Alloying is a long-established strategy to tailor properties of metals for specific applications, thus retaining or enhancing the principal elemental characteristics while offering additional functionality from the added elements. We propose a similar approach to the control of properties of two-dimensional transition metal carbides known as MXenes. MXenes (M n+1 X n ) have two sites for compositional variation: elemental substitution on both the metal (M) and carbon/nitrogen (X) sites presents promising routes for tailoring the chemical, optical, electronic, or mechanical properties of MXenes. Herein, we systematically investigated three interrelated binary solid-solution MXene systems based on Ti, Nb, and/or V at the M-site in a M 2 XT x structure (Ti 2-y Nb y CT x , Ti 2-y V y CT x , and V 2-y Nb y CT x , where T x stands for surface terminations) showing the evolution of electronic and optical properties as a function of composition. All three MXene systems show unlimited solubility and random distribution of metal elements in the metal sublattice. Optically, the MXene systems are tailorable in a nonlinear fashion, with absorption peaks from ultraviolet to near-infrared wavelength. The macroscopic electrical conductivity of solid solution MXenes can be controllably varied over 3 orders of magnitude at room temperature and 6 orders of magnitude from 10 to 300 K. This work greatly increases the number of nonstoichiometric MXenes reported to date and opens avenues for controlling physical properties of different MXenes with a limitless number of compositions possible through M-site solid solutions.

Evidence for a magnetic transition in atomically thin chromium titanium carbide MXene (Cr2TiC2Tx) has been shown experimentally using a combination of magnetometry, spectroscopy, and magnetotransport measurements.

MXenes are a rapidly-expanding family of 2D transition metal carbides and nitrides that have attracted attention due to their excellent performance in applications ranging from energy storage to electromagnetic interference shielding. Numerous other electronic and magnetic properties have been computationally predicted, but not yet realized due to the experimental difficulty in obtaining uniform surface terminations (Tx), necessitating new design approaches for MXenes that are independent of surface groups. In this study, we distinguished the contributions of surface and sub-surface Ti atoms to the electronic structure of four Ti-based MXenes (Ti2CTx, Ti3C2Tx, Cr2TiC2Tx, and Mo2TiC2Tx) using soft x-ray absorption spectroscopy, revealing minimal changes in the spectral features between the parent MAX phase and its MXene when no surface Ti atoms are present in the MXene, such as Mo2TiC2Tx and Cr2TiC2Tx. In contrast, for MXenes with surface Ti atoms, here Ti3C2Tx and Ti2CTx, the Ti L-edge spectra are significantly modified compared to their parent MAX phase compounds. First principles calculations provide similar trends in the partial density of states derived from surface and interior Ti atoms, corroborating the spectroscopic measurements. These results reveal that electronic states derived from sub-surface M-site layers are largely unperturbed by the surface groups, indicating a relatively short length scale over which the Tx groups alter the nominal electron count associated with Ti atoms and suggesting that desired band features should be hosted by sub-surface M-sites that are electronically more robust than their surface M-site counterparts.