Nanostructured Ferecrystal Intergrowths with TaSe₂ Unveiled High Thermoelectric Performance in <i>n</i>-Type SnSe

Ferecrystals, a distinctive class of misfit layered compounds, hold significant promise in manipulating the phonon transport owing to their two-dimensional (2D) natural superlattice-type structure and turbostratic (rotational) disorder present between the constituent layers. Integrating these 2D intergrowth structures as nanodomains embedded in a bulk thermoelectric matrix is a formidable challenge in synthetic chemistry, yet offers groundbreaking opportunities for efficient thermoelectrics. Here, we have achieved an exceptionally high thermoelectric figure of merit, <i>zT</i> ∼ 2.2, at 823 K in <i>n</i>-type Ta and Br-codoped SnSe, by successfully incorporating [(SnSe)_1.15]₇(TaSe₂)₁ ferecrystals with [110] SnSe//[100] TaSe₂ orientation, as nanostructures with modulations in few nm in bulk SnSe solid-state matrix. While the presence of ferecrystal nanostructures induces strong scattering of heat-carrying phonons resulting in an ultralow lattice thermal conductivity (κ_L) of ∼0.18 W m^-1 K^-1 at 773 K, the Ta and Br codoping strategy increases the concentration of <i>n</i>-type charge carriers for enhanced electrical conductivity. Our approach provides a new pathway for damping the phonon transport and enhancing the thermoelectric performance in 2D layered materials.