Carbon K edge spectra of functionalized molybdenum-based MXenes, Mo2CT2 (T = F, OH and O), from first-principles calculations

Abstract

In this study, the effect of the termination groups (T = F, OH, O) on the energy loss near edge structure (ELNES) of carbon K edge in Mo2C MXene at orientation-independent conditions has been investigated using first-principles calculations based on the full-potential linearized augmented plane wave (FP-LAPW) method. The results show that within the YS-PBE0 functional, the Mo2CF2 is a semiconductor with an indirect band gap of 0.723 eV. For Mo2CO2, the indirect band gap increases to 0.17 eV within the screened hybrid functional. The calculation results of ELNES spectra with the affection of core-hole show that, in comparison to pristine Mo2C, as a fingerprint of termination groups, the spectral structures in Mo2CT2 are reproduced at higher energies. Moreover, the spectral features of Mo2CT2 are sensitive to the chemical nature and the location of the T groups on the pristine Mo2C MXene surface. When going from T = O to T = F and, further, to T = OH, the energy separation between the main peaks increases, which is a sign of decreasing the Mo-C bond length, respectively, from T = O to T = F and to T = OH. The comparison of ELNES spectra and the unoccupied densities of states (DOS) reveal that, the origin of the first structure at the carbon K edge of Mo2CT2 is mostly result from electron transition to pz state, while in pristine Mo2C, mainly due to the transition to px + py state. Other structures at higher energies mainly arise from electron transitions to px + py state and partially to pz state. The spectral decomposition of the ELNES into in-plane (l′ = 1, m′ = ± 1) and out-of-plane (l′ = 1, m′ = 0) components also confirms these results. Generally, in both Mo2C and Mo2CT2, the contribution of in-plane element in most of the structures is more considerable.