Electronic and magnetic structure of ultrathin Co9Se8 nanosheets and Co9Se8 bulk from density functional theory calculations

Two-dimensional transition-metal chalcogenides are one of the most interesting new classes of 2D materials. 2D cobalt selenide nanomaterials are being actively pursued and investigated as well, and recently, the first large-scale synthesis of half-unit-cell-thick Co9Se8 nanosheets has been reported. In the present study, using density functional theory calculations and employing the advanced hybrid HSE06 exchange-correlation functional, the electronic and magnetic structure of Co9Se8 nanosheets is investigated. To allow a comparison with bulk, the electronic and magnetic properties of Co9Se8 bulk are investigated as well. The bulk phase is found to be a ferrimagnetic semiconductor. Reducing the dimensions of Co9Se8 bulk to form a half-unit-cell-thick Co9Se8 nanosheet strongly alters the electronic and magnetic structure of the material. The nanosheet is predicted to have a different type of ferrimagnetic ordering and to be a metal with semimetallic features. Analysis of the X–M segment of the nanosheet's band structure, which mainly determines the electronic transport properties of the material, shows that within this segment, the main band below the Fermi level consists of only spin-up states (Se 4p), while the main band above the Fermi level consists of only spin-down states (Se 4p hybridized with Co 3d). This combination points to possibilities of selective conductivity of either spin-up or spin-down electrons and indicates that ultrathin Co9Se8 nanosheets are a most promising candidate for nanoelectronic and spintronic applications.