Rapid Flame Synthesis of Atomically Thin MoO down to Monolayer Thickness for Effective Hole Doping of WSe.

Two-dimensional (2D) molybdenum trioxide (MoO) with mono- or few-layer thickness can potentially advance many applications, ranging from optoelectronics, catalysis, sensors, and batteries to electrochromic devices. Such ultrathin MoO sheets can also be integrated with other 2D materials (e.g., as dopants) to realize new or improved electronic devices. However, there is lack of a rapid and scalable method to controllably grow mono- or few-layer MoO. Here, we report the first demonstration of using a rapid (<2 min) flame synthesis method to deposit mono- and few-layer MoO sheets (several microns in lateral dimension) on a wide variety of layered materials, including mica, MoS, graphene, and WSe, based on van der Waals epitaxy. The flame-grown ultrathin MoO sheet functions as an efficient hole doping layer for WSe, enabling WSe to reach the lowest sheet and contact resistance reported to date among all the p-type 2D materials (∼6.5 kΩ/□ and ∼0.8 kΩ·μm, respectively). These results demonstrate that flame synthesis is a rapid and scalable pathway to growing atomically thin 2D metal oxides, opening up new opportunities for advancing 2D electronics.