"Vendredi 09 juillet"

The laser pulses controlling the ion evaporation in Laser-assisted Atom Probe Tomography (La-APT) can simultaneously excite photoluminescence in semiconductor or insulating specimens [1]. An atom probe equipped with approriate focalization and collection optics can thus be coupled with an in-situ micro-photoluminescence (µPL) bench [2] that can be operated even during APT analysis. Our team has recently developed a coupled µPL-APT instrument operating at 400 kHz, controlled by 150 fs laser pulses tunable in energy in a large spectral range (spanning from deep UV to near IR). Micro-PL spectroscopy is performed using a 320 mm focal length spectrometer equipped with a CCD camera for time-integrated and with a streak camera for time-resolved acquisitions. Such a Photonic Atom Probe (PAP) has been applied to the study of the optical properties of nanoscale emitters in an in-situ correlative microscopy approach. The evolution of the PL signal during the APT analysis is an original source of information. In this work we analyzed specimens containing ZnO/(Mg,Zn)O quantum wells (QWs) of different thicknesses, and we show that it is possible to distinguish the optical signatures of separate QWs distant as few as 20 nm – well below the diffraction limit of the laser [3]. This information is then correlated with the chemical 3D distribution obtained by APT. The analysis of the PL spectral shifts during the APT analysis also allows determining the stress state induced by the electrostatic field [4].

[1] Mancini, Lorenzo, et al. ""Carrier localization in GaN/AlN quantum dots as revealed by three-dimensional multimicroscopy."" Nano letters 17.7 (2017): 4261-4269.

[2] Houard, Jonathan, et al. ""A photonic atom probe coupling 3D atomic scale analysis with in situ photoluminescence spectroscopy."" Review of Scientific Instruments 91.8 (2020): 083704.

[3] Di Russo, Enrico, et al. ""Super-resolution Optical Spectroscopy of Nanoscale Emitters within a Photonic Atom Probe."" Nano Letters 20.12 (2020): 8733-8738.

[4] Dalapati, P., et al. ""In Situ Spectroscopic Study of the Optomechanical Properties of Evaporating Field Ion Emitters."" Physical Review Applied 15.2 (2021): 024014.

Transmission Electron Microscopy (TEM) is a widely used technique for atomic scale imaging and chemical analysis of materials [1]. However, the analysis of elements in trace concentrations, low-Z elements and isotopic selectivity are very difficult or impossible using the typical chemical analysis techniques available in a TEM such as Energy Dispersive X-ray Spectroscopy (EDS) or Electron Energy-Loss Spectroscopy (EELS). Atom Probe Tomography (APT) allows 3D nanoscale chemical imaging and it is capable of distinguishing isotopes. However, the analysed volume is small (~ 10^6 nm^3) and the data is prone to artefacts. Specifically, a precise 3D APT reconstruction requires the local morphology of the sample tip at each instance in time which is often lacking. Furthermore, limited mass resolution and differences in the field evaporation of different elements can also introduce artefacts in the APT data. Secondary Ion Mass Spectrometry (SIMS) is a high-sensitivity technique to detect concentrations down to the ppm range. Moreover, all elements and isotopes in the periodic table can be analysed. However, the main limitation of SIMS is that the quantification is difficult because of matrix effects (i.e. strong variations in ionization yields depending on matrix elements). Relatively large volumes (~ 10^3 um^3) are typically analysed in SIMS and the lateral resolution in SIMS imaging is fundamentally limited to ~ 10 nm by the ion-solid interaction volume. To overcome the limitations of the individual techniques, we present a correlative TEM-APT-SIMS method [2] for the quantification of hydrogen, deuterium and other trace elements (dopants) in passivating contact layers used in silicon photovoltaics. The analysis and data treatment methodologies will be discussed in detail and other new methods to quantify SIMS images will also be briefly introduced [3, 4]. 

This work was partially funded by the Luxembourg National Research Fund (FNR) by the grants C18/MS/12661114 (MEMPHIS) and INTER/SNF/16/11536628 (NACHOS).

References:

[1] D. B. Williams & C. B. Carter, Transmission Electron Microscopy, Springer, US 2009

[2] S. Pal et al, Appl. Surf. Sci, 555, 149650, 2021

[3] S. Eswara et al, MRS Comm., 9, 916–923, 2019

[4] L. Yedra et al, J. Anal. Atom. Spectrom., 36, 56-63, 2021

The transition metal oxides exhibit a variety of interesting properties, such as ferroelectricity, superconductivity, etc. The atomic structural degrees of freedom and the electron’s degrees of freedom (charge and spin) lead to various phenomena through phase transitions. All these orders can be studied through collective excitations. By scanning transmission electron microscopy (STEM) with monochromator, the energy resolution can go as low as 5meV [1], and such low-energy excitations can thus be investigated by electron energy-loss spectroscopy (EELS) [2, 3]. 

In this work, we focus on the low-energy electronic excitations, id est, the plasmon and the d-d excitations on the SrVO3 oxides. The SrVO3 is an attractive earth‐abundant transparent conducting oxide. It is also described as a typical correlated metal where low-energy plasmon fluctuation can also leads to an appreciable renormalization of the low-energy band. Nevertheless, the experimental occurrence of plasmons is poorly reported. The observations of plasmonic type d-d excitations (dipolar contribution, strong surface delocalization in aloof beam) on SrVO3 by EELS are shown in Figure 1. Furthermore, the dispersion curve of the bulk plasmon, as measured for different slabs thicknesses, will be presented and discussed with respect to ab-initio calculation. The Figure 2 shows that the nanostructures based on SrVO3 exhibit the Fabry-Perot type surface plasmon modes. Several surface plasmons modes are visible on “rod-type” geometries, such as half-rod or slit and have been compared to the finite-difference time-domain (FDTD) simulation. We will report energy widths of surface plasmon as low as 50 meV in the sub 0.5eV loss range..It suggests that SrVO3 could be one alternative plasmonic materials due to the high quality factor in the near-infrared range, and thereby providing a reference for searching for better plasmonic materials.

References :

[1] O.L. Krivanek et al., Nature 514 (2014), 209.

[2] A. Gloter et al., Ultramicroscopy 109 (2009), 1333.

[3] V. Mkhitaryan et al., Nano letters 21 (2021), 2444.

Figure 1: (a) Disperson curve and (b) bulk/aloof measurements on SrVO3. (c) Dispersion curve of the plasmon peaks.

Figure 2: The Fabry-Perot type d-d based surface plasmon modes in SrVO3 of (a) half-rod and (c) slit nanostructures. (b) FDTD simulation of (a).