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Register a new userA Synchrotron in the TEM: Spatially Resolved Fine Structure Spectra at High Energies
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Fine structure analysis of core electron excitation spectra is a cornerstone characterization technique across the physical sciences. Spectra are most commonly measured with synchrotron radiation and X-ray spot sizes on the {mu}m to mm scale. Alternatively, electron energy loss spectroscopy (EELS) in the (scanning) transmission electron microscope ((S)TEM) offers over a 1000 fold increase in spatial resolution, a transformative advantage for studies of nanostructured materials. However, EELS applicability is generally limited to excitations below ~2 keV, i.e., mostly to elements in just the first three rows of the periodic table. Here, using state-of-the-art EELS instrumentation, we present nm resolved fine structure EELS measurements out to an unprecedented 12 keV with signal-to-noise ratio rivaling that of a synchrotron. We showcase the advantages of this technique in exemplary experiments.
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High-resolution TEM (HRTEM) is a powerful tool for structure characterization. However, methylammonium lead iodide (MAPbI3) perovskite is highly sensitive to electron beams and easily decompose into lead iodide (PbI2). Universal misidentifications that PbI2 is incorrectly labeled as perovskite are widely exist in HRTEM characterization, which would negatively affect the development of perovskite research field. Here misidentifications in MAPbI3 perovskite calibration are summarized, classified and corrected based on corresponding electron diffraction (ED) simulations. Corresponding crystallographic parameters of intrinsic tetragonal MAPbI3 and the confusable hexagonal PbI2 are also presented clearly. Finally, the method of proper phase identification and some ways to control the radiation damage in HRTEM are provided. This work paves the way to avoid misleadings in HRTEM characterization of perovskite and other electron beam-sensitive materials in the future.
We present the direct measurements of magnetoexciton transport. Excitons give the opportunity to realize the high magnetic field regime for composite bosons with magnetic fields of a few Tesla. Long lifetimes of indirect excitons allow the study kinetics of magnetoexciton transport with time-resolved optical imaging of exciton photoluminescence. We performed spatially, spectrally, and time-resolved optical imaging of transport of indirect excitons in high magnetic fields. We observed that increasing magnetic field slows down magnetoexciton transport. The time-resolved measurements of the magnetoexciton transport distance allowed for an experimental estimation of the magnetoexciton diffusion coefficient. An enhancement of the exciton photoluminescence energy at the laser excitation spot was found to anti-correlate with the exciton transport distance. A theoretical model of indirect magnetoexciton transport is presented and is in agreement with the experimental data.
A systematic study of the impact of annealing on the electronic properties of single InAs/GaAs quantum dots (QDs) is presented. Single QD cathodoluminescence spectra are recorded to trace the evolution of one and the same QD over several steps of annealing. A substantial reduction of the excitonic fine-structure splitting upon annealing is observed. In addition, the binding energies of different excitonic complexes change dramatically. The results are compared to model calculations within eight-band k.p theory and the configuration interaction method, suggesting a change of electron and hole wave function shape and relative position.
One of the puzzling aspects of high temperature superconductors is the prevalence of magnetism in the normal state and the persistence of superconductivity in very high magnetic fields. Generally, superconductivity and magnetism are not compatible. But recent neutron scattering results indicate that antiferromagnetism can appear deep in the superconducting state in an applied magnetic field. Magnetic fields penetrate a superconductor in the form of quantized flux lines each one representing a vortex of supercurrents. Superconductivity is suppressed in the core of the vortex and it has been suggested that antiferromagnetism might develop there. To address this question it is important to perform electronic structural studies with spatial resolution. Here we report on implementation of a high field NMR imaging experiment that allows spatial resolution of the electronic behavior both inside and outside the vortex cores. Outside we find strong antiferromagnetic fluctuations, and localized inside there are electronic states rather different from those found in conventional superconductors.
Diffraction Anomalous Fine Structure (DAFS) spectroscopy uses resonant elastic x-rays scattering as an atomic, shell and site selective probe that gives information on the electronic structure and the local atomic environment as well as on the long range ordered crystallographic structure. A DAFS experiment consists of measuring the Bragg peak intensities as a function of the energy of the incoming x-ray beam. The French CRG (Collaborative Research Group) beamline BM2-D2AM (Diffraction Diffusion Anomale Multi-longueurs donde) at the ESRF (European Synchrotron Radiation Facility) has developed a state of the art energy scan diffraction set-up. In this article, we present the requirements for obtaining reliable DAFS data and report recent technical achievements.
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