The dielectric response of two-dimensional (2D) ${\mathrm{Ti}}_3{\mathrm{C}}_2$ stacked sheets was investigated by high-resolution transmission electron energy-loss spectroscopy and ab initio calculations in the 0.2–30-eV energy range. Intense surface plasmons (SPs), evidenced at the nanometer scale at energies as low as 0.3 eV, are shown to be the dominant screening process up to at least 45-nm-thick stacks. This domination results from a combination of efficient free-electron dynamics, begrenzungs effect, and reduced interband damping. It is shown that, in principle, the SPs energies can be tuned in the mid-infrared, from 0.2 to 0.7 eV, by controlling the sheets' functionalization and/or thickness. This point evidences a new attribute of this new class of 2D materials.

Nanolamellar MAX phase compounds (Cr_0.5V_0.5)_n+1AlC_n are formed with n = 1, 2 and 3, and their 300 K structure is studied in detail by high-resolution neutron diffraction. While the n = 1 compound is found to have complete disordering of vanadium and chromium in the metallic layers, the n = 2 and 3 compounds show strong tendency for these elements' ordering, with the layer in the 2a(0,0,0) site of (Cr_0.5V_0.5)₃AlC₂ fully occupied by vanadium. The thermal expansion dependency of temperature is also studied by neutron diffraction for 2 < T < 550 K, revealing a negligible thermal expansion below 100 K for all of the compounds.

Valence electron energy-loss (VEEL) spectroscopy in the transmission electron microscope is combined to ab initio calculations to investigate the dielectric properties of multilayer (ML) two-dimensional ${\mathrm{Ti}}_{3}{\mathrm{C}}_{2}{T}_{2}$ ($T\ensuremath{\equiv}\text{OH}$ or F) MXene. Besides evidencing important similarities between the ${\mathrm{ML}\ensuremath{-}\mathrm{Ti}}_{3}{\mathrm{C}}_{2}{T}_{2}$ and TiC valence electron gas behaviors, a clear interband transition characteristic of the most stable site of the $T$ functionalization groups is identified in the VEEL spectrum. This signature, highly dependent on the $T$-group localization on the surface, has a prominent effect on the optical properties of the ML, leading to $40%$ variations in the optical conductivity in the middle of the visible spectrum. Such a dependence could be of crucial interest for optical transparent thin films or sensing applications.