  The monolayer transition metal dichalcogenides have recently attracted much
attention owing to their potential in valleytronics, flexible and low-power
electronics and optoelectronic devices. Recent reports have demonstrated the
growth of large-size 2-dimensional MoS2 layers by the sulfurization of
molybdenum oxides. However, the growth of transition metal selenide monolayer
has still been a challenge. Here we report that the introduction of hydrogen in
the reaction chamber helps to activate the selenization of WO3, where
large-size WSe2 monolayer flakes or thin films can be successfully grown.
