Growth of Low-Defect WSe₂ Film via High-Purity van der Waals Crystal Precursor

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) exhibit exceptional electrical and optical properties, empowering their promising prospects for future nanoelectronics. Despite major advances in n-type 2D semiconductors, the field has yet to synthesize high-mobility p-type 2D TMDs, in particular WSe₂, and systematically query the influence of defects. In this study, we unveil the pivotal role of substitutional impurity defects vis-à-vis the precursor used and growth method employed in defining the quality of 2D p-type WSe₂. Density functional theory calculations suggest the adverse effect of Fe-, Co-, Ni- and Si-substituted W impurity defects on the mobility of WSe₂, whereas defects such as O-, S-substituted Se and Mo-substituted W pose negligible impact. Guided by the theory, we pinpoint van der Waals (vdW) crystals, commonly used in mechanical exfoliation, as the optimal precursor, and develop a facile vdW crystal physical vapor deposition (PVD) method to grow high-purity monolayer 2D WSe₂ film (VPVD-WSe₂) that is continuous across a centimeter scale. A suite of spectroscopies confirms the markedly reduced defect density of the as-synthesized WSe₂ compared to those by typical chemical vapor deposition methods, and by PVD with commercial or hydrothermal precursors. Scanning tunneling microscopy further evidence the ultralow substitutional impurity defect density of VPVD-WSe₂, greatly outperforming the control samples and approaching the mechanically exfoliated counterparts. The VPVD-WSe₂ based field-effect transistors exhibit notable electrical performance with record-high field-effect hole mobility up to 112 cm² V^-1s^-1 at room temperature, exceeding the best-reported monolayer WSe₂ synthesized by chemical vapor deposition and rivaling the mechanically exfoliated 2D WSe₂ flakes.