Boosting Monolayer Transition Metal Dichalcogenides Growth by Hydrogen-Free Ramping during Chemical Vapor Deposition.

The controlled vapor-phase synthesis of two-dimensional (2D) transition metal dichalcogenides (TMDs) is essential for functional applications. While chemical vapor deposition (CVD) techniques have been successful for transition metal sulfides, extending these methods to selenides and tellurides often faces challenges due to uncertain roles of hydrogen (H 2 ) in their synthesis. Using CVD growth of MoSe 2 as an example, this study illustrates the role of a H 2 -free environment during temperature ramping in suppressing the reduction of MoO 3 , which promotes effective vaporization and selenization of the Mo precursor to form MoSe 2 monolayers with excellent crystal quality. As-synthesized MoSe 2 monolayer-based field-effect transistors show excellent carrier mobility of up to 20.9 cm 2 /(V·s) with an on-off ratio of 7 × 10 7 . This approach can be extended to other TMDs, such as WSe 2 , MoTe 2 , and MoSe 2 /WSe 2 in-plane heterostructures. Our work provides a rational and facile approach to reproducibly synthesize high-quality TMD monolayers, facilitating their translation from laboratory to manufacturing.