Synthesis of Large-Area Tungsten Disulfide Films on Pre-Reduced Tungsten Suboxide Substrates

We report a facile method for the synthesis of large-area tungsten disulfide (WS₂) films by means of chemical vapor deposition (CVD). To promote WS₂ film growth, the precursor solution, which includes pre-reduced tungsten suboxides, is prepared by using hydrazine as the strong reducing agent and spin-coated onto the growth substrate. Growth is then carried out in a CVD chamber vaporized with dimethyl disulfide as the sulfur precursor. Although only WS₂ flakes are grown with unreduced tungsten precursors under a hydrogen atmosphere, WS₂ films are readily attained on pre-reduced tungsten suboxide substrates without the need for further reduction by hydrogen, which is noted to induce discontinuity of the grown film. The result presents the coverage of WS₂ to be proportional to the amount of reduced tungsten suboxides, which is revealed by X-ray photoelectron spectroscopy. Furthermore, it is found that the multilayer WS₂ flakes grow along the grain boundary, which allows the analysis of the grain size of WS₂ films by optical microscopy images only. WS₂ field effect transistors are fabricated by conventional photolithography and show an average electron mobility of 0.4 cm² V^-1 s^-1 and a high on/off ratio of 10⁶ at room temperature.