Synergistic Effect of Phonon Localization and Band Convergence in a WSe₂–WTe₂ Superlattice Leads to High Thermoelectric Performance

Abstract

Tuning the chemical bonding characteristics and achieving high band degeneracy by forming a superlattice structure could be an efficient approach to improve thermoelectric performance. This study composes two mechanisms: band convergence and localized vibration originating from antibonding states between neighbouring atoms, responsible for enhancing the thermoelectric figure of merit in a WSe2–WTe2 superlattice. Here, we achieve band convergence and high band degeneracy, leading to a remarkably high Seebeck coefficient of 2331 μV/K, considerably elevating that of the parent material WSe2 (1695 μV/K). The presence of strong W–Te antibonding near the Fermi level along with strong phonon softening of particular out-of-plane ZA mode originating mostly from W–Te bond vibration of the WTe2 sublayer plays a crucial role in reducing lattice thermal conductivity. Additionally, the chemical bonding inhomogeneity with mixed bonding and antibonding characteristics present in the superlattice ignite significant lattice anharmonicity, and the appearance of low energetic optical phonons generates more three-phonon scattering events occurring in a superlattice with more participation of low-lying phonon modes, strongly scatter heat-carrying acoustics phonons. The improved electronic properties and ultralow lattice thermal conductivity lead to a peak n-type ZT ∼ 2.71 at 900 K achieved in the superlattice compared to a WSe2 n-type ZT of 0.68, which is incredible among existing thermoelectric materials.