We develop transparent epitaxial SnO2 films with low thermal conductivity and high carrier mobility by domain engineering using the substrates with low symmetry: intentional control of the domain size and the defect density between crystal domains. The epitaxial SnO2 films on r-Al2O3 (a low symmetry substrate) exhibit a twice higher mobility than the epitaxial SnO2 films on c-Al2O3 (a high symmetry substrate), resulting in twice larger thermoelectric power factor in the SnO2 films on r-Al2O3. This mobility difference is likely attributed to the defect density between crystal domains. Furthermore, both samples exhibit almost the same thermal conductivities (∼5.1 ± 0.4 W m−1 K−1 for SnO2/r-Al2O3 sample and ∼5.5 ± 1.0 W m−1 K−1 for SnO2/c-Al2O3 sample), because their domain sizes are almost the same. The uni-leg type film thermoelectric power generator composed of the domain-engineered SnO2 film generates the maximum power density of ∼54 μW m−2 at the temperature difference of 20 K. This demonstrates that a transparent film thermoelectric power generator based on the domain engineering is promising to run some internet of things sensors in our human society.

For developing high performance thermoelectric materials based on anomalous Nernst effect (ANE), we propose a ferromagnetic metal/ semiconductor multilayer, where ferromagnetic metal/semiconductor interfaces prevent heat carrier transport (thermal conductivity reduction), and are expected to enhance the ANE coefficient additionally. The amorphous Co/Si multilayer displays an extremely-low thermal conductivity of ∼1.5 Wm −1 K −1 due to the interface phonon scattering and undoped Si amorphous layers with low thermal conductivity. Concurrently, the multilayers exhibit 2.4 times larger anomalous term of transverse Seebeck coefficient than Co films. This demonstrates that the nanostructure with the ferromagnetic metal/semiconductor interfaces is beneficial for developing thermoelectric generation based on ANE.

We achieved substantially low thermal conductivity by introducing some crystal disorder into complex material BaSi2 films on Si substrates for realization of a high-performance thermoelectric material. The BaSi2 films/Si exhibited a low thermal conductivity of 0.96 W m−1 K−1 without nanostructuring, which is less than about two-thirds value of bulk BaSi2 and is the lowest among ecofriendly silicide materials. This substantially low thermal conductivity was brought by both the use of complex material with intrinsically low phonon group velocity and the introduction of point defects as the crystal disorder. The first-principles calculations revealed that the point defect modulates the phonon dispersion relation lowering longitudinal acoustic phonon group velocity. However, the transverse acoustic phonon group velocity was increased at the same time, resulting in a negligible change in average acoustic phonon group velocity. This indicated that the crystal disorder effect related to point defects in this system is enhancement of phonon scattering, not lowering phonon group velocity. The BaSi2 films/Si with point defects exhibited a higher thermoelectric power factor (2.9 μW cm−1 K−2) than bulk BaSi2. These results highlight that complex material BaSi2 film/Si with point defects, having substantially low thermal conductivity, is a candidate as a thermoelectric power generator material in the sensor network.

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