Chun-Wei Chen

National Taiwan University, T’ai-pei, Taipei, Taiwan

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Publications (88)439.51 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: A novel organic/graphene/inorganic -heterostructure, consisting of a graphene layer encapsulated by n- and p-type photoactive materials with complementary absorptions, enables the control of dual n- and p-typed transport behaviors of a graphene transistor under selective UV or visible light illumination. A graphene-based p-n junction created by spatially patterned wavelength-selective illumination using the organic/graphene/inorganic heterostructure is also demonstrated. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Advanced Materials 11/2014; · 14.83 Impact Factor
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    ABSTRACT: In this paper, we demonstrate an inspection technique-based on only one ellipsometric parameter, Ψ, of spectroscopic ellipsometry (SE) - for the rapid, simultaneous identification of both the structural quality and thicknesses of large-area graphene films. The measured Ψ spectra are strongly affected by changes in the out-of-plane absorption coefficients (αTM); they are also correlated to the ratio of the intensities of the D and G bands in Raman spectra of graphene films. In additional, the electronic transition state of graphene within the UV regime assists the characterization of the structural quality. We also demonstrated that the intensities and shifts of the signals in Ψ spectra allow clear identification of the structural qualities and thicknesses, respectively, of graphene films. Moreover, this Ψ-based method can be further applied to graphene films coated on various substrates. To the best of our knowledge, this paper is the first to describe the direct application of Ψ spectra obtained through conventional SE to determine the structural qualities of graphene films on different substrates (Cu, fused silica, Si). In addition, mapping of the values of Ψ is a very convenient and useful means of rapidly characterizing both the structural quality and thickness of 2D materials at local areas. Therefore, this Ψ-based characterization method has great potential for application in the mass production of devices based on large-area graphene.
    Analytical chemistry. 07/2014;
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    ABSTRACT: In this study, we systematically investigated the stoichiometric dependence of titanium oxide (TiOx, x=1.56–1.93) as a cathode modifier on the device performance of polymer solar cells. Electronic structures of the synthesized TiOx modifier layers were controlled by tuning the compositions of various O/Ti ratios. The effective cathode work-functions and the corresponding device performances of polymer solar cells are systematically changed as a result of inserting the TiOx modification layers. Interfacial modification of the Al cathode with a low O/Ti ratio of TiOx layer yields the best performing photovoltaic device as a result of a largest built-in potential. The correlation of power conversion efficiencies and carrier dynamics of these devices by inserting various TiOx modification layer is further examined by using the Mott-Schottky analysis and the impedance spectroscopy technique. The consistent result shows an enhanced carrier collection efficiency and a reduced charge recombination rate of the device via adequate band alignment between the photoactive layer and the cathode using the TiOx modification layer with an optimized O/Ti ratio.
    Solar Energy Materials and Solar Cells 06/2014; 125:233–238. · 5.03 Impact Factor
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    ABSTRACT: We report the discovery of superconductivity with an onset temperature of ∼0.6 K in a platinum-silicon interface. The interface was formed by using a unique focused ion beam sputtering micro-deposition method in which the energies of most sputtered Pt atoms are ∼2.5 eV. Structural and elemental analysis by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy reveal a ∼ 7 nm interface layer with abundant Pt, which is the layer likely responsible for the superconducting transport behavior. Similar transport behavior was also observed in a gold-silicon interface prepared by the same technique, indicating the possible generality of this phenomenon.
    Applied Physics Letters 05/2014; 104(21):211604-211604-4. · 3.52 Impact Factor
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    ABSTRACT: This work demonstrated a simple platform for rapid and effective surface-assisted laser desorption/ionization time-of-flight mass spectrometry (SALDI-TOF MS) measurements based on the layer structure of reduced graphene oxide (rGO) and gold nanoparticles. A multi-layer thin film was fabricated by alternate layer-by-layer depositions of rGO and gold nanoparticles (LBL rGO/AuNP). The flat and clean two-dimensional film was served as the sample plate and also functioned as the matrix in SALDI-TOF MS. By simply one-step deposition of analytes onto the LBL rGO/AuNP sample plate, the MS measurements of various homogeneous samples were ready to execute. The optimization of MS signal was reached by the variation of the layer numbers of rGO and gold nanoparticles. Also, the small molecules including amino acids, carbohydrates and peptides were successfully analyzed in SALDI-TOF MS using the LBL rGO/AuNP sample plate. The results showed that the signal intensity, SN(-1) ratio and reproducibility of SALDI-TOF spectra have been significantly improved in comparison to the uses of gold nanoparticles or α-cyano-4-hydroxy-cinnamic acid (CHCA) as the assisted matrixes. Taking the advantages of the unique properties of rGO and gold nanoparticles, the ready-to-use MS sample plate, which could absorb and dissipate laser energy to analytes quite efficiently and homogeneously, has shown great commercial potentials for MS applications.
    Analytica chimica acta 01/2014; 809:97-103. · 4.31 Impact Factor
  • Shao-Sian Li, Chun-Wei Chen
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    ABSTRACT: Polymer solar cells have great potential for offering a cost-effective approach for converting solar energy into electricity compared to traditional inorganic counterparts. Besides the most intensively studied materials for polymer solar cells consisting of conducting polymer and fullerene derivative hybrids, polymer–inorganic nanocrystal (NC) hybrid solar cell devices represent promising alternatives by taking advantage of the relatively high electron mobility, good physical and chemical stability and various morphologies of inorganic NCs. This paper presents a review of the current status and development of polymer–inorganic hybrid solar cells based on metal oxide NCs by focusing the discussion on TiO2 and ZnO. These metal oxide NC materials are promising acceptor candidates because they are environmentally friendly and cheap to be synthesized by using wet chemical methods with a wide range of morphologies, enabling full compatibility with the solution-processable fabrication of polymer solar cells. Substantial progress has been achieved recently in the power conversion efficiencies of polymer–metal-oxide hybrid solar cells through the control of nanoscale polymer–inorganic hybrid morphologies and the improved interfaces between polymers and inorganic nanocrystals. We also reviewed the recently developed state-of-the-art analytical techniques introduced to reveal the nanoscale morphological organization of polymers and NCs in polymer–metal-oxide hybrid solar cells, which provides the understanding of the interplay between controlling nanoscale morphologies of polymer–metal-oxide NC hybrids and photocarrier dynamics and the corresponding device performance. Finally, the main challenges in the development of polymer–metal-oxide hybrid solar cells consisting of both bulk heterojunctions (BHJs) and nanostructured hybrid device architectures are identified, and strategies for improving the device performances are also discussed.
    J. Mater. Chem. A. 08/2013; 1(36).
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    ABSTRACT: A unique "clean-lifting transfer" (CLT) technique that applies a controllable electrostatic force to transfer large-area and high-quality CVD-grown graphene onto various rigid or flexible substrates is reported. The CLT technique without using any organic support or adhesives can produce residual-free graphene films with large-area processability, and has great potential for future industrial production of graphene-based electronics or optoelectronics.
    Advanced Materials 06/2013; · 14.83 Impact Factor
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    ABSTRACT: We present prominent photoresponse of bio-inspired graphene-based phototransistors sensitized with chlorophyll molecules. The hybrid graphene-chlorophyll phototransistors exhibit a high gain of 10^6 electrons per photon and a high responsivity of 10^6 A/W, which can be attributed to the integration of high-mobility graphene and the photosensitive chlorophyll molecules. The charge transfer at interface and the photogating effect in the chlorophyll layer can account for the observed photoresponse of the hybrid devices, which is confirmed by the back-gate-tunable photocurrent as well as the thickness and time dependent studies of the photoresponse. The demonstration of the graphene-chlorophyll phototransistors with high gain envisions a viable method to employ biomaterials for graphene-based optoelectronics.
    Carbon 06/2013; · 6.16 Impact Factor
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    ABSTRACT: Calligraphic counter electrodes: An important photovoltaic application using FeS2 nanocrystal (NC) pyrite ink to fabricate a counter electrode as an alternative to Pt in dye-sensitized solar cells is demonstrated. FeS2 NC ink exhibits excellent electrochemical catalytic activity and remarkable electrochemical stability. ITO=indium-doped tin oxide.
    Angewandte Chemie International Edition 05/2013; · 11.34 Impact Factor
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    ABSTRACT: Using cross-sectional scanning tunneling microscope (XSTM) with samples cleaved in situ in an ultrahigh vacuum (UHV) chamber, this study demonstrates the direct visualization of high-resolution interfacial band mapping images across the film thickness in an optimized bulk heterojunction polymer solar cell consisting of nanoscale phase segregated blends of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). We were able to achieve the direct observation of the interfacial band alignments at the donor (P3HT)-acceptor (PCBM) interfaces and at the interfaces between the photoactive P3HT:PCBM blends and the poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) anode modification layer with an atomic-scale spatial resolution. The unique advantage of using XSTM to characterize polymer:fullerene bulk heterojunction solar cells allows us to explore simultaneously the quantitative link between the vertical morphologies and their corresponding local electronic properties. This provides an atomic insight of interfacial band alignments between the two opposite electrodes, which will be crucial for improving the efficiencies of the charge generation, transport, and collection and the corresponding device performance of polymer solar cells.
    Nano Letters 04/2013; · 13.03 Impact Factor
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    ABSTRACT: We present large photoresponse in hybrid graphene-organic molecule transistors, which exhibit high gain and large responsitivity. High-quality graphene phototransistors are achieved via resist-free fabrication and noncovalent bonding of the organic molecules. The photocurrent of the devices is tunable with back gate which enables high controllability by electrical means. The strong photoresponse can be attributed to charge transfer and photogating effect in the layer of organic molecules. High photo-sensitivity in the hybrid graphene-organic molecule transistors is promising for the future development of graphene-based optoelectronic applications.
    03/2013;
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    ABSTRACT: Graphene oxide (GO) demonstrates interesting photoluminescence (PL) because of its unique heterogeneous atomic structure, which consists of variable sp(2)- and sp(3)-bonded carbons. In this study, we report the interaction between the luminescence of GO ranging from a single atomic layer to few-layered thin films and localized surface plasmon resonance (LSPR) from silver nanoparticles (Ag NPs). The photoluminescence of GO in the vicinity of the Ag NPs is enhanced significantly due to the near-field plasmonic effect by coupling electron-hole pairs of GO with oscillating electrons in Ag NPs, leading to an increased PL intensity and a decreased PL decay lifetime. The maxima 30-fold enhancement in PL intensity is obtained with an optimized film thickness of GO, and the luminescence image from a single atomic layer GO sheet is successfully observed with the assistance of the LSPR effect. The results provide an ideal platform for exploring the interactions between the fluorescence of two-dimensional layered materials and the LSPR effect.
    Nanoscale 01/2013; · 6.73 Impact Factor
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    ABSTRACT: In this study, we find that the optical anisotropy of graphene films could be used as an alternative quality factor for the rapid characterization of large-area graphene films prepared through chemical vapor deposition. We develop an angle-variable spectroscopic method to rapidly determine the optical anisotropy of graphene films. Unlike approaches using Raman scattering spectroscopy, this optical anisotropy method allows ready characterization of the structural quality of large-area graphene samples without the application of high-intensity laser irradiation or complicated optical setups. Measurements of optical anisotropy also allow us to distinguish graphene samples with different extents of structural imperfections; the results are consistent with those obtained from using Raman scattering spectroscopy. In addition, we also study the properties of graphene-based transparent conductive films at wide incident angles because of the advantage of the optical anisotropic properties of graphene. The transmittance of graphene is much higher than that of indium tin oxide films, especially at large incident angles.
    Analytical Chemistry 01/2013; · 5.82 Impact Factor
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    ABSTRACT: Band gap opening and engineering is one of the high priority goals in the development of graphene electronics. Here, we report on the opening and scaling of band gap in BN doped graphene (BNG) films grown by low-pressure chemical vapor deposition method. High resolution transmission electron microscopy is employed to resolve the graphene and h-BN domain formation in great details. X-ray photoelectron, micro-Raman and UV-Vis spectroscopy studies revealed a distinct structural and phase evolution in BNG films at low BN concentration. Synchrotron radiation based XAS-XES measurements concluded a gap opening in BNG films, which is also confirmed by field effect transistor measurements. For the first time, a significant band gap as high as 600 meV is observed for low BN concentrations and is attributed to the opening of Π-Π* band gap of graphene due to isoelectronic BN doping. As-grown films exhibit structural evolution from homogeneously dispersed small BN clusters to large sized BN domains with embedded diminutive graphene domains. The evolution is described in terms of competitive growth among h-BN and graphene domains with increasing BN concentration. The present results pave way for the development of band gap engineered BN doped graphene based devices.
    ACS Nano 12/2012; · 12.03 Impact Factor
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    ABSTRACT: Transmission electron microscopy (TEM) is a powerful tool for imaging nanostructures, yet its capability is limited with respect to the imaging of organic materials because of the intrinsic low contrast problem. TEM phase plates have been in development for decades, yet a reliable phase plate technique has not been available because the performance of TEM phase plates deteriorates too quickly. Such an obstacle prohibits in-focus TEM phase imaging to be routinely achievable, thus limiting the technique being used in practical applications. Here we present an on-chip thin film Zernike phase plate which can effectively release charging and allow reliable in-focus TEM images of organic materials with enhanced contrast to be routinely obtained. With this stable system, we were able to characterize many polymer solar cell specimens and consequently identified and verified the existence of an unexpected nanoparticle phase. Furthermore, we were also able to observe the fine structures of an Escherichia coli specimen, without staining, using this on-chip thin film phase plate. Our system, which can be installed on a commercial TEM, opens up exciting possibilities for TEM to characterize organic materials.
    ACS Nano 12/2012; · 12.03 Impact Factor
  • Cheng-Kuang Lee, Chun-Wei Pao, Chun-Wei Chen
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    ABSTRACT: A comprehensive insight into the correlations of the nanoscale organizations of polymer and nanocrystals in polymer/inorganic nanocrystal bulk heterojunction (BHJ) hybrid solar cells is the key toward nanomorphology control for improving device performance. In this study, we investigated the organizations of both the polymer and nanocrystals in polymer/inorganic nanocrystal hybrid solar cells by performing multiscale molecular simulations of P3HT:TiO2 nanocrystal BHJs incorporating nanocrystals with two different dimensionalities, namely, zero-dimensional nanoparticles (NPs), and one-dimensional nanorods (NRs). We reveal that nanocrystal dimensionality has significant impacts on the polymer/nanocrystal organizations for polymer/inorganic nanocrystal hybrid blends. One-dimensional nanocrystals, such as TiO2 NRs, can effectively enhance the polymer degree of crystallinity as a result of preferential polymer chain alignment along the axial dimension of the NRs, thereby promoting hole transport; in addition, the elongated, anisotropic NRs significantly reduce the probability of electron hopping, and maintain a high specific interfacial area for efficient exciton dissociation. Therefore, the present study demonstrates the possibility of the nanoscale morphology control of polymer/inorganic nanocrystal BHJ hybrid blends via tuning the nanocrystal shapes, which is potentially helpful for developing next-generation polymer/inorganic nanocrystal hybrid electronic devices such as solar cells or thin film transistors.
    Energy & Environmental Science 12/2012; 6(1):307-315. · 11.65 Impact Factor
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    ABSTRACT: We report the discovery of an enhancement of the random laser action in a nanocomposite comprising reduced graphene oxide nanoflakes and ZnO nanorods. We show that both emission intensity and lasing threshold exhibit an obvious improvement. Based on our theoretical calculations, the mechanism underlying the enhanced stimulated emission can be attributed to coupling between the optical transition and the surface plasmon resonance of the reduced graphene oxide nanoflakes, induced by the ZnO nanorod surface roughness. The approach we describe here will be very useful for the future development of high-efficiency optoelectronic devices and offers an alternative route for application of reduced graphene oxide.
    Optics Express 11/2012; 20(23):A799-805. · 3.55 Impact Factor
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    ABSTRACT: It is known that the nanoscale morphological organization of donors or acceptors in bulk heterojunction (BHJ) solar cells is critical to device performance and strongly affects carrier generation, transporting, and collection. This work demonstrates the dependence of nanocrystal dimension-ality and organization on the polymer nanomorphology in P3HT:TiO 2 hybrid bulk heterojunctions, which were revealed using grazing-incidence X-ray diffraction (GIXRD) using a synchrotron X-ray beam and electron tomography. We further performed a multiscale molecular dynamic simulation to understand the morphological orientation of a polymer blended with TiO 2 nanoparticles (NPs) or nanorods (NRs). The correlation between polymer nanoscale morphology and the dimensionality and anisotropy of nanocrystals in P3HT:TiO 2 hybrids clearly explains the observation of different optical absorption and carrier transport behaviors in directions perpendicular or parallel to the film substrate. Our results provide crucial information toward understanding the interplay between nanocrystal dimensionality and polymer morphology in developing organic/inorganic hybrid electronic devices such as thin film transistors (TFTs) or photovoltaics (PVs).
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    ABSTRACT: This study investigates the strong photoluminescence (PL) and x-ray excited optical luminescence observed in nitrogen-functionalized two dimensional graphene nano-flakes (GNFs:N), which arise from the significantly enhanced density of states in the region of Π states and the gap between Π and Π* states. The increase in the number of the sp2 clusters in the form of pyridine-like N-C, graphite-N-like and the C=O bonding, the resonant energy transfer from the N and C atoms to the sp2 clusters was found to be responsible for the blue-shift and the enhancement of the main PL emission feature. The enhanced PL is strongly related to the induced changes of the electronic structures and bonding properties, which were revealed by the x-ray absorption near-edge structure, x-ray emission spectroscopy and resonance inelastic x-ray scattering. The study demonstrates that PL emission can be tailored through appropriate tuning of the nitrogen and oxygen contents in GNFs, and pave the way for new optoelectronic devices.
    The Journal of Physical Chemistry C 07/2012; 116:16251-16258. · 4.84 Impact Factor
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    ABSTRACT: This paper presents an innovative approach to fabricating controllable n-type doping graphene transistors with extended air stability by using self-encapsulated doping layers of titanium suboxide (TiOx) thin films, which are an amorphous phase of crystalline TiO(2) and can be solution processed. The nonstoichiometry TiOx thin films consisting of a large number of oxygen vacancies exhibit several unique functions simultaneously in the n-type doping of graphene as an efficient electron-donating agent, an effective dielectric screening medium, and also an encapsulated layer. A novel device structure consisting of both top and bottom coverage of TiOx thin layers on a graphene transistor exhibited strong n-type transport characteristics with its Dirac point shifted up to -80 V and an enhanced electron mobility with doping. Most interestingly, an extended stability of the device without rapid degradation after doping was observed when it was exposed to ambient air for several days, which is not usually observed in other n-type doping methods in graphene. Density functional theory calculations were also employed to explain the observed unique n-type doping characteristics of graphene using TiOx thin films. The technique of using an "active" encapsulated layer with controllable and substantial electron doping on graphene provides a new route to modulate electronic transport behavior of graphene and has considerable potential for the future development of air-stable and large-area graphene-based nanoelectronics.
    ACS Nano 06/2012; 6(7):6215-21. · 12.03 Impact Factor

Publication Stats

1k Citations
439.51 Total Impact Points

Institutions

  • 2003–2014
    • National Taiwan University
      • • Department of Materials Science and Engineering
      • • Institute of Polymer Science and Engineering
      T’ai-pei, Taipei, Taiwan
  • 2012
    • Academia Sinica
      • Institute of Atomic and Molecular Sciences
      Taipei, Taipei, Taiwan
  • 2008–2010
    • Rutgers, The State University of New Jersey
      • Department of Materials Science and Engineering
      New Brunswick, NJ, United States
  • 2004
    • Tamkang University
      • Department of Physics
      Taipei, Taipei, Taiwan