Search

To develop high-performance organic light-emitting organic field-effect transistors (LE-OFETs), a fundamental problem in organic semiconductors is to compromise light luminescent efficiency for high carrier mobility and vice versa. Therefore, LE-OFETs can avoid this problem by separating the light-emission and carrier-transport functions. Here, a bilayer LE-OFET composed of a tetracene crystal as a carrier transporter (bottom crystal) and a 4-(dicyanomethylene)-2-methyl-6-( p-dimethylaminostyryl)-4 H-pyran (DCM1)-doped tetracene crystal as a light emitter (top crystal) was fabricated. Red light-emission color, which is distinct from the green emission color of tetracene, was detected in the top crystal. Light emission from the top layer was prohibited when an insulating thin film was inserted between the two crystals. These observations indicate that excitons are formed in the bottom crystal and transferred to the top crystal, emitting reddish light. Bilayer LE-OFETs have the advantage of providing both high current density and a bright emission for high-performance light-emitting FETs.

Organic semiconductors have attracted much attention for low-cost, flexible and human-friendly optoelectronics. However, achieving high electron-injection efficiency is difficult from air-stable electrodes and cannot be equivalent to that of holes. Here, we present a novel concept of electrode composed of a bilayer of tetratetracontane (TTC) and polycrystalline organic semiconductors (pc-OSC) covered by a metal layer. Field-effect transistors of single-crystal organic semiconductors with the new electrodes of M/pc-OSC/TTC (M: Ca or Au) show both highly efficient electron and hole injection. Contact resistance for electron injection from Au/pc-OSC/TTC and hole injection from Ca/pc-OSC/TTC are comparable to those for electron injection from Ca and hole injection from Au, respectively. Furthermore, the highest field-effect mobilities of holes (22 cm 2  V -1  s -1 ) and electrons (5.0 cm 2  V -1  s -1 ) are observed in rubrene among field-effect transistors with electrodes so far proposed by employing Ca/pc-OSC/TTC and Au/pc-OSC/TTC electrodes for electron and hole injection, respectively.One of technological challenges building organic electronics is efficient injection of electrons at metal-semiconductor interfaces compared to that of holes. The authors show an air-stable electrode design with induced gap states, which support Fermi level pinning and thus ambipolar carrier injection.

Nanoporous graphene- based electric-double-layer transistors (EDLTs) are successfully fabricated. Transport measurements of the EDLTs demonstrate that the ambipolar electronic states of massless Dirac fermions with a high carrier mobility are well preserved in 3D nanoporous graphene along with anomalous nonlinear Hall resistance and exceptional transistor on/off ratio. This study may open a new avenue for device applications of graphene., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Two new regiospecific biphenyl end-capped bithiazole co-oligomers, BP2Tz(in) and BP2Tz(out), have been synthesized for application in thin film field effect transistors (TFTs). BP2Tz(in) with a 2,2'-bithiazole central unit exhibits a field effect hole mobility as high as 3.5 cm(2) V(-1) s(-1). Green light emission is demonstrated for highly balanced ambipoar TFTs based on both BP2Tz(in) and BP2Tz(out).

Three new 2-positional pyrene end-capped oligothiophene co-oligomers, BPynT (n = 1, 2, 3), have been synthesized for application in organic field effect transistors (OFETs). BPy2T showed the highest hole mobility of 3.3 cm(2) V(-1) s(-1) in a single crystal OFET and a good photoluminescence efficiency of 32% in the crystalline state. A green light emission was observed for the OFET based on a BPy2T single crystal.

High quality bulk samples of anthracene (AN) doped with potassium (K) in 1 : 1 and 2 : 1 stoichiometries were successfully prepared by a method involving a room temperature solid-state mechanical diffusion process prior to intercalation reactions during heat treatment, and their physical properties were studied using both magnetic and optical measurements. The transfer of almost one electron from K to AN in K1(AN) was confirmed by SQUID and ESR measurements. A pronounced magnetic hump centered at 150 K associated with antiferromagnetic interactions was observed, which can most likely be interpreted in terms of on-site Coulomb repulsions of the Mott insulating states. Optical spectra of K1(AN) clearly showed the insulating states, as well as the electron occupation of the LUMO-derived band of AN. Our results demonstrated tuning of the ground state of a typical bulk hydrocarbon by alkali metal intercalation.

Optoelectronic devices based on layered materials such as graphene have resulted in significant interest due to their unique properties and potential technological applications. The electric and optoelectronic properties of nano GaTe flakes as layered materials are described in this article. The transistor fabricated from multilayer GaTe shows a p-type action with a hole mobility of about 0.2 cm(2) V(-1) s(-1). The gate transistor exhibits a high photoresponsivity of 10(4) A/W, which is greatly better than that of graphene, MoS2, and other layered compounds. Meanwhile, the response speed of 6 ms is also very fast. Both the high photoresponsivity and the fast response time described in the present study strongly suggest that multilayer GaTe is a promising candidate for future optoelectronic and photosensitive device applications.

Electrically manipulating electron spins based on Rashba spin-orbit coupling (SOC) is a key pathway for applications of spintronics and spin-based quantum computation. Two-dimensional electron systems (2DESs) offer a particularly important SOC platform, where spin polarization can be tuned with an electric field perpendicular to the 2DES. Here, by measuring the tunable circular photogalvanic effect (CPGE), we present a room-temperature electric-field-modulated spin splitting of surface electrons on InN epitaxial thin films that is a good candidate to realize spin injection. The surface band bending and resulting CPGE current are successfully modulated by ionic liquid gating within an electric double-layer transistor configuration. The clear gate voltage dependence of CPGE current indicates that the spin splitting of the surface electron accumulation layer is effectively tuned, providing a way to modulate the injected spin polarization in potential spintronic 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 © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Taking advantage of ultrahigh electric field generated in electric-double-layer transistors (EDLTs), we investigated spin-orbit interaction (SOI) and its modulation in epitaxial trilayer graphene. It was found in magnetotransport that the dephasing length L(φ) and spin relaxation length L(so) of carriers can be effectively modulated with gate bias. As a direct result, SOI-induced weak antilocalization (WAL), together with a crossover from WAL to weak localization (WL), was observed at near-zero magnetic field. Interestingly, among existing localization models, only the Iordanskii-Lyanda-Geller-Pikus theory can successfully reproduce the obtained magnetoconductance well, serving as evidence for gate tuning of the weak but distinct SOI in graphene. Realization of SOI and its large tunability in the trilayer graphene EDLTs provides us with a possibility to electrically manipulate spin precession in graphene systems without ferromagnetics.

A prototype Mott transistor, the electric double layer transistor with a strained CaMnO(3) thin film, is fabricated. As predicted by the strain phase diagram of electron-doped manganite films, the device with the compressively strained CaMnO(3) exhibits an immense conductivity modulation upon applying a tiny gate voltage of 2 V., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We present a comparative study of high carrier density transport in mono-, bi-, and trilayer graphene using electric double-layer transistors to continuously tune the carrier density up to values exceeding 10(14) cm(-2). Whereas in monolayer the conductivity saturates, in bi- and trilayer filling of the higher-energy bands is observed to cause a nonmonotonic behavior of the conductivity and a large increase in the quantum capacitance. These systematic trends not only show how the intrinsic high-density transport properties of graphene can be accessed by field effect, but also demonstrate the robustness of ion-gated graphene, which is crucial for possible future applications.

Using ionic-liquid (IL) gating in electric-double-layer transistors (EDLTs), we investigate field-effect electrical transport properties of ultrathin epitaxial films of a topological insulator (TI), Bi(2)Te(3). Because of their extreme thinness, the Bi(2)Te(3) films show a band gap opening and resulting semiconducting transport properties. Near room temperature, an obvious ambipolar transistor operation with an ON-OFF ratio close to 10(3) was observed in the transfer characteristics of liquid-gated EDLTs and further confirmed by a sign change of the Hall coefficients. Modulation of the electronic states and a phase transition from a semiconducting conduction (dR(xx)/dT < 0) to a metallic transport (dR(xx)/dT > 0) were observed in the temperature-dependent resistance of the ultrathin Bi(2)Te(3) channel, demonstrating that the liquid gating is an effective way to modulate the electronic states of TIs.

Analysis of IR and Raman spectra of Ar@C(60) and Kr@C(60) shows that the incorporation of noble gas atoms causes a blue shift of low energy vibrations, which have radial character, and a red shift of higher energy ones which have a tangential character movement. The mechanism of these phenomena is explained on the basis of ab initio numerical experiments with DFT and MP2 procedures. Methodological discussions are advanced, altogether with a scheme for the estimation of the van der Waals interaction between fullerene and noble gas, based on the frequency shifts., (This journal is © the Owner Societies 2011)

The electric field effect in ferromagnetic semiconductors enables switching of the magnetization, which is a key technology for spintronic applications. We demonstrated electric field-induced ferromagnetism at room temperature in a magnetic oxide semiconductor, (Ti,Co)O(2), by means of electric double-layer gating with high-density electron accumulation (>10(14) per square centimeter). By applying a gate voltage of a few volts, a low-carrier paramagnetic state was transformed into a high-carrier ferromagnetic state, thereby revealing the considerable role of electron carriers in high-temperature ferromagnetism and demonstrating a route to room-temperature semiconductor spintronics.

Superconductivity at interfaces has been investigated since the first demonstration of electric-field-tunable superconductivity in ultrathin films in 1960(1). So far, research on interface superconductivity has focused on materials that are known to be superconductors in bulk. Here, we show that electrostatic carrier doping can induce superconductivity in KTaO(3), a material in which superconductivity has not been observed before. Taking advantage of the large capacitance of the self-organized electric double layer that forms at the interface between an ionic liquid and KTaO(3) (ref. 12), we achieve a charge carrier density that is an order of magnitude larger than the density that can be achieved with conventional chemical doping. Superconductivity emerges in KTaO(3) at 50 mK for two-dimensional carrier densities in the range 2.3 × 10(14) to 3.7 × 10(14) cm(-2). The present result clearly shows that electrostatic carrier doping can lead to new states of matter at nanoscale interfaces.

The electric-double-layer (EDL) formed at liquid/solid interfaces provides a broad and interdisciplinary attraction in terms of electrochemistry, photochemistry, catalysts, energy storage, and electronics because of the large interfacial capacitance coupling and its ability for high-density charge accumulation. Much effort has recently been devoted to the fundamental understanding and practical applications of such highly charged EDL interfaces. However, the intrinsic nature of the EDL charging, whether it is electrostatics or electrochemistry, and how to distinguish them are far from clear. Here, by combining electrical transport measurements with electrochemical impedance spectroscopy (EIS), we studied the charging mechanisms of highly charged EDL interfaces between an ionic liquid and oxide semiconductor, ZnO. The direct measure for mobile carriers from the Hall effect agreed well with that from the capacitance-voltage integration at 1 Hz, implying that the pseudocapacitance does not contribute to carrier transport at EDL interfaces. The temperature-frequency mapping of EIS was further demonstrated as a "phase diagram" to distinguish the electrostatic or electrochemical nature of such highly charged EDL interfaces with densities of up to 8 × 10(14) cm(-2), providing a guideline for electric-field-induced electronic phenomena and a simple method for distinguishing electrostatic and electrochemical charging in EDLTs not only based on a specific oxide semiconductor, ZnO, but also commonly applicable to all types of EDL interfaces with extremely high-density carrier accumulation.

The electric-double-layer (EDL) formed at liquid/solid interfaces provides a broad and interdisciplinary attraction in terms of electrochemistry, photochemistry, catalysts, energy storage, and electronics. Especially in recent years, much effort has been devoted to the fundamental understanding and practical applications of transistor configurations with EDLs because of their ability for high-density charge accumulation. However, to exploit additional new functionalities of such an emerging interface is not only of great importance but also a huge challenge. Here, we demonstrate that, by introducing protic ionic liquid (PIL) as the gate dielectric for ZnO EDL transistors (EDLTs), small and chemically active ions, such as protons and hydroxyls, can serve as an adsorption medium to extend the interfacial functionalities of EDLTs. By selectively driving the H(+) or OH(-) groups onto ZnO channel surfaces with an electric field, the charged adsorbates interact with surface atoms in different adsorption mechanisms, showing remarkable variations in electron transport and providing a possibility for the recognition of surface polarity. Most significantly, the large hysteresis in the transfer characteristics of PIL-EDLTs makes the device available and promising for nonvolatile proton memory devices via surface hydrogenation and dehydrogenation processes. Such a finding provides us with new opportunities to understand liquid/solid heterogeneous interface phenomena and to extend the practical functions of EDLs through controllable interfacial interaction.

Liquid/solid interfaces are attracting growing interest not only for applications in catalytic activities and energy storage, but also for their new electronic functions in electric double-layer transistors (EDLTs) exemplified by high-performance organic electronics, field-induced electronic phase transitions, as well as superconductivity in SrTiO(3) (ref. 12). Broadening EDLTs to induce superconductivity within other materials is highly demanded for enriching the materials science of superconductors. However, it is severely hampered by inadequate choice of materials and processing techniques. Here we introduce an easy method using ionic liquids as gate dielectrics, mechanical micro-cleavage techniques for surface preparation, and report the observation of field-induced superconductivity showing a transition temperature T(c)=15.2 K on an atomically flat film of layered nitride compound, ZrNCl. The present result reveals that the EDLT is an extremely versatile tool to induce electronic phase transitions by electrostatic charge accumulation and provides new routes in the search for superconductors beyond those synthesized by traditional chemical methods.

Electric field control of charge carrier density has long been a key technology to tune the physical properties of condensed matter, exploring the modern semiconductor industry. One of the big challenges is to increase the maximum attainable carrier density so that we can induce superconductivity in field-effect-transistor geometry. However, such experiments have so far been limited to modulation of the critical temperature in originally conducting samples because of dielectric breakdown. Here we report electric-field-induced superconductivity in an insulator by using an electric-double-layer gating in an organic electrolyte. Sheet carrier density was enhanced from zero to 10(14) cm(-2) by applying a gate voltage of up to 3.5 V to a pristine SrTiO(3) single-crystal channel. A two-dimensional superconducting state emerged below a critical temperature of 0.4 K, comparable to the maximum value for chemically doped bulk crystals, indicating this method as promising for searching for unprecedented superconducting states.

The synthesis of a mg amount of pure argon containing fullerene allowed the synthesis of the first endohedral superconductors with critical temperatures lower than expected, an indication of the strong influence of the argon atom on the C60 cage.

Organic thin-film transistors are attracting a great deal of attention due to the relatively high field-effect mobility in several organic materials. In these organic semiconductors, however, researchers have not established a reliable method of doping at a very low density level, although this has been crucial for the technological development of inorganic semiconductors. In the field-effect device structures, the conduction channel exists at the interface between organic thin films and SiO(2) gate insulators. Here, we discuss a new technique that enables us to control the charge density in the channel by using organosilane self-assembled monolayers (SAMs) on SiO(2) gate insulators. SAMs with fluorine and amino groups have been shown to accumulate holes and electrons, respectively, in the transistor channel: these properties are understood in terms of the effects of electric dipoles of the SAMs molecules, and weak charge transfer between organic films and SAMs.

The configuration of La ions of La(2)@C(80) in the [80]fullerene cage was investigated by use of quantum chemical calculations. We found that the D(3)(d)() configuration is the global minimum in total energy, being more stable by 1.9 kcal/mol than the D(2)(h)() configuration, which has been considered to be the most stable. The potential energy surface calculation clarified that La ions travel between 10 equivalent D(3)(d)() positions through D(2)(h)() positions and consequently form pentagonal dodecahedral trajectory, which is in good agreement with the previous synchrotron radiation structural study. The experimental and theoretical investigation of the Raman spectrum revealed that the symmetry of molecular vibration is dramatically reduced simply by encapsulation of two La ions, and resulting vibrational modes were successfully assigned. The Raman peak at 163 cm(-)(1) was interpreted as the in-phase synchronously coupled mode of the [80]fullerene cage elongation and the La-La stretching, rather than a conventional and naive assignment as a metal-to-cage vibration mode.

We describe the HPLC separation and identification of N@C60 and N2@C60. These species were observed after eleven sequential HPLC separations. Their retention times are in the same range as those of the other noninteractive endohedral species of C60, such as noble gas endohedral C60. The separation factors of these endohedrals were evaluated by using a mixture of hexane/toluene as eluent. We note that this is the first evidence for the N2@C60 molecule existing in the form of endohedral C60 complex.

A sample of C60 containing ca. 9% Kr@C60 has been used to form crystalline (0.09Kr@C60/0.91C60).(NiII(OEP)).2C6H6 whose X-ray crystal structure reveals that the Kr atom is centered within the carbon cage and does not produce a detectable change in the size of the fullerene.

The isolation of a new fullerene dimer, C(122)H(4), and its structural characterization by (13)C NMR and (1)H NMR spectroscopy and by UV/vis and IR spectroscopy are reported. The structure of this dimer consists of two fullerene cages, which are directly connected through two C-C bonds and two methylene bridges. Consequently, adjacent hexagonal faces of the two fullerene cages are arranged in a face to face manner. Molecular orbital calculations indicate that the proximity of the fullerene cages results in significant through space overlap in both the HOMO and LUMO. As a consequence of this overlap, the electrochemistry of the dimer shows electronic communication with stepwise reduction of each cage.