Spatial-Temporal Imaging of Anisotropic Photocarrier Dynamics in Black Phosphorus

As an emerging single elemental layered material with a low symmetry in-plane crystal lattice, black phosphonis (BP) has attracted significant research interest owing to its unique electronic and optoelectronic properties, including its widely tunable bandgap, polarization-dependent photorespOnse and highly anisotropic in-plane charge transport. Despite extensive study of the steady-state charge transport in BP, there has not been direct characterization and visualization of the hot carriers dynamics in BP immediately after photoexcitation, which is crucial to understanding the:-performance of BP-based optoelectronic devices. Here we-use the newly developed scanning ultrafast electron microscopy (SUEM) to directly visualize the motion of photoexcited hot carriers on the surface of-BP in both space and-time: We observe-highly anisotropic in-plane-,diffusion of hot holes with a 15 times higher diffusivity along the armchair (x-),direction than that,along-, the zigzag (y-) direction: Our results provide direct evidence of anisotropic hot,carrier transport in BP and demonstrate the capability of,SUEM to resolve ultrafast hot carrier dynamics in layered two-dimensional materials.