Research

Ternary oxides are widely used to fabricate photoelectrodes for their chemical stability, low cost, and tunable optoelectronic properties. Nevertheless, in-depth examinations of their polaronic transport are hampered by the polycrystalline nature of most oxide thin films. We developed an economical approach to deposit highly epitaxial films of ternary metal oxides, which reduce the grain boundary density and enable the anisotropic dynamics to be investigated.

The ionic bonding character in metal oxides gives electrons the tendency to self-trap on specific metal cations, induce distortion to the surrounding lattice, and form the so-called small polarons. We use transient absorption spectroscopy to track the temporal evolution of these polaronic states using different excitation energies. We also perform high-resolution resonant inelastic X-ray scattering (RIXS) to directly observe the shift of phonon energies upon small polaron formation.

Given the lower electronegativity of nitrogen compared to oxygen, the chemical bondings in transition metal nitrides have a greater extent of covalency that should improve the charge transport. However, semiconducting metal nitrides remain relatively underexplored due to the challenges in material synthesis. Utilizing reactive sputter deposition, we prepare a growth environment with highly reactive nitrogen atoms from which metastable nitride thin films can be grown.

Halide perovskites (ABX3) exhibit strong light absorption and can be easily synthesized by solution-based methods. We are interested in the influences of intentionally incorporated defects on the optoelectronic properties and the relative phase stabilities. For instance, excitons formed by optical excitation can disrupt local structural ordering and create self-trapped exciton (STE) states.  Understanding the interplay between electrons and the lattice will expand the application prospects of these emerging functional materials.

Although photoelectrochemical and photocatalytic methods were originally developed for water-splitting and solar fuel production, they have gained attention recently for their applicability in synthetic chemistry. Not only can metal oxides replace traditional organometallic catalysts, they sometimes also provide unique reaction selectivity. To realize the full potential of photoelectrochemistry, we aim to resolve the electron transfer kinetics and the reaction mechanism on the photoelectrodes surfaces.