Novel Janus 2H-Tl<sub>2</sub>SSe Monolayer with Ultralow\nLattice Thermal Conductivity and Outstanding Thermoelectric Performance

Understanding the lattice dynamics from the perspective\nof chemical\nbonds and phonon transport is essential for finding and designing\nhigh-efficiency thermoelectric (TE) materials and achieving applications.\nIn this work, we have constructed a novel two-dimensional (2D) Janus\n2H-Tl<sub>2</sub>SSe from a 2D 2H-TlS monolayer for the first time.\nThe intrinsic low lattice thermal conductivity of the Janus 2H-Tl<sub>2</sub>SSe monolayer is attributed to the coexistence of weak chemical\nbonding and strong phonon anharmonicity, combining first-principles\ncalculations and Boltzmann transport theory. The crystal orbital Hamilton\npopulation (COHP) analysis reveals that the weak Tl–S and Tl–Se\nchemical bonding, stemming from the filled antibonding orbitals, results\nin a low average phonon group velocity (<i>v</i><sub>g</sub>) and acousto–optic coupling. Through further investigating\nthe scattering rates of various scattering channels, we have demonstrated\nthat acousto–optic coupling plays a crucial role in providing\nimportant scattering channels for three-phonon scattering. Furthermore,\nthe Janus structure of the 2H-Tl<sub>2</sub>SSe monolayer breaks the\noriginal symmetry, resulting in additional anharmonicity. The low <i>v</i><sub>g</sub> and strong phonon scattering enable the Tl<sub>2</sub>SSe monolayer to exhibit a low room-temperature lattice thermal\nconductivity (κ<sub>L</sub>) of 1.88 W mK<sup>–1</sup>. With the further inclusion of electron–phonon (el–ph)\nscattering, our research reveals that the Tl<sub>2</sub>SSe monolayer\npresents a high <i>ZT</i> of 2.31 at 700 K, attributed to\nits low κ<sub>L</sub> and excellent electronic transport performance\nfor n-type doping. Our work demonstrates that Janus Tl<sub>2</sub>SSe monolayer has been identified as a novel thermoelectric material\nfor promising medium-temperature applications.