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

Abstract

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 Mo₂C as well as the surface-terminated Mo₂CT₂ (T = O, F, and Cl). Our findings indicate that 2H-Mo₂C 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 Mo₂C 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-Mo₂C, whereas the Cl atom energetically stabilizes on 1T-Mo₂C. Meanwhile, with certain configurations, 2H-Mo₂CF₂ and 1T-Mo₂CF₂ with slightly different energies could exist simultaneously. The Mo₂CO₂ 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-Mo₂CO₂ are responsible for its promising mechanical properties. Interestingly, the topological properties of 2H-Mo₂CO₂ at a wide range of strains (−10% to 12%) are reported. Moreover, 2H-Mo₂CF₂ is metallic through the range of calculation. Meanwhile, originally semiconducting 1T-Mo₂CF₂ and 1T-Mo₂CCl₂ 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.