Controlled Growth of High-Quality Monolayer WS₂ Layers on Sapphire and Imaging Its Grain Boundary

TLDR

WS₂, a structural analogue of MoS₂, has attracted interest for its indirect‑to‑direct band‑gap tunability, giant spin splitting, and valley physics, yet its batch production remains underdeveloped compared to MoS₂. The study demonstrates that high‑crystalline mono‑ and few‑layer WS₂ flakes and complete layers can be synthesized on sapphire with domain sizes exceeding 50 × 50 µm² using low‑pressure CVD. Using low‑pressure chemical vapor deposition with a minor H₂ carrier gas and a mild oxidation step, the authors grow WS₂ on sapphire, controlling nucleation to produce large, single‑crystalline domains. They find that adding H₂ tailors flake shape from jagged to straight‑edge triangles while preserving single crystallinity, that intersecting triangles arise from multiple nuclei indicating polycrystallinity, and that mild oxidation reveals grain boundaries, highlighting the importance of nucleation control for large‑scale single‑crystalline WS₂ suitable for nanoelectronics, optoelectronics, and solar energy conversion.

Abstract

Atomically thin tungsten disulfide (WS2), a structural analogue to MoS2, has attracted great interest due to its indirect-to-direct band-gap tunability, giant spin splitting, and valley-related physics. However, the batch production of layered WS2 is underdeveloped (as compared with that of MoS2) for exploring these fundamental issues and developing its applications. Here, using a low-pressure chemical vapor deposition method, we demonstrate that high-crystalline mono- and few-layer WS2 flakes and even complete layers can be synthesized on sapphire with the domain size exceeding 50 × 50 μm(2). Intriguingly, we show that, with adding minor H2 carrier gas, the shape of monolayer WS2 flakes can be tailored from jagged to straight edge triangles and still single crystalline. Meanwhile, some intersecting triangle shape flakes are concomitantly evolved from more than one nucleus to show a polycrystalline nature. It is interesting to see that, only through a mild sample oxidation process, the grain boundaries are easily recognizable by scanning electron microscopy due to its altered contrasts. Hereby, controlling the initial nucleation state is crucial for synthesizing large-scale single-crystalline flakes. We believe that this work would benefit the controlled growth of high-quality transition metal dichalcogenide, as well as in their future applications in nanoelectronics, optoelectronics, and solar energy conversions.