Layer-by-layer epitaxy growth of thickness-controllable two-dimensional tungsten disulfide

Bilayer transition metal dichalcogenides (TMDs) balance the high mobility of single layers with the high state density of multilayers and therefore have promising application prospects in high-performance electronics. However, the layer-controlled growth of 2D materials is still confronted with challenges such as poor repeatability between different labs and a limited understanding of the growth mechanism at the atomic scale. Herein, we report a new carbon-assisted chemical vapor deposition process that can realize the growth of WS₂ sheets with high yield, precise thickness controllability, and repeatability. We show that carbon can act as a reducing agent and catalyst that preferentially reacts with the WO₃ precursor to form intermediate WO_3−x products with low-valence state W. The resulting oxycarbide gas has a low surface adsorption energy when deposited on the surface of as-grown WS₂, which provides nucleation sites for the subsequent layer of WS₂ growth and leads to the vertical growth of WS₂ sheets. The growth mechanism is thoroughly investigated. Electrical transport measurements show that the produced bilayer WS₂ possesses a high carrier mobility (up to 58 cm²·V⁻¹·s⁻¹) and small subthreshold swing (estimated to be 148 mV/decade), which are among the best reported results for TMDs produced using CVD.