Event

Abstract: 

Spatial direction of energy flow, namely carrier and heat transport, in 2D materials is critical for developing these materials into next-generation optoelectronic and thermoelectric applications. However, current understanding at the single- and few-atom layer limit is largely informed by intrinsic charge and phonon transport mechanisms and does not consider the effects of nanoscale morphological variations, such as material edges and localized strain, which are particularly in mechanically exfoliated systems. The resulting modification of the electronic wave functions, band energetics, and phonon dynamics can compete with or even overwhelm intrinsic properties. I investigate this interplay of intrinsic and extrinsic effects in a model system, anisotropic black phosphorus (BP). The BP lattice is orthorhombic and corrugated and hosts orthogonal in-plane charge and phonon transport. Interrogating the nanoscale morphological modifications and dynamics requires sub-100 nm spatial resolution, which is prohibitive to conventional optical microscopies. I combine polarization-dependent photoemission electron microscopy (PEEM) and ultrafast transmission electron microscopy (UEM) to investigate the morphology-induced modification of the electronic structure and coherent acoustic phonon transport in BP with ~50 nm and ~nm resolution, respectively.  Using PEEM, I show that the optical selection rules of BP are modified at flake edges, resulting in a rotated transition dipole moment. With UEM, I show that the anisotropy-driven mixing of longitudinal and transverse acoustic phonon modes results in a strongly reduced group velocity of coherent acoustic phonons, which cannot be explained by mode-averaged diffusive transport models and underscores the need for heat propagation models beyond diffusive transport. 

Bio:

Dr. Prakriti P. Joshi is an experimental physical chemist currently working as a Kadanoff-Rice Postdoctoral Fellow in the group of Prof. Sarah King in the James Franck Institute and Department of Chemistry at the University of Chicago. She obtained her B.S. in chemistry and biological chemistry at the University of Chicago where she worked under Prof. Laurie Butler before completing her PhD in Chemical Physics at Columbia University with Prof. Xiaoyang Zhu. Her graduate work focused on using ultrafast spectroscopy methods to investigate the structural dynamics of ferroelectric-like charge screening in lead halide perovskites, which is critical for designing new classes of defect-tolerant semiconductors. Her current research uses ultrafast electron microscopy methods to uncover the interplay of nanoscale extrinsic morphological effects with intrinsic electronic structure and phonon behaviors in 2D materials. Her research interests lie in directly interrogating and controlling phonons, or solid-state vibrations, with spatial resolutions pushing the few-unit cell and atomistic limit to direct heat, charge, and ion transport for energy harvesting and storage applications.