In Situ Visualization of Optoelectronic Behavior of Grain Boundaries in Monolayer WSe2 at the Nanoscale

Optoelectronic properties of two-dimensional transition metal dichalcogenides are critically influenced by grain boundaries (GBs). However, optoelectronic behavior of the GBs has not been fully understood in part due to the limited sensitivity and spatial resolution of conventional analytical tools. Herein, we report a detailed investigation of the optoelectronic behavior of GBs in monolayer (1L) WSe2 using a powerful combination of tip-enhanced optical microscopy and electrical-mode atomic force microscopy. Our study reveals that the work function, charge accumulation, and excitonic photoluminescence quenching at the GBs vary significantly with the tilting angle of the merged single crystalline 1L WSe2 flakes. Furthermore, our experimental results are supported by density functional theory calculations of band structure and density of states, which indicate that the variation in optoelectronic behavior is caused by the dislocation structures and midgap states generated in the GBs. The novel insights into optoelectronic understanding of 1L WSe2 gained in this study are directly relevant for applications in high-performance optoelectronic devices.