Reiko Azumi

Publications (38)99.57 Total impact

• Article: Simple push coating of polymer thin-film transistors.

  Mitsuhiro Ikawa, Toshikazu Yamada, Hiroyuki Matsui, Hiromi Minemawari, Jun'ya Tsutsumi, Yoshinori Horii, Masayuki Chikamatsu, Reiko Azumi, Reiji Kumai, Tatsuo Hasegawa
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  ABSTRACT: Solution processibility is a unique advantage of organic semiconductors, permitting the low-cost production of flexible electronics under ambient conditions. However, the solution affinity to substrate surfaces remains a serious dilemma; liquid manipulation is more difficult on highly hydrophobic surfaces, but the use of such surfaces is indispensable for improving device characteristics. Here we demonstrate a simple technique, which we call 'push coating', to produce uniform large-area semiconducting polymer films over a hydrophobic surface with eliminating material loss. We utilize a poly(dimethylsiloxane)-based trilayer stamp whose conformal contact with the substrate enables capillarity-induced wetting of the surface. Films are formed through solvent sorption and retention in the stamp, allowing the stamp to be peeled perfectly from the film. The planar film formation on hydrophobic surfaces also enables subsequent fine film patterning. The technique improves the crystallinity and field-effect mobility of stamped semiconductor films, constituting a major step towards flexible electronics production.
  Nature Communications 11/2012; 3:1176. · 7.40 Impact Factor
• Article: Effect of subphase temperature on the phase-separated structures of mixed Langmuir and Langmuir-Blodgett films of fatty acids and hybrid carboxylic acids.

  Satoshi Watanabe, Ryo Okuda, Reiko Azumi, Hideki Sakai, Masahiko Abe, Mutsuyoshi Matsumoto
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  ABSTRACT: We examined the phase-separated structures of the mixed Langmuir-Blodgett (LB) films of C(k)H(2k+1)COOH (HkA: k=17, 21) and C(m)F(2m+1)C(10)H(20)COOH (FmH10A: m=6, 8) fabricated isothermally or after isobaric thermal treatments. Under isothermal fabrication conditions, disks and wire-type domains formed in the H17A/F6H10A LB films at high and low fabrication temperatures, respectively, because the line tension and dipole-dipole interaction were comparable with each other. The thermodynamically stable phases of H21A/F6H10A LB films at high and low fabrication temperatures were disks and polygonal domains, respectively. Isobaric thermal treatments of the Langmuir films affected the domain size and not the domain shape when the transfer temperature was the same. Isobaric thermal treatments were effective in controlling the domain size. The thermodynamically stable phases of both H17A/F8H10A LB films and H21A/F8H10A LB films were nanowires in the range of the fabrication temperatures studied. Isobaric thermal treatments of the Langmuir films did not affect the domain shape significantly and affected the domain size in both the LB films studied. The change in the value m of FmH10A was more effective in controlling the phase-separated structures of the mixed LB films than the value of k of HkA.
  Journal of Colloid and Interface Science 07/2011; 363(1):379-85. · 3.07 Impact Factor
• Article: Soluble fullerene-based n-channel organic thin-film transistors printed by using a polydimethylsiloxane stamp.

  Yoshinori Horii, Mitsuhiro Ikawa, Masayuki Chikamatsu, Reiko Azumi, Masahiko Kitagawa, Hisatoshi Konishi, Kiyoshi Yase
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  ABSTRACT: A polydimethylsiloxane stamp was applied for the first time to the fabrication of n-channel thin-film transistors based on soluble small molecule organic semiconducting materials. The stamping method was found to facilitate film transfer onto a gate insulator surface irrespective of its surface free energy. We used [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) and C(60)-fused N-methylpyrrolidine-meta-dodecyl phenyl (C60MC12) as n-channel materials. The stamped thin-film transistors of C60MC12 achieved a high electron mobility of 0.39 cm(2)/(V s) and a current on-off ratio of 1 × 10(7). The mobility of the stamped C60MC12 thin-film transistors did not depend much on the surface free energy of the SiO(2) gate insulator with and without surface treatment using a silane-coupling reagent. In particular, the stamped C60MC12 thin-film transistor exhibited a relatively high mobility of 0.1 cm(2)/(V s) on a high energy surface of untreated SiO(2). In addition, a complementary inverter composed of an n-channel and a p-channel stamped thin-film transistor was demonstrated for the first time, which exhibits a maximum gain of 63 at a supply voltage of 50 V.
  ACS Applied Materials & Interfaces 03/2011; 3(3):836-41. · 4.53 Impact Factor
• Article: Anisotropic field-effect hole mobility of liquid crystalline conjugated polymer layers formed on photoaligned polyimide films

  Kenji Sakamoto, Takeshi Yasuda, Kazushi Miki, Masayuki Chikamatsu, Reiko Azumi
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  ABSTRACT: We have fabricated organic field-effect transistors (OFETs) with a highly oriented active layer of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophene] (F8T2), which was formed with the help of a photoaligned polyimide film. Photoalignment is an attractive technique for integrating OFETs with aligned active layers on the same substrate, because of its potential capability of two-dimensional alignment patterning. The F8T2 layer formed on the photoaligned polyimide film showed an absorption dichroic ratio greater than 15 after annealing at 285 °C. Top-gate/bottom-contact-type OFETs with a parylene gate insulating layer exhibited an enhanced hole mobility (0.016 cm2 V−1 s−1) along the alignment direction of the F8T2 backbone structure and a suppressed one (0.002 cm2 V−1 s−1) along the perpendicular direction. This result shows that the photoaligned polyimide film is an attractive alignment layer for fabricating and integrating OFETs with aligned active layers. Current-direction-dependent bias stress effect was observed for those OFETs; when the current flowed parallel to the alignment direction of the F8T2 backbone structures, a larger negative threshold voltage shift was observed. This anisotropic bias stress effect was discussed on the basis of a microstructure model of aligned F8T2 films.
  Journal of Applied Physics 01/2011; 109(1):013702-013702-7. · 2.17 Impact Factor
• Article: Complementary Inverters Based on Soluble P- and N-Channel Organic Semiconductors.

  Masayuki Chikamatsu, Yoshinori Horii, Ming Lu, Yuji Yoshida, Reiko Azumi, Kiyoshi Yase
  IEICE Transactions. 01/2011; 94-C:1845-1847.
• Article: High-Performance Solution-Processed n-Channel Organic Thin-Film Transistors Based on a Long Chain Alkyl-Substituted C60 Derivative

  Yoshinori Horii, Koichi Sakaguchi, Masayuki Chikamatsu, Reiko Azumi, Kiyoshi Yase, Masahiko Kitagawa, Hisatoshi Konishi
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  ABSTRACT: We report on high-performance solution-processed n-channel organic thin-film transistors based on a long-chain alkyl-substituted fullerene derivative, C60-fused N-methylpyrrolidine-meta-dodecyl phenyl (C60MC12), by surface modification of an insulator. C60MC12 films were fabricated on self-assembled monolayer (SAM)-treated gate insulators by spin-coating, which was facilitated by hydrophilic patterning of the edge of the substrate. X-ray diffraction revealed that the crystallinity of C60MC12 films was improved by SAM treatment. The octadecyltrichlorosilane-treated device demonstrated a high mobility of 0.4--0.5 cm2 V-1 s-1, which is comparable with those of amorphous silicon thin-film transistors. An improvement in bias stress stability by the SAM treatment was also observed.
  Applied Physics Express 09/2010; 3(10):1601. · 3.01 Impact Factor
• Article: LIQUID CRYSTALLINE BEHAVIORS OF SUBSTITUTED OLIGOTHIOPHENE BINARY MIXTURES

  Reiko Azumi, Mutsuyoshi Matsumoto, Takashi Yamada, Hideki Sakai, Masahiko Abe
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  ABSTRACT: Thermal behaviors were investigated of the binary mixtures of α,ω-disubstituted oligothiophenes. Smectic A phase, which is absent in the pure state of each component, was observed in the mixture of dicyano- and dimethylterthiophenes. On the other hand, the mixture of dicyanoterthiophene and terthiophene dicarboxaldhyde did not exhibit a new mesophase. Platelet texture was observed in the mixture of terthiophene dicarboxaldhyde and dimethylterthiophene although the two components did not mix well.
  Molecular Crystals and Liquid Crystals 07/2010; 406(1):181-186. · 0.58 Impact Factor
• Article: Patterning of J-aggregated dyes using directed self-assembly on micro- and nanopatterned templates fabricated from phase-separated mixed Langmuir-Blodgett films.

  Satoshi Watanabe, Hirobumi Shibata, Shin Horiuchi, Reiko Azumi, Hideki Sakai, Masahiko Abe, Mutsuyoshi Matsumoto
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  ABSTRACT: We report a useful technique for the deposition of dyes in the form of J-aggregates using directed self-assembly on the micro- and nanopatterned templates fabricated from the phase-separated mixed Langmuir-Blodgett (LB) films. The patterns of the mixed LB films can be tuned by adjusting the intermolecular interactions between the film-forming molecules. We used the mixed LB films containing silane coupling agent for the fabrication of micro- and nanopatterned templates, taking advantage of the difference between the surface free energy of the patterned regions and that of the self-assembled monolayers of the silane coupling agent. Atomic force microscopy showed that dyes were deposited by casting, spin-coating and the LB technique in accordance with the patterns of the original mixed LB films. Emission spectroscopy revealed that J-aggregates were formed in the patterned films. We succeeded in obtaining nanopatterns of a cyanine dye in the form of J-aggregates on the templates.
  Journal of Colloid and Interface Science 11/2009; 343(1):324-9. · 3.07 Impact Factor
• Article: Multi-layered oriented polyfluorene films.

  Shuichi Nagamatsu, Masahiro Misaki, Yuji Yoshida, Reiko Azumi, Nobutaka Tanigaki, Kiyoshi Yase
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  ABSTRACT: Multilayered oriented polyfluorene (PF) films were obtained by applying thermal treatment procedure to a multilayered PF film constructed with fluorene derivatives layer formed on top of a highly oriented friction-transferred crystalline poly(9,9-dioctylfluorene) (PF8) film. The orientations in the multilayered PF films were investigated by polarized photoluminescence (PL) spectroscopy and grazing incident X-ray diffraction (GIXD) analysis. The results of the multilayered PF film constructed with spin-coated PF8 on friction-transferred PF8 indicate that the rearrangement of the upper PF8 layer is induced from the orientation of lower PF8 layer by thermal treatment at the nematic phase temperature. Polarized green emission from the multilayered oriented PF film was demonstrated using the blend of PF8 and poly(9,9-dioctylfluorene-co-benzothiadiazol) (F8BT) as green light emitter for upper layer. By this method, the polarized emission color can be tuned using polymer blends for upper layer similar to the liquid-crystalline polymer arrangement without using different materials as an underlying layer such as the rubbed polyimide.
  The Journal of Physical Chemistry B 05/2009; 113(17):5746-51. · 3.70 Impact Factor
• Article: High-Performance n-Type Organic Thin-Film Transistors Based on Solution-Processable Perfluoroalkyl-Substituted C60 Derivatives

  Masayuki Chikamatsu, Atsushi Itakura, Yuji Yoshida, Reiko Azumi, Kiyoshi Yase
  12/2008;
• Article: Phase-separated structures of mixed Langmuir-Blodgett films of fatty acid and hybrid carboxylic acid.

  Hideto Kimura, Satoshi Watanabe, Hirobumi Shibata, Reiko Azumi, Hideki Sakai, Masahiko Abe, Mutsuyoshi Matsumoto
  [show abstract] [hide abstract]
  ABSTRACT: Phase separation often occurs in mixed Langmuir-Blodgett (LB) films. Usually circular domains at the micrometer length scale form in the LB films. The size and shape of the domains are governed by a compromise between two competing interactions of line tension and dipole-dipole interaction. An attempt was made to control the line tension by varying systematically the hydrophobic moieties of the film-forming molecules. Phase-separated structures of two-component mixed LB films of fatty acid [C(k)H(2k+1)COOH (HkA)] and hybrid carboxylic acid [C(m)F(2m+1)C(n)H(2n)COOH (FmHnA)] were investigated. IR spectra of the mixed LB films of H17A and F8H10A revealed that the alkyl chains were in an all-trans conformation and that the molecular orientation remained unchanged when the two components were mixed. Nanowires formed in the mixed LB films of HkA and F8H10A. The width of the nanowires increased with an increase in k. Domain size and shape in the mixed LB films of H17A and FmHnA depended strongly on the values of m and n. Circular domains at the micrometer length scale formed in the region m + n < 16. In contrast, domains at the nanometer length scale formed in the region m + n > or = 16 except for F6H10A. These results were explained by using a lattice model that considers the effect of the hydrophobic moieties of fatty acid and hybrid carboxylic acid on the line tension.
  The Journal of Physical Chemistry B 12/2008; 112(48):15313-9. · 3.70 Impact Factor
• Article: Micro- and nanopatterned copper structures using directed self-assembly on templates fabricated from phase-separated mixed Langmuir-Blodgett films.

  Satoshi Watanabe, Hideto Kimura, Takahiro Sato, Hirobumi Shibata, Fumitaka Sakamoto, Reiko Azumi, Hideki Sakai, Masahiko Abe, Mutsuyoshi Matsumoto
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  ABSTRACT: We report a versatile method to confine metal thin films in micro- and nanopatterns using directed self-assembly on the templates fabricated from phase-separated mixed Langmuir-Blodgett (LB) films. The pattern of the mixed LB films can be tuned by adjusting intermolecular interaction between the film-forming molecules in the LB films and by varying the fabrication conditions of the films such as the mixing ratio, subphase temperature, and surface pressure. We use the patterned LB films for templates to confine metal in patterned regions, taking advantage of the difference between the surface free energy of the patterned regions and that of the self-assembled monolayer of the silane coupling agent. Au nanoparticles are confined onto the patterned films as a catalyst for the succeeding Cu electroless deposition. The atomic force microscopic images, Auger electron spectra, and scanning Auger electron maps of a Cu-deposited film show that Cu is selectively deposited on the patterns of phase separation of the original mixed LB films.
  Langmuir 08/2008; 24(16):8735-41. · 4.19 Impact Factor
• Article: Very thin photoalignment films for liquid crystalline conjugated polymers: Application to polarized light-emitting diodes

  Kenji Sakamoto, Kazushi Miki, Masahiro Misaki, Koichi Sakaguchi, Masayuki Chikamatsu, Reiko Azumi
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  ABSTRACT: Photoaligned polyimide films with different film thicknesses were prepared on quartz substrates, and uniaxially aligned glassy poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) layers were formed on top. The photoluminescence polarization ratio of the PFO layer rapidly increased with increasing polyimide film thickness, and beyond a thickness of 1.6 nm, it was saturated at ∼ 11. This result shows that the 1.6-nm-thick photoaligned polyimide film works as a good alignment layer for PFO. We succeeded in fabricating a polarized light-emitting diode with a polarization ratio of 29 at 459 nm and a brightness of 700 cd/m2 by using a 2.8-nm-thick polyimide photoalignment layer.
  Applied Physics Letters 10/2007; 91(18):183509-183509-3. · 3.84 Impact Factor
• Article: Ambipolar organic field-effect transistors based on a low band gap semiconductor with balanced hole and electron mobilities

  Masayuki Chikamatsu, Takefumi Mikami, Jiro Chisaka, Yuji Yoshida, Reiko Azumi, Kiyoshi Yase, Akihiro Shimizu, Takashi Kubo, Yasushi Morita, Kazuhiro Nakasuji
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  ABSTRACT: The authors have demonstrated the thin-film properties and the ambipolar transport of a delocalized singlet biradical hydrocarbon with two phenalenyl radical moieties (Ph2-IDPL). The organic field-effect transistors (OFETs) based on Ph2-IDPL exhibit ambipolar transport with balanced hole and electron mobilities in the order of 10−3 cm2/V s. The Ph2-IDPL film is an organic semiconductor with a low band gap of 0.8 eV and has small injection barriers from gold electrodes to both the highest occupied molecular orbital and the lowest unoccupied molecular orbital. A complementary metal-oxide-semiconductor-like inverter using two identical Ph2-IDPL based ambipolar OFETs shows a sharp inversion of the input voltage with high gain.
  Applied Physics Letters 07/2007; 91(4):043506-043506-3. · 3.84 Impact Factor
• Article: Monolayers assembled from a glycolipid biosurfactant from Pseudozyma (Candida) antarctica serve as a high-affinity ligand system for immunoglobulin G and M.

  Tomohiro Imura, Seya Ito, Reiko Azumi, Hiroshi Yanagishita, Hideki Sakai, Masahiko Abe, Dai Kitamoto
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  ABSTRACT: A carbohydrate ligand system has been developed which is composed of self-assembled monolayers (SAMs) of mannosylerythritol lipid-A (MEL-A) from Pseudozyma antarctica, serving for human immunoglobulin G and M (HIgG and HIgM). The estimated binding constants from surface plasmon resonance (SPR) measurement were Ka = 9.4 x 10(6) M(-1) for HIgG and 5.4 x 10(6) M(-1) for HIgM, respectively. The binding site was not in the Fc region of immunoglobulin but in the Fab region. Large amounts of HIgG and HIgM bound to MEL-A SAMs were directly observed by atomic force microscopy.
  Biotechnology Letters 07/2007; 29(6):865-70. · 1.68 Impact Factor
• Article: Organic Memory Device Based on Carbazole‐Substituted Cellulose

  Makoto Karakawa, Masayuki Chikamatsu, Yuji Yoshida, Reiko Azumi, Kiyoshi Yase, Chikanobu Nakamoto
  Macromolecular Rapid Communications 06/2007; 28(14):1479 - 1484. · 4.60 Impact Factor
• Article: Crystal structure of friction-transferred poly(2,5-dioctyloxy-1,4-phenylenevinylene).

  Shuichi Nagamatsu, Masahiro Misaki, Masayuki Chikamatsu, Tatsumi Kimura, Yuji Yoshida, Reiko Azumi, Nobutaka Tanigaki, Kiyoshi Yase
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  ABSTRACT: Poly(2,5-dioctyloxy-1,4-phenylenevinylene) (DOPPV) was found to form a highly oriented film by a friction-transfer technique. Structural investigation of friction-transferred DOPPV was studied by means of polarized ultraviolet-visible (UV-vis) absorption spectroscopy, polarized photoluminescence (PL) spectroscopy, and synchrotron-sourced grazing incident X-ray diffraction (GIXD) analysis. The polarized UV-vis absorption and PL spectra indicate clear axial alignment. DOPPV backbones in friction-transferred film are highly aligned along the drawing direction of the friction-transfer. Further information of the molecular arrangement in friction-transferred DOPPV film was investigated by both the out-of-plane and the in-plane GIXD analyses with synchrotron source. The DOPPV molecules in friction-transferred films were perfectly arranged three-dimensionally: the backbones aligned along the drawing direction of friction-transfer, the alkyl side chains lay in the film plane, and the planar backbones were arranged parallel to the film surface. Additionally, two neighboring DOPPV molecules along the direction of inter-backbones separation by alkyl side chains were found to be shifted with respect to one another by the mean distance of half of a monomeric repeat.
  The Journal of Physical Chemistry B 05/2007; 111(17):4349-54. · 3.70 Impact Factor
• Article: Structure and Electrical Properties of Unsubstituted Oligothiophenes End-Capped at the β-Position

  Jiro Chisaka, Ming Lu, Shuichi Nagamatsu, Masayuki Chikamatsu, Yuji Yoshida, Midori Goto, Reiko Azumi, Masafumi Yamashita, Kiyoshi Yase
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  ABSTRACT: The relationship between molecular structure, molecular packing in bulk, film structure, and electrical characteristics were investigated for the unsubstituted oligothiophene pentamer and hexamer, both of which have terminal thiophene units linking at the β-position (βnT; n = 5, 6). To reveal the molecular geometry and molecular packing of βnTs, X-ray single-crystal analysis was carried out. For the electrical characterization, organic field-effect transistors (OFETs) based on the βnT thin films were fabricated at various substrate temperatures. Film structure was investigated by X-ray diffraction and atomic force microscopy. Highest field-effect mobility of OFET was 0.050 cm2 V-1 s-1 for β6T at a substrate temperature of 120 °C and 0.020 cm2 V-1 s-1 for β5T at a substrate temperature of 90 °C, which are slightly smaller than those of α-oligothiophene isomers with the same number of thiophene rings. β6T OFET characteristics depend on the film morphology such as grain size, while the dependence of β5T OFET on the morphology is slight. The different behavior of β6T and β5T can be interpreted by their different molecular orientation with respect to the film surface.
  04/2007;
• Article: Structure, Physical Properties and Thin-Film Transistor Characteristics of Sexithiophene Isomers

  Ming Lu, Reiko Azumi, Masayuki Chikamatsu, Yuji Yoshida, Kiyoshi Yase
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  ABSTRACT: We have synthesized unsubstituted oligothiophenes with both terminal thiophene rings linked at β-positions. The correlation between molecular structure, character of a single molecule such as conjugation length, and physical properties as solid has been investigated. Moreover, the influence on electric properties of their thin films has been investigated. It was revealed that π-conjugation length of the molecule considerably influences the field-effect transistor characteristic.
  Molecular Crystals and Liquid Crystals 01/2007; 472(September 01):137/[527]-143/[533]. · 0.58 Impact Factor
• Article: Color-variable organic light-emitting device by external light irradiation

  Koichi Sakaguchi, Takeru Oosawa, Masayuki Chikamatsu, Yuji Yoshida, Reiko Azumi, Kiyoshi Yase
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  ABSTRACT: The authors demonstrate a color-variable organic light-emitting device by external light irradiation, where an organic photoelectric conversion layer out of titanyl phthalocyanine is inserted between blue- and green-emitting layers. By near-infrared (780 nm) laser irradiation, the emission color is varied from blue to green. The color coordinate in Commission Internationale de l’Eclairage chromaticity is varied from (0.201, 0.193) to (0.302, 0.445). The color change mechanism can be interpreted by the change in carrier balance caused by the photocarrier generation in the device, resulting in the change in charge recombination site.
  Applied Physics Letters 11/2006; 89(22):223520-223520-3. · 3.84 Impact Factor