Rational control of WSe₂ layer number via hydrogen-controlled chemical vapor deposition

Tranistion metal dichalcogenides are a promising family of materials for electronics and optoelectronics, in part due to their range of bandgaps that can be modulated by layer number. Here, we show that WSe₂ can be selectively grown with one, two, or three layers, as regulated by a one-step hydrogen-controlled chemical vapor deposition (H-CVD) process involving cyclical pulses of H₂ flow. The physical and vibrational properties of the resulting mono-, bi-, and tri-layer WSe₂ films are characterized by atomic force microscopy and Raman spectroscopy. Modifying the H-CVD process to include more than three H₂ pulses results in thicker WSe₂ films, however the thickness of these films is not well controlled and feature small, bulk-like pyramidal islands. Transmission electron microscopy analysis reveals that most of these islands exhibit a spiral structure and appear to be grown via screw-dislocation-driven growth, similar to other works. Therefore, the H-CVD process is demonstrated to be a powerful tool for controlling the layer thickness of WSe₂, but its practicality is limited to the few-layer regime.