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Seminar | X-Ray Science Division

Nonequilibrium Carrier-Lattice Dynamics Probed by Terahertz Spectroscopy and Ultrafast Electron Diffraction

XSD/TRR Special Presentation

Abstract: Understanding and manipulating energy conversion processes among light, electricity, and heat lies at the heart of modern technologies such as photovoltaics and thermoelectrics. Many of these processes intrinsically involve nonequilibrium electronic and atomic-scale responses, with dynamics spanning femtoseconds to seconds.

In this talk, I will present recent studies that directly probe nonequilibrium carrier dynamics and carrier–lattice coupling in a broad range of energy-relevant materials. First, I will present charge separation dynamics in hybrid lead-halide perovskite thin films studied by terahertz emission spectroscopy. Upon ultrafast photo-excitation we observe THz radiation emitted by the perovskite thin films directly reflecting the time-dependent currents in the material. Analysis of the radiated terahertz fields enables a highly sensitive probe of the strong electron–phonon coupling in these thin films. Furthermore, the emitted terahertz field permits direct access into the individual carrier mobilities in the perovskite thin films quantified via an all-optical means for the first time. Second, I will present femtosecond electron diffraction measurements on colloidal nanocrystals, with which we investigate coupling between photogenerated hot carriers and the nanocrystal lattice. We find that the size of the nanocrystal and its surface properties exhibit critical effects on the carrier–lattice coupling dynamics for both metal and semiconductor nanocrystals. Notably, we uncover a phonon bottleneck effect in a core/shell quantum dot system, revealed by the observation of anomalously slow lattice heating response. In addition, we resolve heat dissipation in plasmonic nanocrystals after photo-excitation and show it takes place first through interfacial heat transfer at the nanocrystal/ligand interface.