Synthesis of uniform single layer WS₂ for tunable photoluminescence

Two-dimensional transition metal dichalcogenides (2D TMDs) have gained great interest due to their unique tunable bandgap as a function of the number of layers. Especially, single-layer tungsten disulfides (WS₂) is a direct band gap semiconductor with a gap of 2.1 eV featuring strong photoluminescence and large exciton binding energy. Although synthesis of MoS₂ and their layer dependent properties have been studied rigorously, little attention has been paid to the formation of single-layer WS₂ and its layer dependent properties. Here we report the scalable synthesis of uniform single-layer WS₂ film by a two-step chemical vapor deposition (CVD) method followed by a laser thinning process. The PL intensity increases six-fold, while the PL peak shifts from 1.92 eV to 1.97 eV during the laser thinning from few-layers to single-layer. We find from the analysis of exciton complexes that both a neutral exciton and a trion increases with decreasing WS₂ film thickness; however, the neutral exciton is predominant in single-layer WS₂. The binding energies of trion and biexciton for single-layer WS₂ are experimentally characterized at 35 meV and 60 meV, respectively. The tunable optical properties by precise control of WS₂ layers could empower a great deal of flexibility in designing atomically thin optoelectronic devices.