The optoelectronic properties of atomically thin transition-metal dichalcogenides are correlated with the presence of structural imperfections in the materials, which are not necessarily detrimental for certain applications. Using laser-illuminated microwave impedance microscopy, we were able to perform simultaneous spatial and temporal photoconductivity imaging in two types of WS2 monolayers with different defect densities. For chemical-vapor deposited (CVD) samples, the diffusion length and carrier lifetime, extracted from the spatial profile and temporal relaxation of microwave signals, respectively, are in good agreement with the diffusion equation and Einstein relation. Time-resolved experiments indicate that the critical process for photo-excited carriers is the escape of holes from trap states. As a result, the photoconductivity is weaker in exfoliated monolayers with lower disorders than the more defective CVD samples. Our work reveals the intrinsic time and length scales of electrical response to photo-excitation in van der Waals materials.
Keji Lai is an associate professor at the Department of Physics at the University of Texas Austin. His research includes experimental condensed matter physics, nanoscale electromagnetic imaging, complex oxides, nano-materials, and transport in low-dimensional systems.