Spatial-Temporal Imaging of Anisotropic Photocarrier Dynamics in Black Phosphorus

TL;DR

This paper employs scanning ultrafast electron microscopy to directly visualize and characterize the highly anisotropic hot carrier dynamics in black phosphorus, revealing significant directional differences in carrier diffusion.

Contribution

It introduces the first direct visualization of hot carrier motion in black phosphorus using SUEM, highlighting anisotropic diffusion properties in this layered 2D material.

Findings

Hot holes diffuse 15 times faster along the armchair direction.

SUEM effectively captures ultrafast hot carrier dynamics.

Anisotropic hot carrier transport is confirmed in black phosphorus.

Abstract

As an emerging single elemental layered material with a low symmetry in-plane crystal lattice, black phosphorus (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 photo-excited 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.