Electron Energy Loss Structures of Terminated Scandium and Hafnium MXene Monolayers from First-Principles Calculations

In this study, the energy loss near edge structure (ELNES) of two-dimensional transition metal carbides and nitrides, namely 'MXenes', has been calculated in the density functional theory using the full-potential linearized augmented plane wave method. Calculations of band structure demonstrate that, in agreement with other studies, although MAX phases (bulks) and pristine MXenes (monolayers) are all metallic, some functionalized MXenes are semiconductors. For the first time, we have calculated the ELNES spectra of carbon and nitrogen K-edges in MAX M2AlX phases, pristine M2X MXenes and functionalized MXenes M2XT2 (M = Sc, Hf; X = C, N; T = F, OH, O) at magic angle conditions. Compared to the M2AlX bulks, the differences of energy between the main peaks in carbon and nitrogen K edges of M2X monolayers increase due to the slightly smaller bond lengths that can be used as a fingerprint for monolayers. The structures in ELNES spectra are sensitive to the chemical nature and the location of the T groups on the MXene surface. The energy position of the first main peak and the separation between the main peaks in the ELNES spectra of M2XT2 increase from T = O to T = OH and to T = F surface groups, respectivelyshows that the M-X bond length decreases from T = O to T = OH and to T = F. This short bond in M2XF2 leads to enhanced M-X interaction. The main features in carbon and nitrogen K edges correspond to the peaks in the unoccupied densities of states (DOS), namely sigma* and pi* states; however, the contribution of sigma* states is dominant. Furthermore, for accurate insight into the excitonic effects (electron-hole coupling) in core edges, we have used the solving of the equation of motion of the two-particle Green's function, the Bethe-Salpeter equation (BSE) to determine these effects on the K x-ray absorption near edge structure (XANES) of semiconductors Sc2CF2, Sc2C(OH)(2), Sc2CO2 and Hf2CO2 MXenes. Calculations have been performed in two different levels of theory, with and without considering the excitonic effects. The excitonic effects considerably result in shifting the spectral features to the lower energies and changing in the intensity and overall shapes of the absorption spectra.