Spectroscopic evidence in the visible-ultraviolet energy range of surface functionalization sites in the multilayerTi3C2MXene

Abstract

Valence electron energy-loss (VEEL) spectroscopy in the transmission electron microscope is combined to ab initio calculations to investigate the dielectric properties of multilayer (ML) two-dimensional ${\mathrm{Ti}}_{3}{\mathrm{C}}_{2}{T}_{2}$ ($T\ensuremath{\equiv}\text{OH}$ or F) MXene. Besides evidencing important similarities between the ${\mathrm{ML}\ensuremath{-}\mathrm{Ti}}_{3}{\mathrm{C}}_{2}{T}_{2}$ and TiC valence electron gas behaviors, a clear interband transition characteristic of the most stable site of the $T$ functionalization groups is identified in the VEEL spectrum. This signature, highly dependent on the $T$-group localization on the surface, has a prominent effect on the optical properties of the ML, leading to $40%$ variations in the optical conductivity in the middle of the visible spectrum. Such a dependence could be of crucial interest for optical transparent thin films or sensing applications.