2D materials, MXenes, optical properties, density functional theory, many-body methods

We use time-dependent density functional theory together with the HSE06 hybrid functional to investigate the optical and excitonic properties of two-dimensional transition metal carbides, MXenes. We determine reliable optical gaps, optical absorbance spectra, and exciton features for a set of eight semiconducting MXenes. The optical gaps of Sc2CF2, Cr2CF2, Cr2C(OH)2, and anti-ferromagnetic Mn2CO2 (1.9 - 2.3 eV) lie in the energy region of visible (VIS) light. Sc2C(OH)2, Ti2C, Ti2CO2, and ferromagnetic Mn2CO2 with smaller optical gaps (0.4 - 1.2 eV) well absorb solar radiation, including VIS light. Moreover, Ti2C and ferromagnetic Mn2CO2 show high monolayer absorbance of 10 - 20% in the 1 - 3 eV energy range. Finally, we analyse the excitons in MXenes and find that the first bright excitons of Sc- and Ti-based MXenes are strongly localized in k-space while the corresponding excitons of Cr- and Mn-based systems are delocalized.