Structural Stability and Electronic Structure of (Ti,Ta)2C MXenes

Abstract

Graphene-like two-dimensional early transition metal carbides, nitrides, or carbonitrides with a metal-to-nonmetal atomic ratio of (n+1)/n called MXenes have many excellent electronic, optical, and electrochemical properties, such as metallicity, high electrical conductivity, larger charge density, high capacitance, efficient lithium, sodium, and potassium ions intercalability, etc. First-principles Density Functional Theory (DFT) calculations are performed to study the structural relationship of (Ti, Ta)₂C MXenes to their electronic structure, magnetism, and optical properties. Calculations show carbide MXenes with a single transition metal, namely, titanium carbide (Ti₂C) and tantalum carbide (Ta₂C), and an ordered carbide MXene with double transition metals, namely, titanium tantalum carbide (TiTaC) are non-magnetic and metallic. The density of states at the Fermi level for each of (Ti, Ta)₂C MXenes is dominated by the outermost d-orbitals of transition metal and the 2p orbital of carbon. The dielectric response function is a photon energy-dependent anisotropic quantity. In the static limit, the imaginary parts of the dielectric functions vanish for each of the (Ti, Ta)₂C MXenes. However, the real part of the static dielectric constant is as high as two orders of magnitude for tantalum-containing MXenes and almost half of that for titanium-containing single transition metal carbide MXene.