Nanoscale Heterogeneities in 2D Ti₃C₂T_x MXene Crystals Revealed by TERS

Ever since the discovery of two-dimensional Ti₃C₂ MXenes, there has been a tremendous interest in the study of their structure, properties, and applications. This class of 2D transition metal carbides and nitrides are produced by the exfoliation of bulk MAX phases and can have a wide variety of stoichiometry and composition, which allows control over their properties. Since they are easily dispersed in water, they can easily be processed into a variety of forms such as powders, free-standing films, aerogels, etc. which also allows for tunability of their properties. MXenes are well-known for having a very high metallic conductivity, hydrophilic nature due to a number of surface terminations, high mechanical stiffness and strength, and tunable plasmonic, electrochromic, and optoelectronic properties. These properties combined with the high surface area of 2D MXenes have led to their utilization in battery anodes due to their exceptional capacitive charge storage. They are shown to have use in composites, as transparent conducting electrodes in LEDs and photovoltaics, EM wave shielding, and in gas and electrochemical sensors among other applications. However, a detailed understanding of their optical and electronic properties at the nanoscale level remains a key open question which is relevant to their utilization for a wide range of applications.<br/> <br/>Tip-enhance Raman Spectroscopy (TERS) is a technique which uses a nanoscale plasmonic scanning probe to generate Raman signal from a nanoscale focal volume, thereby providing deep sub-diffraction limited optical spatial resolution proportional to that of Atomic Force Microscopy. TERS has been used to study nanoscale heterogeneities in other 2D materials such as graphene, transition metal dichalcogenides as well as their van der Waals heterostructures. Here, we report for the first time TERS imaging of mono- and few-layer crystals of 2D Ti₃C₂T_x MXenes (Tx is surface groups such as =O, -OH and -F) deposited on gold substrate with 785 nm and 830 nm excitation wavelengths, matching the transverse plasmon resonance of 2D Ti₃C₂T_x located at 780-800 nm. TERS spectra collected from the monolayers are strongly dominated by the intense peak A1g (Ti, C, T_x) at 203 cm^-1. As the number of layers increase, relative intensity of the resonant 126 cm^-1 and 725 cm^-1 peaks as compared to that of the 203 cm^-1 peak also increase, though the absolute intensity of the peaks decrease. In addition, we observed peculiar TERS response from wrinkles in MXene sheets, which commonly appear in crystals of Ti3C2Tx (and other 2D materials) deposited from colloidal suspensions. TERS spectra of the wrinkles featured a strongly enhanced absolute intensity of the 126 cm^-1 and 725 cm^-1 peaks, even in wrinkles that were up to 20 nm high. Using TERS for nanoscale spectroscopic characterization of Ti₃C₂T_x allows collecting fast Raman maps with deep sub-diffraction resolution with a laser power density on the sample about an order of magnitude lower compared to previously conducted confocal microRaman measurements. In addition, we show that the intensity of the TERS response from the mono- to few-layer crystals of Ti₃C₂T_x can be used to track early stages of degradation in ambient conditions, well before noticeable morphological changes start to appear in these crystals. In summary, TERS imaging of 2D Ti₃C₂T_x MXenes has enabled detailed nanoscale spectroscopic characterization which is essential to the understanding of their crystal structure and properties.