Nanostructure engineering has been extensively applied to ZnO in an effort to improve its performance in thermoelectric material, solar cell, and nanogenerator applications. Nano-structured ZnO bulks are limited by their inherently low mobility caused by the high density of grain boundaries and interfaces. In this study, a hybrid micro/nano structure composed of nearly coherent grain boundaries with a low misorientation degree among the nanograins was successfully fabricated in Zn 1Àx Al x O (x ¼ 0, 0.01, 0.02, 0.03, 0.04 mol) bulks via hydrothermal synthesis and spark plasma sintering. Despite the large amount of nanograin boundaries and interfaces in the resulting material, a high carrier mobility value (50.7 cm 2 V À1 s À1 ) was obtained in the x ¼ 0.2 sampleclose to the level shown by ZnO single crystals and far higher than that of its ordinary nano-structured counterparts (<15 cm 2 V À1 s À1 ). A reduced thermal conductivity value of 2.1 W m À1 K À1 at 1073 K was also obtained in the micro/nano-structured x ¼ 0.02 bulk due to extremely effective scattering at boundaries and interfaces also present in the nano-structured counterparts. After the simultaneous optimization of both electrical and thermal transport properties, the micro/nanostructured x ¼ 0.02 sample showed a high ZT value (up to 0.36) at 1073 K. The proposed micro/nanostructure may also be applicable to other thermoelectric materials for further ZT enhancement.

Transparent amorphous oxide semiconductors (TAOSs) based transparent thin-film transistors (TTFTs) with high field effect mobility (μFE) are essential for developing advanced flat panel displays. Among TAOSs, amorphous (a-) SnO2 has several advantages against current a-InGaZnO4 such as higher μFE and being indium free. Although a-SnO2 TTFT has been demonstrated several times, the operation mechanism has not been clarified thus far due to the strong gas sensing characteristics of SnO2. Here we clarify the operation mechanism of a-SnO2 TTFT by electric field thermopower modulation analyses. We prepared a bottom-gate top-contact type TTFT using 4.2-nm-thick a-SnO2 as the channel without any surface passivation. The effective thickness of the conducting channel was ~1.7 ±0.4 nm in air and in vacuum, but a large threshold gate voltage shift occurred in different atmospheres; this is attributed to carrier depletion near at the top surface (~2.5 nm) of the a-SnO2 due to its interaction with the gas molecules and the resulting shift in the Fermi energy. The present results would provide a fundamental design concept to develop a-SnO2 TTFT.

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