Titanium sulphene: Two-dimensional confinement of electrons and phonons giving rise to improved thermoelectric performance

We investigated the thermoelectric properties of titanium sulphene, namely, few-layered TiS ₂ nanosheets, by using density functional theory and the Boltzmann transport equations. The Seebeck coefficient of titanium sulphene was found to increase if the thickness fell below 14 triple layers (∼8 nm) and for the monolayer becomes 40% larger than that of the bulk TiS ₂. This behavior is attributed to an enhancement in the density of states near the conduction band minimum in the monolayer. Moreover, the acoustic phonon band of the monolayer is more flat than that of the bulk, which results in a 37% reduction of the acoustic phonon group velocity and was beneficial for a low lattice thermal conductivity. Therefore, the combined effects from quantum confinement of both electrons and phonons could lead to a significant enhancement in thermoelectric performance in the two-dimensional titanium sulphene.