Unique optical and plasmonic in MXenes: surface functional group effects from first principles

MXenes have attracted great attention in recent years due to their impressive photoelectric properties. In particular, the rich and adjustable set of surface groups offer endless possibilities for enhancing the optical and plasmonic properties of the typical Mo₂CT_x MXenes. Using density functional theory and many-body perturbation theory, we present first-principle calculations that capture all of the significant microscopic mechanisms underlying surface plasmon decay, thereby quantitatively evaluating the effects of various surface functional groups (T = -H, -O, -F) on the dielectric response, optical and plasmonic properties of Mo₂CT_x MXenes. In the low-frequency limit, the results show that the frequency-dependent momentum relaxation time of Mo₂CH₂ is an order of magnitude higher than that of Mo₂CO₂. Thus, surface functional groups can effectively reduce optical-phonon scattering in lower frequencies, thereby modulating the low scattering rate in Mo₂CT_x to extend to a larger frequency range. Interestingly, significant differences in light transmittance within 2 to 2.6 eV photon frequency range can guide laboratory analysis of the types of functional group adsorbed on the surface of the prepared MXenes monolayer. In addition, -H functional group modulates the plasmon frequency of MXenes to a wider visible than graphene. Mo₂CH₂ exhibits highly confined plasmon with long propagation lengths at much higher frequencies, all of which are influenced by temperature. Our results complement experimental studies and will provide theoretical support for the surface group design of MXenes.