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.

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.

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

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.

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.

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

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.

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.

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.

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.

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.

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.

The fabrication technology for emerging printed and flexible electronics is currently suboptimal. Although inkjet technology is a powerful tool for producing single-walled carbon nanotube (SWCNT) film transistors, inkjet printing at moderate temperatures (< 100 degrees C) of complementary logic gates that are suitable for plastic substrates has yet to be realized. In this study, we describe how we surmounted these difficulties by combining SWCNTs and room-temperature-processable polymer dopant. We have successfully inkjet-printed SWCNT complementary inverters at 90 degrees C, thereby improving the performance limit of SWCNT electronics. (C) 2011 The Japan Society of Applied Physics

We report a significant lowering of the Schottky barrier height (SBH) in nickel (Ni)/rubrene devices by the insertion of a tetrafluoro-tetracyanoquinodimethane (F(4)TCNQ), molybdenum trioxide (MoO3), or tetracyanoquinodimethane (TCNQ) layers at the device junction. Devices with F(4)TCNQ and MoO3 layers show ohmic-like characteristics, whereas the device with the TCNQ layer shows a low SBH (0.26 eV). The SBH of Ni/rubrene device without the acceptor layers is 0.56 eV. We explain the SBH lowering by the electron accepting properties of the thin layers. Such layers can be used to fabricate molecular spintronics devices with ohmic contacts for effective electrical spin injection. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4745778]