Anomalous reduction in black phosphorus phonon velocities driven by lattice anisotropy

Phonon transport in solids determines how heat can be utilized or dissipated in materials. Conventional methods for investigating phonon transport use experimental techniques that spatially average over material morphology. However, as electronic devices continue to shrink, material boundaries have an increasing contribution to phonon dynamics. Here we use a novel ultrafast electron microscopy technique to directly interrogate the phonon dynamics of two-dimensional black phosphorus (BP) on the nanoscale. BP has intrinsic anisotropic phonon transport with distinct phonon velocities along the two principal crystal directions, zigzag (ZZ) and armchair (AC). We are able to directly observe the coherent acoustic phonon (CAP) group velocities in these two directions, confirming previous non-localized experiments. We also observe that the group velocities of CAPs are strongly influenced by the direction of propagation. In particular, edge-launched CAPs that propagate 31 degrees relative to the AC lattice direction have a group velocity that is significantly lower than those two directions and deviate from a mode-averaged phenomenological model that does not account for lattice anisotropy. Using Gaussian Approximation Potential calculations, we find that, because of the strong anisotropy of the BP lattice, the crossing of transverse acoustic (TA) and longitudinal acoustic (LA) phonon modes results in anomalously low LA group velocities in directions between the AC and ZZ directions. This suggests that BP is an ideal model platform to explore morphology and directional control of heat transport, particularly for miniaturized optoelectronic and thermoelectric applications.