Decoupling carrier and phonon scattering in n-type Bi₂Te_2.7Se_0.3 thermoelectrics via atomic layer deposition of TiO₂ nanoshells

Breaking the intrinsic coupling between carrier and phonon transport remains a critical bottleneck limiting further enhancement of Bi₂Te₃-based thermoelectric materials. Herein, we report an interface engineering strategy employing atomic layer deposition (ALD) to construct TiO_2–x nanoshells on Bi₂Te_2.7Se_0.3 powders, followed by spark plasma sintering to form engineered interfacial layers in bulk pellets. By controlling the ALD cycle number, the thickness, crystallinity, and dispersion of TiO_2–x interfaces were tuned to optimize energy filtering and phonon scattering. Conformal amorphous TiO_2–x coatings with thicknesses ranging from 1.5 nm to 8.0 nm were obtained, whereas excessive deposition led to agglomeration and weakened the interfacial effects. The optimized interface enabled selective scattering of low-energy carriers while preserving carrier mobility, resulting in an enhanced Seebeck coefficient without severe loss of electrical conductivity. Simultaneously, enhanced phonon scattering effectively reduced lattice thermal conductivity. The optimized specimen fabricated with 100 ALD cycles achieved the best balance between electrical and thermal transport, yielding a maximum zT of 0.92 at 460 K, corresponding to a 27% improvement compared with pristine Bi₂Te_2.7Se_0.3 bulk pellet. This work demonstrates that precise ALD-enabled interface engineering provides a scalable route to decouple carrier and phonon transport for high-performance thermoelectrics.