Browsing by Author "Chen, Yang-Yuan"

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Controlled mechanical cleavage of bulk niobium diselenide to nanoscaled sheet, rod, and particle structures for Pt-free dye-sensitized solar cells
(2014)
Ibrahem, Mohammed Aziz
;
Huang, Wei-Chih
;
Lan, Tian-wey
;
Boopathi, Karunakara Moorthy
;
Hsiao, Yu-Chen
In this study, we report a one-step process for the preparation of NbSe2 nanosheets, nanorods and nanoparticles from pristine materials under the effects of shear and friction forces. Nevertheless, simple and facile methods for the large-scale syntheses of well-defined NbSe2 nanostructures in high yield have yet to be realized and that will have a great impact in a wide range of applications. For example, developing platinum (Pt)-free and highly efficient counter electrodes is meaningful and necessary for the cost reduction of dye-sensitized solar cells (DSSCs). By integrating this approach with a simple method of thin film preparation (spray coating) allowed us to prepare large-area, conductive, semitransparent flexible thin films of NbSe2. We have used microscopic and macroscopic methods to examine the morphologies, compositions, crystallinity, and electrical and optical properties of the converted NbSe2 nanostructures. DSSCs with NbSe2 nanosheet counter electrodes (CEs) achieved a conversion efficiency of 7.73%, superior to an efficiency of 7.01% for Pt-based CEs. Our NbSe2 nanostructure provides a cost-effective CE alternative to the noble metal Pt in DSSCs.
Enhancing thermoelectric performance by Fermi level tuning and thermal conductivity degradation in (Ge1-xBix)Te crystals
(2019-06-13)
Wei, Pai-Chun
;
Cai, Cheng-Xun
;
Hsing, Cheng-Rong
;
Wei, Ching-Ming
;
Yu, Shih-Hsun
In this work, a high thermoelectric figure of merit, zT of 1.9 at 740 K is achieved in Ge1-xBixTe crystals through the concurrent of Seebeck coefficient enhancement and thermal conductivity reduction with Bi dopants. The substitution of Bi for Ge not only compensates the superfluous hole carriers in pristine GeTe but also shifts the Fermi level (E-F) to an eligible region. Experimentally, with moderate 6-10% Bi dopants, the carrier concentration is drastically decreased from 8.7 x 10(20) cm(-3) to 3-5 x 10(20) cm(-3) and the Seebeck coefficient is boosted three times to 75 mu VK-1. In the meantime, based on the density functional theory (DFT) calculation, the Fermi level E-F starts to intersect with the pudding mold band at L point, where the band effective mass is enhanced. The enhanced Seebeck coefficient effectively compensates the decrease of electrical conductivity and thus successfully maintain the power factor as large as or even superior than that of the pristine GeTe. In addition, the Bi doping significantly reduces both thermal conductivities of carriers and lattices to an extremely low limit of 1.57W m(-1)K(-1) at 740 K with 10% Bi dopants, which is an about 63% reduction as compared with that of pristine GeTe. The elevated figure of merit observed in Ge1-xBixTe specimens is therefore realized by synergistically optimizing the power factor and downgrading the thermal conductivity of alloying effect and lattice anharmonicity caused by Bi doping.
High quantity and quality few-layers transition metal disulfide nanosheets from wet-milling exfoliation
(2013)
Ibrahem, Mohammed Aziz
;
Lan, Tian-wey
;
Huang, Jing Kai
;
Chen, Yang-Yuan
;
Wei, Kung-Hwa
Low-cost and large-area electronic applications require the deposition of active materials in simple and inexpensive techniques at room temperature; properties usually associated with polymer films. In this study, we demonstrate a simple, low-cost and environmentally friendly method for the high-yield production of two dimensional nanosheets of semiconducting transition metal disulfides, integrated with the route towards the solution-processed deposition of MoS2 and WS2 thin films. The resulting materials with high purity and no contamination or distortion in their structural and electronic properties were confirmed with different microscopic and macroscopic methods. As a proof of concept, we utilize these layered transition metal disulphide films as electron extraction layers in an inverted structure organic solar cell, prepared at relatively low annealing temperatures (<= 150 degrees C). A promising power conversion efficiency with high stability is achieved, which makes these proposed buffer layers quite attractive for next-generation flexible devices requiring high conductivity and transparency, as well as wide range of other potential applications.
Size effect on multiferroic TbMn2O5 nanorods
(2015-05-07)
Yang, Chun-Chuen
;
Tung, Yung-Hsiang
;
Chen, Yi-Jung
;
Hsu, Ting-Wei
;
You, Pang-Yen
The effect of the size of TbMn2O5 on their magnetoelectric properties is studied using a field emission transmission electron microscope, ac magnetic susceptibility, relative dielectric constant, specific heat capacity, and x-ray diffraction at various temperatures. A bulk sample and nanorods with two axial lengths, < L-C >, are investigated. Nanorods with < L-C > of 216 nm and 64 nm crystallize with orthorhombic Pbam symmetry as the bulk material. The axial direction of the two sizes of nanorod is parallel to the c axis of the crystal. No particular radial direction is observed. The critical size for the magnetic ordering of TbMn2O5 is found to be between < L-C > - 64 nm and 216 nm. Experiments on specific heat capacity verified these results. No similar phenomenon concerning the ferroelectric property is observed. At T = 30K and 35 K, the < L-C > - 216 nm sample yields a charge-ordering-like x-ray diffraction peak, which may be associated with the magnetically induced charge ordering superlattice. (C) 2015 AIP Publishing LLC.
Thermoelectric Figure-of-Merit of Fully Dense Single-Crystalline SnSe
(2019-03-01)
Wei, Pai-Chun
;
Bhattacharya, Sriparna
;
Liu, Yu-Fei
;
Liu, Fengjiao
;
He, Jian
Single-crystalline SnSe has attracted much attention because of its record high figure-of-merit ZT approximate to 2.6; however, this high ZT has been associated with the low mass density of samples which leaves the intrinsic ZT of fully dense pristine SnSe in question. To this end, we prepared high-quality fully dense SnSe single crystals and performed detailed structural, electrical, and thermal transport measurements over a wide temperature range along the major crystallographic directions. Our single crystals were fully dense and of high purity as confirmed via high statistics Sn-119 Mossbauer spectroscopy that revealed < 0.35 at. % Sn(IV) in pristine SnSe. The temperature-dependent heat capacity (C-p) provided evidence for the displacive second-order phase transition from Pnma to Cmcm phase at T-c approximate to 800 K and a small but finite Sommerfeld coefficient gamma(0) which implied the presence of a finite Fermi surface. Interestingly, despite its strongly temperature-dependent band gap inferred from density functional theory calculations, SnSe behaves like a low-carrier-concentration multiband metal below 600 K, above which it exhibits a semiconducting behavior. Notably, our high-quality single-crystalline SnSe exhibits a thermoelectric figure-of-merit ZT similar to 1.0, similar to 0.8, and similar to 0.25 at 850 K along the b, c, and a directions, respectively.