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 Nb 2 C also can be terminated by S atoms. In this study, 20 types of sulfur terminated MXenes (M 2 XS 2 , 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 V 2 CS 2 , Cr 2 NS 2 , Nb 2 CS 2 , Hf 2 CS 2 , and Ta 2 CS 2 are dynamically stable. Formation enthalpy calculations indicate that two-dimensional V 2 CS 2 , Nb 2 CS 2 , Ta 2 CS 2 , Hf 2 CS 2 , and Cr 2 NS 2 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 V 2 CS 2 , Nb 2 CS 2 , and Cr 2 NS 2 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 Ta 2 CS 2 and Hf 2 CS 2 bilayers systems are up to 0.326 and 0.36 eV/atom, respectively. This indicates that Ta 2 CS 2 and Hf 2 CS 2 are relatively difficult to separate into single layers, which can be attributed to the localized p-orbitals of S atoms. Ta 2 CS 2 is a direct band gap semiconductor with band gap of 0.36 eV. Cr 2 NS 2 system is ferromagnetic material among MXenes, with the average magnetic moment of Cr atoms of 1.60 μ B . This research provides new viewpoints for the synthesis and surface termination of MXenes. [ABSTRACT FROM AUTHOR]