L'origine de l’activité catalytique remarquable des NPs Au demeure mal définie, en particulier celle des NPs inférieures à 5 nm [1]. Cette activité est généralement attribuée aux atomes sous-coordinés d’arêtes et de coins de nanoparticules de formes cristallines supposées statiques et parfaites dans les conditions de réaction. Dans cette contribution, en étudiant à l’échelle atomique la dynamique structurale sous H2 de NPs d’Au de taille inférieure à 7 nm par MET in situ, nous montrons que cette vision est erronée pour les NPs « ultrapetites » (~3 nm), avec la mise en évidence de la grande mobilité des atomes d'or à la fois en surface et en volume.

La figure 1 montre l’évolution structurale de deux NPs Au supportées sur anatase TiO2, l’une petite (~4,5 nm) et l’autre ultrapetite (~3 nm), initialement sous 40 Pa d’Argon à 400°C , puis pendant le refroidissement de 400 à 25 °C sous 105 Pa de H2. Les deux NPs ont une structure CFC et une morphologie de type octaèdre tronqué sous Ar (Fig 1.a et d). Sous H2, la petite NP d’Au conserve sa structure cristalline, sa morphologie et son orientation par rapport au support, même jusqu’à 25°C (Fig 1.b et c). A l’inverse, l’ultrapetite NP d’Au évolue vers une structure présentant un ordre icosaèdrique dès l’introduction de H2 à 400°C (Fig 1.e et f). La caractérisation MET ex situ des NPs Au traitées sous H2 dans un réacteur à lit fixe dans des conditions de pression et de température proches des observations in situ confirme la dépendance en taille de la réactivité des NPs Au sous H2 (Fig 2). Si les NPs de taille supérieure à 4 nm présentent très majoritairement une structure CFC (Fig 2.a et c), les structures des NPs de taille inférieure à 4 nm sont différentes de CFC et variables (principalement des structures icosaèdriques et déformées) (Fig 2.e-h). Les observations MET in situ et ex situ concordantes ont été corroborées par des études de dynamique moléculaire ab initio qui seront également présentées.

[1] Catalysis by Gold G.C. Bond, C.Louis, D.Thompson, Imperial College Press, 2006.

In plasma-enhanced chemical vapour deposition (PECVD), the radicals and atoms generated by the plasma have the effect, among others, of changing the surface energies of the growing materials, which allows one to prepare objects that would not stabilise in other conditions. This is the case of the Sn-Cu-catalysed Si nanowires (NWs) we present in this talk: without plasma, the liquid Sn, used by the vapour-liquid-solid mechanism [1], is so unstable that no growth is allowed, while in presence of the H atoms generated by the plasma,  the Sn droplet stabilises and growth can start [2]. Yet, this method is up to now the only way of growing 2H SiNWs (a metastable polytype with interesting optical properties [3]) (Fig. 1) [4]. The goal of our present in-situ study is to understand how this metastable phase can be preferred to the stable one (the 3C polytype).

However, no electric field is compatible with HRTEM, which forbids plasma under the beam. To circumvent this incompatibility, we implemented, on the H2 line of our in-situ TEM (“NanoMAX”, a modified Thermo Fisher Titan Environmental TEM), an electron-cyclotron-resonance plasma source (Aura-wave from SAIREM), to remotely generate the H atoms necessary for the growth. Thanks to this unique setting, we obtained Sn-Cu-catalysed SiNWs in-situ, with 2H regions (Fig. 2) [5]. 

The talk presents the effect on NW growth of switching the plasma on and off, and shows movies of the nucleation of the Si 2H metastable polytype recorded at atomic resolution.

Acknowledgements:

This work was funded by the ANR: TEMPOS-NanoMAX (ANR-10-EQPX-50); HexaNW (ANR-17-CE09-0011). We kindly thank P. Roca i Cabarrocas, M. Foldyna (LPICM), F. Glas, F. Panciera (C2N), for fruitful discussions, and also the CIMEX at École polytechnique.

References:

[1] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964), p. 89

[2] S. Misra, et al., J. Phys. D: Appl. Phys. 47 (2014), p. 393001

[3] M. Amato, et al., Nano Lett. 16 (2016), p. 5694

[4] W. Wang, PhD thesis, Institut Polytechnique de Paris, École polytechnique (2021)

[5] É. Ngo, PhD thesis, Institut Polytechnique de Paris, École polytechnique (2021)

The advance of fast pixelated detectors has allowed performing 4D-STEM, where by measuring the shift of the transmitted beam, the electric field distribution inside the sample can be accessed, as demonstrated for p-n junctions in lamella specimens prepared from bulk or thin-film semiconductor samples [1-3]. However, the detection of the transmitted beam can be affected by diffraction contrast. In thin semiconducting nanowires (NWs) a rotation of the crystal is often present and this change in orientation of the crystal across the NW modifies the diffraction condition, potentially affecting the algorithms used to detect the beam shift. 

Here, we present in-situ biasing 4D-STEM experiments to study the electrical properties of n-i-p/p+n+/n GaN NW photodetectors grown by MBE [4]. We show that artefacts due to crystal rotation in the NWs can be reduced or removed by a combination of in-situ biasing, different algorithms to detect the beam shift, and orienting the sample off-axis to reduce diffraction contrast. We were able to measure electric fields on the order of few MV/cm in a tunnel junction (p+/n+) with high spatial resolution, consistent with doping levels around 〖10〗^19 〖cm〗^(-3). The electric field profiles are compared to 3D finite element calculations and electron beam induced current (EBIC) maps, which also reflect the internal field distribution. We measure the current-voltage (IV) response of the NW at different stages of the experiment to better understand the effect of the electron beam exposure on the electrical properties.

This work illustrates the potential of 4D-STEM measurements combined with in-situ biasing to reveal the internal electric field quantitatively, which is linked with the electrically active doping concentrations and surface band bending. These properties are otherwise difficult to access in semiconductor NWs at nm length scales, and are crucial for development of new devices with carefully designed electrical properties.

Acknowledgements: ERC, CNRS, CEA

References:

[1] N. Shibata et. al., Scientific Reports 5 (2015), 10040.

[2] L. Bruas et. al., Journal of Applied Physics, 127 (2020), 205703.

[3] A. Beyer et. al., Nano Letters, 21 (2021), 2018-25.

[4] S. Cuesta et al. Nano Letters, 19 (2019), 5506-5514.

In this contribution, we will present results on the optical and the structural properties of a monolayer of semiconducting transition metal dichalcogenide (TMD) encapsulated with hexagonal boron-nitride (h-BN), at the tens of nanometers scale. In this work we the correlate optical spectra at the nanoscale with structural and chemical maps. In this way, we connect the information usually available separately from optical diffraction-limited techniques, such as photoluminescence (PL) [1], and from high-resolution techniques, such as electron microscopy [2] or scanning tunneling microscopy [3].

To achieve this, we used combined high spatial and spectral resolution techniques in a scanning transmission electron microscope (STEM), such as illustrated in Fig. 1. Electron energy loss spectroscopy (EELS) was used to obtain optical absorption (low-loss range) and chemical analysis (core-loss range). Cathodoluminescence (CL), a nanoscale coun-terpart of PL [4], was used to measure light emission at the tens of nanometers scale. The two types of spectral data were acquired in the same regions. This gave access to, for example, the local Stokes shift (SS), which is the difference between absorption and emission energies. This quantity gives information about the physical mechanism in-volved in the absorption and emission processes [1]. In addition to these spectral meas-urements, spatial structural information was gathered using either atomically-resolved imaging or diffraction mapping for local strain measurements. Moreover, chemical maps were measured using core-loss EELS.

Using these combined techniques, we detected spatially localized emission from excitonic complexes in WS2 monolayers maintained at 150 K, with bright spots as small as few tens of nanometers (Fig. 2). The presence of such bright spots was linked to the chemi-cal environment, displaying small patches of contaminants (such as C, O, N, or Si), prob-ably coming from sample preparation [5].

References
[1] Kolesnichenko et al. 2D Mater. 7 025008 (2020)
[2] Chang et al. Appl. Miscosc. 49, 10 (2019)
[3] Schuler et al. Sci. Adv., 6, eabb5988 (2020)
[4] Kociak and Zagonel, Ultramicroscopy, 176, 112-131 (2017)
[5] Bonnet et al. arXiv :2102.06140 [cond-mat.mes-hall]

Today's progress in semiconductor technology and electronic device miniaturization, necessitate the exact estimation of electrostatic properties like local electric field and electronstatic potential to enhance the performance and functionality of nano-devices. Though, the direct observation and precise measurements of local electric fields are still quite challenging, 4D scanning transmission electron microscopy (4D STEM) technique is known as a sensitive detection tool to map the internal electric fields arising from nanoscale features like p-n junctions [1-4]. Moreover, the development of fast pixelated detectors based on direct electron detection technology, has improved the feasibility of 4D STEM through their higher acquisition speed, high dynamical range and fast readout. 

In this work, we aim to map the internal electric field of highly doped (1019 atom/cm3) Ge p-n junction nanowires prepared by gold catalyzed chemical vapour deposition (CVD). 4D-STEM measurements with careful tuning of the parameters, with/without precession and in different crystal orientation, are applied to get the maximum spatial resolution, necessary for the built-in electric field detection in the junction. Also, for the first time, in-situ electrical measurements are exploited to monitor the I-V characteristics of the nanowires during different steps of the 4D STEM experiments to verify if the system is electrically modified during 4DSTEM data acquisition. Moreover, reverse and forward biased data are acquired to separate the contribution of electrical characteristics and the ones due to artifacts in the reconstructed data. MerlinEM fast pixelated detector is used for STEM data acquisition. Post-acquisition data analysis is performed by Python and MATLAB based scripts to reconstruct the STEM images from the data sets and investigate the effect of experimental parameters. Moreover, scanning electron microscopy (SEM) based Electron beam Induced current (EBIC) is also used to verify the position of the junction and compared with the STEM data. Also, the data from EBIC can be exploited to acquire higher resolution data by 4D STEM, focusing on the small region of the nanowire containing the field. Different methods for data analysis, including Center of Mass (CoM) and template matching (TM) is applied on the data and pros and cons of each method will be investigated.

Ferrofluids, which are suspensions of magnetic nanoparticles in a liquid, have many current and potential applications in the biomedical field (e.g. improved MRI agents, magnetic hyperthermia) and in the technological industry (e.g. permanent magnets, magnetic memory). When the nanoparticles have a large enough magnetic moments, magnetic dipole interactions give rise to different assemblies of nanoparticles. Indeed, theoretical simulations [1] have predicted the existence of nanoparticles assembled in chains, rings, or a combination of both. These assemblies modify the macroscopic properties of the ferrofluid significantly. Thus, in order to achieve a better understanding of these materials, we aim to study the correlation between the magnetic properties and the self-organisation. To that goal, we have used Cryogenic Transmission Electron Microscopy (Cryo-TEM) and Electron Holography to study a suspension composed of nanoflowers of cobalt ferrite with 25nm diameter, which are known to present a large magnetic anisotropy and their ferrofluidic solution in water have shown efficient magnetic hyperthermia [2]. 

Cryo-TEM allows visualising the self-organisation of the ferrofluid. By rapidly plunge-freezing a drop of the sample in liquid ethane, we show the theoretically predicted assemblies (fig. 1) and establish the statistical occurrence of each shape. Several assemblies were further investigated by Electron Holography, a TEM-based technique, that allows retrieving the magnetic induction at the nanoscale by interfering the transmitted electron beam with a reference beam and measuring the phase shift. We quantitatively mapped the in-plane magnetic induction within several assemblies and deduced the magnetization orientation of each particle (fig 2.). While dipolar magnetic interactions are expected to align dipolar moments top to tail within a chain, we show that the relative orientation of particle magnetization varies from parallel to anti-parallel configuration, suggesting a significant contribution of the particle’s magnetic anisotropy. This contribution cannot, therefore, be neglected when performing Monte-Carlo simulations of magnetic properties for ferrofluids presenting such a large magnetic anisotropy.