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

We report a facile method for the synthesis of large-area tungsten disulfide (WS2) film 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 reduction agent and spin-coated onto the growth substrate. Growth is then carried out in a CVD chamber vaporized with dimethyl disulfide (DMDS) as the sulfur precursor. While only WS2 flakes are grown with un-reduced tungsten precursor under hydrogen atmosphere, WS2 film is readily attained on pre-reduced tungsten suboxide substrates without the need for further reduction by hydrogen, which is noted to induce discontinuity of 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 analysis of the grain size of WS2 film by optical microscopy image only. WS2 field effect transistors are fabricated by conventional photolithography show an average electron mobility of 0.4 cm2V-1s-1 and a high on/off ratio of 106 at room temperature.