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Self-assembly of hierarchical MoSx/CNT nanocomposites (2 < x < 3): towards high performance anode materials for lithium ion batteries
(2013-07-09)
Shi, Yumeng
;
Wang, Ye
;
Wong, Jen It
;
Tan, Alex Yuan Sheng
;
Hsu, Chang-Lung
Two dimension (2D) layered molybdenum disulfide (MoS2) has emerged as a promising candidate for the anode material in lithium ion batteries (LIBs). Herein, 2D MoSx (2 <= x <= 3) nanosheet-coated 1D multiwall carbon nanotubes (MWNTs) nanocomposites with hierarchical architecture were synthesized via a high-throughput solvent thermal method under low temperature at 2006 degrees C. The unique hierarchical nanostructures with MWNTs backbone and nanosheets of MoSx have significantly promoted the electrode performance in LIBs. Every single MoSx nanosheet interconnect to MWNTs centers with maximized exposed electrochemical active sites, which significantly enhance ion diffusion efficiency and accommodate volume expansion during the electrochemical reaction. A remarkably high specific capacity (i.e., >1000 mAh/g) was achieved at the current density of 50 mA g(-1), which is much higher than theoretical numbers for either MWNTs or MoS2 along (similar to 372 and similar to 670 mAh/g, respectively). We anticipate 2D nanosheets/1D MWNTs nanocomposites will be promising materials in new generation practical LIBs.
Monolayer MoSe2 Grown by Chemical Vapor Deposition for Fast Photodetection
(2014-08-01)
Chang, Yung-Huang
;
Zhang, Wenjing
;
Zhu, Yihan
;
Han, Yu
;
Pu, Jiang
Monolayer molybdenum disulfide (MoS2) has become a promising building block in optoelectronics for its high photosensitivity. However, sulfur vacancies and other defects significantly affect the electrical and optoelectronic properties of monolayer MoS2 devices. Here, highly crystalline molybdenum diselenide (MoSe2) monolayers have been successfully synthesized by the chemical vapor deposition (CVD) method. Low-temperature photoluminescence comparison for MoS2 and MoSe2 monolayers reveals that the MoSe2 monolayer shows a much weaker bound exciton peak; hence, the phototransistor based on MoSe2 presents a much faster response time (<25 ms) than the corresponding 30 s for the CVD MoS2 monolayer at room temperature in ambient conditions. The images obtained from transmission electron microscopy indicate that the MoSe exhibits fewer defects than MoS2. This work provides the fundamental understanding for the differences in optoelectronic behaviors between MoSe2 and MoS2 and is useful for guiding future designs in 2D material-based optoelectronic devices.
Enhancing the electrocatalytic water splitting efficiency for amorphous MoSx
(2014-03-26)
Hsu, Chang-Lung
;
Chang, Yung-Huang
;
Chen, Tzu-Yin
;
Tseng, Chien-Chih
;
Wei, Kung-Hwa
Amorphous molybdenum sulfide (MoSx) materials have been considered as cheap and promising catalysts for hydrogen evolution reaction (HER). In this contribution, we report that the amorphous MoSx catalysts prepared by the low temperature thermolysis of the (NH4)(2)MoS4 precursors on carbon clothes (catalyst loading: 3.2 mg/cm(2)) exhibit a Tefal slope of 50.5 mV/dec and a high exchange current density of 1.5 x 10(-3) mA/cm(2) in 0.5 M H2SO4 solutions. Spectroscopic studies of the amorphous MoSx catalysts show that the increase of HER efficiency is positively correlated to the concentration of S-2(2-) species, providing strong evidence to support the argument that S-2(2-) is an active species for electrocatalytic HER. Additionally, the method for preparing catalysts is simple, scalable and applicable for large-scale production. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Large-Area Synthesis of Highly Crystalline WSe2 Mono layers and Device Applications
(2014-01-01)
Huang, Jing-Kai
;
Pu, Jiang
;
Hsu, Chang-Lung
;
Chiu, Ming-Hui
;
Juang, Zhen-Yu
The monolayer transition metal dichalcogenides have recently attracted much attention owing to their potential in valleytronics, flexible and low-power electronics, and optoelectronic devices. Recent reports have demonstrated the growth of large-size two-dimensional MoS2 layers by the sulfurization of molybdenum oxides. However, the growth of a transition metal selenide monolayer has still been a challenge. Here we report that the introduction of hydrogen in the reaction chamber helps to activate the selenization of WO3, where large size WSe2 monolayer flakes or thin films can be successfully grown. The top-gated field-effect transistors based on WSe2 monolayers using ionic gels as the dielectrics exhibit ambipolar characteristics, where the hole and electron mobility values are up to 90 and 7 cm(2)/Vs, respectively. These films can be transferred onto arbitrary substrates, which may inspire research efforts to explore their properties and applications. The resistor-loaded inverter based on a WSe2 film, with a gain of similar to 13, further demonstrates its applicability for logic-circuit integrations.
Comparative study on MoS2 and WS2 for electrocatalytic water splitting
(2013-09-19)
Chen, Tzu-Yin
;
Chang, Yung-Huang
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Hsu, Chang-Lung
;
Wei, Kung-Hwa
;
Chiang, Chia-Ying
Replacing Pt by earth abundant catalysts is one of the most important tasks toward potential large-scale HER applications. Among many potential candidates, low cost and earth abundant transition metal dichalcogenides such as MoS2 and WS2 have been promising as good H-2 evolution electrocatalysts when they are engineered into the structures with active sites. In this work, we have performed systematic studies on the catalytic reactivity of both MoS2 and WS2 materials produced by one-step and scalable thermolysis from (NH4)(2)WS4 and (NH4)(2)MoS4 precursors respectively. Structural analysis shows that these materials prepared at a higher thermolysis temperature exhibit higher crystallinity. The H-2 evolution electrocatalysts efficiency for the MoS2 prepared at a lower temperature is higher than those at higher temperatures, where amorphous MoS2 or S-2(2-) species instead of crystalline MoS2 is the main active site. By contrast, crystalline WS2 prepared at high temperature is identified to be the key reaction site. Both catalysts display excellent efficiency and durability as an electrocatalyst operating in acidic electrolytes. This work provides fundamental insights for further design and preparation of emergent metal dichalcogenide catalysts, beneficial for the development in clean energy. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
A crystalline low-bandgap polymer comprising dithienosilole and thieno[3,4-c] pyrrole-4,6-dione units for bulk heterojunction solar cells
(2011-06-08)
Yuan, Mao-Chuan
;
Chou, Yi-Jen
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Chen, Chia-Min
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Hsu, Chang-Lung
;
Wei, Kung-Hwa
We have used Stille coupling polymerization to synthesize a new low-bandgap conjugated polymer, PDTSTPD, that consists of an electron-rich dithieno[3,2-b:2',3'-d]-silole (DTS) unit and an electron-deficient thieno[3,4-c]pyrrole-4,6-dione (TPD) moiety. The polymer exhibited an excellent thermal stability, crystalline characteristics, a broad spectral absorption, and a deep highest occupied molecular orbital (HOMO) energy level, resulting from combination of the rigid TPD and the coplanar DTS units in the polymer backbone. Moreover, the presence of the silicon atoms along the polymer chain ensured PDTSTPD having strong interchain stacking and good hole mobility. An optimal device incorporating the PDTSTPD:PC(71)BM blend at a weight ratio of 1:1 provided a power conversion efficiency of 3.42%. (c) 2011 Elsevier Ltd. All rights reserved.
Highly Efficient Electrocatalytic Hydrogen Production by MoSx Grown on Graphene-Protected 3D Ni Foams
(2013-02-06)
Chang, Yung-Huang
;
Lin, Cheng-Te
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Chen, Tzu-Yin
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Hsu, Chang-Lung
;
Lee, Yi-Hsien
Label-free detection of DNA hybridization using transistors based on CVD grown graphene
(2013-03-15)
Chen, Tzu-Yin
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Phan Thi Kim Loan
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Hsu, Chang-Lung
;
Lee, Yi-Hsien
;
Wang, Jacob Tse-Wei
The high transconductance and low noise of graphene-based field-effect transistors based on large-area monolayer graphene produced by chemical vapor deposition are used for label-free electrical detection of DNA hybridization. The gate materials, buffer concentration and surface condition of graphene have been optimized to achieve the DNA detection sensitivity as low as 1 pM (10(-12) M), which is more sensitive than the existing report based on few-layer graphene. The graphene films obtained using conventional PMMA-assisted transfer technique exhibits PMMA residues, which degrade the sensing performance of graphene. We have demonstrated that the sensing performance of the graphene samples prepared by gold-transfer is largely enhanced (by 125%). (C) 2012 Elsevier B.V. All rights reserved.
Layer-by-Layer Graphene/TCNQ Stacked Films as Conducting Anodes for Organic Solar Cells
(2012-06-01)
Hsu, Chang-Lung
;
Lin, Cheng-Te
;
Huang, Jen-Hsien
;
Chu, Chih-Wei
;
Wei, Kung-Hwa
Large-area graphene grown by chemical vapor deposition (CVD) is a promising candidate for transparent conducting electrode applications in flexible optoelectronic devices such as light-emitting diodes or organic solar cells. However, the power conversion efficiency (PCE) of the polymer photovoltaic devices using a pristine CVD graphene anode is still not appealing due to its much lower conductivity than that of conventional indium tin oxide. We report a layer-by-layer molecular doping process on graphene for forming sandwiched graphene/tetracyanoquinodimethane (TCNQ)/graphene stacked films for polymer solar cell anodes, where the TCNQ molecules (as p-dopants) were securely embedded between two graphene layers. Poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM) bulk heterojunction polymer solar cells based on these multilayered graphene/TCNQ anodes are fabricated and characterized. The P3HT/PCBM device with an anode structure composed of two TCNQ layers sandwiched by three CVD graphene layers shows optimum PCE (similar to 2.58%), which makes the proposed anode film quite attractive for next-generation flexible devices demanding high conductivity and transparency.
Growth selectivity of hexagonal-boron nitride layers on Ni with various crystal orientations
(2012)
Lee, Yi-Hsien
;
Liu, Keng-Ku
;
Lu, Ang-Yu
;
Wu, Chih-Yu
;
Lin, Cheng-Te
Layered hexagonal-boron nitride (h-BN) films were synthesized by chemical vapor deposition (CVD) on Ni foils using ammonia borane as a precursor. Confocal Raman spectroscopy and electron backscatter diffraction (EBSD) were used to probe the effect of underlying Ni crystals with various orientations on growth behaviors of h-BN layers. The growth of the h-BN layers strongly depends on the Ni crystal orientations, where the growth rate of h-BN is larger on Ni(100)-like crystal surfaces but the growth on Ni(111)-like surfaces is not detectable, suggesting that Ni (100)-like facets are likely to promote the growth of h-BN compared with Ni (111)-like surfaces. The observation is in clear contrast to the reported growth of h-BN on Ni(111) in an ultrahigh vacuum environment. The as-grown CVD h-BN films on Ni exhibit a layered structure as revealed by atomic force microscopy (AFM). Thin h-BN layers are found on the Ni domain with a low growth rate. The observation of h-BN growth on various Ni grains may provide insights for the control of thickness, size and morphology of CVD h-BN films.

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