Browsing by Author "Chang, Yung-Huang"

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Band Alignment of 2D Transition Metal Dichalcogenide Heterojunctions
(2017-05-18)
Chiu, Ming-Hui
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Tseng, Wei-Hsuan
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Tang, Hao-Ling
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Chang, Yung-Huang
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Chen, Chang-Hsiao
It is critically important to characterize the band alignment in semiconductor heterojunctions (HJs) because it controls the electronic and optical properties. However, the well-known Anderson's model usually fails to predict the band alignment in bulk HJ systems due to the presence of charge transfer at the interfacial bonding. Atomically thin 2D transition metal dichalcogenide materials have attracted much attention recently since the ultrathin HJs and devices can be easily built and they are promising for future electronics. The vertical HJs based on 2D materials can be constructed via van der Waals stacking regardless of the lattice mismatch between two materials. Despite the defect-free characteristics of the junction interface, experimental evidence is still lacking on whether the simple Anderson rule can predict the band alignment of HJs. Here, the validity of Anderson's model is verified for the 2D heterojunction systems and the success of Anderson's model is attributed to the absence of dangling bonds (i.e., interface dipoles) at the van der Waal interface. The results from the work set a foundation allowing the use of powerful Anderson's rule to determine the band alignments of 2D HJs, which is beneficial to future electronic, photonic, and optoelectronic devices.
Band Gap-Tunable Molybdenum Sulfide Selenide Monolayer Alloy
(2014-07-09)
Su, Sheng-Han
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Hsu, Yu-Te
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Chang, Yung-Huang
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Chiu, Ming-Hui
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Hsu, Chang-Lung
Comparative study on MoS2 and WS2 for electrocatalytic water splitting
(2013-09-19)
Chen, Tzu-Yin
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Chang, Yung-Huang
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Hsu, Chang-Lung
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Wei, Kung-Hwa
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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.
Direct probe of heterojunction effects upon photoconductive properties of TiO(2) nanotubes fabricated by atomic layer deposition
(2010-06-04)
Chang, Yung-Huang
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Liu, Chien-Min
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Tseng, Yuan-Chieh
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Chen, Chih
;
Chen, Chia-Chuan
This study investigated Schottky- and ohmic-contact effects upon the photoresponses of ITO/TiO(2)/Si and Ti/TiO(2)/Si nanotube-based photodiodes. The TiO(2) tube arrays were fabricated by atomic layer deposition (ALD) and shaped by an anodic aluminum oxide (AAO) template on a p-type Si substrate. The contact area between the electrode (Ti or ITO) and the TiO(2)'s tip was varied by tuning the tube's inner wall thickness with ALD, providing a direct and systematic probe of the heterojunction effects upon the photodiodes' responses. Results show that the Ti/TiO(2)/Si diode exhibits a highly thickness-dependent photoresponse. This is because the photocurrent is driven by the p-n junction at TiO(2)/Si alone and it faces no retarding at the ohmic contact of Ti/TiO(2). For the ITO/TiO(2)/Si diode, the Schottky contact at ITO/TiO(2) regulates photocurrent overriding TiO(2)/Si as a result of higher efficiency in photogeneration, leading to the opposite response compared with the Ti/TiO(2)/Si diode. Respective energy band diagrams are provided to support the statements above, and a consistent picture is obtained for both time response and quantum efficiency measurements.
Direct probe of heterojunction effects upon photoconductive properties of TiO2 nanotubes fabricated by atomic layer deposition
(2010-06-04)
Chang, Yung-Huang
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Liu, Chien-Min
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Tseng, Yuan-Chieh
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Chen, Chih
;
Chen, Chia-Chuan
This study investigated Schottky- and ohmic-contact effects upon the photoresponses of ITO/TiO2/Si and Ti/TiO2/Si nanotube-based photodiodes. The TiO2 tube arrays were fabricated by atomic layer deposition (ALD) and shaped by an anodic aluminum oxide (AAO) template on a p-type Si substrate. The contact area between the electrode (Ti or ITO) and the TiO2's tip was varied by tuning the tube's inner wall thickness with ALD, providing a direct and systematic probe of the heterojunction effects upon the photodiodes' responses. Results show that the Ti/TiO2/Si diode exhibits a highly thickness-dependent photoresponse. This is because the photocurrent is driven by the p-n junction at TiO2/Si alone and it faces no retarding at the ohmic contact of Ti/TiO2. For the ITO/TiO2/Si diode, the Schottky contact at ITO/TiO2 regulates photocurrent overriding TiO2/Si as a result of higher efficiency in photogeneration, leading to the opposite response compared with the Ti/TiO2/Si diode. Respective energy band diagrams are provided to support the statements above, and a consistent picture is obtained for both time response and quantum efficiency measurements.
Effect of Geometric Nanostructures on the Absorption Edges of 1-D and 2-D TiO2 Fabricated by Atomic Layer Deposition
(2013-05-08)
Chang, Yung-Huang
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Liu, Chien-Min
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Cheng, Hsyi-En
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Chen, Chih
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材料科學與工程學系
2-Dimensional (2-D) TiO2 thin films and 1-dimensional (1-D) TiO2 nanotube arrays were fabricated on Si and quartz substrates using atomic layer deposition (ALD) with an anodic aluminum oxide (AAO) template at 400 degrees C. The film thickness and the tube wall thickness can be precisely controlled using the ALD approach. The intensities of the absorption spectra were enhanced by an increase in the thickness of the TiO2 thin film and tube walls. A blue-shift was observed for a decrease in the 1-D and 2-D TiO2 nanostructure thicknesses, indicating a change in the energy band gap with the change in the size of the TiO2 nanostructures. Indirect and direct interband transitions were used to investigate the change in the energy band gap. The results indicate that both quantum confinement and interband transitions should be considered when the sizes of 1-D and 2-D TiO2 nanostructures are less than 10 nm.
Enhanced Electrocatalytic Activity of MoSx on TCNQ-Treated Electrode for Hydrogen Evolution Reaction
(2014-10-22)
Chang, Yung-Huang
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Nikam, Revannath D.
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Lin, Cheng-Te
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Huang, Jing-Kai
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Tseng, Chien-Chih
Molybdenum sulfide has recently attracted much attention because of its low cost and excellent catalytical effects in the application of hydrogen evolution reaction (HER). To improve the HER efficiency, many researchers have extensively explored various avenues such as material modification, forming hybrid structures or modifying geometric morphology. In this work, we reported a significant enhancement in the electrocatalytic activity of the MoSx via growing on Tetracyanoquinodimethane (TCNQ) treated carbon cloth, where the MoSx was synthesized by thermolysis from the ammonium tetrathiomolybdate ((NH4)(2)MoS4) precursor at 170 degrees C. The pyridinic N- and graphitic N-like species on the surface of carbon cloth arising from the TCNQ treatment facilitate the formation of Mo5+ and S-2(2-) species in the MoSx, especially with S-2(2-) serving as an active site for HER. In addition, the smaller particle size of the MoSx grown on TCNQ-treated carbon cloth reveals a high ratio of edge sites relative to basal plane sites, indicating the richer effective reaction sites and superior electrocatalytic characteristics. Hence, we reported a high hydrogen evolution rate for MoSx on TCNQ-treated carbon cloth of 6408 mL g(-1) cm(-2) h(-1) (286 mmol g(-1) cm(-2) h(-1)) at an overpotential of V = 0.2 V. This study provides the fundamental concepts useful in the design and preparation of transition metal dichalcogenide catalysts, beneficial in the development in clean energy
Enhancing the electrocatalytic water splitting efficiency for amorphous MoSx
(2014-03-26)
Hsu, Chang-Lung
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Chang, Yung-Huang
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Chen, Tzu-Yin
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Tseng, Chien-Chih
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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.
Fabrication and Characteristics of Self-Aligned ZnO Nanotube and Nanorod Arrays on Si Substrates by Atomic Layer Deposition
(2010)
Chang, Yung-Huang
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Wang, Shun-Min
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Liu, Chien-Min
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Chen, Chih
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材料科學與工程學系
Vertically self-aligned ZnO nanorods and nanotubes are fabricated on Si substrates by atomic layer deposition with the assistance of anodic aluminum oxide at 250 degrees C. These nanostructures are equal in height, isolated, and vertical to the Si substrate. With 550 deposition cycles, we can fabricate regular arrays of ZnO nanorods with an average diameter of 70 nm and with a height of 470 nm. In particular, the wall thickness of the nanotubes can be controlled precisely by using the atomic layer deposition approach. The measured wall thickness is 18.5 perpendicular to 1 nm after 250 deposition cycles, which yields a growth rate of 0.075 nm/cycle. A polycrystalline structure for both ZnO nanorods and nanotubes was confirmed by a transmission electron microscope and selected area diffraction pattern. Compared with the ZnO films and nanorods, the fabricated ZnO nanotubes exhibit an excellent performance on photoluminescence characteristics due to their larger surface area. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3489953] All rights reserved.
Facile Fabrication of TiO(2) Nanorod Arrays for Gas Sensing using Double-Layered Anodic Oxidation Method
(2012-01-01)
Lin, Hsiao-Wei
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Chang, Yung-Huang
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Chen, Chih
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材料科學與工程學系
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Department of Materials Science and Engineering
Self-organized anodic titanium dioxide nanorod arrays were fabricated using a two-stage anodization method from superimposed Ti-Al layers deposited on quartz substrates. The morphology of nanorods could be controlled by the anodizing conditions of the aluminum (first stage) and the titanium layer (second stage). The first stage of anodization process controls the diameter and density of nanorods and the second stage of anodization controls the height of nanorods. Therefore, we could fabricate nanorod arrays with desired aspect ratio and density under specific anodizing conditions. Furthermore, we used TiO(2) nanorod arrays as the field ionization anode of a gas sensor, and the morphology of nanorods has shown to have a high correlation with the working voltage of gas sensing. The TiO(2) nanorods possess good selectivity and low working voltage under different atmosphere and their working electric field is lower than gas sensing using carbon nanotubes and ZnO nanowires as the anode. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.013201jes].
Facile Fabrication of TiO2 Nanorod Arrays for Gas Sensing using Double-Layered Anodic Oxidation Method
(2012-01-01)
Lin, Hsiao-Wei
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Chang, Yung-Huang
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Chen, Chih
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材料科學與工程學系
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Department of Materials Science and Engineering
Self-organized anodic titanium dioxide nanorod arrays were fabricated using a two-stage anodization method from superimposed Ti-Al layers deposited on quartz substrates. The morphology of nanorods could be controlled by the anodizing conditions of the aluminum (first stage) and the titanium layer (second stage). The first stage of anodization process controls the diameter and density of nanorods and the second stage of anodization controls the height of nanorods. Therefore, we could fabricate nanorod arrays with desired aspect ratio and density under specific anodizing conditions. Furthermore, we used TiO2 nanorod arrays as the field ionization anode of a gas sensor, and the morphology of nanorods has shown to have a high correlation with the working voltage of gas sensing. The TiO2 nanorods possess good selectivity and low working voltage under different atmosphere and their working electric field is lower than gas sensing using carbon nanotubes and ZnO nanowires as the anode. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.013201jes].
Field emission properties of CNT-ZnO composite materials
(2009-02-01)
Huang, Chien-Sheng
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Yeh, Chun-Yu
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Chang, Yung-Huang
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Hsieh, Yi-Min
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Ku, Chien-Yeh
CNT-ZnO composite materials were successfully grown by thermal chemical vapor deposition (thermal CVD). First, Multiwalled Carbon nanotubes were synthesized in the temperature range of 500-700 degrees C. After coating Au nanoparticles on such grown CNTs, ZnO nanowires were synthesized via Zn evaporation method at 500 degrees C. SEM images showed that these CNT-ZnO composites were spinous, and XRD analyses showed that the spinal ZnO were crystalline. The turn-on field of CNT-ZnO composite materials with 500 degrees C growth temperature of underlying CNTs was 3.72 V/mu m, which was 5.58 V/mu m for CNTs grown at 500 degrees C. It was also found that the higher temperature (<550 degrees C) the underlying CNTs were grown at, the better field emission properties CNT-ZnO composite materials have. The WIG ratio from Raman spectroscopy of CNTs decreased as the growth temperature of CNTs increased. The graphitization of underlying CNTs played an important role in the Field emission of CNT-ZnO composite materials. (c) 2008 Elsevier B.V. All rights reserved.
Gap States at Low-Angle Grain Boundaries in Monolayer Tungsten Diselenide
(2016-06)
Huang, Yu Li
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Ding, Zijing
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Zhang, Wenjing
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Chang, Yung-Huang
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Shi, Yumeng
Two-dimensional (2D) transition metal dichalcogenides (TMDs) have revealed many novel properties of interest to future device applications. In particular, the presence of grain boundaries (GBs) can significantly influence the material properties of 2D TMDs. However, direct characterization of the electronic properties of the GB defects at the atomic scale remains extremely challenging. In this study, we employ scanning tunneling microscopy and spectroscopy to investigate the atomic and electronic structure of low-angle GBs of monolayer tungsten diselenide (WSe2) with misorientation angles of 3-6 degrees. Butterfly features are observed along the GBs, with the periodicity depending on the misorientation angle. Density functional theory calculations show that these butterfly; features correspond to gap states that arise in tetragonal dislocation cores and extend to distorted six-membered rings around the dislocation core. Understanding the nature of GB defects and their influence on transport and other device properties highlights the importance of defect engineering in future 2D device fabrication.
Giant photoluminescence enhancement in tungsten-diselenide-gold plasmonic hybrid structures
(2016-05-01)
Wang, Zhuo
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Dong, Zhaogang
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Gu, Yinghong
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Chang, Yung-Huang
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Zhang, Lei
Impressive properties arise from the atomically thin nature of transition metal dichalcogenide two-dimensional materials. However, being atomically thin limits their optical absorption or emission. Hence, enhancing their photoluminescence by plasmonic nanostructures is critical for integrating these materials in optoelectronic and photonic devices. Typical photoluminescence enhancement from transition metal dichalcogenides is 100-fold, with recent enhancement of 1,000-fold achieved by simultaneously enhancing absorption, emission and directionality of the system. By suspending WSe2 flakes onto sub-20-nm-wide trenches in gold substrate, we report a giant photoluminescence enhancement of similar to 20,000-fold. It is attributed to an enhanced absorption of the pump laser due to the lateral gap plasmons confined in the trenches and the enhanced Purcell factor by the plasmonic nanostructure. This work demonstrates the feasibility of giant photoluminescence enhancement in WSe2 with judiciously designed plasmonic nanostructures and paves a way towards the implementation of plasmon-enhanced transition metal dichalcogenide photodetectors, sensors and emitters.
Growth Mechanism of Self-Assembled TiO2 Nanorod Arrays on Si Substrates Fabricated by Ti Anodization
(2012)
Chang, Yung-Huang
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Lin, Hsiao-Wei
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Chen, Chih
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材料科學與工程學系
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Department of Materials Science and Engineering
The growth rate and the growth mechanism are investigated by examining the results of scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) on TiO2 nanorods fabricated in the second anodization process through the nanopores of anodic aluminum oxide. Due to the strong electric field of 1.86 GV/m to 1.33 GV/m, the average ultra-fast growth rate of 250 nm/s is observed, and the abrupt increase of height reaches to 85.2% of its destined height in the early growth stage of 0.6 s. Then the growth rate decreases when the electric field decays with the increase of height, and it is interrupted by the dielectric breakdown under an electric field of 0.65 GV/m. According to the XPS analyzes, the TiO2 species on the outer shell of nanorods are observed, and the sub-oxides, Ti2O3 and TiO, are exhibited after Ar+ ions sputtering. It is proposed that the bottom growth mechanism is identified by the analyzes upon SEM and XPS results. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.034209jes] All rights reserved.
Growth Mechanism of Self-Assembled TixWyO Nanotubes Fabricated by TiW Alloy Anodization
(2018-01-01)
Chang, Yung-Huang
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Chen, Yuan-Tsung
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Huang, Chien-Sheng
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Lu, Chia-Ling
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Lee, Shih-Hung
Nanostructures, such as nanodots and nanorods, have been fabricated by the anodization process with an aid of anodic aluminum oxide (AAO) for several decades. However, tubular nanostructures have not been reported by using an AAO-assistant anodization technology until now. In this study, we successfully synthesized self-organized TixWyO nanotubes with cap structures for the first time by using a two-step anodization process from Al-coated TiW alloy layers. The TixWyO nanotubes were observed only in the shaped pores of AAO with large enough pore size, and otherwise the nanostructures would become solid rods due to the aggregation of TixWyO. The wall of nanotubes was similar to 30% inside the pores and the other similar to 70% penetrated into the AAO template. Furthermore, no tubular nanostructures were found unless the percentage of W element in TiW alloys is larger than 50%, attributing to the formation of a void by oxygen bubbles arising from the oxidation reaction of tungsten metal. Besides, the height of the nanotubes is limited by an electric field of 0.82 GV/m due to the dielectric breakdown effect. Finally, a bottom-up growth mechanism at the oxide/metal interface is proposed, based on the observation of the cap structure on the top of the tubular architecture. (C) The Author(s) 2018. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License.
Heterointerface Screening Effects between Organic Monolayers and Monolayer Transition Metal Dichalcogenides
(2016-02)
Zheng, Yu Jie
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Huang, Yu Li
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Chenp, Yifeng
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Zhao, Weijie
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Eda, Goki
The nature and extent of electronic screening at heterointerfaces and their consequences on energy level alignment are of profound importance in numerous applications, such as solar cells, electronics etc. The increasing availability of two-dimensional (2D) transition metal dichalcogenides (TMDs) brings additional opportunities for them to be used as interlayers in "van der Waals (vdW) heterostructures" and organic/inorganic flexible devices. These innovations raise the question of the extent to which the 2D TMDs participate actively in dielectric screening at the interface. Here we study perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) monolayers adsorbed on single-layer tungsten diselenide (WSe2), bare graphite, and Au(111) surfaces, revealing a strong dependence of the PTCDA HOMO-LUMO gap on the electronic screening effects from the substrate. The monolayer WSe2 interlayer provides substantial, but not complete, screening at the organic/inorganic interface. Our results lay a foundation for the exploitation of the complex interfacial properties of hybrid systems based on TMD materials.
Highly Efficient Electrocatalytic Hydrogen Production by MoSx Grown on Graphene-Protected 3D Ni Foams
(2013-02-06)
Chang, Yung-Huang
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Lin, Cheng-Te
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Chen, Tzu-Yin
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Hsu, Chang-Lung
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Lee, Yi-Hsien
Large-Area Synthesis of Highly Crystalline WSe2 Mono layers and Device Applications
(2014-01-01)
Huang, Jing-Kai
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Pu, Jiang
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Hsu, Chang-Lung
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Chiu, Ming-Hui
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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.
Low-temperature growth of ZnO nanorods in anodic aluminum oxide on Si substrate by atomic layer deposition
(2007-01-15)
Yang, Ching-Jung
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Wang, Shun-Min
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Liang, Shih-Wei
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Chang, Yung-Huang
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Chen, Chih
Low-temperature growth of self-organized ZnO nanorods on Si substrate is achieved using anodic aluminum oxide and atomic layer deposition at 250 degrees C without catalyst or seed layer. Photoluminescence spectrum indicates that the ZnO nanorod arrays exhibit a blue/green luminescence at 480 nm. In addition, the nanorod arrays demonstrate excellent field-emission properties with a turn-on electric field of 6.5 V mu m(-1) and a threshold electric field of 9.8 V mu m(-1), which are attributed to the perfectly perpendicular alignment of ZnO nanorods to the Si substrate. (c) 2007 American Institute of Physics.