The Thermoelectric Performance of Monolayer Penta-NiN₂ with Four-Phonon Scattering

The exploration of two-dimensional semiconductors exhibiting poor lattice heat conduction offers a viable pathway for developing high-efficiency thermoelectric materials. Calculations incorporating four-phonon scattering reveal that monolayer penta-NiN₂ possesses remarkably low lattice thermal conductivity, indicating its significant potential for thermoelectric applications. Here, we systematically evaluate the thermoelectric properties of monolayer penta-NiN₂ by combining first-principles calculations with Boltzmann transport theory, with particular emphasis on the role of four-phonon scattering. Structural analysis confirms its stability and semiconducting nature with a direct band gap. The inclusion of four-phonon scattering leads to a pronounced reduction in the lattice thermal conductivity, from 51.87 W/mK (three-phonon only) to 4.27 W/mK at room temperature. Electronic transport analysis reveals that hole doping yields a substantially higher power factor (2659 μW/cm/K²) than electron doping (1141 μW/cm/K²). Consequently, the optimal ZT reaches 7.57 for hole doping and 1.83 for electron doping with four-phonon processes taken into account, both significantly higher than those obtained under the three-phonon-only scheme. These results establish monolayer penta-NiN₂ as a promising thermoelectric candidate and highlight the critical importance of incorporating four-phonon scattering for a reliable description of thermal transport in penta-2D materials.