Colloidal transition Metal Oxide Semiconductor Nanocrystals

Transition metal oxide semiconductors are a richly diverse class of materials with optical, electronic, chemical, and magnetic properties that vary widely depending on composition, stoichiometry, and crystal structure. These materials are therefore useful in a variety of applications ranging from solar energy conversion  to information storage to thermoelectrics. As with other semiconductors, shrinking transition metal oxide materials down to the nanoscale can cause new or enhanced properties to emerge due to increased surface area-to-volume ratio or quantum confinement effects. However, most methods for synthesizing nanocrystalline metal oxide materials rely on hydrothermal or sol-gel techniques that often result in products with broad distributions in nanocrystal size, shape, and composition and poor colloidal stability. We develop synthetic methods that provide precise control over size, shape, composition, cation distribution and crystal  structure  of metal oxide nanocrystals by tuning precursor, solvent, and ligand chemistry to control the relative rates of the hydrolysis and condensation reactions that govern metal oxide formation in the solution phase.

polarons in photoexcited metal oxide semiconductors

Due to the low dispersion of bands comprised of 3d orbitals, charge carriers in first-row transition metal oxide are prone to localization, specifically in the form of polarons. A polaron is a quasiparticle composed of a charge carrier coupled to a proximal lattice distortion that effectively traps the charge in a local potential well. We use a combination of temperature-and time-resolved spectroscopic measurements and density functional theory computations to study the mechanisms by which polarons form upon photoexcitation of transition metal oxide semiconductors as well as the processes and timescales that govern the decay of these localized excited states. We are particularly interested in the role that thermally populated phonon modes play in mediating photo-induced polaron formation in these systems.

Transition metal oxide nanomaterials as photocatalysts

Due to their high surface area and general redox stability, nanostructured metal oxide semiconductor films can be excellent photocatalysts or photoelectrodes for desirable photo(electro)chemical transformations, such as reduction of carbon dioxide or protons and oxidative degradation of organic pollutants in water. The nanoscopic interface between the metal oxide semiconductor and liquid electrolyte or solvent largely determines the photoelectrochemical behavior of these photoelectrodes, but the structure and chemistry of this interface are poorly understood and therefore difficult to control. We are exploring how the photoelectrochemical and photocatalytic properties of nanostructured and colloidal metal oxide semiconductors depend on the composition and ligand chemistry of their surfaces.