Role of Surface Termination in the Structural and Electronic Properties of Sc₂CTₓ MXene

Graphene-like layered transition metal carbides, nitrides, or carbonitrides, called MXenes, obey the stoichiometric formula of M ₙ₊₁ n + 1 X ₙ n T ₓ x , where M is an early transition metal such as scandium (Sc), n is a natural number, X is C, N, or CN, and T ₓ x is a functional group such as –O, –F, or –OH that passivates the surface of the MXene. The electronic structure of bare Sc ₂ 2 C and functionalized Sc ₂ 2 CT ₓ x MXenes are explored by performing first-principles density functional theory (DFT) calculations. The bare Sc ₂ 2 C is metallic, but less stable than its passivated structure. The Sc ₂ 2 C MXene has an interlayer 2D electron gas not bound to Sc or C atoms but free to move, making it an electride. DFT calculations show that functionalization can open an energy gap in Sc ₂ 2 CT ₓ x MXenes. The size and type (direct versus indirect) of the bandgap vary with the functional groups, which provides a means for opening and tuning of the band gap.