Browsing by Author "Sheu, Yu-Miin"

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Double thermoelectric power factor of a 2D electron system
(2018-06-20)
Zhang, Yuqiao
;
Feng, Bin
;
Hayashi, Hiroyuki
;
Chang, Cheng-Ping
;
Sheu, Yu-Miin
Two-dimensional electron systems have attracted attention as thermoelectric materials, which can directly convert waste heat into electricity. It has been theoretically predicted that thermoelectric power factor can be largely enhanced when the two-dimensional electron layer is far narrower than the de Broglie wavelength. Although many studies have been made, the effectiveness has not been experimentally clarified thus far. Here we experimentally clarify that an enhanced two-dimensionality is efficient to enhance thermoelectric power factor. We fabricated superlattices of [N unit cell SrTi1-xNbxO3 vertical bar 11 unit cell SrTiO3](10)-there are two different de Broglie wavelength in the SrTi1-xNbxO3 system. The maximum power factor of the superlattice composed of the longer de Broglie wavelength SrTi1-xNbxO3 exceeded similar to 5 mWm(-1) K-2, which doubles the value of optimized bulk SrTi1- xNbxO3. The present approach-use of longer de Broglie wavelength-is epoch-making and is fruitful to design good thermoelectric materials showing high power factor.
Electrical, Optical, and Thermal Transport Properties of Oxygen-Deficient Amorphous WOx (2.5 < x < 3) Films
(2019-06-27)
Kim, Gowoon
;
Cho, Hai Jun
;
Sheu, Yu-Miin
;
Ohta, Hiromichi
;
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Oxygen-deficient tungsten oxide (WOx) is known as an active material for various future applications such as smart displays, photocatalysts, Li-ion battery, and so on. WOx exhibits versatile properties depending on the valence state of W, which can vary from +6 to +4. Therefore, clarifying the relationship between x, the valence state of W ion, and the material properties of WOx is crucial for discovering more unique device applications. In case of crystalline WOx, since WOx has many different phases, the valence state of W cannot be modulated continuously from +6 to +4. On the other hand, there is no phase boundary in amorphous (a-) WOx, the valence state of W ion can be continuously modulated against x, and the effect of valence state of W on the material properties of WOx can be thoroughly examined. Here, we report the electrical, optical, and thermal properties of a-WOx films with several valence states of +6 (d(0)), +5 (d(1)), and +4 (d(2)) for x ranging from 2.511 to 2.982. Although the +6 dominant films were electrical insulators with optical transparency in the visible region, we found that both optical transmissivity and electrical resistivity decreased drastically with increase in the +5 concentrations, which also enhances the thermal conductivity because heat can be carried by additional conduction electrons. As the +4 state became dominant in the film, the resistivity slightly increased, whereas the low visible transmission was maintained. These results suggest that the redox of tungsten between +6 and +5 is attributed to all versatile properties of a-WOx, which would be of great use for developing unique devices in the future.