J. Phys. Chem. C., Comp. Mater. Sci., 121, 18947-18953 (2017) [pdf]

Atomistic Origin of Phase Stability in Oxygen-Functionalized MXene: A Comparative Study

Avanish Mishra, Pooja Srivastava, Abel Carreras, Isao Tanaka, Hiroshi Mizuseki, Kwang-Ryeol Lee, Abhishek Singh

Oxygen-functionalized MXene, M₂CO₂ (M = group III-V metals), are emergent formidable two-dimensional (2D) materials with a tantalizing possibility for device applications. Using first-principles calculations, we perform an intensive study on the stability of fully O-functionalized (M₂CO₂)MXenes. Depending on the position of O atoms, the M₂CO₂ canexist in two different structural phases. On one side of MXene, the O atom occupies a site which is exactly on the top of the metal atom from the opposite side. On the other side, the O atom can occupy either the site on the top of the metal atom of the opposite side (BB' phase) or on the top of the C atom (CB phase). We find that for M = Sc and Y the CB phase is stable, whereas for M = Ti, Zr, Hf, V, Nb, and Ta the stable phase is BB'. The electron localization function, the atom-projected density of states, the charge transfer, and the Bader charge analyses provide a rational explanation for the relative stability of these two phases and justify the ground state structure by giving information about the preferential site of adsorption for the O atoms. We also calculate the phonon dispersion relations for both phases of M₂CO₂. The BB'-Sc2CO2 and the CB-Ti2CO2 are found to bedynamically unstable. Finally, we find that the instability of BB'-M₂CO₂ (M = Sc and Y) originates from the weakening of M-C interactions, which manifest as a phonon mode with imaginary frequency corresponding to the motion of C atom in the a-b plane. The insight into the stability of these competing structural phases of M₂CO₂ presented in this study is an important step inthe direction of identifying the stable phases of these 2D materials..