MXenes, i.e. 2D transition metal carbide or nitride layers, are one of the largest family of 2D materials. Because, they exhibit a quite unique combination of hydrophilic behaviour and metallic conductivity, they are investigated for a constantly growing number of applications including energy storage, or optoeletronic/biomedical applications. Being obtained from the chemical exfoliation of strongly bonded nanolaminated ceramics, i.e. the MAX phases, MXene layers are intrinsically functionalized by different T-surface groups (e.g. O, OH, F, Cl) inherited from the exfoliation process and which play a pivotal role on their properties. These T-groups are also known to play a crucial role in composites, serving as anchor points for the deposition of active nanomaterials on MXene supports. As a consequence, the properties of MXene multilayers are governed by the complex interplay between structural (i.e. number of layers, average interlayer distance) and surface functionalization effects.
In this context, and focusing on the benchmark Ti₃C₂T_x MXene, we highlight Electron Energy-Loss Spectroscopy (EELS) in the (S)TEM, coupled to Density Functional Theory (DFT) simulations, as a very powerful approach allowing to quantitatively analyse these two aspects. First, we show that valence EELS (VEELS) allows to directly quantify the absolute number of layers in Ti₃C₂T_x stacks for thicknesses below ∼ 10 nm. This can be done using the large change between the surface and bulk modes intensity ratios in the VEEL spectrum, as predicted by DFT, and confirmed experimentally. DFT simulations also show that the bulk plasmon shift gives access to interlayer distance modifications in a given stack with sub-angström sensitivity. Further, the edges corresponding to the excitation of core electrons provide information on a the very local scale. Using the carbon K-edge as a marker, we show that DFT simulations allow to interpret modifications in experimental data in terms of perturbations of either the Ti₃C₂T_x layers surface, or in terms of structural disorder within the layers. These findings can straightforwardly be used to characterize MXene/nanomaterials interactions in composites.