Tip-Enhanced Raman Scattering Imaging of Single- to Few-Layer Ti3C2Tx MXene

MXenes are among the most widely researched materials due to a unique combination of high electronic conductivity and hydrophilic surface, confined in a 2D structure. Therefore, comprehensive characterization of individual MXene flakes is of great importance. Here we report on nanoscale Raman imaging of single-layer and few-layer flakes of Ti₃C₂T_x MXene deposited on a gold substrate using tip-enhanced Raman scattering (TERS). TERS spectra of MXene monolayers are dominated by an intense peak at around 201 cm^-1 and two well-defined peaks at around 126 and 725 cm^-1. Absolute intensities of these peaks decrease with increasing number of layers, though the relative intensity of the 126 and 725 cm^-1 bands as compared to the 201 cm^-1 band increases. The peak positions of the main MXene bands do not significantly change in flakes of different number of layers, suggesting weak coupling between the MXene layers. In addition, we observed stiffening of the 201 cm^-1 vibration over the wrinkles in MXene flakes. Using TERS for nanoscale spectroscopic characterization of Ti₃C₂T_x allows fast Raman mapping with deep subdiffraction resolution at the laser power density on the sample about an order of magnitude lower as compared to confocal Raman measurements. Finally, we demonstrate very high environmental stability of stoichiometric single-layer MXenes and show that the intensity of TERS response from the single- and few-layer flakes of Ti₃C₂T_x can be used to track early stages of degradation, well before significant morphological changes appear.