Highly Selective Synthesis of Monolayer or Bilayer WSe2 Single Crystals by Pre-annealing the Solid Precursor

Two-dimensional layered transition- metal dichalcogenides (TMDs) have attracted intense interest for their layer number-dependent electronic properties and have exciting potential for atomically thin electronics and optoelectronics. The studies to date have primarily been limited to exfoliated materials with limited control of the size, layer number, and yield. Despite considerable efforts to date, it remains a significant challenge to produce large-sized TMD single crystals with precise control of the layer number. Here, we report the robust growth of high-quality WSe2 single-crystal domains with a selectively controlled thickness in a reverse-flow chemical vapor deposition system with a solid precursor. By introducing a preannealing step to tune elemental distribution and volatilization rate of the solid precursor, we stabilize the vapor supply to achieve a highly uniform nucleation and growth, and thus ensure the precise control of the layer number for the highly selective growth of monolayer or bilayer WSe2 single crystals (>500 mu m). The transmission electron microscopy and optical characterizations of the resulting WSe2 single crystals exhibit excellent crystalline quality with systematically tunable optical properties. Electrical transport studies further show that the WSe2 field-effect transistors exhibit p-type semiconductor characteristics with effective hole carrier mobility up to 92 cm(2) V-1 s(-1) in monolayer and 145 cm(2) V-1 s(-1) in bilayer materials at room temperature. This simple approach opens up a new avenue for the highly controlled synthesis of WSe2 atomic layers for both fundamental studies and technological applications.

Automated summary

This study successfully achieved the precise controlled growth of high-quality WSe2 single crystals in a reverse-flow chemical vapor deposition system, with excellent optical properties for both monolayer and bilayer materials. Electrical transport studies showed promising applications as p-type semiconductors for WSe2 field-effect transistors.