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Now showing items 1 - 16 of 106

  • CVD growth of large-area InS atomic layers and device applications

    Tu, Chien-Liang   Lin, Kuang-, I   Pu, Jiang   Chung, Tsai-Fu   Hsiao, Chien-Nan   Huang, An-Ci   Yang, Jer-Ren   Takenobu, Taishi   Chen, Chang-Hsiao  

    Group-III monochalcogenides of two-dimensional (2D) layered materials have attracted widespread attention among scientists due to their unique electronic performance and interesting chemical and physical properties. Indium sulfide (InS) is attracting increasing interest from scientists because it has two distinct crystal structures. However, studies on the synthesis of highly crystalline, large-area, and atomically thin-film InS have not been reported thus far. Here, the chemical vapor deposition (CVD) synthesis method of atomic InS crystals has been reported in this paper. The direct chemical vapour phase reaction of metal oxides with chalcogen precursors produces a large-sized hexagonal crystal structure and atomic-thickness InS flakes or films. The InS atomic films are merged with a plurality of triangular InS crystals that are uniform and entire and have surface areas of 1 cm(2) and controllable thicknesses in bilayers or trilayers. The properties of the as-grown highly crystalline samples were characterized by spectroscopic and microscopic measurements. The ion-gel gated InS field-effect transistors (FETs) reveal n-type transport behavior, and have an on-off current ratio of >10(3) and a room-temperature electron mobility of similar to 2 cm(2) V-1 s(-1). Moreover, our CVD InS can be transferred from mica to any substrates, so various 2D materials can be reassembled into vertically stacked heterostructures, thus facilitating the development of heterojunctions and exploration of the properties and applications of their interactions.
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  • 2D Materials for Large-Area Flexible Thermoelectric Devices

    Kanahashi, Kaito   Pu, Jiang   Takenobu, Taishi  

    The rapid development of the concept of the "Internet of Things (IoT)" requires wearable devices with maintenance-free batteries, and thermoelectric energy conversion based on large-area flexible materials has attracted much attention. Among large-area flexible materials, 2D materials, such as graphene and related materials, are promising for thermoelectric applications due to their excellent transport properties and large power factors. In this Review, both single-crystalline and polycrystalline 2D materials are surveyed using the experimental reports on thermoelectric devices of graphene, black phosphorus, transition metal dichalcogenides, and other 2D materials. In particular, their carrier-density dependent thermoelectric properties and power factors maximized by Fermi level tuning techniques are focused. The comparison of the relevant performances between 2D materials and commonly used thermoelectric materials reveals the significantly enhanced power factors in 2D materials. Moreover, the current progress in thermoelectric module applications using large-area 2D material thin films is summarized, which consequently offers great potential for the use of 2D materials in large-area flexible thermoelectric device applications. Finally, important remaining issues and future perspectives, such as preparation methods, thermal transports, device designs, and promising effects in 2D materials, are discussed.
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  • Synthesis of Large-Area InSe Monolayers by Chemical Vapor Deposition

    Chang, Han-Ching   Tu, Chien-Liang   Lin, Kuang-I   Pu, Jiang   Takenobu, Taishi   Hsiao, Chien-Nan   Chen, Chang-Hsiao  

    Recently, 2D materials of indium selenide (InSe) layers have attracted much attention from the scientific community due to their high mobility transport and fascinating physical properties. To date, reports on the synthesis of high-quality and scalable InSe atomic films are limited. Here, a synthesis of InSe atomic layers by vapor phase selenization of In2O3 in a chemical vapor deposition (CVD) system, resulting in large-area monolayer flakes or thin films, is reported. The atomic films are continuous and uniform over a large area of 1 x 1 cm(2), comprising of primarily InSe monolayers. Spectroscopic and microscopic measurements reveal the highly crystalline nature of the synthesized InSe monolayers. The ion-gel-gated field-effect transistors based on CVD InSe monolayers exhibit n-type channel behaviors, where the field effect electron mobility values can be up to approximate to 30 cm(2) V-1 s(-1) along with an on/off current ratio, of >10(4) at room temperature. In addition, the graphene can serve as a protection layer to prevent the oxidation between InSe and the ambient environment. Meanwhile, the synthesized InSe films can be transferred to arbitrary substrates, enabling the possibility of reassembly of various 2D materials into vertically stacked heterostructures, prompting research efforts to probe its characteristics and applications.
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  • GaN light-emitting device based on ionic liquid electrolyte

    Hirai, Tomoaki   Sakanoue, Tomo   Takenobu, Taishi  

    Ionic liquids (ILs) are attractive materials for fabricating unique hybrid devices based on electronics and electrochemistry; thus, IL-gated transistors and organic light-emitting devices of light-emitting electrochemical cells (LECs) are investigated for future low-voltage and high-performance devices. In LECs, voltage application induces the formation of electrochemically doped p-n homojunctions owing to ion rearrangements in composites of semiconductors and electrolytes, and achieves electron-hole recombination for light emission at the homojunctions. In this work, we applied this concept of IL-induced electrochemical doping to the fabrication of GaN-based light-emitting devices. We found that voltage application to the layered IL/GaN structure accumulated electrons on the GaN surface owing to ion rearrangements and improved the conductivity of GaN. The ion rearrangement also enabled holes to be injected by the strong electric field of electric double layers on hole injection contacts. This simultaneous injection of holes and electrons into GaN mediated by ions achieves light emission at a low voltage of around 3.4V. The light emission from the simple IL/GaN structure indicates the usefulness of an electrochemical technique in generating light emission with great ease of fabrication. (C) 2018 The Japan Society of Applied Physics.
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  • Light emission from organic single crystals operated by electrolyte doping

    Matsuki, Keiichiro   Sakanoue, Tomo   Yomogida, Yohei   Hotta, Shu   Takenobu, Taishi  

    Light-emitting devices based on electrolytes, such as light-emitting electrochemical cells (LECs) and electric double-layer transistors (EDLTs), are solution-processable devices with a very simple structure. Therefore, it is necessary to apply this device structure into highly fluorescent organic materials for future printed applications. However, owing to compatibility problems between electrolytes and organic crystals, electrolyte-based single-crystal light-emitting devices have not yet been demonstrated. Here, we report on light-emitting devices based on organic single crystals and electrolytes. As the fluorescent materials, alpha,omega-bis(biphenylyl) terthiophene (BP3T) and 5,6,11,12-tetraphenylnaphthacene (rubrene) single crystals were selected. Using ionic liquids as electrolytes, we observed clear light emission from BP3T LECs and rubrene EDLTs. (C) 2018 The Japan Society of Applied Physics
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  • Fluorescent Ferroelectrics of Hydrogen-Bonded Pyrene Derivatives

    Anetai, Hayato   Wada, Yoshifumi   Takeda, Takashi   Hoshino, Norihisa   Yamamoto, Shunsuke   Mitsuish, Masaya   Takenobu, Taishi   Akutagawa, Tomoyuki  

    Organic materials with diverse molecular designs show multifunctional properties such as coupled ferroelectric, optical, ferromagnetic, and transport properties. We report the design of an alkylamide-substituted pyrene derivative displaying fluorescent ferroelectric properties coupled with electron transport properties. In solution phase, this compound displayed concentration-dependent fluorescence, whereas in xerogels, a fluorescent green organogel (>0.1 mM) and entangled nanofibers were observed. A discotic hexagonal columnar liquid crystalline phase was observed above 295 K due to intermolecular hydrogen bonding and pi-stacking interactions. The direction of the hydrogen-bonded chains could be inverted by the application of an external electric field along the pi-stacked column, resulting in ferroelectric polarization-electric field (P-E) hysteresis. The local electric field arising from the ferroelectric macrodipole moment arrangement along the pi-stacking direction affected the electron transport properties on the pi-stack of pyrenes, thus confirming the current-switching phenomena according to P-E hysteresis. We report that multifunctional properties such as ferroelectricity, fluorescence, and electron transport switching were successfully achieved in hydrogen-bonded dynamic pi-molecular assemblies.
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  • Novel Functional Devices of Single-walled Carbon Nanotubes

    Takenobu, Taishi  

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  • Self-Aligned and Scalable Growth of Monolayer WSe2-MoS2 Lateral Heterojunctions

    Li, Ming-Yang   Pu, Jiang   Huang, Jing-Kai   Miyauchi, Yuhei   Matsuda, Kazunari   Takenobu, Taishi   Li, Lain-Jong  

    2D layered heterostructures have attracted intensive interests due to their unique optical, transport, and interfacial properties. The laterally stitched heterojunction based on dissimilar 2D transition metal dichalcogenides forms an intrinsic p-n junction without the necessity of applying an external voltage. However, no scalable processes are reported to construct the devices with such lateral heterostructures. Here, a scalable strategy, two-step and location-selective chemical vapor deposition, is reported to synthesize self-aligned WSe2-MoS2 monolayer lateral heterojunction arrays and demonstrates their light-emitting devices. The proposed fabrication process enables the growth of high-quality interfaces and the first successful observation of electroluminescence at the WSe2-MoS2 lateral heterojunction. The electroluminescence study has confirmed the type-I alignment at the interface rather than commonly believed type-II alignment. This self-aligned growth process paves the way for constructing various 2D lateral heterostructures in a scalable manner, practically important for integrated 2D circuit applications.
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  • Large-area WSe2 electric double layer transistors on a plastic substrate

    Funahashi, Kazuma   Pu, Jiang   Li, Ming-Yang   Li, Lain-Jong   Iwasa, Yoshihiro   Takenobu, Taishi  

    Due to the requirements for large-area, uniform films, currently transition metal dichalcogenides (TMDC) cannot be used in flexible transistor industrial applications. In this study, we first transferred chemically grown large-area WSe2 monolayer films from the as-grown sapphire substrates to the flexible plastic substrates. We also fabricated electric double layer transistors using the WSe2 films on the plastic substrates. These transistors exhibited ambipolar operation and an ON/OFF current ratio of similar to 10(4), demonstrating chemically grown WSe2 transistors on plastic substrates for the first time. This achievement can be an important first step for the next-generation TMDC based flexible devices. (C) 2015 The Japan Society of Applied Physics
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  • Strategy for improved frequency response of electric double-layer capacitors

    Wada, Yoshifumi   Pu, Jiang   Takenobu, Taishi  

    We propose a strategy for improving the response speed of electric double-layer capacitors (EDLCs) and electric double-layer transistors (EDLTs), based on an asymmetric structure with differently sized active materials and gate electrodes. We validate the strategy analytically by a classical calculation and experimentally by fabricating EDLCs with asymmetric Au electrodes (1:50 area ratio and 7.5 pm gap distance). The performance of the EDLCs is compared with that of conventional symmetric EDLCs. Our strategy dramatically improved the cut-off frequency from 14 to 93 kHz and this improvement is explained by fast charging of smaller electrodes. Therefore, this approach is particularly suitable to EDLTs, potentially expanding the applicability to medium speed (kHz MHz) devices. (C) 2015 AIP Publishing LLC.
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  • The pursuit of electrically-driven organic semiconductor lasers

    Bisri, Satria Zulkarnaen   Takenobu, Taishi   Iwasa, Yoshihiro  

    Organic semiconductors have many favourable and plastic-like optical properties that are promising for the development of low energy consuming laser devices. Although optically-pumped organic semiconductor lasers have been demonstrated since the early days of lasers, electrically-driven organic lasers have not been realized and have become one of the greatest challenges in the fields of physics, chemistry and materials science. This article highlights the recent progress in the development of electroluminescent devices, as well as new types of materials with the aim of realizing the first electrically-driven organic semiconductor laser.
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  • Organic single-crystal light-emitting field-effect transistors

    Hotta, Shu   Yamao, Takeshi   Bisri, Satria Zulkarnaen   Takenobu, Taishi   Iwasa, Yoshihiro  

    Growth and characterisation of single crystals constitute a major field of materials science. In this feature article we overview the characteristics of organic single-crystal light-emitting field-effect transistors (OSCLEFETs). The contents include the single crystal growth of organic semiconductors and their application to transistor devices. First, we describe various single crystal growth methods that produce different morphologies and geometries of crystals. Using these single crystals we highlight construction and performance of the devices. The single crystal approach not only allows us to study the device performance that reflects the intrinsic nature of the organic semiconductors but also is advantageous to enhancement in the steady device operation. A current-injected laser oscillation in an electronic device configuration remains as a big challenge to be achieved. In this context we briefly mention the spectrally narrowed emissions as well as the possibility of light amplification in the OSCLEFETs.
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  • Monolayer MoSe2 Grown by Chemical Vapor Deposition for Fast Photodetection

    Chang, Yung-Huang   Zhang, Wenjing   Zhu, Yihan   Han, Yu   Pu, Jiang   Chang, Jan-Kai   Hsu, Wei-Ting   Huang, Jing-Kai   Hsu, Chang-Lung   Chiu, Ming-Hui   Takenobu, Taishi   Li, Henan   Wu, Chih-I   Chang, Wen-Hao   Wee, Andrew Thye Shen   Li, Lain-Jong  

    Monolayer molybdenum disulfide (MoS2) has become a promising building block in optoelectronics for its high photosensitivity. However, sulfur vacancies and other defects significantly affect the electrical and optoelectronic properties of monolayer MoS2 devices. Here, highly crystalline molybdenum diselenide (MoSe2) monolayers have been successfully synthesized by the chemical vapor deposition (CVD) method. Low-temperature photoluminescence comparison for MoS2 and MoSe2 monolayers reveals that the MoSe2 monolayer shows a much weaker bound exciton peak; hence, the phototransistor based on MoSe2 presents a much faster response time (<25 ms) than the corresponding 30 s for the CVD MoS2 monolayer at room temperature in ambient conditions. The images obtained from transmission electron microscopy indicate that the MoSe exhibits fewer defects than MoS2. This work provides the fundamental understanding for the differences in optoelectronic behaviors between MoSe2 and MoS2 and is useful for guiding future designs in 2D material-based optoelectronic devices.
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  • Flexible and stretchable thin-film transistors based on molybdenum disulphide

    Pu, Jiang   Li, Lain-Jong   Takenobu, Taishi  

    The outstanding physical and chemical properties of two-dimensional materials, which include graphene and transition metal dichalcogenides, have allowed significant applications in next generation electronics. In particular, atomically thin molybdenum disulphide (MoS2) is attracting widespread attention because of its large bandgap, effective carrier mobility, and mechanical strength. In addition, recent developments in large-area high-quality sample preparation methods via chemical vapour deposition have enabled the use of MoS2 in novel functional applications, such as flexible and stretchable electronic devices. In this perspective, we focus on the current progress in generating MoS2-based flexible and stretchable thin-film transistors. The reported virtues and novelties of MoS2 provide significant advantages for future flexible and stretchable electronics.
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  • Ambipolar organic single-crystal transistors based on ion gels.

    Yomogida, Yohei   Pu, Jiang   Shimotani, Hidekazu   Ono, Shimpei   Hotta, Shu   Iwasa, Yoshihiro   Takenobu, Taishi  

    Ambipolar electric double-layer transistors (EDLTs) using organic single crystals and ion-gel electrolytes are successfully created by optimising the fabrication of gel films. The p- and n-type EDLTs enable us to investigate the HOMO-LUMO gap energy of semiconductors, offering a new method with which to measure it. Copyright =C2=A9 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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  • Large-Area Synthesis of Highly Crystalline WSe2 Mono layers and Device Applications

    Huang, Jing-Kai   Pu, Jiang   Hsu, Chang-Lung   Chiu, Ming-Hui   Juang, Zhen-Yu   Chang, Yung-Huang   Chang, Wen-Hao   Iwasa, Yoshihiro   Takenobu, Taishi   Li, Lain-Jong  

    The monolayer transition metal dichalcogenides have recently attracted much attention owing to their potential in valleytronics, flexible and low-power electronics, and optoelectronic devices. Recent reports have demonstrated the growth of large-size two-dimensional MoS2 layers by the sulfurization of molybdenum oxides. However, the growth of a transition metal selenide monolayer has still been a challenge. Here we report that the introduction of hydrogen in the reaction chamber helps to activate the selenization of WO3, where large size WSe2 monolayer flakes or thin films can be successfully grown. The top-gated field-effect transistors based on WSe2 monolayers using ionic gels as the dielectrics exhibit ambipolar characteristics, where the hole and electron mobility values are up to 90 and 7 cm(2)/Vs, respectively. These films can be transferred onto arbitrary substrates, which may inspire research efforts to explore their properties and applications. The resistor-loaded inverter based on a WSe2 film, with a gain of similar to 13, further demonstrates its applicability for logic-circuit integrations.
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