Inhomogeneous photocarrier dynamics and transport in monolayer MoS2 by ultrafast microscopy

Abstract

Monolayer MoS2 as a member of two-dimensional transition metal dichalcogenides (TMDs) has attracted considerable attention due to its superior optoelectronic properties. Understanding the photocarrier dynamics and transport in these two dimensional systems is beneficial for applications from photovoltaics to sensing. However, various structural defects strongly impact the dynamics and transport of photocarriers. Especially there lacks a precise measuring and understanding of photocarrier transport in TMDs. Here, femtosecond transient absorption spectroscopy and microscopy were employed to study the photocarrier dynamics and transport in monolayer MoS2. Defect correlated photocarrier dynamics are observed across the monolayer MoS2 where exciton formation and nonradiative recombination are the two dominant decay processes. To the best of our knowledge, we report two distinct photocarrier transport regimes in MoS2 for the first time with diffusion coefficients of cm2 s−1 and cm2 s−1, by taking advantages of ultrafast microscopy with ∼20 nm spatial precision and ∼200 fs temporal resolution. These two regimes are ascribed to fast hot photocarrier diffusion and slow phonon-limited thermal diffusion, respectively. The results indicate that the initial fast photocarrier transport is less dependent on structural defects compared to photocarrier relaxation dynamics which may be useful for hot photocarrier extraction applications.