Theoretical Studies on Structural, Electronic, Piezoelectric, and Optical Properties of Janus Sc₂CXY (X ≠ Y, X/Y = F, Cl, Br, and I) MXenes

Using first-principles calculations, the structural, electronic, piezoelectric, and optical properties of Janus Sc₂CXY (X ≠ Y, X/Y = F, Cl, Br, and I) MXenes were comprehensively investigated. All of the studied MXenes are structurally stable. Both Young’s modulus Y and Poisson’s ratio ν are isotropic and can be modulated by the surface-functionalized halogen atoms. In addition, the band gaps of all the Janus Sc₂CXY MXenes can be modulated from 0.638 eV to 1.437 eV by different functional groups. Interestingly, the semiconductor–metal transition for Janus Sc₂CXI (X = F, Cl, and Br) can occur when the compressive strain reaches 6%. Furthermore, because of their symmetric breaking, the Janus Sc₂CXY MXenes exhibit excellent in-plane and out-of-plane piezoelectric properties. High optical absorption is found in the visible and ultraviolet ranges for all the studied MXenes, and it is expected that they can be utilized in the fields of piezoelectric and optoelectronic nanodevices.