Browsing by Author "Chien, Forest Shih-Sen"

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Core-Level Shift of Graphene with Number of Layers Studied by Microphotoelectron Spectroscopy and Electrostatic Force Microscopy
(2014-10-30)
Lin, Chi-Yuan
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Shiu, Hung-Wei
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Chang, Lo-Yueh
;
Chen, Chia-Hao
;
Chang, Chen-Shiung
The dependence of C-1s core-level shift (CLS) of graphene on the number of layer (NL) is significant information to derive the coordination-resolved C1s core-level energy (E-1s) of carbon allotropes. To observe the variation of CLS of graphene with NL, binding energy and work function of graphene with various NL were examined by scanning photoelectron microscopy/spectroscopy and electrostatic force microscopy. E1s of grounded graphene decreased as NL increased. The E-1s of single-layer graphene and multilayer graphene (MLG) is 289.42 and 289.13 eV, respectively. Based on the NL-dependent E1s of graphene, the CLS of C-1s between a free carbon atom and diamond is 0.74 eV, calculated by the bond-order-length-strength correlation theory. The CLS of graphene with NL supported the existence of surface core-level shift (SCLS) on graphite. The magnitude of SCLS in MLG was +0.09 eV. The effective coordination number is 3.68 for bilayer graphene and 4.31 for the surface carbon layer in MLG.
Dual-band multiplexer/demultiplexer with photonic-crystal-waveguide couplers for bidirectional communications
(2006-10-15)
Chien, Forest Shih-Sen
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Cheng, Sue- Cheng
;
Hsu, Yu-Ju
;
Hsieh, Wen-Feng
;
光電工程學系
An ultrashort photonic-crystal-waveguide (PCW) coupler is numerically designed for dual-band multiplexing/demultiplexing (Muxing/Demuxing). The PCW coupler consists of two line defects of reduced rods in a triangular lattice and Muxing/Demuxing is implemented by the distinction between coupling and decoupling of PCWs. Since waves tend to highly localize within the reduced rods, the backward coupling is suppressed and the efficiency of Muxing/Demuxing is enhanced as compared with the void-rod PCWs. The coupler is utilized to build a bidirectional system for dual-band duplex communications. (c) 2006 Elsevier B.V. All rights reserved.
Graphene On Au-Coated SiOx Substrate: Its Core-Level Photoelectron Microspectroscopy Study
(2012-08-01)
Chen, Jhih-Wei
;
Wang, Chiang-Lun
;
Shiu, Hung Wei
;
Lin, Chi-Yuan
;
Chang, Chen-Shiung
"The core-level electronic structures of exfoliated graphene sheets on a Au-coated SiOx substrate have been studied using synchrotron radiation photoelectron spectroscopy (SR-PES) on a micron scale. The graphene's visibility on the Au-coated SiOx substrate was first demonstrated by micro optical characterization, and then an SR-PES study was conducted. Because of the elimination of charging effect, a precise C 1s core-level characterization shows graphitic and contaminated carbon states of graphene. Different levels of Au-coating-induced p-type doping on single-and double-layer graphene sheets were also examined in the C 1s core-level shift. The Au-coated SiOx substrate can be treated as a simple but high-throughput platform for in situ studying graphene under further hybridization by PES. (C) 2012 The Japan Society of Applied Physics"
Hole transit in P3HT:PCBM solar cells with embedded gold nanoparticles
(2014-02-01)
Cheng, Cheng-En
;
Pei, Zingway
;
Hsu, Chia-Chen
;
Chang, Chen-Shiung
;
Chien, Forest Shih-Sen
The electrical properties of polymer solar cells (PSCs) with gold nanoparticles (AuNPs) embedded in the hole transport layers (Hits) were investigated. The PSCs with AuNP-embedded HTLs exhibited 8%, 9.5%, and 22% enhancement in short-circuit current density, open-circuit voltage, and power conversion efficiency, respectively. The low contact resistance at the active layer/Hit interface resulted in the high collection efficiency and low electron-hole recombination. The downshift of the Fermi energy in AuNP-embedded HTLs lowered the energy barrier and thinned the depletion layer at the active layer/HTL interface, accounting for the enhanced collection efficiency. (C) 2013 Elsevier B.V. All rights reserved.
Impacts of high-electroactive-surface-area buckypaper counter electrodes on the photovoltaic performance of dye-sensitized solar cells
(2015-09-09)
Cheng, Cheng-En
;
Lin, Yu-Chang
;
Tsai, Shang-Yi
;
Lin, Zheng-Kun
;
Lee, Ping-Chen
In this paper we study the photovoltaic performance of dye-sensitized solar cells (DSCs) with high-electroactive-surface-area buckypaper (BP) counter electrodes (CEs). BP is made of entangled multi-walled carbon nanotubes and has a high electroactive surface area, which is effectively 11.5 times higher than that of Pt. The high surface area of BP causes a high I-3(-) reduction rate at the CEs and suppresses the charge recombination at the anodes. Accordingly, the open-circuit voltage, the filling factor, and the power conversion efficiency of DSCs with BP CEs are enhanced, and these photovoltaic parameters exhibit a better trend with the increase of solar power compared with conventional DSCs with Pt CEs.
Improvement of the efficiency of dye-sensitized solar cells with fluorinated carbon-based liquid crystal dopant
(2017-08-01)
You, Chang-Feng
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Cheng, Cheng-En
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Lei, Bi-Cheng
;
Jhang, Jia-Cih
;
Yu, Fang-Cheng
The long-term stability of dye-sensitized solar cells (DSSCs) can be improved using a liquid crystal (LC) dopant. However, the high viscosity of the LC dopant reduces the power conversion efficiency (PCE) of these cells. In this study, low-viscosity LC is doped into the DSSCs. The results indicate that the polar fluoro groups of the LCs effectively interact with the cyano groups of the organic solvent in the liquid electrolyte, enhancing the long-term stability of the DSSCs. The viscosity of the LC dopant is a key factor that affects the light-to-electrical energy conversion efficiency of the LC-doped DSSCs. The short-current density and related PCE of the DSSCs is not reduced because of the low viscosity of the doped fluorinated carbon-based LCs. (C) 2017The Physical Society of the Republic of China(Taiwan). Published by ElsevierB.V. All rights reserved.
Influences of thermal annealing on P3HT/PCBM interfacial properties and charge dynamics in polymer solar cells
(2015-12-01)
Cheng, Cheng-En
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Dinelli, Franco
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Yu, Chen-Te
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Shih, Hwa-Wei
;
Pei, Zingway
The effects of thermal annealing on the interfacial properties of poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C-61 butyric acid methyl ester (PCBM) and on the charge dynamics in P3HT: PCBM polymer solar cells (PSCs) are investigated. This study determines that an effective phase separation of the P3HT and PCBM caused by thermal annealing achieves a larger interfacial area for efficient exciton dissociation and a well-defined pn junction with few defect levels at the P3HT/PCBM interface. Additionally, thermal annealing creates a compositional gradient across the P3HT: PCBM films, which enhances the charge transit ability significantly. These improved interfacial properties and efficiency in charge transit ability account for the better power conversion efficiency of P3HT: PCBM PSCs treated with thermal annealing. (C) 2015 The Japan Society of Applied Physics
Liquid crystal-doped liquid electrolytes for dye-sensitized solar cell applications
(2016-04-01)
Huang, Chi-Yen
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You, Chang-Feng
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Cheng, Cheng-En
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Lei, Bi-Cheng
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Jhang, Jia-Cih
We investigate the effects of liquid crystals (LCs) on the power conversion efficiency of dye-sensitized solar cells (DSSCs). The photovoltaic and electrochemical impedance spectra indicate that minute amounts of LC dopant decrease the short-current density of DSSCs because the doped LCs reduce the electrochemical reaction rate between DSSC counter electrode and electrolyte. The doped LCs delay the degradation rates of DSSCs because of the interaction between cyano groups of the doped LCs and organic solvent in the liquid electrolyte. Owing to the molecular interaction, the doped LCs increase the viscosity and stability, thereby inhibiting the evaporation rate of the liquid electrolyte. (C) 2016 Optical Society of America
Optical controlled graphene-based nonvolatile ternary-logic transistor with azobenzene copolymer
(2013-01-07)
Lin, Chi-Yuan
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Chang, Chen-Shiung
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Lin, Jian Hung
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Hsu, Chia-Chen
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Chien, Forest Shih-Sen
We demonstrated optical-controlled graphene-based nonvolatile transistors incorporated with azobenzene copolymer. The transistor was gated by the quasi remnant polarization of azobenzene copolymer, which was built by photo-assisted poling and erased by photo-depoling at room temperature. By taking the nature of polymer electret of azobenzene copolymer, the graphene-based device can perform ternary logic, and the resistance change ratio of the written status "+/- 1" to the erased status "0" was -/+ 60%. The device can retain its statuses against an electric field as high as 0.2 MV/cm because the azobenzene molecules were frozen in copolymer at room temperature. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4773984]
Optical-power-dependent photodesorption kinetics of graphene studied by conductance response
(2015-06-01)
Hsu, Yang-Yang
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Lin, Chi-Yuan
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Lai, Yi-Chun
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Wu, Kun-Rui
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Ng, Kwai-Kong
The photodesorption kinetics of graphene with various UV laser power is studied by conductance response. Analytical expressions of the power-dependent photodesorption kinetics of graphene in ambience are derived. The photodesorption time constant tau(d), steady current, and magnitude of modulation current, can be expressed as functions of the adsorption time constant tau(a), desorption cross section sigma, and photon flux density. Under illumination the steady occupation ratio of adsorbed O-2 on graphene is equal to tau(d)/tau(a). It is suggested that the photodesorption of O-2 on graphene is attributed the injection of photogenerated hot electrons and is restricted by the density of antibonding states of O-2. (C) 2015 Optical Society of America
Platinum-graphene counter electrodes for dye-sensitized solar cells
(2013-07-07)
Cheng, Cheng-En
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Lin, Chi-Yuan
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Shan, Chien-Hsun
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Tsai, Shang-Yi
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Lin, Ko-Wei
This paper describes the photovoltaic performance of dye-sensitized solar cells (DSSCs) containing graphene-incorporated counter electrodes (CEs). The location and thickness of graphene in CEs are optimized to improve the photovoltaic performance of DSSCs, compared with typical Pt CEs. The DSSC, with a Pt/few-layer graphene (FLG) CE, achieved 8% in short-circuit current density and 13% in power conversion efficiency (PCE). Electrochemical impedance spectroscopy shows that the DSSC, with a Pt/FLG CE, exhibits a series resistance lower than that with a Pt CE. The lower series resistance is attributed to the contact resistance at the interface of platinum and fluorine doped tin oxide. The contact resistance is reduced by the formation of the thin platinum-carbon composite layer. It is demonstrated that the consumption of Pt could be reduced with a Pt/FLG CE. However, graphene/Pt CEs resulted in a slow charge-transfer process and consequently a worse photovoltaic performance of DSSCs. (C) 2013 AIP Publishing LLC.
Pt crystalline ultrathin films as counter electrodes for bifacial dye-sensitized solar cells
(2017-01-01)
Cheng, Cheng-En
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Lin, Zheng-Kun
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Lin, Yu-Chang
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Lei, Bi-Chen
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Chang, Chen-Shiung
This study is to develop the Pt crystalline ultrathin films as high-transparent, efficient, and low-Pt-loaded counter electrodes (CEs) for bifacial dye-sensitized solar cells (DSCs). The 1-nm-thick Pt ultrathin films are sputtered on fluorine-doped tin oxide substrates and thermal annealed at 400 degrees C. After annealing, as-prepared amorphous-nanocrystal-mixed Pt films become high-crystalline films with better optical transmittance and electrocatalytic ability to I-3(-) reduction for bifacial DSCs. The rear-to-front ratios of short-circuit current density and power conversion efficiency of DSCs with crystalline ultrathin Pt CEs are as high as 81 and 83%, respectively. (C) 2017 The Japan Society of Applied Physics
Revealing the flexoelectricity in the mixed-phase regions of epitaxial BiFeO3 thin films
(2015-01-28)
Cheng, Cheng-En
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Liu, Heng-Jui
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Dinelli, Franco
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Chen, Yi-Chun
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Chang, Chen-Shiung
Understanding the elastic response on the nanoscale phase boundaries of multiferroics is an essential issue in order to explain their exotic behaviour. Mixed-phase BiFeO3 films, epitaxially grown on LaAlO3 (001) substrates, have been investigated by means of scanning probe microscopy to characterize the elastic and piezoelectric responses in the mixed-phase region of rhombohedral-like monoclinic (M-I) and tilted tetragonal-like monoclinic (M-II,M-tilt) phases. Ultrasonic force microscopy reveal that the regions with low/ high stiffness values topologically coincide with the M-I/M-II,M-tilt phases. X-ray diffraction strain analysis confirms that the M-I phase is more compliant than the M-II,(tilt) one. Significantly, the correlation between elastic modulation and piezoresponse across the mixed-phase regions manifests that the flexoelectric effect results in the enhancement of the piezoresponse at the phase boundaries and in the M-I regions. This accounts for the giant electromechanical effect in strained mixed-phase BiFeO3 films.
Surface-enhanced Raman scattering of graphene with photo-assisted-synthesized gold nanoparticles
(2013-03-11)
Cheng, Cheng-En
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Lin, Chi-Yuan
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Chang, Hao-Yu
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Huang, Chen-Han
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Lin, Hsing-Ying
This paper presents a convenient and reliable method to prepare gold nanoparticles (AuNPs) on graphene. Photo-assisted synthesis (PAS) was employed to grow AuNPs in AuCl4- electrolyte on graphene. The size of AuNPs could be as large as 130 nm. This optical method had a steady growth rate of AuNPs. The distribution of AuNPs was well controlled by focusing the laser for PAS. The minimum diameter of the distribution was approximately 1 mu m. Surface-enhanced Raman scattering of graphene due to AuNPs was observed. Electrical fields near AuNPs calculated by the finite-difference time-domain algorithm ensured that the Raman enhancement was attributed to the localized surface plasmons of AuNPs. (c) 2013 Optical Society of America
Synchrotron Radiation Soft X-ray Induced Reduction in Graphene Oxide Characterized by Time-Resolved Photoelectron Spectroscopy
(2015-06-11)
Lin, Chi-Yuan
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Cheng, Cheng-En
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Wang, Shuai
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Shiu, Hung Wei
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Chang, Lo Yueh
Synchrotron radiation soft X-ray was employed to reduce graphene oxide (GO) films in ultrahigh vacuum. The dissociation of oxygen-containing functional groups, and the formation of sp(2) C-C bonds were revealed by time-resolved in situ X-ray photoelectron spectroscopy, demonstrating the X-ray reduction of GO. The number of C-O bonds of G-O exhibited an exponential decay with exposure time. The X-ray reduction rate of G-O was positively correlated with the intensity of low-energy secondary electrons excited from substrates by soft X-ray, indicating the C-O bonds were dissociated by secondary electrons.
The effect of thermal annealing on the optical and electrical properties of ZnO epitaxial films grown on n-GaAs (001)
(2015-01-01)
Liu, Wei-Rein
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Lin, Bi-Hsuan
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Lin, Chi-Yuan
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Yang, Song
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Kuo, Chin-Chia
Wurtzite ZnO epitaxial layers grown on n-type GaAs (001) by pulsed laser deposition (PLD) exhibited n-type conductivity. Post-growth annealing leads the conversion of carrier type from electron to hole, as revealed by Hall effect measurements, although only moderate structural improvement was observed. The carrier type conversion is attributed to thermally activated arsenic diffusion from the substrate, confirmed by secondary ion mass spectrometry and photoluminescence. The surface electrical properties of both the as-deposited n-type and annealed p-type ZnO epitaxial layers were thoroughly characterized by Kelvin force microscopy (KFM) and electrostatic force microscopy (EFM). The results indicated the existence of a high density of surface states close to the ZnO midgap with a density of a few 10(14) cm(-2) eV(-1). The Fermi levels (E-F) of n- and p-type ZnO epitaxial layers were found to be 1.06 eV below the conduction-band minimum (CBM) and 1.612-1.769 eV above the valence-band maximum (VBM), respectively. The small EF difference between the n- and p-type ZnO epitaxial layers implies Fermi level pinning at the surface of both n-and p-type ZnO epitaxial layers.
UV-treated graphene oxide as anode interfacial layers for P3HT: PCBM solar cells
(2015-07-01)
Cheng, Cheng-En
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Tsai, Cheng-Wei
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Pei, Zingway
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Lin, Tsung-Wu
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Chang, Chen-Shiung
Solution-processable graphene oxide (GO) ultrathin films were introduced as anode interfacial layers (AILs) for polymer solar cells (PSCs). The photovoltaic performance of PSCs containing thermal-and UV-treated GO was comparable to that of PSCs with conventional poly(3,4-ethyledioxythiphene): poly(styrenesulfonate) AILs. UV treatment induced the surface activation of GO; an increase in the work function of UV-treated GO improved the energy band alignment at the GO/poly(3-hexylthiophene) interface, which accounted for the efficient hole collection and photovoltaic performance of PSCs with treated GO.