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Now showing items 17 - 32 of 63

  • Enhancement of luminescence intensity in TMPY/perylene co-single crystals RID G-9009-2011

    Li, Jinpeng   Takaishi, Shinya   Fujinuma, Naohiro   Endo, Katsutoshi   Yamashita, Masahiro   Matsuzaki, Hiroyuki   Okamoto, Hiroshi   Sawabe, Kosuke   Takenobu, Taishi   Iwasa, Yoshihiro  

    A new molecule, 1,3,6,8-tetramethylpyrene (TMPY), with a similar shape to the luminescent material perylene has been successfully synthesized. The co-crystals with perylene doping have been grown and their crystal structure has been clarified by X-ray analysis. Optical spectra indicate that effective energy transfer has been achieved in the doping systems and the luminescence efficiency has reached as high as 78%. The pure TMPY shows p-type characteristics during the FET operation. The hole mobility is up to 0.26 cm(2) V(-1) s(-1).
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  • Ambipolar light-emitting transistors of a tetracene single crystal

    Takahashi, Tetsuo   Takenobu, Taishi   Takeya, Jun   Iwasa, Yoshihiro  

    The first ambipolar light-emitting transistor of an organic molecular semiconductor single crystal, tetracene, is demonstrated. In the device configuration, electrons and holes injected from separate magnesium and gold electrodes recombined radiatively within the channel. By varying the applied voltages, the position of the recombination/emission zone could be moved to any position along the channel. Because of the changes made to the device structure, including, the use of single crystals and polymer dielectric layers and the adoption of an inert-atmosphere fabrication process, the set of materials that can be used for light-emitting transistors has been expanded to include monomeric molecular semiconductors.
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  • Effect of metal electrodes on rubrene single-crystal transistors

    Takenobu, Taishi   Takahashi, Tetsuo   Takeya, Jun   Iwasa, Yoshihiro  

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  • Electron transport in rubrene single-crystal transistors RID A-7097-2008 RID G-9009-2011

    Bisri, Satria Zulkarnaen   Takenobu, Taishi   Takahashi, Tetsuo   Iwasa, Yoshihiro  

    We report a study of impurity effects on the electron transport of rubrene single crystals. A significant improvement of electron carrier mobility up to 0.81 cm(2)/V s is achieved by performing multiple purifications of single crystals and device aging inside an N(2)-filled glove box. The hole/electron mobility ratio obtained is in good agreement with the reported theoretical calculation, suggesting that the intrinsic electron transport of organic semiconductors is also exploitable in a manner similar to that of hole transport. (C) 2010 American Institute of Physics. [doi:10.1063/1.3419899]
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  • High-performance transparent flexible transistors using carbon nanotube films

    Takenobu, Taishi   Takahashi, Tetsuo   Kanbara, Takayoshi   Tsukagoshi, Kazuhito   Aoyagi, Yoshinobu   Iwasa, Yoshihiro  

    Transparent flexible thin-film transistors (tf-TFTs) are an important focus of research since present silicon-based electronics cannot realize such devices. Here, we demonstrate a single-walled carbon nanotube (SWNT) tf-TFTs based on the solution process using transparent electrodes. SWNT tf-TFTs typically exhibit a mobility of 0.5 cm(2)/V s and an on/off current ratio of similar to 10(4). More importantly, these transistors are highly flexible and can be bent to a radius of 7.5 mm without a significant loss in performance. This study therefore represents a major step towards "SWNT transparent plastic electronics."
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  • Optical evidence of Stark effect in single-walled carbon nanotube transistors

    Takenobu, Taishi   Murayama, Yuji   Iwasa, Yoshihiro  

    The effect of an externally applied electric field in single-walled carbon nanotubes was studied using a thin-film transistor configuration. Under the electric field, the optical spectra displayed redshifts and broadening. These phenomena present evidence of the Stark effect in single-walled carbon nanotubes. The finding of the Stark effect suggests the potential use of carbon nanotubes in electro-optic devices for optical communication. (c) 2006 American Institute of Physics.
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  • Ink-Jet Printing of Carbon Nanotube Thin-Film Transistors on Flexible Plastic Substrates

    Takenobu, Taishi   Miura, Noriko   Lu, Sheng-Yi   Okimoto, Haruya   Asano, Takeshi   Shiraishi, Masashi   Iwasa, Yoshihiro  

    Ink-jet printable thin-film transistors (TFTs) on flexible plastic substrates are an important focus of research because present silicon-based electronics cannot realize such devices. In the present study, we fabricated single-walled carbon nanotube (SWCNT) TFTs on plastic substrates using the ink-jet printing method, and realized high-on/off current ratio (similar to 10(4)) and flexibility, respectively. The present study therefore represents a major step towards "flexible SWCNT electronics". (C) 2009 The Japan Society of Applied Physics
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  • High Current Injection into Dynamic p-n Homojunction in Polymer Light-Emitting Electrochemical Cells

    Sakanoue, Tomo   Li, Jinpeng   Tanaka, Hisaaki   Ito, Ryota   Ono, Shimpei   Kuroda, Shin-ichi   Takenobu, Taishi  

    Ions and electrons in blends of polymer-electrolyte can work in ensemble to operate light-emitting electrochemical cells (LECs), in which the unique features of in situ formed p-n homojunctions offer efficient charge injection and transport. However, electrochemical features give rise to significant stability and speed issues due to limited electrochemical stability and low ion mobility, resulting in low brightness and a slow response of LECs. Here, these issues are overcome by the separate control of ionic and electronic charges, using a simple driving pulse superimposed on a small base voltage; ions with slow response are rearranged by a constant base voltage, while a high-voltage pulse, superimposed upon the base, injects electrons/holes which have fast response, with minimal effect on the ions. This scheme successfully injects an extremely high current density of > 2 kA cm(-2) with a balanced electron/ hole ratio, at a high-speed response time of approximate to 50 ns; both properties demonstrate advantages of LECs in making polymers brighter. An in situ electron spin resonance measurement on the LECs further revealed that this impressive performance is due to the highly doped polymers, whose spin density reached 7 x 10(19) spins cm(-3), and an ordered polymer structure in the active layer blend.
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  • Effect of postannealing on the performance of pentacene single-crystal ambipolar transistors

    Takenobu, Taishi   Watanabe, Kazuhiro   Yomogida, Yohei   Shimotani, Hidekazu   Iwasa, Yoshihiro  

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  • Optical Observation of Carrier Accumulation in Single-Walled Carbon Nanotube Transistors

    Takenobu, Taishi   Murayama, Yuji   Shiraishi, Masashi   Iwasa, Yoshihiro  

    Electric-field-induced spectral changes in single-walled carbon nanotubes were studied using a thin-film transistor configuration. As a function of electric field, the optical spectra displayed continuous intensity modulations. This is the direct evidence of carrier accumulation, and the amount of accumulated carriers was quantitatively consistent with the carrier density in the nanoscale wire-form field-effect transistor model.
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  • Chemical hole doping into large-area transition metal dichalcogenide monolayers using boron-based oxidant

    Matsuoka, Hirofumi   Kanahashi, Kaito   Tanaka, Naoki   Shoji, Yoshiaki   Li, Lain-Jong   Pu, Jiang   Ito, Hiroshi   Ohta, Hiromichi   Fukushima, Takanori   Takenobu, Taishi  

    Hole carrier doping into single-crystalline transition metal dichalcogenide (TMDC) films can be achieved with various chemical reagents. However, large-area polycrystalline TMDC monolayers produced by a chemical vapor deposition (CVD) growth method have yet to be chemically doped. Here, we report that a salt of a two-coordinate boron cation, Mes(2)B(+) (Mes: 2,4,6-trimethylphenyl group), with a chemically stable tetrakis( pentafluorophenyl) borate anion, [(C6F5)(4)B](-), can serve as an efficient hole-doping reagent for large-area CVD-grown tungsten diselenide (WSe2) films. Upon doping, the sheet resistance of large-area polycrystalline WSe2 monolayers decreased from 90 G Omega/sq to 3.2 k Omega/sq. (C) 2018 The Japan Society of Applied Physics
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  • Extraction of the contact resistance from the saturation region of rubrene single-crystal transistors

    Imakawa, Masaki   Sawabe, Kosuke   Yomogida, Yohei   Iwasa, Yoshihiro   Takenobu, Taishi  

    A modified transmission-line method (TLM) is proposed to extract the contact resistance from the transistor saturation region. The conventional TLM requires a linear current-voltage characteristic, and this requirement strongly limits its application. In this study, we focused on the pinch-off point of the output characteristics and analyzed the contact resistance using nonlinear output curves. We applied the modified TLM to both p-and n-type rubrene single-crystal transistors and compared the mobility differences in terms of both the intrinsic bandwidth and the extrinsic carrier trap density. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3666236]
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  • Transport Mechanisms in Metallic and Semiconducting Single-Wall Carbon Nanotube Networks RID G-9009-2011

    Yanagi, Kazuhiro   Udoguchi, Hiroki   Sagitani, Satoshi   Oshima, Yugo   Takenobu, Taishi   Kataura, Hiromichi   Ishida, Takao   Matsuda, Kazuyuki   Maniwa, Yutaka  

    A fundamental understanding of the conduction mechanisms in single-wall carbon nanotube (SWCNT) networks is crucial for their use in thin-film transistors and conducting films. However, the uncontrollable mixture state of metallic and semiconducting SWCNTs has always been an obstacle in this regard. In the present study, we revealed that the conduction mechanisms in nanotube networks formed by high-purity metallic and semiconducting SWCNTs are completely different. Quantum transport was observed in macroscopic networks of pure metallic SWCNTs. However, for semiconducting SWCNT networks, Coulomb-gap-type conduction was observed, due to Coulomb interactions between localized electrons. Crossovers among a weakly localized state and strongly localized states with and without Coulomb interactions were observed for transport electrons by varying the relative content of metallic and semiconducting SWCNTs. It was found that hopping barriers, which always exist in normal SWCNT networks and are serious obstacles to achieving high conductivity, were not present in pure metallic SWCNT networks.
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  • Optical characteristic of 5,5 ''-bis(4-biphenylyl)-2,2 ':5 ',2 ''-terthiophene single-crystal thin-film resonator

    Takahashi, Wataru   Maruyama, Kenichi   Li, Jinpeng   Imakawa, Masaki   Takenobu, Taishi  

    We investigated optical characteristics of 5,5 ''-bis(4-biphenylyl)-2,2':5',2 ''-terthiophene (BP3T) single crystals with naturally grown parallel edges. These crystals showed interference modulation spectra due to the Fabry-Perot resonator. From these spectra, we evaluated their resonator quality factors and the refractive index of BP3T. As a result, we obtained a high quality factor of 1700 and a high refractive index of 2.7. (C) 2014 The Japan Society of Applied Physics
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  • Suppression of current hysteresis in carbon nanotube thin-film transistors RID H-2688-2011 RID G-9009-2011

    Tsukagoshi, Kazuhito   Sekiguchi, Masahiro   Ayagi, Yoshinobu   Kanbara, Takayoshi   Takenobu, Taishi   Iwasa, Yoshihiro  

    Source-drain current hysteresis in carbon nanotube film transistors is effectively suppressed by a combination of ultraviolet/ ozone treatment and the thermal evaporation of a protective pentacene film. Thin-film channel transistors fabricated from single-walled carbon nanotubes contain amorphous carbon particles and molecules adsorbed from the atmosphere as charge-trapping sites. Ultraviolet irradiation under exposure to ozone is shown to be effective for eliminating amorphous carbon, and the evaporation of a pentacene layer prevents adsorption from the atmosphere. The combination of these treatments reduces hysteresis in carbon nanotube film transistors.
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  • High Mobility and Luminescent Efficiency in Organic Single-Crystal Light-Emitting Transistors RID A-7097-2008

    Bisri, Satria Zulkarnaen   Takenobu, Taishi   Yomogida, Yohei   Shimotani, Hidekazu   Yamao, Takeshi   Hotta, Shu   Iwasa, Yoshihiro  

    A high-performance ambipolar light-emitting transistor (LET) that has high hole and electron mobilities and excellent luminescence charcteristics is described. By using this device, a conspicuous light-confined edge emission and current-density-dependent spectral evolution are observed. These findings will result in broader utilization of device potential and they provide a promising route for realizing electrically driven organic lasers.
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