Raman Spectroscopy Characterization of 2D Carbide and Carbonitride MXenes

The first step to wider adoption of two-dimensional (2D) materials is understanding their fundamental properties by employing characterization methods, among which Raman spectroscopy plays a unique role, being a fast and nondestructive tool. The number, frequencies, and intensities of the modes (or bands) in the Raman spectrum have been used to identify the 2D materials' crystal lattice, bonding, and even number of layers. MXenes, 2D transition metal carbides, nitrides, and carbonitrides, span diverse chemistries and structures, but only a few Raman spectra have been reported. This work is the first systematic experimental Raman spectroscopy study of the MXene family. We explore the vibrational spectra and provide peak assignments for ten MXenes with varying structures (from 2 to 4 atomic layers of transition metal) and compositions─Ti2CTx, Nb2CTx, Mo2CTx, V2CTx, Ti3C2Tx, Mo2TiC2Tx, Ti3CNTx, Nb4C3Tx, V4C3Tx, and Mo2Ti2C3Tx (terminated with −F, −OH, and ═O) based on the experimental results and previously reported computational studies. We discuss the effects of MXene layer thickness, surface terminations, and MXene's metallic properties on Raman scattering. Additionally, we employ polarized Raman spectroscopy to identify out-of-plane vibrations and explain the higher frequency region of the spectra. Finally, we demonstrate how electrochemical reactions affect molecular Raman scattering through the change in surface terminations. By creating the Raman spectra library of the most frequently used MXenes, we open the door for the use of Raman spectroscopy for fingerprinting and in situ studies of various MXenes.