The hydrogen-induced structural stability and promising electronic properties of molybdenum and tungsten dinitride nanosheets: a first-principles study

Abstract

Utilizing first-principles calculations, we have investigated the structural stabilities and electronic properties of MoS₂-like MoN₂ and WN₂ sheets. We find that unlike their disulfide counterparts, these dinitride sheets are dynamically unstable with strong soft modes. The soft modes of dinitride sheets can be eliminated by surface hydrogenation, and the formed MoN₂H₂ and WN₂H₂ sheets exhibit good dynamical, thermal and mechanical stabilities for both H and T_d phases. It is found that the H-phase structures present a semiconducting behaviour with band gaps of 2.43/2.50 eV for the MoN₂H₂/WN₂H₂ sheets. These band gaps can be further modulated by strain engineering, which induces an indirect-direct–indirect band gap transition in the range of −6 to 6%. On the other hand, the T_d-phase structures are zero-gap semi-metals, whose Dirac cones locate at off-symmetry reciprocal sites. Distinct from graphene, these Dirac cones are primarily composed of d-orbitals of Mo/W atoms, which are distorted with a notably anisotropic character in the MoN₂H₂ and WN₂H₂ systems. Our study demonstrates that transition-metal dinitride nanosheets can be well stabilized by hydrogenation, which brings tunable electronic properties for many potential applications in nano-electronics and nano-devices.