Designing High-Performance Thermoelectrics through Chalcogen Engineering in InSe-based Layered Materials

Abstract

InSe-based van der Waals layered materials InXY (X,Y=S,Se,Te) with a hexagonal phase are investigated by breaking local inversion symmetry to study their structural, mechanical, electronic, and thermoelectric properties. Chalcogen substitution introduces systematic lattice distortions, including reflection symmetry lowering and internal structural relaxation, bond-length asymmetry and enhanced directional bonding, which not only increase structural rigidity and integrity but also contribute to direct-to-indirect band-gap transition driven by sulfur-induced off-centring and achieve a band convergence strategy which provides complementary pathways for controlling the thermopower in InSe-based systems. Importantly, inclusion of spin–orbit coupling (SOC) does not change the electronic or transport properties, as these mainly depend on band dispersion and effective mass, which remain unaffected, therefore, SOC was excluded from subsequent transport calculations without loss of accuracy. In both Te-substituted systems, bonding from In–In and In–Te and antibonding from In–S or In–Se, together with extended orbitals, form a well-defined conductive network that enhances charge delocalisation and long-range connectivity, which essentially creates a robust framework for efficient carrier transport while preserving structural stability. Selection-rule–governed three-phonon processes (AAO, AOO, and OOO), together with Te-induced phonon softening (higher mass, weaker bonding) and emergence of stiffer phonon modes due to S-substitution, enhance anharmonic scattering, shorten phonon lifetimes, and thereby account for the reduced kl in InSe-based systems. Consequently, InSe0.5Te0.5 achieves the highest electron ZT (3.08 at 600 K) and InS0.5Te0.5 the highest hole ZT (1.17 at 600 K), whereas S-only substituted InSe0.5S0.5 exhibits lower ZT than pristine InSe across all temperatures, indicating that S substitution alone does not enhance thermoelectric performance. Overall, targeted chalcogen substitution in InSe-based layered materials effectively tunes electronic and phononic transport, enabling robust structural stability and significantly enhanced thermoelectric performance.