Biaxial stress and functional groups (T = O, F, and Cl) tuning the structural, mechanical, and electronic properties of monolayer molybdenum carbide.

MXenes are a family of novel two-dimensional (2D) materials attracting intensive interest because of the rich chemistry rooted from the highly diversified surface functional groups. This enables the chemical optimization suitable for versatile applications, including energy conversion and storage, sensors, and catalysis. This work reports the study of the crystal energetics, electronic properties, and mechanical properties, and the impacts of strain on the electronic properties of tetragonal (1T) and hexagonal (2H) phases of MoC as well as the surface-terminated MoCT (T = O, F, and Cl). Our findings indicate that 2H-MoC is energetically more stabilized than the 1T counterpart, and the 1T-to-2H transition requires a substantial energy of 210 meV per atom. The presence of surface termination T atoms on MoC intrinsically induces variations in the atomic structure. The calculated structures were selected based on the energetic and thermodynamic stabilities (400 K). The O atom prefers to be terminated on 2H-MoC, whereas the Cl atom energetically stabilizes on 1T-MoC. Meanwhile, with certain configurations, 2H-MoCF and 1T-MoCF with slightly different energies could exist simultaneously. The MoCO possesses the highest mechanical strength and elastic modulus ( = 52 GPa at = 20% and = 507 GPa). The nature of the ordered centrosymmetric layer and the strong bonding between 4 d-Mo and 2 p-O of 2H-MoCO are responsible for its promising mechanical properties. Interestingly, the topological properties of 2H-MoCO at a wide range of strains (-10% to 12%) are reported. Moreover, 2H-MoCF is metallic through the range of calculation. Meanwhile, originally semiconducting 1T-MoCF and 1T-MoCCl preserve their features under the ranges of the strain of -2% to 10% and -1% to 5%, respectively, beyond which they undergo the semiconductor-to-metal transitions. These findings would guide the potential applications in modern 2D straintronic devices.