Exploring low energy excitations with electron microscopy

In the last few years advances in electron microscopy hardware have given us the opportunity to study the vibrational properties of materials at the nanoscale. The vibrational properties of crystalline materials, known as phonons, are collective oscillations of the atomic nuclei and control the way heat and sound are transferred through a material. The vibrational properties of molecules are the stretching, bending and rotational modes of the bonds. In both the crystalline and molecular cases, the vibrational properties are characteristic of the bonding in the material. A new generation of electron microscopes are able to combine atomic resolution imaging with high resolution electron energy loss spectroscopy, allowing vibrational spectroscopy at atomic resolution. It allows us to collect vibrational spectra from volumes of material up to 20 orders of magnitude smaller than any other technique. These experimental developments allow us to study how the vibrational modes vary spatially and will benefit materials design and optimisation. The first of this new generation of microscopes in Europe is housed at the UK SuperSTEM facility, which is the EPSRC National Research Facility for Advanced Electron Microscopy.

In order to interpret the new features in the experimental spectra, it is important to develop a robust theoretical framework. This project will combine experimental data from the new SuperSTEM microscope with quantum mechanical simulations to address some of the fundamental questions rising from these new experimental capabilities. It will build on work published earlier this year in Science Advances by Hage, Nicholls et al. where, for the first time, momentum resolved vibrational spectra from crystalline materials were obtained using a transmission electron microscope. This work will go beyond the previous work on crystalline materials and address molecules and functional groups. It will compare theoretical spectra simulated with the crystalline approach with a novel molecular approach developed in this project. Experimental work will be used to guide and test the approximations used in the scattering theory framework. The experiments will be carried out at, and in collaboration with, SuperSTEM. To start with, prototype materials will be used to test the theoretical predications. Once there is a robust approach to simulating and interpreting experimental spectra, this new knowledge will then be applied to cutting-edge materials problems. The aim of this project is to focus on molecular functional groups, and so this approach be used to look at the surfaces of catalyst particles. The particular catalyst particles are used in fuel cells which are a viable way of reducing the greenhouse gas emissions from road vehicles. The catalyst particles are part of the section of the fuel cell that needs optimising before fuel cells can become a mainstream technology.

This project falls within the EPSRC Physical sciences, Energy and Manufacturing the Future research areas.