Ultra-fast interfacial charge transfer probed using a core-hole clock implementation of resonant inelastic x-ray scattering (RIXS)

Ultra-fast electron transfer between a molecule and a surface to which it is coupled plays a key role in light-harvesting devices such as dye-sensitised solar cells and water-splitting photoelectrochemical cells (the model water splitting photoanode shown in figure 1 being a prime example). An elegant way to probe these charge transfer processes, which typically happen on the low femtosecond timescale is the use of resonant core-level spectroscopy in the form of resonant photoemission (RPES). This non-radiative core-hole decay technique relies on monitoring the photoelectrons emitted when a resonantly excited electron participates in the core-hole decay. But these have a very limited escape depth so the technique can only be applied to systems where the charge transfer interface is the surface. In principle, the photons emitted during core-hole decay carry the same information. The corresponding radiative technique is known as resonant inelastic x-ray scattering (RIXS).

The aim of this proposal is to realise a novel core-hole clock implementation of resonant inelastic x-ray scattering (RIXS) to probe the electron transfer from specific unoccupied molecular orbitals of a dye molecule into the conduction band of a surface to which it is adsorbed. Using this method we should be able to probe charge transfer dynamics on the timescale of the core-hole lifetime (a few femtoseconds) in the same way as resonant photoemission (RPES), only here we are using a photons-in, photons-out technique. This approach is a very promising route to probing ultra-fast charge transfer in realistic systems where the charge transfer interface of molecular devices such as solar cells are typically buried by a transport layer or electrolyte. The series of synchrotron experiments described in this overseas travel grant proposal aim to obtain the data for a definitive reconciliation of the core-hole clock implementations of both RPES and RIXS, and the application of the latter to a buried charge transfer interface.

The proposed research will provide a definitive demonstration of a core-hole clock (CHC) implementation of resonant inelastic x-ray scattering (RIXS) which would provide access to femtosecond charge transfer dynamics at a wide range of interfaces due to much larger mean free path of soft x-ray photons compared with photoelectrons. This will have a direct impact on the research of molecular solar cells, batteries and photoelectrochemical water splitting as it will provide a tool for studying and optimising the charge transfer dynamics at the buried interfaces of real systems. This is fundamentally important because the efficiency with which an excited electron is transferred from the donor to the acceptor (these terms used in the most general sense) the great the efficiency of the device, its stability and overall performance. Other factors play a role of course, but charge transfer lies at the very heart of the functionality.

The project will also assist in the training of three PhD students from whose research projects relate to solar water splitting, dye-sensitised solar cells, and hydrogen generation respectively. At least one PhD student will join the team for each beamtime to be trained in synchrotron-based soft x-ray spectroscopy of technologically important materials, and the advanced technique of resonant inelastic x-ray scattering (RIXS). These individuals, trained to pursue innovative applications of existing techniques and drive the development of new ones will help ensure that the UK maintains its position in advanced synchrotron techniques.