First-Principles Calculations on Thermoelectric Properties of Layered Transition Metal Phosphides MP2 (M = Ni, Pd, Pt)

Very recently, a stable structure of layered transition metal phosphides MP2 (M = Ni, Pd and Pt) has been discovered. This study reveals that these three materials are semiconductor materials with a direct band gap (0.5–0.85 eV) and also have very high hole and electron mobility. The electron mobility of the PtP2 monolayer is surprisingly as high as 14.4 × 104 cm2 V−1 s−1. Based on their excellent electronic properties, herein we predict the thermoelectric (TE) performance of MP2 monolayers by using first-principles and Boltzmann transport equation. The calculation results show that the single-layer NiP2, PdP2 and PtP2 have low lattice thermal conductivity 7.6 W m−1 K−1, 9.0 W m−1 K−1 and 17.8 W m−1 K−1 at room temperature, respectively, which is mainly caused by the small phonon group velocity, low relaxation time, high Gruneisen parameter and large phonon scattering phase space. Because of the ultra-high carrier mobility and the highly degenerate band structure, MP2 monolayers have large Seebeck coefficient, and the value of PdP2 reaches up to 283 μV K−1 at 300 K. The good phonon and electrical transport properties give the MP2 monolayers a high TE figure of merit (ZT). The maximum ZT of NiP2 and PdP2 monolayers at 500 K is 1.52 (p-type) and 1.95 (n-type), respectively, and the corresponding concentration of maximum ZT MP2 monolayers is around 1011 cm−2. Overall, our work indicates that the MP2 monolayers are the promising candidates in TE applications.