First-principles study on the structure and electronic properties of M₂CS (M = Sc, Ti, Y, Zr and Hf, x = 1, 2)

Two-dimensional (2D) transition metal carbides/nitrides, known as MXenes, have attracted extensive attention due to their rich elemental composition and diverse surface chemistry. In this study, the crystal structure, electronic, mechanical, and electronic transport properties of M₂CS (M = Sc, Ti, Y, Zr, and Hf, x = 1, 2) were investigated by density functional theory (DFT). Our results showed that the studied M₂CS except Y₂CS₂ are thermodynamically, dynamically, thermally, and mechanically stable. The p-d hybridization between the M-d state and the C/S-p state of M₂CS is stronger than that of the corresponding M₂CS₂. However, the antibonding state would appear near the Fermi level and thus reduce the thermal stability of the material due to the introduction of sulfur vacancies in the Y-free MXenes studied. In contrast, sulfur vacancies would significantly enhance the bonding states of Y-C and Y-S bonds and improve the stability of Y₂CS. This provides an explanation for the experimentally observed formation of non-stoichiometric Ti₂CS_1.2. The room-temperature electron mobilities of semiconductor Sc₂CS (Y₂CS) along the x and y directions were determined to be 232.59 (818.51) and 628.22 (552.55) cm² V^-1 s^-1, and the room-temperature hole mobilities are only 88.32 (1.64) and 61.75 (17.80) cm² V^-1 s^-1. This work is expected to provide theoretical insights for the preparation and application of S-terminated MXenes.