First-principles study on the electrical and thermal properties of the semiconducting Sc3(CN)F2 MXene

Abstract

The two-dimensional materials MXenes have recently attracted interest for their excellent performance from diverse perspectives indicated by experiments and theoretical calculations. For the application of MXenes in electronic devices, the exploration of semiconducting MXenes arouses particular interest. In this work, despite the metallic properties of Sc₃C₂F₂ and Sc₃N₂F₂, we find that Sc₃(CN)F₂ is a semiconductor with an indirect band gap of 1.18 eV, which is an expansion of the semiconducting family members of MXene. Using first-principles calculations, the electrical and thermal properties of the semiconducting Sc₃(CN)F₂ MXene are studied. The electron mobilities are determined to possess strong anisotropy, while the hole mobilities show isotropy, i.e. 1.348 × 10³ cm² V^-1 s^-1 along x, 0.319 × 10³ cm² V^-1 s^-1 along the y directions for electron mobilities, and 0.517 × 10³ cm² V^-1 s^-1 along x, 0.540 × 10³ cm² V^-1 s^-1 along the y directions for hole mobilities. The room-temperature thermal conductivity along the Γ → M direction is determined to be 123-283 W m^-1 K^-1 with a flake length of 1-100 μm. Besides, Sc₃(CN)F₂ presents a relatively high specific heat of 547 J kg^-1 K^-1 and a low thermal expansion coefficient of 8.703 × 10^-6 K^-1. Our findings suggest that the Sc₃(CN)F₂ MXene might be a candidate material in the design and application of 2D nanoelectronic devices.

Fig. 1. The top view (a) and side view (b) of the Sc₃(CN)F₂ MXene. (c) The Brillouin zone of the 2D hexagonal and orthorhombic lattice, the high symmetry routes Γ → K (Γ → X) and Γ → M (Γ → Y) correspond to the real-space x and y directions, respectively. (d) The band structure of the Sc₃(CN)F₂ MXene based on the orthorhombic cell with the Fermi level located at zero. The atoms are represented by spheres: Sc (green), C (black), N (blue) and F (orange).

Fig. 2. Band structures of three F terminated MXenes Sc₃C₂F₂ (a), Sc₃N₂F₂ (b) and Sc₃(CN)F₂ (c), and the vacuum energy is set as zero. Red solid and black dotted lines represent electronic energy bands from GGA-PBE and HSE06 respectively. The side view of the Sc₃C₂F₂, Sc₃N₂F₂ and Sc₃(CN)F₂ MXenes are shown below each band structure figure respectively.

Fig. 3. Partial density of states for the Sc₃(CN)F₂ MXene.

Fig. 4. (a) Band structures of Sc₃(CN)F₂ in Model 0, 1 and 2, the Fermi level located at zero; (b) the C (black) and N (blue) atoms arrangement models sketch, atoms under the gray semi-transparent interfaces present the second layer atoms.

Fig. 5. The phonon dispersions of the Sc₃C₂F₂ (a), Sc₃N₂F₂ (b) and Sc₃(CN)F₂ (c) MXenes.

Fig. 6. The temperature dependence thermal conductivities for the Sc₃(CN)F₂ MXene along the Γ → M (a) and Γ → K (b) directions with 5 μm flake length with TA, LA and ZA contributions. The temperature dependence thermal conductivities for the Sc₃(CN)F₂ MXene with 1–100 μm flake lengths along the Γ → M (c) and Γ → K (d) directions.

Fig. 7. (a) The temperature dependence of Sc₃(CN)F₂ specific heat. (b) The temperature dependence of the Sc₃(CN)F₂ thermal expansion coefficient.