Covalent surface modifications and superconductivity of two-dimensional metal carbide MXenes

Versatile chemical transformations of surface functional groups intwo-dimensional transition-metal carbides (MXenes) open up a previously unexploreddesign space for this broad class of functional materials. We introduce a generalstrategy to install and remove surface groups by performing substitution andelimination reactions in molten inorganic salts. Successful synthesis of MXeneswith oxygen, imido, sulfur, chlorine, selenium, bromine, and tellurium surfaceterminations, as well as bare MXenes (no surface termination), was demonstrated.These MXenes show distinctive structural and electronic properties. For example,the surface groups control interatomic distances in the MXene lattice, andTi n +1 C n ( n = 1, 2) MXenes terminated with telluride(Te 2− ) ligands show a giant (>18%) in-plane latticeexpansion compared with the unstrained titanium carbide lattice. The surfacegroups also control superconductivity of niobium carbide MXenes.