The stability and electronic properties of hydrogenated Janus MSiGeZ4 (M = Sc-Zn, Y–Ag, Hf–Au; Z = N, P) monolayers: A first-principles study

The stability and electronic properties of the hydrogenated Janus MSiGeZ4 (M = Sc-Zn, Y–Ag, Hf–Au; Z = N, P) monolayers are predicted by the first-principles calculations. There are 12 energetically stable Janus MSiGeZ4 monolayers were selected from 104 candidates. Accordingly, 24 kinds of the semi/full hydrogenated Janus MSiGeZ4 (H-MSGZ) monolayers are predicted to be stable because of the suitable formation, binding and adsorption energy. Then, the electronic properties of semi-hydrogenated and fully hydrogenated Janus MSiGeZ4 were investigated. Based on the analysis of the band structure of the H-MSGZ monolayers, the forbidden band gaps decrease with the hydrogenation degree. Both the intrinsic and hydrogenated Janus MSiGeZ4 monolayers are indirect band gap semiconductors. Among the 12 Janus MSiGeZ4 monolayers, the intrinsic ZnSiGeN4 has the largest band gap of 2.20 eV. Comparing the hydrogenation processes, the band gap of ZnSiGeN4 decreases most dramatical, which change from 2.20 eV (intrinsic ZnSiGeN4) to 0.57 eV (semi-ZnSiGeN4) to 0 eV (full-ZnSiGeN4). Based on density of states analysis, there are many new doping peaks near the Fermi level after the hydrogenation process, and the positions of the peaks are changed, inducing the band gap decrease. The valence band edges of H-MSGZ monolayers are mainly contributed to the hybrid between TM d and N/P p states, while the conduction band edges are mainly contributed by TM d state. Then, the work functions are calculated and analyzed. In the process of hydrogenation, the greatest change of work function is ScSiGeN4, which changes from 6.67 eV (intrinsic ScSiGeN4) to 6.33 eV (semi-ScSiGeN4) and finally to 2.04 eV (full-ScSiGeN4). Overall, the work functions of Janus MSiGeZ4 showed a gradual decrease after hydrogenation because of the hydrogenation reduces the built-in electric field between the H atom and the MSiGeZ4 monolayer surface. Therefore, the hydrogenation can be effective in changing the properties of the material, and imply that H-MSGZ monolayers should have potential for applications in novel electronics, piezoelectric photovoltaics, and photocatalysts.