Cathodoluminescence of stacking fault bound excitons for local probing of the exciton diffusion length in single GaN nanowires

We perform correlated studies of individual GaN nanowires in scanning electron microscopy combined to low temperature cathodoluminescence, microphotoluminescence, and scanning transmission electron microscopy. We show that some nanowires exhibit well localized regions emitting light at the energy of a stacking fault bound exciton (3.42 eV) and are able to observe the presence of a single stacking fault in these regions. Precise measurements of the cathodoluminescence signal in the vicinity of the stacking fault give access to the exciton diffusion length near this location

Nonlithographic epitaxial Sn_xGe_(1–x) dense nanowire arrays grown on Ge(001)

We have grown 1-µm-thick Sn_xGe_(1–x)/Ge(001) epitaxial films with 0 < x < 0.085 by molecular-beam epitaxy. These films evolve during growth into a dense array of Sn_xGe_(1–x) nanowires oriented along [001], as confirmed by composition contrast observed in scanning transmission electron microscopy in planar view. The Sn-rich regions in these films dominate optical absorption at low energy; phase-separated Sn_xGe_(1–x) alloys have a lower-energy band gap than homogeneous alloys with the same average Sn composition

Suppression of Octahedral Tilts and Associated Changes of Electronic Properties at Epitaxial Oxide Heterostructure Interfaces

Epitaxial oxide interfaces with broken translational symmetry have emerged as a central paradigm behind the novel behaviors of oxide superlattices. Here, we use scanning transmission electron microscopy to demonstrate a direct, quantitative unit-cell-by-unit-cell mapping of lattice parameters and oxygen octahedral rotations across the BiFeO3-La0.7Sr0.3MnO3 interface to elucidate how the change of crystal symmetry is accommodated. Combined with low-loss electron energy loss spectroscopy imaging, we demonstrate a mesoscopic antiferrodistortive phase transition and elucidate associated changes in electronic properties in a thin layer directly adjacent to the interface

Scanning Transmission Electron Microscopy in a Scanning Electron Microscope: Electron-Beam Broadening, Contamination, and Investigation of ZIF-8 by Correlative Electron Microscopy

Scanning transmission electron microscopy (STEM) at low electron energies in scanning electron microscopes has several advantages: The lower energies (typically ≤ 30 keV) in scanning electron microscopy (SEM) compared to the commonly used energies of 80 keV - 300 keV in STEM imaging yield enhanced contrast for light materials and reduced knock-on damage. The many detectors in a scanning electron microscope enable correlative imaging of surface and bulk properties of the same specimen regions. Scanning electron microscopes are also more readily available in research laboratories due to the lower price compared to classical STEM instruments. Hence, STEM in scanning electron microscopes, abbreviated as STEM-in-SEM, is a viable alternative to high-energy STEM in dedicated STEM instruments for analyses down to the nanometer scale. However, some characteristics of STEM-in-SEM, like contamination and electron-beam broadening in specimens, are more severe at lower electron energies. Both of these issues are quantitatively analyzed in this work. Another part of this thesis is concerned with studying the surface-mounted metal-organic framework (SURMOF) ZIF-8 by STEM and transmission electron microscopy (TEM) at electron energies of 200 keV and 300 keV. The applied techniques include STEM tomography, nanobeam electron diffraction, and energy-dispersive X-ray spectroscopy (EDXS). Because ZIF-8 is highly susceptible to be damaged by electron irradiation, low-dose imaging conditions need to be used to preserve its crystalline structure. In addition, ZIF-8 is usually prepared on bulk substrates, from which it needs to be detached for TEM imaging by a potentially damaging procedure. To turn this preparation step obsolete, ZIF-8 was deposited directly on TEM grids covered by amorphous carbon films in a layer-by-layer synthesis approach. Future studies on SURMOF growth can profit from the TEM-sample preparation technique and correlative microscopy described in this thesis

Could face-centered cubic titanium in cold-rolled commercially-pure titanium only be a Ti-hydride?

A face-centered cubic (FCC) phase in electro-polished specimens for transmission electron microscopy of commercially pure titanium has sometimes been reported. Here, a combination of atom-probe tomography, scanning transmission electron microscopy and low-loss electron energy loss spectroscopy is employed to study both the crystal structural and chemical composition of this FCC phase. Our results prove that the FCC phase is actually a TiHx (x>1) hydride, and not a new allotrope of Ti, in agreement with previous reports. The formation of the hydride is discussed

Optical properties of metallic nanowires by valence electron energy loss spectroscopy

The determination of intrinsic conductivity of nanowires (NWs) is essential to understand the charge transport behaviour involved in hybrid nanocomposites. These high conductive metallic fillers are good candidate to improve electrical properties of composites in aeronautic industry. The main difficulty is often to achieve the combination of both high spatial resolution and information on the physical properties as electrical conductivity. One of the suitable methods to give the desired information is electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy (STEM) mode, especially in the low-loss region. This is demonstrated by studying the nickel and gold nanowire

Growth of one-dimensional Si/SiGe heterostructures by thermal CVD

4 pagesInternational audienceThe first results on a simple new process for the direct fabrication of one-dimensional superlattices using common CVD chambers are presented. The experiments were carried out in a 200 mm industrial Centura reactor (Applied Materials). Low dimensionality and superlattices allow a significant increase in the figure of merit of thermoelectrics by controlling the transport of phonons and electrons. The monocrystalline nanowires produced according to this process are both one-dimensional and present heterostructures, with very thin layers (40 nm) of Si and SiGe. Concentrations up to 30 at.% Ge were obtained in the SiGe parts. Complementary techniques including transmission electronic microscopy (TEM), selected area electron diffraction (SAED), energy dispersive x-ray spectroscopy (EDS), scanning transmission electron microscopy (STEM) in bright field and high angle annular dark field (HAADF STEM), and energy-filtered transmission electron microscopy (EF-TEM) were used to characterize the nanoheterostructures

Sub-100 nanosecond temporally resolved imaging with the Medipix3 direct electron detector

Detector developments are currently enabling new capabilities in the field of transmission electron microscopy (TEM). We have investigated the limits of a hybrid pixel detector, Medipix3, to record dynamic, time varying, electron signals. Operating with an energy of 60keV, we have utilised electrostatic deflection to oscillate electron beam position on the detector. Adopting a pump-probe imaging strategy we have demonstrated that temporal resolutions three orders of magnitude smaller than are available for typically used TEM imaging detectors are possible. Our experiments have shown that energy deposition of the primary electrons in the hybrid pixel detector limits the overall temporal resolution. Through adjustment of user specifiable thresholds or the use of charge summing mode, we have obtained images composed from summing 10,000s frames containing single electron events to achieve temporal resolution less than 100ns. We propose that this capability can be directly applied to studying repeatable material dynamic processes but also to implement low-dose imaging schemes in scanning transmission electron microscopy.Comment: 11 pages, 6 figures; improve ref formatting + revise tex