Prediction of thermoelectric performance for monolayer HfNI

The full-potential linearization enhanced plane wave method and the semi-classical Boltzmann theory are used to calculate the thermoelectric properties of monolayer HfNI. For monolayer HfNI, the bandgap, which is calculated by Tran–Blaha-modified Becke–Johnson (TB-mBJ), is larger than that of generalized gradient approximation. There is no imaginary frequency for the phonon band structure of monolayer HfNI, which guarantees its dynamic stability. Although monolayer HfNI has two heavy elements Hf and I atoms, due to the shrinkage effect of lanthanides the phonon gap of monolayer HfNI becomes wide and the phonon dispersion phase space decreases, which suppresses the three-phonon dispersion channel. Moreover, monolayer HfNI has a higher lattice thermal conductivity. Finally, when the carrier concentration is 5 × 1018 cm–3, the ZTmax of n-type monolayer HfNI with TB-mBJ is 0.91 at 500 K.