Amorphous two-dimensional black phosphorus with exceptional photocarrier transport properties

Recently, two-dimensional materials have been extensively studied. Due to the reduced dielectric screening and confinement of electrons in two dimensions, these materials show dramatically different electronic and optical properties from their bulk counterparts. So far, studies on two-dimensional materials have mainly focused on crystalline materials. Here we report studies of atomically thin amorphous black phosphorus, as the first two-dimensional amorphous material. Spatially and temporally resolved pump-probe measurements show that large-area and uniform atomic layers of amorphous black phosphorus, synthesized at low temperature, possess a long exciton lifetime of about 400 ps, a room-temperature exciton diffusion coefficient of 5 cm² s⁻¹, which is at least two orders of magnitude larger than amorphous silicon, and an exciton mobility of about 200 cm² V⁻¹ s⁻¹. We also deduce from these values an exciton mean free time of 50 fs, a mean free path of 5 nm, and a diffusion length of 450 nm. These results suggest that amorphous black phosphorus can be potentially used in low-cost optoelectronic devices. (paper)