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.