Thermoelectric properties of X 3 N 2 O 2 (X = Hf, Zr) MXene monolayers: a first-principles study.

MXene monolayers have received increasing attention due to their unique properties, particularly their high conductivity, which shows great potential in thermoelectric materials. In this paper, we present a theoretical study of the thermoelectric properties of X 3 N 2 O 2 (X = Hf, Zr) MXene monolayers, taking electron-phonon coupling into consideration. Owing to their similar geometrical structures, electronic band structures, and phonon dispersions, X 3 N 2 O 2 MXene monolayers exhibit homogeneous electron and phonon transport properties. The conduction band shows multi-valley characteristics which leads to better n-type electron transport properties than p-type ones. The maximum values of the n-type power factor can reach 32 μW cm -1 K -2 for the Hf 3 N 2 O 2 monolayer and 23 μW cm -1 K -2 for the Zr 3 N 2 O 2 monolayer. In terms of phonon transport, the lattice thermal conductivity for the Zr 3 N 2 O 2 monolayer is higher than that for the Hf 3 N 2 O 2 monolayer, due to larger phonon group velocity. Our results show that the Hf 3 N 2 O 2 monolayer is more suitable for thermoelectric materials than the Zr 3 N 2 O 2 monolayer, with optimal n-type thermoelectric figure of merit ( ZT ) values of 0.36 and 0.15 at 700 K, respectively. These findings may be useful for the development of wearable thermoelectric devices and sensor applications based on X 3 N 2 O 2 MXene monolayers.