Optical gaps and excitons in semiconducting transition metal carbides (MXenes)

Abstract

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.