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 recent work has focused on the electronic properties of colloidal graphene QDs and GaSe. GaSe is a layered material with an indirect bandgap in which the direct bandgap is only about 10 meV below the indirect gap. Thus, strong emission is observed even at low temperatures from bulk to few-layer samples. The large crystal-field splitting of the valence bands leads to the possibility of generating excited carriers with a high degree of spin polarization and long-lived spin memory (Figure 1). Graphene QDs are 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 2). With our synthetic collaborator, Professor Liang-shi Li at Indiana University, we are able to study QDs large enough that the lowest energy confined excitons can be understood in terms of confined Dirac fermions. These allow us to address fundamental questions about exciton interactions that would otherwise be difficult to address in extended, gapless graphene. We address nanoscale systems via a variety of 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 3).

Figure 1: Spin dynamics in GaSe: Thin slabs of e-GaSe excited by light propagating along the c-axis and so with electric field perpendicular to the c-axis can emit light at the edges (b) through wave-guided modes. This remote-edge luminescence shows a high degree of linear polarization (c and d) for slabs thicker than about 160 nm (e-j). The linear polarization is along the line from the excitation focal spot to the edge and so represents a rotation of the exciton dipole. This rotation is a manifestation of the spin dynamics of the lowest exciton manifold. (From Yanhao Tang et al., Phys. Rev. Applied 4, 034008 (2015).
Figure 2: Ground state absorption spectra (upper panel) of colloidal graphene quantum dots (inset) consisting of 132 and 168 sp2-hybridized carbon atoms (blue) and ligands (black). Lower panel shows the change in the absorption coefficient times the sample length as a function of probe wavelength and delay relative to a pump pulse. The initial dynamics at 2.2 eV reflect rapid biexciton Auger recombination. The spectral oscilations at a given delay reflect, e.g., exciton-exciton interactions. (From Cheng Sun et al., Phys. Rev. Lett. 113, 107401 (2014).	Figure 3: How do the dynamics of the hydrogen-bond network change at an extended hydrophobic interface compared to a water/air interface? Pump-probe sum-frequency generation (SFG) reveals the timescales for relaxation of the dangling OH (i.e., non-hydrogen-bonded) stretch excited population after excitation by a resonant infrared (IR) pump. The upper left panel shows pump-probe traces at the hydrophoboic fused-silica/octadecylsilane/water interface for pump pulse polarized either in the optical plane (p) or perpendicular to the optical plane (s). At long delays, data for s- and p-polarized pump differ only due to differing amounts of absorbed energy due to the anisotropic orientation of the dangling OH population. Normalization of the long delay signals reveals a difference (shaded blue region) reflecting orientational dynamics. The average and difference between the pump-induced changes for s- and p-polarization are shown in the upper left panel. The average reflects population relaxation, and the difference reflections both population relaxation and the return to the ground and excited-state populations to the equilibrium orientational distribution. We find that the reorientation from a dangling to a hydrogen-bonded configuration occurs with a time constant of about 1.6 ps, about 50% slower than for the water/air interface. (From Shunhao Xiao et al., JACS, in press (2016).)