Effect of Nb-X ionic bonding on the superconductivity of the two-dimensional Nb2SXC (X=O, S, Se, F, Cl, and Br)

The 2D Janus structure, an important derivative of 2D materials, exhibits distinct properties and significant potential in nanodevices. In this study, we focused on the recently synthesized 2D transition metal carbo-chalcogenide Nb₂S₂C [Adv. Mater. 34, 2200574 (2022)]. Through first-principles calculations, we designed five stable 2D Janus Nb₂SXC (X=O, Se, F, Cl, and Br) structures by substituting the top-layer sulfur atoms with X atoms. Both the intrinsic 2D Nb₂S₂C and the five 2D Janus Nb₂SXC structures display promising superconductivity, with an estimated T_c ranging from 1.35 to 12.66 K. The superconductivity is primarily attributed to the strong coupling between the vibration modes of the transverse acoustic branch and the electrons of Nb atoms. Further analysis reveals the significant role of electronegativity in the superconductivity of X elements. For X elements within the same main group, a larger electronegativity corresponds to stronger ionic Nb-X bonds, resulting in further softening of the transverse acoustic mode and enhanced superconductivity. These findings emphasize the crucial contribution of ionic Nb-X bonding in determining the T_c of the 2D Janus Nb₂SXC system, thus expanding the design possibilities for this wide range of superconducting materials.