Surface termination effects on Raman spectra of Ti₃C₂T MXenes: an in situ UHV analysis

Ti₃C₂T MXenes have typically a mixed surface termination of oxygen, hydroxyl and fluorine groups (T). In this work, we investigate the influence of the surface termination on the vibrational properties of Ti₃C₂T by performing thermal desorption and in situ Raman spectroscopy in ultra-high-vacuum (UHV). Significant changes in the Raman spectra occur after annealing above 600 °C, correlated with the desorption of approximately 80% of the fluorine termination, as confirmed by mass spectrometry and X-ray photoemission spectra. In particular, the intense Raman mode at 203 cm^-1, usually attributed to a Ti-C-layer stretching vibration, is strongly damped upon fluorine desorption, while the broad spectral features between 220 and 680 cm^-1, usually attributed to surface group vibrations, are not changing significantly. We show that the Raman spectra and the change induced by fluorine desorption are well represented by the phonon density of states instead of zone-center phonon modes. Disorder-induced Raman scattering strongly contributes to the Raman spectra. Moreover, due to the metallic nature of MXenes, charge density fluctuation scattering contributes as well. We show that the two scattering mechanisms, deformation potential and charge density fluctuation, may lead to opposite interpretations concerning the symmetry of the fluorine-related mode at 203 cm^-1. This study provides new insights into the interpretation of the Raman spectra of MXenes, especially regarding the relation between surface chemistry and vibrational spectroscopy.