  Structural symmetry-breaking that 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 could be achieved by breaking
inversion symmetry via replacing Y atoms in one face of pristine CrY (Y=P, As,
Sb) monolayers with N atoms, i.e., forming Janus Cr2NY monolayers. The
functionalities include spin-gapless, very low work function, inducing carrier
doping and catalytic activity, which are predominately ascribed to the large
intrinsic dipole of Janus Cr2NY monolayers, making them having great potentials
in various applications. Specifically, Cr2NSb is found to be a spin-gapless
semiconductor, Cr2NP and Cr2NHPF could simultaneously induce n- and p-type
carrier doping for two graphene sheets with different concentrations (forming
intrinsic p-n vertical junction), and Cr2NY exhibits excellent electrocatalytic
hydrogen evolution activity, even superior to benchmark Pt. The results confirm
that breaking symmetry is a promising approach for the rational design of
multifunctional 2D materials.
