Naoko Kawasaki

Superconductivity in alkali-metal-doped picene

Efforts to identify and develop new superconducting materials continue apace, motivated by both fundamental science and the prospects for application. For example, several new superconducting material systems have been developed in the recent past, including calcium-intercalated graphite compounds, boron-doped diamond and—most prominently—iron arsenides such as LaO1–xFxFeAs (ref. 3). In the case of organic superconductors, however, no new material system with a high superconducting transition temperature (Tc) has been discovered in the past decade. Here we report that intercalating an alkali metal into picene, a wide-bandgap semiconducting solid hydrocarbon, produces metallic behaviour and superconductivity. Solid potassium-intercalated picene (Kxpicene) shows Tc values of 7 K and 18 K, depending on the metal content. The drop of magnetization in Kxpicene solids at the transition temperature is sharp (<2 K), similar to the behaviour of Ca-intercalated graphite. The Tc of 18 K is comparable to that of K-intercalated C60 (ref. 4). This discovery of superconductivity in Kxpicene shows that organic hydrocarbons are promising candidates for improved Tc values.

Air-assisted High-performance Field-effect Transistor with Thin Films of Picene

A field-effect transistor (FET) with thin films of picene has been fabricated on SiO2 gate dielectric. The FET showed p-channel enhancement-type FET characteristics with the field-effect mobility, mu, of 1.1 cm2 V-1 s-1 and the on-off ratio of >10(5). This excellent device performance was realized under atmospheric conditions. The mu increased with an increase in temperature, and the FET performance was improved by exposure to air or O2 for a long time. This result implies that this device is an air (O2)-assisted FET. The FET characteristics are discussed on the basis of structural topography and the energy diagram of picene thin films.

Trap states and transport characteristics in picene thin film field-effect transistor

Transport characteristics and trap states are investigated in picene thin film field-effect transistor under O2 atmosphere on the basis of multiple shallow trap and release (MTR) model. The channel transport is dominated by MTR below 300 K. It has been clarified on the basis of MTR model that the O2-exposure induces a drastic reduction in shallow trap density to increase both the field-effect mobility μ and on-off ratio. We also found that the O2-exposure never caused an increase in hole carrier density. Actually, a very high μ value of 3.2 cm2 V−1 s−1 is realized under 500 Torr of O2.

Flexible picene thin film field-effect transistors with parylene gate dielectric and their physical properties

Flexible picene thin film field-effect transistors (FETs) have been fabricated with parylene gate dielectric on polyethylene terephthalate substrates. The picene thin film FETs show p-channel output/transfer characteristics and the field-effect mobility μ reaches ∼1 cm2 V−1 s−1 in vacuum. The FET shows a clear O2 gas sensing effect and negligible hysteresis in the transfer curves, indicating a possible application of the transistor as O2 selective gas sensor. Furthermore, it has been found that the parylene gate dielectric can eliminate a reduction in on-state drain current caused by continuous bias-voltage application which is observed if a SiO2 gate dielectric is used.

Low voltage operation in picene thin film field-effect transistor and its physical characteristics

Low voltage operation of picene thin film field-effect transistor (FET) has been realized with 40 nm thick SiO2 gate dielectrics coated by two polymers, Cytop™ and polystyrene. The picene FETs operated in low absolute gate voltage |VG| below 15 V for Cytop™ coated SiO2 and 30 V for polystyrene coated SiO2 gate dielectrics, and they showed a significant O2 gas sensing effect down to ∼10 ppm. Photoemission spectrum clarified that O2 molecules penetrate into the thin films at O2/picene mole ratio of 1: 1. X-ray diffraction pattern of picene thin films showed highly oriented growth on the polymer-coated SiO2.

Hole-injection barrier in pentacene field-effect transistor with Au electrodes modified by C16H33SH

Field-effect transistor with thin films of pentacene has been fabricated with Au electrodes modified by 1-hexadecanethiol (C16H33SH), and the hole-injection barriers have been determined from the temperature dependence of output properties on the basis of the thermionic emission model for double Schottky barriers. The large tunneling barriers are formed by the insulating C16H33SH at the interfaces between the Au electrodes and pentacene thin films.

Transport properties of field-effect transistor with Langmuir-Blodgett films of C60 dendrimer and estimation of impurity levels

Field-effect transistor (FET) device has been fabricated with Langmuir-Blodgett films of C60 dendrimer. The device showed n-channel normally off characteristics with the field-effect mobility of 2.7×10−3cm2V−1s−1 at 300K, whose value is twice as high as that (1.4×10−3cm2V−1s−1) for the FET with spin-coated films of C60 dendrimer. This originates from the formation of ordered π-conduction network of C60 moieties. From the temperature dependence of field-effect mobility, a structural phase transition has been observed at around 300K. Furthermore, the density of states for impurity levels was estimated in the Langmuir-Blodgett films.

An investigation of correlation between transport characteristics and trap states in n-channel organic field-effect transistors

Transport characteristics in n-channel organic field-effect transistors are discussed on the basis of density of states (DOS) for trap states determined with multiple trap and release model. First the trap-free intrinsic mobilities, the activation energies, and total effective DOS for conduction band are determined with the effective field-effect mobility versus temperature plots and total DOS of trap states. Second the general formula for subthreshold swing S applicable to organic field-effect transistors is derived and the surface potentials are determined from the S determined from the transfer curves and the DOS for the trap states according to the general formula.