Symmetry-Breaking-Induced Multifunctionalities of Two-Dimensional Chromium-Based Materials for Nanoelectronics and Clean Energy Conversion

Abstract

Structural symmetry breaking, which could lead to exotic physical properties, plays a crucial role in determining the functions of a system, especially for two-dimensional (2D) materials. Here, we demonstrate that multiple functionalities of 2D chromium-based materials can be achieved by breaking inversion symmetry via replacing $Y$ atoms in one face of pristine $\mathrm{Cr}$Y (Y = $\mathrm{P}$, $\mathrm{As}$, $\mathrm{Sb}$) monolayers with $\mathrm{N}$ atoms, i.e., forming Janus ${\mathrm{Cr}}_{2}\mathrm{N}Y$ monolayers. The functionalities include gapless spin, very low work function, inducing carrier doping, and catalytic activity, which are predominately ascribed to the large intrinsic dipole of Janus ${\mathrm{Cr}}_{2}\mathrm{N}Y$ monolayers, giving them great potential for various applications. Specifically, ${\mathrm{Cr}}_{2}\mathrm{NSb}$ is found to be a spin-gapless semiconductor, ${\mathrm{Cr}}_{2}\mathrm{NP}$ and ${\mathrm{Cr}}_{2}\mathrm{NHPF}$ can simultaneously induce n- and p-type carrier doping for two graphene sheets with different concentrations (forming an intrinsic p-n vertical junction), and ${\mathrm{Cr}}_{2}\mathrm{N}Y$ exhibits excellent electrocatalytic hydrogen-evolution activity, even superior to that of benchmark $\mathrm{Pt}$. The results confirm that breaking symmetry is a promising approach for the rational design of multifunctional 2D materials.