Kazumoto Miwa

A comparative study of organic single-crystal transistors gated with various ionic-liquid electrolytes

We report on a comparative study of rubrene single-crystal field-effect transistors with various ionic-liquid electrolytes used for gate insulators. A systematic correlation is found that mobility of the field-effect transistors increases with decreasing electrostatic capacitance of the electric double layers, as the result of highly reproducible comparisons among tens of samples with the variation of anions in the purified ionic liquids. By optimizing the gating ionic liquid, the highest mobility of the electrolyte-gated organic transistors elevated up to 9.5 cm2/V s, which is only a fraction of the value of intrinsic material property, demonstrating an excellent field-effect switching operation.

V‐Shaped Organic Semiconductors With Solution Processability, High Mobility, and High Thermal Durability

V-shaped organic semiconductors have been designed and synthesized via a large-scale applicable synthetic route. Solution-crystallized films based on such molecules have demonstrated high-performance transistor properties with maximum mobilities of up to 9.5 cm(2) V(-1) s(-1) as well as pronounced thermal durability of up to 150 °C inherent in the V-shaped cores.

High-Speed Flexible Organic Field-Effect Transistors with a 3D Structure

Organic semiconductor materials offer fl exible platforms for charge current due to their weak van der Waals interaction between π -conjugated organic molecules such that the transport of electrons or holes is activated with modest mobility. Making use of such material properties, technologies of fl exible organic fi eld-effect transistors (OFETs) are in the process of developing attractive devices with fl exible, stretchable, light-weight, low-cost, and low-power-consumption switching components, such as active-matrix elements for plastic displays, [ 1–4 ] sensor arrays, [ 5 , 6 ]

Electric-field control of magnetic moment in Pd

Several magnetic properties have recently become tunable with an applied electric field. Particularly, electrically controlled magnetic phase transitions and/or magnetic moments have attracted attention because they are the most fundamental parameters in ferromagnetic materials. In this study, we showed that an electric field can be used to control the magnetic moment in films made of Pd, usually a non-magnetic element. Pd ultra-thin films were deposited on ferromagnetic Pt/Co layers. In the Pd layer, a ferromagnetically ordered magnetic moment was induced by the ferromagnetic proximity effect. By applying an electric field to the ferromagnetic surface of this Pd layer, a clear change was observed in the magnetic moment, which was measured directly using a superconducting quantum interference device magnetometer. The results indicate that magnetic moments extrinsically induced in non-magnetic elements by the proximity effect, as well as an intrinsically induced magnetic moments in ferromagnetic elements, as reported previously, are electrically tunable. The results of this study suggest a new avenue for answering the fundamental question of “can an electric field make naturally non-magnetic materials ferromagnetic?”

Control of magnetic anisotropy in Pt/Co system using ionic liquid gating

The magnetic anisotropy of the Pt/Co system under ionic liquid gating was studied. A comparison of results obtained using samples under the gating and those subjected to mild oxidization by oxygen plasma ashing suggested that the anodic oxidization of the Co layer could be one of the causes of the large modulation observed in the magnetic anisotropy. However, the charge accumulation effect was probably dominant when the Co layer was on the cathode side. The experiments presented here are expected to aid in elucidating the mechanism by which electric fields affect magnetism.

Electric field modulation of magnetic anisotropy in perpendicularly magnetized Pt/Co structure with a Pd top layer

We investigated the electric field effect on magnetic anisotropy in a perpendicularly magnetized Pt/Co system with a top ultrathin layer of nonmagnetic Pd. By applying an electric field to the surface of the ferromagnetic Pd layer, we observed a clear modulation of the perpendicular magnetic anisotropy of the system. This result shows that the magnetic anisotropy can be modulated by an electric field even when nonmagnetic Pd is inserted at the interface formed by the magnetic layer and insulator. The electric field effect of the proximity-induced moment in Pd might contribute to the anisotropy modulation.

Peculiar temperature dependence of electric-field effect on magnetic anisotropy in Co/Pd/MgO system

We report on the temperature dependence of the magnetic anisotropy in Co/Pd/MgO system, in which magnetic moment in Pd is induced by the magnetic proximity effect. We demonstrate that the magnetic anisotropy is modulated by applying an electric field to the Pd surface. At temperatures below 100 K, we find the nonlinear electric-field dependence of the anisotropy with the sign reversal. We obtain a huge anisotropy modulation efficiency of ∼1600 fJ/V m at 10 K.

High-density carrier-accumulated and electrically stable oxide thin-film transistors from ion-gel gate dielectric

The use of indium–gallium–zinc oxide (IGZO) has paved the way for high-resolution uniform displays or integrated circuits with transparent and flexible devices. However, achieving highly reliable devices that use IGZO for low-temperature processes remains a technological challenge. We propose the use of IGZO thin-film transistors (TFTs) with an ionic-liquid gate dielectric in order to achieve high-density carrier-accumulated IGZO TFTs with high reliability, and we discuss a distinctive mechanism for the degradation of this organic–inorganic hybrid device under long-term electrical stress. Our results demonstrated that an ionic liquid or gel gate dielectric provides highly reliable and low-voltage operation with IGZO TFTs. Furthermore, high-density carrier accumulation helps improve the TFT characteristics and reliability, and it is highly relevant to the electronic phase control of oxide materials and the degradation mechanism for organic–inorganic hybrid devices.

Enhanced thermopower in ZnO two-dimensional electron gas

Significance Thermoelectric effects, the conversion of heat into electrical energy, reflect the profile of the electronic density of states. Therefore, the control of the dimensionality is a key strategy to obtaining higher thermoelectric power. The direct demonstration of the enhancement, however, has been elusive, partly because of the difficulties in preparation of low-dimensional materials with simple band structures. Here, we investigated the Seebeck effect of 2D electrons using the electric field effect. The ion-gating experiment and band calculation demonstrated that the field-induced 2D electrons on an oxide semiconductor ZnO exhibit a higher thermoelectric effect than bulk ZnO. Furthermore, our approach of electrically controlling thermopower provides an effective route to exploit the optimum conditions of thermoelectric properties. Control of dimensionality has proven to be an effective way to manipulate the electronic properties of materials, thereby enabling exotic quantum phenomena, such as superconductivity, quantum Hall effects, and valleytronic effects. Another example is thermoelectricity, which has been theoretically proposed to be favorably controllable by reducing the dimensionality. Here, we verify this proposal by performing a systematic study on a gate-tuned 2D electron gas (2DEG) system formed at the surface of ZnO. Combining state-of-the-art electric-double-layer transistor experiments and realistic tight-binding calculations, we show that, for a wide range of carrier densities, the 2DEG channel comprises a single subband, and its effective thickness can be reduced to ∼ 1 nm at sufficiently high gate biases. We also demonstrate that the thermoelectric performance of the 2DEG region is significantly higher than that of bulk ZnO. Our approach opens up a route to exploit the peculiar behavior of 2DEG electronic states and realize thermoelectric devices with advanced functionalities.

Charge modulation infrared spectroscopy of rubrene single-crystal field-effect transistors

Polarized absorption spectra of hole carriers in rubrene single crystal field-effect transistors were measured in the infrared region (725–8000 cm−1) by charge modulation spectroscopy. The absorptions, including the superimposed oscillatory components due to multiple reflections within thin crystals, monotonically increased with decreasing frequency. The spectra and their polarization dependences were well reproduced by the analysis based on the Drude model, in which the absorptions due to holes in rubrene and electrons in the gate electrodes (silicon), and multiple reflections were fully considered. The results support the band transport of hole carriers in rubrene.