Directing Energy Transport at the Nanoscale with Electron Microscopies

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 especially abundant 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 behavior in a model system, anisotropic black phosphorus (BP). Interrogating the nanoscale edge modifications and dynamics requires sub-100 nm spatial resolution, which is prohibitive to conventional optical microscopies but well-suited to electron microscopies. I combine polarization-dependent photoemission electron microscopy (PEEM) and ultrafast transmission electron microscopy (UEM) to investigate the role of edge-modification on the electronic structure and coherent acoustic phonon transport in BP by directly interrogating the above-bandgap optical transitions and photoexcitation-induced lattice dynamics, respectively. Using PEEM, I show that spatially confined wavefunctions at BP flake edges results in a rotated transition dipole moment, which has implications for spatially selective excitation and design of nanostructured 2D materials. With UEM, I show that the bonding anisotropy-driven mixing of longitudinal and transverse acoustic phonon modes results in a strongly reduced group velocity of in-plane coherent acoustic phonons, which cannot be explained by mode-averaged diffusive transport models. These results suggest that intentional design of edge orientations and intrinsic bonding anisotropy offer a novel route towards controlling in-plane acoustic phonon and heat propagation.