Thermoelectric properties of X₃N₂O₂ (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 X₃N₂O₂ (X = Hf, Zr) MXene monolayers, taking electron-phonon coupling into consideration. Owing to their similar geometrical structures, electronic band structures, and phonon dispersions, X₃N₂O₂ 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 Hf₃N₂O₂ monolayer and 23 μW cm^-1 K^-2 for the Zr₃N₂O₂ monolayer. In terms of phonon transport, the lattice thermal conductivity for the Zr₃N₂O₂ monolayer is higher than that for the Hf₃N₂O₂ monolayer, due to larger phonon group velocity. Our results show that the Hf₃N₂O₂ monolayer is more suitable for thermoelectric materials than the Zr₃N₂O₂ 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 X₃N₂O₂ MXene monolayers.