Single-layer MoSeN : a synthetic Janus two-dimensional transition-metal compound grown by plasma-assisted molecular beam epitaxy

Abstract

Two-dimensional (2D) nanomaterials hold immense application potentials such as in high-performance nano-electronics, and asymmetric 2D structures with inherent electric dipoles will extend the application promises. Yet synthesizing asymmetric 2D structures remains challenging. Herein, we report the first synthesis of single-layer (SL) hexagonal (H-) phase polar Janus MoSeN via nitrogen-plasma-assisted molecular beam epitaxy. This is a significant achievement given the incommensurate valence between Mo, Se, and N, and the inherent strain from the Janus architecture. Using an array of compositional and structural characterization methods, we establish the atomic configurations of the synthesized MoSeN SL, confirming that they are 2D Janus transition-metal chalcogen-nitrides rather than alloys. By employing density functional theory calculations and transport measurements, we explore the structural feasibility and offer insights into its electronic properties, demonstrating its metallic behavior with ohmic contact characteristics. Piezoresponse force microscopy measurements reveal vertical piezoelectricity and ferroelectric potentials from the Janus MoSeN SL. Therefore, it exhibits great potential for applications in, e.g. piezoelectric and ferroelectric devices, sensing technologies, and optoelectronic devices. This work not only addresses existing challenges in 2D nanomaterial research but also opens new avenues for the development of advanced functional materials.