Halide-assisted atmospheric pressure growth of large WSe2 and WS2 monolayer crystals

Chemical vapor deposition (CVD) of two-dimensional (2D) tungsten dichalcogenide crystals requires steady flow of tungsten source in the vapor phase. This often requires high temperature and low pressure due to the high sublimation point of tungsten oxide precursors. We demonstrate atmospheric pressure CVD of WSe2 and WS2 monolayers at moderate temperatures (700-850 degrees C) using alkali metal halides (MX where M = Na or K and X = Cl, Br or I) as the growth promoters. We attribute the facilitated growth to the formation of volatile tungsten oxyhalide species during growth, which leads to efficient delivery of the precursor to the growth substrates. The monolayer crystals were found to be free of unintentional doping with alkali metal and halogen atoms. Good field-effect transistor (FET) performances with high current on/off ratio similar to 10(7), hole and electron mobilities up to 102 and 26 cm(2) V-1 s(-1) for WSe2 and electron mobility of 14 cm(2) V-1 s(-1) for WS2 devices were achieved. (C) 2015 Elsevier Ltd. All rights reserved.