The structure–property relationship at interfaces is difficult to probe for thermoelectric materials with a complex interfacial microstructure. Designing thermoelectric materials with a simple, structurally-uniform interface provides a facile way to understand how these interfaces influence the transport properties. Here, we synthesized Bi_2−xSb_xTe₃ (x = 0, 0.1, 0.2, 0.4) nanoflakes using a hydrothermal method, and prepared Bi_2−xSb_xTe₃ thin films with predominantly (0001) interfaces by stacking the nanoflakes through spin coating. The influence of the annealing temperature and Sb content on the (0001) interface structure was systematically investigated at atomic scale using aberration-corrected scanning transmission electron microscopy. Annealing and Sb doping facilitate atom diffusion and migration between adjacent nanoflakes along the (0001) interface. As such it enhances interfacial connectivity and improves the electrical transport properties. Interfac reactions create new interfaces that increase the scattering and the Seebeck coefficient. Due to the simultaneous optimization of electrical conductivity and Seebeck coefficient, the maximum power factor of the Bi_1.8Sb_0.2Te₃ nanoflake films reaches 1.72 mW m⁻¹ K⁻², which is 43% higher than that of a pure Bi₂Te₃ thin film.

Incorporating magnetic nanoparticles in thermoelectric (TE) materials introduce magnetic interfaces with additional electron and phonon scattering mechanism for high TE performance. However, the influence of heterogeneous interfaces between magnetic nanoparticles and TE matrix on electronic and thermal transport remains elusive in the thermo-electric-magnetic nanocomposites. Here, using p-type TE material Bi_0·3Sb_1·7Te₃ (BST) as matrix and magnetocaloric (MC) material La(Fe_0·92Co_0.08)_11.9Si_1.1 (LFS) nanoparticles as a second phase, TE/MC nanocomposites xLFS/BST (x = 0.1%, 0.2%, 0.3% and 0.4%) were synthesized using spark plasma sintering method. The atomic-resolution interfacial structures demonstrate that Te vacancies originating from LFS-BST interfacial reaction decreases the hole concentration of the LFS/BST nanocomposites and enhances the Seebeck coefficient. The LFS/BST nanocomposites exhibit lower thermal conductivity due to enhanced phonon scattering by interfaces and defects. All the nanocomposites have higher ZT than BST matrix, with 0.2%LFS/BST nanocomposite achieving highest ZT = 1.11 at 380 K. At working current 1.4 A, the device fabricated using 0.2%LFS/BST nanocomposite achieves maximal cooling temperature 4.9 K, which is 58% higher than the matrix. Moreover, the MC properties are retained in all the nanocomposites, which make them a promising candidate to achieve high TE performance and dual TE/MC properties for future applications.