Role of Functional Group in Electronic Structure and Optics of Sc₂CTx MXene Electride

Two-dimensional graphene-like layered transition metal carbides, nitrides, or carbonitrides called MXenes obey the stoichiometric formula of M_n+1X_nT_x, where M is an early transition metal, X is C, N, or CN, and Tx is a functional group such as -O, -F, -OH, etc. that passivates the surface of the MXene. As the top-down wet method is the most commonly used method for synthesizing MXenes, one needs to passivate the crystal structure with functional groups such as -O, -F, and -H, bonded to the transition metal. Sc₂CT_x MXene has a unique electronic structure, characterized by anionic electrons that are unbound from any atomic nuclei within the crystal system but reside in the crystal’s periodic lattice vacancies, known as electrides. The electrons occupying those lattice vacancies wander in the interstitial spaces, which can be exploited in current technology that requires materials with low work functions and electron-emitting abilities. Among the few known MXene electrides, Sc₂CTx, a 3d transition metal MXene electride, is the one reported to exhibit semiconducting properties. We perform the first principles Density Functional Theory (DFT) calculations on pristine Sc₂C and its functionalized form. Calculation shows Sc₂C in its preferred crystal structure symmetry is metallic, while the functionalized ones are semiconducting. DFT calculations show that Sc₂CTx is a small band gap semiconductor for oxygen and fluorine terminations, but not for hydrogen termination.

This work is supported by the Department of Energy BES-RENEW award number DE-SC0024611