Cutting edge(s): towards realistic modelling of MXene flakes

Abstract

We investigate the structural, energetic, electronic, and magnetic properties of newly designed and realistic Ti₂C MXene flake models using density functional theory (DFT) calculations. By means of the Wulff construction procedure, we model flakes that closely resemble experimentally synthesized structures, exhibiting a hexagonal morphology dominated by the thermodynamically stable (110)-nonpolar surface. As the flake size increases, structural parameters and relative stability converge toward periodic Ti₂C slab behaviour, with diminishing quantum confinement (QC) effects. Electronic structure calculations reveal a gapless nature across all sizes, with negligible QC effects on optical properties, even under different magnetic configurations. Hybrid HSE06 calculations predict a slight increase in the band gap compared to the PBE functional, yet this gap vanishes in larger flakes, aligning with periodic slab behaviour, while the band edge energy decreases as the flake size increases. Spin-polarized calculations confirm an antiferromagnetic (AFM) ground state for all flakes, with energy differences between the AFM, nonmagnetic (NM), and ferromagnetic (FM) states increasing with increasing size. Notably, for smaller (Ti₂C)_n flakes (n < 90), AFM and FM configurations are nearly degenerate, whereas for larger flakes (n ≥ 90), AFM becomes the definitive ground state. Functionalization of the basal plane with oxygen stabilizes Ti₂CO₂ flakes, inducing structural edge bending, suppressing magnetism, and favouring an NM ground state. The band gaps in functionalized flakes exhibit size-dependent narrowing due to the presence of edge surfaces, diverging from periodic slab trends due to the change in energy of the electronic states. This work provides more realistic, physically meaningful models and offers new insights beyond conventional periodic approaches.