Quentin Ramasse

SuperSTEM Laboratory, Daresbury, UK

Abstract

Title: Pushing the limits of electron energy loss spectroscopy for materials characterisation: From phonons to core losses in real and momentum spaces

A new generation of monochromators has recently pushed the energy resolution of (scanning) transmission electron microscopes into the sub 20meV range. In addition to the obvious increase in resolution which has made exploring the phonon region of the electron energy loss spectrum (EELS) possible, the flexibility of these instruments is proving advantageous for materials science investigations. The energy resolution, beam current and optics can be adjusted seamlessly within a greatly increased range. This contribution will illustrate these possibilities through a number of applications, such as a highly promising A-site deficient Nd2/3TiO3 based perovskite system. Using monochromated core loss EELS it is for instance possible to map individual components of the Ti L2,3 edge at atomic resolution and relate them to structural distortions of great importance for the material’s properties. Furthermore, it is now possible to routinely observe low energy vibrational signals from a variety of materials. Although the physical origin of these signals is very similar to that giving rise to low-energy phonon vibrations in neutron or inelastic X-ray scattering, differences in experimental geometries and selection rules, among other factors, have made the interpretation of phonon spectra in the STEM challenging. A methodology was therefore developed to calculate phonon spectra in a momentum-resolved STEM EELS experiment. This approach promises to provide data akin to triple-axis spectroscopy in neutron scattering, with the additional advantages of the higher spatial resolution and ability to correlate with chemical analysis.

Biography

Quentin Ramasse is the Director of the SuperSTEM Laboratory, the UK National Facility for Advanced Electron Microscopy, and holds a joint Chair in Electron Microscopy at the Schools of Physics and of Chemical and Process Engineering, University of Leeds. He obtained his Ph.D. from the University of Cambridge working on aberration measurements methodologies for STEM. Before his post at SuperSTEM he held a Staff Scientist position at the National Center for Electron Microscopy in Berkeley. He has published extensively in the field of STEM-EELS, with a dual focus on technique development and on applications to a wide range of energy harvesting materials.