Strain and Charge Doping Modulated Optical Emission Signatures of Polycrystalline WSe.

Two-dimensional (2D) semiconductors with direct bandgap in the visible and near-IR spectral range, such as transition metal dichalcogenide (TMD) films, are promising candidates for optoelectronic devices and in light harvesting applications. Large area growth of 2D TMDs through techniques such as chemical vapor deposition (CVD), while offering industrial scalability, suffers major drawbacks due to polycrystalline nature of the grown films. Here, the optical emission signatures of polycrystalline WSe monolayers grown on sapphire substrates using CVD are investigated to identify the intrinsic and extrinsic factors contributing to the photoluminescence (PL) in these films. The epitaxy with the substrate along with other growth space parameters significantly influences the atomic structure of the grain boundaries (GBs) in polycrystalline WSe. While the local charge doping from adsorbed growth precursors is the critical factor influencing PL intensity at mirror twin GBs (MTGBs), the films with tilt GBs (TGBs) show inhomogeneous PL emission due to tensile and compressive strain arising from thermal expansion co-efficient mismatch between WSe and growth substrate. These results are understood from the Bader charge analysis of adatoms and funneling effect in the band structure arising from non-uniform strain landscape. The findings are crucial in the development of highly efficient optoelectronic devices from 2D TMDs.