Grafting nanotechnology on thermoelectric materials leads to significant advances in their performance. Creation of structural defects including nano-inclusion and interfaces via nanostructuring achieves higher thermoelectric efficiencies. However, it is still challenging to optimize the nanostructure via conventional fabrication techniques. The thermal instability of nanostructures remains an issue in the reproducibility of fabrication processes and long-term stability during operation. This work presents a versatile strategy to create numerous interfaces in a thermoelectric material via an atomic-layer deposition (ALD) technique. An extremely thin ZnO layer was conformally formed via ALD over the Bi0.4Sb1.6Te3 powders, and numerous heterogeneous interfaces were generated from the formation of Bi0.4Sb1.6Te3–ZnO core–shell structures even after high-temperature sintering. The incorporation of ALD-grown ZnO into the Bi0.4Sb1.6Te3 matrix blocks phonon propagation and also provides tunability in electronic carrier density via impurity doping at the heterogeneous grain boundaries. The exquisite control in the ALD cycles provides a high thermoelectric performance of zT = 1.50 ± 0.15 (at 329–360 K). Specifically, ALD is an industry compatible technique that allows uniform and conformal coating over large quantities of powders. The study is promising in terms of the mass production of nanostructured thermoelectric materials with considerable improvements in performance via an industry compatible and reproducible route.

Bonding geometry engineering of metal–oxygen octahedra is a facile way of tailoring various functional properties of transition metal oxides. Several approaches, including epitaxial strain, thickness, and stoichiometry control, have been proposed to efficiently tune the rotation and tilt of the octahedra, but these approaches are inevitably accompanied by unnecessary structural modifications such as changes in thin‐film lattice parameters. In this study, a method to selectively engineer the octahedral bonding geometries is proposed, while maintaining other parameters that might implicitly influence the functional properties. A concept of octahedral tilt propagation engineering is developed using atomically designed SrRuO 3 /SrTiO 3 (SRO/STO) superlattices. In particular, the propagation of RuO 6 octahedral tilt within the SRO layers having identical thicknesses is systematically controlled by varying the thickness of adjacent STO layers. This leads to a substantial modification in the electromagnetic properties of the SRO layer, significantly enhancing the magnetic moment of Ru. This approach provides a method to selectively manipulate the bonding geometry of strongly correlated oxides, thereby enabling a better understanding and greater controllability of their functional properties.

No abstract

We investigated the electronic structures of ultrathin SrRuO3 (SRO) films with n = 1, 2, 3, 4, and 8 monolayers (MLs) on SrTiO3 substrates using O K-edge X-ray absorption spectroscopy (XAS). The intensities of the low-energy features reflect the strengths of the Ru 4d-O 2p orbital hybridization. The Ru 4d orbital state evolves with the increasing SRO thickness, exhibiting a crossover at approximately n = 2. For thick SRO films (n ≥ 3), this constitutes a metallic band, while for the 1 or 2 ML film, the band features shift to a higher energy to form a bandgap (> 0.2 eV), reflecting the emergent insulating nature. The polarization dependence of the peak intensities further shows that in the metallic films (n ≥ 3), Ru t2g - O 2p hybridizations are strong and anisotropic with stronger (weaker) equatorial (apical) hybridizations, possibly owing to compressive strain effects from the SrTiO3 substrate, while in thinner films (n ≤ 2), the hybridization effects become weak and rather isotropic because of the localization of Ru 4d orbitals. Thus, the evolution of anisotropic hybridizations in SRO films in the vicinity of the thickness-driven metal-insulator transition was substantiated.