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

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 MoC 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 MoCF is a semiconductor with an indirect band gap of 0.723 eV. For MoCO, 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 MoC, as a fingerprint of termination groups, the spectral structures in MoCT are reproduced at higher energies. Moreover, the spectral features of MoCT are sensitive to the chemical nature and the location of the T groups on the pristine MoC 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 MoCT is mostly result from electron transition to p state, while in pristine MoC, mainly due to the transition to p + p state. Other structures at higher energies mainly arise from electron transitions to p + p state and partially to p 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 MoC and MoCT, the contribution of in-plane element in most of the structures is more considerable.