Vibrational and THz spectroscopy for optimising organic co-crystal semiconductors

Technologies from organic field effect transistors, to photovoltaics, light emitting diodes, and photonic applications (e.g. masers) depend on organic molecular crystals. These crystals, arising from van der Waals interactions between molecules in the solid state, exhibit exceptional optoelectronic properties. Importantly, organic semiconductors are compatible with a shift toward materials with sustainable chemical feedstocks and also promise lightweight and mechanically flexible, wearable devices. Co-crystals consist of two or more molecular units within a single crystal phase. Modifications in the formation and structure of a co-crystal can be used to improve physical properties with applications already demonstrated in explosives, pigments and pharmaceuticals. In organic semiconductors, co-crystals offer an exciting route to control intermolecular charge-transfer effects. Yet, there remain no general microscopic models of charge transport in these versatile chemical systems. The critical role of phonons-quantized collective crystal excitations-in governing charge transport is unresolved.

The project aim will be to unveil phonon effects in organic co-crystals by applying a range of vibrational spectroscopic techniques including terahertz (THz), infrared, Raman and inelastic neutron and X-ray scattering as well as new, emerging nanoscale vibrational spectroscopy using electron energy loss spectroscopy (feasible at only a few facilities worldwide) in tandem with ab initio calculations using density functional theory (DFT) for precise interpretation of experimental spectroscopy. The student will use solution-growth or liquid assisted grinding techniques to prepare co-crystals of organic semiconductors, using work on co-crystals of stilbene/benzene derivatives or perylene/ tetracyanoquinodimethane as starting points. Electron microscopy and electron and X-ray diffraction will be used to characterise the crystalline structures and to also examine evidence of phonon dynamics within the diffuse scattering. Together, these advanced techniques will explore phonon contributions to charge transport and the physical chemistry of co-crystals exhibiting strong electron-phonon coupling.

The student will join trips to SuperSTEM, the EPSRC National Facility for Advanced Electron Microscopy (Cheshire, UK) and to other national facilities (e.g. Diamond Light Source, Oxfordshire, UK) in addition to facilities in the Leeds Electron Microscopy and Spectroscopy Centre, including cryo-microscopy. The project will be embedded within a multidisciplinary environment across the School of Chemistry, the School of Chemical and Process Engineering, and the Bragg Centre for Materials Research. Computational developments will take advantage of the Leeds advanced research computing facilities.