  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 oC)
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
~10 7, hole and electron mobilities up to 102 and 26 cm2 V 1 s-1 for WSe2 and
electron mobility of ~14 cm2 V-1 s-1 for WS2 devices were achieved.
