• Identification interfacial states for direct bandgap semiconductor surfaces of GaAs by developing a novel broadband ESFG. These unique surface spectroscopy approaches will potentially help us understand the interfacial behaviors of other inorganic and organic semiconductors for solar energy conversion.

​

• Tracking surface states dynamics of semiconductor surfaces of GaAs.  The kinetic processes at the GaAs surface include both the population and recombination of the surface states after photoexcitation, in addition to the build-up of the surface-photovoltage (SPV). The SPV competes with the population of the surface states on the p-type GaAs surface.

​

• Revealing surface dark states and their interactions of the surface states of GaAs surfaces.  We have demonstrated surface dark states and their interactions of the surface states at p-type GaAs (001) surfaces with the 2D-ESFG and 2D-ESHG techniques.

​

• Interfacial charge transfer of nanomaterials and semiconductors. We have employed time-resolved IR spectroscopy to understand several interfacial systems of nanoplatelet Ag/TiO2.  The fast injection time is 13.1 ± 1.5 fs, making Ag nanoplatelets a preferred photosensitizer for wide bandgap semiconductors.The platelet has a much larger surface to bulk ratio and affords a much larger surface area for direct contact with the semiconductor. These two factors facilitate strong Ag–TiO2 coupling.

​

• Interfacial charge transfer of 2D nanomaterials, titanium carbide MXene and semiconductor, TiO2 nanorod. Hot electrons are excited only from MXene upon photon absorption at wavelengths far below the TiO2 band gap. The strong electronic coupling between MXene and TiO2 is due to their proximity, and the resulting interactions are likely responsible for the fast electron transfer in the composites. Our results demonstrate a potential of 2D MXene materials in plasmonic applications and provide new insights into the design of MXene-based photocatalysts.

​

• Anisotropic singlet fission of single crystalline hexacene. The two essential steps of singlet fission are the formation of a correlated triplet pair and its subsequent quantum decoherence. We have examined both essential steps in single crystalline hexacene and discovered remarkable anisotropy of the overall singlet fission rate along different crystal axes. The distinct quantum decoherence rates are ascribed to the notable difference on their associated energy loss according to the Redfield quantum dissipation theory.

​

• Vibronic coupling mechanism in singlet fission of pentacene.  We have examined the role of vibronic coupling in singlet fission using polarized transient absorption microscopy and ab initio simulations on single-crystalline pentacene. It was found that singlet fission in pentacene is greatly facilitated by the vibrational coherence of a 35.0 cm–1 phonon, where anisotropic coherence persists extensively for a few picoseconds. This coherence-preserving phonon that drives the anisotropic singlet fission is made possible by a unique cross-axial charge-transfer intermediate state.

​

• Ultrafast energy transfer in singlet fission of hexacene. We have employed transient absorption microscopy to examine dynamical behaviors of triplet excitons. We observed anisotropic recombination of triplet excitons in hexacene single crystals. The triplet exciton relaxations from singlet fission proceed in both geminate and non-geminate recombination. This anisotropy in the triplet−triplet recombination rates was attributed to the interference in the coupling probability of dipole−dipole interactions in the different geometric configurations of hexacene single crystals.