Ultrafast Self-Limited Growth of Strictly Monolayer WSe2 Crystals.

The controllable synthesis of uniform tungsten diselenide (WSe2 ) is crucial for its emerging applications due to the high sensitivity of its extraordinary physicochemical properties to its layer numbers. However, undesirable multilayer regions inevitably form during the fabrication of WSe2 via the traditional chemical vapor deposition process resulted from the lack of significantly energetically favorable competition between layer accumulation and size expansion. This work innovatively introduces Cu to occupy the hexagonal site positioned at the center of the six membered ring of the WSe2 surface, thus filtrates the undesired reaction path through precisely thermodynamical control and achieves self-limited growth WSe2 crystals. The as-obtained WSe2 crystals are characterized as strictly single-layer over the entire wafer. Furthermore, the strictly self-limited growth behavior can achieve the "win-win" cooperation with the synthesis efficiency. The fastest growth (≈15 times of the growth rate in the previous work) of strictly monolayer WSe2 crystals thus far is realized due to the high-efficiency simultaneous selenization process. The as-proposed ultrafast Cu-assisted self-limited growth method opens a new avenue to fabricate strictly monolayer transition metal dichalcogenides crystals and further promotes their practical applications in the future industrial applications.