Dual-optimisation of thermoelectric performance in MSe/MTe (M = Ga, In, and Tl) heterobilayers via structural symmetry breaking and band topology evolution.

This study presents a systematic first-principles investigation into the structural, electronic, and transport properties of MSe/MTe (M = Ga, In, and Tl) heterobilayer series, demonstrating how the deliberate breaking of structural symmetry via isoelectronic substitution simultaneously optimises electronic band topology and suppresses lattice thermal conductivity (κ1) for high-performance thermoelectrics. In all heterobilayers (HBLs), avoided crossings between acoustic and optical modes enhance scattering and reduce phonon velocity, thereby lowering κ1, a process intensified by overlapping frequency ranges and rattling vibrations and specifically suppressed by nearly flat acoustic bands in TlSe/TlTe, which provide an additional scattering phase space, absent in Ga- and In-based systems. The conduction band minimum (CBM) maintains parabolic dispersion at the zone centre, while the valence band maximum (VBM) evolve from a "Mexican-hat" topology in symmetric systems, marked by van Hove singularities and low mobility, to a "double-dome" structure via symmetry breaking and metal-induced orbital delocalization. This exhibits a transition that effectively reduces hole effective mass and enhances both carrier mobility and thermoelectric performance. Additionally, COHP and Bader charge analyses reveal that the interplay between weakened metal-chalcogen interactions and delocalized bonding, most pronounced in TlSe/TlTe, enhances phonon scattering, resulting in ultralow κ1, while robust metal-metal bond-driven "conductive networks" and more localised charge distributions in Ga- and In-based systems sustain the high electrical conductivity essential for optimised thermoelectric performance. Overall, by systematically breaking inversion and out-of-plane mirror symmetry through isoelectronic chalcogen substitution, the MSe/MTe heterobilayer series achieves a dual-optimisation of thermoelectric performance, where restructured valence band topologies enhance carrier mobility, while asymmetric chemical bonding induces strong phonon scattering, yielding peak ZT values of 2.06 (n-type, InSe/InTe) and 1.44 (p-type, TlSe/TlTe) at 800 K.