Publications (34)111.03 Total impact
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Dataset: jp110879d si 001
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Article: Theoretical Characterization of X-ray Absorption, Emission, and Photoelectron Spectra of Nitrogen Doped along Graphene Edges.
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ABSTRACT: k-edge x-ray absorption, emission, and photoelectron spectra of nitrogen doped along graphene edges are systematically investigated by using first-principles methods. In this study we considered pyridinium-like, pyridine-like, cyanide-like, and amine-like nitrogens at armchair and zigzag edges and pyrrole-like nitrogen at armchair edge as well as graphite-like nitrogen at graphene interior site. Our results indicate that nitrogen configuration and its location (armchair or zigzag edge) in nitrogen-doped graphene can be identified via the spectral analysis. Furthermore, some controversial spectral features observed in experiment for N-doped graphene-like materials are unambiguously assigned. We also discuss the effect of interaction between doped nitrogens along the edges on their spectra.The Journal of Physical Chemistry A 12/2012; · 2.95 Impact Factor -
Article: Electronic Structures of N-doped Graphene with Native Point Defects
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ABSTRACT: Nitrogen doping in graphene has important implications in graphene-based devices and catalysts. We have performed the density functional theory calculations to study the electronic structures of N-doped graphene with vacancies and Stone-Wales defect. Our results show that monovacancies in graphene act as hole dopants and that two substitutional N dopants are needed to compensate for the hole introduced by a monovacancy. On the other hand, divacancy does not produce any free carriers. Interestingly, a single N dopant at divacancy acts as an acceptor rather than a donor. Compared with the case of an isolated N dopant in perfect graphene, the electrons donated by substitutional N dopants would be localized significantly when a N-N pair is formed. The N-N interaction and the interference between native point defect and N dopant strongly modify the role of N doping regarding the free carrier production in the bulk pi bands. Our results are qualitatively consistent with the experimental observation that the concentration of free electrons introduced by N dopants would be lower than that of doped N. We have analyzed N K-edge X-ray adsorption spectroscopy (XAS) spectra of N dopants at vacancies and Stone-Wales (SW) defect. We discuss the relation between the calculated XAS spectra and the experimental ones. We also discuss characteristic scanning tunneling microscope (STM) images originating from the electronic and structural modifications by the N dopant-defect complexes. STM image for small negative bias voltage may provide important information about possible active sites for oxygen reduction reaction.05/2012; -
Article: Interplay between nitrogen dopants and native point defects in graphene
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ABSTRACT: To understand the interaction between nitrogen dopants and native point defects in graphene, we have studied the energetic stability of N-doped graphene with vacancies and Stone-Wales (SW) defect by performing the density functional theory calculations. Our results show that N substitution energetically prefers to occur at the carbon atoms near the defects, especially for those sites with larger bond shortening, indicating that the defect-induced strain plays an important role in the stability of N dopants in defective graphene. In the presence of monovacancy, the most stable position for N dopant is the pyridinelike configuration, while for other point defects studied (SW defect and divacancies) N prefers a site in the pentagonal ring. The effect of native point defects on N dopants is quite strong: While the N doping is endothermic in defect-free graphene, it becomes exothermic for defective graphene. Our results imply that the native point defect and N dopant attract each other, i.e., cooperative effect, which means that substitutional N dopants would increase the probability of point defect generation and vice versa. Our findings are supported by recent experimental studies on the N doping of graphene. Furthermore we point out possibilities of aggregation of multiple N dopants near native point defects. Finally we make brief comments on the effect of Fe adsorption on the stability of N dopant aggregation.Phys. Rev. B. 12/2011; 85(16). -
Article: Selective nitrogen doping in graphene: Enhanced catalytic activity for the oxygen reduction reaction
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ABSTRACT: The structural and electronic properties of N-doped zigzag graphene ribbons with various ratios of dihydrogenated to monohydrogenated edge carbons are investigated within the density functional theory framework. We find that the stability of graphitic N next to the edge, which is claimed to play important roles in the catalytic activity in our previous work, will be enhanced with increasing the concentration of dihydrogenated carbons. Furthermore, the dihydrogenated edge carbons turn out to be easily converted into monohydrogenated ones in the presence of oxygen molecules at room temperature. Based on our results, we propose a possible way to enhance the oxygen reduction catalytic activity of N-doped graphene by controlling the degrees of hydrogenation of edge carbons. The characteristic features in the x-ray absorption and emission spectra for each specific N site considered here will also be given.Physical Review B 12/2011; 84(24):245434. · 3.69 Impact Factor -
Article: Organic macromolecular high dielectric constant materials: synthesis, characterization, and applications.
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ABSTRACT: Hyperbranched and dendritic architectures have been targeted for various applications such as sensing, drug delivery, optical limiting, and light harvesting. One interesting development in this area has focused on utilizing the existence of long-range delocalization in hyperbranched structures to achieve high dielectric constants. In this Feature Article, we will review the creation and development of this concept, and we highlight our recent research progress in this aspect. In particular, we discuss (1) synthetic methods for a particular group of hyperbranched polymers; (2) detailed optical and electronic characterization of this group of hyperbranched polymers, revealing the design criteria for achieving a good combination of high dielectric constant and minimum loss in such materials; and (3) the importance and potential applications of these materials.The Journal of Physical Chemistry B 09/2011; 115(46):13419-32. · 3.70 Impact Factor -
Article: Synthesis and self‐assembly of thermotropic block copolymer with long alkyl tethered cage silsesquioxane in the side chain
Journal of Polymer Science Part A Polymer Chemistry 04/2011; 49(12):2653 - 2664. · 3.92 Impact Factor -
Article: Effect of Hydrogen Termination on Carbon K-Edge X-ray Absorption Spectra of Nanographene
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ABSTRACT: Carbon K-edge X-ray absorption spectra of nanographene have been simulated by density functional theory calculations to obtain information on the edge termination by hydrogen. Such information is crucially important to understand and predict functions such as transport and catalysis. Our results show that different edge terminations significantly affect the binding energy of the 1s core-level of C atoms in the vicinity of edges because of the change in chemical bonding and the localized edge states. We find that a shoulder or a peak appears below the π* peak at relatively different positions with respect to the π* peak position in the theoretical spectra of zigzag graphene nanoribbons, depending on the ratio of monohydrogen- to dihydrogen-terminations. We also point out that the two additional features observed between the π* and σ* peaks of an ideal graphene originate from the σ* states of C−H bonding and C−H2 bonding at the edges.The Journal of Physical Chemistry C 03/2011; 115:5392. · 4.80 Impact Factor -
Article: Hierarchical Structure in Nanoscale Thin Films of a Poly(styrene-b- methacrylate grafted with POSS) (PS214-b-PMAPOSS27)
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ABSTRACT: An interesting organic−inorganic diblock copolymer, poly(styrene-b-methacrylate grafted with polyhedral oligomeric silsesquioxane) (PS-b-PMAPOSS), with PS (an average degree of polymerization DP of 214) and PMAPOSS (DP = 27) blocks was prepared: PS214-b-PMAPOSS27. The morphological structures in nanoscale thin films of PS214-b-PMAPOSS27 and their temperature-dependent changes were investigated in detail by performing grazing incidence X-ray scattering (GIXS) with a synchrotron radiation source. GIXS formulas based on structural models were derived for the quantitative analysis of the two-dimensional (2D) scattering data. In addition, the thermal properties of the diblock copolymer were characterized: it was found to be stable up to 330 °C, and to undergo a glass transition at 96 °C (PS block) and a melting transition at 173 °C (PMAPOSS block). The quantitative 2D GIXS analysis found that the diblock copolymer molecules in the thin films undergo phase separation into PS and PMAPOSS domains, which generates a lamellar structure. Surprisingly, in the solvent-annealed films with and without subsequent thermal annealing the lamellar structure is completely oriented in the out-of-plane of the film and is stable up to the degradation temperature. The PMAPOSS layers in the lamellar structure were found to consist of two sublayers, namely a densely ordered layer composed of vertically oriented orthorhombic crystals and a less ordered layer that consists of molecularly stacked layers that are oriented vertically. In both the sublayers, the PMAPOSS block chains have helical conformations and thus are present as molecular cylinders. These molecular cylinders are always oriented in the film plane. On heating, the orthorhombic crystals and the molecularly stacked layers melt over the range 170−190 °C. On subsequent cooling, the orthorhombic crystals and molecularly stacked layers are not fully recovered, which indicates that the formation of orthorhombic crystals and stacked layers in the PMAPOSS domains requires thermal annealing.12/2010; -
Article: A Nitric Acid‐Assisted Carbon‐Catalyzed Oxidation System with Nitroxide Radical Cocatalysts as an Efficient and Green Protocol for Selective Aerobic Oxidation of Alcohols
Advanced Synthesis & Catalysis 10/2010; 352(14‐15):2635 - 2642. · 6.05 Impact Factor -
Article: Pt-free cathode catalysts prepared via multi-step pyrolysis of Fe phthalocyanine and phenolic resin for fuel cells.
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ABSTRACT: Pt-free cathode catalysts for polymer electrolyte membrane fuel cells have been prepared by multi-step pyrolysis of FePc and PhRs, in the best of which show extensively high initial cell performance and good durability compared to other present precious-metal-free cathode catalysts to date.Chemical Communications 09/2010; 46(34):6377-9. · 6.17 Impact Factor -
Article: Synchrotron grazing incidence X-ray scattering study of the morphological structures in thin films of a polymethacrylate diblock copolymer bearing POSS moieties.
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ABSTRACT: The morphological structures in thin films of a diblock copolymer of methyl methacrylate and polyhedral oligomeric silsesquioxane (POSS) functionalized methacrylate (PMMA-b-PMAPOSS) with a volume ratio of 13/87 were investigated in detail by using synchrotron grazing incidence small and wide-angle X-ray scattering (GISAXS and GIWAXS). In addition, its thermal properties were studied. Thin films of this diblock copolymer were found to undergo phase-separation during solvent-annealing with carbon disulfide and post thermal annealing. To quantitatively analyze the scattering data, GISAXS and GIWAXS formulas were derived and applied. Our detailed analysis found that cylinders of PMMA blocks are induced to form in the diblock copolymer films by solvent-annealing and are hexagonally packed in the PMAPOSS matrix, in which the cylinders are oriented vertically with respect to the film plane. In the solvent-annealed films, both the PMMA cylinders and the PMAPOSS matrix are featureless, i.e., amorphous. However, the post thermal annealing process induces aggregation of the POSS moieties, which results in the formation of crystals with an orthorhombic lattice unit cell. These crystals were found to consist of PMAPOSS block chains in a helical conformation in which the molecular PMAPOSS cylinders are aligned in the film plane. The formation of these crystals is induced by the ordering ability of the POSS moieties. The crystals were found to melt above 190 degrees C during heating and subsequent cooling. In contrast, the hexagonally packed structure of the PMMA cylinders in the solvent-annealed and post thermally annealed films was found to be retained during the heating and the subsequent cooling. In addition, the scattering analysis provides detailed structural parameters. The 2D GISAXS and GIWAXS patterns were reconstructed from the determined structural parameters by using the derived scattering formulas, and found to be in good agreement with the experimental patterns. Moreover, a model for the structure of the films of the diblock copolymer is proposed.The Journal of Physical Chemistry B 06/2010; 114(24):8033-42. · 3.70 Impact Factor -
Article: Selective aerobic oxidation of benzylic alcohols catalyzed by carbon-based catalysts: a nonmetallic oxidation system.
Angewandte Chemie International Edition 12/2009; 49(2):436-40. · 13.45 Impact Factor -
Article: Carbon Nitride as a Nonprecious Catalyst for Electrochemical Oxygen Reduction
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ABSTRACT: Nitrogen-doped carbon-based catalysts are increasingly being studied as Pt-free electrodes for oxygen reduction in polymer electrolyte membrane fuel cells. Here, we study the oxygen reduction activity of stoichiometric carbon nitride, which has much higher nitrogen content and is synthesized at lower temperatures, without using ionic or metallic iron. Carbon nitride was studied and characterized via X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, BET specific surface area analysis, and thermogravimetric analysis. Rotating electrode voltammetry in oxygen-saturated sulfuric acid was used to determine the catalytic activity. The onset potential for oxygen reduction by carbon nitride electrodes was 0.69 V (vs NHE) compared to 0.45 V for a carbon black reference electrode. However, the current density was low, possibly due to the low surface area of the material. Blending the carbon nitride with a high surface area carbon black support resulted in a significant improvement in current density and in an increase in onset potential to 0.76 V. The role of surface area was elucidated via cyclic voltammetry. This work confirms that stoichiometric carbon nitride has improved inherent oxygen reduction activity compared to pure carbon, and suggests that Fe coordination sites are not essential for electrochemical oxygen reduction in nitrogen-containing carbon materials.11/2009; -
Article: Hierarchical Self-Assembled Structures from POSS-Containing Block Copolymers Synthesized by Living Anionic Polymerization
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ABSTRACT: Two kinds of polyhedral oligomeric silsesquioxane (POSS)-containing block copolymers (BCPs), namely PS-b-PMAPOSS and PMMA-b-PMAPOSS, were synthesized by living anionic polymerization. A wide range of molecular weights were obtained with a very narrow polydispersity index of less than 1.09. The bulk samples prepared by slow evaporation from a polymer solution in chloroform exhibit well-defined microphase-separated structures with long-range order. Thermal annealing induced hierarchical structures consisting of a smaller length scale ordered crystalline POSS domains within the larger microphase-separated structures. We report detailed structural characterization of these hierarchical structures in bulk and thin films by transmission electron microscopy and grazing incidence wide-angle X-ray scattering (GIWAXS). On the basis of this structural analysis, we propose a model for the formation of an orthorhombic lattice structure through the aggregation of POSS segments which formed a helix-like structure.11/2009; -
Article: One‐Step Direct‐Patterning Template Utilizing Self‐Assembly of POSS‐Containing Block Copolymers
Advanced Materials 06/2009; 21(43):4334 - 4338. · 13.88 Impact Factor -
Article: Hyperbranched polysiloxysilane nanoparticles: surface charge control of nonviral gene delivery vectors and nanoprobes.
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ABSTRACT: New hyperbranched polysiloxysilane (HBPS) materials containing terminal carboxylic acid and quaternary ammonium groups were designed and synthesized to obtain fluorescent-dye-encapsulated nanoparticles. These polymers exhibited desirable characteristics, including amphiphilicity for nanoparticle formation, and contained various terminal groups for surface-charge control on the nanoparticles or for further bioconjugation for targeted imaging. Nanoprobes composed of polysiloxysilane nanoparticles encapsulating two-photon dyes were also prepared for optical bioimaging with controlled surface charge density (zeta potential) for modulation of cellular uptake. Intracellular delivery of these structurally similar polysiloxysilane nanoparticles, with substantially different surface charges, was investigated using confocal and two-photon fluorescence microscopy as well as flow cytometry. Finally, the use of these nanoparticles as efficient gene delivery vectors was demonstrated by means of in vitro transfection study using beta-galactosidase plasmid and pEGFP-N1 plasmid and the most efficient combination was obtained using HBPS-CN30:70.International journal of pharmaceutics 06/2009; 376(1-2):141-52. · 2.96 Impact Factor -
Article: Spin-Assembled Nanolayer of a Hyperbranched Polymer on the Anode in Organic Light-Emitting Diodes: The Mechanism of Hole Injection and Electron Blocking
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ABSTRACT: We introduced a spin-assembled nanolayer of hyperbranched poly(ether sulfone) with sulfonic acid terminal on top of an indium−tin oxide anode in organic light-emitting diodes. This results in great improvement in luminous efficiency, better than that of devices using a commercially available conducting polymer composition as a hole-injection layer. The effect of the nanolayer was investigated by impedance spectroscopy, photovoltaic measurement for built-in-potential, and transient electroluminescence. We concluded that the high luminous efficiency resulted from the efficient electron-blocking by the nanolayer and hole-injection assisted by the accumulation of electrons at the interface. This result implies that, for an efficient hole-injection layer, the electron-blocking capability should be incorporated in addition to the hole-injection and -transport capability.Langmuir 10/2008; 24(21). · 4.19 Impact Factor -
Article: Hierarchical Nanostructures of Organosilicate Nanosheets within Self-Organized Block Copolymer Films
06/2008; -
Article: Synthesis of multiblock hyperbranched‐linear poly(ether sulfone) copolymers
Journal of Polymer Science Part A Polymer Chemistry 06/2008; 46(14):4785 - 4793. · 3.92 Impact Factor
Top co-authors
Top Journals
Institutions
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2007–2013
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Tokyo Institute of Technology
- Organic and Polymeric Materials Department
Tokyo, Tokyo-to, Japan
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2008
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Samsung Advanced Institute of Technology
Osan, Gyeonggi, South Korea
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