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.

Ballistic phonon transport was observed in Si films containing Ge nanodots. In SiGe films containing Ge nanodots, thermal conductivity was drastically reduced close to that of amorphous materials due to alloy phonon scattering and nanodot scattering.

A transparent thermoelectric material requires not only high thermoelectric performance but also high optical transmittance. However, in transparent nanostructured thermoelectric materials, the nanostructure interface brings the trade-off relationship between thermal conductivity and optical transmittance. We propose an approach for the simultaneous control of thermal conductivity and optical transmittance in the epitaxial nanostructured films, where carriers can be smoothly transported. This is realized by the interface design based on the three strategies: (1) a large atomic mass difference at the heterointerface for low thermal conductivity; (2) heterointerface with almost the same refractive index and flat surface for high optical transmittance; and (3) epitaxial heterointerface for smooth carrier transport. We formed epitaxial ZnO/SnO2 multilayer films based on this design guideline. The multilayer films exhibit lower thermal conductivity and higher optical transmittance than an ever reported transparent nanostructured thermoelectric material. These results highlight that this design is promising to realize high-performance transparent nanostructured thermoelectric materials.

It is expected to develop the measurement system to obtain physical/chemical information with nanoscale space resolution related to the non-thermal equilibrium phenomena. In this study, we developed controlled temperature-gradient kelvin force microscopy (T-KFM) to measure the temperature difference (ΔT)-induced vacuum level variation under non-thermal equilibrium. Therein, the biggest issue, difficulty in applying the large ΔT in narrow space (∼100 μm), was solved by introducing “heating and cooling systems” in T-KFM; one sample side is heated using a ceramic heater and the other side is cooled using liquid nitrogen. Using T-KFM, the space distribution of ΔT-induced vacuum level variation was well observed on the scale of hundreds of nanometers in a polycrystalline ZnO film with nanostructures. The obtained image of the ΔT-induced vacuum level variation can reflect a distribution of the thermal properties such as the thermal resistance and thermoelectromotive force. This pronounced technique for obtaining surface potential under T-gradient helps us to comprehend the non-thermal equilibrium phenomena.