Band engineering of honeycomb monolayer CuSe via atomic modification*

Two-dimensional monolayer copper selenide (CuSe) has been epitaxially grown and predicted to host the Dirac nodal line fermion (DNLF). However the metallic state of monolayer CuSe inhibits the potential application of nanoelectronic devices in which a band gap is needed to realize on/off properties. Here, we engineer the band structure of monolayer CuSe which is an analogue of a p-doped system via external atomic modification in an effort to realize the semiconducting state. We found that the H and Li modified monolayer CuSe shifts the energy band and opens an energy gap around the Fermi level. Interestingly, both the atomic and electronic structures of monolayer CuHSe and CuLiSe are very different. The H atoms bind on top of Se atoms of monolayer CuSe with Se-H polar covalent bonds, annihilating the DNLF band of monolayer CuSe dominated by Se pz orbitals. In contrast, Li atoms prefer to adsorb at the hexagonal center of CuSe, preserving the DNLF band of monolayer CuSe dominated by Se pz orbitals, but opening band gaps due to a slight buckling of the CuSe layer. The realization of metal-to-semiconductor transition from monolayer CuSe to CuXSe (X=H, Li) as revealed by first-principles calculations makes it possible to use CuSe in future electronic devices.