Structures and optoelectronic properties of two-dimensional MC6 (M = Ti and Hf) predicted by computational approaches

The group ⅥB elements have good compatibility with carbon to form two-dimensional carbides. In this work, two kinds of two-dimensional metal carbides (2D-MC₆, M = Ti and Hf) were theoretically proposed. Using first-principles calculations, we optimized their structures and checked their dynamical/thermal stabilities. Furthermore, their electronic, mechanical and optical properties were also computationally investigated. Calculations on electronic properties show that the 2D-TiC₆ and 2D-HfC₆ exhibit direct band gap of 0.82 eV and indirect band gap of 1.20 eV, respectively. Calculations on mechanical properties indicate that 2D-TiC₆ and 2D-HfC₆ have good elasticity. By applying biaxial strain or external electric field, the electronic band gap of 2D-TiC₆ and 2D-HfC₆ can be effectively tuned. Due to the moderate direct band gap, the 2D-TiC₆ exhibits smaller exciton binding energy (0.58 eV) and larger absorbed photon flux (0.66 mA cm⁻²) than those of 2D-HfC₆. Particularly, the 2D-TiC₆ shows potential for short wave infrared detection or emission applications when certain tensile strain is applied. This study provides new insight into the rational design of functional nanomaterials.