One-Step Growth of Spatially Graded MoW S Monolayers with a Wide Span in Composition (from x = 0 to 1) at a Large Scale.

Alloying is an effective way to modulate material's properties. In particular, graded alloying within a single domain of two-dimensional transition-metal chalcogenide (2D-TMC) is of great technological importance, for example, for achieving band gap modulations. Here, we report a facile method to grow gradient alloying of MoW S monolayers with large domain sizes and high crystal qualities via the chemical vapor deposition technique. The as-grown MoW S monolayers have a gradient composition of W from x = ∼0 to ∼1 in a single domain with a lateral dimension up to 300 μm, and the span in band gap can be readily tuned. Owing to the grading in band offsets, the compositionally graded MoW S alloy monolayer demonstrates an excellent rectifying effect with the ratio of forward to reverse current up to ∼10. Moreover, phototransistors based on the compositionally graded MoW S monolayers exhibit a high responsivity up to 298.4 A/W in the visible light regime, and particularly a decent responsivity of 28.7 A/W in the near-infrared regime. The control of band gap offset gradient and span in alloyed 2D-TMC semiconductors provides an additional degree of freedom in designing fascinating applications in achieving multifunctional optoelectronic devices on individual substrates.