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 ab initio 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 Mo 2 C as well as the surface-terminated Mo 2 CT 2 (T = O, F, and Cl). Our findings indicate that 2H-Mo 2 C 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 Mo 2 C 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-Mo 2 C, whereas the Cl atom energetically stabilizes on 1T-Mo 2 C. Meanwhile, with certain configurations, 2H-Mo 2 CF 2 and 1T-Mo 2 CF 2 with slightly different energies could exist simultaneously. The Mo 2 CO 2 possesses the highest mechanical strength and elastic modulus ( σ max = 52 GPa at ε b = 20% and E = 507 GPa). The nature of the ordered centrosymmetric layer and the strong bonding between 4 d-Mo and 2 p-O of 2H-Mo 2 CO 2 are responsible for its promising mechanical properties. Interestingly, the topological properties of 2H-Mo 2 CO 2 at a wide range of strains (-10% to 12%) are reported. Moreover, 2H-Mo 2 CF 2 is metallic through the range of calculation. Meanwhile, originally semiconducting 1T-Mo 2 CF 2 and 1T-Mo 2 CCl 2 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.