Stability and electronic properties of sulfur terminated two-dimensional early transition metal carbides and nitrides (MXene)

Experimental results have shown that two-dimensional early transition metal carbides such as Nb2C also can be terminated by S atoms. In this study, 20 types of sulfur terminated MXenes (M2XS2, where M = Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W; X = C, N) are investigated using the density functional theory method. Phonon frequency analysis and molecule dynamic simulations upon 1000 K results show that V2CS2, Cr2NS2, Nb2CS2, Hf2CS2, and Ta2CS2 are dynamically stable. Formation enthalpy calculations indicate that two-dimensional V2CS2, Nb2CS2, Ta2CS2, Hf2CS2, and Cr2NS2 structures have the highest stability and may be easily synthesized. It indicates that MXenes can also be terminated by S atoms, which is influenced by the synthesis process. The binding energies for V2CS2, Nb2CS2, and Cr2NS2 bilayer systems are smaller than 0.18 eV/atom, which indicates they can be easily separated into single layers. In contrast, the binding energies for Ta2CS2 and Hf2CS2 bilayers systems are up to 0.326 and 0.36 eV/atom, respectively. This indicates that Ta2CS2 and Hf2CS2 are relatively difficult to separate into single layers, which can be attributed to the localized p-orbitals of S atoms. Ta2CS2 is a direct band gap semiconductor with band gap of 0.36 eV. Cr2NS2 system is ferromagnetic material among MXenes, with the average magnetic moment of Cr atoms of 1.60 mu(B). This research provides new viewpoints for the synthesis and surface termination of MXenes.