Synthesis of Optically Uniform Single Layer WS₂ for Tunable Photoluminescence

Two-dimensional transition metal dichalcogenides (2D TMDs) are promising materials for next-generation opto-electronic and nano-electronic devices 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 2.1eV 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. As-synthesized few-layer WS2 film shows 3.8 nm thickness considered to be 4 – 6 layers, and the post-laser irradiation enables systematic layer-by-layer thinning of the as-synthesized WS2 down to single-layer. 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 also find WS2 layer dependent intensity variation of the exciton complexes in PL emission. Both a neutral exciton and a trion increases with decreasing WS2 film thickness; however, the neutral exciton is predominant in single-layer WS2. The tunable optical properties by precise control of WS2 layers could empower a great deal of flexibility in designing atomically thin optoelectronic devices. Figure 1