Hidekazu Shimotani

Electric-field-induced superconductivity in an insulator

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.

Liquid-gated interface superconductivity on an atomically flat film.

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.

Electrically induced ferromagnetism at room temperature in cobalt-doped titanium dioxide.

The magnetic properties of a magnetic insulator can be controlled by an electric field at room temperature. 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)O2, by means of electric double-layer gating with high-density electron accumulation (>1014 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.

Discovery of superconductivity in KTaO3 by electrostatic carrier doping

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.

Tunable Carbon Nanotube Thin‐Film Transistors Produced Exclusively via Inkjet Printing

Inkjet printing in electronics production has attracted considerable attention for a wide-range of applications because it is an environmentally friendly and low-cost technique. [ 1–5 ] In addition to excellent charge-transport properties, materials for inkjet printing must meet other key requirements, such as high chemical stability, low-temperature processability, low hysteresis, and low-voltage operation. In the past, materials satisfying these criteria have not been available. Here, we demonstrate low-cost green manufacturing via precisely controlled inkjet printing of singlewalled carbon nanotube (SWCNT) fi lms. This transistor exceeds the performance of conventional organic transistors (a mobility of 1.6 to 4.2 cm 2 V − 1 s − 1 and an on/off ratio of 4 to 5 digits) and is fabricated at moderate temperatures (80 ° C). We also demonstrate the production of exclusively inkjetprinted SWCNT transistors with printable ionic-liquid gate dielectrics. Printable technology has the potential to drastically reduce ecological impact, energy consumption during manufacturing, and wasted materials by controlling the quantity and location of ink deposition. Inkjet technology is exceptionally promising because patterns can be generated without any material waste, leading to drastic reductions in production costs and in environmental impact. Materials for printable electronics must satisfy several requirements, such as high transport properties, chemical stability, and low-temperature processability. Research in this area has been focused largely on organic semiconductors [ 1–5 ] because carrier mobilities comparable to those of amorphous silicon ( ≤ 1 cm 2 V − 1 s − 1 ) are needed to create printable electronics. Although highly crystalline organic

Accessing the transport properties of graphene and its multilayers at high carrier density

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 1014 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.

High-sensitivity photodetectors based on multilayer GaTe flakes

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.

Insulator-to-metal transition in ZnO by electric double layer gating

The authors report high-density carrier accumulation and a gate-induced insulator-to-metal transition in ZnO single-crystalline thin-film field effect transistors by adopting electric double layers as gate dielectrics. Hall effect measurements showed that a sheet carrier density of 4.2×1013cm−2 was achieved. The highest sheet conductance at room temperature was ∼1mS, which was sufficient to maintain the metallic state down to 10K. These results strongly suggest the versatility of electric double layer gating for various materials.

Electrolyte-gated charge accumulation in organic single crystals

Comparative studies of electrolyte-gated and SiO2-gated field-effect transistors have been carried out on rubrene single crystals by experimentally estimating their accumulated charges. The capacitance of the electrolyte gate at 1mHz was 15μF∕cm2, which is more than two orders of magnitude larger than that of the 100-nm-thick SiO2 gate dielectric. The maximum carrier density in the electrolyte gate was 0.33hole∕molecule, which is considerably larger than that in the SiO2 gate. Furthermore, the transfer characteristics of the electrolyte-gate field-effect transistor showed reversible-peak behavior at an accumulated carrier density of 0.23hole∕molecule.

Direct comparison of field-effect and electrochemical doping in regioregular poly(3-hexylthiophene)

We have measured carrier mobility of regioregular poly(3-hexylthiophene) films by both field-effect and electrochemical doping on identical devices, which allowed us a direct comparison between the two doping processes. The carrier mobility of electrochemical doping at low doping levels was lower than that of field-effect doping by two orders of magnitudes, while that of electrochemical doping steeply increased with doping levels, reaching comparable or higher values than that of field-effect doping. These results are attributable to carrier trapping by the Coulomb potentials of dopant anions at low doping levels, demonstrating a significant difference between field-effect and chemical doping.