Ultralow In-Plane Thermal Conductivity in 2D Magnetic Mosaic Superlattices for Enhanced Thermoelectric Performance.

Lowering thermal conductivity via introducing heterointerfaces of heterophase fillings (HPFs) is a common strategy for optimizing thermoelectric performance, but it is always accompanied by deterioration of electrical conductivity. Here we report an ordered magnetic HPF system in a CoSe 2 -SnSe mosaic heterostructure superlattice synthesized by van der Waals confined epitaxial growth (vdWCEG), which realizes a maximized filling amount to decrease in-plane thermal conductivity of SnSe layers and maintain the intact in-plane carrier transport path. The in-plane thermal conductivity of CoSe 2 -SnSe superlattice reaches the lowest range among SnSe-based materials with a value of 0.27 W m -1 K -1 at 850 K, which can be attributed to abundant interfaces between CoSe 2 nanocrystals and SnSe layers. Moreover, the CoSe 2 nanocrystals show superparamagnetic behavior, by which the rotation of magnetic domains provides additional phonon scattering to further decrease in-plane thermal conductivity. By combination with the preserved in-plane electrical conductivity of SnSe layers, an enhanced in-plane ZT value of 0.62 is achieved at 850 K. This vdWCEG approach can also be generally applied to fabricate various other two-dimensional (2D) mosaic heterostructures, providing an avenue for artificial 2D heterostructures with desired functionalities.