Dependence of topological and optical properties on surface-terminated groups in two-dimensional molybdenum dinitride and tungsten dinitride nanosheets

Abstract

Using ab initio methods, the topological and optical properties of surface-functionalized XN₂ sheets (X = Mo, W) were investigated. Based on first principles calculations and the K·p effective model, the existence of topological nodal-line states in potassium-functionalized XN₂ sheets (K₂MoN₂ and K₂WN₂) is reported. This study shows that a nodal line ring exists near the Fermi level in the absence of spin–orbit coupling (SOC). When SOC is included, the band-crossing points are gapped, giving rise to a new nodal ring along Γ–K. This band-crossing is protected due to the existence of mirror reflection and time-reversal symmetry. These calculations demonstrate the inclusion of electron–hole (e–h) interactions, which were further confirmed through the optical absorption of functionalized MoN₂, revealing the presence of strongly bound excitons below the absorption onset where they depend strongly on the terminated surface groups. Moreover, the surface terminated groups change the energy distribution range of the exciton, which can be used to tune the absorption of infrared (IR) and visible light. Interestingly, F₂MoN₂ has several strongly bound excitons, with the first exciton having a binding energy of 1.35 eV, larger than the corresponding one in the transition metal dichalcogenide MoS₂.