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 WO3−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.