Hybrid density functional study of structural and electronic properties of functionalized Ti_n+1X_n (X=C, N) monolayers

Density functional theory simulations with conventional (PBE) and hybrid (HSE06) functionals were performed to investigate the structural and electronic properties of MXene monolayers, Ti_n+1C_n and Ti _n+1N_n (n=1-9) with surfaces terminated by O, F, H, and OH groups. We find that PBE and HSE06 give similar results. Without functional groups, MXenes have magnetically ordered ground states. All the studied materials are metallic except for Ti₂CO₂, which we predict to be semiconducting. The calculated density of states at the Fermi level of the thicker MXenes (n≥5) is much higher than for thin MXenes, indicating that properties such as electronic conductivity and surface chemistry will be different. In general, the carbides and nitrides behave differently with the same functional groups.