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, internal structural relaxation, and bond-length asymmetry with enhanced directional bonding, which not only increase structural rigidity and integrity but also contribute to a direct-to-indirect band-gap transition driven by sulfur-induced off-centering, while both conventional and unconventional band convergence strategies provide complementary pathways for controlling the thermopower in InSe-based systems. In both Te-substituted systems, the coexistence of bonding interactions from In–In and In–Te and antibonding contributions from In–S or In–Se, together with spatially extended orbitals, forms a well-defined conductive network that enhances charge delocalization and long-range connectivity, creating a robust framework for efficient carrier transport while preserving structural stability. Selection-rule-governed three-phonon scattering processes (AAO, AOO, and OOO), together with Te-induced phonon softening arising from higher atomic mass and weaker bonding, and the emergence of stiffer phonon modes due to S-substitution, collectively enhance anharmonic scattering, shorten phonon lifetimes, and suppress lattice thermal conductivity across the InSe-based series. Consequently, InSe_0.5Te_0.5 achieves the highest n-type ZT of 3.08 at 600 K and InS_0.5Te_0.5 delivers the best p-type ZT of 1.17 at 600 K, whereas S-only substituted InS_0.5Se_0.5 exhibits lower ZT than pristine InSe across all temperatures, confirming 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.