Thermoelectric properties for AA- and AB-stacking of a carbon nitride polymorph (C3N4)

Abstract

The transport properties, and electronic structure, for C₃N₄ were investigated, and it was found that the valence band maximum (VBM) and the conduction band minimum (CBM) of AA-stacking of C₃N₄ are located at the A point of the Brillouin zone (BZ), resulting in a direct band gap of approximately 0.69 eV (LDA), 0.870 eV (GGA), 1.237 eV (EV-GGA), and 2.589 eV (mBJ). For AB-stacking, the VBM and the CBM are situated at the Γ point of the BZ, maintaining a direct band gap of approximately 1.204 eV (LDA), 1.357 eV (GGA), 1.680 eV (EV-GGA), and 2.990 eV (mBJ). The calculated electronic band structure was used to determine the thermoelectric properties such as electrical conductivity (σ/τ), electronic thermal conductivity (κ_e/τ), Seebeck coefficient (S), power factor (P), and figure of merit (ZT) using the semi-classical Boltzmann theory as incorporated in the BoltzTraP code. The maximum values of σ for AA-stacking are 3.4 × 10²⁰ (Ω m s)⁻¹ and 1.05 × 10²⁰ (Ω m s)⁻¹, which are achieved at ±0.17 μ(eV) for p-type and n-type materials at 300 and 600 K, respectively. For AB-stacking, the maximum values are 3.8 × 10²⁰ and 3.7 × 10²⁰ (Ω m s)⁻¹ at 300 and 600 K for p-type, whereas they are 2.8 × 10²⁰ (Ω m s)⁻¹ at 300 and 600 K for n-type; these values are achieved at ±0.2 μ(eV) for p-type and n-type. One can see that AA-stacking exhibits the highest S values of approximately 99.0 (μV K⁻¹) for p-type and 98.0 (μV K⁻¹) for n-type, while AB-stacking exhibits the maximum S values of approximately 270 (μV K⁻¹) for p-type and 280 (μV K⁻¹) for n-type. The critical points of S for p-type and n-type are ±0.03 μ(eV) and ±0.08 μ(eV) for AA- and AB-stacking of C₃N₄, respectively. The values of the critical points are the range where AA- and AB-stacking of C₃N₄ is expected to exhibit good thermoelectric properties, and beyond these critical points, S is zero. AA-stacking shows the minimum values of the κ_e between ±0.03 μ(eV), while they are ±0.08 μ(eV) for AB-stacking. Therefore, in these regions, AA- and AB-stacking are expected to exhibit maximum efficiency. The highest peaks of ZT (equal to unity) are confined between ±0.05 μ(eV) for AA-stacking and ±0.1 μ(eV) for AB-stacking.