Spatial-Temporal Imaging of Anisotropic Photocarrier Dynamics 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 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.

The Optical Properties and Plasmonics of Anisotropic 2D Materials

“…In addition to the extensively studied steady‐state and linear optical properties of BP, strong anisotropy has also been demonstrated in the transient and nonlinear optical properties. By means of scanning ultrafast electron microscopy (SUEM), Liao et al achieved a direct visualization of the photoexcited carrier dynamics on the bulk BP surface in both space and time . Figure a presents the SUEM images of carrier diffusion taken at different times after photoexcitation, clearly demonstrating the strong in‐plane anisotropy: the carriers diffuse much faster in the AC direction than in the ZZ direction, regardless of the initial excitation polarization.…”

Scanning ultrafast electron microscopy: A novel technique to probe photocarrier dynamics with high spatial and temporal resolutions

“…SUEM experiences the same charging issue as the normal SEM, so in principle is not suitable to study electrically insulating materials, although the environmentalmode SUEM [29] can be a potential solution. SUEM has been utilized to image ultrafast photocarrier dynamics on the surface of a wide range of materials, including crystalline semiconductors [28,30], semiconducting nanowires [31] and nanocrystals [32], amorphous semiconductors [33], semiconductor p-n junctions [34] and two-dimensional materials [35], and these applications have resulted in intriguing observations such as ballistic transport of photocarriers across a p-n junction [34], superdiffusion of photocarriers in heavily-doped semiconductors [30] and spontaneous spatial separation of electrons and holes in amorphous semiconductors [33]. Whereas there has been an abundance of recent reviews of ultrafast electron microscopy [8,9,[36][37][38], we dedicate this article specifically to SUEM, with an emphasis on the current understanding of various physical processes that contribute to the contrast images observed in SUEM from a users' perspective.…”

“…(b) the temporal evolution of the angle-resolved variance of the hot-holes spatial distribution (defined in the main text), normalized to the initial distribution at 40 ps, quantifying the highly anisotropic diffusion process. (a) and (b) reproduced with permission from reference [35]. © 2017 American Chemical Society.…”

Mid-infrared Polarized Emission from Black Phosphorus Light-Emitting Diodes

“…The atomic crystal structure of BP consists of layered sp 3 -hybridized phosphorus atoms with distinct configurations along two orientations termed the armchair (AC) and zigzag (ZZ) directions. This gives rise to strong in-plane anisotropy for thermal conduction 18,19 , carrier transport and optical absorption 20,21,22,23,24 . In the context of optoelectronics, one of the most interesting properties of BP is its widely tunable bandgap and the fact that it is a direct bandgap material from the bulk down to bilayers.…”