Our research focuses on interactions in reduced dimensional and nanoscale systems, particularly the effects of confinement on the interactions between different degrees of freedom (charge, spin, vibrational) as manifested in electronic and vibrational dynamics. Confined systems of interest include doped and heterostructured semiconductor nanocrystal quantum dots (QDs) and colloidal graphene QDs. Our most recent work has focused on the electronic properties of colloidal graphene QDs, nanoscale particles of sp²-hybridized carbon with narrow size dispersion, in which confinement opens a large gap from the otherwise gapless extended graphene band structure (Figure 1). We address these problems via ultrafast nonlinear optical techniques.

Coherent nonlinear optical processes are especially sensitive to the symmetries of a system (e.g., the breaking of inversion symmetry at the surface of a system with a centrosymmetric bulk structure), while optical techniques are among the few ways to directly access dynamics on the picosecond and sub-picosecond timescales that characterize many processes in condensed phases. Among problems of interest to us is the vibrational dynamics at water surfaces, where the hydrogen bond network is essential to numerous physical, chemical, and biological phenomena and leads to unusual properties of water including sub-picosecond vibrational relaxation (Figure 2).