Transition metal dichalcogenides (TMDs) have recently emerged as atomically thin semiconductors whose application in optoelectronics requires a detailed understanding of their fundamental carrier dynamics. In this talk, I will present our studies of the ultrafast carrier dynamics of TMDs by optical pump-probe spectroscopy and time-resolved photoemission electron microscopy. Optical pump-probe spectroscopy with few-cycle laser pulses elucidates carrier thermalization on the sub-20-fs timescale. The observed dependence of the thermalization time on the carrier density and the sample temperature suggests that carrier thermalization proceeds via both carrier-carrier and carrier-phonon scattering. The <i>n</i><sup>–0.37±0.03</sup>-scaling of the thermalization time with carrier density suggests that equilibration of the non-thermal carrier distribution involves non-Markovian quantum kinetics. Subsequent cooling of the hot Fermi-Dirac carrier distribution proceeds on the ~0.6-ps timescale through carrier-phonon scattering. These measured timescales are supported by ab initio molecular dynamics simulations. While ensemble-averaged measurements provide insight into the elementary carrier dynamics of TMD materials, the inherent spatial heterogeneity of these materials begets the need to address their ultrafast dynamics on the nanoscale. To this end, we have recently interfaced a photoemission electron microscope with a megahertz repetition-rate fiber laser to resolve the dynamics of spatially heterogeneous materials with ~50-fs time- and ~50-nm spatial resolution. When applied to TMDs, we observe enhanced electron cooling rates and reduced exciton-exciton annihilation rates along the edges of the flake, both of which suggest the presence of increased structural disorder along the edges of the flake. These studies shed light on the spatially heterogeneous ultrafast carrier dynamics of TMDs and represent the first step towards correlating carrier dynamics with the various structural defects present in TMDs.

Ultrafast carrier dynamics of transition metal dichalcogenides probed at the nanoscale

256th National Meeting (2018)

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