Monolayer transition metal dichalcogenides (TMDs) have attracted considerable attention as atomically thin semiconductors for the ultimate transistor scaling.For practical applications in integrated electronics,large monolayer single crystals are essential for ensuring consistent electronic properties and high device yield.The TMDs available today are generally obtained by mechanical exfoliation or chemical vapor deposition (CVD) growth,but are often of mixed layer thickness,limited single crystal domain size or have very slow growth rate.Scalable and rapid growth of large single crystals of monolayer TMDs requires maximization of lateral growth rate while completely suppressing the vertical growth,which represents a fundamental synthetic challenge and has motivated considerable efforts.Herein we report a modified CVD approach with controllable reverse flow for rapid growth of large domain single crystals ofmonolayer TMDs.With the use of reverse flow to precisely control the chemical vapor supply in the thermal CVD process,we can effectively prevent undesired nucleation before reaching optimum growth temperature and enable rapid nucleation and growth ofmonolayer TMD single crystals at a high temperature that is difficult to attain with use of a typical thermal CVD process.We show that monolayer single crystals of 450μm lateral size can be prepared in 10 s,with the highest lateral growth rate up to 45 μm/s.Electronic characterization shows that the resulting monolayer WSe2 material exhibits excellent electronic properties with carrier mobility up to 90 cm2 V-1 s-1,comparable to that of the best exfoliated monolayers.Our study provides a robust pathway for rapid growth of high-quality TMD single crystals.