Comparative analysis of Boron, nitrogen, and phosphorous doping in monolayer of semi-metallic Xenes (Graphene, Silicene, and Germanene) - A first principle calculation based approach

In this work, for the first time, a density functional theory (DFT) based detailed theoretical study has been performed on the comparative effects of Boron (B), Nitrogen (N), and Phosphorous (P) on the structural stability, structural and electronic properties of semimetallic Xene (Graphene, Silicene, and Germanene). The B, N, and P doping reduces the structural stability in Graphene whereas the exact opposite trend has been observed in Germanene. In contrast, B and N doping increase the structural stability, and P doping slightly reduces the structural stability in Silicene. This work also systematically demonstrates the impact of introducing doping species in the same and different sub-lattice sites of Xenes, which distinctly influences cohesive energy, bond length, lattice vector, and average buckling distance. A small compressive strain for N doping and small tensile strains for both B and P doping have been observed in Graphene. However, a notably larger change in the lattice structures and thereby relatively higher tensile strains has been observed in doped Silicene and Germanene. Next, the impact of doping on the electronic properties of Xenes has been assessed from the energy band gap, effective masses, and spin-orbital splitting at the band edges. The substitutional doping in Xene breaks the sub-lattice symmetry and subsequently introduces finite band gaps and effective masses at the band edges. The introduction of dopants in the same sub-lattice site leads to larger energy bandgap and effective masses compared to doping at different sub-lattice sites of Xenes with the exception of P doping in Silicene and Germanene, in which an exact opposite trend has been observed. The doping at the same sub-lattice sites demonstrates significant spin-orbit splitting at the band edges of Silicene and Germanene. This work presents an extensive theoretical and design-level insight into the non-metallic doping approach in Xenes to realize semi-metallic to semiconducting transformation, which can facilitate electronic, optoelectronic, and energy storage applications of doped Xenes in the future.