering, and the effect of solution processing parameters and the formation of surface defects was studied. Microstructural differences were observed for the same composition materials under the different processing conditions (Fig. 1). The morphological and compositional properties of the final C12A7 samples are highly affected by their structure and crystallisation.

References

1. S. W. Kim, Y. Toda, K. Hayashi, M. Hirano, H. Hosono, Chem. Mater., 18 (2006) 1938
2. L. Palacios, A. G. De La Torre, S. Bruque et al. , Inorg. Chem., 46 (2007) 4167

Acknowledgement

This work was financially supported by grant of the National Science Centre SONATA 8 2014/15/D/ST8/02630.

Katarzyna BERENT (Krakow, Poland)

08:00 - 18:15 #6964 - MS01-609 Discovery of pyridinic nitrogen defects and single atom spin in graphene.

MS01-609 Discovery of pyridinic nitrogen defects and single atom spin in graphene.

Nitrogen is one of the most important doping elements for carbon materials. N doping was reported to enhance the catalytic ability of carbon materials and can improve the oxygen reduction reaction efficiency. Besides, the N doped graphene are also anticipated for several applications such as n-type transistor, sensor or lithium battery. Distinct N configurations, i.e., graphitic N and pyridinic N, have been predicted to behave different electronic properties. Therefore, to precisely control the type of N doping is considered a critical issue to realize high performance graphene-based devices. Though X-ray photoelectron spectroscopy (XPS) analysis suggests distinct N 1s states for different N configuration, however, the direct link to the specific N defect is still limited by the spacial resolution. Here, we present systematic studies of atomic structures and the atomic EELS studies on graphitic N and pyridinic N defect in graphene by scanning transmission electron microscopy [1]. Figure 1a and 1b show the ADF images and the corresponding atomic models of graphitic and pyridinic N defect in graphene. The energy loss near-edge structures of the graphitic N and pyridinic N defects are shown in Figure 1c. The graphitic N K-edge shows two sharp peaks at 401.4 eV (π*) and 407.6  eV (σ*), while the π* and σ* peak of pyridinic N exhibits significant down shift to 398.0 eV and 406.6 eV.

 

Pyridinic N defects are also found highly reactive to attract individual single transition metals (TM) to the defect sites. The spin state of single TM atoms in graphene defect was studied by core-level electron spectroscopy. Figure 2a and 2b show the ADF images of single Fe atoms anchored at graphene divacancy and at the four pyridinic N defects. We found that the single Fe atom possess high spin at graphene divancacy, while the spin state can be altered to low spin when bonding to pyridinic N (Figure 2c). This work realize the controllable of spin state of an individual TM atom which can be regarded as the smallest component of spintronic devices [2].

 

 

Reference

1. Y. C. Lin et al., Nano Lett. 15, 7408-7413 (2015).
2. Y. C. Lin et al., Phys. Rev. Lett. 115, 206803 (2015).

 

Yung-Chang LIN (Tsukuba, Japan), Kazu SUENAGA

08:00 - 18:15 #7040 - MS01-611 Advanced Microstructure Characterization of β Ti-Nb-Ta-Fe Alloys obtained by Powder Metallurgy.

MS01-611 Advanced Microstructure Characterization of β Ti-Nb-Ta-Fe Alloys obtained by Powder Metallurgy.

In order to obtain advanced biomaterials, with low elastic modulus and acceptable mechanical strength titanium alloys with high contents of refractory materials are used. The addition of niobium, tantalum and molybdenum difficult the manufacturing processes of these alloys. One way to obtain these β-Ti alloys is the powder metallurgy (P/M) that allow obtaining of customized materials. Although it presents intrinsic limitations, such as porosity, lack of diffusion and the increasing of grain size with sintering parameters. The objective of this work was the microstructure characterization of phases and mechanical properties of Ti35Nb10Ta alloy with Fe additions, using transmission electron microscopy (TEM) and selected area electron diffraction (SAD). The distribution of phases and grain orientation maps were determined with an Automatic Crystal Orientation Mapping (ACOM) system installed in a FEI Tecnai F20 TEM with LaB6 gun. An ASTAR NanoMegas system was used for ACOM diffraction data acquisition. The analyzed map step was 10 nm based on a rectangular grid (400 x 200 pixels). The identification of phases and orientations are obtained through image matching between experimental diffraction patterns and calculated templates. The microstructure obtained is composed mainly by β-Ti phase (bcc) in β-stabilizers rich areas (Nb, Ta, Fe), and α+β phase´s region confirmed in TEM image (Fig. 1.a and 1.c) and SAD with orientation relationship ([0001]_a// [110]_β) in β-stabilizers poor areas (Ti rich). The a-Ti (hcp) phase occurs mainly along the grain boundaries, growing inwards. In between it is possible to identify metastable w phase in nanometric scale, confirmed by TEM image (Fig. 1.b) and by spots with orientation relationship with β-Ti matrix in the SAD zone axis [11-20]_w//[1-10]_β (Fig. 1.d). Fig. 2 shows ACOM image of Ti35Nb10Ta alloy sintered at 1250ºC with virtual bright field (BF) combined with Reliability of α+β region (Fig. 2.a) and (b) PhaseMap combined with Virtual-BF image of β (red) and α (green) region (Fig. 2.b). β-Ti phase is mainly observed, with some α+β regions (β-stabilizer poor elements concentration) and higher β-Ti stabilization with Fe addition and sintering temperature. Nanometric ω phase was observed inside β-Ti phase using TEM analysis. TEM and ASTAR provide complementary information both on phase constitution and orientation distribution in nanosized α phase precipitated inside β-stabilizer poor regions.

Conrado Ramos Moreira AFONSO (São Carlos - SP, Brazil), A. M. AMIGÓ, V.b. AMIGÓ

08:00 - 18:15 #7068 - MS01-613 Multi-technique approach to study the corrosion of plasma assisted surface treatments of 316L stainless steel.

MS01-613 Multi-technique approach to study the corrosion of plasma assisted surface treatments of 316L stainless steel.

Different plasma assisted methods can be used to nitriding, carburizing, carbonitriding or to produce a coating over stainless steels. In order to preserve their excellent corrosion resistance is important to  control  these  processes  parameters,  avoiding  the  chromium  depletion of  the  steel  matrix, produced  mainly  due  to longer  process  time  and/or higher temperatures.  This  work  has  the  aim  to analyze  the  corrosion  morphology  and  behavior  under  certain  test  conditions  using  different characterization methods. For  assessing  the  corrosion  resistance,  cyclic  potentiodynamic  anodic  tests  have  been  done.  A three electrodes cell has been used, where the counter electrode is a platinum wire, the reference electrode is a saturated calomel electrode and the treated surface sample is the working electrode. When  the  current  density  reach  an  arbitrary  value  of  200 mA/cm2,  the  potential  is  registered  as E₂₀₀,  and  the  potential  is  swept  in  reverse  direction.  This E₂₀₀ value is used as a parameter  of comparison,   obtaining   the   highest  values for   those   samples   with   the   thickest   modified   layers.   The morphology  observed  for  the nitrided  and  carburized AISI  316L  samples, consists  in  pits  located below  the  modified  layer,   and originated  in  some  non-metallic  inclusions present in the "as received" alloy.  SEM/FIB microscopy was used to identify and characterize this behavior. At present, there are  some  points  of  interest  under  research.  The  interphase  between  the modified layer  and  the  steel  seems  to  be  a  preferable  site  for  pits  to  nucleate and  grow.  Atomic probe  microscopy,  TEM  as  well  as  EBSD  were  used  to  identify  possible  chemical  and  structural singularities in that place. The connection between the interphase and the electrolyte is produced by  channels  originated  in   the inclusions,  but  not  all  of  them  nucleate  the  attack.  Different kind of inclusions have been observed and identified using TEM and EBSD.

L ESCALADA, A.s. GASCO OWENS, F. SOLDERA, M. ÁVALOS, S BRÜHL, S SIMISON (Mar del Plata, Argentina)

08:00 - 18:15 #6299 - MS02-615 Atomic Scale in-situ Studies of Catalytic Reactions between Iron Clusters and Single-walled Carbon Nanotubes.

MS02-615 Atomic Scale in-situ Studies of Catalytic Reactions between Iron Clusters and Single-walled Carbon Nanotubes.

Atomic Scale in-situ Studies of Catalytic Reactions between Iron Clusters and Single-walled Carbon Nanotubes

Kecheng Cao(1), Johannes Biskupek (1), Thomas W. Chamberlain(2), Andrei N. Khlobystov (2) and Ute Kaiser (1)

(1) Electron Microscopy of Materials Science, Central Facility for Electron Microscopy, Ulm University, Albert Einstein Allee 11, Ulm 89081, Germany

(2) School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom

 

Metal nanoparticle catalysts play an important role in the field of catalytics. Understanding the reaction between metal nanoparticles (MNPs) and carbon at the atomic scale contributes to the design of highly efficient catalysts. Single-walled carbon nanotubes (SWNTs) with the characteristics of heat resistance, cleanness and transparency areideal nano-test tubes for MNPs to study their catalytic properties in-situ. Moreover, with the aid of the powerful aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM), several kinds of MNPs are filled into SWNT and have been investigated their reactions with SWNT.The transition metals are varied along groups as well as periods of the periodic table of elements, and characterized systematically to track their interactions with the electron beam and the carbon special environment^1-4. Among them, Os and Ni nanoparticles exhibit the ability of cutting SWNT under electron beam irradiation^{2, 3}. But iron nanoparticles with Fe₃C structure are much more stable that no significant reactions are observed between Fe₃C and SWNT⁴.

In this study we present in-situ irradiation experiments of sub-nanometer sized α- Fe clusters enclosed in SWNTs by means of low-voltage AC-HRTEM. The Fe nanoparticles show different structure to the previously published experiments on Fe₃C nanoparticles⁴, and are observed to be unstable under e-beam irradiation at 80 kV. In our experiments, the particular technique of HRTEM combines imaging tool and irradiation source in one integral experiment. Here, SWNTs are initially filled with Fe₃(CO)₁₂ molecules and then irradiated by election-beam with acceleration voltage 80 kV in TEM. α- Fe nanoparticles are obtained in the lumen of SWNTs. Being irradiated, α- Fe nanoparticles keep changing shape and restructuring the geometric construction of SWNT and ultimately cut it into two parts. Additional experiments regarding the stability under e-beam irradiation at much lower voltages down to 20 kV will be carried out to study influences of knock-on damage and ionization effects. Figure 1 shows a time series of the cutting process. . Thus we confirm that two kinds of clusters, α- Fe nanoparticle and Fe₃C can be obtained by breaking the Fe₃(CO)₁₂ molecules in SWNT and they exhibits distinct catalytic activities that Fe₃C is stable but α- Fe nanoparticle can cut SWNTs. The experiments were conducted using a C_S-corrected FEI Titan 80-300 operated at 80 kV. The dose rate was 1.4*10⁶ e^-/s*nm² and the exposure time was 1.0 second.

[1] Thilo Zoberbier et.al. J. Am. Chem. Soc. 134, 3073−3079 (2012)

[2] Thomas W. Chamberlain et.al. Chem. Sci. 3, 1919–1924 (2012)

[3] Irina V. Lebedeva et.al. Naoscale, 6, 14877-14890 (2014)

[4] Thilo Zoberbier et.al. Small, DOI: 10.1002/smll.201502210 (2016)

Acknowledgment

We gratefully acknowledge the support of the “Graphene Flagship”, DFG SPP “Graphene”, the DFG and the Ministry of Science, Research and the Arts (MWK) of Baden–Wuerttemberg within the frame of the SALVE ( Sub Angstrom Low-Voltage Electron Microscopy ) project, ERC Consolidator Grants and EPSRC and NUST "MISiS" (grant K3-2015-030) for financial support.

 

Kecheng CAO (ULM, Germany), Johannes BISKUPEK, Thomas W. CHAMBERLAIN, Andrei N. KHLOBYSTOV, Ute KAISER

08:00 - 18:15 #6314 - MS02-617 In-situ TEM study of the formation of the smallest possible fullerenes on metal surfaces.

MS02-617 In-situ TEM study of the formation of the smallest possible fullerenes on metal surfaces.

The nucleation and growth of carbon on catalytically active metals is one of the most important techniques to produce a large variety of graphenic nanomaterials. The most prominent species that have been grown on metals by vapor or solid phase deposition are graphene and carbon nanotubes. Small closed cages such as fullerenes have hardly been observed to nucleate catalytically on metal surfaces. The formation of fullerene molecules, in particular under realistic growth conditions is difficult to observe at high spatial resolution. The smallest possible fullerene is C₂₀ which is a dodecahedron composed of 12 pentagons. This molecule with exteme curvature is in a hybridization state between sp² and sp³. Until now, only few studies reported the synthesis of C₂₀ due to its instability and high reactivity. Isolated cages of C₂₀ haven’t been observed to date by electron microscopy.

Here we report the nucleation and growth of spherical carbon cages, some of them corresponding to the smallest possible fullerenes starting with approximately the size of C₂₀, on metal surfaces [1]. The experiments were carried out in-situ in a transmission electron microscope (TEM) by using a heating stage. The samples were prepared on few-layer graphene suspended on standard Cu grids for electron microscopy. Different transition metal layers (Co, Fe, Ru) with thickness of 5 nm were deposited by cathodic sputtering onto the graphene layers. After an initial heating and cooling cycle of the samples, small carbon cages appeared on the graphene layers around the periphery of metallic nanoparticles. Fig.1 shows schematically how the experimental procedure was carried out. A series of examples for the observed structures is shown in Fig. 2. The contrast of these circular features closely resembles the appearance of C₆₀ in TEM images. However, the diameter of most of the observed cages ranges between 0.35 and 0.4 nm which is clearly smaller than the diameter of C₆₀ (0.7nm). No isolated cages were observed; the small cages always appeared as aggregates and in many cases as an ordered arrangement, in particular when the cages were encapsulated by a graphenic shell. The prerequisite for the nucleation of the cages was an uncovered metal surface. The cages persist after cooling to room temperature. In order to identify the elemental composition and the bonding states of the observed cages, electron energy-loss spectra with a monochromated electron beam were taken at energy resolution of 0.2 eV. To relate the observed contrast in the TEM images to fullerene-like clusters, image simulations were carried out by using the EMS (Electron Microscope Simulator) simulation program. Polymerized and unpolymerized C₂₀ were simulated. Fig. 3 shows the calculated appearance of the aggregate of three C₂₀ molecules on a monolayer of graphene. The observations are in accordance with the simulated images of polymerized C₂₀ molecules.

The nucleation of the cages occurs by the dissolution of carbon in the metal at high temperature and the diffusion through the bulk, followed by the segregation on the surface upon cooling. Since the C₂₀ cages are less stable than larger fullerenes, their formation should be driven by kinetics under non-equilibrium conditions. Due to their large curvature and inherent reactivity, the cages tend to polymerize.

[1] F. Ben Romdhane, J. A. Rodríguez-Manzo, A. Andrieux-Ledier, F. Fossard, A. Hallal, L. Magaud, J. Coraux, A. Loiseau, F. Banhart. "The formation of the smallest fullerene-like carbon cages on metal surfaces", Nanoscale, 2016, 8, 2561.

Ferdaous BEN ROMDHANE (STRASBOURG CEDEX), Julio A RODRÍGUEZ-MANZO, Amandine ANDRIEUX-LEDIER, Frédéric FOSSARD, Ali HALLAL, Laurence MAGAUD, Johann CORAUX, Annick LOISEAU, Florian BANHART

08:00 - 18:15 #6323 - MS02-619 Characterization of nanodispersed graphite in mesoporous carbon for supercapacitor applications.

MS02-619 Characterization of nanodispersed graphite in mesoporous carbon for supercapacitor applications.

Starbon®, a family of mesoporous carbonaceous materials, was recently developed at the University of York from polysaccharides (e.g. starch) [1,2]. The novelty and the advantages of these materials include cheap, green and renewable sources, low temperature carbonization processing, avoidance of harmful chemicals, and a tunability of the surface functionality from hydrophilic to hydrophobic. These properties make Starbon® an ideal candidate for applications in catalysis and material absorption [3]. Recently the tunability of the properties of these materials has been successfully extended to their functional properties by ballmixing it with graphite before the carbonization process. This resulted in an enhanced nanocomposite material (see picture) that adds to the porosity of the mesoporous carbon the conductivity of the graphite nanoflakes. This material shows promising characteristics as electrode material in electrochemical double layer capacitors (EDLC or supercapacitor). Here we present an Electron Microscopy study of this enhanced composite material, by combining Electron Diffraction, Electron Energy Loss Spectroscopy and Aberration Corrected TEM/STEM Imaging in correlation with the physical and transport properties exhibited by the materials as electrodes for supercapacitors including I-V curves, Galvanostatic charge-discharge curves and charge retention measurements.

References:
[1] P. S. Shuttleworth, A. Matharu, J. H. Clark, in Polysaccharide Building Blocks, John Wiley & Sons, Inc., 2012, pp. 271285.
[2] V. Budarin, J. H. Clark, J. J. E. Hardy, R. Luque, K. Milkowski, S. J. Tavener, A. J. Wilson, Angew. Chem.Int. Edit. 2006, 45, 37823786.
[3] R. J. White, V. Budarin, R. Luque, J. H. Clark, D. J. Macquarrie, Chem. Soc. Rev. 2009, 38, 34013418.

Acknowledgments: P.S. gratefully acknowledges the Spanish Ministry, Economy and Competitivity  (MINECO) for the concession of a Ramón y Cajal fellowship and a proyecto de I+D+I para jóvenes investigadores  (MAT2014-59674-JIN).

Leonardo LARI (York, United Kingdom), Zlatko NEDELKOSKI, Peter SHUTTLEWORTH, Gary ELLIS, Vitaliy BUDARIN, James CLARK, Vlado LAZAROV

08:00 - 18:15 #6395 - MS02-621 Colloidal Quantum-Dot Heterostructures Studied Using Aberration-Corrected Scanning Transmission Electron Microscopy.

MS02-621 Colloidal Quantum-Dot Heterostructures Studied Using Aberration-Corrected Scanning Transmission Electron Microscopy.

Complex colloidal semiconductor quantum- dot heterostructures, such as core/shell or core/crown nanoplatelets, can now be readily synthesized [1-4]. Such heterostructures significantly enhance the optical properties of the colloidal quantum dots. The optical properties of these colloidal quantum dots depend not only on the morphology of the heterostructure – i.e. the size and shape of the core-, but, more importantly on the chemical nature and the presence of a composition gradient at the heterostructure interfaces, and on the elastic deformation inside the quantum dots that is due to a lattice mismatch between the core and shell/crown materials. The study of these heterostructures through aberration-corrected Scanning Transmission Electron Microscopy (STEM) provides access to their structure down to the atomic scale. High Angle Annular Dark Field STEM images, in particular, provide direct access to the atomic structure of the nanoparticles, and through the contrast of the atomic columns ("Z-contrast” images) to their chemical nature. Therefore, atomic resolution STEM images allow one possible to precisely map the strain fields of the heterostructure. Finally, chemical information accessed through the Z-contrast can be correlated to quantitative STEM-EDX with a spatial resolution of 1 nm. The present talk summarizes such studies conducted on CdSe/Cd(Zn)S core/shell [1,2], and core/crown CdSe/CdS [3,4] and CdSe/CdTe nanoplatelets.

 

References:

[1] Core/Shell Colloidal Semiconductor Nanoplatelets
B. Malher, B. Nadal, C. Bouet, G. Patriarche, B. Dubertret
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 134 (2012) 18591-18598

[2] Colloidal CdSe/CdS Dot-in-Plate Nanocrystals with 2D-Polarized Emission
E. Cassette, B. Mahler, J.-M. Guigner, G. Patriarche, B. Dubertret, T. Pons
ACS NANO 6 (2012) 6741-6750

[3] Efficient Exciton Concentrators Built from Colloidal Core/Crown CdSe/CdS Semiconductor Nanoplatelets
M. D. Tessier, P. Spinicelli, D. Dupont, G. Patriarche, S. Ithurria, B. Dubertret
NANO LETTERS  14 (2014) 207-213

 [4] Type-II CdSe/CdTe Core/Crown Semiconductor Nanoplatelets
S. Pedetti, S. Ithurria, H. Heuclin,  G. Patriarche, B. Dubertret,
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 136 (2014) 16430-16438

Gilles PATRIARCHE (MARCOUSSIS), Konstantinos PANTZAS, Silvia PEDETTI, Michel NASILOWSKI, Mickael TESSIER, Elsa CASSETTE, Benoit MALHER, Benoit DUBERTRET

08:00 - 18:15 #6457 - MS02-623 Van der Waals heterostructures of MoSe2 and graphene studied by transmission electron microscopy.

MS02-623 Van der Waals heterostructures of MoSe2 and graphene studied by transmission electron microscopy.

       Two dimensional layered transition metal dichalcogenides (TMDs) have attracted much attention for future electronics and optoelectronics due to their unique semiconducting features [1]. Nonetheless their properties are strongly influenced by the structural and chemical atomic arrangement in these atomically thin layers. The control and understanding of the atomic structure of synthesized TMD monolayers are thus crucial to exploit the potential properties predicted and/or to be newly discovered. In addition, compared to graphene which is a mono-atomic planar structure, the structural and chemical configuration of the TMD materials can have a lot of variations and it can be a way to tune their semi-conducting features. For instance, a ternary mixture such as Mo_xW_1-xS₂ and vertical/horizontal heterostructures between TMD structures with different chemical components or with other layered structures such as graphene and boron nitride can open the possibility of unique architectures [2]. In particular, vertical heterostructures are promising building blocks for novel semiconducting future materials because these layers have no surface dangling bond and vertically stacked layers are connected with van der Waals (vdW) forces. This allows to create atomically sharp interface with a desired structure design down to single atomic layered scale. Today a lot of efforts have been made to fabricate vdW heterostructures [3].

       In this work, vdW vertical heterostructures of MoSe₂ and graphene are studied using a transmission electron microscopy (TEM). The vdW stacks are fabricated by two step growth process. First graphene is grown by conventional CVD technique on Pt substrate, then followed by MoSe₂ growth via vdW epitaxy by Molecular beam epitaxy (MBE) technique in another reactor. The direct growth approach presents various interests compared to the manual stacking, such as clean interface and large surface production. In addition, using as grown CVD graphene, the obtained stack layers can be easily transferred on appropriate substrates. The synthesized MoSe₂/graphene layers are studied from micron down to atomic scale by several TEM techniques mainly using Low Voltage Aberration Corrected (LVAC) TEM in order to understand the growth mechanism of the vdW epitaxy by MBE and the correlation between grown MoSe₂ layer and graphene substrate. Using (S)TEM techniques, abundant information on synthesized structures can be provided. Local number of layers can be determined by several STEM techniques such as STEM HAADF imaging (Figure 1) and PACBED (Partially averaged convergent beam electron diffraction) [4]. Domains in graphene and MoSe₂ layers were independently recognized together with their local orientation using diffraction information, which allowed to study the local structural relationship between MoSe₂ and graphene substrate. Figure 2 shows a TEM image of stack layer and the orientation of MoSe₂ and graphene is determined by Fourier transform shown in the inset. MoSe₂ layers are often grown oriented to graphene with small range of misorientation 0 to 5°. The edge of MoSe₂ monolayer are observed along the zig-zag line of graphene in the case of non-continuous MoSe₂ monolayers. In addition, typical line defects are observed in a continuous domain (Figure 3a). This line defect consists of a symmetrical mirror structures (Figure 3b and 3c) [5], considered to be related to the stoichiometry control during the growth. Local chemical quantitative analysis by energy dispersive X-ray spectroscopy (EDX) was also applied on MoSe_x in order to exploit the sensitivity of the measurements, which will be a powerful method applicable at multi scale to predict various defect structures influencing their stoichiometry. Finally the MoSe₂ layers grown on CVD graphene with different experimental conditions were characterized using TEM and STEM based techniques. The influence of process parameters on the atomic configuration such as line defects are studied and the crystal mosaicity in MoSe₂ monolayer related to graphene substrate will be discussed by local structural analysis with a theoretical support.

References

[1] Splendiani et al., Nano Lett. 10 (2010) p1271

[2] A. Geim et al., Nature 499 (2013) p419

[3] N. Massicotte, Nature Nanotechnology 11 (2016) p42

[4] Thesis of Y. Martin ; University Joseph Fourier, Grenoble France, 2014

[5] Lehtinen et al., ACS Nano 9 (2015) p3724

Hanako OKUNO (GRENOBLE CEDEX 9), Tuan DAU MINH, Eric ROBIN, Alain MARTY, Helene LE POCHE, Pascal POCHET, Matthieu JAMET

08:00 - 18:15 #6473 - MS02-625 Identifying and Mapping the Polytypes and Orientation Relationships in ZnO / CdSe Core Shell Nanowire Arrays.

MS02-625 Identifying and Mapping the Polytypes and Orientation Relationships in ZnO / CdSe Core Shell Nanowire Arrays.

  Core shell ZnO nanowire (NWs) heterostructures have emerged, over the past decade, as a potential building block for a large variety of nanoscale optoelectronic devices, including self-powered UV photodetectors, dye-sensitized solar cells (DSSCs), and extremely thin absorber (ETA) solar cells. These heterostructures benefit from a high absorption over the UV and visible parts of the electromagnetic spectrum through sophisticated optical processes (i.e., optically guided and radiated modes). In this work, we focused on ZnO / CdSe core shell  NW heterostructures. The ZnO NWs are typically grown by chemical bath deposition on top of a ZnO seed layer deposited by sol-gel process and strongly oriented along the polar c-axis, but with no in-plane orientation. Then, the growth of the CdSe shell was performed by molecular beam epitaxy.

  CdSe is known to crystallize into the two following polytypes: cubic zinc blende (ZB) and hexagonal wurtzite (WZ). Identifying the different polytypes of CdSe by standard characterization techniques, such as x-ray diffraction (XRD) and selected area electron diffraction (SAED) using transmission electron microscopy (TEM), as well as Raman scattering, is complicated in the present case. Both the ZB and WZ crystalline phases have very similar properties: most of the diffraction peaks and phonon modes of the ZB crystalline phase are located very closely to those of the WZ crystalline phase; similarly, the bang gap energy of both crystalline phases is almost identical. In addition to the identification of the different polytypes and potential epitaxial relationships, the spatial information on the local scale through mapping is of great interest, but requires the use and development of advanced TEM-based experiments.

  In order to address these issues, the morphology and structural properties of ZnO / CdSe core shell NW heterostructures are thoroughly investigated by field-emission gun scanning electron microscopy (FEG-SEM), XRD, Raman spectroscopy, TEM-HRTEM (Figure 1) and ASTAR  (Automated crystal phase and orientation mapping in TEM). We show the strong interest in ASTAR for identifying and mapping the different polytypes of the CdSe shell , but also for revealing the occurrence of orientation relationships with the ZnO NWs, as shown in figure 2.

 

ACKNOWLEDGEMENTS 

 

This work was partially supported by the Nanosciences Foundation of Grenoble through the project II-VI Photovoltaic and by the Carnot Institute Energies du Futur through the project CLAPE. Electron microscopy was performed at the CMTC characterization platform of Grenoble INP supported by the Centre of Excellence of Multifunctional Architectured Materials "CEMAM" n°ANR-10-LABX-44-01 funded by the "Investments for the Future" Program.  This work was also supported by the Spanish Ministry of Economy and Competitiveness under the project MAT2015-71035.

Laetitia RAPENNE (GRENOBLE), Vincent CONSONNI, Gilles RENOU, Hervé ROUSSEL, Lionel GERARD, Edgar RAUCH

08:00 - 18:15 #6506 - MS02-627 Investigations of transition metal dichalcogenides with momentum-resolved electron energy-loss spectroscopy.

MS02-627 Investigations of transition metal dichalcogenides with momentum-resolved electron energy-loss spectroscopy.

Investigations of transition metal dichalcogenides with momentum-resolved electron energy-loss spectroscopy

Michael R. S. Huang, Wilfried Sigle, and Peter A. van Aken

Max Planck Institute for Solid State Research, Stuttgart, Germany

When fast electrons pass through a thin film, various solid-state excitations occur through mutual Coulomb interactions, which convey useful information relevant to the fundamental materials properties. Measurements by momentum-dependent electron energy-loss spectroscopy (q-dependent EELS or ω - q map) directly access the dispersion relation, enabling the physical origins of the intrinsic electronic excitations to be explored [1]. This research is concentrated on anisotropic titanium diselenide (TiSe2), a two-dimensional material in the transition metal dichalcogenide group. The ω - q maps were acquired in the Zeiss sub-electron-volt-sub-angstrom microscope (SESAM), which is equipped with a monochromator and the advanced in-column Mandoline energy filter. To enhance the angular resolution, the specimen was purposely raised above the eucentric height, which significantly extends the effective camera length (ECL) beyond the originally achievable specification [2]. Figure 1 shows the characteristic dispersion of TiSe2 recorded along the momentum transfer parallel to the Γ-K direction. The specific selection of scattering vector in reciprocal space is achieved through a narrow slit well positioned in the filter entrance pupil plane. With further increase of the ECL as well as in the spectral magnification, the details of the dispersion become clearly resolved (Figure 2). Similar to other layer- structured crystals such as graphite or molybdenite (MoS2) [3,4], the two dominant features at approximately 6.5 and 19.7 eV can be interpreted as the π and π + σ plasmons, which stem from the collective oscillations of the π and π + σ valence electrons, respectively. Moreover, another weak spectral feature at a lower energy of about 1.9 eV without significant dispersive behavior was also noticed. This excitation could probably be attributed either to the interband transition or to another plasmon resonance as a result of the negative real part of the dielectric function within this regime. However, more comprehensive investigations are required for clarity.

References:
[1] H. Raether, Excitation of plasmons and interband transitions by electrons, Springer Tracts in Modern Physics, v.88 (1980), Springer, Berlin
[2] P. A. Midgley, Ultramicroscopy 76 (1999), p.91
[3] E. A. Tapt and H. R. Philipp, Phys. Rev. 138 (1965), A197
[4] K. Zeppenfeld, Optic Commun. 1 (1969), p119
[5] The research leading to these results has received funding from the European Union Seventh Framework Program [FP7/2007-2013] under grant agreement no. 312483 (ESTEEM2).

Keywords: transition metal dichalcogenides, dispersion, momentum-resolved EELS 

Huang MICHAEL R. S. (Stuttgart, Germany), Sigle WILFRIED, Van Aken PETER A.

08:00 - 18:15 #6532 - MS02-629 Advanced STEM characterisation of composition controlled MoxW1 xS2 mixed transition metal dichalcogenide alloys grown by chemical vapour deposition.

MS02-629 Advanced STEM characterisation of composition controlled MoxW1 xS2 mixed transition metal dichalcogenide alloys grown by chemical vapour deposition.

Nanoengineering of transition metal dichalcogenides (TMDs) (MX₂: M= Mo, W, Nb; X= S, Se, Te) offers exciting new prospects for the production of two‑dimensional nanomaterials with tailored properties¹. In particular, single layer TMD alloys, including Mo_xW_1-xS₂ and MoSe_2(1-x)S_2x (x=0-1), have been shown to have a compositionally modulated electronic structure^5,6,7,8, providing a tunable band gap that could be advantageous for new nanoelectronic, optoelectronic or photonic applications. Powders of such nanostructured materials may also offer improved catalytic capabilities due to optimised edge structures⁹. However, to date, synthesis of these ternary alloys has been limited to exfoliation of flakes from single crystals that are produced by vapour transport using bulk Mo, W, and S^5,6 or MoS₂⁷, offering limited prospects for large-scale manufacturing in the future. 

Here we investigate composition-controlled Mo_xW_1-xS₂ nanoflakes synthesised by atmospheric‑pressure chemical vapour deposition (CVD) using novel Mo and W containing precursors¹⁰. Conventional TEM and EDX analysis, supported by complementary XPS, where used to investigate the shape and thickness of the flakes and demonstrates that the W dopant composition can be varied from as little as a few percent (x=0.98), to over 86% (x=0.14). Through atomic-resolution annular dark field scanning transmission electron microscopy (STEM) using a Cs probe corrected JEOL ARM200F we directly observe the substitution of W atoms for Mo atoms within the MoS₂ lattice. This confirms the synthesis of alloyed dichalcogenides rather than heterostructures, with W randomly distributed throughout the nanoflakes¹¹. This new method for growth of ternary 2D TMD alloys offers improved composition control for application as industrial catalysts, while opening a new avenue for bandgap engineering of monolayers in the future.

 

1. A. Ferrari et al, Nanoscale, 7, 4598 (2015)
2. C. Huang et al, Nature Materials, 13, 1096 (2014)
3. Y. Gong et al, Nature Materials, 13, 1135 (2014)
4. X. Duan et al, Nature Nanotechnology, 9, 1024 (2014)
5. Y. Chen et al, ACS Nano, 7, 4610 (2013)
6. D.O. Dumcenco et al, Nature Communications, 4, 1351 (2013)
7. Z. Lin, APL Materials, 2, 092514 (2014)
8. Y. Gong et al, Nano Letters, 14, 442 (2013)
9. L.P. Hansen, Angewandte Chemie International Edition, 50, 10153 (2011)
10. A. Prabakaran, Chemical Communications, 50, 12360 (2014)
11. A.T. Murdock et al, manuscript in preparation

Adrian MURDOCK, Juan G LOZANO (Oxford, United Kingdom), Arunvinay PRABAKARAN, Frank DILLON, Nicole GROBERT

08:00 - 18:15 #6534 - MS02-631 Structural and chemical characterisation of novel FeS nanostructures for energy storage synthesised using a single-source precursor.

MS02-631 Structural and chemical characterisation of novel FeS nanostructures for energy storage synthesised using a single-source precursor.

Iron sulphide is an interesting material for a wide range of potential applications due to its number of possible phases, high abundance, low cost and non-toxicity [1]. In particular, Fe-S phases are capable of undergoing reversible electrochemical reactions with lithium-ions making Fe-S a potential material for anodes in lithium-ion batteries [2]. Furthermore, by engineering the electrode materials to nanometre scale, there will be (a) a larger surface area of electrode material in contact with the electrolyte per unit mass, allowing a larger flux of Li-ions between the two electrodes, resulting in faster charging and discharging of the battery; (b) shorter diffusion distances of Li-ions which will result in a higher power density; (c) a better accommodation of strain following conversion reactions, thus resulting in longer cycling life. This advantages are even further strengthened if the nanostructures are synthesised in the two-dimensional form, since then the accessible surface area of the materials is dramatically increased. Despite their superior properties, the scarce number of published reports on the synthesis on two-dimensional FeS nanostructures is remarkable [3].

Here we report structural and chemical characterisation of iron sulphide nanoparticles and two-dimensional nanosheets synthesised using a one-pot, fast and facile single source precursor method. This synthesis method offers the possibility of tailoring the design of the nanostructures with slight variations of the synthesis conditions. Conventional transmission electron microscopy (TEM), high resolution TEM, and selected area electron diffraction patterns confirmed that the nanoparticles consisted of high-crystalline quality troilite FeS; while the nanosheets are made of small crystallites with random rotations while still lying on their (0001) basal plane. Scanning-transmission electron microscopy (STEM) and electron energy-loss spectroscopy confirmed that the majority of the crystallites in the nanosheets are troilite FeS, with some residual pyrite also present; and no oxides were formed during synthesis.  Electrochemical tests also indicate that the nanosheets show much larger capacities compared with the nanoparticles.

1. V. Yufit et al.   Electrochim Acta 50, 417 (2004).
2. A. S. Aricò et al.  Nat Mater.4, 366 (2005)
3. X. Rui et al.  Nanoscale 6, 9889 (2014)

Juan G LOZANO (Oxford, United Kingdom), Frank DILLON, Andy NAYLOR, Lok Yi LEE, Chris LIPPARD, Duncan JOHNSTONE, Peter G BRUCE, Nicole GROBERT

08:00 - 18:15 #6536 - MS02-633 Morphology, structure and composition of TiO2-based nanocomposites fabricated by dealloying.

MS02-633 Morphology, structure and composition of TiO2-based nanocomposites fabricated by dealloying.

Abstract:

Nanostructured TiO₂ and many titanates are of tremendous fundamental and technological interests for a wide range of applications not only in areas such as catalysis, but also for energy or hydrogen storage because of their unique structure and large surface area [1–3]. TiO₂ and titanates in form of nanotubes, nanowires or nanobelts can be fabricated by reaction of raw TiO₂ and NaOH [4,5]. Nanostructured TiO₂ can also be directly fabricated by dealloying Al-Ti ribbon using a simple and highly efficient corrosion process [6], where the reaction solution is one of the key factors for the morphology and structure of the final product.

In the present work, several TEM techniques are employed to study the morphology, crystal structure and composition of products from Al_85at.%-Ti_15at.% ribbons dealloyed in NaOH, KOH and HCl solutions. Figure 1, 2 and Table 1 show the analytic results for the dealloyed product in NaOH solution. A polycrystalline layered structure with ~0.75 nm lattice distance is observed (Fig.1). Around 8 at.% Na were identified by EDX analysis of different areas (Table 1). In order to further study the structure of this phase, a radial distribution function (RDF) was calculated based on electron diffraction to provide the distribution of atomic distances and compared to a simulated RDF for crystalline Na₂Ti₆O₁₃ structure, which shows good agreement (Fig.2a). The determined structure is supported by XRD result in Fig.2b, which shows that this product consists of two compositions, Na₂Ti₆O₁₃ phase and some anatase TiO₂. Multivariate statistical analysis of 2500 EDX spectra of an area of 100 nm × 100 nm is used to image the Na₂Ti₆O₁₃/TiO₂ distribution and electron tomography is used to provide three dimensional (3D) information of the morphology. Comparing the structures fabricated by different dealloying solutions will lead to understand the formation mechanism and to better control nanostructures TiO₂-based compounds.

Keywords: TiO₂-based nanocomposites, Dealloying, Layer structure.

 

Acknowledgement: Wu Wang is grateful for the financial support of the China Scholarship Council (CSC) for PhD study.

 

References

[1]         Allen MR, Thibert A, Sabio EM, Browning ND, Larsen DS, Osterloh FE. Chem Mater 2010;22:1220.

[2]         Wagemaker M, Kentgens a PM, Mulder FM. Nature 2002;418:397.

[3]         Bavykin D V., Lapkin A a., Plucinski PK, Friedrich JM, Walsh FC. J Phys Chem B 2005;109:19422.

[4]         Kasuga T, Hiramatsu M, Hoson A, Sekino T, Niihara K. Langmuir 1998;14:3160.

[5]         Kasuga T, Hiramatsu M, Hoson a, Sekino T, Niihara K. Adv Mater 1999;11:1307.

[6]         Xu C, Wang R, Zhang Y, Ding Y. Nanoscale 2010;2:906.

Wu WANG (Karlsruhe, Germany), Di WANG, Xiaoke MU, Christian KÜBEL

08:00 - 18:15 #6560 - MS02-635 Direct Observation of CVD Graphene Growth and Related Surface Dynamics of Active Metal Catalysts by In-situ Scanning Electron Microscopy.

MS02-635 Direct Observation of CVD Graphene Growth and Related Surface Dynamics of Active Metal Catalysts by In-situ Scanning Electron Microscopy.

During the last three years, we have modified the set-up of a conventional scanning electron microscope in order to enable the observation of catalyst surface dynamics under controlled atmosphere and temperature. Using this instrument, we investigate chemical vapor deposition (CVD) growth of graphene on different metal catalysts. Since the experiments are performed in the chamber of a microscope, it is possible to observe a complete CVD process from substrate annealing through graphene nucleation and growth and, finally, substrate cooling in real time at nanometer-scale resolution without the need of sample transfer. The nucleation and growth of single layer graphene can be investigated at temperatures of up to 1000°C, while at the same time, surface dynamics of the active metal catalyst can be imaged and directly related to the catalytic activity (Figure 1).[1] Due to the high sensitivity of the secondary electron signal to changes in the work function and charge transfer at the surface, we are able to visualize different degrees of graphene-substrate coupling [2] as well as the stacking sequence of few layer graphene. The in situ SEM image in Figure 2a and the plot in Figure 2b illustrate the step-wise variation of the contrast that allows identification of up to 9 individual graphene layers on platinum substrates, starting with the brightest first layer in contact with the substrate. In addition, the in situ SEM images of edge misalignment between mutual layers and individual sheets provide real-time information on the evolution of the rotation angle between growing layers and formation of the stacking order. The growth behavior of graphene on nickel, copper and platinum substrates shows characteristic differences that are related to the catalytic activity and carbon solubility of the respective catalysts (Figure 3).

In the case of Cu and Pt substrates, we observe grain orientation dependent growth dynamics. Real-time imaging during growth thus allows us to directly visualize and study the catalytic activity of differently oriented surfaces.            
ESEM observations during graphene growth highlight the dynamic nature of catalysts and reveal the sensitive response of the surface to changes in the chemical potential of the gas phase. In situ scanning electron microscopy furthermore covers the spatial resolution of complementary in situ techniques that provide spectroscopic information, such as ambient pressure X-ray and Raman spectroscopy. It completes the spectroscopic data with visual information and spatially resolved chemical dynamics.

 

References:

[1] Zhu-Jun Wang et al., ACS Nano, 2015, 9 (2), 1506-1519

[2] Piran R. Kidambi et al., Nano Lett., 2013, 13 (10), 4769-4778

Zhu-Jun WANG, Gisela WEINBERG, Rober SCHLÖGL, Marc Georg WILLINGER (Berlin, Germany)

08:00 - 18:15 #6581 - MS02-637 Quantative atomic column mapping of oxygen functionalized two-dimensional Ti3C2 MXene sheets.

MS02-637 Quantative atomic column mapping of oxygen functionalized two-dimensional Ti3C2 MXene sheets.

Two-dimensional (2D) materials, in particular MXenes, are growing in interest as a result of exhibiting excellent energy storage capabilities [1]. Prone to intercalation, high surface to volume ratio as well as originating from a large family of laminar compounds, called MAX phases, offers high chemical tunability. The latter grants a high chemical versatility originating from the many possible combinations of transition metal-carbide or transition metal-nitride bonds, providing a wide range a tunable properties [2]. Additionally, surface functionalization, which naturally occurs in MXene synthesis, has been shown to further alter the properties of MXene [3]. MXene synthesis is performed through chemical etching of the nanolaminated MAX structure, resulting in removal of the A layer, which is separating the transition metal-carbide/nitride layers (MX), and the result is 2D MX layers terminated by functional groups originating from of the etching agent. To date, the most frequently studied MXene is Ti₃C₂, and the etching agent used is HF diluted in water, this leading to OH- and F-termination groups on the MXene [3]. The surface functional groups are identified as disordered after etching [4]. However, effects of thermal treatment of functionalized Ti₃C₂ has to the best of our knowledge not been directly imaged for single sheets. In order to further understand surface functionalization on MXenes at high temperatures, which is imperative for energy storage, a thermal evolution investigation of single Ti₃C₂ sheets was performed in an aberration corrected transmission electron microscope (TEM).

 

In this contribution, we present an investigation of reorganization of the surface functionalized groups on Ti₃C₂ MXene surfaces at high temperatures, employing atomically resolved scanning TEM (STEM) and a high angle annular dark field (HAADF) detector. A powder of Ti₃C₂ was dispersed in ethanol, ultra-sonicated for 1 minute and subsequently filtered on a heating holder chip. Low dose STEM annealing investigations were carried out using a DENS Solution holder in the double-corrected FEI Titan³ 60-300 located in Linköping operating at 300 kV.

 

Fig. 1 shows an atomically resolved STEM image of a single Ti₃C₂ sheet at 500 °C. A statistical analysis was performed, effectively mapping out intensities of the atomic columns, as shown in Fig. 2. A basic Rutherford model of the electron scattering was hypothesized and a Z² intensity dependence was calculated for the atomic columns and possible adatoms, values are shown in Fig. 3a. In Fig. 3b, a colormap is applied on the image in Fig.1, image intensities has been vacuum intensity substracted and normalized to a value defined for a pure Ti+C column. Fig.4 present a structural model of O functionalized Ti₃C₂ MXene based on the mapping the positions corresponding to relative intensities matching 2O adatoms in the Ti+C column. It is clear from the model that O align on top of a single Ti column forming a large hexagonal lattice, as seen in Fig. 4.

 

[1] Barsoum, M. W. (2000). Progress in Solid State Chemistry 28(1–4): 201-281.

[2] Naguib, M., et al. (2011). Adv. Mater. 23(37): 4248.

[3] Wang, X., et al. (2015). J. Am. Chem. Soc. 137(7): 2715.

[4] Xie, Y. et al. (2014) J. Am. Chem. Soc. 136: 6385.

 

The authors would like to acknowledge the funding support from the Kunt and Alice Wallenberg Foundation (KAW) for funding of the electron microscopy laboratory in Linköping. The authors declare no competing financial interest.

Ingemar PERSSON (Linköping, Sweden), Justinas PALISAITIS, Per PERSSON

08:00 - 18:15 #6590 - MS02-639 Microscopic study of TiO2 nanostructures formed by electrochemical method.

MS02-639 Microscopic study of TiO2 nanostructures formed by electrochemical method.

TiO₂ as the material exhibits properties that allow its use in various applications such as photocatalysts, solar cells, gas sensors, and biomedical applications. The typical organic solar cells are produced on transparent conductive layers covered with nanostructured TiO₂. The surface roughness is one of the key parameters for high efficiency. In that sense the nanotube arrays offer numerous advantages.

The initial Ti films were deposited by evaporation (Fig 1) or magnetron sputtering. By variation of deposition conditions, different crystal sizes, orientation and grain sizes were obtained. TiO₂ nanostructures were formed by anodic oxidation on Ti substrates in ethylene glycol based electrolytes. Anodization parameters (anodization time, applied voltage, amount of added water and ammonium fluoride), were varied during preparation of TiO₂ nanostructures. As-prepared TiO₂ films were amorphous, while polycrystalline TiO₂ anatase phase was obtained after heat treatment. The film structure and crystallinity, before and after annealing, were studied by Raman spectroscopy and grazing incidence X-ray diffraction. Scanning electron microscopy was used to determine the morphologies of prepared anodic films.

The diameter, shape and density of the nanostructures were correlated with the processes parameters. The results indicate that geometric characteristics and morphology of prepared TiO₂ critically depend on initial Ti crystal grains sizes as well as anodization conditions (Fig 2).

 

Acknowledgement

This work has been supported by European social fond ESF, Human resources development. This work has been supported in part by Croatian Science Foundation under the project (IP-2014-09-9419).

Kereković IRENA (Zagreb, Croatia), Plodinec MILIVOJ, Juraić KRUNOSLAV, Mandić VILKO, Salamon KREŠIMIR, Meljanac DANIEL, Janicki VESNA, Gracin DAVOR, Gajović ANDREJA

08:00 - 18:15 #6643 - MS02-641 Sn catalysts and Sn dopants for Ge Nanowire Growth.

MS02-641 Sn catalysts and Sn dopants for Ge Nanowire Growth.

Vapor-liquid-solid (VLS) growth of semiconductor nanowires has been extensively studied as an avenue to control composition, crystallinity, strain and doping of nanowires for potential applications in Si device processing. As evidenced by studies of the Ge nanowire/Au catalyst system, an advantage of nanowire growth as compared with bulk synthesis is the generally large departure from equilibrium, which allows for supersaturation and undercooling during growth^1,2 and formation of metastable structures and compositions.^3,4,5  Non-equilibrium growth also offers possiblities for metastable solute trapping of dopants. Here we show results using Sn both as the growth catalyst, and as a dopant in the nanowires using SnCl₄ as the dopant gas. Incorporating Sn into the nanowires offers the possibility of increasing the carrier mobility, and of achieving a direct band-gap for efficient light absorption and emission by pushing the concentration of Sn in Ge beyond the equilibrium value.⁶  Fig. 1 shows the morphology and single crystallinity of nanowires grown using Sn as the catalyst. The catalyst, formed by evaporation of thin Sn layers on Ge substrates, produces wires with typical diameters <10 nm and a <110> growth axis. The Ge-Sn binary eutectic liquid occurs at a composition that is close to pure Sn so that the expected composition of the liquid droplet from which the Ge NW grows is very tin-rich. However, the tip that remains after growth degrades rapidly in the electron beam, preventing reliable EDS analysis. Nanowires that are grown using Au catalysts, with Sn added via the introduction of SnCl₄ gas partway through the growth process, are shown in Fig. 2.  Sn is incorporated into the liquid catalyst droplet, which enlarges the catalyst and the nanowire diameter.  During end-of-growth cool-down Sn and Ge are rejected from the catalyst resulting in tapered ends, with Au remaining at the tip.  Fig. 3 and 4 show the effect of higher and lower concentrations of SnCl₄ gas flow respectively. Higher Sn concentration causes the Sn to precipitate out at the surface of the nanowires. Lower Sn concentration allows the wire to form a coreshell structure with stacking faults, presumably dislocation loops, forming in the outer region. Preliminary EDS results suggest a Sn concentration of 2-5 at%, well above the <1 at% equilibrium value. We are currently investigating the nature of the stacking faults, and the growth parameters needed to achieve high Sn concentrations while minimizing defects.

Acknowlegements:  Financial support is provided by National Science Foundation grant DMR-1206511. Part of this work was performed at the Stanford Nano Shared Facilities.

1. Kodambaka, S., Tersoff, J., Reuter, M. C., and Ross, F. M., Science 316, 729 (2007).

2. Adhikari, H., Marshall, A. F., Goldthorpe, I. A., Chidsey, C. E. D., and McIntyre, P. C., ACS Nano 1, 415 (2007).

3.  Marshall, A. F., Goldthorpe, I. A., Adhikari, H., Koto, M., Wang, Y.-C., Fu, L., Olsson, E., and McIntyre, P. C., Nano Lett. 10, 3302 (2010).

4. Sutter, E. and Sutter, P., Nanotechnology 22, 295605 (2011).

5. Gamalski, A. D., Tersoff, J., Sharma, R., Ducati, C., and Hofmann, S., Phys. Rev. Lett. 108, 255702 (2012).

6. Kouvetakis, J., Menendez, J., and Chizmeshya, A. V. G.,  Annu. Rev. Mater. Res. 36, 497 (2006).

Ann F. MARSHALL (Stanford, CA, USA), Gerentt CHAN, Andrew C. MENG, Michael BRAUN, Paul C. MCINTYRE

08:00 - 18:15 #6699 - MS02-643 Atomic-scale visualization of the growth and structure of MoS2-based hydrodesulfurization catalysts.

MS02-643 Atomic-scale visualization of the growth and structure of MoS2-based hydrodesulfurization catalysts.

Atomic-scale visualization of the growth and structure of MoS₂-based hydrodesulfurization catalysts

 

Lars P. Hansen^*1, Yuanyuan Zhu², Quentin M. Ramasse³, Christian Kisielowski⁴, Christian Dahl-Petersen¹, Michael Brorson¹ and Stig Helveg¹

 

¹Haldor Topsoe A/S, Haldor Topsøes Allé 1, DK-2800 Kgs. Lyngby, Denmark.

²Pacific Northwest National Laboratory, Richland, WA, United States

³SuperSTEM Laboratory, STFC Daresbury, Keckwick Lane, Daresbury WA4 4AD, United Kingdom

⁴Lawrence Berkeley National Laboratory, National Center for Electron Microscopy and Joint Center for Artificial Photosynthesis, Berkeley CA 94708, United States of America.

*email: lpha@topsoe.dk

 

Current environmental legislation calls for the production of clean fuels with ultra-low sulfur contents. To produce such fuels, oil refineries have to process mineral oil by catalytic hydrodesulfurization reactions. The state-of-the-art hydrodesulfurization catalysts are based on highly anisotropic MoS₂ nanocrystals as the active component [1,2]. The MoS₂ structure consists of two-dimensional S–Mo–S slabs which may be stacked to various degrees. The catalytic reactivity of the MoS₂ nanocrystals has been associated with their exposed edges and the catalysis can be further boosted by attaching promoter metals such as Co or Ni to the edges [1,2].  However, a detailed understanding of how these Co-promoted MoS₂ catalysts functions has been lacking, reflecting that information about the abundance and structure of the edge sites on MoS₂-based catalysts has not been accessible.

Recent advancements have made (scanning) transmission electron microscopy (S)TEM a powerful technique for studying industrial-style catalysts at the atomic-level [3-7]. This presentation will outline the benefit of these advancements for the study of MoS₂-based hydrodesulfurization catalysts. First, a differentially pumped electron microscope, dedicated entirely to corrosive sulfur-containing gas reaction environments, was used for time-resolved TEM imaging of the growth of MoS₂ nanocrystals during the sulfidation reaction that transforms a molybdenum oxide precursor into highly dispersed MoS₂ nanocrystals (Figure 1). Specifically, these time-resolved image series provide new information about the evolution of MoS₂ nanocrystals with different size, morphology and stacking and thus uncover mechanisms responsible for the nucleation and growth of the MoS₂ nanocrystals. These in situ observations are beneficially combined with STEM imaging and electron energy loss spectroscopy (EELS) enabling the detection of the catalytic active edge structures and the location of promoter atoms at the single atom level (Figure 2). Thus, the combined use of in situ and single-atom sensitive (S)TEM observations provides new insight into the formation of MoS₂ nanocrystals at the atomic-level that can help to develop the understanding of structure-sensitive properties in industrial-style hydrodesulfurization catalysts.

 

References:

[1] H. Topsøe et al, Hydrotreating Catalysis, vol. 11, Springer, Berlin (1996).

[2] F. Besenbacher et al, Catal. Today, 130 (2008), p. 86.

[3] C. Kisielowski et al, Angew. Chem., Int. Ed. 49 (2010), p. 2708.

[4] L. P. Hansen et al, Angew. Chem., Int. Ed. 50 (2011), p. 1015

[5] L. P. Hansen et al., J. Phys. Chem. C 118 (2014), p. 22768

[6] Y. Zhu et al., Angew. Chem. Int. Ed. 53 (2014), p. 10723.

[7] S. Helveg, J. Catal. 328 (2015), p. 102

Lars Pilsgaard HANSEN (Kgs. Lyngby, Denmark), Yuanyuan ZHU, Quentin M. RAMASSE, Christian KISIELOWSKI, Christian DAHL-PETERSEN, Michael BRORSON, Stig HELVEG

08:00 - 18:15 #6747 - MS02-645 Crystal structure of 1DCuCl@SWCNTs determined by low voltage high resolution transmission electron microscopy.

MS02-645 Crystal structure of 1DCuCl@SWCNTs determined by low voltage high resolution transmission electron microscopy.

A wide variety of metal oxides, metal halides and other materials may be introduced into opened multiwalled or single-walled carbon nanotubes (SWCNT). These experiments have permitted the study of the crystal growth of low-dimensional materials whereby the incorporated crystals are constrained to just a few atomic layers in cross-section by the internal van der Waals surface of the carbon nanotubes. The type of encapsulated crystal and its interaction with the carbon nanotube determines the electronic properties of the entire system. Change in the local crystal chemistry of incorporated crystals has been observed directly by HRTEM and has also been predicted from ab initio calculations. Therefore, the precise the composition and structure data of the encapsulated crystal is important for a correct interpretation of the filled nanotubes electronic properties.

In this work, the structure and composition of SWCNTs filled by CuCl molecules from gas-phase was characterized by High-Resolution Transmission Electron Microscopy (HRTEM), Scanning Transmission Electron Microscopy (STEM) and X-ray Energy Dispersion Spectroscopy (EDS). The measurements were carried out on a Titan 60-300 TEM/STEM microscope (FEI, The Netherlands) at acceleration voltage of 80 kV equipped with monochromator and Cs spherical aberration corrector. Characterization and interpretation of the experimental images were performed by comparison with simulated HRTEM images. The CuCl@SWCNT films were characterized by Raman spectroscopy (Ar–Kr ion laser at the wavelengths of 488 nm (2.54 eV), 514.5 nm (2.41 eV), 568 nm (2.18 eV), and 647 nm (1.92 eV)). The spectral resolution was 0.5 cm^-1.

A series of HRTEM images with different orientations of encapsulated CuCl crystal inside the nanotubes was obtained and analyzed. The chemical composition of the filled nanotubes was evaluated by EDS. It is shown that the composition ratio of copper and chlorine is 1:1. A typical fragment of filled CuCl@SWCNT is shown in Fig. 1. Fast Fourier transform was applied for the interplanar distances and angles between the vectors of reciprocal lattice estimation. The good coincidence between the calculated and experimental values of interplanar distances of encapsulated CuCl was observed for the case of NaCl type crystal lattice deformed along the [110] direction. Simulation of CuCl@SWCNT HRTEM images revealed a good correspondence of calculated and experimental data. It was found the significant changes in the Raman spectra of the CuCl@SWCNT films relative to unfilled nanotubes. The G Raman mode had a large shift (up to 20 cm^-1) and a different shape. This could be explained by efficient charge transfer between the nanotube surface and the encapsulated CuCl crystal.

The work was supported by RSF-15-12-30041.

Andrey OREKHOV (Moscow, Russia), Andrey CHUVILIN, Alexander TONKIKH, Elena OBRAZTSOVA

08:00 - 18:15 #6750 - MS02-647 Study of Thermally Tunable Coupled Magnetic Vortex Oscillators with Lorentz Transmission Electron Microscopy and Differential Phase Contrast Microscopy.

MS02-647 Study of Thermally Tunable Coupled Magnetic Vortex Oscillators with Lorentz Transmission Electron Microscopy and Differential Phase Contrast Microscopy.

Magnetic vortex oscillators are an ideal system to study the dynamics of magnetic systems at very small length scales and over a wide frequency range. Their dynamic behavior shows characteristics known from other fundamental physical systems like the harmonic oscillator [1] and is in many aspects well understood. Their lateral dimension vary from a few microns [2] down to the nanometer scale [3]. Due to their flux closure configuration they are magnetically stable and a potential candidate for high density magnetic logic devices and magnonic crystals [4]. The oscillations, which can be obtained in essentially zero external magnetic field (besides the driving field), exhibit a narrow line width and resonance frequencies starting from the MHz range up to GHz frequencies [5]. They can be excited using magnetic field pulses [6] and electric currents harnessing the Spin Transfer Torques (STT) [5].

Recently, the magnetization dynamics in neighbouring magnetic vortex oscillators coupled via their stray fields come into focus of research [7-10]. The system behaves like damped coupled harmonic oscillators. It has been shown that the dynamics of such systems is strongly influenced by the strength of the magnetostatic interaction given by the distance between the elements and the relative configuration of the core polarizations, e.g., the directions of the out-of-plane magnetization components [9].

Here we present a study of coupled vortices with Lorentz Transmission Electron Microscopy (LTEM) and Differential Phase Contrast Microcopy (DPC) at zero magnetic field. We show a novel technique to control the interaction of two or more vortex oscillators by directly influencing their resonance frequencies. The resonance frequencies depend on the saturation magnetization M_s of the magnetic material, in this case permalloy and is highly dependent on the temperature of the disk. We use Joule heating to electrically manipulate the resonance frequencies of one element to control its excitation by a second neighbouring disk. We systematically mapped the frequency response of both disks for different temperatures to fully understand the behavior of the system.

 

References
[1] S. Zhang and Z. Li, Phys. Rev. Lett. 93, 127204 (2004)
[2] T. Shinjo1, T. Okuno, R. Hassdorf, K. Shigeto, T. Ono, Science 11, 289, 930-932 (2000)
[3] A. Imre, G. Csaba, L. Ji, A. Orlov, G.H. Bernstein, W. Porod, Science 13, 311, 205-208 (2006)
[4] Anjan Barman, Saswati Barman, T. Kimura, Y. Fukuma and Y. Otani, Journal of Physics D: Applied Physics, 43, 422001 (2010)
[5] Matthias Noske, Ajay Gangwar, Hermann Stoll, Matthias Kammerer, Markus Sproll, Georg Dieterle, Markus Weigand, Manfred Fähnle, Georg Woltersdorf, Christian H. Back, and Gisela Schütz, Phys. Rev. B 90, 104415 (2014)
[6] Jung, Hyunsung and Yu, Young-Sang and Lee, Ki-Suk and Im, Mi-Young and Fischer, Peter and Bocklage, Lars and Vogel, Andreas and Bolte, Markus and Meier, Guido and Kim, Sang-Koog, Applied Physics Letters, 97, 222502 (2010)
[7] K. Yu. Guslienko, V. Novosad, Y. Otani, H. Shima, and K. Fukamichi, Phys. Rev. B 65, 024414 (2001)
[8] A. Vogel, M. Martens, M. Weigand, and G. Meier, Appl. Phys. Lett. 99, 042506 (2011)
[9] A. Vogel, A. Drews, T. Kamionka, M. Bolte, and G. Meier, Phys. Rev. Lett. 105, 037201 (2010)
[10] Satoshi Sugimoto, Yasuhiro Fukuma, Shinya Kasai, Takashi Kimura, Anjan Barman, and YoshiChika Otani PRL 106 (2011)

 

Johannes WILD (Regensburg, Germany), Michael VOGEL, Michael MÜLLER, Felix SCHWARZHUBER, Christian BACK, Josef ZWECK

08:00 - 18:15 #6762 - MS02-649 InAs1-xSbx / Al core-shell nanowire epitaxy.

MS02-649 InAs1-xSbx / Al core-shell nanowire epitaxy.

   Hybrid materials with topological classification have received immense attention in recent years and become a focal point of condensed-matter research [1-2]. This development is to a large extent driven by the search for materials hosting Majorana bound states, proposed as building blocks for topologically protected quantum computation [3]. Recently, semi- and superconducting hybrid nanowires (NW) have been shown to be a feasible material combination, fulfilling the criteria of withstanding high magnetic fields while maintaining large spin-orbit interaction of the semiconductors [4]. Prior growth studies of semiconducting InAs NWs with epitaxial superconducting Al shells have given detailed insight into the mechanisms of Al grain growth kinetics and especially how the growth evolution depends on the NW morphology [5]. In general, the quality of the interfaces have been shown to play an essential role in a variety of nanostructured device applications, ranging from photovoltaics to quantum transport, and is therefore an important hybrid material characteristics.

   In this study we present compositional and structural control of InAs_1-xSb_x NWs grown by molecular beam epitaxy (MBE), ranging from pure InAs to pure InSb. We show that the hybrid system, InAs_1-xSb_x with epitaxial Al, offers new possibilities to form alternative epitaxially matched interfacial domains controlled by the Sb influence on the lattice parameter and the broken bond energy of the facets. By changing the molar fraction of Sb, the electrical properties of the semiconducting core can be tuned to desirable properties, which may be relevant for designing topological superconducting materials. An example of the InAs_1-xSb_x/Al hybrid system is shown in figure 1 with x = 0.34. Also presented in this study is a method to grow long and pure wurtzite (WZ) InAs_1-xSb_x NWs having a surprisingly high degree of epitaxial match between Al and the NWs.  

 

References

 [1] Hell, M., et al. ” Time scales for Majorana manipulation using Coulomb blockade in gate-controlled superconducting nanowires”,arXiv:1601.07369[2] Beenakker, C. W. J. (2013). Search for Majorana Fermions in Superconductors.Annual Review of Condensed Matter Physics, Vol 4. J. S. Langer. Palo Alto, Annual Reviews. 4: 113-136.

[3] Alicea, J., et al. “Non-Abelian statistics and topological quantum information processing in 1D wire networks.” Nature Physics 7, 412–417 (2011)

[4] Van Weperen, I., et al. "Spin-orbit interaction in InSb nanowires." Physical Review B 91(20). (2015)

[5] Krogstrup, P., et al. "Epitaxy of semiconductor-superconductor nanowires." Nature Materials 14(4): 400-40. (2015)

Thomas KANNE (Frederiksberg, Denmark), Aske GEJL, Joachim SESTOFT, Erik JOHNSON, Søren SIMONSEN, Jesper NYGÅRD, Peter KROGSTRUP

08:00 - 18:15 #6867 - MS02-651 The oxidation of gallium (II) sulphide.

MS02-651 The oxidation of gallium (II) sulphide.

Introdution

Gallium (II) sulphide (GaS) is a III-VI layered semiconductor, which has recently been exfoliated using liquid-phase exfoliation. As a wide-gap semiconductor it has potential in a range of applications including photodetectors, non-linear optics, optoelectronics, Li ion battery anodes, but also as a catalyst for hydrogen evolution. However, early devices fabricated using GaS stopped functioning after a few weeks. Upon close inspection it was noted that the edges of exfoliated flakes were seen to have oxidised very shortly after exfoliation, and become amorphous, but the cause of device failure was unclear. 

Methods

A combination of ab initio Density Functional Theory (DFT) calculations and High Resolution Scanning Transmission Electron Microscopy (HRSTEM) has been used to study the initiation and progression of oxidation from two potential species: O₂ and H₂O - the latter of particular importance due to potential effects on hydrogen evolution. DFT was implemented in the CP2K code, modelling GaS as monolayer nanoribbons with various edge terminations to explore potential reaction sites and pathways.

Experimentally, GaS was exfoliated in IPA using ultrasonication, resulting in flakes several layers thick. The dispersion was drop-cast onto Au TEM grids, and then aged in two environments: ambient and under de-ionised water, to explore the effect we have observed stark contrasts between flakes exposed to air (Figure 1) and those exposed to water (Figure 2). During ageing, they were investigated with HRSTEM combining high spatial resolution imaging with pixel-by-pixel energy dispersed X-ray (EDX) mapping.

Results

O₂ seems to react with the edges of the flake, substituting into sulphur sites, resulting in a loss of crystalline structure. However this reaction, while initially exothermic at all edges, ceases progressing the oxidation after the first few nm of the edge leaving the rest of the flake pristine, even after over 100 days.

Exposure to DI H₂O, on the other hand, resulted in significant changes after only a couple of weeks. The resulting HAADF contrast was mottled, with small crystallite regions remaining. The brighter regions (corresponding to thicker regions) contain higher concentrations of sulphur, but seem localised more towards edges, although overall there is an increase in the amount of sulphur relative to gallium. This suggests that GaS undergoes a reaction with water resulting in the loss of gallium from the flakes, and our theory is that remaining sulphur redeposits at the step edges. 

Edmund LONG (Dublin, Ireland), Clotilde CUCINOTTA, Andrew HARVEY, Clive DOWNING, Stefano SANVITO, Valeria NICOLOSI

08:00 - 18:15 #6884 - MS02-653 High-resolution TEM study of colloidal cesium lead bromide nanocrystals.

MS02-653 High-resolution TEM study of colloidal cesium lead bromide nanocrystals.

Cesium lead halide perovskites, of the type CsPbX₃ (X=Cl, Br, I), are promising candidate materials for optoelectronics, solar devices and high-energy radiation detection [1-6]. Colloidal CsPbBr₃ nanocrystals with morphology of nanocubes [1], nanoplatelets [2-3] and nanowires [4] have been successfully synthesized in the last few years. Orthorhombic, tetragonal, and cubic phases were reported for bulk CsPbBr₃, the cubic phase being the high-temperature one [6]. The majority of CsPbBr₃ nanocrystals were reported to have cubic phase, e.g.  nanocubes [1] and nanoplatelets [2-3]. On the other hand, nanowires [4] and also nanocubes [5] exhibited orthorhombic phase, the latter ones despite their cubic shape. Here a study of colloidally grown CsPbBr₃ nanocubes (NCs), with edge length of about 40 nm, and large nanosheets (NSs) using high-resolution TEM (HRTEM) and selected area electron diffraction (SAED) shows that nanocrystals with both morphologies share the same orthorhombic phase (ICSD # 97851, see Figure 1). The observations were carried out by using an image Cs-corrected JEOL JEM-2200FS TEM (accelerating voltage = 200 kV). The lattice parameters, a’,  b’, c’, recalculated with respect to the ideal perovskite(a/ , b/ , c/2) for bulk orthorhombic CsPbBr₃ are very similar at room temperature (298 K) and further decreasing of temperature below the transition temperature 361 K increases the discrepancy between them. Here we performed HRTEM and SAED study for CsPbBr₃ NCs at various lower temperatures (Figure 2). The results have shown that the spacing among closely spaced peaks (298 K) increases at low temperature (153 K), indicating a larger discrepancy among the lattice parametes (see the arrows labelled in Figure 2). The orthorhombic CsPbBr₃ NCs and NSs exhibit significantly different facetting: the NCs are enclosed by {1-10}, {110} and {001} planes (Figure 3(a,b)). However, in the growth condition of NSs, the {001} planes are strongly passivated and the growth along [001] is inhibited. The growth in the plane of (001) leads to formation of large nanosheets confined in [001], and extended at the plane (001) and enclosed by {1-10} and {110} (Figure 3(c,d)).

References

[1] L. Protesescu et al., Nano Lett. 15, 3692 (2015)

[2] Y. Bekenstein et al., J. Am. Chem. Soc. 137, 16008 (2015)

[3] Q. Akkerman et al., J. Am. Chem. Soc. 138, 1010 (2016)

[4] D. Zhang et al., J. Am. Chem. Soc. 137, 9230 (2015)

[5] A. Swarnkar et al., Angew. Chem. Int. Ed. 54, 15424 (2015)

[6] M. Rodová et al., J. Therm. Anal. Calorim. 71, 667 (2003)

 

Acknowledgement: The research leading to these results has received funding from the European Union 7th Framework Programme under Grant Agreement No. 614897 (ERC Consolidator Grant “TRANSNANO”).

Zhiya DANG (Genova, Italy), Rosaria BRESCIA, Quinten AKKERMAN, Javad SHAMSI, Mirko PRATO, Liberato MANNA

08:00 - 18:15 #6885 - MS02-655 Low Voltage Imaging of Defects in 2D-Quantum Materials.

MS02-655 Low Voltage Imaging of Defects in 2D-Quantum Materials.

The discovery of extraordinary new quantum materials with striking properties has caused great excitement, and promises to transform signal processing and computation. We have performed integrated research on three materials (1) Graphene (G) - electrons that move as massless particles at a constant speed; (2) Topological Insulators (TI) - mobile surface electrons with spins fixed to the direction of motion; and (3) Nitrogen-vacancy (NV) Centers in Diamond - a single spin stores a bit of quantum information.  Remarkably, the quantum phenomena displayed by these materials persists at room temperature, changing the rules for signal processing and computation and opening the way for quantum electronics.   

 Defects in materials effect the propagation of electrons and holes in graphene and topological insulators act in ways that are totally unlike carriers in conventional semiconductors - they move like two-dimensional (2D) massless, ultra-relativistic electrons, except their speed is much less than the speed of light. Because there is no bandgap, an electron can pass through a potential barrier by temporarily turning into a hole, dramatically reducing scattering and improving coherence. In addition, for topological insulators the direction of the spin of a surface electron is tied to its direction of motion, providing an ideal means to transport spin information. 

We have imaged and characterized high quality graphene-like materials, such as hexagonal boron nitride (hBN) and hybrid graphene-hBN structures (Fig. 1). Compared with mechanical exfoliation, CVD synthesis [1-2] can provide larger areas, with wafer-scale monolayer or multilayer graphene sheets. Aberration-corrected electron microscopy has been used to characterize MBE-grown films with high resolution at low beam voltages (40 & 80kV) to directly visualize structural defects and relate them to performance.   

We use a Cs corrected Zeiss Libra TEM to investigate chemical vapor deposition (CVD) graphene with added copper and mercury defects. With TEM we address the question, where the Hg and Co atoms are placed on the graphene. At the same time, we observe the effect of the copper and mercury on the pi electrons in graphene with Raman spectroscopy. Furthermore, we are interested in graphene based hybrid structures, such as graphene oxide embedded in a vanadium pentoxide nanofiber matrix (Fig. 2). The graphene sheets and the nanofibers have approximately the same thickness, leading to a material with enhanced mechanical performance in comparison to pure vanadium pentoxide and pure graphene oxide sheets.

Application of Low-Voltage Electron Microscopy and its development and future directions will be presented.

.

References:

[1] A. Reina, X. Jia, J. Ho, D. Nezich, H. Son, V. Bulovic, M.S. Dresselhaus, & J. Kong,Nano Lett. 9, 30–35 (2008).

[2] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.H. Ahn, P. Kim, J.Y. Choi, and B.H. Hong,  Nature 457, 706-710 (2009).

[3] This work was supported by the STC Center for Integrated Quantum Materials, NSF Grant No. DMR-1231319.

David BELL (Cambridge, USA), Felix VONCUBE

08:00 - 18:15 #6896 - MS02-657 Seeing nanostructures from a new angle – tomography in a transmission electron microscope.

MS02-657 Seeing nanostructures from a new angle – tomography in a transmission electron microscope.

Research interest in low dimensional materials has expanded immensely in the last decade. The fascinating properties of nanotubes, graphene and transition metal dichalcogenides (TMDCs) has pushed this research toward applications in composites, (opto)-electronics and photovoltaics.

 

Seeing and understanding the physical and chemical structure of these nanostructures is a vital aspect of this growing research area. Using rotational tomography on low dimensional TMDCs, we reveal additional points of view that are not routine in transmission electron microscopy (TEM). Bright field TEM highlights the non-uniformity of tungsten disulphide (WS₂) nanotube tips, as well as revealing structural deformities in the nanotubes as a whole. Complimentary to this, we present high-angle annular dark field (HAADF) scanning TEM tomography, which provides clearer structural interpretation through fewer Bragg scattering events being detected. 2D nanosheets are also characterised via high resolution TEM tomography, allowing for a three dimensional view of a 2D object. As the sample is rotated, further lattice and thickness information is acquired.

 

Having these complimentary tomographic methods available for nanomaterial characterisation will help to improve the accuracy of interpretation and results. Often, 2D TEM images lead to assumptions about the nature of these materials. Tomography at high resolution will reveal more information on the shape and structure of the studied nanomaterials (e.g., ripples) in 3D.

Eoghan O'CONNELL (Limerick, Ireland), Yina GUO, Robert YOUNG, Florian WINKLER, Beata KARDYNAL, Ursel BANGERT

08:00 - 18:15 #6900 - MS02-659 Measuring flexural phonon spectrum of graphene by electron diffraction.

MS02-659 Measuring flexural phonon spectrum of graphene by electron diffraction.

Graphene surface is always corrugated, particularly, when it is not in contact with a substrate [1]. The main reason for this are out-of-plane thermal oscillations (so-called flexural phonons) whose amplitude reaches rather high values in suspended 2D crystals [2]. Studying suspended sheets is of great importance because crystal relief gives us information on the very intrinsic properties of graphene in this case.  Contrariwise, graphene being in contact with a substrate either repeats its relief [3] or demonstrates a stationary state of the flexural phonon spectrum frozen by the interaction with the substrate and, thus, significantly distorted [4]. At the same time, suspended graphene shows the highest charge carriers mobility values and, thus, is the most prospective material for high-speed electronics [5]. And flexural phonons are the main limiting factor of free-standing graphene conductance [6], thus, investigating them is crucial for the developments in this field.

However, it is almost impossible to measure a suspended 2D crystal relief by direct techniques like atomic-force or scanning tunneling microscopies because of the very high flexibility of the ultrathin sheets. Particularly, it is so for the dynamic, rapidly varying relief caused by the thermal oscillations.

Here we present a technique for measuring flexural phonon spectrum in suspended graphene. The technique is based on analysis of electron diffraction patterns [7]. The technique utilizes intensities variations around a diffraction spot observed when crystal is being tilted with respect to the incident electron beam. The measured intensities in the vicinity of a diffraction spot (Fig. 1) vary in different manner depending on the distance from the spot (Fig. 2). A mathematical model of a corrugated 2D crystal representation in the reciprocal space is developed. This model shows a way to extract flexural phonons spectral amplitudes from the experimental data. The main difficulty arises from the relatively broad tails of the microscope point-spread-function, which is defined by the spatial coherency of the electron beam in the case of diffraction imaging. A couple of spectrum reconstruction techniques are suggested which allow measuring the spectrum in a relatively wide range of wave-vector lengths (0.4-4 nm^-1 is achieved in our case).

The directly measured spectrum and reconstructed by different techniques are shown in Fig. 3. The most remarkable is the q^-4 dependence at the right hand part, which is in agreement with the theoretical predictions. This allows direct measuring of the bending rigidity of suspended graphene. At the same time, the dependencies at smaller wave-vectors are considerably weaker than the predicted. This may be a sign of a more significant influence of charge carriers interaction with the lattice distortions on the flexural phonons dynamics. We performed a simulation of the point-spread-function effect on the measurements and also checked the validity of the suggested reconstruction techniques (Fig. 4) and found it all in agreement with our experimental data.

Finally, we introduce a technique for measuring flexural phonon spectrum in suspended graphene. The obtained dependence of the spectral amplitude on the wave-vector can be directly compared with the results of theoretical simulations, which are quite abundant in this field (while, there is a lack of such straightforward experimental data). The found spectrum profile raises questions on the role of different mechanisms involved in the flexural phonon dynamics. Moreover, the technique is applicable to studying of suspended 2D crystals of any other types that can be of great importance in the corresponding fields.

Acknowledgments

The author thanks RFBR (grant no. 16-32-60165) for the partial support of this work and Joint Research Center ‘Materials science and characterization in advanced technology’ with financial support by Ministry of Education and Science of the Russian Federation (Agreement 14.621.21.0007, 04.12.2014, id RFMEFI62114X0007) for the use of their equipment.

References

1. J.C. Meyer, A.K. Geim, M.I. Katsnelson, K.S. Novoselov, T.J. Booth, S. Roth. Nature, 2007, 446, 60-63.

2. A. Fasolino, J. H. Los, M. I. Katsnelson. Nature Materials, 2007, 6, 858-861.

3. C.H. Lui, L. Liu, K.F. Mak, G.W. Flynn, T.F. Heinz. Nature, 2009, 462, 339-341.

4. V. Geringer, M. Liebmann, T. Echtermeyer, S. Runte, M. Schmidt, R. Ruckamp, M. C. Lemme, M. Morgenstern. Phys. Rev. Lett., 2009, 102, 076102.

5. A.S. Mayorov, D.C. Elias, I.S. Mukhin et al. Nano Letters, 2012, 12, 4629-4634.

6. S. Das Sarma, S. Adam, E. Rossi. Rev. of Mod. Phys., 2011, 83, 407-470

7. D.A. Kirilenko, P.N. Brunkov. Ultramicroscopy, 2016, (available on-line, DOI: 10.1016/j.ultramic.2016.03.010)

Demid KIRILENKO (St-Petersburg, Russia)

08:00 - 18:15 #6901 - MS02-661 Low energy electron beam induced cleaning of graphene layers in SEMs.

MS02-661 Low energy electron beam induced cleaning of graphene layers in SEMs.

In order to examine mutually overlapped flakes of two-dimensional crystals such as graphene with an electron microscope we need to obtain a contrast contribution from a single layer of carbon atoms. This task requires increasing the scattering rate of incident electrons by means of a drastic lowering of their energy to hundreds of eV or less.

The effect of airborne contaminants, mainly hydrocarbons, on graphene layers distinguishing in low energy S(T)EMs can be significant and due to their removing from the surface by appropriate in-situ methods is very important from practical point of view. During the scanning of the surfaces by electrons, the image usually gradually darkens because the hydrocarbon layer is deposited on the top. This effect can be described as an electron stimulated deposition, the thermal diffusion of organic molecules around the irradiated area is use as a source of building atoms, precursor. On the other hand, the effect of electron stimulated desorption occurred at the same time, especially at low observation energies, and then depends which process, deposition or desorption, is dominated. Our experiments have showed the fact that prolonged bombardment with electrons in a range of hundreds or even tens eV gradually increases the transmissivity (and decreases the reflectivity) of graphene due to the removal of adsorbed gas molecules providing an ultimate cleaning procedure evidently leading to an atomically clean surface. A sample such as graphene enables one to distinguish this kind of cleaning from radial damage, so this result opens up new possibilities for certain surface studies performed without ultrahigh vacuum.

Experiments have been performed in a FEI SEM/STEM microscope equipped by beam deceleration mode on free standing graphene (Ted Pella^®) and the effects of landing electron energy, samples biasing, electron dose, heating, etc. on the cleaning efficiency of electrons was studied in details. Moreover, the structural damage of the graphene after extremely high electron doses was observed, and the critical values for selected impacted energies were also calculated.

Acknowledgment

The work was financially supported from the European Commission for the Marie Curie Initial Training Network (ITN) SIMDALEE2: Grant No. 606988 under FP7- PEOPLE-2013-ITN.

Eliska MIKMEKOVA (Eindhoven, The Netherlands), Ludek FRANK, Ilona MULLEROVA, Seyno SLUYTERMAN

08:00 - 18:15 #6904 - MS02-663 Gap measurements via low-loss EELS on atomically thin MoxW(1-x)S2 nanoflakes.

MS02-663 Gap measurements via low-loss EELS on atomically thin MoxW(1-x)S2 nanoflakes.

The properties of alloyed materials are a fundamental issue in Materials Science. For years, layered semiconductors of the TX₂ type (T=Mo, W; X=S, Se, Te) have been the subject of a varied range of studies due to their interesting electric, optical, catalytic and structural properties. In this sense, Mo_xW_(1-x)S₂ alloys have been recently reported [1,2]; but most of their properties haven’t been delved into yet. In this contribution, we focus on the local optoelectronic properties of such atomically thin (up to 6-8 layers) of Mo_xW_(1-x) S₂ . These properties have been probed by low-loss EELS measurements [3] and we have examined the bandgap behavior for different alloying degrees of such nanomaterials as well as a function of the number of layers. 

These works have been carried out using a FEI Titan Cs probe-corrected microscope equipped with a monochromator (working at 80 KV and with an energy resolution of ~180 meV).

Figure 1 displays three HRSTEM-HAADF micrographs of three different monolayers of MoS_2, Mo_0.5W_0.5S₂ and WS₂ samples, respectively. The alloying effects of such materials can be easily distinguished from the HAADF image corresponding to the Mo_0.5W_0.5S₂ sample (Fig. 1(a) second image from the left). The areas of reduced number of layers have been selected via optical and low-magnification TEM images and identified for the low-loss measurements, see Fig. 1(b).

            Low-loss EEL spectra, using the spectrum-line mode, have been recorded in regions where different stacks of a few layers can be noticed, see Fig. 2. In each of these stacks, easily recognisable for being the flat regions in the HAADF intensity profile (Figure 2(b)), several spectra have been integrated over a window of 10 to 12 nm. After zero loss peak (ZLP) extraction, the different spectra are fitted to obtain the band-gap value in each of these zones. In parallel, the thickness of these areas has been estimated, using the standard procedure [4]. Finally, these results show a relation between the band-gap of the material and the number of layers for every composition (Figure 2(c)).

All these results will be deeply discussed in the framework of previous experimental (photoluminescence) and theoretical (DFT calculations) works carried out in these material [1,2]. In conclusion, the present studies improve our knowledge of the optoelectronic properties of atomically thin layered alloys of dichalcogenides and provides further insight into the potential applications of these materials.

 

[1] D.O. Dumcenco, H. Kobayashi, Z. Liu, Y.S. Huang, K. Suenaga, Nature Comm. 4, 1351 (2013).

[2] Y. Chen, J. Xi, D.O. Dumcenco, Z. Liu, K. Suenaga, et al., Acs Nano 7, 4610-4616 (2013).

[3] R Arenal, O Stephan, M Kociak, D Taverna, A Loiseau, C Colliex, Phys. Rev. Lett. 95, 127601 (2005).

[4] R.F. Egerton, Electron energy-loss spectroscopy in the electron microscope, Springer, New York, 2011.

 

Acknowledgements:

This work was supported by the project ESTEEM2 (Integrated Infrastructure Initiative - I3, Grant Agreement 312483), the Spanish MINECO (FIS2013-46159-C3-3-P) and from the EU under Grant Agreement 604391 Graphene Flagship. Low-loss EELS studies were developed at the Advanced Microscopy Laboratory (LMA) of Institute of Nanoscience of Aragon (INA) - U. of Zaragoza (Spain).

 

Mario PELAEZ FERNANDEZ (Zaragoza, Spain), Kazu SUENAGA, Raul ARENAL

08:00 - 18:15 #6929 - MS02-665 Structural and Compositional EELS Studies on Doped Carbon Nanostructures used as Cold Field Emitters.

MS02-665 Structural and Compositional EELS Studies on Doped Carbon Nanostructures used as Cold Field Emitters.

Nowadays, more and more microscopes are equipped with cold field emission guns (C-FEG) due to their higher performance. For further improvement of C-FEGs, new emitter materials need to be explored. In this sense, carbon nanotube (CNTs) related materials are very promising candidates as cold field emitter [1]. However, more recently, carbon cone nanotips (CcNT) [2] have been considered as one of the most applicable form of carbon as field emitter, answering most of the technological problems of CNTs installation in a C-FEG [3,4]. For a higher performance of C-FEG, the electronic band structure of carbon nano-objects (tube or cones) can be modulated through the introduction of heteroatoms into the graphene lattice leading to a lower work function [5,6]. This work is devoted to the doping of these carbon nanostructures by nitrogen and/or boron, the deep study of their structure and atomic composition, and to the evaluation of their cold field emission characteristics. In this contribution, we will focus on our recent results related to doping of carbon nanotubes. HRTEM imaging, spatially-resolved EELS and X-ray photoelectron spectroscopy (XPS) studies have been developed on these NTs. After heating treatments, there is no significant structural modification in the CNTs (Fig.1). C-B and C-N bonds are identified at macro scale by XPS (Fig.2 (a)) and at local scale by EELS-STEM (Fig.2 (b), (c)). DFT calculations are also carried out and point out how the CNTs are affected by BN dopants. All these results will be discussed in depth in this contribution.

References:
[1] N. de Jonge et al, Nature 420 (2002),p. 393–395.
[2] R. L. Jacobsen, M. Monthioux, Nature 385 (1997), p. 211-212.
[3] F. Houdellier et al, Carbon 50 (2012), p. 2037-2044.
[4] F. Houdellier et al,Ultramicroscopy 151 (2015), p. 107-115
[5] P. Ayala et al, Reviews of modern physics 82 (2010), p. 1843-1885.
[6] R. Arenal, X. Blase, A. Loiseau, Advances in Physics 59, 101 (2010).

Acknowledgements:
This work was supported by the project ANR LASCAR (ANR-13-BS04-0007), by the European Union Seventh Framework Program under Grant Agreement 312483 – ESTEEM2 (Integrated Infrastructure Initiative – I3), and by the international associated laboratory TALEM (CNRS - U. of Zaragoza). TEM studies were developed in the Advanced Microscopy Laboratory (LMA) of Institute of Nanoscience of Aragon (INA) - U. of Zaragoza (Spain). R.A. acknowledges funding from the Spanish MINECO (FIS2013-46159-C3-3-P), and from the EU under Grant Agreement 604391 Graphene Flagship.

Rongrong WANG, Masseboeuf AURÉLIEN (Toulouse), David NEUMEYER, Marc MONTHIOUX, Alejandro LOPEZ-BEZANILLA, Raul ARENAL

08:00 - 18:15 #6931 - MS02-667 Atomic Scale Characterization on III-V Based Heterostructure Nanowire Interfaces.

MS02-667 Atomic Scale Characterization on III-V Based Heterostructure Nanowire Interfaces.

Quasi-one-dimensional III-V semiconducting nanowires attract enormous attention owing to their physical properties such as tunable direct bandgap, high surface to volume ratio, high carrier mobility, tunable structures. Hence they have  potential application in next generation electronics, sensors, photonics and photovoltaics. Understanding the details such as, atomic scale structure, local chemical stoichiometry and defects in a sub nanometer scale are inevitable when nanowires are intended for devices.

A systematic interfacial investigation on molecular beam epitaxy grown III-V based nanowires such as InAs, GaAs,^[1] in general and in particular, heterostructure nanowires of axially grown Ga_xIn_{1_x}As-InAs and InAs-InAsSb will be presented.^[2] In this work, atomic scale structural interfaces and interfacial chemical composition are analyzed using advanced aberration corrected transmission electron microscopy.  Atomic resolution high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and high resolution energy dispersive x-ray spectroscopy techniques are predominatly used for characterizing the grown nanowires. Our work demonstrates on tailoring the growth, periodicities and the stoichiometry of III-V nanowire heterostructure containing single/multiple electronic barriers involving In,Ga,As and Sb. We aim to provide understanding on the growth of dissimilar interfaces in heterostructure nanowire (see fig 1). In the InAs-InAsSb heterostructure wires,^[1] our work demonstrates the quality of the misfit strained interface in terms of structural abruptness and chemical homogeneity. 

References

1. S. Venkatesan, P. Krogstrup, C. Liebscher, G.Dehm, (Unpublished)

2. S. Venkatesan, M.H. Madsen, H. Schmid, P. Krogstrup, E. Johnson and C. Scheu, Appl. Phys. Lett. 103, 063106, (2013).

Sriram VENKATESAN (Duesseldorf, Germany), Peter KROGSTRUP, Christina SCHEU, Christian LIEBSCHER, Gerhard DEHM

08:00 - 18:15 #6966 - MS02-669 Beam exfoliation of MoS2 layers with a helium ion beam.

MS02-669 Beam exfoliation of MoS2 layers with a helium ion beam.

Precise modification of two dimensional (2D) materials with high resolution will be instrumental in future device fabrication. Such a method is presented here for the removal of layers of MoS_2. The thickness of selected regions of few layer MoS₂ has been altered with a sub-nanometre beam of He⁺_. This ‘beam exfoliation’ of MoS₂ has been characterised and controlled. An investigation of annealing and of the modified regions is now underway. 

MoS₂ is a material in the transition metal dichalcogenide (TMD) family which undergoes a transition from indirect to direct bandgap when going from multilayer to monolayer. This makes it ideally suited to applications in digital electronics as well as numerous photonic applications such as light emitters, photodetectors and solar cells. Excellent mechanical flexibility also provides potential for use in flexible electronics. Even more interesting than the superlative physical properties of 2D materials is the ability to tune these properties with precision. Considerable alteration of MoS₂ behaviour is already possible by tailoring the crystal structure, stoichiometry and geometry. Since there is such a significant difference between the behaviour of single layer and multilayer MoS₂, thinning of MoS₂ has been reported in a number of different manners. These include laser ablation and Ar⁺ plasma etching. 

Few-layer molybdenum disulphide (MoS₂) was obtained by mechanical and liquid exfoliation and transferred to a TEM grid. It was irradiated with He⁺ at 30 keV in a number of different configurations . In the images presented, thinning is observed in the bright regions where the beam did not penetrate the material.

Figure 1 is a 300kV TEM image which shows a region with intact crystal structure demonstrating a thinned front progressing towards a fully milled region. In addition, the crystal structure is clearly intact.

From previous work, it is known that few layer (one to roughly five or six layers) MoS₂ can be milled (cut through completely) with a minimum dose in the approximate range of 10¹⁷-10¹⁸ ions cm^-2. In order to perform rudimentary characterisation on the thinning of material SEM was used to measure the minimum helium ion dose at which altered contrast -and it is implied, thinning- could be observed (of course this does not exclude other possible effects on the complex systems of electron microscope contrast). A variety of SEM operation modes were tested to determine which was the most sensitive to the changes (secondary electron (SE2), Inlens, bright field STEM and dark field STEM). It was found that dark field STEM exhibited the sharpest change in contrast with respect to dose as indicated in figure 2.

Finally figure 3 is a schemtic diagram of the thinning process.

 

Hongzhou ZHANG, Pierce MAGUIRE (Dublin, Ireland), Daniel S FOX, Yangbo ZHOU

08:00 - 18:15 #7042 - MS02-671 Exchange-coupled Sm–Co/Co thin layers; structural and magnetic investigations.

MS02-671 Exchange-coupled Sm–Co/Co thin layers; structural and magnetic investigations.

Exchange-coupled SmCo/Co hard/soft magnetic bilayers with different thicknesses of hard and soft phases were deposited on (100) silicon substrate, using Ultra High Vacuum (UHV) evaporation. Structural properties were investigated by transmission electron microscopy. Magnetic properties were measured, at room temperature, by Polar Magneto-Optical Kerr Effect. Hysteresis loops show single-phase magnetic behavior suggesting a strong exchange coupling between hard and soft phases. The magnetization ratio M_r/M_s and coercive field H_c were found to increase with increasing hard layer thickness. The highest remanence (M_r/M_s = 1) and coercivity (H_c =670 Oe) were obtained when SmCo and Co thicknesses are respectively 2.5 and 0.5 nm. The STEM-HAADF images exhibit that this bilayer was not thoroughly continuous; it is composed of Co elliptical particles surrounded by SmCo. PMOKE imaging shows that the magnetization reversal process is dominated by wall propagation.

Marwen HANNACHI (Tunis, Tunisia), Wajdi BELKACEM, Martiane CABIÉ, Lotfi BESSAIS, Najeh MLIKI

08:00 - 18:15 #6338 - MS03-673 Nitride layers grown on patterned graphene/SiC.

MS03-673 Nitride layers grown on patterned graphene/SiC.

Self-heating of high power GaN devices during their operation is a major drawback that limits the performance. Integration of sheets with very high thermal conductivity material could help in this matter. After some unsuccessful GaN growth experiments carried out directly on graphene, we succeeded to grow nitride layers on patterned graphene/6H-SiC by Metalorganic Chemical Vapour Deposition (MOCVD). The growth is similar to the well-known Epitaxial Lateral Overgrowth method in which the graphene buried stripes are overgrown laterally from the window regions, where AlN could grow on bare SiC with epitaxy. An AlN buffer layer was first deposited on patterned graphene/6H-SiC surface followed by a deposition of  ~ 300 nm thick Al_0.2Ga_0.8N and ~ 1.5 µm thick GaN layer. The AlN buffer deposited onto the graphene stripe was grown in a 3D way (Fig.1a). The heterostructure was studied using aberration-corrected transmission electron microscopy (TEM) methods in combination of electron energy-loss X-ray spectroscopy (EDXS) and electron energy-loss spectroscopy (EELS). TEM specimens were prepared using both conventional and focused ion beam methods.

The most surprising details of this study is the appearance of the AlN/GaN superlattices, which were formed in a self-organised way over the buffer layer. Instead the ternary AlGaN we have superlattice (Fig. 1.b and c) in which the thickness of the AlN/GaN is determined by the available elements from the Al_0.2Ga_0.8N which we wanted to grow. The control sample (without graphene) showed a much more flat AlN buffer and a ternary Al_0.2Ga_0.8N on that without any phase separation. EDXS mapping and also superlattice reflections show however, clearly the complete phase separation in the case the nitride layers are grown on graphene. We suppose, that some excess carbon induced the phase separation.

The detailed TEM studies revealed the AlN nucleation directly on SiC and lateral overgrowth of graphene island as shown in Fig.2a. The high resolution image in Fig.2.b shows three layers of graphene and the AlN that is in epitaxy with SiC. Both interfaces are sharp and no interdiffusion of the elements are observed according to the Si, C (not shown) and Al maps in Fig. 2c  The results show that high quality GaN layer over graphene/SiC can be grown with MOCVD that can serve as templates for high power GaN devices.    

 

Acknowledgements. Authors thank the support of the Hungarian National Scientific Foundation (OTKA) through Grant No. K 108869 and NN118914. B. P thanks to the European Commission for providing support to access the ER-C facility through the ESTEEM2 project.

 

(1)   A. Kovács, M. Duchamp, R.E. Dunin-Borkowski, R. Yakimova, P. L. Neumann, H. Behmenburg, B. Foltynski, C. Giesen, M. Heuken and B. Pécz, Advanced Materials Interfaces, published online: 22 DEC 2014 | DOI: 10.1002/admi.201400230, Vol. 2, Iss. 2, January 21 201597

Bela PECZ (Budapest, Hungary), Andras KOVACS, Rafal E DUNIN-BORKOWSKI, Rositza YAKIMOVA, Michael HEUKEN

08:00 - 18:15 #6356 - MS03-675 Controlled production and growth of hexagonal gold nanostructures during self-assembly on a Ge(001) surface.

MS03-675 Controlled production and growth of hexagonal gold nanostructures during self-assembly on a Ge(001) surface.

Self-organized gold nanostructures on Ge(001) surfaces are currently of special interest due to their applications for mono-molecular electronic devices and the growth of Ge nanowires. The understanding of electrical as well as physical properties of the system is of great importance and is strongly linked to its atomic structure.

Here, we report on studies concerning post-annealing induced nanostructure formation after room temperature deposition of a thin film of Au on Ge(001) in Ultra High Vaccuum (UHV). Deposition of 6 monolayers of Au by Molecular Beam Epitaxy (MBE) resulted in the formation of a continuous Au overlayer which was confirmed to be crystalline by Reflection High Energy Electron Diffraction (RHEED). Just after deposition, the samples were post-annealed in UHV to temperatures ranging from 473 K to 773 K with different cooling rates. The self-organized structures, in the form of Au nanoislands, were characterized by HR-SEM and HR-STEM methods as well as EBSD. It has been found that a preferential island orientation exists along the crystallographic direction of the substrate surface as can be seen in Fig. 1(a),(b). For an annealing temperature close to the eutectic temperature of the Au/Ge system (640 K), a change in size and shape of the Au nanoislands is observed as well as the occurence of the hexagonal phase of gold, indicating eutectic melting of the system as can be seen in Fig. 1. Au(011) orientation of the Au islands with respect to the Ge surface was revealed, independent of the annealing temperature Fig. 1(c),(d). TEM measurements of Au/Ge(001) sample cross sections revealed that the nanoislands created upon annealing at T640 K part of the islands are buried beneath the substrate surface (Fig. 1g), which confirms eutectic AuGe melting. The chemical composition of the Au/Ge interface was uncovered using quantitative atomically resolved HAADF-STEM and indicates the absence of alloying (Fig. 1f and 1h). The crystallographic structure of the Au islands and the presence of hexagonal gold as well as the Au/Ge interface were studied by quantitative atomically resolved HAADF-STEM allowing the determination of the structure.

N.G. and J.V. acknowledge funding from the European Research Council under the 7th Framework Program (FP7), ERC Starting Grant 278510 VORTEX. F.K. acknowledges funding from the Polish National Science Center, grant no. DEC-2012/07/B/ST5/00906. All authors acknowledge financial support from the European Union under the Seventh Framework Program under a contract for an Integrated Infrastructure Initiative (Reference No. 312483-ESTEEM2).

Nicolas GAUQUELIN (Antwerpen, Belgium), Benedykt JANY, Marek NIKIEL, Tom WILHAMMAR, Karel VAN DEN BOS, Sandra VAN AERT, Konrad SZAJNA, Johann VERBEECK, Gustaff VAN TENDELOO, Franziszek KROK

08:00 - 18:15 #6364 - MS03-677 Defect Investigation by Atomic-Resolution STEM of III-V Horizontal Nanowires grown via Template-Assisted Selective area Epitaxy.

MS03-677 Defect Investigation by Atomic-Resolution STEM of III-V Horizontal Nanowires grown via Template-Assisted Selective area Epitaxy.

Scaling of silicon microelectronics is reaching fundamental physical limitations related in particular to the power consumption. A possible solution is represented by III-V semiconductors integrated on Si which have much higher electron mobility and injection velocity. The possibility to grow III-V nan-owires allows also the creation of new transistor concepts for Tunnel Field-Effect Transistors [1], which could lead to an increase in the efficiency of the circuits by reducing the supply voltage and thus the power consumption.
The growth of III-V nanowires by Template Assisted Selective Epitaxy (TASE) (Fig. 1a) [2] has enabled the direct integration of heterostructures on Si (100) for TFET applications. The performance of the de-vice may however be affected by the presence of defects (twins, dislocations, stacking faults, anti-phase boundaries) along the nanowire since every single discontinuity in the crystal can lead to a mod-ification in the electrical properties of the material. The understanding, control and suppression of such defects have always been a very challenging task and a crucial point to obtain well performing devices.
Here, we report the investigation of different III-V compounds, by means of atomic-resolution STEM, starting from GaAs homostructures to InAs-GaSb heterostructures (Fig.1b), allowing detailed charac-terization of defects, strains and compositions. We make use of the large degree of freedom of growth parameters in the TASE technique (temperature, V/III ratio, molar flux of precursors) to prepare nan-owire samples (Fig.1b) with the aim to find the right parameters combination to reduce the defects density. Our analysis confirms a process-dependent defect density in nanowires and shows that de-fect-free nanowires can be indeed obtained by optimizing the growth conditions (Fig.2).

[1]. Riel, H., Wernersson, L.-E., Hong, M. & del Alamo, J. a. III–V compound semiconductor transistors—from planar to nanowire structures. MRS Bull. 39, 668–677 (2014)
[2]. Schmid, H. et al. Template-assisted selective epitaxy of III–V nanoscale devices for co-planar heterogeneous inte-gration with Si. Appl. Phys. Lett. 106, 233101 (2015).

Acknowledgments: This work was supported by the Swiss National Science Foundation (project no. 200021_156746) and E2SWITCH (project no. 619509).

Nicolas BOLOGNA (Zurich, Switzerland), Moritz KNOEDLER, Mattias BORG, Davide CUTAIA, Rolf ERNI, Heike RIEL, Marta ROSSEL D.

08:00 - 18:15 #6389 - MS03-679 New phases in Cu3Si solid solution.

MS03-679 New phases in Cu3Si solid solution.

   

Cu₃Si is used as a catalyst for the production of technologically highly important chlorosilanes, an intermediate compound in the production of ultrapure silicon for the semiconductor industry [1]. Copper silicides and copper germanides have also been studied as materials for applications as contacts and interconnects in Si and Ge-Si electronic devices [2]. Different structures of Cu₃Si and Cu₃(SiGe) have been reported over the years [3,4,5], however, new aspects have been revealed by this study.

   

Nanoobjects of various shapes were prepared by the CVD method using organometallic precursors (SiH­­₄, EtSiH₄, BuSiH₃, and their mixtures with H₂) and copper substrates at temperature of about 500 °C. For comparison and more variability in composition, bulk samples of various compositions (Cu₇₈Si₂₂, Cu₇₇Si₂₃, Cu₇₆Si₂₄, Cu₇₅Si₂₅, Cu₇₄Si₂₆) were prepared by arc melting.

  

Samples were screened by SEM/EDX/EBSD and powder XRD. Selected samples were studied by single-crystal XRD and TEM. TEM was performed on a Philips CM 120 (LaB₆, 120kV) equipped with a NanoMEGAS precession unit DigiStar, an Olympus SIS CCD camera Veleta (2048x2048), and an EDAX windowless EDX detector Apollo XLTW. Precession-assisted electron diffraction tomography (EDT) in microdiffraction setup was used to acquire data for structure characterization of nanoobjects.

   

In the Cu₃Si solid solution (Fig. 1), two variants were identified sharing the same average structure (P6₃/mmc, a=4.06Å, c =14.66Å). The structures of the two variants (diagonal (D) and off-diagonal (O)) differ in the placement of satellite reflections, which are caused by strong modulation of the honeycomb copper layers. The D and O variants are most likely stabilized by composition. The D-variant was observed in bulk sample with composition of Cu₇₇Si₂₃ and also in the nanoplatelets prepared by CVD on Cu-substrates, whereas the O-variant was present in the samples richer in silicon. Moreover, additional periodicity along c-axis was detected in Cu₃Si compared to Cu₃(SiGe) with c=7.33Å (Fig. 2). Temperature experiments are currently under progress, and will be also presented.

  

  

[1] Bernard, F.; Souha, H.; Gaffet, E. Mater. Sci. Eng., A 2000, 284, 301–306.

[2] An, Z.; Ohi, A.; Hirai, M.; Kusaka, M.; Iwami, M. Surf. Sci. 2001, 493, 182–187.

[3] Solberg, K. J. Acta Crystallogr. 1978, A34, 684–698.

[4] Wen, Y. C.; Spaepen, F. Philos. Mag. 2007, 87, 5581–5599.

[5] Palatinus, L.; Klementová, M.; Dřínek, V.; Jarošová, M.; Petříček, V., Inorg. Chem. 2011, 50, 3743–3751.

[6] The study was supported by the Czech Science Foundation under project No. 15-08842J.

Klementová MARIANA (Prague, Czech Republic), Cinthia CORREA, Vladislav DŘÍNEK, Petr BRÁZDA, Jaromír KOPEČEK, Lukáš PALATINUS

08:00 - 18:15 #6434 - MS03-681 Automated in situ transmission electron microscopy experiments.

MS03-681 Automated in situ transmission electron microscopy experiments.

In situ transmission electron microscopy (TEM) involves the application of a stimulus to a specimen in the TEM while changes to the specimen are recorded using imaging, diffraction or spectroscopic techniques. However, in most previous in situ TEM studies the apparatus that was used to apply a stimulus did not communicate with the software or hardware that was used to control the TEM and collect data.

.

Important criteria for in situ TEM experiments include minimisation of irradiation dose and avoidance of user bias, resulting in the need to work quickly - and ideally in an automated way. A direct interface between a setup used to apply a stimulus and an interface used to control the TEM is therefore crucial. We have implemented plug-ins for Digital Microcrograph (DM), which can be used to communicate directly with a GPIB bus compatible setup (Fig. 1 a) and external Labview-based software that can then be used to control the stimulus applied to the specimen (e.g., temperature regulation). Values of the applied stimulus and signals measured from the specimen are recorded and added to the tags and titles of TEM images.

.

We have studied silicon oxide-based resistive switching devices in situ in the TEM using a movable W needle and recorded bright-field (BF) TEM images with different voltages applied to the specimen (Figs 1 b-d). A DM script was used to apply a voltage ramp and to measure the current flowing through the sample in an automated way for each applied voltage. By using this approach, we were able to follow the formation and destruction of a conductive path across the SiO_x layer and to correlate it with a measured change in conductivity.

.

A second experiment involved in situ electrical biasing of a solar cell and recording a map of electron beam induced current (EBIC) inside the TEM. DM plug-ins were used to record the current generated by the electron beam while scanning the active layer of a μc-Si:H solar cell (Fig. 2 a). The same script was used to measure the current across the sample as the electron beam was scanned across the specimen and a voltage applied to the solar cell. Simultaneously acquired scanning TEM and EBIC maps are shown in Figs 2 (c-d).

.

A further in situ TEM experiment performed on a biased solar cell involved the acquisition of off-axis electron holograms to determine changes in electrostatic potential across the active layer. An external stimulus such as an applied bias can be applied to such as specimen to remove the unwanted mean inner potential contribution from the results. For each applied voltage, a hologram was acquired from the area of interest on the specimen, the stage was moved to record a vacuum reference hologram and it was then returned to the same sample area. This approach was used to record a series of amplitude and phase images from electron holograms of an electrically biased Si:H solar cell (Fig. 2 e) and to extract phase profiles across the top ZnO contact, the p-doped Si layer and the amorphous intrinsic layer (top right of Fig. 2 e).

.

We are grateful to Michael Farle and AG Farle at the University of Duisburg-Essen for technical help. We also acknowledge the European Union under the Seventh Framework Programme under a contract for an Integrated Infrastructure Initiative (Reference 312483 ESTEEM2) and the European Research Council for an Advanced Grant (Reference 320832 IMAGINE).

Martial DUCHAMP (Jülich, Germany), Vincent VIGNÈRES, Gautier DUFOURCQ, Vadim MIGUNOV, Rafal E. DUNIN-BORKOWSKI

08:00 - 18:15 #6471 - MS03-683 FIB patterning for position-controlled nanowire growth.

MS03-683 FIB patterning for position-controlled nanowire growth.

Semiconductor nanowire (NW) based heterostructures are a promising material system for next generation optoelectronic devices, such as flexible solar cells and light emitting diodes [1]. Their reduced contact area and surface strain relaxation allow for epitaxial growth on lattice-mismatched substrates, a key advantage for integration of different III-V semiconductors with existing silicon-based technology.

 

Position-controlled NWs can be grown in ordered arrays on Si to improve uniformity and device integration. This is commonly performed by using a SiO₂ thin film as a mask. Patterning of circular holes in the mask (Fig. 1(a)) allows for site-specific NW growth in predefined patterns and positions. To date, this is performed using lithography techniques such as electron beam lithography or nanoimprint lithography [2]. Important processing parameters include oxide thickness, hole diameter and pattern pitch, requiring several steps to be optimized in order to achieve a high yield of uniform NWs [3]. Additionally, the catalytic particle is rarely centered in the hole, leading to undesirable asymmetry in the NW cross-sections [4].

 

In this work, the parameter space for direct patterning of NW growth substrates by focused ion beam (FIB) is explored (Fig. 1). Self-catalyzed GaAsSb NWs were grown using molecular beam epitaxy (MBE) on a FIB patterned Si(111) substrate with 40 nm thermal oxide, where hole size, dose and Ga-beam overlap were systematically varied (Fig. 1(a-c)). It is expected that a higher degree of flexibility and control can be attained using FIB compared to the conventionally used resist-based patterning techniques. In addition, patterning by FIB leads to Ga implantation in both Si and SiO₂, which could positively affect the self-catalyzed NW growth and the properties of the NW-substrate system in a unique way.

 

After MBE growth, three distinct growth regimes can be recognized, present in all arrays (Fig. 1(d-e)): The smallest (10 nm pattern) diameter row features a high yield (≤ 80%) of straight NWs. As the hole diameter increases there is initially a transition to more parasitic crystal growth and finally multiple (2-5) NWs grow within each hole. As the dose increases between arrays in each column, the patterned diameter for these transitions decreases proportionally. The results demonstrate that using FIB the parameter space can be mapped out efficiently within a single growth session and that growth can be tuned between aligned single NWs, 2D parasitic crystals and multiple NWs per hole. Transmission electron microscopy and electrical testing of single NWs directly on the growth substrate [5] will be used to refine the structural analysis and study the electrical properties of these NWs. It is expected that in addition to the flexibility of FIB patterning, III-V NWs grown on FIB-patterned Si will exhibit novel properties due to the implantation of Ga and the altered NW-substrate interface.

 

References:

[1] Joyce, H. J. et al., Prog. Quant. Electron., 35, 23–75 (2011), DOI: 10.1016/j.pquantelec.2011.03.002

[2] Munshi, A. M. et al., Nano Lett., 14, 960–966 (2014), DOI: 10.1021/nl404376m

[3] Plissard, S. et al., Nanotechnology, 22, 275602 (2011), DOI: 10.1088/0957-4484/22/27/275602

[4] Nilsen, J. S. et al., J. Phys. Conf. Ser., 644, 012007 (2015), DOI: 10.1088/1742-6596/644/1/012007

[5] Fauske, V. T. et al., J. Microsc., (2015, in press), DOI: 10.1111/jmi.12328

 

Acknowledgements: This work is supported by the Research Council of Norway through funding for the NorFab (197411) facility and the FRINATEK (214235) program.

Aleksander Buseth MOSBERG (Trondheim, Norway), Dingding REN, Bjørn-Ove FIMLAND, Antonius Theodorus Johannes Van HELVOORT

08:00 - 18:15 #6513 - MS03-685 In-situ propagation of a Cu phase in germanium nanowires observed by transmission electron microscopy.

MS03-685 In-situ propagation of a Cu phase in germanium nanowires observed by transmission electron microscopy.

Semiconductor nanowires (NWs) are promising candidates for many device applications ranging from electronics and optoelectronics to energy conversion and spintronics. However, typical NW devices are fabricated using electron beam lithography and therefore source, drain and channel length still depend on the spatial resolution of the lithography. In this work we show fabrication of NW devices in a transmission electron microscope (TEM) where we can obtain atomic resolution on the channel length using in-situ propagation of a metallic phase in the semiconducting NW. The corresponding channel length is independent on the lithography resolution. We show results on semiconducting NW devices fabricated on two different electron transparent Si₃N₄ membranes: a calibrated heater chip from DENs solution [1] and homemade membranes where the NW-metal contact is locally heated by Joule heating [2]. We demonstrate a real-time observation of the metal diffusion in the semiconducting NW. First we present results on in-situ propagation of a copper-germanium phase in Ge NWs while monitoring the system temperature [3] and by Joule heating while measuring the current through the device. We study the kinetics and rate limiting step by monitoring the position of the reaction front as a function of time. Second we will show characterization of the formed phase at atomic length scales with different (S)TEM techniques (electron diffraction tomography, energy dispersive X-ray spectroscopy, HR(S)TEM) to understand how the metal atoms diffuse and incorporate into the germanide phase at the reaction front and how these parameters relate to the electrical properties of the same interface. Using EDX and diffraction characterization we find that an orthorombic Cu₃Ge phase is created in the reacted NW part, see Fig. 2,3. Furthermore, both Cu and Ge are diffusing in opposite directions. Both EDX and diffusion studies indicate that the reaction proceeds via surface diffusion along the Cu₃Ge segment.

Refrences

[1]  http://denssolutions.com/products/nano-chip

[2] M. Mongillo, P. Spathis, G. Katsaros, P. Gentile, M. Sanquer and S. De Franceschi, ACS, Nano, 5,

      7117-7123 (2011).

[3] T. Buchhart, A. Lugstein, Y. J. Hyun, G. Hochleitner and E. Bertagnolli, Nano. Lett, 9, 3739-3742 (2009).

 

 

 

Acknowledgements

Financial support from the French ANR for the “COSMOS” project is acknowledged. We thank B. Fernadez and T. Fournier for their technical support.

Khalil EL HAJRAOUI (Grenoble), Clemens ZEINER, Eric ROBIN, Stéphanie KODJIKIAN, Alois LUGSTEIN, Jean-Luc ROUVIÈRE, Martien DEN HERTOG

08:00 - 18:15 #6543 - MS03-687 Self-Induced Compositional Variations in GaAs/AlGaAs Core-Shell Nanowires.

MS03-687 Self-Induced Compositional Variations in GaAs/AlGaAs Core-Shell Nanowires.

Semiconductor nanowires (NWs) have promising properties for optoelectronic devices such as solar cells and light emitting diodes. For the development of such NW-based devices, the correlation between structural features, composition, and optoelectronic properties of the NWs must be well understood. This task can be challenging as the growth can induce large NW-to-NW variations. As a statistical meaningful sampling at the required spatial resolution by multiple techniques would be very time consuming, correlated studies where the exact same NWs are characterized optically and structurally by different techniques are an alternative [1]. In this study, the same single self-catalyzed GaAs/AlGaAs core-shell NW is studied using micro-photoluminescence (µ-PL) and transmission electron microscopy (TEM). After conventional TEM, cross-sections of two regions of the same NW were made using focused ion beam (FIB) to obtain a 3D impression of the NW. To study variations in the shell, a cross-section was made perpendicular to the growth direction from the lower half of the NW, and to study variations in the tip a section was made perpendicular to the -1-12-direction from the top of the NW (Fig. 1(a)). The cross-sections were studied using high-angle annular dark-field scanning TEM (HAADF STEM) and quantitative electron-dispersive x-ray spectroscopy (EDS) to reveal compositional variations in the different directions of the NW.

 

The NWs in the growth batch are mostly defect free zinc blende (ZB), with stacking faults and a wurtzite (WZ) region towards the tip area (Fig. 1(b-c)). µ-PL of 17 studied NWs shows a signal at the ZB GaAs free exciton energy at 12 K. About half of the NWs also have an additional PL signal at higher energy, as can be seen in Fig. 1(e). Compositional variations in the AlGaAs shell of the NWs could possibly explain this high-energy PL emission in the 1.6-1.8 eV energy range [2]. In both cross-sections (Fig. 1(d) and Fig. 2) this type of structure, with narrow Al-rich and Al-deficient bands parallel to the facets of the NW, is visible. Quantitative EDS maps based on the zeta-method [3] (Fig. 3(a)) shows that the Al concentration in the Al deficient bands for the observed widths is too high to explain the sharp PL emission in the range 1.6-1.8 eV. In addition to the shell, the tip region also depicts compositional variations. These features were only visible in the cross-section normal to the -1-12 - direction (Fig. 1(d) and 2(b)) and not apparent by conventional TEM imaging (Fig. 1(c)). Quantitative EDS (Fig. 3(b)) shows that the Al concentration is varying within the tip. Correlated studies on the very same NW including µ-PL, conventional TEM, FIB preparation in different directions and quantitative EDS are required to visualize and explain self-induced compositional variations and peculiar optical characteristics within these GaAs/AlGaAs core-shell NWs.

 

 

[1] J. Todorovic et al., Nanotechnology 22.32 (2011), 325707.         

[2] J. S. Nilsen et al., Journal of Physics: Conference Series 644 (2015), 012007

[3] M. Watanabe and D. B. Williams, Journal of Microscopy 221 (2006), 89-109.

 

 

Acknowledgements: The Research Council of Norway for the support to the NorFab (197411) and the NORTEM (197405) facilities, as well funding from the NANO2021 (239206) and FRINATEK (214235) programs.

Julie Stene NILSEN (Trondheim, Norway), Aleksander Buseth MOSBERG, Andreas GARMANSLUND, Johannes Frøhaug REINERTSEN, Abdul Mazid MUNSHI, Dheeraj DASA LAKSHMI NARAYANA, Bjørn-Ove FIMLAND, Helge WEMAN, Antonius Theodorus Johannes VAN HELVOORT

08:00 - 18:15 #6547 - MS03-689 Nanostructure and luminescence of Ga and Fe-doped IZO´s.

MS03-689 Nanostructure and luminescence of Ga and Fe-doped IZO´s.

The study of transparent semiconducting oxides (TCO) constitutes a large field of research due to its applications as transparent electrodes in transistors, flat panel displays, solar cells, sensors, etc. [1, 2]. Most of the interest is aimed to optimize both conductivity and transparency in the visible region of commercial indium tin oxide (ITO), with lower production costs. In this sense, the In₂Zn_kO_3+k system seems to be one of the best candidates. Moriga et al [3] reported the discovery of nine members in this system. The physical properties of these phases are a function of the k value. Likewise these phases allow doping with other elements, which provides greater flexibility when designing materials [4, 5].

 In this work, In_2-xM_xZn₇O₁₀ (M = Ga and Fe, and 0≤x≤0.5) materials have been prepared by the ceramic method. The k=7 term crystallizes in the space group R-3m, the introduction of dopant decreasing the lattice parameter without changing the crystalline structure. HRTEM images show these materials formed by ordered layers of InO octahedra sharing edges with layers of (InZn_k)O_k+1⁺ composition along c-axis, in such a way that the (001) plane of the ZnO structure and the (111) In₂O₃ plane are epitaxially equivalent to the (001) plane of Zn_kIn₂O_k+3. The existence of extended defects such as twins, dislocations and disordered intergrowths were observed.

Cathodoluminescence (CL) measurements show the existence of two emission bands, one associated to defects whose intensity and width vary depending on the chemical composition of the material. The second issue is the band edge of the material, which is present in the undoped sample, and it disappears and reappears depending on dopant concentrations. EDS spectroscopy confirms the presence of Ga and Fe dopants. A deeper study by means of atomic resolved microscopy has been performed in order to stablish the structure-properties relationship.

References:

[1] C.G. Granqvist, Solar Energy Materials & Solar Cells. 91 (2007) 1529-1598

[2] G.B. Palmer, K.R. Poeppelmeier, T.O. Mason, Chem. Mater .9 (1997) 3121-3126.

[3] T. Moriga, D. D. Edwards, T. O. Mason, G. B. Palmer, K. R. Poepperlmeier, J. L. Schindler, C. R. Kannewurf, I. Nakabayashi, J. Am. Ceram. Soc. 81 (1998) 5, 1310-1316

[4] R. Wang, A. W. Sleight, R. Platzer y J. A. Gardner, J. Solid StateChem. 122, (1996) 166-175.

[5] A. Ambrosini, S. Malo, K. R. Poeppelmeier, M. A. Lane, C. R. Kannewurf y T. O. Mason, Chem. Mater. 14 (2002) 58-63.

Javier GARCÍA-FERNÁNDEZ, Almudena TORRES-PARDO, Julio RAMIREZ-CASTELLANOS, Ana CREMADES, Javier PIQUERAS, Jose M GONZÁLEZ-CALBET (MADRID, Spain)

08:00 - 18:15 #6570 - MS03-691 Analysis of core/shell nanoparticles by electron microscopy techniques.

MS03-691 Analysis of core/shell nanoparticles by electron microscopy techniques.

Core-shell nanoparticles are being intensively studied due to their exceptional properties like quantum dot confinement. In these nanoparticles, their optical and electronic properties can be modulated changing their dimensions [1]. In particular, materials such as CdSe/ZnS or InP/ZnS are extensively used in a variety of applications such as biochemical sensors [2], light emitting diodes [3] or photovoltaic devices [4]. In these nanoparticles, the shell has the function of avoiding the re-absorption of the light emitted by the core of the particle [5]. Recently, special attention is paid to InP/ZnS nanoparticles to replace the CdSe cores because of the harmful consequences in the environment and health due to the presence of Cd.     

In this communication, we analyse core/shell nanoparticles of different compositions by electron microscopy techniques. In particular, we have studied CdSe/ZnS and InP/ZnS nanoparticles with diameter of the core of approx. 2.7 nm and 0.6 nm thick shells. Initial analyses have been carried out by high resolution transmission electron microscopy (HRTEM). Fig 1 a) shows an HRTEM image of CdSe/ZnS nanoparticles, where one of the particles has been marked. The observed particles have been found to be very homogeneous in shape and dimensions The measured average size of the observed particles is 3 nm approx., which agrees with the designed value. However, and as it can be observed, the core and the shell cannot be distinguished with this technique. The small thickness of the shell is not expected to produce a layer with a noticeably different lattice parameter than the core. Because of this, the samples have been analysed by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), where the intensity in the images can be related to the atomic number Z of the atoms in the material. Fig. 1 b) shows an HAADF-STEM image of CdSe/ZnS nanoparticles. As it can be observed, a clear interface between the core and the shell is not noticed, but it can be seen that the edge of the particle shows smaller intensity than the core. This can be related to two reasons. On the one hand, in these particles, the Z number of the material in the shell is smaller than in the core (Z_ZnS = 46, Z_CdSe = 82). However, it should also be considered that the electron beam finds a smaller amount of material at the edge of the particle, what would cause a reduction of intensity. Because of this, the correlation of the area of reduced intensity with the ZnS shell is not straightforward. In order to investigate the possibility to distinguish the core and the shell in these nanoparticles, structures with different sizes of core and shell are being studied by HAADF-STEM. Image simulations will be carried out in order to help with the interpretation of these images, to allow the correlation of the structural characteristics of these nanoparticles with their optoelectronic properties.

Acknowledgements: This work was supported by the Spanish MINECO (projects TEC2014-53727-C2-1-R, -2-R and CONSOLIDER INGENIO 2010 CSD2009-00013) and Junta de Andalucía (PAI research group TEP-946). The research leading to these results has received co-funding from the European Union.

References:

[1] S. Baskoutas, A.F. Terzis, Journal of Applied Physics, 99 (2006) 013708.

[2] D. Vasudevan, R.R. Gaddam, A. Trinchi, I. Cole, Journal of Alloys and Compounds, 636 (2015) 395-404.

[3] A. Rizzo, Y. Li, S. Kudera, F. Della Sala, M. Zanella, W.J. Parak, R. Cingolani, L. Manna, G. Gigli, Applied Physics Letters, 90 (2007) 051106.

[4] A.J. Nozik, Physica E: Low-Dimensional Systems and Nanostructures, 14 (2002) 115-120.

[5] F. Meinardi, A. Colombo, K.A. Velizhanin, R. Simonutti, M. Lorenzon, L. Beverina, R. Viswanatha, V.I. Klimov, S. Brovelli, Nature Photonics, 8 (2014) 392-399.

Natalia FERNÁNDEZ-DELGADO (Cádiz, Spain), Miriam HERRERA-COLLADO, Pedro RODRÍGUEZ-CANTÓ, Rafael ABARGUES, E MOYA LÓPEZ, Juan MARTÍNEZ-PASTOR, Sergio MOLINA

08:00 - 18:15 #6589 - MS03-693 Precision top-down delayering of microelectronics devices using broad-beam argon ion milling.

MS03-693 Precision top-down delayering of microelectronics devices using broad-beam argon ion milling.

The semiconductor industry is a dynamic, rapidly growing manufacturing sector. In 2015, global sales of semiconductor products increased 9.9% and reached a record US $335.8 billion [1]. Constantly evolving microelectronic device designs continue to grow more complex, more compact, and smaller. For example, three-dimensional (3D) NAND flash memory cells are layered vertically in three-dimensional stacks, which provides much greater cell density and increases memory capacity [2]. Such design complexity makes maintaining manufacturing quality standards a consistent challenge for the industry and failure analysis plays a critical role in meeting that challenge. Failure analysis can determine the root cause of a defective device; it enables failure identification and characterization, as well as provides feedback for product and process improvement. Many failure analysis techniques, both nondestructive and destructive, have been developed in the past five decades [3, 4]:

·         nondestructive techniques: electrical measurement and testing, infrared and X-ray examination, and optical or electron microscopy evaluation

·         destructive techniques: chemical etching, mechanical polishing, plasma etching, and delayering

Many techniques are untenable for multilayer devices. For example, Samsung found that etching agents are too aggressive and the company enumerated several major structural failures that can occur in NAND flash memory related to etching or particle contamination. Delayering is a popular choice because it allows top-down, whole chip characterization. However, delayering 3D devices with multiple layers is difficult. The main challenge presented by a vertical stack is looking through a stack of many dissimilar layers. Instrumentation that employs low energy, broad beam, argon ion milling using a top-down delayering technique can help to expose a defect region for further analysis [5]. The work presented is a new development in semiconductor device delayering for failure analysis using low energy, broad-beam argon ion milling. 

References

1. Rosso, D. (2016, February 1). Semiconductor Industry Association - Global semiconductor sales top $335 billion in 2015. Retrieved from http://www.semiconductors.org/news/2016/
   02/01/global_sales_report_2015/global_semiconductor_sales_top_335_billion_in_2015.
2. Shimpi, A. L. (2013, August 21). Samsung’s V-NAND: Hitting the reset button on NAND scaling. Retrieved from http://www.anandtech.com/show/7237/samsungs-vnand-hitting-the-reset-button-on-nand-scaling.
3. Lakshminarayanan, V. (2001). Failure analysis techniques for semiconductors and other devices. Retrieved from Mobile Dev & Design website: http://mobiledevdesign.com/news/
   failure-analysis-techniques-semiconductors-and-other-devices.
4. Crockett, A., Almoustafa, M., & Vanderlinde, W. (2004). Plasma delayering of integrated circuits. Microelectronics Failure Analysis Desk Reference, 4, 243-25. 

Pawel NOWAKOWSKI (Export, USA), Kristin OLEXA, Mary RAY, Paul FISCHIONE

08:00 - 18:15 #6627 - MS03-695 SEM based electro-optical characterization of core-shell LEDs and simulation of imaging including CL and EBIC excitation inside ensembles.

MS03-695 SEM based electro-optical characterization of core-shell LEDs and simulation of imaging including CL and EBIC excitation inside ensembles.

Three dimensional (3D) nano- and microstructures (NAMs) are attracting a lot of attention and are discussed regarding several applications, especially in optoelectronics and sensors. For example GaN based 3D light emitting diodes (LEDs) with a core-shell geometry are supposed to have substantial advantages over conventional planar LEDs: The active area along the sidewalls of hexagonal GaN pillars can considerably be increased by high aspect ratios - leading to a lower current density inside the InGaN quantum well (QW) at the same operation current per substrate area. ^[1]

Thus related methods are requested for characterization of local electro-optical properties with a high spatial resolution on single structures as well as in ensembles. Usually, electron microscopy is employed to investigate the geometry and properties of such 3D-NAMs and for mapping of vertical features by an SEM a certain sample tilt (e.g. about 30°) is needed. Investigation of single 3D-LEDs by electron beam induced current (EBIC) using an SEM based manipulator setup proves the presence of a pn-junction and doping type of the core and shell, while cathodoluminescence (CL) gives an insight to the optical properties of the QW ^[2]. But in contrast to SEM on planar regions the interactions of the electron probe are significantly affected by the 3D geometry and the surrounding of the NAMs.

In ensembles of 3D-NAMs a certain portion of incident electrons are scattered into neighbor structures and conventional SEM signals (SE, BSE, CL, X-ray emission) are partly shadowed. This interaction is affecting the SEM imaging contrast and the probed signal also includes contributions which are not related to the material properties at the electron beam spot. As such parasitic signals are generated quite close to the original region of the interaction most (global) SEM detectors cannot separate them from the original source. In particular scattering events occur in an enlarged volume of the sample (of the substrate and NAMs) leading to a reduced excitation density and parasitic effects, e.g. this causes a significant contribution of defect related yellow luminescence (YL)

We present results of InGaN/GaN core-shell LEDs obtained with an FE-SEM which is equipped with SE, In-Beam SE, low-kV BSE, EBIC and monochromatic CL detection as well as a piezo controlled manipulator setup, see Figure 1. A modified parabolic collection mirror enables measuring luminescence from planar samples up to 4’’ in a tilted view up to 30°. For a quantitative interpretation of CL and EBIC measurement values and image contrasts, the physical modeling of SEM images and spatially resolved energy transfer by a probe spot is necessary. This is performed using the simulation program MCSEM ^[3]. It models the different stages of image formation and generates SEM images of complex NAM shapes using e.g. GaN as model material. Aspects of the simulation are the electron probe formation, a 3D model of the specimen structure, the interaction of electron probe and solid state by means of scattering trajectories, the emission of secondary electrons, and different types of electron detectors, see Figure 2 and Figure 3. An insight to CL and EBIC imaging is gained by evaluating the scattering energy deposited in a distinct volume inside the NAMs as an imaging signal - this is related to the generation rate of electron-hole pairs inside the respective volume of the semiconductor.

Consent to the experiments this simulation reveals an edge contrast and shadowing of signals by the ensemble as well as scattering of primary electrons inside the ensemble of 3D-NAMs. A quantitative comparison is possible by the absorbed current (EBAC). Artefacts of the EBIC are also demonstrated by the simulation, in particular edge contrast by a reduced generation rate and parasitic signals by scattering from neighbor structures.

 

1. Mandl, M. et al. Group III nitride core-shell nano- and microrods for optoelectronic applications. Phys. status solidi - Rapid Res. Lett. 7, 800–814 (2013). doi: 10.1002/pssr.201307250a

2. Ledig, J. et al. Characterization of the internal properties of InGaN/GaN core-shell LEDs. Phys. status solidi 213, 11–18 (2016). doi:10.1002/pssa.201532485

3. Frase, C. G., Gnieser, D. & Bosse, H. Model-based SEM for dimensional metrology tasks in semiconductor and mask industry. J. Phys. D. Appl. Phys. 42, 183001 (2009). doi: 10.1088/0022-3727/42/18/183001

Johannes LEDIG (Braunschweig, Germany), Carl Georg FRASE, Frederik STEIB, Jana HARTMANN, Hergo-Heinrich WEHMANN, Andreas WAAG

08:00 - 18:15 #6668 - MS03-697 Analytical electron microscopy characterization of light-emitting diodes based on ordered InGaN nanocolumns.

MS03-697 Analytical electron microscopy characterization of light-emitting diodes based on ordered InGaN nanocolumns.

Self-assembled nanocolumns (NCs) with InGaN/GaN disks constitute an alternative to conventional light emitting diodes (LED) planar devices [1]. However, their efficiency and reliability are hindered by a strong dispersion of electrical characteristics among individual nanoLED. Polychromatic emission derives from an inhomogeneous distribution of indium concentration due to the inherent tendency of InGaN alloys to develop composition fluctuations as a function of the polarity of the growth crystallographic planes [2]. The recent development of selective area growth of NCs by molecular beam epitaxy has allowed the achieving of highly homogeneous and controllable GaN/InGaN NCs with improved crystalline quality and higher control over the indium distribution [3].

 

In this work, we present the characterization performed on LEDs based on ordered NCs with InGaN active disks (figure 1). The detailed structural characterization of the nanostructures has been performed by scanning transmission electron microscopy (STEM) carried out on an aberration-corrected JEOL-JEMARM200 microscope. High crystal quality of the NCs is set by the analysis of atomically-resolved high angle annular dark field (HAADF) images. The indium distribution within the InGaN disks is studied by EDS elemental mapping while the polarity of the semiconductor NCs is followed by locating the nitrogen atomic columns in annular bright field (ABF) images while (figure2). Direct correlation of the optical and structural properties on a nanometer-scale was achieved using low temperature cathodoluminescence (CL) spectroscopy in an FEI STEM Tecnai F20 [4].

References

 

[1] R. Penn et al., Am. Mineral. 83 (1998) p.1077

[2] J. F. Banfield, et al., Science 289 (2000), p. 751

[3] M. P. Boneschanscher et al., Science 344 (2014), p.1377

[4] Ž. Gačević et al., Phys.Rev. B (2016) accepted

 

Almudena TORRES-PARDO (Madrid, Spain), Žarko GAČEVIĆ, Noemi GARCÍA-LEPETIT, Marcus MÜLLER, Sebastian METZNER, Ana BENGOECHEA-ENCABO, Steven ALBERT, Frank BERTRAM, Peter VEIT, Juergen CHRISTEN, Enrique CALLEJA, Jose M. GONZÁLEZ-CALBET

08:00 - 18:15 #6687 - MS03-701 Diamond-based MOSFETs: Bandgap interface profiling by STEM-EELS.

MS03-701 Diamond-based MOSFETs: Bandgap interface profiling by STEM-EELS.

In the view of developing high performance metal-oxide-diamond field effect transistor (diamond MOSFET), recent reports presents different approach in the choice of the gate material and, in particular, on the dielectric layer [1-2]. However, a nanometric analysis of the band levels is necessary to understand the electron dynamic across the MOS stack. Recently, STEM-EELS have been revealed as reliable technique to estimate the bandgap of SiO₂ dielectric materials. Nevertheless, its applicability has been limited to SiN_x materials.

To evaluate electron transitions near the bandgap energy in the EELS spectra, researchers have to overcome several experimental difficulties. Here, we evaluate the effect of Čerenkov radiation and volume plasmon-related peaks in low-loss range of EELS spectra. In this study, STEM-EELS techniques are used to analyse the O-terminated diamond/Al₂O₃ interface (similarly to previous studies presented in SiO₂ by other authors [3]). Indeed, volume plasmons (VP) and Čerenkov (Ch) radiation contribution are evidenced in Fig.1, which also shows diamond-related D1-2 peaks and Al₂O₃-related peak A1 however, in the Al layer, only plasmon-related peaks are revealed. Probe position of the previously presented EELS spectra are shown in Fig.2, as numbered dots. Figure 2 shows 001-BF TEM micrography of the diamond/Al₂O₃/Al layers. Inset of Fig.2 shows HREM micrography of the diamond/Al₂O₃ interface, revealing variations in the crystalline quality of the low temperature (100ºC) ALD-deposited Al₂O₃ layer.

In this work, we present a methodology to evaluate the influence of the Čerenkov losses and plasmon-related peaks in the low-loss EELS spectra. In some cases, such peaks are shown to mask the interband-related transition. Indeed, in such cases, Čerenkov-related and plasmon-related peaks have to be deconvoluted and removed, in order to accurately apply the linear-fit method [3], which allows calculating the diamond/oxide bandgaps.

The previously described methodology allows determining the bandgap variations in the oxygen-terminated diamond/oxide interfaces.

 

[1] S. Cheng, L. Sang, M. Liao, J. Liu, M. Imura, H. Li, and Y. Koide, Appl. Phys. Lett. 101, 232907 (2012).

[2] A. Maréchal, M. Aoukar, C. Vallée, C. Rivière, D. Eon, J. Pernot, and E. Gheeraert, Appl. Phys. Lett. 107 (14), 141601 (2015).

[3] Jucheol Park, Sung Heo, Jae-Gwan Chung, Heekoo Kim, HyungIk Lee, Kihong Kim, and Gyeong-Su Park,  ULTRAMICROSCOPY 109 (9), 1183 (2009).

José PIÑERO, Daniel ARAÚJO (, Spain), Pilar VILLAR, Julien PERNOT

08:00 - 18:15 #6713 - MS03-703 Visualization of ordering phenomena in di-fluorinated sexiphenyl by HRTEM.

MS03-703 Visualization of ordering phenomena in di-fluorinated sexiphenyl by HRTEM.

Asymmetrically, di-fluorinated para-sexiphenyl (6P-F2) can be used as a model system of a polar conjugated molecule for the growth on an inorganic semiconductor forming a hybrid inorganic/organic system (HIOS) for opto-electronic applications. The two fluorine atoms are positioned terminally at only one end of the 6P molecule. This asymmetry triggers a dipole moment causing strong interaction with the electric field of the polar crystal structure of ZnO serving as inorganic substrate. The molecule/substrate interaction is expected to be much stronger than that for pure 6P or for symmetrically fluorinated 6P-F4 [1].

Based on the findings for non-fluorinated 6P which was found to form a well ordered crystalline structure when grown on ZnO(10-10) [2], structure models of the 6P-F2/ZnO interface are created to predict the high-resolution TEM image contrast. In a first approach, only potential 6P-F2 bulk structures derived from the β-6P structure were considered. 6P molecules were replaced by 6P-F2 molecules in three different ways (see structure models of Fig. 1a) to c) (left column)). In Fig. 1a) a tip-to-end ordering is considered where the fluorinated tip of the molecule points always in the same direction possibly towards the substrate surface. In Fig. 1b) an alternating ordering is realized. Only every second molecule points downwards the others point upwards. The alternating arrangement is realized for two directions being the horizontal direction and the direction normal to the projection plane. The third type of ordering is a tip-to-tip and end-to-end one (see Fig. 1c). The difference between these 3 types is highlighted by the green arrow markers symbolizing the fluorinated tips. For comparison, the crystal structure of pure 6P is seen in Fig. 1d).

HRTEM image contrast simulations were performed with the JEMS software package [3]. The results are given in the 3^rd and 4^th column of Fig. 1. According to the findings published in [2] a large defocus df up to 1000 nm underfocus was applied for gaining high phase contrast for the low spatial frequency details of the 6P-F2 structure. Moreover, the sample thickness t was varied up to 100 nm in steps of 25 nm. Here the results for t = 100 nm are given.

A clear indication for ordering is only seen in Fig. 1c viz. for the tip-to-tip / end-to-end alignment. The other types (Fig. 1a and b) show fringes almost identical to those of pure 6P (Fig. 1d). Also, for df = -1000 nm the image contrast seems directly interpretable in terms of basic structural features. The dark lines trace the position of the center of mass of the next neighbor 6P-F2 or 6P molecules.

Having a more careful look now at Fig. 1c where a clear indication for ordering is seen. The contrast transfer for tip-to-tip / end-to-end alignment simulated for df = -500 nm promotes two dark fringes for the end-to-end position of the molecules (see yellow arrows in Fig. 1c) whereas the fluorinated tips do not exhibit any fringes. For df = -1000 nm a faint fringe appears at the tip-to-tip position (cf. red arrows) while the distance between the two dark fringes increases.

Conclusively, only the tip-to-tip / end-to-end alignment of 6P-F2 can be clearly identified by HRTEM imaging at large underfocus and the defocus condition strongly influences the visibility of structural details and interpretability of the HRTEM image contrast.

With respect to HIOS applications, the arrangement of 6P-F2 on either the polar (0001) or the nonpolar (10-10)ZnO surface will be studied in further detail. A potential influence of the polar/nonpolar substrate surface on the arrangement of the polar 6P-F2 molecules will be considered.

References:

[1]    M. Sparenberg, A. Zykov, P. Beyer, L. Pithan, C. Weber, Y. Garmshausen, F. Carla`, S. Hecht, S. Blumstengel, F. Henneberger, S. Kowarik, Phys. Chem. Chem. Phys. 16 (2014) 26084.

[2]    H. Kirmse, M. Sparenberg, S. Sadovef, A. Zykov, S. Kowarik, S. Blumstengel, accepted for publication in Cryst. Growth Des. (2016).

[3]    P.A. Stadelmann, http://cimewww.epfl.ch/people/Stadelmann/jemsWebSite/jems.html

Holm KIRMSE (Berlin, Germany), Mino SPARENBERG, Sergey SADOFEV, Sylke BLUMSTENGEL, Christoph T KOCH

08:00 - 18:15 #6727 - MS03-705 Quantitative and non-destructive defect metrology for beyond Si semiconductors.

MS03-705 Quantitative and non-destructive defect metrology for beyond Si semiconductors.

Electron channeling contrast imaging (ECCI) is a powerful scanning electron microscopy (SEM) technique for the visualization and analysis of crystalline defects like dislocations and stacking faults. Distortion of the crystal lattice of a material due to the presence of such defects causes the variation of the backscattered electron intensity, allowing their visualization.  ECCI has been demonstrated to be a fast and robust method for assessing the density and Burgers vector of different defect types in various materials with reliability comparable to that of transmission electron microscopy [1, 2]. However, in contrast to TEM, ECCI can be used for the non-destructive investigation of large areas.

This fact makes the technique particularly interesting for the semiconductor industry, where defect metrology techniques for the non-destructive analysis of (Si)Ge and III/V compounds with  dislocation densities below 10⁵cm^-2, are crucial to support CMOS scaling beyond the 10nm node.

In order to analyze such lowly defective samples, areas exceeding about 200 x 200µm² in size need to be examined to ensure proper statistics. For this purpose we acquire a set of tiles that can be stitched into a single image, thereby leading to an image resolution allowing for the detection of single threading dislocations. The FEI software application MAPS, dedicated to the automated acquisition of high resolution images from large areas, is used to record a set of ECC images that are further processed and examined for the presence of defects. It is important to note that for proper ECC imaging (i.e. maximum channeling contrast at a defect site), the sample needs to be oriented close to the Bragg condition. This is facilitated by tilting and rotating the sample according to the electron channeling pattern (Fig. 1) acquired by scanning the sample at low magnification. The investigated area of the tilted specimen is maintained in focus during the acquisition through the interpolation of the settings.  Imaging conditions such as accelerating voltage and beam current were fine-tuned in advance using a sample with higher defect density (Fig. 2). A retractable below-the-lens backscatter electron (BSE) detector is used to record the individual ECC images.

Using the above described procedure we analyzed in detail the density and distribution of threading dislocations in blanket SiGe layers of different defect densities. Our results reveal that in case of dedicated strain relaxed buffer layers the surface appears basically defect free over several tens of micrometers, only locally one can observe individual defects and pile-ups of threadings reaching the layer surface. Results are verified by defect decoration using chemical etching followed by optical etch pit detection.

Our work demonstrates that ECCI in combination with automated image acquisition provides quantitative information on defect density and distribution on systems foreseen for future semiconductor devices.

 

[1] S. Zaefferer and N-N. Elhami, Acta Mat. 75 (2014) 20-50.

[2] I. Gutierrez-Urrutia and D. Raabe, Scripta Mat. 66 (2012) 343-346.

Anna PROKHODTSEVA (Brno, Czech Republic), Tomas VYSTAVEL, Andreas SCHULZE, Matty CAYMAX

08:00 - 18:15 #6741 - MS03-707 Control of Polarity, Structure and Growth Direction in Sn-Seeded GaSb Nanowires.

MS03-707 Control of Polarity, Structure and Growth Direction in Sn-Seeded GaSb Nanowires.

Among III-V semiconductor materials GaSb is highly interesting for several device applications such as optoelectronics.¹ The epitaxial growth of GaSb nanowires has mainly been done using Au as the seed material which demonstrated several limitations like direct nucleation and crystal structure tuning.² In this work we have investigated the epitaxial growth of Sn-seeded GaSb nanowires directly nucleated on GaSb (111)A substrates with controlled Ga- and Sb-polarities, and study their structural and physical properties.

Nanowires are grown by metal organic vapor phase epitaxy (MOVPE) in a standard low pressure (100 mbar) horizontal MOVPE reactor (Aixtron 200/4). Sn-seed particles are formed in-situ by using Tetraethyltin (TESn) precursor at 530°C. Following the particle formation, the reactor temperature was changed to the nanowire growth temperature in the range of 490-570°C. Trimethylgallium (TMGa) and trimethylantimony (TMSb) are used as precursors for nanowire growth.

The polarity of the nanowires is confirmed by aberration-corrected scanning transmission electron microscopy (STEM) as can be seen in Figure 1. It is shown that there are differences in the structural properties (i.e. growth direction, composition of the seed particle, and crystal purity) of Ga- and Sb-polar nanowires; and their growth mechanism is studied. In addition, the formation of inclined twins which only occurs in the Sb-polar nanowires is explained by using simulation methods. Also high-angle annular dark-field (HAADF-) STEM image simulation is employed for realization of subtle features in the experimental images, as shown in Figure 2. In this Figure, presence of twin boundaries which is not perpendicular to the zone axis is proven. Finally, photoluminescence response of the Sn-seeded GaSb nanowires is compared with their Au-seeded counterpart, suggesting incorporation of Sn atoms from the seed particle into the nanowires.³

 

 

References

¹ A.G. Milnes and A.Y. Polyakov, Solid-State Electronics 36, 803 (1993).

² M. Jeppsson et al, J. Cryst. Growth 310, 5119 (2008).

³ R.R. Zamani et al, submitted (2016).

 

Acknowledgements

The authors thank the scientific staff in DTU-Cen and the access to the microscopy facilities. Additionally, RRZ and KAD acknowledge the European Research Council (ERC) for funding the “NEWIRES” project under grant agreement number 336126.

Reza R. ZAMANI (Lund, Sweden), Sepideh GORJI GHALAMESTANI, Jie NIU, Niklas SKÖLD, Kimberly A. DICK

08:00 - 18:15 #6761 - MS03-709 In-situ operation of oxide-based memories in a transmission electron microscope.

MS03-709 In-situ operation of oxide-based memories in a transmission electron microscope.

There is a great deal of activity in the development of new memory technologies that can be used to provide the required density and reliability for future generations of data storage [1, 2]. At this time there is a bewildering array of proposed systems each with advantages and disadvantages. One of the problems with the development of these types of materials systems is that it is not clear exactly how these devices function and as a consequence, it is difficult to select the best combinations of materials to provide the best overall performance.

In this presentation we will present results that have been obtained on a range of different TaO and Ta2O5 OxRAM structures that have processed using reactive and RF deposition physical vapour deposition (PVD). The focus on this work is the mapping of oxygen and results will be presented that have been obtained by a range of different techniques including aberration-corrected high-resolution annular bright-field (ABF) scanning transmission electron microscopy (STEM) imaging, EDS (Energy dispersive X-Ray Spectroscopy) or Electron Energy Loss Spectroscopy (EELS) for the measurement of oxygen concentration (atoms) and electron holography and differential phase contrast (DPC) for the distribution of electrostatic potential caused by the distribution of the oxygen. An example is shown in Figure 1 where a (a) high resolution ABF STEM and (b) HAADF STEM image of a TaO/Ta2O5 stack with TiN top and bottom electrodes can be seen. Figures 1(c) and (d) show EELS spectra that have been acquired for the N, Ti, O and Ta regions. Figure 1(e) and (f) show quantitative maps and profiles respectively. Here the variations in the Ta and O concentrations can be observed across the active region of the device. In this presentation we will highlight the best techniques for measuring the variations of oxygen in the devices and discuss which stacks have the best stoichiometry for memory device applications.

The key to the performance of these devices is the movement of oxygen during switching. As a consequence it is necessary to perform these observations in situ in the TEM to avoid problems with locating the conducting filaments during specimen preparation and additional issues such as device retention and other modifications that could occur during specimen preparation. In this presentation we will present results on specimens that have been switched using a dedicated in-situ holder in the electron microscope. Figure 2 shows how a movable probe is placed onto a FIB-prepared specimen with nm-scale accuracy by using a Nanofactory biasing system. An electrical pulse is then used to switch the specimen in situ in the TEM. In this presentation we will show how to avoid common problems such as device heating and electrical shorting of the specimen from redeposition during preparation by focused ion beam milling. Finally in this presentation we will show results obtained on our TaO specimens that have been switched in situ in the microscope.

Acknowledgements: EV thanks the Labex Minos ANR-10-LabEx-55-01 for funding this PhD position. DC thanks the European Union for the ERC Starting Grant 306535 “Holoview”. These experiments were performed on the Platform Nanocharacterisation at Minatec.

References
[1] Waser, R. et al. Nature materials, 6 (2007) 833-840.
[2] M. J. Lee et al. Nature materials, 10, (2011) 625-630.

Edouard VILLEPREUX (Grenoble), David COOPER, Zineb SAGHI, Anne ROULE, Mirielle MOUIS

08:00 - 18:15 #6786 - MS03-711 Strain at interfaces of the InAs/AlSb heterostructure for quantum cascade lasers.

MS03-711 Strain at interfaces of the InAs/AlSb heterostructure for quantum cascade lasers.

Devices based on epitaxially grown multilayers can be highly sensitive to structural and chemical properties at interfaces, especially when the active zones are of nanometric size. The InAs/AlSb system is particularly sensitive to this phenomenon due to the lack of common atomic species between the two materials. While the lattice mismatch between the two material layers is moderate (1.3%), the formation of chemical bonds at interfaces that are different from those existing in layers such as Al-As or In-Sb may result in large and very-localized strains. The lattice parameters of AlAs bonds (a_AlAs=0.566 nm) or InSb bonds (a_InSb=0.6479 nm), as bulk materials, are 6.6% smaller or 6.9% higher than that of InAs (a_InAs=0.6058 nm), respectively. The stress states at interfaces are therefore highly dependent on their dominant bond type, being tensile for AlAs-like interfaces or compressive for InSb-like interfaces. In a previous study, a first estimation of the composition at interfaces in the InAs/AlSb system was obtained by combining strain analysis with chemical ones [1]. It was shown that the formation of the tensile AlAs-type interface is spontaneously favored compared to InSb-type, which is attributed to its higher thermal stability.

In this work we focused on the meaning of the measured strain at interfaces in order to get quantitative data on their atomic composition and thus to improve the understanding of interface formation. In this perspective, a strain analysis achieved by using the geometrical phase analysis (GPA) was performed on atomically-resolved Z-contrast images acquired by HAADF-STEM (Fig. 1). Experimental strain profiles were then compared to those obtained from simulated images, with a focus on the effect of convolution due to the mask used in the GPA treatment. Very high negative strains at the scale of two atomic planes are observed from experimental images, both at the InAs-on-AlSb and at the AlSb-on-InAs interfaces. The comparison with simulated strain profiles highlights the strong AlAs-type character of the interfaces when they are spontaneously formed within the chosen growth conditions (Fig. 2). The strain states at interfaces of these structures were also investigated by using the density functional theory (DFT) and compared to the ones predicted by the theory of linear elasticity (Fig.3).

 

This work is supported by the French national project ANR NAIADE (ANR-11-BS10-017) and by ESTEEM2

 

 

[1] J. Nicolai et al., J. Appl. Phys. 118, 035305 (2015)

Maxime VALLET (Toulouse), Yann CLAVEAU, Bénédicte WAROT-FONROSE, Christophe GATEL, Nicolas COMBE, Cesar MAGEN, Roland TEISSIER, Alexei BARANOV, Anne PONCHET

08:00 - 18:15 #6807 - MS03-713 Epitaxy of GaN nanowires on graphene.

MS03-713 Epitaxy of GaN nanowires on graphene.

Nitride nanowires (NWs) are today actively explored as an active material for a large number of optoelectronic devices (light emitting diodes, photodetectors, solar cells). The NWs are usually grown on bulk crystalline substrates (Si, sapphire) but these substrates impose their properties which may not be adapted to the device functionality (low electrical or thermal conductivity, opacity, weigh, rigidity, cost,...). Recently, graphene has been proposed as an attractive candidate to grow III-V semiconductor NWs [1]. Graphene is transparent, flexible and it has high thermal and electrical conductances and it can be synthesized at low cost on large areés. Furthermore, graphene films are easily transferable to almost any carrier substrate, including amorphous and/or flexible materials.

In this study, epitaxial growth of GaN nanowires on graphene is demonstrated using molecular beam epitaxy without any catalyst or intermediate layer. We have grown GaN NWs on isolated mono-crystalline graphene flakes transfered onto SiO2/Si carrier substrates. The nanowires grow vertically and X-Ray diffraction (XRD) show that the nanowires have grown along their c-axis. We have observed by SEM that locally, the NW factes have the same in-plane orientation over several µm². The 10-10 facets of the NWs have been determined by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Cross-sections have been prepared by focussed ion-beam (FIB). HAADF images of NW-graphene/silica interface show that the base of the NWs is defect-free. Selected area electron diffraction (SAED) patterns taken from different zones show identical patterns indicating that the nanowires from all the zones have same in-plane orientation. Since it is well established that the hexagonal periphery of the graphene flakes corresponds to zig-zag edges of the graphene lattice, we can conclude that the epitaxial relationship is 11-20 GaN parallel to the graphene zig-zag. It leads to an apparent mismatch of 29% between the graphene and the GaN wurtzite structure. This value is very large. We show that an in-plane super cell coincidence between 4 unit cells of graphene and 3 unit cells could lead to a more favorable mismatch of -3.1%. The optical properties of NWs arrays have been probed by photoluminescence spectroscopy.

 

[1] Munshi A.M., Dheeraj D.L., Fauske V.T., Kim D.C., Van Helvoort A.T., Fimland B.O., Weman H. NanoLetters, 12 (9), 4570 (2012)

ACKNOWLEDGEMENTS

We acknowledge Agence Nationale de la Recherche (ANR), program of investment for the future, TEMPOS project (n°ANR-10-EQPX-50) for having funded the acquisition of the NANOTEM platform (Dualbeam FIB-FEG FEI SCIOS system and TEM-STEM FEI Titan Themis equipped with the Super-X Chemistem EDX detectors) used in this work

Vishnuvarthan KUMARESAN, Ludovic LARGEAU (Marcoussis), Ali MADOURI, Frank GLAS, Hezhi ZHANG, Fabrice OEHLER, Antonella CAVANNA, Andrey BABICHEV, Noelle GOGNEAU, Maria TCHERNYCHEVA, Jean-Christophe HARMAND

08:00 - 18:15 #6812 - MS03-715 Phase formation and growth behavior of Co-Ge thin films.

MS03-715 Phase formation and growth behavior of Co-Ge thin films.

     The potential use of germanide thin films as self-aligned metallization in Ge-based Complementary Metal-Oxide Semiconductor (CMOS) technology has drawn interest in the solid-state reactions of Ge and metal films [1-3]. In order to integrate these germanide thin films as a contact material, a complete analysis of the solid-state reactions of amorphous Ge (a-Ge) and metals is required. The solid-state reactions in the Co-Ge thin films systems previously examined by in situ x-ray diffracting annealing experiments only probed the crystal structures present in the film and did not identify the morphological variations of the process [1]. Furthermore, to the authors knowledge previous works Co-Ge thin film systems have only analyzed systems with Ge in excess of Co. 

     In this study, the phase formation and crystallization behavior of a-Ge and Co thin film layers are investigated by ex situ transmission electron microscopy (TEM) coupled with in situ x-ray diffraction (XRD) annealing experiments. Four different specimens were prepared, alternating the stacking order and film thickness (200 nm and 30 nm), in order to explore the influence of the free surface on the crystallization and the phase reactions with Co in excess of a-Ge. Thin film layers of a-Ge and Co were deposited via electron beam evaporation onto (100) silicon wafers with the native oxide film present on the surface (~1–2 nm). The thin film specimens were characterized by TEM before and after annealing during the phase formation process. Bright field TEM images of the 200 nm Co on top of 30 nm a-Ge stack are shown in Figure 1: (a) as-deposited, (b) annealed at 280°C and (c) 400°C. Upon heating to 280°C, the a-Ge layer reacts with the Co layer to form CoGe, which then undergoes an additional phase transformation to CoGe₂ with continued heating to 400°C. 

     The influence of the free surface (^fs) on the crystallization process was examined on the 200 nm a-Ge and 30 nm Co film stack. The scanning electron microscopy images of the specimens after annealing to 400°C are shown in Figure 2. (a) The specimen with a-Ge at the free surface (a-Ge ^fs) shows two distinct contrast indicating the presence of two different crystalline phases near the surface, while (b) the specimen with Co at the free surface (Co^fs) shows a more homogeneous contrast throughout the surface. Both specimens have voids visible at the surface, which formed during annealing. By viewing the specimens in cross-section, the presence of large voids is visible in (c) the specimen a-Ge^fs, but the voids are not present in (d) the specimen with Co^fs. The resulting formation of voids in only one of two specimens indicates the free surface has a direct influence of the phase formation process.

 

Citation Information:

[1] K. Opsomer, D. Deduytsche, C. Detavernier, R.L. Van Meirhaeghe, a. Lauwers, K. Maex, and C. Lavoie, Appl. Phys. Lett. 90, 2005 (2007).

[2] W. Knaepen, S. Gaudet, C. Detavernier, R.L. Van Meirhaeghe, J.J. Sweet, and C. Lavoie, J. Appl. Phys. 105, (2009).

[3] T.H. Phung, R. Xie, S. Tripathy, M. Yu, and C. Zhu, J. Electrochem. Soc. 157, H208 (2010).

 

Acknowledgments:

Ce travail a bénéficié d’une aide de l’Etat gérée par l’ANR au titre du programme d’Investissements d’Avenir A*MIDEX portant la référence ANR-11-IDEX-0001-02 

Carlos ALVAREZ (MARSEILLE CEDEX 20), Maxime BERTOGLIO, Khalid HOUMMADA, Alain PORTAVOCE, Ahmed CHARAI

08:00 - 18:15 #6894 - MS03-717 In-Situ Formation of Crystallographically-Oriented Semiconductor Nanowire Arrays via Selective Vaporization.

MS03-717 In-Situ Formation of Crystallographically-Oriented Semiconductor Nanowire Arrays via Selective Vaporization.

Mass production of high-quality semiconductor nanowire arrays (SNAs) with precisely controlled orientation and structure is essential for their application in nanoelectronics and optoelectronics. Here, we present a single-step approach for large-scale fabrication of [0001]-oriented SNAs via direct heating of their bulk components in a H₂/Ar atmosphere. Real-time imaging during in-situ SEM experiments clearly demonstrates that the SNAs form through a selective vaporization process with respect to the crystallography of wurtzite crystals. We propose that the selective vaporization relies on the low symmetry of the wurtzite structure with large differences between surface energies of low-index planes. Further, we demonstrate that the approach can be extended to zinc-blende type structures through a cation exchange process. Finally, the high-quality of the SNAs is demonstrated by fabrication of photodetectors that present outstanding optoelectronic performances. We believe that our method represents a transformative new fabrication platform for large-scale fabrication of orientated SNAs with novel functionalities.

Xing HUANG (Berlin, Germany), Yongqiang YU, Travis JONES, Hua FAN, Lei WANG, Jing XIA, Zhujun WANG, Xiangmin MENG, Marc WILLINGER

08:00 - 18:15 #6912 - MS03-719 EELS analysis of cation intermixing around LaAlO3/SrTiO3 interfaces.

MS03-719 EELS analysis of cation intermixing around LaAlO3/SrTiO3 interfaces.

The pioneering work by Ohtomo and Hwang1 reported the formation of an electron gas with a large charge carrier density at the interface between two band insulators, LaAlO₃ (LAO) film on SrTiO₃ (STO) substrate. However the mechanisms of charge transfer and transport in this system are still not clearly established.

Epitaxial films with 3 and 5 u.c. thickness were grown by Pulse Laser Deposition. The oxygen partial pressure during deposition was fixed at 10^-4 Torr, and the temperature reached 750°C. The parameter misfit between the substrate (aSTO = 3.905 A) and the film (aLAO = 3.791 A using a pseudo cubic description) did not induce strain relaxations via interfacial misfit dislocations as no dislocations could be detected along the foil observed for both samples, as seen on Figure 1. The 5 u.c. sample exhibited a conductive interface while for the 3 u.c. film the resistance exceeded our instrumental limits (> 100 MΩ). Thus, consistently with previous studies, the critical thickness lies between 3 and 5u.c. These samples are good candidates to investigate structural and/or chemical differences between conductive and insulating samples.

 EELS profiles across the interface of the selected samples were used to deduce the contribution of Ti³⁺ to the Ti-L_2,3 absorption edges. A minimum valence of Ti^3.9+ (+/- 0.05) was found located in the first unit cell below the interface of both samples (Figure 2). This would lead to a maximum theoretical density of free charge carriers of 6.6x10¹³ (+/- 3.28x10¹³) cm^-2 if we assume that all the carriers originate from Ti⁴⁺ reduction. Experimental measurements of Hall coefficient on the 5 u.c. sample below 10 K revealed a 2D charge carrier density (≈ 3x10¹⁴ cm^-2) that was comparable to theoretical density (3.3x10¹⁴ cm^-2). However, 2D charge carrier density at room temperature (n > 1.2x10¹⁵ cm^-2), was much higher than the density calculated based on EELS valence measurements. This suggests that the conduction was not purely bidimensional. The hypothesis of a quasi 2D conduction zone restricted to the first layers above and below the interface, still underestimates the charge carrier density with respect to the Hall measurements This would confirm the 3D nature of the conducting layer. 

At the partial pressure of 10^-4 Torr used during the PLD growth, no signature of oxygen vacancies could be detected in the O-K edge recorded in the substrate and around the interface by EELS, as observed on Figure 3. The interfacial O-K EELS spectra reflect intermixing rather than oxygen vacancies. Although a low level of oxygen non-stoichiometry is not excluded, it would be insufficient to explain the sheet resistance measured and the differences between the 3 and 5 u.c. samples.

The full presentation will combine this analysis with elemental profiles and strain analysis obtained by Medium-Energy Ion Scattering (MEIS), and additional electrical measurements to give a rather complete description of these films. Neither electronic reconstruction nor anionic vacancies alone can explain the carrier density observed. Intermixing is demonstrated in the two samples, excluding a donor doping scenario as single mechanism. The measured c/a ratio are larger than those predicted by epitaxial strains obtained from an elastic calculation taking intermixing into account. This indicates that compressive electrostatic forces developed around the interface, and extended deeper into the substrate in the 3 u.c. sample, reducing the confinement and diluting the interfacial charge carrier. A complex competition between donor doping, structural distortions and reconstruction, and ionic compensation is revealed.

We thank the French  METSA network and the European 7th framework program ‘‘ESTEEM2’’ for financial support.

Hicham ZAID, Marie-Hélène BERGER, Richard AKROBETU, Alp SEHIRLIOGLU, Denis JALABERT, Michael WALLS (LPS, Orsay)

08:00 - 18:15 #6937 - MS03-721 Atomic scale study of Cu2O/ZnO heterojunction interfaces by TEM, STEM and DFT.

MS03-721 Atomic scale study of Cu2O/ZnO heterojunction interfaces by TEM, STEM and DFT.

Atomic scale study of Cu₂O/ZnO heterojunction interfaces by TEM, STEM and DFT

Sandeep Gorantla¹, Jiantuo Gan¹, Ole Martin Løvvik^1,2, Spyros Diplas², Kristin Bergum¹, Bengt G. Svensson¹, Edouard Monakhov¹,  Phuong D. Nguyen¹, Anette E. Gunnæs¹

¹ Department of Physics, Center for Materials Science and Nanotechnology, University of Oslo, Norway

² Department of Materials and Nanotechnology, SINTEF, Oslo, Norway

 

Cuprous oxide (Cu₂O) is a low-cost, nontoxic semiconductor interesting for photovoltaic applications. Together with ZnO it forms a p-n junction diode.  With a theoretically estimated conversion efficiency of 18 % ^[1] it is envisaged as a potential material system for the next generation thin-film based solar cells. However, experimentally reported efficiencies are only 2-5 %. In order to better understand the microstructural factors that limit their practical achievable efficiency, Cu₂O/ZnO model systems were grown and studied by conventional TEM and C_s probe corrected scanning TEM (STEM) methods and interfaces evaluated using Density Functional Theory (DFT). For the model systems, c-axis single crystal ZnO substrates (both Zn- and O polar) were used and the Cu₂O films were grown by both ceramic and reactive RF magnetron sputtering methods ^[2]. Cross section specimens were prepared in order to study the films and Cu₂O/ZnO heterojunction interfaces.

Regardless of the polarity, in reactive sputtering case, the (111) [1-10] Cu₂O || (001) [110] ZnO orientation relation was observed consistent with an epitaxial relationship across the Cu₂O\ZnO interface. However, only in O-polar ceramic sputtering case, additional (110) Cu₂O || (001) ZnO orientation relationship was also observed. The STEM investigations revealed the presence of an unexpected ~ 5 nm thick polycrystalline interfacial CuO layer between the ZnO substrate and Cu₂O film, as shown in figure 1. The presence of CuO is unexpected because the deposition conditions were optimized for the Cu₂O film growth. Repeated growth experiments have, however, confirmed the findings.  Interestingly, despite the presence of this interfacial layer, epitaxial twinning of the Cu₂O films was observed in all the specimens except for the case with O-polar ZnO, ceramic grown Cu₂O film. Also notable, the films grown by reactive sputtering were more dense relative to films grown by ceramic sputtering ^[2].The CuO grains in the interfacial layer were observed to have certain preferred orientations with respect to the substrate i.e. textured growth. Geometric Phase Analysis evaluation of the CuO/ZnO STEM images clearly showed an array of misfit dislocations along their interface reducing the strain in the epitaxial CuO film, as shown in figure 2. DFT calculations were carried out based on the experimentally observed Cu₂O/CuO/ZnO interfaces. It was found that the formation of CuO on ZnO and Cu₂O on CuO is energetically favorable compared to the Cu₂O/ZnO interface even when dislocations are not taken into consideration on the CuO/ZnO interface. The total lattice strain of (111) Cu2O/ (100) CuO/ (001) ZnO is lower than the calculated lattice strain between (111) Cu₂O and (001) ZnO. It can be concluded that the driving force for formation of the ~ 5 nm thick CuO interface layer is strain induced. Such a CuO interfacial layer is detrimental for Cu₂O/ZnO p-n heterojunction diode efficiency and its existence can explain the large gap between the experimental and theoretical reported conversion efficiencies.

Acknowledgements: This work was conducted under the research project ES483391 Development of a Hetero-Junction Oxide-Based Solar Cell Device (HeteroSolar), financially supported by the Research Council of Norway (RCN) through the RENERGI program.

[1] T. Minami, Y. Nishi, T. Miyata, App. Phys. Express 6, 044101, 2013.

[2] J. Gan, S. Gorantla, H. N. Riise, Ø. S. Fjellvåg, S. Diplas, O. M. Løvvik, B. G. Svensson, E. V. Monakhov, and A. E. Gunnæs,  App.   Phys. Lett. 2016 (in press).

 

 

 

Sandeep GORANTLA (Oslo, Norway), Jiantuo GAN, Ole Martin LØVVIK, Spyros DIPLAS, Kristin BERGUM, Bengt SVENSSON, Edouard MONAKHOV, Phuong NGUYEN, Anette GUNNAES

08:00 - 18:15 #6988 - MS03-723 Strain measurements at AlGaN/GaN HEMT structures on Silicon substrates.

MS03-723 Strain measurements at AlGaN/GaN HEMT structures on Silicon substrates.

High Electron Mobility Transistors (HEMTs) based on AlGaN/GaN are of great interest due to their high electrical performance and the related applications. The high carrier density, electron mobility, breakdown voltage, and the good thermal stability of AlGaN/GaN are great benefits for high power and high frequency technologies. The high electron mobility is a consequence of a two dimensional electron gas (2DEG) which is formed at the interface between GaN and AlGaN. The source, drain, and gate of the transistors are realized by metal contacts on top of the semiconductor. Strain in the transistor structures may arise due to thermal processing steps or applied passivation layers on top of the HEMT structure. Especially the thermal processes may cause strain due to the mismatch in the coefficients of thermal expansion between the metal contacts and the GaN/AlGaN. Additional stress may be induced by the substrate material. In order to reduce material costs, such as those associated with power electronic applications, the usage of silicon substrates to replace the expensive silicon carbide and sapphire are under development. The disadvantage of GaN on silicon is the lower quality of the deposited AlGaN/GaN layers caused by the mismatch of the lattice parameters which differ by 17%. This can cause higher defect densities and residual strain in the AlGaN/GaN epi-layers.

In this work the local residual strain distribution in the AlGaN/GaN layers of HEMT structures is characterized. Investigations were conducted utilizing Nano Beam Electron Diffraction (NBED) which is a well-established and sensitive method for strain analysis in semiconductors. The experiments were performed with an image Cs-corrected TEM (Titan³ G2 60-300, FEI) equipped with a 3-condenserlens system and a small condenser aperture (10 µm) which is crucial for NBED experiments. The NBED data were further analyzed using the FEI Epsilon Nanobeam Diffraction Strain Analysis Package (v1.1.0.39).

Fig. 1a shows a dark field STEM overview of a normally-on AlGaN/GaN HEMT representing the typical arrangement of the source, gate, and drain metal contacts and the field plate. In Fig. 1b a detailed image of the gate contact can be seen where the AlGaN-layer on top of the GaN substrate is visible. Fig. 1c presents the elemental mapping of the gate Schottky contact in order to depict the different contact metals and partially the field plate. The contact is out of gold with a thin layer of nickel metallization underneath to form the Schottky contact to the active layer. No abnormalities at this gate structure could be found by TEM and EDX analysis. Nevertheless, electrical measurements of the investigated HEMT show a significant gate-drain leakage current. Consequently, despite no irregularities were discovered with high resolution TEM of the interface region of the gate contact, NBED results showed local strain at the area (Fig. 2b). The compressive strain in [002] direction starts at the Schottky interface of the gate structure and runs through the AlGaN layer to the GaN bulk material. This may implicate a low resistance electron path from the gate into the 2DEG and must be further investigated.

David POPPITZ (Halle, Germany), Andreas GRAFF, Michél SIMON-NAJASEK, Mikael BROAS, Frank ALTMANN

08:00 - 18:15 #6153 - MS04-725 Two new microcrystalline (oxo)nitridosilicates with complex crystal structures determined by combination of TEM and synchrotron micro diffraction.

MS04-725 Two new microcrystalline (oxo)nitridosilicates with complex crystal structures determined by combination of TEM and synchrotron micro diffraction.

Nitridosilicates and oxonitridosilicates doped with rare earth elements are well known as luminescent materials, e.g. in phosphor-converted LEDs.^[1] The synthesis of such complex multinary materials often leads to inhomogeneous microcrystalline samples that contain unknown compounds. With respect to structure-property relationships, precise structure determination is essential to predict and understand luminescence properties.

Two novel La,Ba-(oxo-)nitridosilicates were found in a microcrystalline sample. They are coherently intergrown which renders structure determination challenging. The two phases were identifies by selected-area electron diffraction (SAED) and X-ray spectroscopy (EDX). Suitable (not intergrown) single crystals of each phase were located in TEM images. Lattice parameters and the chemical composition were determined. Both phases show hexagonal metrics and exhibit the same c lattice parameter; however, they differ in the a lattice parameter and the chemical composition (Fig. 1). Single-crystal data of these crystallites on TEM grids were collected using sub-micron synchrotron beams (ID11, ESRF, Grenoble).^[2] Corresponding structure solution yielded two structure models. These were confirmed by HRTEM along characteristic directions, including corresponding simulations, as well as Z-contrast imaging (STEM-HAADF, Fig. 1). The latter shows heavy atom sites (La, Ba). For both compounds, the plane groups of HRTEM images (along [001]: p3) and the symmetry of SAED patterns pointed to the Laue class 6/m. The distribution of elements with lacking scattering contrast such as O/N and Ba/La on mixed occupied sites were analyzed by bond-valence sum calculations.

The oxonitridosilicate Ba₂₄La₅₄Si₁₂₉N₂₄₀O₃ (a = 17.49 Å, c = 22.70 Å) shows an interrupted, centrosymmetric 3D framework of vertex- and edge-sharing Si(O,N)₄ tetrahedra. The crystal structure can be described assuming two slab-like building blocks (Fig. 2A). Slab a consists of two layers of dreier rings that are interconnected via pairs of Si(O,N)₄ tetrahedra. Slab c forms asymmetric dreier, vierer, fünfer, sechser and siebner rings of Si(O,N)₄ tetrahedra and contains the edge-sharing Si(O,N)₄ tetrahedra. These two blocks are interconnected via an additional layer (part b) that formally consists of isolated groups of four Si(O,N)₄ tetrahedra.

The second phase is a nitridosilicate (Ba₂₉La₈₀Si₁₇₃N₃₃₀ with a = 20.19 Å, c = 22.68 Å). Comparable to Ba₁₂La₁₄Si₄₃N₇₆O₅, the crystal structure consists of an interrupted 3D network of SiN₄ tetrahedra, which can also be divided in different parts. The main difference is part c, especially concerning the arrangement and linkage of the edge sharing SiN₄ tetrahedra. The difference is caused by the pairs of Si(O,N)₄ tetrahedra parallel to (Fig. 2B, white tetrahedra) in Ba₂₄La₅₄Si₁₂₉N₂₄₀O₃ which are replaced by three SiN₄ tetrahedra linked over one Si atom (Fig. 2C) in Ba₂₉La₈₀Si₁₇₃N₃₃₀. The pronounced tendency towards intergrowth of both phases is due to the fact that part a and b are very similar in both structures.

[1]        M. Zeuner, S. Pagano, W. Schnick, Angew. Chem. Int. Ed. 2011, 50, 7754-7775.

[2]        F. Fahrnbauer, T. Rosenthal, T. Schmutzler, G. Wagner, G. B. M. Vaughan, J. P. Wright, O. Oeckler, Angew. Chem. 2015, 127, 10158-10161; Angew. Chem. Int. Ed. 2015, 54, 10020-10023.

Lukas NEUDERT (Munich, Germany), Schultz PETER, Dajana DURACH, Oliver OECKLER, Wolfgang SCHNICK

08:00 - 18:15 #6181 - MS04-727 Defects and strain analysis of GaAs/Si nanostructures from high-resolution HAADF-STEM images.

MS04-727 Defects and strain analysis of GaAs/Si nanostructures from high-resolution HAADF-STEM images.

The increasing demand for high-performance, ultra-small size electronics and photonics requires the development of new nanostructured materials. Promising candidates for this purpose are monolithically integrated GaAs nanocrystals, selectively grown by metal organic vapor phase epitaxy (MOVPE), on top of Si nano-tips and nano-pillars (Fig. 1,2) with ~ 60 nm and ~ 40 nm diameter, respectively. Growth on substrate nanopatterns eliminates threading dislocations similar to “aspect ratio trapping” in submicron trenches and pits [1]. It may even prevent plastic strain relaxation by misfit dislocations in contrast to the growth of thin films on planar substrates by taking advantage of the compliant substrate effects [2]. Consequently, it is important to study the local atomic defects at the GaAs/Si interface and their evolution with changing shape/size of the substrate pattern, and to analyze the residual elastic strain of the grown crystals by means of the scanning transmission electron microscopy (STEM) technique in order to find the maximum crystal size below which elastic relaxation is favorable.

STEM investigation of the GaAs/Si nanostructures has been performed on lamellas prepared by focused ion beam (FIB). From the analysis of aberration-corrected high-resolution high-angle annular dark-field (HAADF) STEM images the presence of 60º misfit dislocations (MDs), as well as 90º and 30º Shockley misfit partial dislocations (MPDs) at the GaAs/(001)Si tip interface has been revealed (Fig. 3). The latter generate extrinsic and intrinsic stacking faults, respectively, which propagate through the entire crystal. Gliding of a group of 30º Shockley MPDs along parallel successive slip planes produces a nanotwin. In the GaAs crystals grown on (001) Si pillars two intrinsic stacking faults, lying in {111}-type planes, meet at the interface and build a V-shaped defect (Fig. 4). According to the literature [3], such V-shaped configuration of stacking faults suggests their formation upon island coalescence. These stacking faults border at the interface a stair-rod dislocation, which is formed by the interaction of two 30º Shockley MPDs located in the Si pillar. Among the observed defects, the most efficient ones for relaxing misfit strain are 60º MDs, whereas 30º MPDs release strain only partially. Geometrical phase analysis (GPA) [4] has been also applied to estimate the misfit strain in the GaAs crystals.

[1] J.G. Fiorenza et al., ECS Trans. 33 (2010) 963-976.

[2] P. Zaumseil et al., J. Appl. Phys. 112 (2012) 043506.

[3] E.W.Z. Liliental-Weber, J. Washburn, Defect Control in Semiconductors, edited by K. Sumino, Elsevier, Amsterdam, (1990) pp. 1295-1305.

[4] M.J. Hÿtch et al., Ultramicroscopy 74 (1998) 131-146.

Roksolana KOZAK (Dübendorf, Switzerland), Ivan PRIETO, Oliver SKIBITZKI, Yadira ARROYO-ROJAS DASILVA, Marta ROSSELL, Rolf ERNI, Thomas SCHROEDER, Hans VON KÄNEL

08:00 - 18:15 #6202 - MS04-729 Revisiting the EELS analyses and its coupling with multi-wavelength Raman spectroscopy: the case of hydrogenated amorphous carbon thin films.

MS04-729 Revisiting the EELS analyses and its coupling with multi-wavelength Raman spectroscopy: the case of hydrogenated amorphous carbon thin films.

       Thanks to the long-term stability of their properties, hydrogenated amorphous carbon (a:C-H)  thin films are very promising materials for numerous applications including coatings for spatial applications.¹ In order to improve their performances, a full understanding of their local chemistry is highly required. Fifteen years ago, according to the seminal work of Ferrari et al.,² EELS was the most used technique to get such kind of quantitative information on these materials. Nowadays the complexity of the physics phenomena behind EELS is well known³ and this technique is regarded as time-consuming and difficult to interpret properly. Other optical techniques such as Raman spectroscopy are now clearly favored by the scientific community. However they still lack of the spatial resolution that EELS in a STEM offers for getting direct chemical information.

      

       a-C:H thin films, with a thickness around 300 nm, were deposited on a Si wafer and submitted to isothermal annealing at 500°C with different annealing times up to 2500 minutes. The hydrogen content was monitored by multi-wavelength (MW) Raman using a set of reference materials. To determine the sp² fraction (sp² %) from core-loss EELS, the R ratio (R = I_π*(ΔE)/I(_{π*(ΔE)+σ*(ΔE)}) was determined first by taking into account the asymmetry of the π* character (Fig. 1a).^4,5 This value was then normalized by the maximum R value (R_REF) that could be obtained from a HOPG sample in the same experimental condition using relativistic calculations (Fig. 1b).⁶ When needed, this method was also slightly modified to take into account the contribution of heterospecies. In addition, the mass density and the oxygen content was derived from low-loss and core-loss spectra, respectively.

 

       The EELS C-K edge spectra (Fig. 2a) present all a typical signature of amorphous carbons. However, the intensity of the massif above 292 eV differs from sample to sample and clearly highlights a slight variation of the sp² %. The samples annealed 2500 min also presents a supplementary peak (red arrow in Fig. 2a), which is related to the oxidation of the thin film. As expected, the sp² % increases with the annealing time (Fig. 2b). This effect is related to the H desorption of the thin films as monitored by Raman spectroscopy. Two samples do not follow this trend: the as-deposited sample and the sample annealed 2500 minutes. This latter presents a strong oxidation, leading to a decrease of the sp² %. On the other hand, the as-deposited sample shows variation of the C-K edge fine structures (Fig. 3a) highlighting chemical inhomogneities in the thin film. This sample presents a strong gradient of the sp² % induced by the deposition process (Fig. 3b) which is cured with the annealing time.

              All these results will be detailed together with the influence of the oxidation on the chemical and physical properties. In addition, the coupling of MW Raman, infrared and EELS spectroscopies to extract a wealth of chemical information will be discussed. Our results provide a complete combination of C-hybridization, spatial elemental analyses and structural defects studies for shedding light on these complex materials.^7,8

 

1. A. Rusanov et al., Carbon. 81, 788–799 (2015) ; 2. A.C. Ferrari et al., Phys. Rev. B 62 (16), 11089 (2000) ; 3. P. Schattschneider et al., Phys. Rev. B 72, 045142 (2005) ; 4. N. Bernier et al., J. Electron Spectrosc. Relat. Phenom. 164, 34–43 (2008) ; 5. J. Titantah, D. Lamoen, Phys. Rev. B. 70, 075115 (2004) ; 6. F. Bocquet et al., Ultramicroscopy. 107 81–94 (2007); 7. L. Lajaunie, C. Pardanaud, C. Martin, P. Puech, C. Hu, M.J. Biggs and R. Arenal, Submitted; 8. We acknowledge funding from the EU under Grant Agreement 312483-ESTEEM2, Grant Agreement 604391 Graphene Flagship and the Spanish Ministerio de Economia y Competitividad (FIS2013-46159-C3-3-P).

Luc LAJAUNIE (Zaragoza, Spain), Cédric PARDANAUD, Céline MARTIN, Puech PASCAL, Cheng HU, Mark James BIGGS, Raul ARENAL

08:00 - 18:15 #6269 - MS04-731 Unusually high iron enrichment in hard dental tissues of coypu.

MS04-731 Unusually high iron enrichment in hard dental tissues of coypu.

Living organisms possess a unique capability to form complex bio-minerals with very diverse compositions and structures. Organic matrix and crystalline constituents are closely linked together in unique material constructions that are formed under conditions of moderate temperature, pressure and pH value. Many of these bio-minerals show excellent physical and mechanical properties [1, 2] that cannot be reproduced in the laboratory.   

Rodents have long opposing pairs of continuously growing incisors that are worn down by gnawing. The front surface of the incisors is enamel consisting of 96 wt% of inorganic material; the inner part is softer dentine that forms the bulk of the teeth [3]. The surfaces of incisors of different rodent species show a characteristic orange-brown color and are identified with the presence of iron [4].

In our study, the microstructure and the chemical composition of continuously growing incisors from the coypu (Myocastor coypus Molina) were investigated in detail using energy-dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS) combined with scanning transmission electron microscopy (STEM) imaging at high spatial as well as high energy resolution. VG HB501UX, Zeiss SESAM and Jeol ARM200F microscopes were used and the resulting data were combined with knowledge obtained from mechanical testing experiments.

A layer with variable thickness, which has not been observed before, was detected on the outer surface of the incisors as marked on the optical micrograph image (Fig. 1). An annular dark field (ADF)-STEM image of a cross-sectional view showing the interface between the Fe-rich surface layer (Fe-SL) and Fe-rich enamel (Fe-E) is shown in Figure 2a. Analytical investigations of this surface layer showed much higher amounts of iron compared to the concentration values reported in the literature up to now. Within the iron-rich surface layer we surprisingly detected multiple iron containing varieties where iron is present in predominantly the 3+ valence state, as revealed by studies of electronic structure. O-K fine structural features suggest the presence of different intermixing levels of Fe phosphate and Fe oxide/hydroxide (Fig. 2b).

The present discoveries will greatly enhance the understanding of the function of the incorporation of iron at the nanoscale level and its effect on the microstructural properties.

 

[1] UGK Wegst and MF Ashby, Philos Mag 84 (2004), 2167.

[2] AP Jackson and JFV Vincent, J Mater Sci 25 (1990), 3173.

[3] BA Niemec in “Small animal dental, oral & maxillofacial disease” (2010), Manson Publishing Ltd, London.

[4] EV Pindborg JJ Pindborg and CM Plum, Acta Pharmacol 2 (1946), 294.

Vesna SROT (Stuttgart, Germany), Ute SALZBERGER, Birgit BUSSMANN, Julia DEUSCHLE, Masashi WATANABE, Bostjan POKORNY, Ida JELENKO TURINEK, Alison F. MARK, Peter A. VAN AKEN

08:00 - 18:15 #6294 - MS04-733 Morphologies of a solvent cast polystyrene-polybutadiene-polystyrene (S-B-S) triblock copolymer characterised by TEM, AFM and Energy Filtered SEM.

MS04-733 Morphologies of a solvent cast polystyrene-polybutadiene-polystyrene (S-B-S) triblock copolymer characterised by TEM, AFM and Energy Filtered SEM.

The most common method of characterisation of the morphologies of block copolymers is transmission electron microscopy (TEM) of ultrathin sections because of the nanoscale of the domains in the morphologies of this block copolymer[1]. The contrast of polymer phases is often enhanced by heavy metal element staining using, typically, OsO₄. TEM provides imaging at high resolution at the cost of time-consuming and challenging specimen preparation by cryo-ultramicrotoming and the negative staining which is not only toxic but also introduces artifacts[2]. Furthermore, the 2D projection by TEM imaging makes direct interpretation of a nanoscale complex material difficult when the specimen sections thickness (~100nm) is considered. Atomic force microscopy (AFM) can characterise a multiphase material by detecting localised variation in mechanical properties, e.g. friction, adhesion and modulus, and has been used to identify the morphology in block copolymers in some recent studies without staining [3]. Combined AFM and TEM characterisation of multiphase copolymers has been developed to characterise the complex microstructures of block polymers in a complementary way [3,4]. Novel holders were designed to facilitate sample preparation for both techniques and to direct correlation of the topographical information provided by the AFM better to interpret the two-dimensional images provided by TEM. However, the lateral resolution of AFM is significantly worse than TEM limited by the scanning tip although the depth resolution is super. An alternative approach is to use the recent energy-filtered scanning electron microscopy (EFSEM) technique, which is based upon the energy spectroscopy of detected secondary electrons (SE)[5].  In this work, the S-B-S triblock copolymer cast from toluene, cyclohexane and ethyl acetate was characterised in order to understand the effect of casting solvent on the solid state morphology of this triblock copolymer. The morphology varies from continuous to spherical and lamella structures ranging from a few nm to ~100nm. Preliminary results have shown complementary morphologies provided by AFM and TEM, as shown in Figures 1 and 2. The contrast mechanisms from the three methods and consequences for the morphology determination will be discussed, as well as the sample preparation methods.

References:

1, L.C.Sawyer, D.T.Grubb, G.F.Meyers. Polymer Microscopy,3^rd ed.,Springer-Verlag, 2008 (Chapter4).

2,T.M.Chou, P.Prayoonthong, A.Aitouchen, M.Libera. Nanoscale artifacts in RuO₄-stained poly(styrene), Polymer 43(2002), 2085–2088

3, H.Ott, V.Abetz, V.Altstadt, Y.Thomanna, A.Pfau. Comparative study of a block copolymer morphology by transmission electron microscopy and scanning force microscopy. J Microsc. 205 (2002), 106–108.

4, Y.Thomann, R.Thomann, M.Ganter, G.Bar, & R.Mühlhaupt, Combined ultramicrotomy for AFM and TEM using a novel sample holder. J. Microsc.195 (1999), 161–163.

5. R.C.Masters, A.J.Pearson, T.S.Glen, F.-C. Sasam, L. Li, M. Dapor, A.M. Donald, D.G. Lidzey, C.Rodenburg. Sub-nanometre resolution imaging of polymer–fullerene photovoltaic blends using energy-filtered scanning electron microscopy. Nature communications 6(2015):6928 

Zhaoxia ZHOU (Loughborough, United Kingdom), Scott S DOAK, Dave B GRANDY, Kerry J ABRAMS, Cornelia RODENBURG, Nicole WESTON, Douglas J HOURSTON

08:00 - 18:15 #6295 - MS04-735 Valence states of new Mn coordination sites at the ferromagnetic domain walls of TbMnO3 thin films.

MS04-735 Valence states of new Mn coordination sites at the ferromagnetic domain walls of TbMnO3 thin films.

Bulk TbMnO₃ is a classic multiferroic material that combines antiferromagnetic ordering below 42 K and, below 27 K, a spin spiral transition with inversion symmetry breaking below 27 K that causes ferroelectricity [1]. However, it was recently demonstrated that strained TbMnO₃ thin films grown on SrTiO₃ (001) display an induced ferromagnetic behavior absent in the bulk material. This net magnetic moment arises from a two-dimensional ferromagnetic phase synthesized at the ferroelastic (orthorhombic) domain walls (DW), which nucleate to accommodate the huge epitaxial strain induced by the substrate, and thus, scales with the DW density [2]. These ferromagnetic DWs are characterized by a spatially-ordered substitution of alternate Tb sites by Mn atoms along the pseudocubic [100] growth direction, inducing magnetic frustration and a spin canted ferromagnetic ground state. First-principle calculations point to two different types of Mn atoms occurring at the boundary planes, Mn(I) and Mn(II), with different magnetic moments and crystal environments, i.e. tetrahedral and a quasi-square-planar O coordinations, respectively [see Fig.1(a)].

 

This work presents the detailed aberration-corrected Scanning Transmission Electron Microscopy (STEM) study of the different structural environments and electronic properties of Mn cations located at the Tb sites of the DW structure. For this purpose, atomically resolved High Angle Annular Dark Field (HAADF), Annular Bright Field (ABF) imaging and fine structure Electron-Energy Loss Spectroscopy (EELS) in plane view and cross sectional configurations have been combined. ABF imaging evidences that the two first neighbor O coordinations for Mn(I) and Mn(II) sites predicted theoretically are present in plane-view specimens, see Fig. 1(b). Furthermore, the analysis of the O K edge fine structure has allowed us to map the nominal Mn oxidation state [3] using atomic-resolution STEM-EELS, see Figure 2. This experiment has confirmed the decrease of the overall Mn valence at the domain wall previously reported and a fine modulation of the electronic state between the tetrahedrally-coordinated Mn(I) and the square-planar-coordinated Mn(II) replacing Tb cations, as predicted by DFT calculations.

 

References

[1] Y. Kimura et al., Nature 426, 55 (2003).

[2] S. Farokhipoor, C. Magén, et al., Nature 515, 379 (2014).

[3] M. Varela et al., Physical Review B 79, 085117 (2009).

Roger GUZMAN, Jorge IÑIGUEZ, Saeedeeh FAROKHIPOOR, Beatriz NOHEDA, César MAGÉN (Zaragoza, Spain)

08:00 - 18:15 #6306 - MS04-737 Quantitative analysis of a model pharmaceutical material, theophylline, by transmission electron microscopy.

MS04-737 Quantitative analysis of a model pharmaceutical material, theophylline, by transmission electron microscopy.

Modern electron microscopy (EM) techniques and hardware offer some of the highest attainable spatial resolutions for crystal imaging, making EM one of the best tools for microstructural analysis of a wide variety of materials. Organic materials, specifically pharmaceuticals, for which microstructure is a key part of their functionality would make ideal candidates for EM analysis, as it could provide useful feedback at various stages during drug development to check the presence of desired crystalline polymorphs, assess mixing quality, quantify crystal lattice defects and identify contamination. However, the major drawback to the use of EM is the high sensitivity of organic crystalline materials to electron beam exposure. Employing fluence rates that are typically used for inorganic samples would destroy all traces of crystallinity in most pharmaceutical materials[1]. Low-dose techniques have been used for many years to analyse beam sensitive samples[2] using EM, but with recent improvements in CCD camera sensitivities, microscope stabilisation and current control, combined with existing low-dose techniques, there is great opportunity for EM to be used for detailed organic materials analysis. Initially, the limits and flexibility of low-dose techniques need to be tested and documented, both qualitatively and quantitatively. From this, a deeper understanding of the damage mechanisms at play can be drawn out, informing on the limits of organic crystalline materials analysis by EM and indicating appropriate damage mitigation strategies which could be employed in future studies.

 
Initial experiments with a model pharmaceutical, theophylline, observed by transmission electron microscopy (TEM) considered the effects of changing sample and electron beam conditions[3]. The aim was to determine the conditions which resulted in the highest critical dose (CD) (the dose where the intensity in a given diffraction spot decays to 1/e of its highest value). Since then, further experiments have been undertaken, including tests at 300 kV and a range of temperatures, from 93 K to 423 K. The ideal conditions identified use a high accelerating voltage with a graphene support for improved heat and electronic conduction, at a reduced sample temperature if necessary (only minor CD improvement in theophylline). Figure 1 shows the critical doses measured for the selection of variables investigated. Using this knowledge of theophylline’s limits, lattice imaging has been attempted using a number of different techniques. Both bright field TEM and STEM imaging modes have been used and their results compared. Figure 2 shows a bright field STEM image of theophylline. Of note is that STEM results were acquired at a total electron fluence several times higher than the average CD of theophylline in TEM mode, suggesting an inherent damage reduction when analysing organic samples in STEM mode. Future work will focus on the use of improved hardware for direct lattice imaging in low-dose TEM and determining the best conditions for STEM mode to exploit the potential differences between imaging modes.

 
The authors would like to thank Dr. Ian Ross of the University of Sheffield, and Prof. Bill Jones and Dr. Mark Eddleston of the University of Cambridge.

 
[1] M S’ari, J Cattle, N Hondow, H Blade, S Cosgrove, RMD Brydson, AP Brown, ‘Analysis of Electron Beam Damage of Crystalline Pharmaceutical Materials by Transmission Electron Microscopy’, Journal of Physics: Conference Series, 644, (2015)
[2] DT Grubb, ‘Radiation Damage and Electron Microscopy of Organic Polymers’, Journal of Materials Science, 9, 1715 – 1736, (1974)
[3] J Cattle, M S’ari, N Hondow, P Abellán, A Brown, R Brydson, ‘Transmission Electron Microscopy of a Model Crystalline Organic, Theophylline’, Journal of Physics: Conference Series, 644, (2015)
[4] J Cattle, M S’ari, N Wilkinson, N Hondow, A Brown, R Brydson, ‘Prospects for High Resolution Analytical Electron Microscopy of Organic Crystalline Materials’, Microscopy and Microanalysis, 21, (2015)

James CATTLE (Leeds, United Kingdom), Mark S'ARI, Patricia ABELLÁN, Quentin RAMASSE, Nicole HONDOW, Andy BROWN, Rik BRYDSON

08:00 - 18:15 #6322 - MS04-739 Electron microscopy analysis of flash-annealed CuZr based bulk metallic glass.

MS04-739 Electron microscopy analysis of flash-annealed CuZr based bulk metallic glass.

Bulk metallic glass (BMG) is an amorphous material with no long-range order. Still, topological and chemical short-range or medium-range order is expected to occur. The unique atomic structures of BMG lead to interesting physical and mechanical properties that make them useful for potential applications. To circumvent the limited ductility of BMG, the concept of heterogeneous microstructure by forming composites has recently been used [1]. One route to achieve a composite structure is thermal treatment of the BMG. Here we present the structure of flash-annealed CuZr based BMG studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) methods including electron diffraction and fluctuation electron microscopy (FEM).

During the flash-annealing process the structure of BMG samples is modified by heating to different target temperatures above glass transition temperature and subsequent rapid cooling in a water bath leading to changes in the atomic structure. Cu₄₄Zr₄₄Al₈Hf₂Co₂ samples heated to 898 K and 916 K with a mean rate of 67 K/s were studied in a Zeiss Supra 55VP SEM at 20 kV as well as a Philips CM200 TEM operating at 200 kV.

SEM observation of the 916 K sample using back-scattered electrons reveals both an amorphous and a crystalline part each taking up about 50 percent of the sample with a transition area of amorphous material containing crystallites of different size in between (Figure 1). Figure 2 results from the TEM study of a FIB lamella prepared from a single crystallite as shown in Figure 1. The associated diffraction pattern (DP) with superlattice reflections indicates the presence of the B2 ordered structure. It is interesting to note that in crystalline CuZr based materials, devitrified from the amorphous structure, Cu₁₀Zr₇ and CuZr₂ structures are expected to occur.

To obtain structural information of the samples flash-annealed to 898 K, fluctuation electron microscopy was applied since no indication of crystallization was found. Figure 3a illustrates a corresponding DP with integrated intensity using the software PASAD [2] indicating a fully amorphous structure. Tilted dark field (DF) images show intensity variations in form of speckles as a result of local structural correlations (Figure 3b). Statistical analysis of a series of DF images acquired at different scattering vectors k and angles φ yields mean intensity and normalized variance as a function of k. The normalized variance is defined as V(k) = (<I(k,r)²>/<I(k,r)>²) – 1, with I being the image intensity and <> meaning the average over sample position r. Figure 4 shows intensity and variance curves for the 898 K sample using an objective aperture of 10 µm. While the maximum of mean intensity is located at about 4.0 nm^-1 the normalized variance shows a significant shift to larger k values. Similar curves were obtained using different spatial resolution by varying the objective aperture size.

Based on our results flash-annealing of CuZr based BMGs facilitates the formation of the B2 ordered crystalline structure as a metastable phase. In the amorphous phase of all samples the presence of a significant medium range order can be concluded from peaks in the normalized variance curve of DF FEM analysis. The maxima position of the normalized variance (~ 5 nm^-1) is different to that found in Zr₅₀Cu₄₅Al₅ BMG at about 4.0 nm^-1 reported in literature [3].

 

We kindly acknowledge financial support by the Austrian Science Fund (FWF): [I1309, J3397].

 

References

[1] J. Eckert, J. Das, S. Pauly, C. Duhamel, Journal of Materials Research 22, 285 (2007).

[2] C. Gammer, C. Mangler, C. Rentenberger, H. P. Karnthaler, Scr Mater 63, 312 (2010).

[3] J. Hwang, Z. H. Melgarejo, Y. E. Kalay, I. Kalay, M. J. Kramer, D. S. Stone, P. M. Voyles, Phys. Rev. Lett. 108, 195505 (2012)

Christoffer MÜLLER (Mödling, Austria), Christian EBNER, Christoph GAMMER, Konrad KOSIBA, Benjamin ESCHER, Simon PAULY, Jürgen ECKERT, Christian RENTENBERGER

08:00 - 18:15 #6423 - MS04-741 Phase identification of complex grain boundary precipitation in a high Cr and Ni superalloy upon direct-aging.

MS04-741 Phase identification of complex grain boundary precipitation in a high Cr and Ni superalloy upon direct-aging.

The need for corrosion-resistant alloys applied to support increasingly harsh environments at high temperatures resulted in the development of high chromium-containing nickel-base superalloys. It is well established in several complex multicomponent alloy systems, the concurrence of several modes of precipitation phenomena. Perhaps the most intriguing is the occurrence of concomitant general precipitation (GP, homogenous and heterogeneous) and discontinuous mode of precipitation (DP). While the former is controlled by lattice diffusion the latter is driven by grain boundary (GB) diffusion and migration. It is well established that the mechanical and corrosion properties depends critically on the phase stability under service conditions. This study is focused on identification of the different phases precipitated at a specific grain boundary.  Here, the precipitation takes place in an high performance superalloy 33 with austenitic matrix and chemical composition (wt.%) of 32,75Cr-32,53Fe-31,35Ni-1,49Mo-0,54Cu-0,4N-0,012C-0,63Mn-0,30Si. For this purpose, a JEOL JEM-ARM200CF aberration-corrected scanning transmission electron microscope (STEM) has been used at Lehigh.

Figure 1A presents a complex precipitation product at a specific GB in this alloy upon direct-aging at 700 °C for 100 h. Different phases have nucleated and grown, driven by the diffusion of a moving GB. The two diffraction patterns of Figs 1B and 1D correspondto both grains across the GB and the dark field (DF) TEM image of Fig. 1C shows the adjacent grain being consumed by the advancement of the GB reaction front of the DP.  In a previous study [1], X-ray elemental maps show the degree of partition of different substitutional elements (Fe, Ni, Cr, Mo) and interstitial N. In fact, one can identify more than two phases in this precipitate colony. In general, such complex GB precipitation products depend on the structure and thermodynamic properties of individual GBs [2].

Figure 2 shows a high angle annular dark field (HAADF) image of the region boxed in red in Fig. 1A.  This TEM session was conducted in a ChemiSTEM FEI Titan at UFRJ with the foil upside down respect to Fig 1A. This study reveals that the discontinuous precipitation at grain boundary results in three different phases. The precipitate indicated by n° 1 at the GB reaction front itself has been identified as η phase, Cr₃Ni₂SiN, with diamond cubic crystal structure. At the original position of the GB, however, two different phases have precipitated: precipitate n° 2 is a M₂₃C₆ carbide with FCC structure and precipitate n° 3 is an intermetallic σ phase with tetragonal structure. The precipitation of the latter is associated with embrittlement [3]. XEDS spectra and elemental mapping have corroborated such phase identification [1].

References

[1] Spadotto, J.C., Watanabe, M., Solórzano, I.G. (2016). Submitted to Microscopy&Microanalysis 2016.

[2] Solorzano, I. G., Purdy, G. R., Weatherly, G. C. (1984). Acta Metall. 32, 1709–1717.

[3] Sourmail, T. (2001). Mater. Sci. Technol. 17, 1–14.

Acknowledgements: This research was sponsored by CNPq (Brazil) and NSF-MWN (US: DMR-0303429) joint CIAM Program. One co-author acknowledges the financial support from FAPERJ. The authors are grateful to the Nucleo de Microscopia Electronica (UFRJ) for access to their TEM.

Julio Cesar SPADOTTO, Masashi WATANABE, Jean DILLE, Ivan Guillermo SOLÓRZANO (Rio de Janeiro, Brazil)

08:00 - 18:15 #6444 - MS04-743 Effect of processing parameters on microstructure and mechanical properties of additively manufactured stainless steel.

MS04-743 Effect of processing parameters on microstructure and mechanical properties of additively manufactured stainless steel.

Additive manufacturing (AM) allows for the building up of bulk structures through the fusion of powder particles layer by layer using a high power laser. This novel technique is practical for the fabrication of complex structures without any mold or tool, but this flexibility is accompanied by the delicate balance of AM processing and post-processing parameters which are to be optimized to ensure the desired microstructure in order to obtain the required mechanical properties [1-4]. The current study is aimed at understanding the effect of the AM processing parameters used for the preparation of AM samples on the resulting microstructure, its anisotropy according to growth direction, as well as its implications on the mechanical properties.

Bulk structures of conventional 316L grade stainless steel (with Cr-16.5-18.5%, Ni-10-13%, Mo 2-2.5%, traces of Mn, Si, P, S, C)  were produced by AM through selective laser melting (SLM), followed by an annealing heat treatment for material internal stress release. Two sets of processing conditions with variations in parameters such as the incident laser energy, the laser spot size and the direction of growth were varied, and the resulting effect on the microstructures both before and after the heat treatment are studied through diverse microscopy techniques such as SEM, EBSD and TEM. The variation in the phase distributions, grain size and porosity are analyzed as a function of the process parameters. Mechanical properties are measured by tensile tests completed by microhardness. Additionally, XRay Diffraction (XRD) has been used for identifying the phases present in the alloy and also for measuring residual stress and crystallographic texture.

The presence of the pores with unmolten particles has been observed in all samples produced with lower laser energy (Fig. 1). Alongside, the layers formed immediately on top of such pores show different microstructure, with the new phase observed consistently on the same side of the porosity in all samples (Fig. 2). The origin of this phase separation is expected to be from the differential heat dissipation (during fabrication) through the pore as opposed to the bulk material. The mechanical properties of the bulk structures (tensile stress, microhardness) showed that increasing the laser power results to an increase in the yield point. Fracture surface (Fig. 3) shows clearly presence of the pores which influences largely the mechanical properties such as ductility.

Overall, through the variation of the process parameters, the trend in evolution of the microstructural and mechanical properties is elucidated aimed at optimizing the parameters for AM products of 316L stainless steel.

1. H. D. Carlton, A. Haboub, G. F. Gallegos, D.Y. Parkinson, A. A. MacDowell, Materials Science & Engineering A, 651 (2016) 406–414

2. J. A. Cherry, H. M. Davies, S.Mehmood, N. P. Lavery, S. G. R. Brown, J. Sienz, International Journal of Advanced Manufacturing Technology, 76 (2015) 869–879

3. T. M. Mower, M. J. Long, Materials Science & Engineering A, 651 (2016) 198–213

4. A. Yadollahi, N. Shamsaei, S.M.Thompson, D.W.Seely, Materials Science&EngineeringA, 644 (2015) 171–183

Kaushik VAIDEESWARAN (Neuchâtel, Switzerland), Olha SEREDA, Youness ZANGUI, Hervé SAUDAN, Lionel KIENER, Massoud DADRAS

08:00 - 18:15 #6518 - MS04-745 Temperature rise during laser-induced self-organization of nanoparticle gratings revealed by Raman microspectroscopy and electron microscopy.

MS04-745 Temperature rise during laser-induced self-organization of nanoparticle gratings revealed by Raman microspectroscopy and electron microscopy.

Self-organization of metallic nanoparticles (NPs) has recently been reported upon visible continuous-wave (cw) laser exposure [1]. The self-organized structures are Ag NP gratings embedded in a thin TiO₂ film deposited on glass. Such composite structures exhibit singular visual effects that can find applications in secured traceability. The related optical properties directly depend on the NP size distribution, the average grating period, the organization rate and the TiO₂ thickness and refractive index. These sample features appear to be largely controlled by the temperature rise that occurs during the laser-induced self-organization process. The aim of the present contribution is to estimate the plasmon-induced temperature rise which appears to be strongly influenced by the laser scanning speed. To do so, Raman microspectroscopy and various modes of transmission electron microscopy (TEM) are used. The latter allow accurate information to be acquired on the NP size distributions resulting from different temperature rises, on their localization in the film and on the phase and chemical changes that occur in the film and the substrate surrounding NPs. Finally, we show how such thermal effects can be considerably decreased when using femtosecond (fs) laser pulses to initiate the NP self-organization.

The TiO₂ thin layer used in this work is initially mesoporous and amorphous and contains small silver NP of 1-3 nm as described in a previously published article [1]. The self-organized growth of silver NPs is implemented by scanning a laser beam focused on the sample surface at a constant speed. Post mortem Raman microspectroscopy shows that TiO₂ remains amorphous or adopt successively anatase, both anatase and rutile or only rutile crystalline forms for increasing laser scanning speeds, which was confirmed by high resolution TEM micrographs. Further in situ Raman microspectrocopy characterizations also attest an increase in temperature from 200°C to 750°C from low speed to higher speed in a range where anatase is formed; This increase of the temperature when the scanning speed increases was totally unexpected.  In addition to TEM crystallographic characterization, scanning electron microscopy (SEM) appeared to be useful to identify different morphologies for anatase and rutile nanocrystals and to study changes in the nanocrystal density as a function of speed.

Scanning TEM (STEM) micrographs and electron energy loss spectroscopy (EELS) analysis of sample cross-sections prepared by focused ion beam (FIB) give further interesting information about the in-depth structure of samples. Ag nanoparticles are located below the TiO₂ film (Fig. 1a) made of TiO₂ nanocrystals immersed in a Si-based amorphous phase, in a new interfacial thin amorphous layer mixing both Ti from the initial film and Si from the glass substrate (Fig. 1b). A three-dimensional reconstruction of the film sample from a series of FIB-SEM experiments confirms that all Ag NPs are rather spherical and located in a single plane just below the nanocrystalized TiO₂ layer. High angle annular dark field scanning TEM (HAADF STEM) imaging was used to study systematically non-monotonous changes in the NP size distribution with the temperature rise for many samples.

All studies that we have performed so far point out that the temperature rise can be considered as a drawback since it affects the integrity of the supporting material; we present here few results obtained with fs laser pulses in order to investigate a way to self-organize metallic NPs without high temperature rise in order to preserve the substrate and give the ability to work on softer substrates like plastic ones. Self-organization can successfully be obtained without altering the substrate top surface (Fig. 1 c-d). Ag NPs remain localized in the TiO₂ films, which is only locally crystallized around the grown NPs, as attested by STEM-diffraction maps recorded in TEM.

To conclude, this paper demonstrates the interest of a multimodal application of TEM techniques in order to provide a thorough study of the 3D nanostructure and chemical composition of complex samples made of Ag NP gratings embedded in a nanocrystallized TiO₂film, which result from laser-induced self-organization processes. It provides crucial information on thermal effects that drive the laser-induced self-organization process.

Z. LIU, G. VITRANT, L. SAVIOT, M. MARCO-DE-LUCAS, T. EPICIER, M. BUGNET (Ontario, Canada), Y. LEFKIR, S. REYNAUD, J. SIEGEL, M. GARCIA-LECHUGA, J. SOLIS, N. DESTOUCHES

08:00 - 18:15 #6519 - MS04-747 Deconvolution of EDS steel spectra using low acceleration voltages and low energy X-ray lines.

MS04-747 Deconvolution of EDS steel spectra using low acceleration voltages and low energy X-ray lines.

One of the most important materials used in industry is steel. Its fine microstructure consisting of different phases and inclusions, has led to the development of new steel alloys that push the analytical requirements for spatial resolutions down to the 100 nm scale. In X-ray microanalysis either using energy or wavelength dispersive spectrometry (EDS, WDS), low accelerating voltages, e.g. ≤ 6 kV, can fulfill this requirement. At low accelerating voltages only the L lines for most steel alloying elements (e.g. Cr, Mn, Fe, Co, Ni) can be determined as their K lines can no longer be excited. Qualitative and quantitative analysis of low energy L lines is very challenging due to the presence of absorption edges within the bremsstrahlung background, the energy dependence of the efficiency and the uncertainties of absorption effects [1]. Peak overlaps, furthermore are considerable for EDS and can even be significant for WDS.

Due to these effects this type of analysis poses difficulties for EDS and WDS. To evaluate and compare these challenges for both techniques, a systematic study was performed on steels at acceleration voltages of 15 kV and 5-6 kV [1,2]. Spectra and net intensities of 15 steel samples covering a wide range of concentrations of the major elements were simultaneously acquired on a JEOL JXA8530F electron microprobe equipped with 5 WD spectrometers and a Bruker XFlash ED spectrometer.

The results of the 15 kV spectra show that for elements with a concentration of < 1 wt.% the WDS results are more accurate when significant peak overlaps occur in ED spectra (Mn, Co, Cu). For elements without peak overlaps, EDS results were comparable (Si, Mo) to WDS [2]. At an acceleration voltage of 6 kV, the deconvolution results of ED spectra show  agreement with the experimental spectra, fig. 1. Since the quantification of L lines is more sensitive to the quantification procedure and fundamental parameters (e.g. mass absorption coefficient) used, the comparison focuses on  k-ratios instead of the quantified mass fractions. Fig. 2 shows a good agreement between EDS and WDS k-ratios, substantiating the peak deconvolution procedure shown in fig. 1.

 

References:

[1] Llovet et al, IOP Conf. Series: Materials Science and Engineering 32, 2012, 012014

[2] PT Pinard et al, Micosc. Microanal. 21 (Suppl 3), 2015, 1879

Ralf TERBORG (Berlin, Germany), Tobias SALGE, Philippe PINARD, Silvia RICHTER

08:00 - 18:15 #6678 - MS04-749 Atomically-resolved insight of unusual Sr-Mn(V) oxyhydroxide.

MS04-749 Atomically-resolved insight of unusual Sr-Mn(V) oxyhydroxide.

Inorganic compounds containing Mn(V) in tetrahedral coordination are known to show strong optical absorption, producing turquoise- to green-colored compounds [1]. The existence of hypermanganate anion MnO₄^3- has been found, for example, in the Ba₅(PO₄)_3-x(MnO₄)_xCl [2]. However, Mn(V) is very rarely found in oxides. The ability of Mn to be stabilized in mixed oxidation states leads in turn to interplay among spin, charge, and orbital degrees of freedom. In this sense, it has been recently reported that the electrochemical activity of the Mn(IV)/Mn(V) couple, plays a very important role in the discharge capacity of the nanostructured Li₄Mn₂O₅ material [3].

 

In this work we show the characterization of a Sr-Mn (V) oxide with blue-greenish color and nominal composition Sr₂(MnO₄)(OH) [4]. Atomically-resolved high angle annular dark field (HAADF) images acquired in a JEOL JEMARM200cF microscope shows that the Sr-Mn oxide crystallizes according to the apatite-type structure [5]. In this structure, each Mn ion is coordinated by distorted oxygen tetrahedral giving rise to isolated MnO₄ tetrahedra (Figure 1). Evidence of Mn(V) was shown by high energy resolution Electron Energy Loss Spectroscopy (EELS) (Figure 2a). However, contrast variations in the hexagonal tunnels of the apatite structure suggest either the direct imaging of light OH groups in the HAADF image or the presence of a heaviest element in these positions. Image simulation is used to elucidate the origin of such contrast because of the low stability of the sample under the electron beam prevents point-by-point spectroscopic analysis (Figures 2b and 2c).

 

References

[1] P. Jiang et al. Inorg. Chem. 52 (2013), pp.1349

[2] M. Uchida, J. Phys. Soc. Japan, 70 (2001), pp.1790

[3] M. Freire et al. Nat. Mat. 15 (2016), pp. 173

[4] E. J. Baran et al. Monatshefte für Chemie, 100 (1969), pp. 1674

[5] E. Banks et al. Inorg. Chem. 4 (1965), pp. 78

Isabel GÓMEZ-RECIO (MADRID, Spain), Almudena TORRES-PARDO, María HERNANDO, Aurea VARELA, Marina PARRAS, Jose Maria GONZALEZ-CALBET

08:00 - 18:15 #6703 - MS04-751 Correlative Raman spectroscopy and scanning electron microscopy of thermosetting carbon nanotube composite microstructures.

MS04-751 Correlative Raman spectroscopy and scanning electron microscopy of thermosetting carbon nanotube composite microstructures.

In recent years carbon nanotubes (CNT) have attracted significant research into their processing, properties and applications due to their extraordinary mechanical [1], electrical [2] and thermal properties [3]. Incorporating CNTs into polymer matrices to produce composite materials is one strategy to harness the potential of these materials. The development of a powder based processing route for thermosetting nanocomposites allows the manufacture of materials with high loading fractions (up to 20wt%) of CNTs [4]. Typically, nanocomposites with randomly dispersed CNTs show a decline in strength and plateau in elastic modulus beyond a few volume percent CNTs, as well as severe embrittlement [5, 6]. However, in recent work, the highest strength and modulus of the powder-based composites increased up to the highest loading [4]. Differential interference contrast (DIC) optical reflective microscopy of these nanocomposites have revealed a texture with domains on the length scale of the original powder particles, suggesting the migration of resin out of the nanocomposite particles to fill voids at particle interfaces. Raman spectroscopy combined with scanning electron microscopy is a powerful tool to identify the presence of epoxy rich regions and variations in CNT density. The correlated chemical and morphological analysis provides insight into the unique microstructure of the nanocomposite not possible by elemental analysis methods, such as EDS. Using correlated Raman and SEM techniques, the relationship between CNT loading on the “grain size” is quantified and calibrated resulting in an enhanced understanding of how the microstructure affects the macro mechanical properties of the nanocomposite.

 

References

[1] Salvetat, J.P., et al., Mechanical properties of carbon nanotubes. Applied Physics A, 1999. 69(3): p. 255-260.

[2] Popov, V.N., Carbon nanotubes: properties and application. Materials Science and Engineering: R: Reports, 2004. 43: p. 61–102.

[3] Hone, J., et al., Thermal properties of carbon nanotubes and nanotube-based materials. Applied Physics A, 2002. 74(3): p. 339-343.

[4] Herceg, T.M., et al., Thermosetting nanocomposites with high carbon nanotube loadings processed by a scalable powder based method. Composites Science and Technology, 2016. 127: p. 62-70.

[5] Coleman, J.N., et al., Small but strong: A review of the mechanical properties of carbon nanotube–polymer composites. Carbon, 2006. 44(9): p. 1624-1652.

[6] Yeh, M.-K., T.-H. Hsieh, and N.-H. Tai, Fabrication and mechanical properties of multi-walled carbon nanotubes/epoxy nanocomposites. Materials Science and Engineering: A, 2008. 483-484: p. 289-292.

 

Acknowledgements

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 642890

 

Sandra FISHER (London, United Kingdom), Milo SHAFFER

08:00 - 18:15 #6729 - MS04-753 Microstructure Analysis of Transient Liquid Phase Bonded Joints with Sn based Rapidly Solidified Braze Ribbons.

MS04-753 Microstructure Analysis of Transient Liquid Phase Bonded Joints with Sn based Rapidly Solidified Braze Ribbons.

Transient liquid phase (TLP) bonding is a diffusion based joining process that has been applied to different metallic systems. It is based on diffusion of filler metal constituents into the base material and/or vice-versa leading to isothermal solidification. Combining isothermal solidification with a subsequent solid state homogenisation treatment offers the possibility of producing bonds that are almost chemically identical to the base material and have no significant microstructural discontinuity at the bond line [1]. In this work, braze ribbons as filler material were produced by melt spinning process resulting in flexible ribbons with homogenous element distribution, which in the end will enable equally homogenous melting characteristics of the braze material [2]. The aim of this work is to analyse the microstructure of the braze ribbons to understand their melting and solidification behaviour in a TLP bonding process.

In this work, we present the microstructure analysis of two braze ribbons made of Sn78Cu22 and Sn75Cu20Ge5. Germanium was added for improving the glass forming properties. Electron backscatter diffraction (EBSD) and SEM EDS analysis were performed on a JEOL field emission SEM JSM-7000F equipped with EDS and EBSD detectors by EDAX. The TEM lamellae were prepared by the focused ion beam technique (FIB) using an FEI single beam FIB Strata 205. Scanning transmission electron microscope (STEM) investigations were conducted using a 200 kV field emission in column corrected omega filter TEM Zeiss Libra 200 FE equipped with HAADF and EDS detectors from Fischione and Bruker, respectively.

EBSD shows the differences in grain morphology after rapid solidification by the melt spinning process for the Sn75Cu20Ge5 braze ribbon (figure 1). It has been also revealed by EBSD analysis that metastable hexagonal CuSn phase could be found in the Sn matrix of the Sn75Cu20Ge5 braze ribbon. More detailed analyses for both braze ribbons were done by means of STEM combined with EDS analysis. These results revealed that CuSn phases in a Sn matrix can be found in both braze ribbons. Furthermore, cross sections of the Sn75Cu20Ge5 braze ribbon show different phases as well as different microstructures (figure 2). Crystalline CuSn phases were formed in a Sn matrix with amorphous Ge at the grain boundaries, whereas amorphous SnG areas were formed in the upper and lower part of the cross sections respectively. Moreover, DSC measurements were conducted to determine the melting behaviour of two ribbons and the measurement curves reveal two exothermic peaks up to 450 °C [3].

The brazing processes were performed in high vacuum by putting each braze ribbon in between aluminium cast material AlSi7Mg0.3. To achieve a uniform contact between the filler and base material during brazing, each joint was statistically loaded with p = 5.3 N/mm² during the brazing process. SEM EDS analysis was used to reveal the phases present at the brazed joints. As can be seen in figure 3, all phases are enclosed in a Sn matrix for both brazed joints. The Sn diffuses into the aluminium base material and the aluminium diffuses into the brazed joint material (see inset). In the Sn78Cu22 braze joint, intermetallic Cu_xAl_y phases can be detected. In the Sn75Cu20Ge5 braze joint, smaller CuAl precipitates spread over the joint with addition of GeAl. The results suggest that Ge induces a change of the intermetallic CuAl phase, while Ge binds a part of the Al.

Acknowledgments:

The authors wish to thank Kevin Kistermann for his assistance in preparing the samples for EBSD measurements. This study was part of the Collaborative Research Centre SFB 1120 “Precision Melt Engineering” at RWTH Aachen University and funded by the German Research Foundation (DFG).

References:

1. W.F. Gale & D.A. Butts (2004) Transient liquid phase bonding, Science and Technology of Welding and Joining, 9:4, 283-300

2. Kalinkichenka, S, Rascherstarrte nanokristalline Magnesiumlegierungen für die Wasserstoffspeicherung, Dissertation TU Dresden, 2011

3. K. Bobzin, M. Öte, S. Wiesner, L. Pongratz, J. Mayer, A. Aretz , R. Iskandar, A. Schwedt, Charakterisierung von Erstarrungsprozessen während des TLP-Bondings mit rasch erstarrten Lotbändern aus dem Melt-Spinning-Verfahren, 2016 WTK Chemnitz und IOP conference series

Riza ISKANDAR (Aachen, Germany), Ludwig PONGRATZ, Stefanie WIESNER, Mehmet ÖTE, Alexander SCHWEDT, Kirsten BOBZIN, Joachim MAYER

08:00 - 18:15 #6832 - MS04-755 Imaging molecular sieve 3A.

MS04-755 Imaging molecular sieve 3A.

The framework of many zeolites has been studied using different TEM techniques even though these materials are easily damaged by the electron beam¹. Until now the mechanisms for radiation damage in zeolites are not fully understood due to the complexity of their structure except for some works which have given a general indication on its origin^1-4. Despite the fact that each work used different zeolites it can be derived from them that the main factors for the stability of a zeolite under the electron beam are the Si/Al ratio, the size of cations and the water content. In this work we present the investigation of molecular sieve 3A (LTA framework) by HRTEM, with a Si/Al ratio of 1, and Na and K cations (Sigma Aldrich). Although the Si/Al ratio is unity i.e. the material is highly sensitive to electron irradiation² our results show that with electron dose rates up to 1000 e/nm²s the atomic structure of such material can be obtained (observation timespan ~1200 s) independent of the electron energy (80 keV, 200 keV or 300 keV). The latter indicates that by using an electron energy above 80 keV electronic exitations (e.g. radiolysis) and knock-on damage are simultaneously taking place during electron irradiation thus no dominance of either radiation mechanism, these results are in agreement with previous findings reported by Ugurlu et al.⁴. Although the structure of zeolite could be obtained, damage invariable occurred during recording, however, at a slow rate. In conclusion, these results are one step foward for the understanding of radiation damage mechanisms in molecular sieves, and a step towards imaging the cation positions.
 
 
[1] Bursill L.A., Lodge E.A., Thomas J.M. Nature 286 (1980)
[2] Csencsits R., Gronsky R. Ultramicrosocpy 23 (1987)
[3] Bursill L.A., Thomas J.M., Rao K.J. Nature 289 (1981)
[4] Ugurlu et al. Physical Review B 83 (2011)

Gerardo ALGARA-SILLER (Berlin, Germany), Thomas LUNKENBEIN, Robert SCHLÖGL, Marc Georg WILLINGER

08:00 - 18:15 #6970 - MS04-757 Microstructure of stainless steel produced by direct metal laser sintering.

MS04-757 Microstructure of stainless steel produced by direct metal laser sintering.

Microstructure of stainless steel produced by direct metal laser sintering
C. Labre1, 2, A. L. Pinto1, G. Solórzano2
1. Brazilian Center of Physics Research, Rio de Janeiro, Brazil.
2. Department of Chemical and Materials Engineering, PUC-Rio de Janeiro, Brazil.
Keywords: Additive manufacturing, direct metal laser sintering, 3D
Additive manufacturing or rapid prototyping is a layer-by-layer fabrication technique based on selective fusing and consolidating thin layers of loose powders with a scanning laser beam, for building three-dimensional (3D) objects [1]. Among the various methods of additive manufacturing the direct metal laser sintering (DMLS) had a big progress in the recent years and as an alternative manufacturing route to produce components, it attracted great attention due to the easy processing, high process speed and capability of producing complex shaped metallic objects. The present study focuses on the microstructural characteristics of the 15-5 PH stainless steel powder and a component built by DMLS from the same powder (composition: Cr 16,4wt%, Ni 4wt%, Cu 3,7 wt%, Mn 1,3wt%, Si 0,045wt%, Mo 0,5wt%, Nb 0,35wt%, C 0,045wt% and Fe (bal.)). For this purpose, JEOL JSM 7100FT scanning electron microscope (SEM), LYRA3 FEG SEM and electron backscattered diffraction (EBSD) have been used at Brazilian Center of Physics Research (CBPF).
Figure 1A shows that the 15-5 PH powders are in spherical shape and the particle size are in the ranged of 20 to 50μm. Preliminary EBSD results of the slice view of Figure 1B show that the grains are randomly oriented (Figure 1C), each orientation is indicated by a specific color, and it has a higher concentration of grain size of up to 7μm (Figure 1D). Grain orientation map shows that there is not a preferential crystallographic orientation.
Figure 2A shows a SEM image of the cross section of an object produced by DMLS. A laser scanning was repeated to build the subsequent layers until the objects is completed. Arrow indicates the building direction. The grain orientation map presents columnar grains randomly oriented melt pool boundaries (Figure 2B). The grain size distributions show a higher concentration of grain size below 5m (Figure 1C). The grain size of the object is reduced comparing with the grain size of the original powder.
Reference
[1] Journal of Materials Processing Technology 213 (2013) 2126 - 2136.

Cilene LABRE, Andre PINTO, Guillermo SOLÓRZANO (Rio de Janeiro, Brazil)

08:00 - 18:15 #6976 - MS04-759 atomic-resolution analysis of the structure and dopants of beam sensitive ordered porous materials.

MS04-759 atomic-resolution analysis of the structure and dopants of beam sensitive ordered porous materials.

With the modern transmission electron microscopes (TEM) sub-angstrom resolution has become readily achievable overcoming the lateral resolution problem. However, there are many cases where its fully potential cannot be totally exploited due the strong interaction between the high energetic electron beam and the material. Zeolites, zeotypes and metal organic framework (MOFs) suffer from this interaction resulting in an irreversible framework disruption through a radiolytic mechanism[1].

Since the pioneers electron microscopy observations of zeolites in the 70’s, its application and image resolution has been always been limited by this high sensitivity making impossible to fully exploit the TEM capabilities. However, in recent years with the introduction of the spherical aberration correctors and paying special attention to the electron dose, images with unprecedented resolution has been able to be obtained, making feasible the truly identification of the atoms conforming the structure.

In here, it will be presented atomic-column resolution images which will provide new information on guest species[2] within the zeolite cavities, as well as the observation of structural defects[3] providing new information about the ion exchange properties of these solids. Figure 1 displays the data obtained after introducing rare earth metals into ETS-10.

Furthermore, it will be shown how aberration corrected STEM provide unique information dealing with the analysis of new complex materials[4]; making able the observations of pores linked by single four-rings, double four-rings or oxygen linkers.

Acknowledgements

The authors would like to thank the European Union Seventh Framework Programme under grant agreement no. 312483 (ESTEEM2, Integrated Infrastructure Initiative).

References

[1] I. Díaz, A. Mayoral, TEM studies of zeolites and ordered mesoporous materials, Micron, 42 (2011) 512–527.

[2] A. Mayoral, T. Carey, P.A. Anderson, A. Lubk, I. Diaz, Atomic Resolution Analysis of Silver Ion-Exchanged Zeolite A, Angew. Chem. Int. Ed., 50 (2011) 11230–11233.

[3] A. Mayoral, J. Coronas, C. Casado, C. Tellez, I. Díaz, Atomic Resolution Analysis of Microporous Titanosilicate ETS-10 through Aberration Corrected STEM Imaging, Chem. Cat. Chem., 5 (2013) 2595–2598.

[4] M. Mazur, P. Wheatley, M. Navarro, W. Roth, M. Položij, A. Mayoral, P. Eliasova, P. Nachtigall, J. Cejka, R. Morris, Synthesis of unfeasible zeolites, Nat. Chem., 8 (2016) 58–62.

 

 

Alvaro MAYORAL (Zaragoza, Spain), Jennifer READMAN, Marta NAVARRO, Russell E. MORRIS, Isabel DÍAZ

08:00 - 18:15 #6985 - MS04-761 A multimodal and multiscale approach to investigate the micronsized organisation of a very complex biological material : the wheat grain.

MS04-761 A multimodal and multiscale approach to investigate the micronsized organisation of a very complex biological material : the wheat grain.

Nowadays, a large set of microscopy tools are commonly used to investigate the nanostructure of complex biological or synthetic materials with a multiscaled organisation: epi-fluorescence and laser scanning confocal microscopes (LSCM), scanning and transmission electron microscopes (SEM, TEM), atomic force microscopy (AFM), or Raman imaging. In particular, transmission electron microscopy (TEM) allows observation of the internal structures of natural systems (grains, cells and tissues) with a high spatial resolution. However, such observation requires in parallel dedicated sample preparation steps to prepare the fragile hydrated biological samples with successive steps including chemical or physical fixation, dehydration or vitrification of the water content, resin-inclusion and contrast-enhancement staining. Its well-known these steps may induce artefacts that reduce the image resolution and valuable information, but are often considered as the unique way to access to the ultrastructural information.

We propose here to explore alternative routes for the ultrastructure investigation of the grain tissues, as an example of a very complex biological material, with the main objective to reduce the preparation steps as most as possible while adapting the same preparation to be process with several microscopy tools: TEM, SEM, AFM, Raman imaging, Fluorescence imaging, X-ray microscopy. Depending on the successfulness of this approach, a set of complementary experiments from the same zone of a native sample can alloy to connect the plant ultrastructure with a high spatial resolution to accurate chemical information.

The multimodal and multiscale approach has been fully tested to study wheat grains tissues. We will show how unique features in the wheat outer layers organisation have been revealed due to both the capacity of keeping the native tissue “true contrast”, free from any chemical artefacts, or the native mechanical stress between the components, exempt from any alteration from resin infiltration. Moreover, taking advantage from the capacity of keeping the wheat tissue in its native shape, the mechanical properties of the native wheat outer layers (intenal and external pericarp, aleurone and sub-aleurone layers and starchy endosperm) have been characterised by AFM at the nonometer scale, see Figure 1. Additionally, the chemical composition has been acquired by Raman mapping from the different zones of interest, giving in an unique way the capability to understand the natural assemblies of biopolymers and mineral nutrients at the sub-cellular level in regard to their chemical, physicochemical and structural properties.

 

 

REFERENCES

C. Karunakaran, C.R. Christensen, C. Gaillard, R. Lahlali, L.M. Blair, V. Perumal, S.S Miller, A.P. HitchcockIntroduction of soft X-ray spectromicroscopy as an advanced technique for plant biopolymers research. PLoS One, 2015 26;10 (3) pages: e0122959.

Berquand A., B. Bouchet and C. Gaillard: Investigating the ultrastructure and mechanical properties of wheat grain tissues using optical microscopy and HarmoniX, Application Note, n°AN122, Veeco Instruments (2008)

Cédric GAILLARD (NANTES)

08:00 - 18:15 #6376 - MS05-763 Application of different imaging techniques for characterization and visualization of micro­ and nanostructural elements in Allvac 718Plus superalloy.

MS05-763 Application of different imaging techniques for characterization and visualization of micro­ and nanostructural elements in Allvac 718Plus superalloy.

Allvac 718Plus (718Plus) is a newly developed Ni-based superalloy, high strength, corrosion resistant and has improved higher temperature performance compared to the Inconel 718 superalloy. The 718Plus superalloy is used for applications in power generation, aeronautics and aerospace. The combination of a different chemistry and adequate heat treatments causes precipitation mainly the γ′ and δ or η- phases. The 718Plus microstructure consists of a γ matrix with spherical precipitates of ordered face centred cubic γ'-Ni₃(Al,Ti) type phase, some orthorhombic δ-Ni₃Nb and hexagonal η-Ni₃Ti, η*-Ni₆AlNb or Ni₆(Al,Ti)Nb particles precipitated at the grain boundaries.

The aim of this study was characterization of phases and 3D visualization of microstructural element morphology using several microscopy techniques, mainly HRSTEM-HAADF, STEM-EDX spectrometry. The present work concerns also the application of TEM and FIB-SEM electron tomography for imaging and evaluation of qualitative and quantitative information about microstructure of materials. Performed analysis in atomic level (HRSTEM-HAADF) of selected precipitates in the 718Plus superalloy revealed the complex nature of these precipitates, as shown in Fig.1. X-ray spectroscopic imaging (STEM-EDX), enables the mapping of local concentrations of selected chemical elements. This technique was used for qualitative and quantitative evaluation of η- phase precipitates with resolution about of 0.2 nm. STEM-EDX maps of selected elements forming η particles in 718Plus superalloy are presented in Fig.2.

The electron tomography is currently a relatively new technique in materials science that uses a TEM or FIB-SEM to 3D imaging of microstructural elements in various engineering materials. The TEM electron tomography technique enables obtaining 3D model of the investigated object(s) from the multiple 2D projection images, acquired over a range of viewing directions (±75°). The TEM investigation was performed on FIB lamella by a Cs-corrected Titan³ G2 60-300 with EDX ChemiSTEM™ technology, which allowed to achieve high X-ray signal over a large tilt angle of sample  and collect a tomographic series of 2D elemental maps at the angular range from -40° to +60° (with a step of 4°) of tilting the sample. The STEM-EDX imaging by ChemiSTEM™ provides new opportunities for 3D visualization of changes in the concentrations of particular chemical elements in nanoparticles or analysis of the microstructure of thin foils. Tomographic reconstruction of a tilt series images was performed using SIRT method, which allowed visualizing the three-dimensional distribution of selected elements (Al, Cr) in the analysed volume. Application of elemental maps imaging (STEM-EDX) acquired during tilting the sample was used for 3D imaging of coherent γ′ precipitates in 718Plus superalloy (Fig.3a).

FIB-SEM tomography is based on a serial slicing technique using a FIB-SEM dual beam workstation. Dual-beam SEM enables the acquisition of serial images with small (few nanometers) and reproducible spacing between the single imaging planes - because no mechanical stage tilting is necessary between the FIB milling and the electron beam SEM imaging steps. Meso-scale FIB-SEM tomography technique, was used for characterization of spatial distribution and metrology of the η- phases in 718Plus superalloys with resolution of 12 nm. SEM backscattered electrons (BSE) image of 718Plus superalloy presents different shapes of the η- phase precipitated in the γ matrix. Fig. 3b shows three-dimensional visualization of FIB-SEM tomographic reconstructed η- phase. Fig. 3b shows at different angle of view a morphology of selected η particles precipitated at the γ grain boundary. Platelets, occasionally as a lamellar structure at grain boundaries and in the grains were observed in the microstructure of the 718Plus. The η- phase precipitates at the γ grain boundaries had much higher thickness (270 nm) than the thickness of the η- phase plates (56 nm) in lamellar precipitates inside γ grains (Fig. 3b). Application of HRSTEM-HAADF imaging and tomographic techniques (STEM-EDX, FIB-SEM) allowed for visualization the precipitates in 718Plus superalloy, their quantitative assessment, spatial distribution and morphology. 

Acknowledgement: The authors acknowledge the financial support from EU 7FP under Grant Agreement 312483 - ESTEEM2 

Adam KRUK (Krakow, Poland), Aleksandra CZYRSKA-FILEMONOWICZ

08:00 - 18:15 #6485 - MS05-765 HAADF STEM and EELS Analysis of Li(Ni0.8Co0.15Al0.05)O2 Cathode Held at High Voltages.

MS05-765 HAADF STEM and EELS Analysis of Li(Ni0.8Co0.15Al0.05)O2 Cathode Held at High Voltages.

Ni rich Li(Ni_0.8Co_0.15Al_0.05)O₂ commonly known as NCA is being used commercially as Li-ion battery cathode material for its high discharge capacity. [1] The stability and related safety concerns at high charge voltages limit the use of NCA to 3.6 V charge voltage corresponding to 0.5 extracted Li.  In order to extract a higher amount of Li, high charge voltages to 4.75 V are required. The bulk R-3m layered structure of NCA does not change, however new surface phases are formed wich are induced by oxygen loss. To utilize the full potential of NCA, high-voltage studies of surface phases, their chemical evolution and their mechanisms are needed.  We present here the evolution of surface phases in NCA held at constant voltages up to 4.75V.

The surface phases of NCA were observed in a cold cathode field emission aberration corrected JEOL ARM (at Lehigh University) for HAADF STEM imaging. The spatial resolution of the STEM images was 0.07 nm. EELS as well as HAADF STEM imaging were carried out using a cold cathode aberration corrected HITACHI HD2700C TEM and GATAN Enfina EELS spectrometer with an energy resolution of 0.5 eV. A HAADF STEM image of a NCA particle (held at 4.75V for 2 weeks) oriented along [010] zone axis of layered (R-3m) structure is shown in Figure 1.  Atomic–resolution images from different regions of the particle were obtained revealing structural inhomogeneity with two different surface phases. The HAADF STEM image from region 1 (Figure 2a) shows that the bulk layered structure (R-3m) is maintained up to the surface, but with almost 1/3 of transition metal (TM) ions (mostly Ni) occupying to the Li layer as determined from the intensity profile (Figure 2b). In region 2 the surface shows a HAADF contrast corresponding to “rocksalt (RS) type” phase (Figure 2c), however with a higher O content than for stoichiometric NiO.  The presence of two different phases within the same particle shows that the surface of NCA particles is highly inhomogeneous. Also, this large scale migration of TM ions (1/3 of TM moving to Li layer) from its original octahedral site to a tetrahedral Li site is driven by the presence of oxygen vacancies. This is consistent with the earlier report which shows that the diffusion barrier for TM migration is reduced when initial configuration of TM octahedron is five coordinated (MO_5, deficient in one O) instead of regular six coordinated (MO₆).[2]  Oxygen loss as measured by EELS indeed accompanied these surface phase transformations.  A surface with NiO chemistry is observed only within 1-2 nm from the surface.  A characteristic EELS feature of pristine NCA is the presence of O-pre-peak, 12.0 eV from the main O peak, which occurs due to a transition from O 1s to a hybridized state formed by O 2p and Ni 3d states. For NiO, this pre-peak is smaller and located 7.5 eV from the main O peak.  This pre-peak is almost missing at the surface as shown for a sample held at 4.75V (Figure 3a).  The O edge at this voltage resembles that of NiO phase (Figure 3b) with a lower pre-peak intensity and average energy position of 7.5 eV. The O/TM atomic ratio of 1.6 measured for the surface phases and averaged over several particles show that the surface is oxygen deficient as compared to pristine NCA (2). Also, a reduction in Ni valence, measured from Ni L₃/L₂ ratio is observed at the surface (Ni^+2.4) as compared to pristine NCA (Ni⁺³). Both of these values are between pristine NCA and NiO (O/TM ratio of 1 and Ni²⁺ valence state).  So, the reduction in Ni observed at 4.75V is corroborated by a shift of Ni L edge with respect to bulk NCA (Figure 3c). [3]

References:

[1] S. Hwang, et. al. Appl. Phys. Lett. 105 (2014) 103901 (4pp)

[2] D. Qian, et. al. Phys. Chem. Chem. Phys. 16 (2014) 14665 (4pp)

[3]The funding for this work is provided by NECCESS, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001294.                    

Pinaki MUKHERJEE, Dong SU, Nathalie PEREIRA, Glenn AMATUCCI, Frederic COSANDEY (Piscataway, USA)

08:00 - 18:15 #6502 - MS05-767 EELS analysis of He bubbles in ODS Steel and vanadium.

MS05-767 EELS analysis of He bubbles in ODS Steel and vanadium.

 

The understanding and the assessment of neutron irradiation in nuclear materials is critical in the design of the next-generation nuclear fission reactors. Here, one of the most promising structural and fuel cladding materials, an oxide-dispersion-strengthened (ODS) steel was implanted with He and Fe ion in order to simulate the transmutant He and the damage (He/dpa) caused by neutron irradiation. The fine distribution of Y-Ti-O nanoparticles (NPs, 1-20 nm) in the Fe-Cr ferritic matrix is expected to improve thermal and mechanical properties. STEM-EELS was used to investigate the structure and chemistry of these NPs and the He bubbles generated. The ODS material (Fe-14Cr-1W-0.3TiH₂- 0.3Y₂O₃, wt.%) was prepared by mechanical alloying of Fe-Cr-W-TiH2 and Y₂O₃ powders followed by hot extrusion. Ion irradiation was carried out at 500°C, producing 5 dpa damage (Fe) with 1000 appm/dpa He implantation. Core loss spectrum-images were denoised using principal component analysis [1]. Implanted He is shown to be trapped in some Y-Ti-O NPs (fig. 1) although bubbles also exist outside the NPs. The ion irradiation has also changed the Cr distribution, removing the Cr-shell observed around the NPs in non-irradiated ODS samples (fig. 1) [2,3] and rendering the Cr distribution in the metallic phase generally more heterogeneous.

 

The He-K line (21.218 eV for free atoms) shifts to higher energy in the bubbles (ΔE = 0.5 to 4 eV); this is shown to be correlated with the He density. He quantification has been carried out with three different methods: spatial difference, curve-fitting and Multivariate analysis (MVA) methods (see below), as well as hybrid approaches combining the latter two methods which are proving promising for the elimination of the problems associated with each method on its own. The density and pressure values are found to reach 105 nm^-3 and 8 GPa respectively, although the pressure measurement is only semi-quantitative given that the error bars can reach 30%. The curve-fitting method allows us to map the He-K energy position and intensity, yielding the density, in individual bubbles (fig. 2). The spectral response of individual bubbles can be separated in an SI containing many bubbles at different densities using independent component analysis (ICA: example shown in figure 3) or vertex component analysis (VCA). Bubbles larger than 4 nm are shown to be under-pressurized or at equilibrium with the Fe-Cr matrix. Below 3.5 nm, the He pressure is shown to increase markedly, passing into the over-pressurised regime.

In the presentation we will also describe experiments and analysis on similar bubbles implanted in vanadium at 700 and 1000°C. Here the size/pressure relation is much less clear, but the MVA approach is similarly promising.

 

We thank the French ‘‘Contrat de Programme de Recherche: ODISSEE’’ funded by AREVA, CEA, CNRS, EDF and Mécachrome under contract n070551, the METSA network and the European 7th framework program ‘‘ESTEEM2’’ for financial support. We also thank the ANR programme EPIGRAPH and all its memebers for their contribution to the vanadium studies.

 

[1] www.hyperspy.org

[2] A. Hirata et al. Nature Materials 2011;10:922.

[3] V. Badjeck et al. J. of Nucl. Mat. 456 (2015) 292

Michael WALLS (LPS, Orsay), Nathalie BRUN, Vincent BADJECK

08:00 - 18:15 #6505 - MS05-769 Reduction of Fe antisite defects in LiFePO4 for Li-ion batteries.

MS05-769 Reduction of Fe antisite defects in LiFePO4 for Li-ion batteries.

Triphylite (LiFePO₄ or LFP) has raised as promising cathode material in lithium-ion batteries, due to its superior safety, high theoretical capacity (~170  mAhg^-1), high stability, and suitable operating voltage (~3.4 V). Olivine has a structure with Pnma space group, with lithium ions confined in the channels (M1 site) formed by interconnecting FeO₆ octahedra (M2 site) and PO₄ tetrahedra (see Figure 1 top). One of the limiting factors in ionic conductivity in these sample is the presence of Fe antisite defects (Fe in M1 site) blocking Li diffusion. This affects the hydrothermal synthesis of LiFePO₄ (lowest cost method but able to produce large scale material). We have found that the hydrothermal synthesis involves indeed an intermediate vivianite phase, which inevitably creates Fe-antisite defects. However, these can be progressively removed by cation exchange, assisted by a nanometer-thick Li rich amorphous layer at the surface, which acts as a second phase to exchange Li and Fe atoms with (Figure 1). To study the presence of antisite defects we have combined neutron scattering with HAADF-STEM, XPS, and EELS. The presence of Fe antisites in the M1 site can easily be spotted in aberration corrected HAADF images (see Figure 2) and the amount of defects quantified by comparison with image simulations [1]. We have found that the defects can be reduced to ~2% by prolonging the synthesis procedure to 5 hr, while reducing at the same time the amorphous layer thickness at the surface. Moreover, the time of treatment can further be largely reduced (to approx. 30 min) by making use of Ca⁺ ions added during the synthesis, which favors the Fe antisites removal while keeping optimal charge characteristics [2]. This is achieved by two concomitant effects: (i) the Fe-antisite defects aggregate at the surface of the LiFePO₄ crystal during crystal reduction in size, (ii) the increase in the surface area, which further exposes the Fe-antisite defects. Figure 2 compares the HAADF images from the samples synthesized in 30 min. using Ca⁺ and without Ca⁺, respectively [3].

ACKNOWLEDGEMENTS

[1] Paolella A. et al. Cation exchange mediated elimination of the Fe-antisites in the hydrothermal synthesis of LiFePO₄. Nano Energy 16, 256–267. doi:  10.1016/j.nanoen.2015.06.005 (2015)

[2] Paolella A. et al. Accelerated Removal of Fe-Antisite Defects while Nanosizing Hydrothermal LiFePO₄ with Ca²⁺. Nano Letters, Article ASAP. doi: 10.1021/acs.nanolett.6b00334 (2016)

[3] European Union FP7 Grant Agreement n. 265073 ITN-Nanowiring, and FP7 Grant Agreement n. 312483 ESTEEM2 for Integrated Infrastructure Initiative – I3

Giovanni BERTONI (Parma, Italy), Andrea PAOLELLA, Stuart TURNER, Karim ZAGHIB

08:00 - 18:15 #6528 - MS05-771 Factors limiting the doping efficiency in atomic layer deposited ZnO:Al thin films: a dopant distribution study by transmission electron microscopy and atom probe tomography.

MS05-771 Factors limiting the doping efficiency in atomic layer deposited ZnO:Al thin films: a dopant distribution study by transmission electron microscopy and atom probe tomography.

Transparent conducting oxides (TCOs), such as indium tin oxide (ITO),  are commonly used as transparent electrodes in a wide variety of devices, such as in displays and solar cells. ZnO has been reported to be a promising alternative TCO for ITO, because of its lower cost. As the conductivity of intrinsic ZnO films is too low for the applications in mind, doping the ZnO film is essential, the most common dopant being Al. Atomic layer deposition (ALD) is an emerging technique for the deposition of doped ZnO thin films, allowing for accurate thickness control and excellent conformality on high aspect ratio topologies. Due to the self-limiting half-reactions and cyclic nature of the ALD process, not only  the aforementioned characteristics can be met, but also the amount and distribution of dopants can be controlled by selecting the precursors (i.e. the Zn or Al precursors) for each individual half-cycle. However, thus far, the maximum conductivity that can be obtained in Al-doped ZnO (ZnO:Al) thin films prepared by ALD appears to be limited by the low doping efficiency of Al.

To better understand the origin of this limitation, the 3-dimensional distribution of Al atoms in ZnO films has been examined using a combination of Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT). For this study, three ZnO:Al films with different Al:Zn ratios were grown sequentially in one film stack, and capped and separated by intrinsic ZnO films. A diagram of the stack is shown in Fig. 1a. This geometry allowed a single APT or TEM measurement to collect data on all three doped films, keeping the analytical conditions identical. BFTEM studies (Fig 1b) showed that for high Al concentrations the ZnO grains are interrupted, while they continue across the lower doped layers. Scanning TEM – High Angle Annular Dark Field (HAADF) imaging and 2-D EDX mappings allows for revealing the aluminum distribution as a function of film depth, showing that the Al-doped layers follow the surface topography of the ZnO grains during growth Fig. 1c,d. However, TEM is limited in providing 3-D dopant distributions, on the one hand because of the limited sensitivity of EDX, on the other hand because of the projection of rough interfaces in a 2-D image. The latter is illustrated in Fig. 2a: individual Al-doped layers can clearly be discerned for larger interspacings, but are poorly recognizable in layer ‘AZO-3’.

One-dimensional depth profiles extracted from cylindrical sub-volumes of the 3D APT data (Fig. 2 b) are presented in Fig. 2c. These 1D profiles show that the peaks in Al concentration are no δ-functions, as might be expected from the binary nature of the ALD process. Instead, the peaks have a full width at half maximum (FWHM) of ~2 nm. The 3-dimensional dopant distribution can be used to explain the dependencies of resistivity and doping efficiency on growth recipes used. When the local Al density is too high, the doping efficiency is limited by two proposed limiting factors: the solid solubility limit of Al atoms in a ZnO matrix and the disorder-induced carrier localization.

Marcel VERHEIJEN (Eindhoven, The Netherlands), Yizhi WU, Devin GIDDINGS, Ty PROSA, David LARSON, Fred ROOZEBOOM, Erwin KESSELS

08:00 - 18:15 #6533 - MS05-773 TEM investigation of the effects of cycling on electrochemically stable hybrid Pt@NbOx nanocatalysts.

MS05-773 TEM investigation of the effects of cycling on electrochemically stable hybrid Pt@NbOx nanocatalysts.

Renewable energies and fuel cell technologies will likely play a major role in reducing our dependency on fossil fuels. In particular, proton exchange membrane fuel cells (PEMFCs) are suitable for use in both domestic and automotive applications. This sophisticated technology requires a functional nanostructured material that contains platinum to catalyse both the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) to produce water, electricity, and heat. [1] Unfortunately, in spite of their remarkable potential, the high cost and degradation of platinum-based catalytic materials have been a barrier to widespread adoption of this type of technology. Reducing platinum content while simultaneously improving the durability will therefore require novel approaches in the catalyst design and its characterization at nanoscale level. [2]

Transmission Electron Microscopy (TEM) techniques have continued to play a major role in the design and characterization of PEMFC materials. Using an aberration-corrected microscope (Titan cubed 80-300), we investigated the structural stability over the lifetime of a proposed fuel cell cathode material containing nano-particulate platinum on a NbOx-carbon hybrid support. The characterization of this material included a series of ex-situ TEM analyses before or after accelerated stress tests that cycled the sample 30,000 times between 0.6 and 1.0 V in an electrochemical cell.

The histograms shown in Fig1a and b reveal that some metal particle coarsening occurred during the 30,000-cycle test. The as-prepared catalyst contained 1–5 nm particles, the majority of which were determined to be bimetallic, as determined by electron energy loss spectroscopy (EELS), with some Pt-rich particles and Nb-rich grains (Fig. 1c). After 30,000 cycles, microanalysis data confirms that the cycling treatment caused some agglomeration of the small Pt-rich particles. Analysis of the EELS oxygen K ionization edge indicates that multiple niobium oxidation states are present in the system, predominantly Nb(V) before electrochemical cycling (Fig 1c). Although conventional image comparisons between the initial and final state of the hybrid catalyst suggest that the Pt particle size increased marginally, in order to obtain an improved understanding of the material’s degradation, morphological and structural evolution of the particles and hybrid support were tracked using the so-called Identical Location TEM technique [3]. The results in figure 2 indicate that no large-scale degradation of the hybrid support took place, suggesting carbon corrosion was minimized. In addition, only minor changes occurred to the average particle size, demonstrating the excellent stability of the metallic particles. These findings demonstrate that highly dispersed Pt/NbO_x on carbon support material is a promising electrocatalyst for PEMFC.

[1] J. Wu, X. Z. Yuan, J. J. Martin, H. Wang, J. Zhang, J. Shen, S. Wu, Walter Merida, “A review of PEM fuel cell durability: Degradation mechanisms and mitigation strategies”, J. Power Sources, vol 184, no.1, pp 104-119, 2011

[2] B. Corain, G. Schmid, N. Toshina, Metal Nanoclusters in Catalysis and Materials Science: The Issue of Size Control, Elsevier, Amsterdam, 471 p, 2008

[3] K. J.J. Mayrhofer, J. C. Meier, S. J. Ashton, G. K.H. Wiberg, F. Kraus, M. Hanzlik, M, Arenz, “Fuel cell catalyst degradation on the nanoscale” Electrochemistry Communications, vol. 10, no. 8, pp.1144-1147, 2008

Lidia CHINCHILLA, David ROSSOUW (Dundas, Canada), Tyler TREFZ, Natalia KREMLIAKOVA, Gianluigi BOTTON

08:00 - 18:15 #6550 - MS05-775 Insight into CuInS2 thin films by electron microscopy.

MS05-775 Insight into CuInS2 thin films by electron microscopy.

Over the last decades the interest in materials for renewable energy applications has increased considerably. One promising material is copper indium disulfide, CuInS₂, which has a band gap of 1.5 eV and a high absorption coefficient.^[1] Due to the electronic and optical properties CuInS₂ is not only attractive for solar cells but also for solar-driven water splitting devices and in photo catalysis to generate H₂ as an energy carrier.^{[2, 3]} For these applications, a large surface area and a high crystallinity are beneficial. CuInS₂ thin films with such structural features can be grown by several wet chemical approaches. One is a solvothermal synthesis route ^{[4, 5]} which uses moderate temperatures of around 150 °C and common chemicals as precursors e.g. CuSO₄ ∙ 5H₂O and InCl₃.

In the present work the used sulfur source Thioacetamide, which is toxic, has been replaced with L-Cysteine, a natural amino acid. The other solvothermal conditions have been kept as reported in literature. The CuInS₂ films were grown on Fluorine-doped tin oxide (FTO) coated glass substrates and investigated using X-ray diffraction (XRD) and electron microscopic techniques such as scanning electron microscopy (SEM), (scanning) transmission electron microscopy ((S)TEM), energy-dispersive X-ray (EDX) spectroscopy and electron energy loss spectroscopy (EELS). SEM was done on a Zeiss Merlin microscope equipped with a Bruker EDX system. (S)TEM was performed on a FEI Titan Themis operated at 300 kV and equipped with a SuperX EDX system. EELS was done on a FEI Titan 80 – 300 kV and a Gatan Tridiem image filter. The aim of the work is to study the reaction path and to determine the microstructure of the films.

For the sulfur source L-Cysteine a different surface morphology is obtained compared to films grown with Thioacetamide as sulfur source. Thioacetamide gives an open flower-like surface topology (Figure 1(c)) whereas L-Cysteine leads to films consisting of agglomerates of small nanoparticles (Figure 1(a,b)). As visible in the SEM images there are also multiple larger agglomerates built up from nanoparticles on top of the film. For both sulfur sources the Chalcopyrite structure of CuInS₂ is proven by XRD (not shown) and selected area electron diffraction (SAED, Figure 2 (d)). For increasing precursor concentration the film thickness increases. STEM images of CuInS₂ films grown with a medium precursor concentration are shown in Figure 2. EDX maps show that the films contain mainly Cu, In and S. Quantification of several areas gives Cu:In 1:1 with a lack in sulfur. Compared to a scratched TEM sample, where a stoichiometric composition of Cu:In:S 1:1:2 is obtained, this results indicate that sulfur is not stable and leaves the sample during FIB preparation.

Since the Cu²⁺ of the precursors CuSO₄ must be reduced to Cu⁺ to form CuInS₂, the oxidation state of Cu was examined with EELS (Figure 3 (c)). The reaction product which forms between CuSO₄ and L-Cysteine without heating consists of an amorphous matrix in which crystalline nanoparticles with a size of a few nanometers are embedded (Figure 3 (a)). Compared to literature^[6] the EELS data indicate an oxidation state of +1 for copper. This means that L-Cysteine is sufficient to reduce Cu²⁺ to Cu⁺. Furthermore SAED shows distinct d-values for Cu_xS phases (Figure 3 (b)).

References

[1]        B. Tell, et al., Physical Review B 1971, 4, 2463.

[2]        J. Luo, et al., Nano Letters 2015, 15, 1395.

[3]        Y. Wang, et al., Materials Letters 2014, 137.

[4]        S. Peng, et al., Journal of Alloys and Compounds 2009, 481.

[5]        A. Wochnik, et al., J Mater Sci 2012, 47.

[6]        F. Hofer, et al., Ultramicroscopy 1987, 21.

Anna FRANK (DUSSELDORF, Germany), Angela WOCHNIK, Christina SCHEU

08:00 - 18:15 #6586 - MS05-777 Electron microscopy studies of Silicon Radial junction for stable and highly efficient thin film solar cells.

MS05-777 Electron microscopy studies of Silicon Radial junction for stable and highly efficient thin film solar cells.

Owing to their enhanced light trapping and anti-reflection effect, silicon nanowires (SiNWs) provide an effective platform for developing a new generation of low-cost and efficient solar cells. By decoupling the light absorption and carrier collection directions, SiNWs enable the use of ultra-thin intrinsic layers for high efficiency PIN radial junction solar cells fabricated on the NW matrix. Apart from reducing the material consumption, using thinner absorber layers provides higher built-in field and hence a better separation of carriers. Consequently, the cells operate as random arrays of microscopic PIN diodes connected in parallel with optical and electrical properties of microstructural elements strongly depending on their dimensions, occurrence of defects or structural imperfections. [1,2] From a fundamental point of view, it is expected that overall performances of the cells could be limited by weak diode elements related to local variations of microstructure.

Along this line, the goal of this work is to perform a comprehensive analysis using both TEM and the STEM-HAADF imaging modes, together with the STEM-HAADF electron tomography. This type of analysis provides reliable information regarding the morphology and the crystallographic structure of the radial junction (RJ) which allows us to propose hypotheses, firstly on the nucleation and the growth of SiNWs processes and second how conformaly the hydrogenated amorphous (a-Si:H) layer and ITO(Indium Tin Oxide) layers cover the SiNWs. In order to access the characteristics of each component prior to the TEM observations, an FIB (Focused Ion Beam) preparation technique was used to obtain thin lamellas on each studied radial junction sample. Figure 1(a) shows an SEM image of the complete PIN radial junction solar cell after the deposition of intrinsic and n-type a-Si:H and Fig. 1(c) after the ITO sputtering. Both images were taken at positions marked on the sample photo in Fig. 1(b). The TEM observations performed in bright field mode on transversal FIB cross-sections (see Fig. 2) for both samples allowed the identification of the SiNW- core, the a-Si:H layers (intrinsic and n-type) as well as the ITO. The typical total thickness of the the a-Si:H was estimated to be around 120 nm. Concerning the sputtered ITO layer, the observation allowed seeing that the deposition is done in an inhomogeneous manner with thickness varying from 50 to 90 nm around the RJ. Regarding the morphology of the radial junction, several tomographic studies were performed in STEM-HAADF imaging mode on a radial junction solar cell without an ITO layer. A detailed analysis of the reconstructions suggests that the diameter of the Si core NWs is varying from 22 nm at the bottom to 10-12 nm at the top. Concerning the a-Si:H layers, we have observed that they are not perfectly homogeneous, but have their  thickness varying from 140 nm at the top of the radial junction to 65 nm at its bottom.

 Reference:

[1] S. Misra, et al. J. Phys. D: Appl. Phys. 47 (2014) 39(3001)

[2] S. Misra, L. Yu, M. Foldyna, P. Roca i Cabarrocas. IEEE JPV, 5, 40 (2015)

 Acknowledgements: This work received support from the French state managed by the National Research Agency under the Investments for the Future program under the reference ANR-10-EQPX-50-NANOTEM, as well as from the project Solarium (ANR-14-CE05-0025).

Ileana FLOREA (Palaiseau), Soumyadeep MISRA, Martin FOLDYNA, Raul ARENAL, Jean-Luc MAURICE, Linwei YU, Pere ROCA I CABARROCAS

08:00 - 18:15 #6602 - MS05-779 In-situ thermal stability and reactivity monitoring of Au nanoparticles using Cs-Corrected environmental TEM.

MS05-779 In-situ thermal stability and reactivity monitoring of Au nanoparticles using Cs-Corrected environmental TEM.

        For the production of novel or more efficient catalysts, the link between nanoparticle (NP) structure and catalytic performances need to be understood. Such an understanding requires tools that allow the "observation" with single-atom sensitivity of the surface of real catalysts in response to reaction conditions. Indeed, when nanocatalysts operate in gas environment and high temperature, they do not remain static but undergo dynamic atomic-scale processes (surface restructuration, oxidation...) which directly influence catalytic properties. In this work, we have studied the thermal stability of Au-Cu nanocatalysts under oxidative and reductive environments and/or at high temperature using environmental aberration corrected TEM (ETEM).

 

        Au NPs with controlled sizes, compositions and morphology were synthesized by pulsed laser deposition. Environmental gas TEM of nanocatalysts at the atomic scale was undertaken in an aberration-corrected JEM-ARM200F TEM with in situ gas/temperature conditions achieved by using a MEMS-based nanoreactor (Protochips Inc.) and equipped with a high performance Gatan OneView camera enabling the capture of both high quality images and high-speed videos of in situ events (512 x 512 pixels at 300 fps).

   Figure 1 compares TEM images of Au NPs (a) at room temperature in vacuum, (b) at 300°C under O₂ atmosphere and (c) at 300°C under H₂ atmosphere. At room temperature, in vacuum, NPs are stable but once a gas, O₂ and/or H₂ is injected, coalescence phenomena appear. The coalescence continues at 100°C and 200°C. An additional phenomenon appears at 200°C with O₂ that does not with H₂: particle faceting. At 300°C, under O₂, all particles are well facetted (truncated octahedron). When we cool down the sample with O₂, the faceting disappears. Thus, particles size and morphology are clearly dependent on temperature and gas nature with an increase in temperature favoring particle coarsening and exposure to an O₂ atmosphere leading to more facetted Au NPs.

   Figure2 shows TEM images of Au NPs at 300°C (a) which are all facetted at this temperature (truncated octahedron) and at constant temperature of 850 °C at different times (b,c,d). The NPs become more spherical at higher temperature and began to evaporate at 850°C. One can determine the surface energy of the evaporating particles by means of the Kelvin equation². There is a size dependence of the surface energy of the NPs, indeed, the surface energy increases with the size. For a particle radius greater than 4nm, one cannot apply the Kelvin model. Thereby, in order to understand the underlying mechanism of the behavior of a nanoparticle in this size range, simulations using the tight binding model are in progress. Also, by alloying gold with copper, we are studying the phase reactivity of AuCu nanoparticles as well.

 

 

 

 

 

 

 

 

 

 

 

References

 

1)       Prunier H. et al., Phys. Chem. Chem. Phys., 2015,17:28339-28346.

2)       Sambles J.R et al., Proc. R. Soc. Long. 1970, A 318, 507.

Adrian CHMIELEWSKI (Paris), Hélène PRUNIER, Jaysen NELAYAH, Hakim AMARA, Jérôme CREUZE, Damien ALLOYEAU, Guillaume WANG, Christian RICOLLEAU

08:00 - 18:15 #6618 - MS05-781 Heterointerfaces TEM characterization of buffer layers in KF treated CIGS solar cells. Towards a new buffer layer?

MS05-781 Heterointerfaces TEM characterization of buffer layers in KF treated CIGS solar cells. Towards a new buffer layer?

Cu(In,Ga)Se₂–based (CIGS) solar cells are thin film devices achieving nowadays the highest conversion efficiency (22.3% at the laboratory scale [1]), surpassing any other thin-film or multi-crystalline silicon technology. A heterojunction is obtained in these solar cells by depositing a cadmium sulfide (CdS) buffer layer (n-type semiconductor) on the CIGS absorber layer (p-type semiconductor). The strong conversion efficiency increase over the past three years is due to the introduction of a potassium fluoride treatment (KF evaporation) between the CIGS coevaporation stage and the CdS chemical bath deposition stage [2]. The understanding of the KF deposition stage on the solar cell performance is still under debate, due at least partly to the lack of material characterization at the nanometer scale.

We first present the characterization of two samples with the same CIGS absorber, with and without KF treatment. Transmission electron microscopy (TEM) conducted on cross section lamellae prepared by focused ion beam (FIB) allowed us to evidence several material changes induced by the KF treatment: compared to the untreated sample, the KF-treated sample shows a depletion of Cu, Ga, and Se and a segregation of K and O at grain boundaries close to the CIGS/CdS interface (figure 1). Without KF the CdS/CIGS interface is abrupt and homogenous in the longitudinal direction. The KF treated sample exhibits a 5 nm-thick layer containing mainly Cd, S, In, Se and O at the CdS/CIGS interface, as well as few 50 nm large CdSe particles (figure 2). This ultra-thin interface layer could result in p-CIGS/n⁺-CdIn₂(S,Se)₄ type/n-CdS heterojunction, explaining the possibility to reduce the CdS thickness without loss of performance, as already reported in [2].

To go deeper in the understanding of the benefit of such a layer, two other samples were synthesized: both with the same CIGS layer but either with a CdS or a CdIn₂S₄ buffer layer. The CdS layer was obtained by chemical bath deposition while the CdIn₂S₄ layer was deposited by physical vapor deposition (PVD). TEM results concerning the sulfur diffusion in CIGS and the crystalline structure of CdIn₂S₄ layer will be presented [3].

 

[1] Solar Frontier. Press release (2015)

[2] Chirilă, A.et al., Nature materials 12, 1107-1111 (2013)

[3] The authors want to thank the French national network METSA, the CLYM centre in Lyon and the Hubert Curien Program “Pessoa” to provide them TEM and FIB facilities. 

Eric GAUTRON (Nantes), Thomas LEPETIT, Sylvie HAREL, Ludovic ARZEL, Lionel ASSMANN, Agathe FRELON, R-Ribeiro ANDRADE, Sascha SADEWASSER, Thierry DOUILLARD, Thierry EPICIER, Nicolas BARREAU

08:00 - 18:15 #6648 - MS05-783 STEM-EELS valence mapping and charge relaxation in LiFePO4 cathode.

MS05-783 STEM-EELS valence mapping and charge relaxation in LiFePO4 cathode.

LiFePO₄ has emerged as an important cathode material for Li-ion batteries because of it stability and high rate capabilities.  It is now well established that lithiation-delithiation occurs via a two-phase reaction. At high charge/discharge rates, the process of nucleation and growth of a two phase reaction is too slow and a non-equilibrium single phase reaction has been proposed followed by relaxation into LiFePO₄ and FePO₄ end product phases [1].  In this study, we studied relaxation mechanisms and determined the spatial distribution of lithiated/delithiated phases by STEM/EELS spectrum imaging.

LiFePO₄ particles from partially charged or discharge electrodes were observed with a cold cathode field emission Hitachi HD2700C STEM and Gatan Enfina EELS spectrometer. The energy resolution of the combined STEM/EELS system was 0.5eV.  The energy was calibrated with respect to the main O-K peak at 539 eV. Typical EELS spectrum for LiFePO₄ and FePO₄ are shown in Fig.1a and 1b respectively. A characteristic feature of delithiated FePO₄ phase is the presence of an oxygen pre-peak marked by an arrow in Fig.1b. This pre-peak in non existant in LiFePO4.  The existence of this O prepeak has been attributed to a transition from O 1s to 2p hybridized state with Fe 3d [2].  In addition the change in Fe valence state from LiFe²⁺PO₄ to Fe³⁺PO₄ is accompanied with a shift to higher energy of Fe-L₃ peak position of about 1.5 eV.  In this study we have quantified the existence of these two lithiated and delithiated phases from the shift in Fe-L₃ peak energy, Fe L₃/L₂ peak intensity ratio and from quantification of normalized O pre-peak intensities.  Measurements made from about 50 particles reveal two clusters of data with average Fe-L₃ peak energy of 708.2 eV and 709.8 eV with O pre-peak intensity ratio of 0.037 and 0.16 respectively.  These two data clusters correspond to fully lithiated LiFePO₄ and delithiated FePO₄ phases.  The spectrum images of the lithiated LiFePO₄ and delithiated FePO₄ expressed as the normalized O pre-peak intensity are shown in Fig.2a and 2b respectively revealing uniform lithiation throughout the particles, i.e. the particles are either fully lithiated or fully delithiated in accordance with the non-equilibrium solid solution transformation path followed by relaxation. An ADF-STEM image taken from an area with many particles and the corresponding phase distribution map are shown in Fig.3a and 3b respectively, revealing a non-uniform distribution of phases with agglomeration of fully lithiated and delithiated regions that include many nanoparticles clusters. 

References

[1]        F. Omenya et al. Adv. Energy Mater. 4 (2014) 1401204 (9pp)

[2]        M.K. Kinyanjui et al. J. Phys. Condens.Matter, 22 (2010) 275501 (8pp)

[3]        The funding for this work is provided by NECCESS, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001294.

Joseph SCHNEIDER-HAEFNER, Dong SU, Yuxuan WANG, Jiye FANG, Fredrick OMENYA, Natasha CHERNOVA, Frederic COSANDEY (Piscataway, USA)

08:00 - 18:15 #6653 - MS05-785 Effect of grain size, precipitation and texture on mechanical and corrosion properties of ECAPed AZ80.

MS05-785 Effect of grain size, precipitation and texture on mechanical and corrosion properties of ECAPed AZ80.

Magnesium alloys have increasing engineering applications due to its high specific strength, for instance in automobile, aerospace and electronics industries [1]. Grain refinement of AZ80 magnesium alloy was achieved by Equal Channel Angular Pressing (ECAP) at 573K for 1, 2 and 4 passes using Bc route [2]. Backpressure of 100MPa was applied to increase the workability and uniformity of the samples [3]. Grain diameter was reduced from 18±0.5 µm to ~6±0.5 µm and further reduced when low processing temperature of 423K was used. SEM/EBSD showed B-fibre texture was obtained after 4 passes of the ECAP process (Figure 1). Number of passes had no significant effect on grain refinement but improve the homogeneity of the samples. Hardness was increased approximately 15% after ECAP. Corrosion resistance of the ECAPed samples was decreased due to the increase in the volume fraction of the grain boundaries and the presence of the uneven distribution of Mg₁₇Al₁₂ precipitates [4]. Grain orientation also affected corrosion rate to some extent. The morphology and composition of the corrosion products was studied using SEM and EDX (Figure 2). The main corrosion products could be a mixture of magnesium hydroxide (Mg(OH)₂) and magnesium carbonate (MgCO₃).

 

 

References:

1)    A. Yamamoto, R. Tamai, H. Tsubakino, Magnesium: Proceedings of the 6th International Conference, magnesium Alloys and Their Applications, 22 Apr 2005, 568

2)    K. Nakashima, Z. Horita, M. Nemoto, T.G Langdon, Materials and Science Engineering A, 281, 2000, 82

3)    R.E. Lapovok, Metally, Issue 1, 2004, 44

4)    G. Song, Advanced Engineering Materials, 7(7), 2005, 563

 

 

 

 

 

 

Yau Yau TSE (Loughborough, United Kingdom), Kaiyuan ZHANG, Yang DU, Diana MARADZE, Yang LIU

08:00 - 18:15 #6654 - MS05-787 Quantitative approach to twin boundaries in Cu3Pt nanoparticles.

MS05-787 Quantitative approach to twin boundaries in Cu3Pt nanoparticles.

Imperfections of the crystal structure, such as partial ordering, formation of core-shell and Pt rich skin improve the electro-catalytic activity of Pt-based intermetallic nanoparticles used as a catalyst material in low-temperature fuel cells [1, 2]. Impact on the surface reactivity by the core–shell nanostructures could be explained with the induced lattice strain [3, 4]. In the case of twinned structure, the lattice strain significantly influences the interatomic distances and consequently the energy levels of bonding electrons, which determines the catalytic, electrical and optical properties [5]. 

In this work we studied the local structure and chemistry near the lamellar (111) twin boundaries in sol-gel prepared intermetallic Cu₃Pt nanoparticles, specially tailored [6] to exhibit high amount of polysynthetic twins.  Using HAADF-STEM (Jeol ARM 200 CF) imaging in conjunction with image simulations we extracted the information about chemical composition and strain on the level of individual atom columns. Experimental Cu-Pt column intensities were measured using modified approach by LeBeau and Stemmer [7]. Experimental intensities were compared to the intensities from simulated images where chemical composition and thickness of individual Cu-Pt columns were varied.

In Fig. 1a a LAADF micrograph (20 – 80 mrad) of (111) twin boundaries in 60 nm sized Cu₃Pt nanoparticle is displayed. Enlarged image of lamellar twins is shown in Fig. 1b. The bright contrast around planar defect indicates strain. From HAADF (70 – 180 mrad) images (Fig. 2a) the intensity ratios were extracted and correlated to the average chemical composition of individual columns. Fig. 2b is the distribution of normalized intensities of atomic columns around the twin boundary where a significant decrease in the intensity near the boundary indicating depletion in Pt content. For conversion of intensities to Pt/Cu ratio HAADF images were simulated using a multislice method with frozen phonon approximation (QSTEM software). Using DFT calculations the energy and lattice relaxation were calculated as a function of local chemical composition near the twin boundaries. Taking into account the local strain and chemical composition variation the influence of twin boundaries on electrocatalytic properties will be discussed in details.

 

Acknowledgement

The authors acknowledge funding from the Slovenian Research Agency (J2-6754 and P2-0393)

References

[1] J. Greeley, et al, Nature Chemistry, 1 (2009), p. 552.

[2] D. Wang, et al, Nature Mater., 12 (2013), p. 81.

[3] C. Kuo, et al, ChemSusChem, 6, 10 (2013), p. 113.

[4] P. Strasser, et al, Nat. Chem., 2 (2010), p. 454.

[5] J. Wu, et al, J. Am. Chem. Soc., 134, 29 (2012), p. 11880.

[6] M. Bele, et al, Chem. Comm., 50, 86 (2014), p. 13124.                                       

[7] J. LeBeau and S. Stemmer, Ultramicroscopy 108 (2008), 1653–1658.

Goran DRAZIC (Ljubljana, Slovenia), Francisco RUIZ ZEPEDA, Anja LAUTAR, Primoz JOVANOVIC, Marjan BELE, Miran GABERSCEK

08:00 - 18:15 #6662 - MS05-789 The Importance of Nano-Confinement in Nanoporous Catalysts: Atom Probe Tomography and FIB/SEM study of surface segregation.

MS05-789 The Importance of Nano-Confinement in Nanoporous Catalysts: Atom Probe Tomography and FIB/SEM study of surface segregation.

To improve the understanding of catalytic processes, the surface structure and composition of the active materials need to be determined before and after reaction. Morphological changes may occur under reaction conditions and can dramatically influence the reactivity and/or selectivity of a catalyst. Gold-based catalysts with different architectures are currently being developed for selective oxidation reactions at low temperatures. Specifically, nanoporous Au (npAu) with a composition of Au₉₇-Ag₃ is obtained by dealloying a Ag₇₀-Au₃₀ bulk alloy. Recent studies highlight the efficiency of npAu catalysts for methanol oxidation as well as the importance of the residual Ag in the catalytic process. Ozone is used to activate the catalysts before methanol oxidation. In this work, we studied the morphological and compositional changes occurring at the surface of Au-based catalysts of different compositions.

To get better insight of the Ag distribution within the Au backbone, we first analysed nanoporous Au catalysts (composition: Au₉₇-Ag₃) by atom probe tomography (APT). APT is a powerful technique to characterize the composition and 3D structure of materials at the atomic-scale, but the presence of pores make the analysis and reconstruction difficult. New developments in sample preparation are required, and we were able to successfully image npAu samples by atom probe tomography and analyse the segregation of Ag atoms in the npAu sample (Fig. 1). Complimentary experiments were performed on a bulk sample of the same composition, and XPS and APT experiments confirm the surface segregation of Ag (as silver oxide species) after ozone treatment, which is then reduced after exposing the catalyst to reaction conditions. Further experiments were performed on bulk Ag₇₀-Au₃₀ samples which were exposed to ozone and reaction conditions. Ozone induces the segregation of Ag at the surface, which forms a distinct black layer of silver oxides. Below this oxide, a homogeneous Ag-depleted region (Ag₅₆-Au₄₄) can be observed, and extends over a depth of a few μm (the depth depends on the duration of the ozone treatment). As it can be seen on Fig. 2, this region undergoes severe morphological changes, and the bulk sample becomes porous. FIB cross-section analysis proves the segregation behaviour and long-range diffusion of Ag in bulk samples, as compared to the nanoscale-segregation observed by APT, correlating previous observations by E-TEM. The nanoconfinement induced by the specific architecture of the nanoporous sample is then responsible of the long term stability and efficiency of the catalyst.

 This study highlights the importance of ozone treatment in the segregation of Ag at the surface, which can dramatically influence the local chemistry and morphology of a catalyst. The combination of APT, FIB/SEM and XPS allows studying the surface and subsurface compositional and morphological changes of the sample after various physicochemical treatments, and also allows the segregation behaviour of Ag in different Au-based catalysts to be correlated.

Cedric BARROO, Nare JANVELYAN, Branko ZUGIC, Juergen BIENER, Austin AKEY, Cynthia FRIEND, David BELL (Cambridge, USA)

08:00 - 18:15 #6689 - MS05-791 Revealing nanoscale morphology of organic solar cell blend by analytical electron microscopy.

MS05-791 Revealing nanoscale morphology of organic solar cell blend by analytical electron microscopy.

Organic photovoltaic as future technology with cost-efficient production and printability is a promising research field. New materials and studying improved synthesis techniques over the past years lead to efficiencies of more than 13 % by organic solar cell [1]. In order to improve device efficiencies, the conversion rate of photo-generated excitons to electron-hole pairs should be increased. This critical aspect, depends on the complex morphology of distributed donor and acceptor materials.

In this work we resolve the morphology of an organic solar cell, where the active layer is a blend of two small molecules; ZnPc (ZnC₃₂H₁₈N₈) as donor and C₆₀ as acceptor_.  The investigation of nanoscale morphology and phase distribution is conducted using Energy Dispersive X-ray spectroscopy (EDX) and Energy selective Backscattered (EsB) imaging in SEM [2]. The results are confirmed using analytical TEM.

The unique aspect of this work is substituting complicated TEM method by SEM with these advantages:

• Possibility of resolving chemical composition in a real solar cell in contrast to commonly deposited blend layer on TEM grids, that allows to study the complete stack of glass substrate, Indium tine oxide (ITO) electrode, electron and hole transport layers and aluminum top electrode.

• No influence of thick (90 nm) layer of heavy ITO, which is under the donor acceptor blend layer, on the TEM study.

• Gaining insight into the height distribution and roughness beside the lateral distribution (as in TEM)

To correlate morphology and material contrast we combined EDX data of the pure materials (composition of specific structure) with EsB detector mapping (strong contrast, unknown corresponding material). EDX spectrum of two material phase is shown in Figure 1, the same features where imaged by EsB detector, as it can be seen in Figure 2. Taking advantage of this combined data, we overcome poor EDX lateral and depth resolution. Since exciting Zn-K using high energy electrons, deteriorates the spatial resolution and damage the material.

Furthermore, we manage to observe the phase morphology, despite the very close mass average and quite similar chemical composition of the two phases.

In principle the technique can be extended to 3D mapping by use of slice-and-view approaches. Finally, our analytical TEM (EELS and EDX) investigations proved the corresponding morphology of each phase.

 

Acknowledgement:

Authors thank A. Garitagoitia Cid for SEM supply of the images in figure 2. This work was supported by the German Science Council Center of Advancing Electronics Dresden (cfaed).

 

References:

[1] Heliatek record on February 8^thhttp://www.heliatek.com/en/press/press-releases

[2] “Energy-filtered backscattered imaging using Low Voltage SEM” A. G. Cid, M. Sedighi, M. Löffler, W. F. van Dorp and E. Zschech. (submitted)

Mona SEDIGHI (Dresden, Germany), Markus LÖFFLER, Ehrenfried ZSCHECH

08:00 - 18:15 #6701 - MS05-793 Characterization of Solution-Annealed and Thermally-Treated Alloy 600 Intergranular Oxidation using Advanced Analytical Electron Microscopy.

MS05-793 Characterization of Solution-Annealed and Thermally-Treated Alloy 600 Intergranular Oxidation using Advanced Analytical Electron Microscopy.

The susceptibility of Ni-Cr-Fe alloy (Alloy 600) to intergranular stress corrosion cracking (IGSCC) in light water reactor (LWR) primary water environment is well-known. However, SCC initiation mechanism is still unclear and under debate, especially the key parameters responsible for the general and localised “internal” oxidation susceptibility of Alloy 600 in LWR primary environment require elucidation to develop a mechanistic understanding of the phenomena and to assess their relative importance. 

It is well-known that the precipitation of intergranular M₇C₃ carbide network improves Alloy 600 Stress Corrosion Cracking resistance in PWR primary water environment. However, the main reason for the beneficial effect of intergranular carbides is still uncertain. In this study, the effect of grain boundary carbides on the preferential intergranular oxidation susceptibility and local grain boundary (GB) migration of Alloy 600 has been studied in detail. Solution-annealed (SA) and thermally-treated (TT) Alloy 600 oxidation coupons were exposed for 120 hours in hydrogenated steam environment at 480°C. This oxidation system successfully simulated PWR oxide morphologies [1-3]. A combination of field emission gun (FEG) scanning electron microscopy (SEM), focused ion beam (FIB) microscopy and analytical electron microscopy (AEM) techniques have been used to characterize in detail the type and extent of preferential oxidation associated with the development of SCC initiation sites.

The surfaces of the oxidized Alloy 600SA and Alloy 600TT specimens were evaluated in an FEI Magellan XHR FEG-SEM using both secondary electron (SE) and backscattered electron (BSE) modes. High resolution SEM characterization revealed a marked difference between the two thermal treatments especially in terms of HAGB undulations and oxide GB structure, Fig. 1. Surface oxide morphology was correlated with intergranular oxidation susceptibility and local GB migration by using FIB cross-sections analysis and TEM examination (Fig. 2).  Further AEM analyses were performed using the FEI Titan G2 80-200 aberration-corrected S/TEM with Super EDX.  Detailed scanning transmission electron microscope (STEM) - energy dispersive x-ray (EDX) microanalysis of FIB TEM lift-out specimens containing at least one grain boundary revealed the presence of a Cr-Fe rich oxide at the grain boundaries and a marked microchemical redistribution in the near-surface region for both SA and TT Alloy 600 (Fig. 3, Fig. 4). However, a noticeable difference in terms of intergranular oxide penetration and local grain boundary migration was observed when comparing the solute segregation/depletion and Intergranular oxidation between the SA and TT specimens.

References:

[1] F. Scenini, R. C. Newman, R. A. Cottis, R. J. Jacko, “Alloy 600 oxidation studies related to PWSCC.” In: Proceedings of the 12^th Int’l. Symp. on Environmental Degradation of Materials in Nuclear Power System- Water Reactors (2005).

[2] G. Bertali, F. Scenini, M. G. Burke, “Oxidation Studies of Alloy 600 in Low Pressure Hydrogenated Steam  In: Proceedings”, In: proceedings of the 16^th international symposium on environmental degradation of materials in nuclear power system- water reactors (2013).

[3] G. Bertali, F. Scenini, M.G. Burke, “Advanced Microstructural Characterization of Alloy 600 Intergranular Oxidation.”, Corrosion Science 100 (2015), 474-483.

Giacomo BERTALI (manchester, United Kingdom), Fabio SCENINI, Grace BURKE

08:00 - 18:15 #6765 - MS05-795 Analysis of capping with GaAsSbN thin layers in (un)coupled InAs/GaAs multi quantum dot layers for enhanced solar cells.

MS05-795 Analysis of capping with GaAsSbN thin layers in (un)coupled InAs/GaAs multi quantum dot layers for enhanced solar cells.

Nowadays, stacked InAs/GaAs quantum dots (QDs) capped with layers different than GaAs are being applied in photovoltaic and photodetector technologies due to their potential for tailoring the optical properties and enhancing the device efficiency.^1,2 The particular use of GaAsSbN capping layers (CLs) allows controlling the QD size as well as the electron and hole confinement potentials in a wide range.³ In addition, multi-coupled QD structures formed by vertically aligned QDs represent a very interesting approach, which enhances the photoluminescence (PL) intensity and reduces the spectral width by utilizing carrier tunneling among QDs. However, in this type of nanostructures the control of the QD composition, size and alignment are important parameters as they affect the electronic coupling and the carrier lifetime. In this work, we analyze the effect of using 2.5 nm-thick capping layers of GaAs_0.76Sb_0.2N_0.04 in an (un)coupled 10 stacked layer structure of InAs/GaAs QD by transmission electron microscopy (TEM) techniques. For this, we have compared three samples: two samples with uncoupled QDs (the spacer thickness is 40 nm) with (SbN-u) and without CL (Ref), and a coupled sample with CL and a spacer thickness of 10 nm (SbN-c). PL and photocurrent (PC) data are also used in the discussion.

Cross sectional g200 dark field (DF) images sensitive to composition have been used to obtain a statistic of the buried QD size (Figure 1). For the uncoupled samples, our measurement on more than 90 QDs revealed an unexpected similar height for Ref and SbN-u samples (4.7 nm) while the average diameter presents a slight increase in sample SbN‑u with respect to Ref sample (from 17 to 19 nm). The panorama changes totally for SbN-c sample, where the average height and diameter increases layer by layer from 4.7 to 5.2 nm and from 23 to 34 nm, respectively. The relative volume rises quickly (Figure 1) up to the 4^th layer, and after slowly, becoming 7 times higher in the upper layer. The shield effect of Sb in the QD decomposition is not appreciated. Indeed, EDX measurements revealed only tiny traces of Sb in the CLs that we related with the typical delay observed in the Sb incorporation due to the CL small thickness. In addition, strain maps from high resolution TEM images acquired on the [110] pole axis were calculated. The comparison between the uncoupled samples discloses that N is incorporated in the CL. The same occurs in the coupled sample where a high compressive strain appears over the QDs (Figure 2). More important, the strain into the QDs decreases in a similar way that the volume increases. Finite element simulations suggest a modification of segregation process during the growth, with an increase of Ga incorporation in the QDs of the upper layers.

Acknowledgments

We acknowledge the Spanish MICINN–MINECO for funding through Project MAT2013-47102-C2, and SCCYT-UCA for technical support.

References

¹ D. Gonzalez et al., Solar Energy Materials & Solar Cells 145 (2016) 154–161

² A. D. Utrilla et al., Solar Energy Materials & Solar Cells 144 (2016) 128–135

³ J. M. Ulloa et al.,Appl. Phys. Lett. 100, 013107 (2012)

 

Daniel F. REYES (Chiclana de la Frontera, Spain), Veronica BRAZA, Antonio D. UTRILLA, Teresa BEN, Alvaro GUZMAN, Adrian HIERRO, Jose M. ULLOA, David GONZALEZ

08:00 - 18:15 #6800 - MS05-797 Combining TEM and APT for a better understanding of super high efficiency Cu(In,Ga)Se2 thin film solar cells.

MS05-797 Combining TEM and APT for a better understanding of super high efficiency Cu(In,Ga)Se2 thin film solar cells.

The Sharc25 EU H2020 project aim is to push the efficiency of Cu(In,Ga)Se2 (CIGS) thin film solar cells to their maximum theoretical limit. Sharc25 consists in a large consortium to take under consideration both theoretical and experimental aspects. One of the strategies to achieve this goal is to improve and understand the CIGS absorber material deposited by co-evaporation.

The polycrystallinity of the CIGS absorber layer is one of the key elements which plays a crucial role in its efficiency. We combine here atom probe tomography (APT) (Fig. 2) and transmission electron microscopy (TEM) to study the distribution of alkali elements at sub-nanometer scale and with statistic relevancy. 3D APT atomic mapping of grain boundaries (GB) is here completed by systematic STEM-EDS analyses of GB all along the CIGS absorber layer (Fig. 1). Issues on samples preparation for APT and TEM will be also discussed.

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 641004

Celia CASTRO (UMR 6634 CNRS, Université et INSA, Rouen), Mohit RAGHUWANSHI, Philippe PAREIGE, Sébastien DUGUAY, Emmanuel CADEL, Wolfram WITTE, Philip JACKSON, Dimitrios HARISKOS, Friedrich KESSLER, Stephan BUECHELER, Romain CARRON, Enrico AVANCINI, Benjamin BISSIG, Ayodhya TIWARI

08:00 - 18:15 #6809 - MS05-799 The Influence of Gold on Corrosion Resistance of PtCu3/C Nanoparticles Composite.

MS05-799 The Influence of Gold on Corrosion Resistance of PtCu3/C Nanoparticles Composite.

One of the applications of Pt-based nanoparticles on high surface area carbon (HSAC) support composites is as a catalyst in polymer electrolyte membrane fuel cells (PEMFC) [1]. Yet the full commercial implementation of the technology has been hindered by the composite cathode which exhibits a low catalytic activity for oxygen reduction reaction (ORR), a poor stability in electrochemical environment and is expensive due to use of Pt. One solution to those issues is to alloy Pt with other transitional metals, which will not only reduce the composite costs, but also increase the catalytic activity for ORR [2]. Chemical ordering of the Pt-M/HSAC (M=Metal) composite has shown to improve the stability of the nanoparticles, as well as to enhance the catalytic activity for ORR. [3]. Recent work has shown that adding a small amount of gold can improve the durability of Pt and Pt-based nanoparticles [4,5], as well as inhibit the oxidation of HSAC support [5].

We have used Aberration Corrected Scanning Transmission Electron Microscopy (AC-STEM) to study the effect of gold on the Pt-skin partially ordered intermetallic PtCu₃/HSAC composite. Au doped PtCu₃/C and PtCu₃/C composites were compared side to side, before and after an electrochemical cycling treatment. Fig. 1 shows scanning transmission electron microscopy annular dark filed images and energy dispersive x-rays maps of the two composites after 200 cycles of electrochemical activation (EA, 0.05 – 1.35 V_RHE, 300 mVs^-1). Less porous nanoparticles and lower Cu dissolution are observed in the Au doped PtCu₃/C composite. Surface modifications were also observed in non-porous nanoparticles after performing 10 000 cycles of severe degradation (0.4 – 1.4 V, 1 Vs^-1): (1) thickening of the Pt skin, (2) faceting and (3) surface dislocations (Fig. 2). These surface modifications may have influence on stress and nanoparticle stability. Overall, the Au doped PtCu₃/C composite exhibited higher Cu retention and better durability compared to its binary analogue.

 

References:

[1] M. K. Debe, Nature 486 (2012) 43−51.

[2] J. Greeley, I. E. L. Stephens, A. S. Bondarenko, T. P. Johansson, H. A. Hansen, T. F. Jaramillo, J. Rossmeis, I. Chorkendorff and J. K. Nørskov, Nature Chemistry 1 (2009) 552 – 556.

[3] M. Bele, P. Jovanovič, A. Pavlišič, B. Jozinović, M. Zorko, A. Rečnik, E. Chernyshova, S. Hočevar, N. Hodnik and M. Gaberšček, Chem. Commun. 50 (2014) 13124-13126.

[4] J. Zhang, K. Sasaki, E. Sutter, R. Adzic, Science 315 (2007) 220−222.

[5] M. Gatalo, P. Jovanovič, G. Polymeros, J. P. Grote, A. Pavlišič, F. Ruiz-Zepeda, V. S. Šelih, M. Šala, S. Hočevar, M. Bele, K. J. J. Mayrhofer, N. Hodnik, M. Gaberscek. ACS Catal. 6 (2016) 1630–1634.

 

Acknowledgments:

The authors would like to acknowledge support from Slovene Research Agency and NATO SfP “Durapem” project.

Francisco RUIZ-ZEPEDA (Ljubljana, Slovenia), Matija GATALO, Primož JOVANOVIČ, Marjan BELE, Goran DRAŽIĆ, Miran GABERŠČEK

08:00 - 18:15 #6825 - MS05-801 Nanostructured MnxOy as catalyst for Oxygen Reduction Reaction.

MS05-801 Nanostructured MnxOy as catalyst for Oxygen Reduction Reaction.

Nowadays the development of new and clean energy resources represents one of the major scientific challenges, due to the growing concerns about global warming. The electrochemical Oxygen Reduction Reaction (ORR) occurring at the cathode of fuel cells and metal-air batteries is one of the key limits for their further development and requires electrocatalysts to increase its efficiency. Manganese oxides are among the most interesting non-precious metal-based catalysts due to their low cost, relatively high abundance, low environmental impact and considerable electrocatalytic activity.

In this work we present nanostructured manganese oxides in the form of xerogels (obtained by means of the sol-gel plus freeze-drying techniques) and in the form of nanofibers (obtained by means of the electrospinning technique). They were synthesized by employing manganese acetate as the Mn source and by employing environmental friendly (water is the used solvent) templating agents, such as agar and polyethylene oxide, for xerogels and nanofibers respectively.

To investigate the oxidation process forming the manganese oxides species, structural and morphological characterizations (in-situ X-ray diffraction, field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM)) were performed on both the nanostructures.

Concerning manganese oxide xerogels, in situ X-ray diffractograms were recorded while heating in air the as-prepared material from room temperature up to 725 °C, at a heating rate of  2.5°C/min. Figure (1a) shows the phase evolution of the xerogel as a function of the temperature. From this image it can be observed that Mn₃O₄ nucleates and grows at temperatures between 175 °C and 475 °C, temperature at which the α-Mn₂O₃ phase starts to evolve; the latter completes its formation at 700 °C approximately, while Mn₃O₄ is progressively disappearing. From Rietveld refinements of all diffractograms, major differences were found in the crystal size of the two phases, where the average size for Mn₃O₄ was 21± 1 nm, while it was 65.8±0.8 nm for α-Mn₂O₃.

By means of bright field TEM and selected area electron diffraction (SAED) techniques, two different morphologies were observed, i.e. nanosized grains and large dendritic structures, composed of rectangular domains of less than 100 nm (Figure 2). The SAED pattern is composed by two different patterns, that are clearly distinguished. The first corresponds to complete and low-intensity rings, which were indexed as Mn₃O₄, and that could, a priori, be assigned to the nanosized grains. The second pattern of bright spots was assigned to the large dendritic structure, and indexed as Mn₂O₃ along the [201] axis zone. The TEM and SAED results are in agreement with the Rietveld analysis.

Regarding manganese oxide nanofibers, in-situ X-ray diffractograms were recorded while heating in air the as-prepared material from room temperature up to 725 °C, at a heating rate of  2.5°C/min. Figure (1b) shows the phase evolution of the nanofibers as a function of the temperature. From this image is observed that Mn₃O₄ starts to nucleate and grow at temperatures between 125 °C and 575 °C, temperature at which the α-Mn₂O₃ phase start to evolve, while Mn₃O₄ is progressively disappearing.

From FESEM and bright field TEM characterizations (Figure 3), it is evident that the nanofibers are composed by nanosized grains of few nanometers.

The catalytic performance of the two nanostructured catalysts were characterized by means of the rotating ring disk electrode technique by using 4-electrodes measurements. Both xerogels and nanofibers showed extremely good performances for the oxygen reduction reaction, with n values between 3.5 and 3.7, meaning a predominant 4-electrons reduction pathway.

In conclusion, nanostructured xerogels and nanofibers of manganese oxides were prepared through the sol-gel plus freeze-drying and the electrospinning techniques respectively, and by using biocompatible and biodegradable precursors. The obtained materials were composed by Mn₃O₄, Mn₂O₃, or a mix of both, depending on the temperature. Both materials showed good catalytic performance for the ORR, making them good candidates as low-cost and green catalysts for applications in electrochemical devices.

Jose Alejandro MUÑOZ TABARES (Torino, Italy), Gian Paolo SALVADOR, Luisa DELMONDO, Matteo GEROSA, Adriano SACCO, Nadia GARINO, Giulia MASSAGLIA, Marzia QUAGLIO, Candido Fabrizio PIRRI, Angelica CHIODONI

08:00 - 18:15 #6858 - MS05-803 Decorated Carbon Nanotubes for Electrochemical Energy Storage (EES) systems.

MS05-803 Decorated Carbon Nanotubes for Electrochemical Energy Storage (EES) systems.

Electrochemical Energy Storage (EES) systems have various applications ranging from large-scale generation and transmission-related systems, to distribution networks, customer/end-user sites or electrical mobility and transportation. [1] Still higher performance and more environmentally acceptable component materials are continuously required. This can only be overcome by major advances in materials with nanomaterials playing a central role. Lithium-Ion batteries, by far the most common form of EES system, share three common features: two electrodes and an ionically conducting electrolyte which provides means of transporting ions to and from the component electrodes. The electrodes comprise a current collector and an active material. Improvement in the design of one or more of these features may lead to EES systems with improved performance, delivering higher energy densities and characteristics such as increased capacity, charge/discharge rates and cycle lifetime.

Along this line the key-issue addressed by the present work relates to the assessment of the batteries performances when nanostructured electrode materials are considered for both the anode and the cathode. Grown directly on the current collector (copper for anode and respectively aluminum for cathode) substrate, thin walled carbon nanotubes in the form of very dense vertically aligned (VACNT) carpets are considered as ideal support matrix (current collector) for the insertion active materials due to their efficient electron transport pathways and stable mechanical support. [3] The present work focuses in presenting preliminary 2D analysis based on advanced electron microscopy techniques (TEM, HR-TEM, STEM) coupled with the spectroscopy ones (EELS and EDX), dedicated to the characterization of both anode and cathode decorated carbon nanotubes based electrodes. The different analyses allowed the validation of the CNTs carpets fabrication, which becomes nanostructured current collector upon their coupling (decoration) with Si nanoparticles for anodes and metallic lithiated nanoparticles (LiMO₂) for cathode. The electrochemical experiences performed on the nanostructured system confirmed the devices performance. Regarding the anode fabrication, the synthesis of CNTs directly on the copper substrate is visible from figure 1a) that using a dHF-CVD technique one can obtain a homogenous and very dense carpet of VACNT having a 60µm length. The HR-TEM analysis allowed identifying a double walled structure for the nano-tube with a diameter size of 3-4nm presenting surface defects which act as anchorage sites for the eletrodeposition of the nanoparticles. Concerning the nano-tubes decoration with silicon nanoparticles the process has been carried following an innovative approach using ionic liquid with SiCl₄ as precursor for Si. These hybrid nanostructures are expected to enhance the electrode capacity since the VACNT acts as both mechanical support and electrical conductor while silicon nanoparticles as high capacity insertion anode material. Figure 2b) illustrates silicon decorated CNTs. The HR-TEM analyses on several areas on the current collector allowed evidencing that the deposition is obtained homogenously on all nanotubes with nanoparticles sizes ranging from 5 to 30nm. Regarding the cathode fabrication, similar TEM analyses have been performed to validate the synthesis of CNTs carpets on aluminum foil substrate as well as their subsequent decoration with LiMO₂ nanoparticles.

 

References:

[1] Nanostructured Materials for Electrochemical Energy Production and Storage; Springer (2009) eds. David J. Lockwood, doi: 10.1007/978-0-387-49323-7 or (b)

[2] A.K. Shukla and T. Prem Kumar; WIREs Energy Environ 2013, 2: 14–30, doi: 10.1002/wene.48

[3].A. Gohier, B. Laïk, K. Kim, J. Maurice, J-P. Pereira-Ramos et al, Advanced Materials 84 (2012) 2592-2597

 

Acknowledgements:

Authors gratefully acknowledge funding from Chaire Development Durable Ecole-Polytechnique-EDF.

 

Mariam EZZEDINE (Palaiseau), Ileana FLOREA, Costel-Sorin COJOCARU

08:00 - 18:15 #6882 - MS05-805 2D-TEM investigations of CNTs synthetized within vertical-PAA templates for devices applications.

MS05-805 2D-TEM investigations of CNTs synthetized within vertical-PAA templates for devices applications.

Since their discovery in 1991 carbon nanotubes (CNTs) become one of the most emblematic type of nanostructures due to their unique electrical, mechanical, thermal and optical properties. Despite these promising and astonishing properties, the critical subject of their manipulation remains a primordial challenge. Very expensive and time consuming techniques, like e-beam lithography, have been developed in order to precisely control their growth, or to obtain a nano-object from the etching of a bulk material. Such techniques are suitable as proofs of concept; however the object-by-object manipulation approach is unrealistic because they are not compatible with a massive integration. On the other hand the miniaturization of the fabricated devices is reaching a bottleneck since the scaling-down for enhancing their performance is approaching many physical limitations. The efforts concentrated to achieve device fabrication by continuous miniaturization are nowadays struggling with performance decline in the electrical conductivity transports values and a significantly loss of reliability. Self-organized templates such as porous anodic alumina (PAA) templates provide several advantages for controlling the nanostructures growth.[1,2] Its well-ordered structure and the confinement imposed by the nanopores the PAA template offer a promising approach for cost-effective, stable and efficient fabrication of carbon nanotubes based devices.[3]

Here we present a complete 2D analysis based on advanced electron microscopy techniques devoted to the full characterization of both the PAA structure and the as-grown CNTs using a dHF-CVD (double-Hot Filament assisted CVD) synthesis method. More exactly, we combine the FIB (Focused Ion Beam) preparation technique with advanced TEM characterization techniques such as STEM-EDX and EELS spectroscopy for the assessment of an accurate correlation between the synthesis parameters and the morphological, structural and chemical characteristics of both the PAA structure and the as-grown CNTs.

In a first step, the TEM analysis of different PAA cross-sections prepared using the FIB technique, allowed us accessing precise characteristics such as the pore length (800nm) and their diameter (30nm) as well as the inter-pore distance (30nm) (see figure1). A more detailed analysis on the bottom part of the PAA structure helped us evidencing the presence of a branched nanopores structure product of an exponential voltage decrease process, applied in order to thin the oxide barrier layer at the bottom of the pores.

For the CNTs, we first examined the impact of the catalyst pretreatment step performed prior to the CNTs growth step. Secondly, by varying the hot-filaments power applied during the growth, we investigated the impact of the additional gas phase activation conditions over the synthesized carbon nanostructures. The results revealed that the pretreatment conditions determine the catalyst distribution at the bottom pores of the PAA membranes, with a strong impact on the CNTs growth within the PAA templates. Another important finding concerns the amount of defects incorporated into the grown CNTs walls which could be related to the hot-filament power applied during the synthesis.

 

References:

[1] Maune, H.T., et al., Nature Nanotechnology, 2010. 5 (1): p. 61-66.

[2] Choi, W.B., et al., Applied Physics Letters, 2001. 79(22): p. 3696-3698.

[3] Kim K.-W., et al., Adv. Mater. 2014, 26, 4363–4369.

 

Acknowledgements:

This work received support from the French state managed by the National Research Agency under the Investments for the Future program under the reference ANR-10-EQPX-50-NANOTEM.

Ileana FLOREA (Palaiseau), Leandro Nicolas SACCO, Marc CHATELET, Costel-Sorin COJOCARU

08:00 - 18:15 #6892 - MS05-807 Microstructure-Thermal Conductivity Relationship in Pressureles Sintered AlN Ceramics for Energy Applications.

MS05-807 Microstructure-Thermal Conductivity Relationship in Pressureles Sintered AlN Ceramics for Energy Applications.

Aluminum nitride (AlN) ceramics are attractive materials for microelectronic packaging due to its high thermal conductivity, low dielectric constant and good matching of thermal expansion coefficient [1-4]. However, it is difficult to sinter AlN due to its strong covalent bonding. Densification of AlN powder has been attempted by various techniques. For full densification, rare earth and/or alkaline earth oxides are often added as sintering aids during the fabrication of AlN ceramics [4]. To investigate the effect of microstructure on the thermal properties and sintering behaviour, commercially available AN powder (Grade H, Tokuyama Co. Ltd., Japan) was sintered pressureless by using commercially available Sm₂0₃ and Yb₂0₃ powders (purity > % 99, Treibacher Industrie, AG, Austria ) as sintering additives and graphene as a secondary phase to improve the thermal properties.

The powder mixture was milled by plenatory ball mill in ethanol for 1.5 hours at 300 cycles / min. Then, the ethanol in the slurry was evaporated. After drying, the powders were pressed by handpress at 50 MPa to obtain pellets. The pellets were cold isostatically pressed (CIP'ed) under 200 MPa. The specimens were then placed into a BN crucible and sintered at 1800 — 1860^oC in a graphite furnace with a flowing nitrogen gas.

The heat capacities of the samples were measured by DSC (NETZSCH STA 449F3) whereas the thermal diffusivities of the samples were measured by laser flash technique (Netzsch-LFA 457). The sintered samples were characterized by employing XRD, SEM and TEM techniques. XRD patterns of the samples were recorded using a (Rigaku Rint 2200, Tokyo, Japan) monochromatic CuKα radiation. Scanning electron microscope (SEM) investigations were carried out using a ZEISS SUPRA 50 VP microscope. For TEM investigations 200 kV field emission TEM (JEOL JEM-2100F) equipped with STEM high angle annular dark field (STEM-HAADF) detector (Model 3000, Fischione), electron energy loss spectrometer (EELS) and energy filter (Gatan ^TM GIF Tridiem), and energy dispersive spectrometer (EDS) (JEOL JED-2300T) was used.

Three different liquid phase addition caused to obtain different intergranular phase formation resulting in a decrease in the thermal diffusivity of the materials with the addition of Yb₂0₃ (Fig 1a). When the micstructure was investigated with STEM HAADF (Fig 1 (b, c and d)), the grain boundary intergranular films found to have a very different behaviour. In this study, property-microstructure realtionship for AlN ceramics and AlN-graphene composites will be presented as a function of different sintering addities, different sinterin conditions and heat treatment procedures.

References

[1] C. Yun et al. (2015) Ceramics International 41, 8643.

[2] R.R. Tummala, (1991), J. Am, Ceram. Soc. 74, 895.

[3] L.M. Sheppard, (1990), Am. Ceram. Soc. Bull. 69, 1801.

[4] A.V. Virkar, (1989), J. Am. Ceram. Soc. 72 2031.

[5] J . Yoshikawa et al. (2005), J. Am. Ceram. Soc.88 3501.

Servet TURAN (Eskisehir, Turkey), Alper CINAR, Pinar KAYA

08:00 - 18:15 #6895 - MS05-809 Characterization of the glass-coated CoSb3 thermoelectric material by electron microscopy.

MS05-809 Characterization of the glass-coated CoSb3 thermoelectric material by electron microscopy.

  Over the last years alternative sources of energy took on special significance. Durability, reliability and maintenance-free operation make the devices for power generation based on thermoelectric materials very attractive. Beside high efficiency, one of the main requirements for thermoelectric materials is the stability of the properties during the long-time operating at elevated temperatures. Doped cobalt triantimonides (CoSb₃) are used as components of thermoelectric devices at temperature range about 400–600 ˚C. The main difficulty of the CoSb₃ application is the degradation of its thermoelectric properties as a result of antimony sublimation and material oxidation at elevated temperatures. To prevent these processes protective coatings are foreseen.

   The objective of this work was to characterize the glass coating/CoSb₃ interface and to determine its influence on the CoSb₃ stability at elevated temperatures. The borosilicate glass coatings with different chemical compositions were applied on the CoSb₃ substrates by dipping and then fired in air at temperature up to 700 ˚C.  To determine oxidation resistance, coated samples were oxidized at 600 ˚C in air. Then samples were examined by SEM/EDS and TEM/EDS. Merlin Gemini II of ZEISS (SEM) as well as a probe Cs-corrected Titan³ G2 60-300 equipped with ChemiSTEM™ system were used to investigate the oxidized samples. TEM lamellae were prepared by FIB facility. Phase identification was performed by STEM-EDS and SAED electron diffraction supported with the JEMS software. Surfaces, fractures facets and cross-sections were analyzed to assess the quality of the coatings, adherence to the underlying substrate and glass coating/CoSb₃ interface structure.

   Depending on the chemical composition of the glass (the content of the network modifiers), different effects were observed at the glass coating/CoSb₃ interface. The results of the study showed that effective protection for CoSb₃ against oxidation at 600 ˚C in air was possible only if:

1)    No crystallization near glass/substrate interface occurred. During coating firing antimony oxides were formed and reacted with the glass. However, glasses with too high Sb₂O₃ content (more than 50%) tends to crystallize. Air trapped in the voids in the crystallization zone caused degradation of the substrate by oxidation during annealing at elevated temperatures.

2)    Porosity at the glass coating/CoSb₃ interface caused by the antimony sublimation during firing was as small as possible (Fig. 1).

3)    Bonding mechanism due to mutual solubility of the glass to the substrate was involved. The Sb₂O₃ and SiO₂ form an eutectic type phase diagram with low eutectic temperature. Increased antimony concentration in the inner part of the coating (Fig. 1) denotes the scale dissolving in the liquid phase.

4)    Mechanical bonding was developed (Fig. 2).

5)    Chemical bonding was of less importance in studied case. The Cu precipitates observed on the SEM/EDS maps near the interface indicated that following reaction could be involved:

3 CuO_(glass) + 2 Sb_(substrate) = Sb₂O_3(glass) + 3 Cu.

However the glass coating/CoSb₃ interface as shown in the TEM image (Fig. 3) is sharp and shows no transition zone, characteristic for the chemical bonding.

   Summarizing, the borosilicate glass with high titania content was found to be an effective protection for CoSb₃ during the exposure to air at 600 ˚C. The glass coating was an effective barrier for oxygen diffusion into the material and for antimony sublimation, therefore chemical and phase composition of the substrate was not affected by the oxidation. Good coating/substrate adhesion was ensured mainly due to mutual solubility and mechanical bonding mechanisms.

 

Acknowledgments The study was supported by the AGH-UST statutory project (no. 11.11.110.299). The authors wish to acknowledge Mr Adam Gruszczynski, MSc. for FIB lamellae preparation.

Kinga ZAWADZKA, Oleksandr KRYSTHAL, Marek NOCUŃ, Elżbieta GODLEWSKA, Aleksandra CZYRSKA-FILEMONOWICZ (KRAKOW, Poland)

08:00 - 18:15 #6898 - MS05-811 Study of the Mg Insertion in Mn-based Spinel and Birnessite Structures Upon Electrochemical Cycling.

MS05-811 Study of the Mg Insertion in Mn-based Spinel and Birnessite Structures Upon Electrochemical Cycling.

The search for low cost and environmental friendly intercalation cathode materials offering high power density in rechargeable ion-exchange batteries is driven by the limitations of the existing Li-ion technology. At present, the use of the lithium metal as a negative electrode is restricted to the use of specific polymer electrolytes¹ which hinder the formation of the dendrites. Therefore, the graphite negative electrodes are employed. However, they reduce the theoretical capacity density from 2046 mAh cm^-3 to ~850 mAh cm^-3 and drastically increase costs. Here, the application of multivalent battery technology that pairs an intercalation cathode with the metal electrode thus allowing for higher energy density and lower costs, is desired².

Among the candidates, Mg metal that possesses high volumetric specific capacity of 3833 mAh cm^-3, exhibits no dendrite growth on deposition², is safe to handle in ambient atmosphere and largely available, is of special interest³. Various materials have shown the initial promise for multivalent intercalation, including Chevrel phase Mo₆S₆, layered V₂O₅, graphitic fluoride, etc. However, owing to the limited mobility of Mg ions and possible concurrent insertion of water and/or protons, the cycling stability of these host materials has been shown insufficient.

The promising candidates for the cathode materials that display higher voltages than Chevrel phases are variants of manganese oxide. In this study we have chosen a MgMn₂O₄ spinel and (Mg_xNa_y)Mn₂O₄ birnessite phases. These materials crystal structures employ different mechanisms for keeping the stability upon cycling and allow for Mg de/insertion, which was performed in magnesium nitrite aqueous electrolyte. The aim of the study was to investigate the possible Mg insertion mechanisms in both materials prior assembling a battery by correlating the cyclic voltammetry (CV) results with the structural and compositional changes of these cathode materials by S/TEM at the atomic level upon increasing number of cycles.

 The spinel phase was prepared by the delithiation of the commercially available LiMn₂O₄ spinel in 0,1 M Mg(NO₃)₂ aqueous electrolyte and its following magnesiation. The Mg containing birnessite phase was synthesized by the rout described by Aronson and coworkers via Na-birnessite phase⁴. The STEM-EDX confirmed partial exchange of Na over Mg with the Mg occupying fully the smaller particles and only the outer shells of the larger particles. Same behavior was observed in the spinel material, where the small particles had a higher Mg content than the large particles (above 200 nm). Both materials were then put through the CV tests to explore the Mg de/insertion mechanisms. Plots in Fig 1 (a,c)  show that both spinel and birnessite structures can reversibly insert the Mg ions. The complete stabilization of the CV curve was observed in both materials at around the 20^th cycle (Fig. 1 a,c). STEM-ABF images (Fig. 1 b,d) were taken from the material after the third cycle, when the initial changes of structure due to the Mg de/insertion took place. The ABF technique allowed for the visualization of the lighter Mg and O atoms that can be vaguely seen in case of spinel structure. The ABF imaging of birnessite, in its turn, confirmed the presence of extra O atoms belonging to the crystal water interlayer that has been reported to play a crucial role in the layered cathode materials by enhancing the ion diffusion as well as suppressing the Mn²⁺ dissolution⁵.

 

¹ L. Damen, J. Hassoun, M. Mastragostino, B. Scrosati, J. Power Sources 195, 6902, (2010)

² H. D. Yoo, I. Shterenberg, Y. Gofer, G. Gershinsky, N. Pour, D. Aurbach, Energy Environ. Sci. 6, 2265, (2013)

³ J. Muldoon, C. B. Bucur, T. Gregory, Chem. Rev. 114, 11683, (2014)

⁴ B. J. Aronson, A. K. Kinser, S. Passerini, W. H. Smyrl and A. Stein, Chem. Mater. 11, 949, (1999)

⁵ K. W. Nam, S. Kim, E. Yang, Y. Jung, E. Levi, D. Aurbach and J. W. Choi, Chem. Mater. 27, 3721 (2015)

Elena TCHERNYCHOVA (Ljubljana, Slovenia), Ana ROBBA, Miran GABERŠČEK, Robert DOMINKO

08:00 - 18:15 #6924 - MS05-813 Ni-based superalloy: crystalline orientation mapping and gamma-gamma’ phases discrimination with the iCHORD method.

MS05-813 Ni-based superalloy: crystalline orientation mapping and gamma-gamma’ phases discrimination with the iCHORD method.

Electron backscatter diffraction (EBSD) is routinely employed as a characterization tool to obtain individual grain orientations, local texture and phase identification. However, in the case of γ - γ’ Ni-based superalloys, the EBSD technique allows mapping the orientations but fails discriminating the two phases because their diffraction signature is too similar. A coupling with EDX analysis for instance helps to identify the two phases but suffers from the lack of spatial resolution of the EDX maps.  Another way to discriminate the two phases is to use the BSE images, sensible to the chemistry, but because of the difference in the geometry of acquisition of the EBSD images and the BSE images, superposition of the two information is complicated by strong  spatial distortions. In this context, any new technique that can lead to an easier phase and orientation mapping would be welcome, especially to resolve the fine secondary γ’ precipitates (typically few tens of nanometers).

We proposed recently the iCHORD method (for ion CHanneling ORientation Determination), aiming at constructing orientation maps based on the well-known channeling contrast phenomenon observed in a rotation series of ionic images (see figure 1) [1]. The proof-of-concept of the iCHORD method was established using a titanium nitride specimen (TiN) and further tests were conducted on different metallic materials. In the case of Ni-based superalloys, the channeling contrast is very strong and allows obtaining quite easily orientation maps comparable in all aspects to the one obtained by the EBSD technique. Furthermore, when using the ion beam to scan the surface, a secondary ion signal can be detected and used for imaging the sample. In the case of γ - γ’ superalloys, this secondary ion signal provides a strong contrast between the γ phase and the γ’ phase (see figure 2). Because of the identical geometry of acquisition for the iCHORD orientation maps and the secondary ion images, both information can be easily superimposed. It allows discriminating between the two phases and giving their crystallographic orientations (see figure 3). Moreover, the spatial resolution of the secondary ion images is around few tens of nanometers, which is far better than the resolution of ~1 µm of EDX maps.

To conclude, the advantages of using ion images to study Ni-based γ - γ’ superalloys are discussed, particularly the benefits taken from the higher spatial resolution and the ease of mixing orientation and phase information.

 

 

References:

[1] Crystal Orientation Mapping via ion channeling: an alternative to EBSD

C. Langlois, T. Douillard, H. Yuan, N.P. Blanchard, A. Descamps-Mandine, B. Van de Moortèle, C. Rigotti, T. Epicier, Ultramicroscopy 157 65-72 (2015)

Cyril LANGLOIS (Villeurbanne Cedex), Marie-Agathe CHARPAGNE, Sébastien DUBAIL, Thierry DOUILLARD, Nathalie BOZZOLO

08:00 - 18:15 #6936 - MS05-815 Texture, microstructure and mechanical anisotropy in SLM processed superalloys.

MS05-815 Texture, microstructure and mechanical anisotropy in SLM processed superalloys.

Samples of Nickel-based (IN738LC and Hastelloy X) and Cobalt-based (Mar-M509) alloys were built by selective laser melting (SLM). To evaluate the anisotropy in the mechanical behavior of the material due to layer-wise build up, specimens were built with their cylinder axis (loading direction) oriented either parallel to the building direction, or perpendicular to the building direction. After building up the specimens by SLM, they were investigated either under the “as-built” condition or after heat treatment. The analysis of microstructural anisotropy in SLM made specimens was done by using EBSD, EDX and X-ray texture analysis methods, and then correlated with anisotropic material behavior observed during tensile and creep testing at room temperature and 850°C. All Ni-based samples possess the same general texture, with the majority of grains forming one single component of a cube texture with one of the cubic axes parallel to the building direction, and another cubic axis parallel to the laser scanning direction. The Young’s modulus determined during tensile testing is lowest parallel to the building direction and parallel to the laser scanning direction. By applying suitable laser scanning strategies, the possibility to switch from transverse anisotropic to transverse isotropic properties and reverse is demonstrated for triple layered tensile samples. While the Ni-based alloys exhibit coarse and elongated grains with a sharp texture and thus a pronounced mechanical anisotropy, the Co-based alloy shows smaller grains with only moderate structural and mechanical anisotropy. These differences are discussed with respect to different recovery and recrystallization behavior of the two groups. Furthermore, recrystallisation heat treatment leads to a weakening of the texture and a reduction in mechanical anisotropy. The anisotropy of Young’s modulus was modeled based on the single crystal elastic tensor and the measured crystallographic preferred orientations, and compares well with the data from tensile tests.

References:

Michael Cloots, Karsten Kunze, Peter J. Uggowitzer, Konrad Wegener (2016) Microstructural characteristics of the nickel-based alloy IN738LC and the cobalt-based alloy Mar-M509 produced by selective laser melting, Materials Science and Engineering: A658, 68-76, 10.1016/j.msea.2016.01.058. 

Thomas Etter, Karsten Kunze, Fabian Geiger, Hossein Meidani (2015) Reduction in mechanical anisotropy through high temperature heat treatment of Hastelloy X processed by Selective Laser Melting (SLM), IOP Publishing 82(1) (ICOTOM 17)  10.1088/1757-899X/82/1/012097 

Fabian Geiger, Karsten Kunze, Thomas Etter (2016) Tailoring the texture of IN738LC processed by selective laser melting (SLM) by specific scanning strategies, Materials Science and Engineering: A661, 240-246, 10.1016/j.msea.2016.03.036.

Karsten Kunze, Thomas Etter, Jürgen Grässlin, Valery Shklover (2015) Texture, anisotropy in microstructure and mechanical properties of IN738LC alloy processed by selective laser melting (SLM), Materials Science and Engineering: A620, 213-222, 10.1016/j.msea.2014.10.003.

Karsten KUNZE (Zürich, Switzerland), Thomas ETTER, Fabian GEIGER, Michael CLOOTS

08:00 - 18:15 #6949 - MS05-817 Decoupling of valence and coordination number contributions at perovskite surfaces.

MS05-817 Decoupling of valence and coordination number contributions at perovskite surfaces.

Perovskite oxide nanostructures are on the forefront of technology due to the wide spectrum of possible applications pertinent to renewable energy sources, such as water-splitting, solar cells, fuel cells, batteries, and catalysis. In particular, the exceptional properties for the oxygen reduction reaction in catalysis have been detailed recently in a volcano plot and the results reveal that orthorhombic, Jahn-Teller distorted LaMnO₃ perovskite nanoparticles are the leading, non-noble metal candidate for enhanced catalytic activity on the cathode electrode of fuel cells [1]. Since the functional properties of these nanoparticles lie on their active surfaces, our approach involves a detailed structural and chemical evaluation of the surfaces on the atomic scale to determine what/where the reaction centres are. Subsequently, the morphology of the particles can be optimised to maximise the number of these reaction centres, allowing us to attain the highest possible performance of perovskite catalysts.

From structural transmission electron microscopy (TEM) data it was determined that polar facets exist on crystallites, which lead to assumptions on possible surface reconstruction/relaxation. However, high resolution TEM indicated that the atomic terminations of several surfaces remained defect-free up to the very surface with no visible reconstruction taking place [2], as shown in Figure 1. Next, the surface and subsurface of the working perovskite catalyst was probed by high spatial and temporal resolution electron energy-loss spectroscopy (EELS) in scanning TEM mode. The results revealed that the surface shows different character than the bulk. Tan et al. has previously shown that different oxidation states of Mn can be probed at neighbouring sites in the same compound [3] but it was also theoretically predicted that such a change can be attributed to coordination number differences as well [4]. Indeed, the extracted experimental information by EELS for the pristine LaMnO₃ powder was analysed utilising density functional theory calculations under the optic matrix elements approximation, as shown for the Mn L₃ peak in Figure 2, and the shift to lower energies of the Mn L_3,2 edge was found to be a convolution of both changes in oxidation state and in the number of nearest neighbours (coordination).

 

References

 

[1] J. Suntivich, H. A. Gasteiger, N. Yabuuchi, H. Nakanishi, J. B. Goodenough, and Y. Shao-Horn,

Nat Chem 3  546-550 2011.

[2] E. A. Ahmad, V. Tileli, D. Kramer, G. Mallia, K. A. Stoerzinger, Y. Shao-Horn, A. R. Kucernak,

and N. M. Harrison, J. Phys. Chem. C 119  16804-16810 2015.

[3] H. Tan, S. Turner, E. Yucelen, J. Verbeeck, and G. Van Tendeloo, Phys. Rev. Lett. 107 107602 2011.

[4] E. A. Ahmad, G. Mallia, D. Kramer, V. Tileli, A. R. Kucernak, and N. M. Harrison, Phys.

Rev. Lett. 108  259701 2012.

Vasiliki TILELI (Lausanne, Switzerland), Ehsan AHMAD, Ross WEBSTER, Giuseppe MALLIA, Martial DUCHAMP, Kelsey STOERZINGER, Yang SHAO-HORN, Rafal DUNIN-BORKOWSKI, Nicholas HARRISON

08:00 - 18:15 #6968 - MS05-819 Electron microscopy of novel technical superconducting materials.

MS05-819 Electron microscopy of novel technical superconducting materials.

The improvement of characteristics of modern superconducting devices could be achieved by changing technologies or utilization of new materials. The electron microscopy (EM), electron diffraction (ED) and microanalysis (MA) investigations of Nb-Sn based superconducting wires used for ITER project demonstrated the severities to form A15 Nb₃Sn structure over all volume of superconducting wires. Similar wires might be used for upgraded Large Hadron Collider (LHC) and Very Large Hadron Collider (VLHC). The Sn content in the large part of the superconducting wires could be less then 20 at% which could lead to the deviation of crystal structure of wires from A15 and consequent drop of electrophysical properties. The superconducting wires were formed by bronze method [1] and Nb₃Sn phase was grown during solid phase reaction of Sn solved in the bronze with Nb inserts. One possible reason of Sn depletion is the diffusion barrier appeared on the Nb/bronze interface at the early stages of superconducting wires formation. It was found that this barrier is simply uniform and slowly growing layer of Nb₃Sn adopted A15 crystal structure. The EDX elemental mapping with the help of SuperX detector (Bruker, US) in Osiris (FEI, US) STEM (Fig.1) unambiguously demonstrated the absence of Sn diffusion along Nb grain boundaries. After A15 Nb₃Sn reached critical thickness the Nb-Sn grains with Sn content less than 18 at% began to form. HAADF STEM study of these Sn depleted grains in C_s probe corrected TITAN 80-300 TEM/STEM (FEI, US) at 300 kV (Fig.2) indicated that these grains contained high density of antisite defects start to form and it was Nb substituted for Sn atoms. That conclusion was made by the estimation of intensities of atomic columns as it was done in [2] and comparison with simulated images (P.Stadelmann’s JEMS software was used for image simulations). To increase Sn content in the grains, the changes of technology were proposed. Another possibility is the utilization of new materials and possible candidates are FeTeX (where X- Se or S) superconductors. The X-ray crystal structural analysis, TEM, HAADF STEM, electron diffraction and microanalysis (TITAN 80-300 TEM/STEM) were applied to the study of FeTeX based single crystals and thin films on different substrates. One of HRSTEM images is presented in Fig.3. It was found that there are uncertainties in Fe1 and Fe2 position in FeTeX compounds and ordering of S atoms. The misfit between the FeTeX films and substrates were released through misfit dislocation or an intermediate layer at the film/substrate interface. Thus the FeTeX films were stress free and critical temperature Tc should not change due to structural modifications.

 

[1] E.Dergunova, A.Vorobieva, I.Abdyukhanov, K.Mareev, S.Balaev, R.Aliev, A.Shikov, A.Vasiliev, M.Presnyakov, A.Orekhov Physics Procedia (2012) 36, 1510.

[2] S.Van Aert, K.J.Batenburg, M.D.Rossell, R.Erni, G.Van Tendeloo Nature (2011) 470, 374.

Alexander VASILIEV (Moscow, Russia), Igor KARATEEV, Mikhail KOVALCHUK, Mikhail PRESNYAKOV

08:00 - 18:15 #6990 - MS05-821 A Review on Interface Engineering and Thin Film Nanotechnology for Low-Cost High Efficient Photovoltaic (PV) Solar Cell Devices.

MS05-821 A Review on Interface Engineering and Thin Film Nanotechnology for Low-Cost High Efficient Photovoltaic (PV) Solar Cell Devices.

Currently, the major challenge of the research and development (R&D) in the field of Photovoltaic (PV) renewable energy is to lower the cost per Wp of electricity generation and to improve the conversion efficiency of the solar cell devices.

In comparison with the crystalline silicon technology, silicon (Si) thin film nanotechnology hold the promise of reducing the solar cell cost through lowering the material amounts and energy requirement of the manufacturing process.

In this review, several aspects of the material engineering at interfaces and process optimisation for low-cost high-efficient silicon based thin film photovoltaic (PV) solar cells are discussed.

A basic understanding of the thin film growth technique using a plasma enhanced chemical vapour deposition (PECVD) system coupled with analytical nanostructural characterisations of Si thin film solar cells with particular emphasis on hydrogenated amorphous and microcrystalline p-i-n single and double (tandem) junction silicon solar cells are presented.

Nanostructural defects including microvoids and cracks at interfaces – as potential extrinsic effects on the hydrogenated microcrystalline (uc-Si:H) solar cells performance – are studied.

Analytical nanostructures of an ultrathin interfacial buffer amorphous hydrogenated germanium (a- Ge:H) layer at the P+- type a-Si:H/Transparent Conducting Oxide (TCO) are investigated using STEM and EDS. It was shown that such an interfacial buffer layer could improve the electrical performance and enhance the efficiency of the a-Si:H single junction solar cells.

Mohamed SAAD (Belfast, United Kingdom), J KIM, P KOZLOWSKI, J OTT, D SADANA, T.c CHEN

08:00 - 18:15 #6996 - MS05-823 TEM investigation of Li2MnSiO4 microstructure at different states of charge of Li-ion battery cell.

MS05-823 TEM investigation of Li2MnSiO4 microstructure at different states of charge of Li-ion battery cell.

The study concerns dilithium manganese orthosilicate nanomaterial (Li₂MnSiO₄ - LMS), which is a promising new candidate for cathodes in Li-ion systems. The material is characterized by high theoretical capacity, working voltage and low production costs. Unfortunately, LMS material cannot be successfully applied in commercial systems yet, due to its structural instability in initial charging/discharging cycles of a battery cell. The reasons of structural changes (amorphization) of the material, that occur during electrochemical reaction in the cell, are still not clear. In order to find a stage of the electrochemical process in which a destruction of long-range ordering in LMS crystalline structure occurs, we conducted ex-situ TEM observations at different states of charge of a battery cell. To facilitate microscopic observations of particular LMS grains, a working electrode was prepared by using TEM support grid as a current collector. Thanks to that, the same sample could be used in TEM observations and in potentiostatic charging/discharging processes. Our study (Fig.1) shows that destruction of Li2MnSiO4 crystalline structure takes place at potentials lower (e.g. 3.5 V) than oxidation reaction of manganese ions (i.e. 4.2 V). It suggests that, in opposition to previous literature reports, amorphization process of the material is connected with other, and not yet specified, electrochemical reactions.

Marta GAJEWSKA (Krakow, Poland), Michał ŚWIĘTOSŁAWSKI, Marcin MOLENDA

08:00 - 18:15 #6287 - MS06-825 3D-TEM studies of hierarhical graphene-composite aerogels for ultra-long-life Li-ion batteries.

MS06-825 3D-TEM studies of hierarhical graphene-composite aerogels for ultra-long-life Li-ion batteries.

Among all potential application fields of graphene, most important are still the electrochemical  energy-storage devices, in particular the Li-ion batteries (LIB). Despite of all the efforts, the performance of the graphene-based Li-ion batteries is still far from satisfactory, considered their insufficient electrical capacity, rate performance and long-term stability (1). In the present work, we present the technique allowing the production of hierarchical graphene-based composite aerogels as binder-free anodes for the ultra-long-life Li-ion batteries (LIBs).  Our approach is to increase an active area of the composite aerogel for the Li-ion uptake (or adsorption)by the introduction of spacers between the graphene sheets. The spacers are the Mo_xS_y particles of three different size ranges:  sub-nanometer (iii), a few nanometers (ii) and several hundreds of nanometer large amorphous carbon balls (i) filled with nanometer-sized Mo_xS_y (see schematic in Figure 1). The incorporation of these poly-dispersed particles as spacers between the graphene sheets results in the hierarchically porous aerogel.  Such structures as anodes in LIBs possess high capacity, 1069 mAh/g at 0.35 A/g, rate performance, 425 mAh/g at 10 A/g and 304 mAh/g at 50 A/g, and show ultra-long stability.   

In the present work, Mo_xS_y-particle loaded aerogels before and after the Li-ion uptake  were studied by HRTEM / HRSTEM. Morphological and elemental analyses of the particle populations were performed (Figure 2).  Moreover, all three groups of the spacers were analyzed for their spatial arrangement in relationship to graphene sheets by HRTEM and HRSTEM tomography, revealing the fully homogeneous spatial distribution of two groups of the small-sized clusters on the sheets and the hierarchical nature of the largest spacers consisting of the polymer balls uniformly filled with the small clusters. TEM analyses confirmed that all three kinds of the spacers are very effective in the preventing the graphene sheets from coalescence. Both, graphene sheets and the spacers can intake Li-ion, contributing to the charge-discharge cycles and are as such responsible for the long-life electrochemial performance.

1. J. Liu, Charging graphene for energy, Nat. Nanotechnol.2014,  9, 739-741.

Alla SOLOGUBENKO (Zurich, Switzerland), Guobo ZENG, Elena TERVOORT, Fabian GRAMM, Markus NIEDERBERGER

08:00 - 18:15 #6367 - MS06-827 Identification of Laves- and Z-phase formed in 9%Cr ferritic alloy after long-term thermal treatment.

MS06-827 Identification of Laves- and Z-phase formed in 9%Cr ferritic alloy after long-term thermal treatment.

High-performance reduced-activation ferritic–martensitic steels (EUROFER 97) with 9%Cr (WMnVTa) content have been envisaged for application as a structural material for operations at up to 650°C in future fusion reactors as well as in Generation IV fission reactors [1]. The thermal stability of steel’s microstructure is a basis for its application in structural components exposed for long time at a very hostile environment, which includes high neutron fluxes at high temperatures, as the one expected during operation of future fusion reactors. The desired mechanical properties of the EUROFER 97 were achieved by the formation of M₂₃C₆, TaC, and VN precipitates on the grain and lath boundaries during alloy fabrication and thermal treatment [1]. The long term annealing for up to 100.000h leads to the changes in this desired precipitates distribution and formation of coarse Z- and Laves phases which might influence the mechanical properties of the steel. Transmission Electron Microscopy (TEM) Energy Dispersive X-ray analysis (EDX) and Electron Energy Loss Spectroscopy has been used for identification and to study the morphology, structure, and chemical composition of new phases formed during long-term thermal treatment.

Fig. 1 shows TEM investigation of precipitate’s distribution in the specimen annealed at 550°C for 76.000h. The different phases are imaged with different colours in 2 dimensional elemental map (b). The investigation reveals the presence of tungsten rich precipitates additionally to the typical for EUROFER 97 three phases: M₂₃C₆, TaC, and VN phases. These new W-rich precipitates have been identified as Laves phase with (FeCr)W₂ composition. The corresponding EDX spectrum shown in Fig. 3a demonstrates typical Laves phase composition.

Fig. 2 shows a distribution of main compositional elements in a specimen annealed at 600°C for 35000 h, which was obtained using EDX elemental mapping. The investigated area is marked by a square in the HAADF image (Fig. 2). The new precipitates identified by this temperature are V-rich Z-phase. The smaller precipitates of M₂₃C₆ imaged with red colour have a size that varies from 50 nm to 120 nm. The faceted Z-phase have been found to be up to 700 nm in size. Ta and V are present in small precipitates of MX type (VN and TaC [1]) as well as in the Z-phase. Based on these analytical results, it was found that the Z-phase can be differentiated well from other phases, which also may contain Cr, V, and Ta or a combined composition of these elements [2]. M₂₃C₆ and VN precipitates in ferritic steels can have a contrast similar to that of the matrix in HAADF and bright field images, they can only be studied using 2-dimensional analytical imaging.

The results demonstrate that precipitation mechanism of secondary phases changes in the narrow temperature range between 550°C and 600°C. Formation Z-phase precipitates with Cr2TaVN2 composition was detected only at 600° after annealing for as longer as 30.000h.

[1] M. Klimenkov, et. al Progress in Nuclear Energy 57 (2012) 8-13

[2] H. K. Danielsen Ph.D. thesis (2007) Technical University of Denmark

Michael KLIMENKOV (Eggenstein-Leopoldshafen, Germany), Ute JÄNTSCH, Jan HOFFMANN, Michael RIETH

08:00 - 18:15 #6384 - MS06-829 Atomic structure and magnetic circular dichroism of antiphase boundary defects in NiFe2O4 thin films.

MS06-829 Atomic structure and magnetic circular dichroism of antiphase boundary defects in NiFe2O4 thin films.

      The complex and interesting properties of ferrimagnetic spinel ferrite thin films are of great fundamental interest, as well as being of practical importance for applications in spintronic devices and ultra-high-density recording media. The presence of antiphase boundary (APB) defects is responsible for reduced spin polarization and magnetism in spinel ferrites^[1,2]. There is also considerable discussion about the relationship between the atomic structures of APBs and their magnetic properties. Whereas atomic structures of many APBs have been determined using high-resolution transmission electron microscopy and high-angle annular dark field (HAADF) imaging^[3], local measurements of magnetic properties at APBs on the nm scale have not yet been achieved.

      Electron magnetic circular dichroism (EMCD) was demonstrated experimentally in 2006 for a specific diffraction geometry^[4]. Since then, the spatial resolution of EMCD has been improved to approximately 1 nm using nanobeam diffraction^[5]. In 2013, we developed a site-specific EMCD method for magnetic structure determination and achieved EMCD spectra with high signal-to-noise ratio^[6]. Here, we combine site-specific EMCD in nanobeam diffraction mode with high-resolution HAADF imaging, in order to simultaneously determine the magnetic circular dichroism and atomic structure of APBs in NiFe₂O₄ thin films.

      We find the atomic structures of APBs that are formed on {111} planes by a crystallographic translation of 1/4a[0-11]  using HAADF imaging. The EMCD signals at such defects were obtained using an electron beam with a diameter of ~1 nm and compared with signals obtained from a perfect single crystalline region under the same illumination and acquisition conditions. We demonstrate experimentally that the strength of the magnetic circular dichroism at APBs is suppressed significantly when compared with that in the perfect area. The capability of EMCD at 1 nm spatial resolution enable us to correlate our experimental magnetic circular dichroism spectra from local defects with corresponding structural and chemical information recorded at the atomic scale, opening the door to experimental investigations of the relationship between atomic structure and magnetic properties of local defects in materials.

References

[1] D.T. Margulies, et al, Phys. Rev. Lett., 79 (25), 5162, 1997

[2] W. Eerenstein, et.al. Phys.Rev.B.68.014428(2003)

[3] K.P. McKenna, et.al, Nat. Commun. 5,5740 (2014).

[4] P. Schattschneider, et al, Nature 441, 486–488 (2006).

[5] J. Salafranca, et al, Nano Lett., 12: 2499, 2012

[6] Z.Q.Wang, X.Y. Zhong, et al, Nat. Commun. 4, 1395 (2013).

Acknowledgements

This work is financially supported by the National Basic Research Program of China (2015CB921700), the National Natural Science Foundation of China (51471096), the Tsinghua University Initiative Scientific Research Program and partly supported by JST under Collaborative Research Based on Industrial Demand ”High Performance Magnets: Towards Innovative Development of Next Generation Magnets”. This work made use of the resources of the National Centre for Electron Microscopy in Beijing and the Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons in Forschungszentrum Jülich. RDB is grateful to the European Research Council for an Advanced Grant. We are grateful to Prof. R. Yu, Prof. J. Zhu, Prof. C. L. Jia, Prof. J. Yuan, Dr. M. Bornhöfft and Mr. D. S. Song for valuable discussions

Zechao WANG (Beijing, China), Xiaoyan ZHONG, Lei JIN, Hideto YANAGIHARA, Eiji KITA, Hanbo JIANG, Rafal E DUNIN-BORKOWSKI

08:00 - 18:15 #6392 - MS06-831 Atomic scale structure and local chemistry of CoFeB-MgO perpendicular spin injector.

MS06-831 Atomic scale structure and local chemistry of CoFeB-MgO perpendicular spin injector.

The development of spin light emitting diode (spin-LED) with spin injectors with perpendicular magnetic anisotropy (PMA) is a prerequisite for the conversion of carrier spin polarization to the circular polarization (PC) of photon without magnetic field for practical applications. In our previous study, a maximum PC at zero field (20% at 25K, 8% at 300K) was reported with an ultrathin perpendicular MgO/CoFeB spin injector with Ta capping layer [1]. To achieve a good PMA, post-annealing is indispensable with a narrow optimal window around 250°C. Recent work shows that the PMA can be increased by 20%, by replacing Ta layer with a Mo layer [2]. Moreover, Mo capping increases the temperature stability of the spin injector, allowing post annealing up to 425°C.

The specific crystal structure, local chemistry and local bonding of all staking layer that make up the perpendicular spin injector remain unclear, hindering the establishment of the relationships between the material properties and the nanoscale structure. The interfacial anisotropy depends critically on the crystal structure. The composition, chemical states, and local defects in CoFeB and in MgO are known to be critical for transport and magnetic properties. Experimentally the structural and chemical issues in CoFeB-MgO have been addressed utilising HRTEM, EDS, XPS, secondary-mass spectroscopy. A consistent picture has not emerged yet due to the difficulty of probing local atomic details. In particular there is many contradictory results on the fate of B following annealing, including diffusion into Ta [3], segregation at the CoFe interface [4], B diffusion into MgO forming a magnesium boride phase [5].

Here, we combine HRTEM, aberration corrected STEM (HAADF and BF) and spatially resolved EELS to follow the structure and the local chemistry of MgO\ CoFeB capped with Ta or Mo, before and after annealing up to 400°C. The spin injectors consist in 2.5 nm of MgO / 1.2 nm CoFeB / 5 nm Ta or Mo deposited on a GaAs-based LED (Fig 1). For FeCoB layer caped with Ta at temperature higher than 250°C, annealing favours the diffusion of B into Ta when CoFe crystallises then Ta diffuses trough the CoFe layer into the MgO barrier (Fig 2). For CoFeB layer caped with Mo, boron stays in the CoFe layer close to the CoFe-Mo interface (Fig 3). Mo diffusion into the CoFe layer also occurs, but for higher temperature than for Ta diffusion. In extension we'll show that for both structures, annealing has a strong influence on the Fe/Co ratio at the MgO-CoFe interface and at the CoFe-capping layer (Fig 2 and 3). The influence of the structure and of local chemical composition on the properties of the spin injector will be discussed.

 

[1] S. H. Liang et al., Phys. Rev. B 90 (2014) 085310

[2] T. Liu et al., Sci. Rep. 4 (2014) 5895

[3] X. Kozina et al., Appl. Phys. Lett. 96 (2010) 072105

[4] J. Burton et al. , Appl. Phys. Lett. 89 (2006) 142507

[5] Z. Q. Bai et al Phys. Rev. B 87 (2013) 014114

Bingshan TAO, Xavier DEVAUX (NANCY), Philippe BARATE, Pierre RENUCCI, Bo XU, Julien FROUGIER, Michel HEHN, Stéphane MANGIN, Henri JAFFRES, Jean-Marie GEORGE, Xavier MARIE, Xiufeng HAN, Zhanguo WANG, Yuan LU

08:00 - 18:15 #6412 - MS06-833 Scanning electron microscopy study of rhodium, palladium and platinum foils treated in NH3-air flow.

MS06-833 Scanning electron microscopy study of rhodium, palladium and platinum foils treated in NH3-air flow.

     Ammonia oxidation with air on platinum catalyst gauzes is widely used in chemical industry for synthesis of nitric acid. It is well known that during this process the gauzes undergo deep structural rearrangement of surface layers (catalytic etching) leading to a loss of platinum and decrease of catalytic activity. To determine the role of individual metals: Pt, Pd and Rh in the catalytic etching of platinum catalyst gauzes during the NH₃ oxidation, we carried out detailed investigation of the surface microstructure of platinum, palladium and rhodium catalysts treated in the reaction medium (NH₃+O₂). Polycrystalline Pt, Pd and Rh foils with the size of 10 х 5 х 0.04 mm were used as the catalyst samples. Each sample was assembled into a package with four platinum gauzes required to maintain standard conditions of the NH₃ oxidation process. The platinum catalyst gauzes were made from a polycrystalline wire (d ≈ 82 μm) with the chemical composition (in wt.%) 81% Pt, 15% Pd, 3.5% Rh and 0.5% Ru. A laboratory flow reactor made of a quartz tube with the inner diameter of 11.2 mm was used at the feed (ca. 10% NH₃ in air) flow rate 880-890 l/h, the gauze temperature 860±5 °C and total pressure about 3.6 bar. The surface microstructure was studied using a scanning electron microscope (SEM) JSM-6460 LV (Jeol) in the mode of secondary electrons at beam energy 25 keV.

     The SEM study of polycrystalline Pt, Pd and Rh foils after treatment at T~860°C for 5 h in the reaction medium (~10% NH₃ in air) revealed differences in the surface microstructure of these samples. The O₂ reaction with Rh during the catalytic oxidation of NH₃ over rhodium foil results in deep rhodium oxidation followed by the formation of a continuous layer of Rh₂O₃ crystals with the size 1-2 μm. Fast reaction of gaseous NH₃ molecules with O atoms of rhodium oxide leads to the formation of oxygen vacancies and movement of Rh atoms to the surface of the oxide crystals. Rh atoms quickly migrate over the oxide surface and desorb into the gas phase. Increased concentration of Rh atoms in the near-surface gas layer initiates the formation and gradual growth of elongated pyramidal Rh crystals with low concentration of defects. Such continuous processes lead to the formation of a solid layer of pyramidal Rh crystals with the sizes 0.3-0.5 μm at the base, 0.05-0.1 μm at the end and the length 1-2 μm (Fig. 1a,b). The O₂ interaction with Pd during the NH₃ oxidation on palladium foil leads to intense dissolving of oxygen atoms at defects and in the metal lattice, whereas the resulting oxide PdO quickly decomposes under these conditions. The reaction of gaseous NH₃ molecules with absorbed oxygen atoms O_abs with the formation of gaseous NO results in local overheating of the surface initiating the release of metal atoms to the surface.  Intense release of metal atoms from the grain boundaries leads to the formation of extended voids between the grains. Adsorbed Pd atoms quickly migrate over the metal surface and get incorporated into energetically the most favorable sites. Due to these processes, pits, pores and crystalline facets grow on the surface, whereas grains are gradually reconstructed into faceted crystalline agglomerates with through pores formed due to the growth and merging of pits. So, dramatic structural reconstruction of the foil surface layer (catalytic etching) with the formation of a rough layer takes place during the catalytic NH₃ oxidation with air on Pd. This rough layer contains microcrystals and porous agglomerates with the size of ~10-20 μm containing pores with the diameter 1-2 μm separated by voids with the width ~1-10 μm (Fig. 2a,b). The O₂ interaction with platinum during the NH₃ oxidation over Pt foil results in removal of the surface carbon impurities followed by dissociative chemisorption of oxygen on the surface. It is well known that oxygen dissolution in the Pt lattice with the formation of oxide phases is substantially slower than on Pd and Rh. Small amount of oxygen atoms can be absorbed at the grain boundaries and other defects. The NH₃ reaction with O_abs at these defects initiates release of a few Pt atoms to the surface leading to weak etching of the platinum foil surface. The catalytic NH₃ oxidation on Pt at T~860°C for 5 h results in minor structural reconstruction of the foil surface layer related to the formation of grain boundaries and shallow parallel furrows with the width 1-2 μm covered with crystalline facets (Fig. 3a,b).

Acknowledgement

This work was supported by Russian Academy of Sciences and Federal Agency of Scientific Organizations (project 44.1.17).

Aleksei SALANOV (Novosibirsk, Russia), Natalia KOCHUROVA, Elena SUTORMINA, Lyubov ISUPOVA, Valentin PARMON

08:00 - 18:15 #6417 - MS06-835 Electron Radiolytic Triggering Metal-Insultor Transformation in VO2 Nanowire.

MS06-835 Electron Radiolytic Triggering Metal-Insultor Transformation in VO2 Nanowire.

Vanadium dioxide (VO₂) is a correlated-electron materials that, in the strain-free state, undergoes a first-order metal–insulator phase transition (MIT) at T_c=341 K with a change in conductivity of several orders of magnitude. The MIT is accompanied by a structural phase transition from the high-temperature rutile phase (metallic, R) to the low-temperature monoclinic phase (insulating, M). Although VO₂ is a promising functional material to build up micro sensors, switches and actuators, the precise localization of MIT is still unavailable, and the M/R domain wall evolution is out of control. There are big challenges for device design at micro/nano scale. In this work, we demonstrate how we locally trigger nano-scale metal-insulator transition of vanadium dioxide with a focused electron beam. Basically, the electron beam provides a nano-sized probe for local oxygen vacancies doping to control the transition temperature in nano-area of the VO₂ nanowires. Our results shed lights on understanding the mechanism of MIT in VO₂ and developing VO₂-based MEMS device with better flexibility.

 

Individual single-crystal VO₂ nanowire cantilever is fabricated to be free from various strains and mounted in a transmission electron microscope (TEM) as shown in Fig. 1a. One end of the single crystal nanowire is attached on the copper substrate and the other end is free standing. It was originally an entire monoclinic phase (M₁) at room temperature (25 °C). At elevated temperature of 68 °C, metallic rutile phase (R) is more stable, the entire pristine VO₂ nanowire abruptly transformed from M₁ phase into R phase. That is, no metallic-insulator domains are observed. However, as 200 KeV electron beam in TEM was employed to radiate at the free end tip of a VO₂ nanowire of 3.7 µm long with cross sectional area of 7.1×10^-2 μm² with the dose of about 7.2×10⁵ Å^-2, R phase was found to preferentially nucleate just at the irradiated site at 62.2 °C, obviously lower than 68 °C in the pristine VO₂ nanowire (Fig. 1b). The enlarged image (Fig. 1c) shows the coexistence of the R-phase and M-phase domains. Fig. 1d shows that the transition temperature can be controlled with the electron dose. When the total dose is lower than a critical dose, no metallic-insulator domains are observed, but the transition temperature decreases accordingly. The domain size and structure of metallic-insulator can be controlled with irradiation of electron beam. Fig.1e shows a VO₂ nanowire marked the electron beam irradiation sites at room temperature. Fig. 1f shows the dark field image of the designed domain structure of the nanowire. Once the metallic R phase domain forms in the tip of VO₂ nanowire, the rest of insulator M phase will transform into the R phase gradually rather than abruptly. The growth kinetics of R phase can then be controlled by the temperature. Fig. 2a is a series of TEM images to show the temperature dependent transformation of insulator M phase to metallic R phase, while Fig. 2b shows the transformation fraction of R phase vs. temperature. The transformation kinetics and the mechanism of the radiolytic triggering MIT in VO₂ nanowire will be discussed in detail in the talk.

 

Acknowledgement The authors acknowledge the support by the National Natural Science Foundation of China (Grants Nos. 11374028 and U1330112). M.L.S. acknowledges the Cheung Kong Scholars Programme of China. The authors acknowledge very useful discussion with Li-Min Liu. The authors also thank Bin Zhang and Jin-Hua Hong for help with EELS measurement. The authors are also grateful to Rui-Wen Shao and Yong-He Li for assistance during in situ electrical measurement with Nanofactory holder.

Zhen-Hua ZHANG, Hua GUO, Wen-Qiang DING, Xiao-Xiang KE, Fu-Rong CHEN, Man-Ling SUI (Beijing, China)

08:00 - 18:15 #6443 - MS06-837 Aberration-corrected STEM and EELS investigations of grain boundaries in an optimised BaTiO3 based PTCR ceramic.

MS06-837 Aberration-corrected STEM and EELS investigations of grain boundaries in an optimised BaTiO3 based PTCR ceramic.

Positive temperature coefficient of resistivity (PTCR) effect is a property found in polycrystalline materials which can transform from a low resistance state to a high state of resistance in response to heat. Accordingly, this effect has found extensive applications in sensing technologies such as self-regulating heating elements, current sensors and sensors for the detection of air flow, liquid level and temperature changes.^[1] Among the various materials exhibiting PTCR effect to date, the most favoured material group is Barium Titanate (BaTiO₃) based or quasi-BaTiO₃ based ternary perovskite compounds where the temperature at which this switch in behaviour occurs, near the ferroelectric-paraelectric Curie transition temperature (T_c), and the magnitude of the switch can be controlled and optimised via the addition of different dopants and/or changes in the processing conditions.^{[2, 3]}

The role of grain boundaries in these ceramics has been strongly deliberated in previous studies with most of the experimental evidence towards the role of grain boundaries established by macroscopic studies, allowing the interpretation of grain-boundary resistivity in terms of equivalent circuit diagrams.^{[4, 5]} Yet, direct visualisation and mapping studies of the PTCR behaviour on the nanoscale has been missing. Here, we identify the grain boundaries as the pivotal region of interest by reporting clear evidence of non-linear changes in electrical potential (via Kelvin probe force microscopy (KPFM)) observed locally across single grain boundaries, explicating their central role in this phenomenon.

Several studies have suggested that chemical diffusion, and segregation at the grain boundaries could play a part in creating the PTCR effect, but attempts to provide evidence of this chemical heterogeneity have so far been unsuccessful.^{[6, 7]} We employed aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) to investigate chemical inhomogeneity and electronic structure changes across grain boundaries on the atomic scale. These studies have revealed the presence of localised PbTiO₃-like formation (Figure 1) and striking changes in Ti-L_2,3 and O-K ELNES features approaching the grain boundary (Figure 2). Due to the clear chemical and intrinsic change in symmetry identified, a quantitative analysis of the crystal field splitting (CFS) was carried out across the grain boundary. We present the remarkable CFS trend, suggestive of octahedral distortion, transitioning across single grain boundaries.

The state-of-the-art microscopy techniques involved in this investigation have allowed us to unravel the complexity of this PTCR ceramic. As a result, we provide a logic interpretation of the BaTiO₃-like grain interior and PbTiO₃-like grain boundary regions in terms of an enhanced normal polarisation component at the grain boundary interface, which according to the modified Heywang-Jonker model^{[8, 9]} augments electronic transport in the ferroelectric phase and enhances the magnitude of resistivity jump at T_c. We demonstrate the idea that a confined grain boundary region which exists in such a strategic manner offers a novel route towards engineering better performing PTCR devices.

 

 References

[1] B. M. Kulwicki, , Journal of Physics and Chemistry of Solids. 1984, 45(10), 1015-1031.

[2] D. Y. Wang, K. Umeya, , J Am Ceram Soc. 1990, 73(3), 669-677.

[3] J. Nowotny, M. Rekas, , Ceram. Int. 1991, 17(4), 227-241.

[4] D. C. Sinclair, A. R. West, , Journal of Applied Physics. 1989, 66, 3850.

[5] F. D. Morrison, D. C. Sinclair, A. R. West, , Journal of the American Ceramic Society. 2001, 84, 531.

[6] L. Affleck, C. Leach, , Journal of the European Ceramic Society. 2005, 25(12), 3017-3020.

[7] J. Hou, Z. Zhang, W. Preis, W. Sitte, G. Dehm, , Journal of the European Ceramic Society. 2011, 31(5), 763-771.

[8] W. Heywang, , Journal of the American Ceramic Society. 1964, 47(10), 484.

[9] G. H. Jonker, , Solid-State Electronics. 1964, 7(12), 895-903.

 

Kristina HOLSGROVE (Belfast, United Kingdom), Demie KEPAPTSOGLOU, Alan DOUGLAS, Quentin RAMASSE, Eric PRESTAT, Sarah HAIGH, Amit KUMAR, Marty GREGG, Miryam ARREDONDO

08:00 - 18:15 #6496 - MS06-839 Piezoelectric thin films investigated by dark-field electron holography and in-situ biasing.

MS06-839 Piezoelectric thin films investigated by dark-field electron holography and in-situ biasing.

Ferroelectric and piezoelectric oxides have a large number of applications in memories and micro-electro-mechanical systems (MEMS). Recently, a piezoelectronic transistor (PET) has been proposed as an alternative to the conventional metal-oxide-semiconductor (CMOS) transistor [1]. Therefore, there is a growing need for the characterisation of piezoelectric thin films at the nanoscale.

We investigated Pb(Zr,Ti)O₃ (PZT) and Pb(Mg_1/3Nb_2/3)O₃−PbTiO₃ (PMN-PT) epitaxial thin films containing c- and a-domains (or 90° domains) using transmission electron microscopy (TEM) techniques. Experiments were carried out using the I2TEM-Toulouse microscope (Hitachi HF-3300) equipped with a cold field emission gun, an image corrector (B-COR) and multiple electron biprisms. Firstly, dark-field electron holography [2] was carried out to map the residual deformations due to the epitaxial constraints. Secondly, in-situ biasing was conducted using a picoindenter PI 95 (Hysitron) to observe directly the switching of ferroelectric domains.

We found that the residual deformation and rotation fields depend on the inclination of neighbouring a-domains. Fig. 1(a) shows an example of dark-field electron hologram obtained on a tetragonal Pb(Zr_0.2,Ti_0.8)O₃ thin film grown on SrTiO₃. The investigated region contains two main a-domains with opposite inclination, forming a hat shape. The rotation map in Fig. 1(b), retrieved from the hologram, reveals two rotation gradients in the direction perpendicular to the a-domains, increasing towards the interface. The rotation field is guided by partial a-domains located in the central region. These gradients are related to the lattice mismatch at the domain walls and the growth constraints onto the substrate [3].

Fig. 2(a) shows an in-situ biasing experiment conducted on a Pb(Zr_0.2,Ti_0.8)O₃ thin film using a diamond probe (doped) placed in contact with the surface of the film. The SrTiO₃ substrate (Nb doped) is connected to the ground. The application of the electric field favours the formation of c-domains (vertical polarisation) at the expense of the two a-domains (horizontal polarisation) located just below the probe. When applying -15V (Fig. 2(b)), the a-domains become narrower and then they are completely erased at -25V (Fig. 2(c)). New a-domains are created on the left side of the probe which can be due to a redistribution of the deformation or a deviation of the electric field from the vertical direction (stronger in-plane component) in this region.

Finally, Fig. 3(a) shows an experiment conducted on the same film but with a gold top electrode. The region showed here contains two a-domains indicated by dashed lines. When applying +10V (Fig. 3(b)), it was observed that c/c domain walls move laterally through the film until they form close pairs. The two domain walls on the right side moved quickly towards each other and finally disappeared (Fig. 3(c)). The c/c domain walls on the left side were slower (still exist after 60s in Fig. 3(c)) which might be related to defects or pre-existing domain walls.

 

[1]  D.M. Newns et al, Advanced Materials, 24, 3672–3677 (2012)

[2]  M.J. Hÿtch et al, Nature, 453, 1086–1089 (2008)

[3]  G. Catalan et al, Nature Materials, 10, 963-967 (2011)

 

Acknowledgments

This work was funded through the European Metrology Research Programme (EMRP) Project IND54 Nanostrain. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. This work has been supported by the French National Research Agency under the "Investissement d'Avenir" program reference No. ANR-10-EQPX-38-01. The authors acknowledge the "Conseil Regional Midi-Pyrénées" and the European FEDER for financial support within the CPER program.

Thibaud DENNEULIN (TOULOUSE CEDEX), Nicole WOLLSCHLÄGER, Martin HŸTCH

08:00 - 18:15 #6504 - MS06-841 Quantitative investigation of the all round shape memory effect in a Ni51Ti49 alloy by TEM orientation imaging.

MS06-841 Quantitative investigation of the all round shape memory effect in a Ni51Ti49 alloy by TEM orientation imaging.

Ni-Ti shape memory alloys are being investigated for a new application which focuses on the artificial sphincter for human implantation. In this application, the all round shape memory effect (ARSME) [1], which describes the effect of a dramatic and repeatable shape change from the parent (austenite) phase to an opposite shape in the product (martensite) phase, controls the performance of the alloy for its utilization.

According to the Ni-Ti phase diagram, several precipitates can appear during thermal treatment, with the metastable Ni₄Ti₃ type being the most relevant one for controlling the SME. These precipitates can be produced by appropriate ageing and normally have a lenticular shape and remain coherent with the matrix if they do not grow too large, leading to small but important strain fields around the precipitates [2]. For the alloy with ARSME, the way of training is by constrained ageing. The different strain state on the outside and inside of the bended alloy ribbon will cause the formation of different variants of the Ni₄Ti₃ precipitates, and this difference leads to the final result of the ARSME. Although Nishida et al. have already clarified the origin of this effect, their conclusions are mainly based on direct acquisition of images from different conditions and then matching those with the macroscopic performance of the alloy [3]. Obtaining quantitative and statistically relevant data on these precipitates, however, remains a problem due to their small size and the four orientation variants in which they can appear.

In this study, a Ni₅₁Ti₄₉ alloy was fabricated by arc melting from Ni and Ti powders with fast solidification, followed by different sets of treatment which include constrained aging and stress free aging. The ARSME varies when aging conditions are changed in both time and temperature. DSC was used to investigate the thermal behavior and TEM was used for studying the microstructure of the alloys. The new technique of automated crystal orientation and phase mapping (ACOM-TEM from NanoMEGAS^®) is used to investigate the precipitates at nanoscale resolution. The electron diffraction spot patterns are collected with an external CCD camera, capturing the diffraction pattern on the small retractable phosphorous screen inside the column, while the sample area of interest is scanned by a nanoprobe electron beam. Local crystallographic orientation and/or phase are identified through an algorithm that compares the recorded electron diffraction spot patterns with pre-simulated kinematical templates for all possible orientations and/or phases of a number of pre-selected structures. This technique provides clear orientation and phase distribution information yielding quantitative information on the precipitates’ dispersion.

Since this is the first attempt to apply this technique to nanosized Ni₄Ti₃ precipitates, a parameter optimization was performed before the quantification and based on reliability maps and shape and size measures of detected precipitates when compared to BF TEM images. The best conditions include an incoming beam precession of 0.2°, a spot size 9 and a 30 μm C2 aperture (Tecnai G2 microscope equipped with field emission gun). The template generation for the Ni₄Ti₃ structure should be performed with an excitation error of 0.2, intensity scale 14 and step count 120 while for the template generated for the Ni-Ti B2 matrix an excitation error of 0.5 and step count 120 is preferred.

Figure 1 gives one example on one of the stress free samples to show the result of orientation and phase mapping of Ni₄Ti₃ precipitates in the Ni-Ti B2 matrix. Figure 2 shows the result of orientation and phase mapping of a stress assisted sample. The precipitate density in figure 1b is calculated to be 17%, while density in figure 2b is increased to 21%. From these images, such quantitative comparisons between different fabrication processes can be obtained, which will lead to better understanding and improvement of the ARSME in Ni-Ti. 

[1] M. Nishida, T. Honma, Scripta Metallurgica, 18 (1984) 1293-1298.

[2] W. Tirry, D. Schryvers, Nat Mater, 8 (2009) 752-757.

[3] M. Nishida, C.M. Wayman, T. Honma, Scripta Metallurgica, 18 (1984) 1389-1394.

Xiayang YAO (Antwerp, Belgium), Yuanyuan LI, Shanshan CAO, Xiao MA, Xin-Ping ZHANG, Dominique SCHRYVERS

08:00 - 18:15 #6530 - MS06-843 Structural special features of the gold nanolayer sputtered on the glass surface modified by surface ion exchange and chemical etching.

MS06-843 Structural special features of the gold nanolayer sputtered on the glass surface modified by surface ion exchange and chemical etching.

     The research data are related to X-ray structure analysis of the gold nanolayers (50, 100 and 200 nm thick) on glass surface modified by the surface ion exchange, chemical etching or by treatment combining two indicated above methods. The soda lime silica glass has been modified by the special Surface Ion Exchange Paste (SIEP) [1-3]. Na⁺/Li⁺ ion exchange with SIEP has included next stages: glass surface degreasing and washing; the SIEP laying on the glass surface; thermal treatment at ~ 300º C for 15 – 20 min.; washing with running and distilled water. The chemical etching of glass have been carried out with the composition containing hydrofluoric acid [4]. The glass surface morphology has been observed by the SEM JSM-6460 (Jeol, Japan); the gold nanolayers were deposed on glass surface by the special device JVC-1600 (Jeol, Japan).

     X-ray data have been measured with diffractometer D8 Advance (CuKa radiation, one-dimensional detector Lynx–Eye with nickel filter). The range of the measurement was 2q = 10 – 120° with step 0,02° and acquisition interval 35,4 с. The program Topas 4.2 (Bruker AXS, Germany) and initial structural data of inorganic base ICSD, FIZ Karlsruhe, Germany have been also used in calculations.  

     X-ray data analysis allows make conclusion that the surface modification of the soda lime silica glass by the surface ion exchange and chemical etching has a substantial influence upon the size of gold crystal grains (Figure 1 i). Gold crystal grains have elongated shape in direction . The increase of the gold nanolayer thickness leads to decreasing of the crystal grains size.  The least size have been related to gold nanolayer 200 nm thick on glass surface modified by combined method indicated above: in direction average calculated grains size is ~ 14.7 nm; grains size averaged in directions , and is ~ 6.8 нм.

Authors express thanks to N.V. Bulina for technical support.                               

References

1. A.A. Sidelnikov, D.V. Svistunov, O.N. Sidelnikova Patent RF 2238919, (2004) 1-3.

2. O.N. Sidel’nikova, A.N. Salanov Glass and Ceramics, 64 (2007) 425 – 428.

3. O.N. Sidelnikova, G.A. Pozdnyakov, A.N. Salanov A.N., A.N. Serkova A.N. Glass Tech.: European J. of Glass Sc. and Tech. Part A, 52 (2011) 15 – 22.

4. E.K. Lazareva, T.M. Chelsova, A.A. Vernyj Inventor’s sertificate USSR 948926A (1982).

 

Olga SIDELNIKOVA, Aleksei SALANOV (Novosibirsk, Russia), Dmitriy YATSENKO, Alexandra SERKOVA

08:00 - 18:15 #6561 - MS06-845 TEM study of the explosive crystallization process in perovskite PZT-based heterostructures.

MS06-845 TEM study of the explosive crystallization process in perovskite PZT-based heterostructures.

Explosive crystallization (EC) of materials has been extensively studied both theoretically and experimentally in the end of 20^th century. Now EC can be used for investigations of fast phase transitions [1] and controlled fabrication of heterostructures, for example, by the laser annealing [2]. In this case, location of the heat source depends on an absorption coefficient of an annealing substance. In transparent amorphous material high energy, laser pulses give rise to ionization due to multiphoton absorption, which initiates melting with further crystallization. The power density to start the process falls in the range of tenths TW/cm² [3]. The use of ultrashort pulses provides locality of the process seeding while the surrounding layers accept almost unperturbed even if ablation of material accrues [4]. In this work we have studied of the EC process in ferroelectric precursor films after multipulse femtosecond laser annealing.

For the study we used films of lead zirconate-titanate (PZT) with the thickness of 700nm deposited on a platinized silicon substrate (Pt (80 nm)/SiO₂ (300nm)/Si (300μm)) by RF magnetron sputtering. Wavelength of the laser radiation was set at 800nm.The laser pulses have duration of 100fs, and the repetition rate of 80MHz. To anneal the ferroelectric film the latter was exposed to the laser radiation for the period τ_A from 0.1s to 1.2s. The power density at the annealed spot ranges from 1.0 to 2.0 MW/cm². Cross-sections for TEM were prepared by focused ion beam (FIB) in a FEI Helios. TEM investigations were carried out in a Tecnai G²30ST equipped by a HAADF detector for STEM mode and an EDX detector at accelerating voltage of 300kV and Tecnai Osiris equipped by Bruker Super-X system at accelerating voltage of 200kV.

General TEM views of the cross-sectional selected annealed microstructures are shown in Fig.1. The marked areas are semicircles with the center at the surface of the film. In the annealed areas, the structure is changed to a granulated one. The size of grains varies from 10 to 200nm. The most unexpected peculiarity of these images is the semispherical shape of crystallized areas. Although the heat source is located at the bottom PZT/Pt interface, the center of semi-spheres is located at the top interface (surface) of the PZT film. Calculations of the interplanar distances showed that the perovskite in this area has an increased tetragonality (1.357±0.05) compared to the tabulated value of 1.02.

Cross-sectional transmission electron microscopy (TEM) images obtained for different crystallization times allow us to consider the crystallization propagation within the film. Although the femtosecond pulses are practically not absorbed by the film and result in an ultrafast laser induced heating of Pt-layer, the crystallization is seeded at the surface of the film and propagates to the heat source at the film/Pt interface (Fig. 2.).

The source of the heat is localized at the bottom interface. However, the heat propagates very fast and the temperature of both the top and the bottom interfaces are almost equal due to a very small thickness of the layer. At the same time, the film at the bottom interface undergoes high strain due to the difference in thermal expansion of Pt and PZT layers. The strain increase results in an increase of activation energy, which suppresses crystallization at the bottom interface. As a result, crystallization starts from the top interface i.e. the surface of the film.

This work was performed using the equipment of the Shared Research Center IC RAS and particularly supported by the Ministry of Education and Science of Russian Federation (State task no. 11.144.2014) and Grant no.14.Z50.31.0034, p220.

References:

1)    N. Zheludev, Nature Photonics 1 (2007) 551.

2)    D.N.Khmelenin, O.M.Zhigalina, K.A.Vorotilov, I.G.Lebo. Phys. of Solid State, 2012, 54, 5, 999-1001

3)    N.YuFirsova, E.D.Mishina, A.S.Sigov, S.V.Senkevich, I.P.Pronin, A.Kholkin, I. Bdikin, Yu.I.Yuzyuk, Ferroelectics 433 (2012) p.164.

4)    A.S.Elshin, I.P.Pronin, O.M.Zhigalina, M.Yu.Presnyakov, D.N.Khmelenin, E.D.Mishina, V.I.Emel’yanov. Solid State Comm., 2015, 224, 5.

Olga ZHIGALINA (Moscow, Russia), Dmitrii KHMELENIN, Mikhail PRESNIAKOV, Igor PRONIN, Andrew ELSHIN, Elena MISHINA

08:00 - 18:15 #6566 - MS06-847 Atomic scale studies of La/Sr ordering in La2-2xSr1+2xMn2O7 single crystals.

MS06-847 Atomic scale studies of La/Sr ordering in La2-2xSr1+2xMn2O7 single crystals.

Many fascinating properties of materials depend strongly on the local chemical environment. This is the case for many complex oxides, such as materials with colossal magnetoresistance, where small variations of composition at the atomic scale can affect drastically the macroscopic properties. The main objective of the present work is to analyze the local chemical composition with atomic resolution and to find out if any underlying chemical order is in any way connected to the magnetic properties of double perovskite La_2-2xSr_1+2xMn₂O₇ (LSMO) manganite oxides. For these compounds, charge and orbital ordering are observed for some doping values near x = 0.50 [1, 2].  

For this purpose, we have use aberration corrected scanning transmission electron microscopy (STEM) combined with electron energy-loss spectroscopy (EELS) measurements and also theoretical simulations. We have compared different compositions within three distinct magnetic regions of the phase diagram: a ferromagnetic metallic sample with x=0.36, an insulating, antiferromagnetic (AF) x=0.56 and an additional AF x=0.50 sample which also exhibits charge ordering.

High angle annular dark-field (HAADF) images, also known as Z-contrast, confirm that our single crystals exhibit high crystal quality. No secondary phases or defects are observed. Figure 1 displays an atomic resolution image obtained with the c-axis perpendicular to the electron beam of a x=0.50 sample. The perovskite (P)-like planes and the rock salt (R)-like planes are clearly observed, highlighted in green and red, respectively, on the image. The P-like planes exhibit a slightly high contrast, suggesting a possible La enrichment. EELS atomic resolution maps (inset) support a high degree of La segregation on those planes, while R-like planes are Sr rich. However, due to dechanneling of the beam, detailed image simulations are essential to accurately quantify the local chemical composition in an atomic column-by-atomic column fashion. For all our samples, we find a significant degree of long-range chemical ordering, which increases in the AF range. However, ordering is not complete and it cannot explain by itself the macroscopic electronic ordering phenomena [3].

 

Acknowledgements: Research at Oak Ridge National Laboratory and at Argonne National Laboratory was sponsored by the US Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. Research at Univ. Complutense was supported by the European Research Council. This work was supported in part by DOE grant No. DE-FG02-09R46554. The authors are thankful to M. Watanabe for the PCA plug in for Digital Micrograph.

References

[1] Q. Li, K.E  Gray, H. Zheng, H. Claus, S. Rosenkranz, S.N. Ancona, R. Osborn, J.F. Mitchell, Y. Chen & J.W. Lynn (2007). “Reentrant orbital order and the true ground state of LaSr₂Mn₂O₇”. Phys. Rev. Lett. 98 167201 (2007).

[2] J.F. Mitchell, D.N. Argyriou, A. Berger, K.E. Gray, R. Osborn & U. Welp. “Spin, Charge, and Lattice States in Layered Magnetoresistive Oxides”. J. Phys. Chem. B  105 (44) 10731-11052 (2001).

[3] “Atomic scale studies of La/Sr ordering in colossal magnetoresistant La_2-2xSr_1+2xMn₂O₇ single crystals”. M. A. Roldan, M. P. Oxley, Q. A. Li, H. Zheng, K. E. Gray, J. F. Mitchell, S. J. Pennycook, M. Varela. Microscopy & Microanalysis 20, 1791-1797 (2014).

[4] M. Bosman, M. Watanabe, D.T.L. Alexander, D.T.L. & V.J. Keast. “Mapping chemical and bonding information using multivariate analysis of electron energy-loss spectrum images”. Ultramicroscopy 106, 1024–1032 (2006).

Manuel ROLDAN (Thuwal, Saudi Arabia), Mark OXLEY, Qiang LI, Kenneth GRAY, John MITCHELL, Stephen PENNYCOOK, Maria VARELA

08:00 - 18:15 #6569 - MS06-849 Full structural and chemical characterization of the uniaxial relaxor SBN-67 (Sr0.67Ba0.33Nb2O6).

MS06-849 Full structural and chemical characterization of the uniaxial relaxor SBN-67 (Sr0.67Ba0.33Nb2O6).

SBN belongs to the tetragonal tungsten bronze (TTB) family of uniaxial ferroelectric relaxor materials, with a single component polarization vector pointing along the tetragonal c axis. The high dielectric constants observed and polarization controlling around the ordering temperature 'T_m' make of relaxors promising systems for applications such as electrocaloric, pyroelectric, piezoelectric/electrostrictive devices and sensors.

The idealized structure of SBN, with the generalized formula A₂B₄C₄Nb₂Nb₈O₃₀, has three types of structural channels along the c axis, formed by two types of crystallographically independent NbO₆ octahedra. Disorder due to presence of voids on A cation sites (occupied by Sr atoms) and random distribution of Ba and Sr on B cation sites are thought to be responsible for the relaxor behavior, via the formation of random electric fields and, subsequently, polar nanodomains oriented in the only easy polarization axis c, without a structural change occurring in the phase transition. In this work we present structural and chemical studies of a SBN single crystal by means of Selected Area Electron Diffraction (SAED), High Angular Annular Dark Field (HAADF), Annular Bright Field (ABF) and Electron Energy Loss Spectroscopy (EELS) along the [001] and [100] zone axes. Experiments were carried out in non-aberration corrected (Phillips CM30, JEOL J2100, JEOL2010F) and probe-corrected (Titan³ G2, ARM cF200) TEMs.

An incommensurate structural modulation in SAED experiments along the [100] direction already points to structural disorder (Fig. 1). Diffuse scattering in the form of streaking patterns in the main reflections can also be related to strain induced by the presence of vacant sites. HAADF imaging along the [100] zone axis directly shows in real space the presence of cationic vacancies in A-type sites. Elongation of the atomic columns in the center of the B pentagonal sites in HAADF images along the [001] zone axis (Fig. 2) is compatible with the presence of Ba and Sr randomly occupying that crystallographic position. EELS spectrum imaging along the [001] zone axis (Fig. 3) directly confirmed the mixing of Sr/Ba in the B-type sites. With these results, the causes for local charge disorder in the structure are mapped in direct space with atomic column resolution. ABF imaging allowed mapping the oxygen sublattice in the structure, achieving the full structural characterization of the material.

Acknowledgement: research leading to these results received financial support from the Spanish Ministry of Economy and Competitivity via the projects Consolider-IMAGINE (CSD2009-20013), MAT2010-16407 and MAT2013-41506.

Lluís LÓPEZ-CONESA (Barcelona, Spain), José Manuel REBLED, Alicia RUIZ-CARIDAD, Guilhem DEZANNEAU, Almudena TORRES-PARDO, Luisa RUIZ-GONZÁLEZ, José Maria GONZÁLEZ-CALBET, Sonia ESTRADÉ, Francesca PEIRÓ

08:00 - 18:15 #6571 - MS06-851 TEM/STEM study of rapidly quenched hard magnetic Nd-Fe-B ribbons.

MS06-851 TEM/STEM study of rapidly quenched hard magnetic Nd-Fe-B ribbons.

The increase of coercivity and simultaneously reducing the amount of heavy rare earth elements of Nd-Fe-B magnets is of great economic and scientific interest. Both, the grain size of the hard magnetic Nd₂Fe₁₄B phase and the presence of a grain boundary (GB) phase and its chemical composition have a crucial influence on the coercivity of sintered Nd-Fe-B magnets [1], [2]. Besides the sinter processing also the production route of rapid solidification, also called melt-spinning, for Nd-Fe-B magnets satisfies the demand of the industry for magnets with high coercivity and energy product [3]. The variation of the process parameters has a significant influence on the microstructure, like grain size and occurring phases. Two isotropic Nd-Fe-B melt-spun ribbons, ms-A and ms-B, with an average grain size of about 19 nm (STDV = 6 nm) (ms-B) and 60 nm (STDV = 22 nm) (ms-A) and a coercive field ranging from µ₀H_c = 0.6 T (ms-A) to 1.0 T (ms-B) were investigated in a nanoanalytical TEM/STEM study carried out on an analytical field emission transmission electron microscope (TEM) (FEI Tecnai F20) at 200 kV, which is equipped with a high angle annular dark field detector (HAADF), a silicon drift energy dispersive X-ray detector (EDX) from EDAX and a Gatan Tridem GIF electron energy loss spectrometer (EELS). Conventional TEM sample preparation (cutting, thinning, ion milling) was conducted, in order to investigate large sample areas. For detailed nanoanalytical investigations Focused Ion Beam (FIB) samples were prepared in an FEI Quanta 200 3D DualBeam-FIB using the lift-out technique. A TEM bright field (BF) image shows the large grains of sample ms-A (Fig.1a). Besides selected area electron diffraction (SAED) analysis (Fig1.b), the occurring phases were identified with Fast Fourier Transformation (FFT) (Fig.1c) of High Resolution (HR) TEM images (Fig.1d). The [100], [110] and [111] lattice fringes of the single crystalline (Pr,Nd)₂(Fe,Co)₁₄B phase (φ) are visible in Fig.1d. Beside the 2-14-1 phase two further Fe dominating phases Fe-1 and Fe-2 were found. Two single crystalline bcc-Fe (α) grains of phase Fe-1, which contains mostly Fe and small amounts of Co and O, are displayed in Fig.1d. This phase shows characteristic dotted morphology, were else the Fe-2 phase has a homogeneous contrast like the Nd-Fe-B phase. Besides Fe also significant amounts of Nb and O were found in this phase. Large area EDX analysis have indicated about 40 % of the grains to be one of these two Fe phases and only 60 % are Nd-Fe-B grains and the ratio of the two Fe phases is approximately 50/50. The smaller grain size of sample ms-B is observed in the TEM-BF image (Fig.2a). The [001] lattice plains of an Nd-Fe-B grain was indexed with an FFT (Fig.2c) of the TEM Dark Field (DF) image (Fig.2d). An EELS line scan (ls) over an interface of two Nd-Fe-B grains shows no change in the chemical composition, implying that a grain boundary phase is not present in this material (Fig.3b,d). On the basis of the information on the microstructure obtained by this TEM study a numerical micromagnetic finite element model was created to simulate the influence of microstructural features like grain size and other occurring phases (Fig.3c). The micromagnetic simulation of the demagnetization curve of randomly oriented grains with direct intergranular coupling shows a decrease of the coercive field with increasing grain size (Fig.3a), which is in good agreement with the measured coercive field of the two samples.

 

[1]           G. A. Zickler, P. Toson, A. Asali, and J. Fidler, Phys. Procedia, vol. 75, pp. 1442–1449, 2015.

[2]           P. Toson, G. A. Zickler, and J. Fidler, Phys. B Condens. Matter, Oct. 2015.

[3]           J. J. Croat, J. F. Herbst, R. W. Lee, and F. E. Pinkerton, J. Appl. Phys., vol. 55, no. 6, p. 2078, 1984.

Acknowledgements: The funding from the European Community´s Seventh Framework Programme (FP7-NMP) under grant agreement no. 309729 (ROMEO) is acknowledged.

Gregor A. ZICKLER (Vienna, Austria), Josef FIDLER, Ahmad ASALI, David BROWN, Johannes BERNARDI

08:00 - 18:15 #6582 - MS06-853 Multiphase material sample preparation using broad-beam Ar ion milling for EBSD analyses.

MS06-853 Multiphase material sample preparation using broad-beam Ar ion milling for EBSD analyses.

When compared to monophase materials, multiphase materials can offer an improvement in properties or introduce new properties. For example, tungsten, carbides, and nitrides do not have many applications as monophase materials; these metals are brittle, which can override their other desirable properties, such as hardness, high temperature stability, and wear. However, when these materials are dispersed in a soft matrix, such as low-carbon steel, nickel, or cobalt, and form a composite, the multiphase materials are then characterized by desirable mechanical properties, such as ductility, strength, and resistance to creep and fatigue [1,2]. Other types of multiphase materials are protective surface coatings, such as tribological and antioxidation applications, and microelectronics devices [3].

Knowledge of the phases present in the material, their distribution, and their fractions is fundamental to materials characterization. A common investigative approach for engineered materials is electron backscatter diffraction (EBSD) combined with energy dispersive spectroscopy (EDS). EBSD provides information about crystallographic orientations or misorientations and allows the study of grain boundaries, deformation, and recrystallization. EDS can define the chemical composition of the materials. When combined, these techniques permit phase identification with high accuracy [4].

Sample preparation of multiphase materials for EBSD and EDS can be challenging, especially when the different phases have very different characteristics. For example, if one phase is hard and brittle and the second phase is soft and ductile, the rate of material removal during mechanical polishing can vary widely. In addition, the hard particles removed from the first phase can act as a grinding medium and tear the soft matrix of the second phase. The result is a surface inappropriate for EBSD analyses. Electrolytic etching is another sample preparation technique that is not ideal for multiphase materials; it often requires the use of toxic or dangerous chemicals, it can cause differential etching of the sample surface, and it can give the sample surface a varied topography, which will produce a shadowing effect in the EBSD/EDS analysis.           

The goal of this work is to illustrate how the use of low energy, broad-beam argon ion milling can improve and facilitate multiphase material sample preparation. Several examples are discussed, including hard particles within a soft matrix and a multiphase protective layer on a metal substrate.

References

1. Fogagnolo, J., Robert, M., & Torralba, J. (2006). Mechanically alloyed AlN particle-reinforced Al-6061 matrix composites: Powder processing, consolidation and mechanical strength and hardness of the as-extruded materials. Materials Science and Engineering: A, 426(1-2), 85-94. doi:10.1016/j.msea.2006.03.074
2. Maetz, J., Douillard, T., Cazottes, S., Verdu, C., & Kléber, X. (2016). M₂₃C₆ carbides and Cr₂N nitrides in aged duplex stainless steel: A SEM, TEM and FIB tomography investigation. Micron, 84, 43-53. doi:10.1016/j.micron.2016.01.007
3. Nuri, K., & Halling, J. (1993). Multi-phase materials and their surface contact behaviour with reference to friction and wear. Wear, 160(2), 213-219. doi:10.1016/0043-1648(93)90423-j
4. Nowakowski, P., Ubhi, H. S., & Mathieu, S. (2015). Investigation of phases and textures of binary V-Si coating deposited on vanadium-based alloy (V-4Cr-4Ti) using electron backscatter diffraction. IOP Conference Series: Materials Science and Engineering, 82(1), 012061. doi:10.1088/issn.1757-899x

Pawel NOWAKOWSKI (Export, USA), James SCHLENKER, Mary RAY, Paul FISCHIONE

08:00 - 18:15 #6584 - MS06-855 Topological insulator Sb2Te3/Bi2Te3 heterostructures: structural properties.

MS06-855 Topological insulator Sb2Te3/Bi2Te3 heterostructures: structural properties.

In a topological insulator (TI) the bulk electronic band structure behaves like an ordinary band insulator. However, at the surface topologically protected surface states occur, which give rise to a spin-locked, dissipation-less electronic transport. Hence, topological insulator materials, such as Sb₂Te₃ or Bi₂Te₃, are of great interest for spintronic devices or quantum computing. In order to achieve the dissipation-less transport, the Fermi level has to be exactly tuned within the Dirac cone like band structure at the surface.  To date intrinsic doping by vacancies or antisite defects renders Sb₂Te₃ and Bi₂Te₃ p- and n-type, respectively, which results in hole or electron transport in the bulk. Recently we solved this problem by growing p-n junctions made of Sb₂Te₃ and Bi₂Te₃ [1]. In this approach the carrier concentration at the surface is reduced by formation of the space charge layer at the buried heterointerface.

Structure-wise the X₂Te₃ (X=Bi, Sb) rhombohedral unit cell consists of three Te-X-Te-X-Te quintuple layers, which are linked by van der Waals forces.  In order to achieve layers of high structural perfection on Si(111) substrates  careful control of the growth parameters of the molecular beam epitaxy is required. In particular, we could demonstrate that the suppression of twin domains, which are the most prominent structural defects, is possible by van der Waals epitaxy [2].

Here we report on advanced scanning transmission electron microscopy and energy dispersive X-ray studies on MBE grown Sb₂Te₃/Bi₂Te₃ heterostructures. Figure 1 displays STEM bright-field and dark-field images, where the quintuple layers within the heterostructures and a highly perfect interface to the Si substrate are atomically resolved. The EDX measurement in Figure 2 reveals, that at the heterointerface a Sb and Bi gradient extends over about 4 nm, effectively introducing a ternary compound as interlayer. Corresponding electrical transport measurements demonstrate the tunability of the intrinsic carrier concentration by variation of the thickness of the individual films [3].

[1]   Eschbach M, Młyńczak E, Kellner J, Kampmeier, J, Lanius, M, Neumann, E., Weyrich C, Gehlmann M, Gospodaric P, Döring S, Demarina N, Luysberg M, Biehlmayer G, Schäpers, T,  Plucinski L, Blügel S, Morgenstern M, Schneider C M, Grützmacher D. Nature Communications. 2015;6(May):8816. doi:10.1038/ncomms9816.

[2]   Kampmeier J, Borisova S, Plucinski L, Luysberg M, Mussler G, Grützmacher D. Crystal Growth and Design. 2015;15(1):390-394. doi:10.1021/cg501471z.

[3]   Lanius, M,  Mussler, G.,  Kampmeier, J,  Weyrich, C.,  Schall, M.  Kölling, S.,  Schüffelgen, P., Neumann, E., Luysberg, M.,  Koenraad, P.  Schaepers, T.,  Grützmacher, D.  accepted for publication in Crystal Growth and Design. doi: 10.1021/acs.cgd.5b01717

Martina LUYSBERG (Jülich, Germany), Martin LANIUS, Jörn KAMPMEIER, Christian WEYRICH, Sebastian KÖLLING, Melissa SCHALL, Peter SCHÜFFELGEN, Elmar NEUMANN, Gregor MUSSLER, Paul M. KOENRAD, Thomas SCHÄPERS, Detlev GRÜTZMACHER

08:00 - 18:15 #6656 - MS06-857 EMCD investigation of the Verwey-transition in magnetite.

MS06-857 EMCD investigation of the Verwey-transition in magnetite.

Magnetite is of interest in physics as well as in chemistry, for example because of its surface chemistry [1]. Furthermore, the investigation of the magnetic properties is worthwhile as the material exhibits a magnetic phase transition (the so-called Verwey-transition) at 125 K [2]. Magnetite shows a drop in its magnetisation when cooled down below the transition temperature, together with a change in electronic and structural properties. This behaviour was subject to a number of investigations [2]. However, the details of the transformation mechanism are still under discussion.

EMCD (energy-loss magnetic chiral dichroism) is a versatile technique to investigate magnetic properties in the TEM on a nanoscale [3]. When performing EMCD on magnetite, the Fe L_2,3-edge, acquired using two different scattering conditions, is compared. The difference signal corresponds to the magnetic properties of the investigated material. Using current TEMs, EMCD can be performed in-situ with a high spatial resolution, thus giving information about local changes of the magnetic moments [4].

We used a FEI Tecnai TF20 TEM, operated at 200 kV, equipped with a Gatan GIF Tridiem, and a Gatan Cryo-Transfer sample holder to investigate magnetite samples close to the transition temperature. Thus, not the whole specimen was transformed. Figure 1(a) shows a bright field image of the investigated sample area. Due to variations from the perfect stoichiometry, regions that already transformed are visible alongside sample regions still in the high-temperature phase. The diffraction patterns shown in Fig. 1(b,c) were acquired at two different regions, marked in Fig. 1(a), to check whether the material has already transformed.

The EMCD spectra that were recorded in the two marked sample regions are depicted in Fig. 2. It can be seen that in the non-transformed region a strong EMCD signal of 13.5% is visible, while in the transformed region no EMCD effect is observed. This indicates changes in the local magnetic moments due to the phase transition. The results are faced to Bloch-wave simulations [5], carried out for the same settings used in the EMCD measurements. Thus, the influence of different channelling conditions due to structural changes induced by the Verwey-transition is compared to the measured EMCD signal. It is shown that the difference in the EMCD effect is mainly caused by local changes of the magnetic moments and not by different channelling conditions due to the structural changes.

EMCD in combination with the versatility of a wide variety of analytical techniques in a TEM is an ideal tool to gain knowledge about the magnetic properties of a material on the nanoscale. The necessary combination of experiments with simulations (as elastic scattering strongly influences the measured signal) provides insight into the mechanisms of magnetic phase transitions.

[1] Weiss and Ranke, Prog. Surf. Sci 70 (2002) 1

[2] Walz, J. Phys. Condens. Matter 15 (2002) R285

[3] Schattschneider et al., Nature 441 (2006) 486

[4] Schattschneider et al., PRB 78 (2008) 104413

[5] Löffler and Schattschneider, Ultram. 110 (2010) 831

[6] The authors thank U. Diebold and G. Parkinson for providing the sample. A. Steiger-Thirsfeld is acknowledged for preparing the FIB-lamellae for TEM investigation.

Walid HETABA (Mülheim an der Ruhr, Germany), Michael STÖGER-POLLACH

08:00 - 18:15 #6659 - MS06-859 Structural evolution of Pt/CNF nanocomposites as a part of fuel cell’s gas-diffusion electrodes.

MS06-859 Structural evolution of Pt/CNF nanocomposites as a part of fuel cell’s gas-diffusion electrodes.

A search of new nanostructural materials (such as gas-diffusion layers, electrodes, catalysts, membranes etc.) for a polybenzimidazole-membrane fuel cell remains a crucial task nowadays. Reducing of noble metals content has become one of the most significant goals in development of fuel cells while a balanced union of electron and ion conductivity along with gas permeability and catalytic activity of electrodes serves as an essential condition of the hydrogen-air fuel cell efficiency [1].

In this study carbon nanocomposites of nanofiber nonwoven mats, produced by electrospinning and decorated with Pt, after work inside membrane electrode assembly as gas-diffusion cathode at 160-180 ^оС were investigated by analytical TEM and STEM methods [2].

Initially a nanofiber surface is evenly covered with a thick layer of Pt nanoparticles of anisotropic elongated shape with a number of sub angstrom atomic steps on their surface (Fig. 1a). This type of defects plays an important part in platinum catalytic activity [2,3].

The investigations of such a nanocomposite after work as a gas-diffusion cathode in a fuel cell revealed significant changes in the structure of the platinum layer. After several hours of work at a standard fuel cell working temperature (160 ^oC) the structure of metal nanoparticles changes slightly (Fig. 1b). Metal nanoparticles preserve their shape elongated in direction and specific distribution on fibers surface same as in initial nanocomposites. However, in this case platinum is covered with a thin amorphous layer. Some of the platinum nanocrystals demonstrate the signs of partial melting and a loss of initial acicular shape as well as the decrease of surface defects.

After several days of work at standard and high fuel cell working temperature (160-180 ^oC) the structure of catalyst  changes considerably (Fig. 1c) and can be observed as melted conglomerates of unspecified shape and size of 40-100 microns (Fig. 2) with a cover of thin (5-10 nm) amorphous layer.

Acknowledgements

This work was performed using the equipment of the Shared Research Center IC RAS and partially supported by RFBR grant # 14-29-04011 ofi-m.

 

1. Y. Wang, K.S. Chen, J. Mishler, S.C. Cho, X.C. Adroher. Appl. Energ., 2011, 88, Pp. 981-1007.

2. I.I. Ponomarev, Iv.I. Ponomarev, I.Yu. Filatov, Yu.N. Filatov, D.Yu. Razorenov, Yu.A. Volkova, O.M. Zhigalina, V.G. Zhigalina, V.V. Grebenev, N.A. Kiselev. Dokl. Phys. Chem. 2013, 448, P. 23.

3. V.G. Zhigalina, О.М. Zhigalina, I.I. Ponomarev, D.N. Khmelenin, D.Yu. Razorenov, Iv.I. Ponomarev, N.А. Kiselev. Nanomaterials and nanostructures — XXI century. 2012, 4, Pp. 36-40.

Victoria ZHIGALINA (Moscow, Russia), Olga ZHIGALINA, Igor PONOMAREV, Ivan PONOMAREV, Dmitriy RAZORENOV, Kirill SKUPOV

08:00 - 18:15 #6675 - MS06-861 The effect of spark plasma sintering on structure and phase stability in half-Heusler thermoelectric alloys.

MS06-861 The effect of spark plasma sintering on structure and phase stability in half-Heusler thermoelectric alloys.

Thermoelectric (TE) materials have the capability to directly convert a temperature difference into an electric voltage across the material and vice versa. Half-Heusler (HH) compounds are a promising group of TE materials. They are ternary intermetallic compounds, with the general formula XYZ – X and Y transition metals (e.g., X=Ti, Zr, Hf and Y=Ni, Co) and Z a metal or metalloid (e.g., Sn, Sb). The electronic structure and charge carrier concentration can be manipulated by atomic substitution on each crystallographic site to enhance the TE properties. However, further increase of efficiency of the alloys is limited by the relatively high thermal conductivity. Nanostructuring is reported to reduce the thermal conductivity as a result of an enhancement of the phonon scattering, thus increasing the efficiency of the TE materials [1].

Spark plasma sintering (SPS) is reported to give rapid densification of powders into bulk specimens and retain nanostructures (grain boundaries) from the starting powders. In the current study, n-type XNiSn alloys with different (Ti, Zr, Hf) compositions on the X-site were prepared by ball-milling of arc-melted and thermally annealed ingots, followed by SPS for 10 minutes, with temperatures between 850 and 1100 ^oC and applied pressures between 65 and 80 MPa. The SPS prepared alloys were investigated with a combination of X-ray diffraction (XRD) and scanning and transmission electron microscopy (SEM and TEM) techniques, supported by density functional theory (DFT) thermodynamic calculations.

In all SPS samples, graphite is present in the surface regions. At the interface between the graphite and the XNiSn alloys, carbides are present as illustrated in figure 1 for the HfNiSn alloy – sintered at 1100 ^oC and 80 MPa. In agreement with previous reports on HH studies, single phase HH does not form in any of the alloy systems [2]. Secondary phases are distributed along compositional HH boundaries as illustrated in figure 2 from the TiNiSn alloy, sintered at 900 ^oC and 80 MPa. Phase separation of HH phases, with different composition X_i and HH lattice parameter, is generally evident by splitting of the HH reflections in the XRD diffractograms. In the case of the TiNiSn alloy, such splitting of the HH reflection is not observed; however, we find compositional variations of the HH phase from the nominal composition – consistent with the variations in the shades of grey seen in the HH regions in figure 2. In addition to the HH and secondary phases seen in the figure, grains of full-Heusler TiNi₂Sn exist in the alloy.

REFERENCES

[1]  S. J. Poon, D. Wu, S. Zhu, W. Xie, T. M. Tritt, P. Thomas and R. Venkatasubramanian, Journal of Materials Research 26 (2011), 2795;

[2]  E. Rausch, S. Ouardi, U. Burkhardt, C. Felser, J. M. Stahlhofen and B. Balke, Condensed Matter – Materials Science (2015), e-print arXiv: 1502.03336v1 [cond-mat.mtrl-sci]

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial support by the NANO2021 programme and THELMA research project (project no. 228854) of the Research Council of Norway (NFR).

Raluca TOFAN (OSLO, Norway), Cristina ECHEVARRIA-BONET, Kristian BERLAND, Matthias SCHRADE, Magnus SØRBY, Bjørn Christian HAUBACK, Clas PERSSON, Øystein PRYTZ, Vidar HANSEN, Anette Eleonora GUNNÆS

08:00 - 18:15 #6680 - MS06-863 Chemical reactivity between sol-gel deposited Pb(Zr, Ti)O3 layers and their GaAs substrates.

MS06-863 Chemical reactivity between sol-gel deposited Pb(Zr, Ti)O3 layers and their GaAs substrates.

The combination on the same wafer of materials having different physical properties is a key challenge. In particular, functional oxides of the perovskite family are very attractive for applications in the micro-optoelectronic field since they present a variety of physical properties (ferroelectricity, ferromagnetism, superconductivity, high Pockels coefficients, …). They are classically grown on SrTiO₃ (STO) substrates, but molecular beam epitaxy (MBE) allows their epitaxy on Si and GaAs platforms, providing that specific interface engineering strategies are used. However, even in this case, chemical reactions between the growing oxide and the substrate strongly impact the epitaxy process and impose using specific growth procedures that are detrimental to the oxide crystal quality. Amongst the oxides that can be integrated on STO templates, Pb(Zr,Ti)O₃ (PZT) is of particular interest because of its high remanent polarization (P_r), its low coercive field (E_c) and its outstanding piezoelectric properties

In this contribution, we report the study of sol-gel prepared PZT thin films on STO/GaAs templates grown by molecular beam epitaxy (MBE). The spin-coated layers were calcined at 350°C under air for 5 minutes to dty the sol film. The crystallization of the PZT have been studied in function of post-annealings at three different temperatures ranging (405°C, 420°C and 510 °C). High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDXS) have been used to study the structure and the chemical composition of the layers. High resolution HAADF images coupling to geometrical phase analysis (GPA) show that PZT is locally strained on the STO/GaAs during the first steps of crystallization. Complete crystallization of the PZT laeyrs cannot be achieved due to a significant oxidation of the GaAs substrate which enhances As diffusion through the structure and further formation of parasitic PbAs and SrAs compounds.               

ACKNOWLEDGEMENTS

We acknowledge Agence Nationale de la Recherche (ANR), program of investment for the future, TEMPOS project (n°ANR-10-EQPX-50) for having funded the acquisition of the NANOTEM platform (Dualbeam FIB-FEG FEI SCIOS system and TEM-STEM FEI Titan Themis equipped with the Super-X Chemistem EDX detectors) used in this work

Benjamin MEUNIER, Ludovic LARGEAU (Marcoussis), Philippe REGRENY, José PENUEALS, Romain BACHELET, Bertrand VILQUIN, Baba WAGUE, Guillaume SAINT-GIRONS

08:00 - 18:15 #6692 - MS06-865 Mn 4+ holes localization around divalent cations in low-hole doped manganites.

MS06-865 Mn 4+ holes localization around divalent cations in low-hole doped manganites.

Magnetotransport properties of low Ca doping La_1-xCa_xMnO₃ perovskite related manganites have been described on the basis of the existence of isolated metallic (M)-double exchange (DE) ferromagnetic (FM) nanoclusters embedded in a long range insulating (I) matrix. Taking into account the DE mechanism, long range FM order and M conduction are expected across the La_1-xCa_xMnO₃ series. However, the real behaviour differs from this ideal description as an I state is observed for x£0.2. This can be understood by assuming that the electronic itinerancy is not spread to the whole sample but restricted to FM-M clusters embedded in the matrix. In spite of much effort devoted to their study, the behavior at atomic level is far from being satisfactorily explained. Here we report direct experimental evidence in La_0.9Ca_0.1MnO₃, by means of STEM-EELS characterization in a JEOL ARM 200 cFEG aberration corrected microscope, that holes and divalent cations tend to be closely correlated in the sample. Figure 1 shows a characteristic HAADF image and corresponding EELS compositional maps showing a heterogeneous distribution of La and Ca. The study of the Mn oxidation state suggests a shift towards Mn³⁺ or Mn⁴⁺ when the environment is La and Ca richer, respectively (figure 2).^[1] This Ca-hole attraction results in the formation of small clusters confined to one or two unit cells and would provide an answer to the long-standing problem of the origin of the electric and magnetic behavior in the low calcium region of the La_1-xCa_xMnO₃ system.

Luisa RUIZ-GONZALEZ (MADRID, Spain), Raquel CORTÉS-GIL, Daniel GONZALEZ-MERCHANTE, Jose Maria ALONSO, José M. GONZALEZ-CALBET

08:00 - 18:15 #6694 - MS06-867 Revealing Lattice Oxygen Exchange During CO Oxidation over 6H-BaFeO3-δ Catalyst Nanoparticles by Diffraction and Atomic Resolution Techniques.

MS06-867 Revealing Lattice Oxygen Exchange During CO Oxidation over 6H-BaFeO3-δ Catalyst Nanoparticles by Diffraction and Atomic Resolution Techniques.

Eliminating or reducing the amount of noble metals and rare earths in catalysts is a primary issue to reach the goal of sustainable development. The substitution of noble metals by 3d metals is receiving increasing attention. Related to this strategy, ABO₃ perovskite oxides (A=lanthanide and B=Fe, Co, Mn) are being essayed as catalysts in many reactions. Substitution of La by an alkaline earth has been reported to modify the catalytic activity of such perovskites.^[1] To balance the charge deficiency produced by the incorporation of divalent ions into the A sublattice, either the B element increases its oxidation state or oxygen vacancies are created or, even more, both processes occur simultaneously. In general terms, a positive effect on the catalytic activity is observed on methane, propene and hydrogen oxidation reactions within the whole temperature range, after the addition of barium in Mn, Fe and Co perovskites. On the basis of these ideas, we have considered of interest exploring the catalytic behavior of a lanthanide-free, BaFeO_3-δ perovskite in CO oxidation, putting emphasis on the influence of redox pre-treatments on catalytic activity and on the possible involvement of lattice oxygen in the process. This particular process was firstly of interest in automotive pollution control devices but more recently for cutting-edge technologies related to production of hydrogen for fuel cells. Though it is admitted that the catalytic behavior of perovskites is intimately related to the nature of their defects, studies focused on the detailed analysis of the nature of such defects at atomic scale are systematically lacking. Therefore, this contribution aims at illustrating the large potential of diffraction data combined with AC-STEM to establish structure-function correlations. As an illustration, we have explored the involvement of lattice oxygen in the catalytic activity of BaFeO_3-δ perovskite for the CO oxidation.

BaFeO_3-δ (0.2<δ<0.4) depicts a 6H perovskite hexagonal structural type, with Fe both in (III) and (IV) oxidation states and oxygen nonstoichiometry accommodated by random distribution of anionic vacancies.^[2] Analysis of the redox properties of 6H-BaFeO_2.78 reveals the involvement of two types of lattice oxygen atoms in the CO oxidation over BaFeO_3-δ. On its hand, an exhaustive study combining different diffraction techniques (in-situ high temperature SAED, X-ray diffraction, powder neutron diffraction) and aberration-corrected STEM techniques, both in imaging and spectroscopic modes, have provided us with an atomically resolved picture of the accommodation of oxygen-non-stoichiometry. Thus, mapping of Ba, Fe and O atomic columns, Figure 1, have allowed us identifying the distribution of all the elements in the sample in the ...cchcch... sequence expected for the 6H-polytype. No systematic differences have been observed between the hexagonal and cubic layers in BaFeO_3-δ this meaning, in agreement to NPD and ABF results, that there is not a preferential distribution of the oxygen vacancies along the different layers. From the whole set of results obtained it has been possible to propose the role of the oxygen deficiency responsible for the low temperature CO oxidation activity of BaFeO_3-δ.

[1] Yamazoe, N.; Teraoka, Y.; Catal. Today 1990, 8, 175-199.

[2] Parras, M.; Vallet-Regi, M.; Gonzalez-Calbet, J. M.; Grenier, J. C. J. Solid State Chem. 1989, 83, 121-131.

Isabel GÓMEZ-RECIO (MADRID, Spain), Achraf EL HADRI, Raquel CORTES-GIL, Aurea VARELA, Marina PARRAS, Angel GUTIERREZ, F. Javier GARCÍA-GARCÍA, Eloy DEL RIO, Jose Juan CALVINO, Juan Carlos HERNADEZ, Juan Jose DELGADO, Jose Antonio PÉREZ-OMIL, Jose Maria GONZALEZ-CALBET

08:00 - 18:15 #6700 - MS06-869 Accommodation of oxygen deficiency in La0.5Ca2.5Mn2O7-d and LaSr2Mn2O7-d Ruddlesden-Popper Manganites.

MS06-869 Accommodation of oxygen deficiency in La0.5Ca2.5Mn2O7-d and LaSr2Mn2O7-d Ruddlesden-Popper Manganites.

Ruddlesden-Popper (RP) (AO)(ABO₃)_n mixed oxides have recently attracted considerable insight¹ as a consequence of their potential for low-dimensional physic arising from their structural configuration which results from the ordered intergrowth between n ABO₃ perovskite (P) blocks and one AO rock-salt (RS) layer. This ordered arrangement of different chemical and structural unities induces intriguing behaviours that can be modified by controlling the number of intergrowing unities. For instance, colossal magnetoresistance and ferromagnetic ordering have been reported by Moritomo² et al in n=2 La_2-2xSr_1+2xMn₂O₇ system. However, it should be noticed the difficulty to stabilize pure high ordered terms, which are frequently obtained as disordered intergrowths between the basic unities. We have prepared and characterized ordered n=2 terms for the La_0.5Ca_2.5Mn₂O₇ and LaSr₂Mn₂O₇ compositions. For instance, figure 1 shows characteristic HAADF and ABF images atomically resolved for the strontium sample. In addition, compositional variations at the anionic sublattice are scarce compared to other manganese related perovskite systems in which different superlattices have been described as a consequence of the ordering on non-occupied oxygen positions. In this work, we have also focused on the stabilization and characterization of new oxygen deficient La_0.5Ca_2.5Mn₂O_7-d and  LaSr₂Mn₂O_7-d  phases.³ The ensemble of XRD, ND, SAED, HREM and aberration corrected HAADF, ABF images as well as EELS techniques allows proposing a topotactic reduction process in both systems through different oxygen sites, apical and equatorial, for the strontium and calcium samples, respectively.

[1] Mulder, A. T.; Benedek, N. A.; Rondinelli, J. M.; Fennie, C. J. Adv. Funct. Mater. 2013, 23, 4810.

[2] Moritomo, Y.; Asamitsu, A.; Kuwahara, H.; Tokura, Y. Nature 1996, 380, 141.

[3] Ruíz-González, M. L.; González Merchante D.; Cortés-Gil, R.; Alonso, J. M.; Martínez, J. L.; Hernando, A.; González-Calbet, J. M.; Chem. Mater. 2015, 27, 1397.

Raquel CORTÉS-GIL (Madrid, Spain), Daniel GONZALEZ-MERCHANTE, Jose M ALONSO, Luisa RUIZ-GONZALEZ, Jose M GONZALEZ-CALBET

08:00 - 18:15 #6711 - MS06-871 Real structure of highly oriented Ge-Sb-Te thin films investigated by Cs-corrected STEM.

MS06-871 Real structure of highly oriented Ge-Sb-Te thin films investigated by Cs-corrected STEM.

The phase-change effect in a wide class of tellurium-based chalcogenide compounds allows for the fast and reversible transition between crystalline and amorphous states that possess a large contrast in material properties. Application examples include optical storage media, phase-change RAM. Ge-Sb-Te (GST) compounds are often used as a model system for this behavior, and have thus received wide-spread attention in literature [1-3]. Recent interest in the material contrast switching behavior of GST has extended to the reversible switching of highly oriented layered superlattices [3]. The exact nature of the crystalline transition is still under debate, and detailed insight into the high-temperature trigonal phases in an epitaxial environment is required in order to accurately interpret experimental data. While such epitaxial thin films of GST are typically prepared by MOCVD or MBE, the here presented experimental results were obtained by pulsed laser deposition (PLD) from compound targets onto various substrates at elevated temperatures. The aim of the work presented is thus to investigate the microstructure of of GST thin films deposited by PLD, with particular focus on the interface formation, as well as to characterize the stacking sequences and defect structures in the trigonal phases produced [4].

GST thin films were deposited from a stoichiometric Ge₂Sb₂Te₅ compound target onto various silicon substrates in a range of temperatures from 110 to 280 °C. The thin films deposited on amorphous surface layer and chemically cleaned substrates possessed closed surfaces and low roughness. While electron and x-ray diffraction data shows that all films are in the trigonal phase, some were found to be polycrystalline, while others possess are clear uniform epitaxial relation towards the substrate. Furthermore, a comparison of average chemical compositions by STEM-EDX revealed that the relative concentration of Ge rapidly declines at deposition temperatures above 200 °C.

Two atomic resolution images of polycrystalline columnar grain growth of GST by PLD are shown in Fig. 1. As can be seen in Fig. 1(a), when deposited onto a flat amorphous interface layer, the crystalline grains above can be misoriented towards the substrate, and the onset of systematic layering of the characteristic van der Waals (vdW) layers is delayed. Fig. 1(b) shows the disordered stacking in the bulk of a crystallite deposited onto cleaned Si(111) at 110 °C, with 7, 9 and 11-layered building blocks and Z-contrast correlating with Ge₁Sb₂Te₄, Ge₂Sb₂Te₅ and Ge₃Sb₂Te₆, respectively. Fig. 2 shows the interface of GST on superficially cleaned Si(111) deposited at 110 °C (Fig. 2(a)) and 280 °C (Fig. 2(b)). All epitaxial thin films possess a  surface passivation Te/Sb layer, as well as a vdW gap immediately above [4,5]. We thus find a strong correlation between grain morphology, surface passivation and substrate treatment, as well as characteristic stacking disorder and chemical reordering of the Ge-rich layers.

We thank Mrs. A. Mill for her assistance in the FIB preparation. The financial support of the European Union and the Free State of Saxony (LenA project; Project No. 100074065) is gratefully acknowledged.

[1] U. Ross, A. Lotnyk et al., Appl. Phys. Lett. 104, 121904 (2014).

[2] A. Lotnyk, S. Bernütz et al., Acta Mater. 105, 1 (2016).

[3] J. Momand, R. Wang et al., Nanoscale 7, 45 (2015).

[4] U. Ross, A. Lotnyk et al., Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.03.159

[5] A. Koma, Thin Solid Films 216, 72 (1992).

Ulrich ROSS, Andriy LOTNYK (Leipzig, Germany), Erik THELANDER, Bernd RAUSCHENBACH

08:00 - 18:15 #6721 - MS06-875 Scanning transmission electron microscopic studies of calcium silicate hydrates.

MS06-875 Scanning transmission electron microscopic studies of calcium silicate hydrates.

Cement-based materials play a vital role in our modern day concrete environment. During service life, they are often in contact with different aqueous solutions, which lead to structural and chemical changes and in turn, a possible deterioration of this material. Transport processes and intrinsic material properties are key components of such reactions. Evaluations on transport mechanisms, as well as shrinking and creep processes, require a detailed picture of the pore system. To this day, it has not been fully understood yet even though the nanometer small gel pores of the calcium silicate hydrate (CSH) phase have a significant impact on material performance. This calls for additional characterizations on a much smaller scale. However, the gel pore structure of the CSH is complex, heterogeneous and inaccessible making evaluations problematic. For this purpose, an analytical approach on multiple scales was applied. To determine the potential impact of gel pores on material properties and thus, performance, a structural characterization of a model cement paste was performed using scanning transmission electron microscopy (STEM) along with gas adsorption measurements (multi-point BET) in addition to standard mineralogical techniques. The model cement paste consisted of synthesized and hydrated tricalcium silicate. For this purpose, the focused ion beam (FIB) preparation technique was used, which is a fairly novel method concerning cementitious building materials. The challenge was the preparation of the FIB-lamella due to the water content, the unstable nature of the sample’s constituents and the high porosity of 20-30 %. Manual adjustments had to be performed throughout the entire milling process in order to prevent the disintegration of the lamella. Furthermore, it was necessary to build multiple robust frames by depositing high amounts of platinum. The lamella was analyzed with the FEI Titan 80-300 TEM in low-dose (80 keV) mode. STEM images reveal the complex structure of the CSH. It consists of 3-5 nm thin CSH foils oriented to one another in a honeycomb structure. The pore size spans from only a few nanometers up to 50 nm with a maximum at 15 nm. These gel pores touch and intersect each other producing a sponge-like conduit network. A similar pore width distribution was also observed with BET. Multiple selected area diffraction (SAED) patterns reveal the volatile nature of this material. The first pattern showed a broad diffuse ring between 2.5 and 3 Å, which can be attributed to CaO. In the following diffraction patterns, this ring becomes more pronounced with many small individual reflexes (Si-Si 1.6 Å) and further rings appear. These observations point towards a crystallization induced by the incident beam. This study shows that such advanced techniques can also be applied to building materials. The information gained is not only valuable to basic research of cementitious materials but also to applied problems related to deterioration processes. Further research and practice will be devoted to improve the FIB handling and technique for cement-based materials and to the analysis of other cement-based compounds, in order to investigate the relationship between nanoscale features and deterioration reactions.

Olivia WENZEL (Eggenstein-Leopoldshafen, Germany), Matthias SCHWOTZER, Torsten SCHERER, Venkata S. K. CHAKRAVADHANULA, Andreas GERDES

08:00 - 18:15 #6726 - MS06-877 Atomic resolution studies of La0.7Sr0.3MnO3/BaTiO3 multiferroic tunnel junctions.

MS06-877 Atomic resolution studies of La0.7Sr0.3MnO3/BaTiO3 multiferroic tunnel junctions.

In complex oxide heterostructures, the interplay between magnetic, electric and transport properties often results in novel functionalities [1-4]. The introduction of controlled densities of oxygen vacancies may allow further tuning of the interface magnetic structure, providing a new path towards enhanced device functionalities. In this work, we have fabricated multiferroic tunnel junctions combining a La_0.7Sr_0.3MnO₃ (LSMO) ferromagnetic manganite electrodes with BaTiO₃ (BTO) ferroelectric tunnel barriers. High quality heterostructures have been produced by high oxygen pressure RF sputtering system. In particular, we have grown samples where an ultrathin layer of a La-Sr cuprate with a nominal composition of oxygen deficient La_0.84Sr_0.16CuO_3-x (LSCO) is inserted in between the BTO and the top LSMO electrode. The transport properties of such asymmetric LSMO/BTO/LSCO/LSMO junctions should be highly tunable via the application of external fields, since the physical properties of cuprates are highly sensitive to doping.

Aberration-corrected scanning transmission electron microscopy (STEM) combined with electron energy-loss spectroscopy (EELS) has been used to study the structure, chemistry and electronic properties of our systems, with atomic resolution in real space. These techniques constitute powerful tools to inspect cuprate/manganite interfaces, where charge transfer may take place [5].

Low magnification images such as the one in figure 1(a) show that the resulting magnetic tunnel junctions exhibit flat layers over long lateral distances. Atomic resolution images in figure 1(b) and 1(c) exhibit a high crystal quality and coherent growth. Figure 2(a) displays atomic resolution compositional maps obtained across the stacking, for the different elements La, Ti, Mn, Ba, Cu and O. Figure 2(b) exhibits the O K edge signal across the heterostructure, normalized and integrated laterally. A clear dip is observed within the cuprate layers, suggesting that the LSCO ultrathin film stores a large density of oxygen vacancies in the structure. Such thin cuprate layers are insulating, ensuring an electronic screening asymmetry. This behavior could yield values in excess of 10⁴ % upon polarization switching, since ferroelectric switching would involve the motion of those oxygen vacancies. Vacancy accumulation would also affect the electronic structure of the interfaces. In fact, values of the tunneling magnetoresistance (TMR) of 125% have been obtained along with a change in the sign of the TMR when the BTO polarization is reversed. This finding (which is not observed in symmetric LSMO/BTO/LSMO structures) suggests that polarization switching has an effect on the spin polarization of the interface, related to the presence of the LSCO layer and the O vacancies. In summary, our results suggest that tailoring the spin polarization at interfaces by growing asymmetric heterostructures with O vacancy–rich thin cuprate layers may provide new ways to design the spin filtering devices of the future.

Acknowledgements

Research at ORNL supported by U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Materials Sciences and Engineering Division. Research at UCM supported by Fundacion BBVA. We acknowledge financial support by Spanish MICINN through grants MAT2011-27470-C02, MAT2015-66888-C3-3-R and ConsoliderIngenio 2010 - CSD2009-00013 (Imagine), by CAM through grant S2009/MAT-1756 (Phama).

References 

[1] A. Ohtomo and H. Y. Hwang, Nature 427, 423-426 (2004).

[2] V. Garcia, M. Bibes et al., Science327,1106–1110 (2010).

[3] E. Y Tsymbal, Condensed Matter15, R109-R142 (2003).

[4] J. D. Burton  and E. Y. Tsymbal, Phys. Rev. B80, 174406 (2009).

[5] J. Salafranca et al., Phys. Rev.Lett. 112, 196802 (2014).

Mariona CABERO (Madrid, Spain), Ana PEREZ-MUÑOZ, David HERNÁNDEZ, Zohuair SEFRIOUI, Radu ABRUDAN, Sergio VALENCIA, Steve John PENNYCOOK, Carlos LEÓN, Maria VARELA, Jacobo SANTAMARÍA

08:00 - 18:15 #6781 - MS06-879 Microscopy study of the front and back sides of platinum catalyst gauzes used in ammonia oxidation.

MS06-879 Microscopy study of the front and back sides of platinum catalyst gauzes used in ammonia oxidation.

     Ammonia oxidation with air on platinum catalyst gauzes is widely used in chemical industry for synthesis of nitric acid. It is well known that during this process the gauzes undergo deep structural rearrangement of surface layers (catalytic etching) leading to the platinum loss and catalytic activity decrease. To reveal the mechanism of the catalytic etching of platinum catalyst gauzes during the NH₃ oxidation, we studied in detail the surface microstructure of the front and back sides of platinum catalyst gauzes used in ammonia oxidation. The platinum catalyst gauzes used in the study were made from a polycrystalline wire with d ≈ 82 μm with the chemical composition (in wt.%) 81% Pt, 15% Pd, 3.5% Rh and 0.5% Ru. A laboratory flow reactor made of quartz tube with the inner diameter of 11.2 mm was used at the feed (ca. 10% NH₃ in air) flow rate 880-890 l/h, the gauze temperature 860±5 °C and total pressure ca. 3.6 bar. A pack of four gauzes was loaded into the reactor to maintain standard conditions of the catalytic process. The surface microstructure was studied using a scanning electron microscope (SEM) JSM-6460 LV (Jeol).

     Substantially different surface layer microstructure of the front and back sides of the polycrystalline wire in the first gauze relative to the gas flow after the treatment in the reactor at T ≈ 860 °C for 50 h in the reaction medium (ca.10% NH₃ in air) was observed after the SEM study. SEM images of the wire surface for the front and back sides of the gauze are shown in Figures 1 and 2, respectively. Images b and c in Figures 1 and 2 were obtained from the central part of the wires shown in Fig. 1a and Fig. 2a, respectively. The SEM images demonstrate that the front side of the wire was etched much more significantly than the back side due to the strong effect of the gas feed on this side of the wire. The wire surface on the front side of the gauze was covered by a continuous corrosion layer consisting of crystalline agglomerates with the sizes 5–15 μm separated by deep voids with the width 1–10 μm (Fig. 1a). The agglomerates had different shapes, crystalline faceting and contained through pores with the diameter 1–5 μm (Fig. 1b). The surface of these agglomerates consisted of crystalline facets without large defects (Fig. 1c). On the surface of the wire from the back side of the gauze weak etching was observed only at the wire interweaving places whereas the major part of the wire surface looked relatively smooth (Fig. 2a). Granular structure with 1–13 μm grains (Fig. 2b) separated by grain boundaries containing 200–400 nm pores was observed in the central part of the wire (Fig. 2a). Crystalline planes with the height ~ 100 nm and many dark spots with the diameter 50–150 nm were observed on the surface of the grains (Fig. 2c). Some of them had pyramidal shape resembling the shape of etching pits at the places where screw dislocation exit to the surface. The concentration of these pits is 4.0 x 10⁸ cm^-2, which is close to the dislocation density in platinum.

     The obtained data indicate that the size of agglomerates on the front side of the gauze (5–15 μm) is close to that of grains observed on the back side (1–13 μm). This result seems to suggest that the etching develops in the course of gradual growth and transformation of the grains into crystalline agglomerates during the growth and merging of etching pits at the grain boundaries. Through pores with the size of 1–5 μm inside the agglomerates may be formed during merging of growing etching pits on the surface and in the bulk of the grains. The emergence and growth of the pits can be related to the reaction of ammonia molecules with oxygen atoms absorbed at the grain boundaries, dislocations and other surface defects. The reaction of gaseous NH₃ molecules with absorbed oxygen atoms O_abs with the formation of gaseous NO results in local overheating of the surface initiating the release of metal atoms to the surface. Intense release of metal atoms from pits at the grain boundaries forms extended voids between the grains. Metal atoms released from the defects quickly migrate over the grain surface and are gradually incorporated at the energetically most favorable sites. As a result, the grains are gradually reconstructed into faceted crystalline agglomerates with through pores formed due to the growth and merging of pits. When these processes go on for a long time, a rough corrosion layer including crystalline agglomerates with through pores separated by deep extended void is formed (Fig. 1).

Acknowledgement

This work was supported by RussianAcademy of Sciences and Federal Agency of Scientific Organizations (project 44.1.17).

Aleksei SALANOV (Novosibirsk, Russia), Evgenii SUPRUN, Elena SUTORMINA, Lyubov ISUPOVA, Valentin PARMON

08:00 - 18:15 #6817 - MS06-881 TiO2 nanotubes array decorated by Ag and modified in reduction atmosphere for photo-catalytic application using visible light.

MS06-881 TiO2 nanotubes array decorated by Ag and modified in reduction atmosphere for photo-catalytic application using visible light.

TiO₂ nanostructures with promising physical and chemical properties, such as a high specific surface area, a high photo-activity and environmental stability, but also the low costs of synthesis inspire numerous studies over the past three decades. Titania nanostructures can be used for photo-catalysis, in solar cells (DSSC) and sensors, still it can absorb light and behave as photo-catalytically active only under irradiation with UV light since TiO₂ has a relatively large band-gap (3.2 eV for anatase and 3 eV for rutile phase). So, one of the key parameters necessary to develop enhanced photo-catalytic properties in the visible and near-IR regions is to optimize the band-gap of TiO₂ materials towards the bandgap of monocrystal silicon. In our study we use Ag nanoparticles and hydrogenation to manipulate with the bandgap, photo-absorption of TiO₂ nanotubes with the aim to enhance photo-catalytic properties.

TiO₂ nanotubes array (TiNTA) was synthesized by anodization of titanium foils as broadly used method for production of nanotubes arrays with different morphology and dimensions of nanotubes. In case of decorated nanotubes, the silver particles were obtained by the photo-reduction of AgNO₃ under UV light. Pure and silver-decorated nanotubes were additionally heat treated in hydrogen. For the structural characterization we used conventional and analytical transmission electron microscopy (TEM) including high angle annular dark field imaging (HAADF), scanning electron microscopy (SEM) techniques, X-ray diffraction (XRD) and Raman spectroscopy. Using UV–ViS-NIR spectroscopy we studied bandgaps and the photo-catalytic activity (on a model organic compound - wastewater) for all the samples before and after hydrogenation.

We found that decoration of TiNTA with Ag broadened the light absorption also in visible range due to the surface plasmon resonance of Ag nanoparticles. Ag nanoparticles with different shapes and diameters are attached on TiO₂ array surface (Fig. 1) as well as inside the nanotubes (Fig. 2). Therefore, the frequency of their plasmonic oscillations covers a wide range, permitting absorbance over a visible and UV part of solar spectrum. Moreover, the hydrogenation at high temperature was shown as essential to considerably increase the absorption of light in the visible and near-IR regions and enhance photo-catalytic activity. This is a consequence of the synergetic effects between the silver and the defects at the surface of TiO₂ nanoparticles (the most probably oxygen vacancies) that increase the efficiency of the formation of electron–hole pairs and the charge transfer to the surface of the nanoparticles.

Acknowledgement

This work has been supported in part by Croatian Science Foundation under the project (IP-2014-09-9419), Croatia (MZOŠ)-Germany(DAAD) bilateral project and in part by European social fond ESF, Human resources development.

Andreja GAJOVIC (Zagreb, Croatia), Milivoj PLODINEC, Ivana GRČIĆ, Marc WILLINGER

08:00 - 18:15 #6822 - MS06-883 Structure and non-stoichiometry in Ca3-xFe15+xO27-y Hexaferrite.

MS06-883 Structure and non-stoichiometry in Ca3-xFe15+xO27-y Hexaferrite.

Recent studies led in the system Ca-Fe-O, and especially by considering the formation of intergrowths (CaFe₂O₄)(FeO)n [1], put forward a strong dependence in temperature so much point of structural view as electronics of these complex oxides with mixed valence Fe²⁺/Fe³⁺. In particular, it was demonstrated that CaFe₅O₇ ferrite_, n = 3 member of the series, exhibits a structural monoclinic/orthorhombic reversible transition in 360K associated with a transition of charge order [2].

New crystal chemistry studies by playing on possible non stoichiometry, so anionic as cationic, around the composition CaFe₅O₇, revealed the existence of a new structure which the stacking mode is related to the large family of hexaferrite. The structural resolution of this compound was performed by combining electronic diffraction and X-ray diffraction data on powder. In particular, a tomographic approach in precession electron mode was carried out and will be presented. This structure which is also described as a mechanism of intergrowths will be detailed and illustrated by the high-resolution imaging (HREM, HAADF). The electronic properties will be also exposed. The latter put forward two magnetic transitions over the room temperature: the first one around 350K, sensitive to the actual stoichiometry of the sample, and the second one in 460K [3].

[1]- B. Malaman, H. Alebouyeh, F. Jeannot, A. Courtois, R. Gerardin, O. Evard. Materials Research Bulletin. 1981, Vol 16, 1139-1148.

[2]- C. Delacotte, F. Hüe, Y. Bréard, S. Hébert, O. Pérez, V. Caignaert, J.M. Greneche, D.  Pelloquin. Inorganic Chemistry. 2014, Vol 53, 10171-10177.

[3]- L. Monnier, C. Moussa, C. Delacotte, P. Boullay, A. Maignan and  D. Pelloquin (in preparation)

[4] – L. Palatinus, V. Petricek, CA. Correa. Acta Crystallographica A-Foundation and Advances. 2015, Vol 71, 235-244.

Laurine MONNIER (Caen), Phillipe BOULLAY, Denis PELLOQUIN

08:00 - 18:15 #6870 - MS06-885 Epitaxial BaTiO3 on Si and SiGe for low power devices: nanoscale characterization of the film and its interface with the semiconductor by HAADF and EELS in STEM.

MS06-885 Epitaxial BaTiO3 on Si and SiGe for low power devices: nanoscale characterization of the film and its interface with the semiconductor by HAADF and EELS in STEM.

Introduction. In the late 1990s, the success of SrTiO₃ epitaxial growth on Si by molecular beam epitaxy (MBE) opened a path for integrating complex oxides on Si-based platforms. In particular, ferroelectric perovskite oxides offer promising perspectives to improve or add functionalities on-chip like low power logic devices or integrated photonics [1]. BaTiO₃, the prototypical ferroelectric perovskite oxide, is a good candidate for these applications [2, 3]. However, regarding practical devices, integrating a perovskite oxide epitaxially on Si by MBE is still in its infancy. One key point is the control of the crystalline orientation, which determines the polarization orientation within the thin film. Playing with experimental parameters of growth and the composition of the Si _xGe_1-x semiconductor substrate are means to manipulate the competition between compressive stress from epitaxy and tensile stress from thermal expansion. In order to support MBE growth strategies, aberration-corrected scanning transmission electron microscopy has been investigated to determine both the strain and the chemical state of epitaxial BaTiO₃ thin films.

 

Experiment. BaTiO₃ was epitaxially grown on Si _xGe_1-x (x=1 and x = 0.20) substrates either using an SrTiO₃ buffer layer to reduce both thermal and lattice mismatches between BaTiO₃ and Si or without any buffer on Si_0;2Ge_0.8. The complex oxides were grown by MBE using Sr, Ba and Ti effusion cells. Details on the growth are given in refs [1,4]. BaTiO₃ was grown directly on strained Si_0.8Ge_0.2/Si substrates using a barium passivation. STEM-HAADF images were collected on a FEI Titan Low-Base 60-300 probe corrected microscope and the data treated using the geometric phase analysis (GPA). STEM-EELS data were also acquired with a special attention to Ba, Ti and O elements.

 

Main results. In a first part, we will first describe the crystalline structure and cationic composition studied at the nanoscale in BaTiO₃/SrTiO₃/Si heterostructures. The effect of oxygen pressure will be discussed. We show that the lattice parameter profile evolution within the thickness of the BaTiO₃ films is clearly associated with modifications of the cation stoichiometry within the thickness and that there is a clear impact of the oxygen pressure on both lattice parameter and composition profiles. In a second part, we will discuss the particular epitaxial state of BaTiO₃ grown directly on Ba-passivated strained Si_0.8Ge_0.2.

 

References.

[1] L. Mazet et al., A review of molecular beam epitaxy of ferroelectric BaTiO₃ films on Si, Ge and GaAs substrates and their applications, Science and Technology of Advanced Materials. 16, 036005 (2015)

[2] S. Salahuddin, S. Datta, Use of Negative Capacitance to Provide Voltage Amplification for Low Power Nanoscale Devices, Nano Letters. 8, 405–410 (2008)

[3] S. Abel et al., A strong electro-optically active lead-free ferroelectric integrated on silicon, Nature Communications, 4, 1671 (2013)

[4] L. Mazet et al., Structural study and ferroelectricity of epitaxial BaTiO₃ films on silicon grown by molecular beam epitaxy, Journal of Applied Physics. 116, 214102 (2014) 

Sylvie SCHAMM-CHARDON (Toulouse cedex4), Cesar MAGEN, Lucie MAZET, Robin COURS, Martin FRANK, Vijay NARAYANAN, Catherine DUBOURDIEU

08:00 - 18:15 #6878 - MS06-887 Interface study of metal nitride films on MgO and Al2O3 substrate using CS-corrected TEM.

MS06-887 Interface study of metal nitride films on MgO and Al2O3 substrate using CS-corrected TEM.

 

Transition metal nitrides have found wide-spread applications in the cutting- and machining-tool industry due to their extreme hardness, thermal stability and resistance to corrosion. The increasing demand of these nitrides requires an in-depth understanding of their structures at the atomic level. This has led to some experimental and theoretical researches [1-6].

In this paper, we will present our recent results on the atomic and electronic structures of the interface between various metal nitride thin films (CrN, VN and TiN) on MgO and Al₂O₃ substrate using C_S-corrected HRTEM and STEM, EELS/EDXS, quantitative atomic measurement and diffraction analysis as well as theoretical calculations. Interfacial detailed atomic and electronic structures are revealed and compared. Interface induced phenomena between nitride films and substrates are unveiled [1-4].

Figure 1 shows two typical interfacial atomic structures, frequently accompanied by interface atom reconstruction and interface dislocations to accommodate the misfit strains between two lattices. Particular studies on the effects of N defects in the metal nitride (CrN) film have been carried out. Combining the image analysis and spectrum analysis, some generalized conclusions are derived. (i) a relationship between the lattice constant and N vacancy concentration in CrN is established [5], (ii) the ionicity of CrN crystal is correlated with the N vacancy concentration; (iii) especially, a direct relationship between electronic structure change (L₃/L₂ ratio) and elastic deformation (lattice constants) in CrN films has been experimentally deduced, indicating that the elastic deformation may trigger a noticeable change in the fine structure of Cr-L_2,3 edge (Figure2). The experiment demonstrates an indirect approach to acquire electronic structure changes during the elastic deformation. The effect of randomly distributed defects in the films has been explored in a quantitative way using electron diffraction, complemented with HRTEM and EELS analysis. Some quantitative relations are also established [5].

 

 

[1]. R. Daniel et al, Acta Materialia 58(2010), p. 2621.

[2]. Z. L. Zhang, et al  Physical. Review B 82(R)(2010) p060103-4

[3]. Z. L. Zhang, et al Journal of Applied Physics, 110(2011)p043524-4

[4]. Z.L. Zhang et al  Physical Review B  87 (2013) p014104.

[5]. T.P. Harzer, et al Thin Solid Films 545 (2013) p154–160

[6]. P. Wan, Z.L. Zhang, et al,  Acta Mater 98(2015)119–127

[7]. Acknowledgement: Gabriele Moser and Herwig Felber are gratefully acknowledged for their help with sample preparation, thanks are given to Dr. Hong Li for ab-initio calculations.  Thanks are also given to Rostislav Daniel and Christian Mitterer in Montanuniversität  Leoben, Leoben, Austria for preparing the materials, and to Gerhand Dehm (Max-Planck-Institut für Eisenforschung) for helpful discussion.

Zaoli ZHANG (Leoben, Austria)

08:00 - 18:15 #6917 - MS06-889 Ferroelectric/dielectric composite tunnel junctions: influence of the stacking sequence on their microstructure.

MS06-889 Ferroelectric/dielectric composite tunnel junctions: influence of the stacking sequence on their microstructure.

Increasing the tunnel electroresistance of ferroelectric tunnel junctions can be achieved by replacing the single ferroelectric barrier by a ferroelectric/dielectric bilayer. For a given thickness of the layers, the stacking sequence (ferroelectric/dielectric or dielectric/ferroelectric) can lead to different resistivity.

In this paper, we study composite tunnel junctions based on the Mn-BiFeO3/SrTiO3 bilayer, deposited on a LaSrMnO3 (LSMO) buffer, grown on (001)-oriented SrTiO3 (STO) substrates (fig. 1). The samples were grown by PLD. The nominal thicknesses are 0.8nm for SrTiO3 and 2.8 nm for Mn-BiFeO3 (BFO).

TEM/HRTEM micrographs reveal the homogeneity of the bilayers. The sharpness and roughness of the interfaces are then studied by mean of Cs-corrected HAADF-STEM for both stacking sequences.

STO/BFO/LSMO (fig. 1a): for this configuration, both BFO/LSMO and STO/BFO interfaces appear flat, suggesting a homogeneous thickness of the BFO layer of 2.8 nm. The top surface of STO exhibits steps of half perovskite-cell height leading to an effective STO thickness of about three to four unit-cell; somewhat thicker than the awaited one.

BFO/STO/LSMO (fig. 1b): whereas the STO/LSMO interface looks flat and sharp, the BFO/STO interface exhibits a roughness of the order of one perovskite unit-cell, indicating that the effective STO layer thickness is somewhat inhomogeneous, but close to the awaited 0.8nm thickness. The BFO surface looks flat, suggesting that the effective BFO thickness varies along the bilayer but also remains close to the 2.8nm awaited thickness.

Deeper insights on the interfaces sharpness are further obtained by ELNES measurements at the Mn L23 , Fe L23 , Ti L23 and O-K edges. Peculiar fingerprints of the Fe and Ti L23-edges are observed at the bilayer interfaces suggesting slight changes of the crystal field in their vicinity.

Frédéric PAILLOUX (FUTUROSCOPE CEDEX), Matthieu BUGNET, Arnaud CRASSOUS, Stéphane FUSIL, Vincent GARCIA, Manuel BIBES, Gianluigi BOTTON, Agnès BARTHÉLÉMY, Jéröme PACAUD

08:00 - 18:15 #6935 - MS06-891 Influence of Cu content on the structural and morphological features of TixCuy intermetallic thin films for biosignals acquisition.

MS06-891 Influence of Cu content on the structural and morphological features of TixCuy intermetallic thin films for biosignals acquisition.

Thin films synthesized by Physical Vapor Deposition (PVD) are currently studied to be used in different types of sensors [1,2]. Among these, sensors and electrodes used for biomedical devices are particularly important since they allow converting one or more measured signals of any living tissue into other quantities, usually an electrical signal. Beyond the electrical response, a biomedical sensor should also be biocompatible and totally innocuous for the patient. Additionally some degree of antibacterial effect is considered as a further asset for the sensor. The Ti-Cu system perfectly meets such requirements, Ti being biocompatible, while Cu is relevant for its antibacterial character. Objective of this study is to deeply characterize the nature and microstructure of Ti-Cu films in order to better understand and optimize the sensor response.
Films are deposited by a PVD magnetron sputtering process from a composite Ti-Cu target. Their chemistry, morphology and fine microstructure are characterized by X-Ray diffraction, Rutherford Backscattering Spectrometry, Scanning and Transmission Electron Microscopies.
Results show that three main zones were distinguished, in relation with the Cu/Ti atomic ratio into the films. SEM reveals that the morphology of micrometer- thick films changed from a columnar to an amorphous-like microstructure. XRD diffraction indicated that the hcp-Ti structure dominates for low Cu/Ti ratios. For higher Cu contents, formation of Ti-Cu intermetallic phases was noticed which becomes more clear and obvious for the third zone (Cu contents above 75 at.%). HR-TEM and STEM observations confirm the presence of nanocrystallites embedded into an amorphous matrix (Fig. 1). A further chemical characterisation allowed identifying the nature of  intermetallics, which contributed to explain sensors’ electrical behaviours (Fig. 2).

[1] C. Lopes; M. Vieira; J. Borges; J. Fernandes; M. S. Rodrigues; E. Alves; N. P. Barradas; M. Apreutesei; P. Steyer; C. J. Tavares; L. Cunha; F. Vaz; "Multifunctional Ti-Me (Me = Al, Cu) Thin Film Systems For Biomedical Sensing Devices"; Vacuum (2015).
[2] C. Lopes, P. Fonseca, T. Matamá, A. Gomes, C. Louro, S. Paiva, F. Vaz; “Protective Ag:TiO2 thin films for pressure sensors in orthopedic prosthesis: the effects of composition, structural and morphological changes on the biological response of the coatings”, Journal of Materials Science: Materials in Medicine; 25 (2014); 2069-2081.

Claudia LOPES, Siddhardha KONETI, Lucian ROIBAN, Joel BORGES, Thierry EPICIER, Filipe VAZ, Philippe STEYER (VILLEURBANNE CEDEX)

08:00 - 18:15 #6961 - MS06-893 Plan view STEM analysis of the domain structure in anisotropically strained epitaxial K_0.95Na_0.05NbO₃ ferroelectric films with giant piezoelectricity.

MS06-893 Plan view STEM analysis of the domain structure in anisotropically strained epitaxial K_0.95Na_0.05NbO₃ ferroelectric films with giant piezoelectricity.

In the past years lead free K_xNa_1-xNbO₃ ceramics with huge d₃₃-values, comparable to that of PbZr_1-xTi_xO₃, have been demonstrated [1]. The high piezoelectric coefficients are generally attributed to the presence of a monoclinic phase at the morphotropic phase boundary and the associated symmetry reduction. A recent work by Schwarzkopf et al. [2] reports that monoclinic phases can also be stabilized in ferroelectric perovskite thin films by utilizing anisotropic epitaxial strain. Another advantage of using epitaxial strain is to engineer by an appropriate choice of the substrate and film composition the arrangement and size of domains, which in turn influences on the piezo- and ferroelectric material properties. Ferroelectric thin films and their domain structure are commonly characterized by combining X-ray diffraction (XRD) and piezoresponse force microscopy (PFM). While XRD gives useful crystallographic information, its local resolution is limited and a detailed study of the domains is impossible. On the other hand PFM has sufficient spatial resolution but does not provide structural information.

Plan-view transmission electron microscopy (TEM) with its ability to study structural data with high accuracy at atomic resolution is in principle ideally suited to gain detailed information on the domain structure of the film. There is, however, one major challenge. In order to preserve the as-grown strain state of the film and thus its domain structure plan-view TEM samples with a thick substrate layer are necessary. Since TEM is a projective method it images typically the projected potential averaged along the beam direction including both film and substrate for a plan-view observation geometry. In this paper we will show that this challenge can be overcome by appropriate imaging conditions using high resolution scanning TEM (STEM) annular dark field (ADF) imaging. Due to the channeling effect and the reduced depth of focus in STEM mode we are able to obtain information mainly from the monoclinic distorted film, while the substrate gives rise to a uniform background intensity.

As an example we study a 26 nm thin film of K_0.95Na_0.05NbO₃, which was epitaxially grown on top of a (110) NdScO₃ substrate and shows giant piezoelectricity. PFM reveals a complex periodic domain structure with the coexistence of two different monoclinic domains (yellow and purple stripes in Fig. 2 a). According to XRD these domains differ from each other by the orientation of the in-plane monoclinic distortion. Based on the structural data provided by XRD and PFM we created a supercell consisting of a K_0.95Na_0.05NbO₃ film, which contains two domains with a difference of the in-plane monoclinic distortion of 2β=8 mrad, on top of a 150 nm thick rigid NdScO₃ substrate without any lattice distortion (Fig. 1 a). The chosen thickness of the substrate is a minimum value necessary to preserve the as-grown strain state of the film. Frozen phonon simulations reveal that if the incident electron probe is transmitted first through the film, then the final high resolution STEM ADF image pattern directly reflects the domain structure of the K_0.95Na_0.05NbO₃ film with the evaluated shear angle 2β as in the supercell (Fig. 1 b). However, if the electron beam direction is inverted, i.e. the beam is transmitted first through the substrate, then no lattice distortion corresponding to the domain structure of the film is found in the obtained high resolution STEM ADF image. An analysis of the evolution of the electron probe inside the supercell (Fig. 1 c) shows that this 3-dimensional sensitivity is caused by the electron channeling effect. In the present case the high resolution pattern in the final image is produced basically in the first 20-30 nm of transmitted material. After this thickness the wave function of the electron beam is widely dispersed, irrespectively of the initial position of the electron probe. Thus the subsequent part of the specimen mainly contributes to a uniform background intensity in the final image.

Consequently, the frozen phonon simulations demonstrate the feasibility of our approach to study domains in epitaxially strained ferrolectic films by plan-view high resolution STEM ADF imaging. An experimental example for the STEM analysis of the domain structure for the above mentioned K_0.95Na_0.05NbO₃ film is shown in Fig 2 c. A model of the complex domain pattern including the sub-domain structure will be discussed in the paper.

 

[1] Wang et al., J. Am. Chem. Soc. 136, 2905 (2014)

[2] Schwarzkopf et al., J. Appl. Cryst. 49, 375 (2016)

Toni MARKURT (Berlin, Germany), Jutta SCHWARZKOPF, Dorothee BRAUN, Martin SCHMIDBAUER, Martin ALBRECHT

08:00 - 18:15 #6982 - MS06-895 Microstructure investigation of Allvac 718Plus superalloy after heat treatment with temperature gradient.

MS06-895 Microstructure investigation of Allvac 718Plus superalloy after heat treatment with temperature gradient.

ATI Allvac 718Plus® superalloy (718Plus alloy) exhibits high strength and good corrosion resistance in high temperatures. The 718Plus alloy was designed to increase the maximal temperature of application without strong increase of production costs. It was achieved by improving the microstructure stability up to 700 °C and was possible due to the changes in the main strengthening mechanism of the 718Plus alloy.

The 718Plus alloy typical chemical composition is as follows: Ni-18Cr-10Fe-9Co-5.1(Nb+Ta)-2.7Mo-1W-0.7Ti-1.5Al-0.03C (wt%). Those elements create multiple phases and proper characterisation of those phases is the key for understanding properties of the 718Plus alloy. The 718Plus alloy microstructure consists of a γ matrix (Ni-base solid solution) with ordered face centred cubic γ’-Ni₃(Al,Ti) type phase, some orthorhombic δ-Ni₃Nb and hexagonal η-Ni₃Ti, η*-Ni₆AlNb or Ni₆(Al,Ti)Nb particles precipitated mainly on the grain boundaries [1,2].

The aim of this study was to investigate the microstructural changes, with a special focus on the evolution of secondary phases, during the multiple-step heat treatment conditions. Establishing such understanding will allow to define an optimal heat treatment route for the best alloy performance and stability, and can provide major savings in production costs.

 

Several microscopy techniques were used for microstructure investigations, mainly SEM, TEM/HRTEM and STEM-EDX spectrometry. Phase identification was performed by XRD, EDX and electron diffraction (SAED, nD) supported by JEMS. The samples were prepared by conventional jet electropolishing and by FIB techniques. The research was conducted utilizing Merlin G20 TWIN (SEM) and a probe Cs corrected Titan³ Cubed G2 60-300 with a ChemiSTEM system. It provided a possibility of very detailed analyses mainly focused on identification, morphology and chemical composition of the phases strengthened the 718Plus alloy. STEM imaging using HAADF contrast and EDX mapping were used for characterization of the particles’ nanostructure down to the atomic level.

The as-received 718Plus alloy microstructure (Figs 1, 2) consists of spherical γ’-Ni₃(Al,Ti) phase particles and various plate-like precipitates, some of them with a very complex structure. Observations in TEM dark-field revealed stripes of additional phase inside some of the plate-like precipitates (Fig. 3), which were not observed in the bright-field images. Fig. 4 shows HRSTEM-HAADF image of plate-like precipitate at atomic level, as seen along [110] axis. Identification by electron diffraction and EDX showed that it is a hexagonal η-Ni₃Ti phase enriched in Nb (possibly η*-Ni₆AlNb or Ni₆(Al,Ti)Nb phase), however some reflections indicated a presence of a different phase. It seems to be orthorhombic δ-Ni₃Nb phase (“white stripes”), which has similar chemical composition (Ni, Nb) but different crystal structure. Unambiguous identification of phases forming complex plate-like particles is in progress.

[1] O.M. Messé et al. : On the precipitation of delta phase in ALLVAC® 718Plus, Philosophical Magazine, 94(2014) 1132-1152

[2] E.J. Pickering et al.: Grain-boundary Precipitation in Allvac 718Plus, Acta Materialia 60 12012)2757-2769

 

Acknowlegments: The authors acknowledge Pratt & Whitney, USA for providing the material used in this investigation and for the financial support.

Sebastian LECH (Cracow, Poland), Adam KRUK, Bogdan RUTKOWSKI, Agnieszka WUSATOWSKA-SARNEK, Aleksandra CZYRSKA-FILEMONOWICZ

08:00 - 18:15 #6239 - MS07-897 Prospective scintillators for low-energy BSE detectors.

MS07-897 Prospective scintillators for low-energy BSE detectors.

Cerium activated bulk single crystals of yttrium aluminium garnet (YAG:Ce) Ce_x:Y_3-xAl₅O₁₂ are widely used as scintillators for the detection of backscattered electrons (BSE). In the electron microscopy research of nanomaterials, biomaterials or semiconductors, low energy (units of keV) electron beam imaging is often necessary. Because BSE detectors are mostly non-accelerating or low-accelerating, electrons with approximately the same energy as primary beam (PB) have to be detected. However, commonly used YAG:Ce single crystal strongly loses its light yield (LY) with the decrease of the PB energy [1]. As possible available alternatives for this application, bulk single crystals of YAlO₃:Ce (YAP:Ce) and CRY018 can be predicted. However, similar LY sink can be expected also with these scintillators.

There are two main reasons, why this occurs. Firstly, slower electrons don’t have enough energy to pass through the relatively thick conductive layer on the scintillator surface. Therefore, thinner conductive layer has to be used. Secondly, commonly available scintillators suffer from structural defects that are created mostly due to surface damage (as a result of its grinding, polishing, purification or contamination) or already during the own bulk single crystal growth.

The influence of all of these defects on cathodoluminescence (CL) properties can be eliminated by the scintillators in form of thin single crystalline films because, as shown previously [2], the concentration of these defects decreases with the decreasing temperature of the crystal growth. Therefore, single crystalline epitaxial films have attracted a lot of attention recently because the growth temperature of these films is about a half (1000 °C) of the bulk ones (2000 °C). Moreover, appropriate doping of the garnet structure can suppress the influence of the defects on the CL properties.

For the purpose of this work, bulk single crystals of YAG:Ce, YAP:Ce and CRY018 were studied. Results were compared with those of promising cerium activated single crystalline films of gadolinium aluminium gallium garnet (GAGG:Ce) Ce_x:Gd_3-xGa_yAl_5-yO₁₂. These films were grown by the isothermal dipping liquid phase epitaxy onto YGG substrates from lead-free BaO-B₂O₃-BaF₂ flux [3]. These specimens were coated with conductive layers of different composition and different thicknesses. Properties of these layers are in the table shown in Table 1.

The specimens were excited by an electron beam with energy in range from 0.8 to 10 keV using a specialized CL apparatus [4]. In this energy range, CL LY of YAG:Ce were measured for coating layers of different composition and different thicknesses (Fig. 1). Moreover, CL spectra (Fig. 2) and CL intensity decays (Fig. 3) have been measured for all presented specimens.

It was shown, that the coating layer with thickness of only units of nm has to be used to allow low-energy BSE penetrating the layer without significant losses. Moreover, it was shown, that the GAGG:Ce film with balanced Ga content shows excellent scintillation properties where the effect of unwanted structural defects was suppressed, the spectrally corrected CL LY value exceeded 160 % of the commercially available bulk YAG:Ce single crystal, and CL decay was dominated by a fast component with 50 ns decay time which is close to that of Ce³⁺ (5d-4f) photoluminescence decay. Thanks to these excellent CL properties at PB energy of 10 keV, GAGG:Ce single crystalline films are new prospective scintillators suitable for low-energy BSE detectors. This research is in progress, therefore other results at different PB energies will be presented at the conference.

Acknowledgements: The research was supported by the Technology Agency of the Czech Republic (TE01020118), by Czech Science Foundation (projects GA16-05631S, GA16-15569S) and by Ministry of Education, Youth and Sports of the Czech Republic (project LO1212) and by European Commission (project CZ.1.05/2.1.00/01.0017). The authors thank the company CRYTUR, s.r.o., for supplying with the specimens of bulk single crystals.

References:

[1] G. F. J. Garlick, Brit. J. Appl. Phys. 13 (1962) 541–547.

[2] M. Nikl, E. Mihokova, J. Pejchal, A. Vedda, Y. Zorenko, K. Nejezchleb, Phys. Status Solidi B 242 (2005) R119-R121.

[3] M. Kucera, K. Nitsch, M. Kubova, N. Solovieva, M. Nikl, J.A. Mares, IEEE Trans. Nucl. Sci. 55 (2008) 1201.

[4] J. Bok, P. Schauer, Rev. Sci. Instrum. 82 (2011) 113109.

Ondřej LALINSKÝ (Brno, Czech Republic), Petr SCHAUER, Miroslav KUČERA, Zuzana LUČENIČOVÁ, Martin HANUŠ

08:00 - 18:15 #6251 - MS07-899 VEELS investigation of perovskite manganite interfaces.

MS07-899 VEELS investigation of perovskite manganite interfaces.

As promising candidates for magnetic storage and magnetic field sensing applications at room temperature Sr-doped LaMnO₃ (LSM) perovskite manganites show a very large negative magnetoresistance [1]. These materials exhibit a colossal magnetoresistance (CMR) which is very sensitive to the behaviour of the interface, due to the lattice misfit between substrate and thin film, and hence to the induced strain.

The investigated LSM thin film synthesised from La_0.8Sr_0.2MnO₃ powder was prepared by pulsed laser deposition (PLD). The 140 nm thick LSM layer was epitaxially grown on a single crystalline LaAlO₃ (LAO) (100) substrate. The preparation of a cross section specimen for TEM analysis was done by focused ion beam (FIB) followed by subsequently Ar⁺ ion polishing. The analytical TEM investigations were performed by using a FEI TECNAI G20 TEM equipped with a Gatan GIF 2001 energy filter and a field-emission FEI TECNAI F20 TEM equipped with a Gatan GIF Tridiem energy filter. In this work, we will present a study of the microstructure as well as the optical and electronic properties of LaSrMnO₃-LaAlO₃ interfaces at different temperatures and operating voltages. High-resolution (HR) TEM, high-angle annular dark field (HAADF) STEM and analytical methods, such as valence electron energy loss spectrometry (VEELS) and electron magnetic chiral dichroism (EMCD) were applied.

The bright field (Fig. 1B) and the dark field (Fig. 1C) image convey the impression of a non-uniform distribution in the LSM layer. This fact is confirmed by means of the HAADF image in Fig. 1A. A columnar structure is distinguished in the thin film. In addition, spot splitting perpendicular to the interface occurs from the lattice mismatch as shown in the Fourier transform (FT) (Fig. 1D) of the HRTEM image (Fig. 1E) recorded at the interface. It was observed that the columnar growth mechanism starts directly at the interface within a heavily strained region as reported in [2].

The EELS investigations of the LaSrMnO₃-LaAlO₃ interface are shown in Fig. 2. In the case of determining optical properties, high beam energy alters the valence EELS (VEELS) spectrum by exciting retardation losses and therefore, we reduced the operation voltage in order to eliminate these effects [3]. The Cerenkov contribution is observed at 200 keV in LAO and LSM in Fig. 2, while these losses are vanished at lower beam energy. The EELS spectrum after zero-loss peak (ZLP) removal is shown for LAO in Fig. 2A and for LSM in Fig. 2B at 40 keV and 200 keV, respectively. A pre-measured zero-loss peak was used for the ZLP removal of the VEELS signal. However, some artefacts at the bandgap can be seen in the insertion above in Fig. 2A. The VEELS spectrum image in Fig. 2C exhibits the bandgap transition at the LaSrMnO₃-LaAlO₃ interface with a beam energy at 60 keV.

 

Acknowledgements

We thank the FWF (Austrian Science Foundation) for financial support under the project F4501-N16.

 

References

[1] A. M. Haghiri-Gosnet, J. Wolfman, B. Mercey, C. Simon, P. Lecoeur, M. Korzenski, M. Hervieu, R. Desfeux, and G. Baldinozzi, ‘Microstructure and magnetic properties of strained La0.7Sr0.3MnO3 thin films’, Journal of Applied Physics (2000), vol. 88, no. 7, pp. 4257–4264.

[2] G. Van Tendeloo, O. I. Lebedev, and S. Amelinckx, ‘Atomic and microstructure of CMR materials’, Journal of Magnetism and Magnetic Materials (2000), vol. 211, no. 1, pp. 73–83.

[3] M. Stöger-Pollach, ‘Optical properties and bandgaps from low loss EELS: Pitfalls and solutions’, Micron (2008), vol. 39, pp. 1092-1110.

 

Wolfgang WALLISCH (Wien, Austria), Michael STÖGER-POLLACH, Edvinas NAVICKAS, Andreas STEIGER-THIRSFELD, Johannes BERNARDI

08:00 - 18:15 #6440 - MS07-901 Can transverse plasmonic fields be revealed by differential phase contrast?

MS07-901 Can transverse plasmonic fields be revealed by differential phase contrast?

Surface plasmons give rise to a wide range of applications from molecular sensors [1] over novel circuit designs [2] to the design of meta-materials with highly unusual optical properties [3]. Of particular importance are localized surface plasmons (LSPs) that are confined to the surface of nanoparticles as they can give rise to a significant enhancement of electromagnetic fields in the vicinity of the nanostructure. Because LSPs typically are confined to the nanometer regime, TEM is ideally suited for mapping those charge oscillations.

So far, the predominant method of studying plasmon oscillations has been EELS, which allows mapping the strength of selected resonance modes by measuring the energy loss probability of the probe beam for different LSP energies. In a non-relativistic approximation, this energy loss is brought about by the component of the electric field along the optical axis (and, in principle, the magnetic component perpendicular to the optical axis) of the excited plasmon resonance. Thus, it is impossible to gain any information about the electric field in the viewing plane (i.e., perpendicular to the optical axis). Precisely this component can, however, be studied using differential phase contrast (DPC) [4,5].

DPC exploits the fact that electrons subject to an electromagnetic field are deflected according to the Lorentz force. Any deflection along the optical axis gives rise to a change in kinetic energy and, hence, shows up in EELS. Any deflection perpendicular to the optical axis, however, changes the direction of the electron's momentum, but not its magnitude (in first order approximation). This gives rise to a shift in the electron's momentum distribution. The final momentum distribution, after passing the nanostructure, can then conveniently be measured in the TEM's diffraction plane. Compared to a reference measured without field, the displacement of the transmitted beam shows a shift that is proportional to the field integrated along the electron trajectory.

Here, we used the MNPBEM toolbox [6,7] to simulate the plasmonic response of a 200x50x50 nm³ Ag nanorod to the electron beam (see fig. 1). From the data of the surface charges and currents, we then calculated the EELS maps (see fig. 1) and in-plane deflections along a line parallel to the nanorod (see fig. 2) for different plasmonic modes. The EELS maps show the typical excitation probabilities for the first two modes with two and three maxima. The in-plane electric field components show a similar behavior in general, although the local extrema are less pronounced. The DPC deflections are found to be in good agreement with the electric field with some small differences close to the center of the rod which can be attributed to the cumulative nature of the DPC deflections as well as retardation effects. The absolute magnitude of the DPC deflections in fig. 2 is of the order of 0.1 µrad at 300 keV which, albeit small, should be measurable with latest generation TEMs when using large camera lengths and/or the LACBED technique. In addition, the deflections can be increased, e.g., by using a lower acceleration voltage.

This work shows that it should be feasible to determine all three components of the electromagnetic field caused by plasmons using a combination of DPC and EELS using state-of-the-art TEMs. This will open up new possibilities for understanding and designing novel plasmonic devices.

 

Acknowledgements: Financial support by the Austrian Science Fund (FWF) under grant nr. J3732-N27 and by the NSERC is gratefully acknowledged.

 

[1] Willets & Duyne, Ann. Rev. Phys. Chem. 58 (2007) 267
[2] Ozbay, Science 311 (2006) 189
[3] Zheludev & Kivshar, Nat. Mater. 11 (2012) 917
[4] Rose, Ultramicroscopy 2 (1977) 251
[5] Lohr et al., Ultramicroscopy 117 (2012) 7
[6] Hohenester & Trügler, Comp. Phys. Commun. 183 (2012) 370
[7] Hohenester, Comp. Phys. Commun. 185 (2014) 1177

Stefan LÖFFLER (Wien, Austria), Edson P BELLIDO, Isobel C BICKET, Gianluigi A. BOTTON

08:00 - 18:15 #6562 - MS07-903 ZnO Nanostructures for Mid-IR Plasmonics.

MS07-903 ZnO Nanostructures for Mid-IR Plasmonics.

Degenerate metal oxide nanocrystals (NCs) are promising systems to expand the significant achievements of plasmonics into the infrared (IR) range¹. We report on the tunable mid IR Plasmon induced in degenerate Al and Ga doped ZnO (AZO and GZO) nanocrystals. The NCs were obtained by Low Energy Cluster Beam Deposition (LECBD). By varying the Al and Ga content from 3 to 9 at.% within the particles we are able to tune the plasmon wavelength from 3 to 4 μm.   However, the plasmon resonances are characterized by an unusually large damping, which originates from two mechanisms. The first one is the Oriented Attachment (OA) process (cf. figure 1, B)³. When NCs attach by epitaxy, the resulting structure has a lower symmetry, which induces a shift and broadening of the plasmon resonance. Embedding the particles in an Al₂O₃ matrix has prevented the OA, and hence the damping was reduced along with broadening. The second mechanism is the partial activation of the dopants. We have observed that less than half of the dopants actually participate to the electron gas². The cause of the partial activation is related to the position of the dopants within the particles. It has been proposed that the damping is larger if the dopants are homogeneously distributed⁴. In the present work we investigate the possibility of mapping the spatial distribution of dopants within the nanocrystals usingFEI-TITAN ETEM equipped with High resolution Gatan GIF (see Figure 2). We subsequently anneal the nanocrystals to let them reach the thermodynamic equilibrium. The distribution of dopants, and its consequences on the plasmon resonances is then investigated [5,6].

References
[1] G.V. Naik, V.M. Shalaev, A. Boltasseva, “Alternative Plasmonic Materials: Beyond Gold and Silver”, Adv. Mat. 2013, 25, 3264-3294.
[2] M. K. Hamza, J.-M. Bluet, K. Masenelli-Varlot, B. Canut, O. Boisron, P. Melinon and B. Masenelli. ”Tunable mid IR plasmon in GZO nanocrystals”. Nanoscale, 2015, 7, 12030
[3] D. Hapiuk, B. Masenelli, K. Masenelli-Varlot, D. Tainoff, O. Boisron, C. Albin and P. Melinon.”Oriented Attachment of ZnO Nanocrystals ”. J. Phys. Chem. C, 2013, 117, 10220–10227.
[4]  S. D. Lounis, E. L. Runnerstrom, A. Bergerud, D. Nordlund  and D. J. Milliron, J. Am. Chem. Soc., 2014, 136, 7110–7116.
[5] V. C. Holmberg, J. R..Helps, K. A. Mkhoyan, D. J. Norris “Imaging Impurities in Semiconductor Nanostructures”, Chem. Mater., 2013, 25 (8), pp 1332–1350
[6] Thanks are due to the CLYM (Centre Lyon - St-Etienne de Microscopie, www.clym.fr) for access to the microscope.

Mohamed HAMZA TAHA (Villeurbanne), Jean BLUET, Karine MASENELLI-VARLOT, Cyril LANGLOIS, Thierry EPICIER, Matthieu BUGNET, Bruno CANUT, Olivier BOISRON, Patrice. MELINON, Bruno MASENELLI

08:00 - 18:15 #6957 - MS07-905 Hyperbolic Plasmons in the Topological Insulator Bi2Se3.

MS07-905 Hyperbolic Plasmons in the Topological Insulator Bi2Se3.

The surface plasmon excitation is one of the popular field of research due to their high potential to be applicable in sensor¹ or information technologies², cancer research³ etc. These coherent delocalized electron oscillations are common in metal-dielectric interfaces. However, recent studies show they also exist in highly doped semiconductors, conducting oxide system or graphene, in summary mostly the systems with high carrier mobility. The next question can be asked for the insulator interfaces.

Bismuth Selenide became popular material system due to its recently discovered topological insulator property, in which it behaves as an insulator in the bulk state and metallic at the surface state. This also makes Bi₂Se₃ as a potential candidate for surface plasmons,so that the first example of Dirac plasmons on Bi₂Se₃ was observed in 2013 in 0.5 – 1 eV energy loss region⁴.

The Dirac state is not the only reason of the existence of a plasmon resonance in Bi₂Se₃. It is a highly anisotropic with hypabolic dispersion Tetradymites crystal structure, which also makes the dielectric properties highly anisotropic which can allow the plasmon excitation⁵. In this study we would like to show energy filtered transmission electron microscopy (EFTEM) and a finite-difference frequency-domain (FDTD) study for investigating Bi₂Se₃ nanoplates and try to find an explanation for the existing plasmon excitations.

For the experimental observation of Hyporbolic dispersion in Bi₂Se₃, EFTEM study was carried out using FEG-TEM Sub-Electron-Volt-Sub-Ångstrom-Microscope (Zeiss SESAM) in the 200 kV equipped with an electrostatic monochromator and the in-column MANDOLINE filter with 0.2 eV slit width. As shown in Fig. 1 localized excitations exist at surface of Bi₂Se₃ crystal and they show different collective modes in different energies. These results supported by FDTD simulations to be able to explain the interplay between the edge plasmons and surface plasmons.   

References:

1.   Wang, Z.; Cheng, Z.; Singh, V.; Zheng, Z.; Wang, Y.; Li, S.; Song, L.; Zhu, J., Stable and Sensitive Silver Surface Plasmon Resonance Imaging Sensor Using Trilayered Metallic Structures. Analytical Chemistry 2013, 86, 1430-1436.

2.   Kosmeier, S.; De Luca, A. C.; Zolotovskaya, S.; Di Falco, A.; Dholakia, K.; Mazilu, M., Coherent control of plasmonic nanoantennas using optical eigenmodes. Sci. Rep. 2013, 3.

3.   Cai, W.; Gao, T.; Hong, H.; Sun, J., Applications of gold nanoparticles in cancer nanotechnology. Nanotechnology, Science and Applications 2008, 2008, 17-32.

4.   Di Pietro, P.; OrtolaniM; LimajO; Di Gaspare, A.; GilibertiV; GiorgianniF; BrahlekM; BansalN; KoiralaN; OhS; CalvaniP; LupiS, Observation of Dirac plasmons in a topological insulator. Nat Nano 2013, 8, 556-560.

5.   Esslinger, M.; Vogelgesang, R.; Talebi, N.; Khunsin, W.; Gehring, P.; de Zuani, S.; Gompf, B.; Kern, K., Tetradymites as Natural Hyperbolic Materials for the Near-Infrared to Visible. ACS Photonics 2014, 1, 1285-1289.

Acknowledgment:

The research leading to these results has received funding from the European Union Seventh Framework Programme [FP7/2007-2013] under grant agreement n°312483 (ESTEEM2). W. V. d. B. acknowledges the Carl Zeiss Foundation.

Cigdem OZSOY KESKINBORA (Stuttgart, Germany), Nahid TALEBI, Hadj BENIA, Christoph KOCH, Peter VAN AKEN

08:00 - 18:15 #6101 - MS08-907 Valence state analysis on iron in minerals of Earth’s lowermost mantle by electron energy loss spectroscopy.

MS08-907 Valence state analysis on iron in minerals of Earth’s lowermost mantle by electron energy loss spectroscopy.

Determining the ratio of ferrous and ferric iron concentrations in lower mantle minerals is essential to understand the geophysical and geochemical properties of the deep Earth. The two iron-bearing phases in the lower mantle are bridgmanite (Brg, (Mg,Fe)SiO₃) and ferropericlase (Fp, (Mg,Fe)O) where the valence state and partitioning of iron have a strong impact on the physical and chemical properties [1,2]. Samples consisting of a Brg and Fp mineral assemblage have been previously synthesized from Al-rich olivine composition [3]. Brg turns into an amorphous matrix after decompression to atmospheric pressure and Fp remains crystalline with grain sizes ranging from 50 – 400 nm. To understand the correlation between the distribution and oxidation states of iron, characterization methods with a high spatial resolution are required. Electron energy loss spectroscopy (EELS) in the transmission electron microscope (TEM) is a useful method to investigate Fe³⁺/ ∑Fe in iron-rich minerals [4]. Things to take care of during acquisition are e.g. beam damage of the material or electron-beam-induced oxidation of iron. EELS analysis of Brg and Fp phases synthesized at different temperatures and pressures prepared by focused ion beam (FIB) sectioning will be discussed.

 

[1] Badro et al., Science 300 (2003) 789.

[2] Prescher et al., Earth and Planetary Science Letters 399 (2014) 86.  

[3] Piet et al., submitted

[4] van Aken et al., Physics and Chemistry of Minerals 29 (2002) 188.

Teresa DENNENWALDT (Lausanne, Switzerland), Hélène PIET, James BADRO, Philippe GILLET, Cécile HÉBERT

08:00 - 18:15 #6102 - MS08-909 TEM analysis of stress induced defects in baddeleyite xenocrysts from the Phalaborwa complex.

MS08-909 TEM analysis of stress induced defects in baddeleyite xenocrysts from the Phalaborwa complex.

Large deposits of baddeleyite (natural zirconia) were found in the Phalaborwa complex and mainly in the foskorite ore zone and to a lesser extent in the carbonatite ore body [1]. Baddeleyite has a monoclinic zirconia (ZrO₂) structure at room temperature and undergoes a martensitic transformation to a tetragonal structure at approximately 1100⁰C depending on the ambient pressure.  Transformation of the zirconia, on cooling, results in a 5% volume change in the unit cell as well as 8-14% increase in strain energy [2]. This volume change and strain energy during the transformation could result in crystal twinning and/or cracking during catastrophic changes in temperature and pressure especially during the emplacement process. Lumpkin [3] has commented on the lack of a detailed microstructural analysis for natural zirconia which would possibly provide information on the nature of primary and secondary physical alteration mechanisms, and hence the geothermal history of the material. Recent reports on the microanalysis for Phalaborwa zirconia produced data on the origin of secondary inclusions [4] as a result of physical alteration during the emplacement process.  Considering the thermal history of the material it is expected that there should be evidence for stress induced damage. In this paper, the nature of stress induced defects in baddeleyite xenocrysts will be presented and discussed.

 

Sections, approximately 2 mm in thickness, were cut from a Phalaborwa baddeleyite xenocryst using a diamond wire saw and polished to a 0.25 μm finish. TEM sections   were prepared in an FEI Helios NanoLab as well as a Gatan precision ion polishing system. The TEM sections were imaged and analysed using JEOL ARM 200F and JEM 2100 TEMs operated at 200 kV.

 

Both {001} and {011} twin domains were identified using SAD patterns. TEM bright field (BF) images for the {011} twin domains are shown in figure 1. Simulation of the diffraction pattern for the central twin domain shows a 3% elongation in the [001] direction indicating the possibility of strained material in this domain. The twin domains analysed in this work for the Phalaborwa baddeleyite are orders of magnitude larger and do not “pinch out” as described by Kerschofer et al [5]. Figure 2 presents evidence for small loop-like defects formed along what is assumed to be an individual ion track from a fission product recoil (indicated by the dashed lines). The entire field of view is covered by randomly distributed small defect structures which have not yet been identified. Figure 3 shows a few large extended defects which are believed to be oxygen interstitial complexes. These defects are similar in nature to those observed in synthetic stabilized zirconia which has been irradiated with high fluences of swift heavy ions followed by thermal annealing [6]. These high fluences are improbable for natural material and hence the damage is believed to be stress induced followed by thermal annealing.  These results have thus shown that stress induced damage is in fact present in the baddeleyite xenocryst and further analysis will be required to determine the exact characteristics of these defects. The results could possibly provide valuable information on the geochronological thermal history of the material

 

References

[1] JH Nielsen et al., Zirconium and Zirconium Alloys 26 (2000), p. 621.

[2] MTD Wingate and W Compston, Chemical Geology 168 (2000), p.75.

[3] GR Lumpkin, J. Nucl. Mater. 274 (1999),  p206.

[4] ME Lee et al., Proc. Microsc. Soc. South. Afr.42 (2012), p57.

[5] L Kerschhofer et al., Earth. Planet. Sci. Lett. 179 (2000), p219.

[6] K Yasuda et al., J. Nucl. Mater. 319 (2003), p74.

Mike LEE (Port Elizabeth, South Africa), Jacques O'CONNELL, Arno JANSE VAN VUUREN

08:00 - 18:15 #6439 - MS08-910 3D Elemental Mapping of Non-metallic Inclusions in Japanese Sword with FIB-SEM / EDS System.

MS08-910 3D Elemental Mapping of Non-metallic Inclusions in Japanese Sword with FIB-SEM / EDS System.

  Japanese swords are made of raw steel produced by smelting iron sand. The raw steel made by the Tatara method contains less phosphorus and sulphur, and higher concentrations of non-metallic inclusions than modern steel. By analyzing non-metallic inclusions the source of the iron sand and the heat process used during processing Japanese swords have been investigated [1,2]. The purpose of this study is to reveal the 3D distribution of non-metallic inclusions to understand the solidification process. The 3D distribution was observed with a Focused Ion Beam - Scanning Electron Microscope (FIB-SEM). The sample was repeatedly sliced with an FIB-SEM to expose a new surface which was then automatically analyzed using EDS. The 3D distribution was reconstructed from the acquired EDS data to analyze solidification processes of the non-metallic inclusions in the edge. There are structures in which aluminum and calcium-rich areas are wrapped in silicon-rich areas [3]. In this study, structures of non-metallic inclusions in back side of a Japanese sword were analyzed. The sample was a Japanese sword with the signature of Bizen Osafune Katsumitsu (property of M. Kitada). It was made in Japan in the 16th century.

  The 3D distribution of non-metallic inclusions was determined using a JIB-4610F (FIB-SEM, JEOL Ltd.) and an EDS (by Thermo Fisher Scientific). The SEM conditions were as follows; accelerating voltage: 10 kV, probe current: 10 nA, whereas the FIB processing conditions; accelerating voltage: 30 kV, probe current: 10 nA, and ion dose: 100 nC/μm².

  Cross-sectional macrograph of the sword after etching is shown in Figure 1 a). The secondary electron image (SEI) of the analysis area indicated by the red square in Figure 1 a) is shown in Figure 1 b). Figure 1 c) is an example of backscattered electron image (BEI) of the non-metallic inclusion (red circle of Figure 1 b)) analyzed with the FIB-SEM / EDS. An image reconstructed from BEIs is shown in Figure 2 a). 3D reconstructed images of the non-metallic inclusions and interspace in the non-metallic inclusions extracted by volume rendering are shown in Figures 2 b) and c), respectively. The size of the analyzed volume was about X: 15μm, Y: 44 μm, and Z: 15 μm. The slice pitch was 100 nm. Each analysis took 20.5 minutes. BEI and EDS elemental maps of oxygen, aluminum, silicon, calcium, titanium, and iron obtained from the 90th slice are shown in Figure 3 a). A superimposed 3D elemental map of oxygen (green) and iron (black) is shown in Figure 3 b). Therefore, grains in the non-metallic inclusion are iron oxide. Figure 3 c) is a superimposed 3D elemental map of aluminum (green), silicon (yellow), calcium (cyan) and titanium (magenta). The elemental distribution in the non-metallic inclusions was clearly observed three dimensionally as shown in Fig.3 c). Elements excepting Fe and O described above in the non-metallic inclusion are unevenly distributed as shown in Fig. 3 c). That is, many oxides containing Al, Si, Ca and Ti are distributed around iron oxide grains. And interspace exists between the oxides.

  The back side of a Japanese sword is comprised of core steel. Unlike the blade known as edge steel, core steel is not heavily forged. Therefore, the non-metallic inclusions in the back side of the Japanese sword keeps the constitution of raw materials in some degree. And the shape of the non-metallic inclusion also keeps and remains the same as the original material. Interspaced distances between inclusions also remain the same. As to the distribution of the elements, many iron oxides of several micrometers were encapsulated by the non-metallic inclusions comprised of Al, Si, Ca and Ti oxides. The titanium distribution in the non-metallic inclusions suggests that the source of the iron sand contained an iron titanate; for example, ilumenite (FeTiO3).

 

References

[1] M. Kitada, Fine structures of a Japanese Sword Fabricated in the Late Muromachi Era (16th Century)，　Uchida-Roukakuho   Tokyo (2008) 27-36. p. 317

[2] M. Kitada，Microstructure of Sword Fabricated in Europe in 17-18th Century， Bulletin of The Faculty of Fine Arts, Tokyo University of The Arts, (2012) 37-50.

[3] H. Matsushima, M. Kitada and G. Brunetti. MCM12 (2015) p. 531

Hideki MATSUSHIMA (TOKYO, Japan), Masahiro KITADA, Minoru SUZUKI, Toshiyuki KANAZAWA

08:00 - 18:15 #6446 - MS08-911 Laser induced yellowing of stonework: a combined TEM imaging and STEM-EELS study on model samples.

MS08-911 Laser induced yellowing of stonework: a combined TEM imaging and STEM-EELS study on model samples.

Nd-YAG Q-Switched laser devices operating at 1064 nm have been considered in the 1990s as the most promising tool for cleaning stone sculptures, and more particularly eliminating indurated black gypsum crusts. However, the spreading of this laser technology has been undermined because of the yellow hue it occasionally conveys to the cleaned surfaces as seen on Figure 1. Especially in France, this yellowing effect is considered as a major esthetic issue by conservators and the laser technique has gradually disappeared from the restoration sites. This discoloration issue remains partly unexplained: a currently admitted hypothesis states that the iron containing compounds present in the black crusts would transform, upon laser irradiation, into yellow iron-rich nanophase(s) that would re-deposit on the cleaned substrate. To verify this hypothesis, model black crusts have been elaborated by mixing hematite α-Fe₂O₃ and gypsum CaSO₄.2H₂O in different proportions. The model crusts were irradiated using a Nd-YAG QS laser resulting in ablation of numerous particles in a visible smoke and the color of the samples shifted instantaneously from red to a bright yellow. Transmission electron microscopy (TEM) has been used to characterize the morphology of the nanostructures generated by the laser, both in the smoke and on the surface of the samples as observed in Figure 2. In addition, the chemical composition of the neo-formed nanophases was determined by aberration corrected scanning transmission electron microscopy in combination with electron energy loss spectroscopy (STEM-EELS) and high angle annular dark field imaging (HAADF) as seen on Figure 3. It was found that both the surface of the samples and the ablated micro-materials are covered by an irregular nano-film and by dispersed spherical nanoparticles, all containing iron and oxygen. These results ascertain the link between the yellowing effect and the presence of iron containing nanophases after irradiation.

Marie GODET (Paris), Nicolas GAUQUELIN, Véronique VERGÈS-BELMIN, Christine ANDRAUD, Mandana SAHEB, Judith MONNIER, Eric LEROY, Julie BOURGON, Johan VERBEECK, Gustaaf VAN TENDELOO

08:00 - 18:15 #6853 - MS08-912 Imaging and characterization of HgS nanoclusters in natural organic matrix: challenges and results.

MS08-912 Imaging and characterization of HgS nanoclusters in natural organic matrix: challenges and results.

Methylmecury is the environmental form of neurotoxic mercury that is biomagnified in the food chain. Methylation rates are reduced when the metal is sequestered in crystalline mercury sulfides or bound to thiol groups in macromolecular natural organic matter. Mercury sulfide minerals are known to nucleate in anoxic zones, by reaction of the thiol-bound mercury with biogenic sulfide, but not in oxic environments. However, recent work, using indirect characterization methods such as EXAFS spectroscopy, provided evidence that mercury sulfide forms from thiol-bound mercury alone in aqueous dark systems in contact with air. To confirm such new result, proof of mercury sulfide precipitation using direct characterization methods was needed. Such direct imaging, using TEM techniques, was however challenging due to the scarce occurrence and tininess of mercury sulfide particles in heterogeneous an beam-sensitive natural soil samples. First results of HRTEM imaging of HgS nanoclusters of particles in Hg-contaminated humic acid from Elliott reference soil, combined with diffraction technique to assess the of HgS polymorph, will be presented. Specific challenges due to the nature of the samples will be discussed.

Anne-Claire GAILLOT (NANTES Cedex 3), Cyprien LEMOUCHI, Martine LANSON, Alain MANCEAU, Kathryn L. NAGY

08:00 - 18:15 #6906 - MS08-913 3D study of pore space morphology of the shales.

MS08-913 3D study of pore space morphology of the shales.

       One of the goals of gas and oil shales study is the analysis of multiscale porosity. Pores are located in the areas, containing organic compounds, particularly kerogen, and in the mineral matrix. The analysis of porosity is required for the characterization of geological cores and development of hydrocarbon extraction methods. The parameters, which have to be determined, are pore size distribution, total pores volume and their connectivity. Different methods are used for these investigations and the most popular are the mercury intrusion porosimetry and the X-ray tomography. However, these methods are limited in space resolution and do not reveal completely the pore space at the micro- and nanoscale. Nowadays one of the frequently used methods of the porous material characterization is the scanning electron microscopy (SEM) combined with the focused ion beam (FIB), so-called “slice-and-view” method: it allows to reconstruct the three-dimensional (3D) microstructure of a sample, in particular its porous space, by successive imaging (by SEM) and slicing (by FIB) the object. An important step of 3D reconstruction is the SEM images segmentation, which allows identifying pores (images binarization). Often this task is complicated due to peculiarities of SEM images contrast formation and specific appearance of pores in SEM images. The solution of these problems and completion of all processing steps results in generation of pores surface model and that allows to get quantitative characteristics of pore space, including the connectivity of pores by skeletonization of the internal pore space.

       The study of pore space was performed on a number of shales from different areas of Bazhenov formation and these samples were in various katagenesis stages. Bazhenov formation is one of the largest Russian shale formation (Western Siberia) with unconventional hydrocarbon reserves, formed by sediments of the seabed in the late Jurassic and early Cretaceous period. The composition of the Bazhenov formation rocks is characterized by a large volume fraction of organic matter, in which the kerogen dominates. Other shales components, determined by X-ray diffraction (XRD), SEM and energy-dispersive X-ray spectroscopy, were silica minerals series (e.g., quartz), carbonates, clay minerals and pyrite. Moreover, the components of the shales, namely mineral component and organic compounds, contain pores of different types, which sizes vary over a wide range (nano- and micrometer ranges), and their distribution is nonuniform.

       Helios (FEI, USA), Scios (FEI, USA) and Versa 3D (FEI, USA) DualBeam (FIB/SEM) systems with the registration of secondary electrons (SE) and back-scattered electrons (BSE) were used in this study. Avizo and Amira software (FEI, USA) was used for image processing, 3D reconstruction and analysis.

       3D reconstruction of one of the shale samples is shown in Fig. 1. The overall microstructure consists of three main components: mineral matrix, pore-kerogen space and pyrite inclusions. This sample has a high concentration of the kerogen at a high katagenesis stage (MK₃) and, therefore, high density of pores of different sizes. The pores sizes in the kerogen were in the range of 0.01 µm to 0.5 µm and in cross-sections they looked mostly roundish. More irregular pores were in mineral matrix and typically they were larger in size. The automatic segmentation by threshold level in this case was not always possible. The automatic segmentation of pores was challenging and, therefore, an advanced image processing, including image filtering, combined with more complicated segmentation methods were used. The result of 3D reconstruction of the pore space in the kerogen is presented in Fig. 2. Pores volume distribution was calculated on the basis of the 3D binarized image dataset. Next procedure was the skeletonization of pore space and the estimation of pores connectivity. After 3D reconstruction it was easy to estimate the volume fractions of the kerogen, pyrite and mineral matrix in the sample.

 

       This work was supported by Ministry of Education and Science of the Russian Federation under the contract RFMEFI58114X0008.

Alexey MIKHUTKIN, Evgeniy PICHKUR, Igor KARATEEV, Mikhail SPASENNYKH, Alexander VASILIEV (Moscow, Russia)