Spectroscopic Signatures of Layer Rotation and Grain Boundaries in WSe₂

Two-dimensional transition metal dichalcogenides, a novel class of 2D layered materials, exhibit interesting optical and electronic properties that are markedly different from bulk characteristics. Artificially stacked bilayers and defects in monolayer transition metal dichalcogenides allow us to explore unique quantum effects in these 2D semiconductors. Here we report spectroscopic signatures of twisted bilayer WSe₂ and different types of grain boundaries in monolayer WSe₂. 2D bilayers obtained by stacking atomically thin monolayers with specific twist angle form Moiré superlattices which offers an excellent platform to achieve artificial quantum materials with emergent optical and electronic properties. The twist angle modifies the band structure, interlayer coupling in transition metal dichalcogenides and offers new excitonic properties such as characteristic of Moiré excitons, exciton-phonon coupling etc. We fabricated the twisted bilayer WSe₂ samples with twist angles ranging from 0 to 60⁰. The Raman spectroscopy suggest how interlayer coupling changes with twist angle. The generic quantum effects on electron and exciton transport across the grain boundary on monolayer WSe₂ is still unclear. Here using photoluminescence mapping we have investigated the different types of grain boundaries in monolayer WSe₂such as mirror and tilt grain boundaries. Along with identification of grain boundary angle, these maps also provide useful insights into defect-induced quenching and doping-induced enhancement of photoluminescence for various grain boundary configurations.