Time-Resolved Probes of Lattice Dynamics in Technologically Relevant Semiconductors

The role of thermal energy deposition in nanoparticles and bulk semiconductors with nanoscopic features has not been substantively probed, yet can impact material function. I will present studies of ultrafast optical-pump, x-ray diffraction probe as well as infrared-pump electronic-probe performed on semiconductor nanocrystal (NC) colloidal dispersions as functions of particle size, polytype, and pump intensity to examine lattice response in materials such as CdSe and lead halide perovskites. In x-ray experiments, shifts of Bragg peaks relate lattice heating and peak amplitude reduction conveys transient lattice disordering (or melting). Intraband and Auger-derived heating is clearly observed for low fluences, and disordering was observed upon absorption of larger numbers of photons excitations per NC on average. Perovskites are shown to undergo solid-solid phase transitions followed by melting at elevated fluences. Infrared-pump experiments offer insights into thermal dissipation within these compositions including sub-lattice coupling and interfacial thermal conductance. These findings suggest a need to take into account nanomaterial physical stability and transient electronic structure for high intensity excitation applications such as lasing and solid-state lighting.