NaCuX (X = Se, Te): Novel 2D Materials Simultaneously With Ultra-low Thermal Conductivity and High Thermoelectric Figure of Merit

Based on first principles calculations and Boltzmann transport theory, the electronic structures and thermoelectric properties of two-dimensional NaCuSe and NaCuTe were systematically investigated. The results show that the monolayers are direct semiconductors with narrow band-gaps of 1.18 and 1.53 eV, respectively. Also, they simultaneously exhibit high Seebeck coefficient (0.21 ∼ 0.88 mV/K), high conductivity (∼ 105 Ω−1m−1), and high power factor (0.98 ∼ 1.18 mW/mK2). Moreover, owing to low group velocity and high phonon-phonon scattering rate, they also possess low lattice thermal conductivities of 0.10 ∼ 0.23 W/mK. As a result, these two monolayers exhibit high thermoelectric figure of merit of 0.46 ∼ 0.73 at 300 K, and rise even further to 1.37 ∼ 3.67 at 700 K.