Surface passivation induced a significant enhancement of superconductivity in layered two-dimensional MSi₂N₄ (M = Ta...

Two-dimensional (2D) transition metal di-nitrides (TMN₂) have been arousing great interest for their unique mechanic, electronic, optoelectronic, and magnetic properties. The recent successful growth of monolayer MSi₂N₄ (M = Mo and W) further motivates us to explore new physics and unusual properties behind this family. By using first-principles calculations and Bardeen-Cooper-Schrieffer theory, we predicted the existence of the superconductivity in single-layer (SL) 1T- and 1H-TaN₂ with superconducting transition temperatures (T_c) of ∼0.86 and 1.3 K. Specifically, the T_c could be greatly enhanced to ∼24.6 K by passivating the TaN₂ monolayer with Si-N bilayers. Furthermore, the superconductivity could be increased to ∼30.4 K via substituting lighter Nb for Ta. This enhancement of superconductivity mainly stems from the softer vibration modes consisting of in-plane Ta/Nb vibrations mixed with Si-xy vibrations. The superconductivity can be further tuned by applying external strains and carrier doping. This enhancement strategy of surface passivation and light atom substitution would suggest a new platform for 2D superconductors and provide an instructive pathway for next-generation nanoelectronics.