Ultra-low lattice thermal conductivity and high thermoelectric performance in chiral phonon-protected...

Heterostructures have recently emerged as suitable candidates for energy applications owing to their intriguing high electronic properties and low thermal conductivity. Traditionally, experimentally observable chiral phonons, protected by the crystal hexagonal symmetry in two-dimensional honeycomb lattices and originating primarily at the Brillouin zone corner, play a critical role in achieving ultra-low lattice thermal conductivity (κ_l). It is mainly attributed to the three-fold rotational symmetry-driven circularly polarized chiral phonons that restrict certain scattering processes and reduce thermal resistance. Herein, we identify an innovative mechanism that not only preserves phonon chirality but also minimises lattice thermal conductivity by satisfying certain phonon selection rules and enhancing anharmonic scattering, particularly in the long wavelength limit. This study encompasses first-principles calculations and solutions to the Boltzmann transport equation, along with various thermal transport theories to investigate the structural, thermal and electronic transport properties of group-IV-based dichalcogenides in hexagonal phase MSe₂ (M = Mo and W) and their heterostructures (HS), i.e. MoSe₂/WSe₂ and MoSeTe/WSeTe. Interestingly, phenomena such as membrane effect and hybridization of acoustics and low-lying optical phonons combined with phonon branching and phonon bunching are identified to execute an imperative role in suppressing thermal transport, resulting in a low κ_l value of 2.83 W m^-1 K^-1 in MoSe₂/WSe₂ and an ultra-low κ_l of 0.5 W m^-1 K^-1 in MoSeTe/WSeTe HS. The disparity in computing carrier mobility using deformation potential theory (DPT) is addressed herein, and to rectify the inconsistency, we additionally incorporate the Fröhlich interaction, which accounts for longitudinal optical phonons in estimating the carrier mobility accurately. Overall, the present work exploits the intrinsic properties of chiral phonons through breaking the crystal inversion symmetry, providing important insights into the rational design of low-dimensional thermoelectric materials.