Akira Kawakami

Preparation of carbazole polymer thin films by electron-assisted deposition of 3-(n-carbazolyl)propyl acrylate

Polymeric thin films having carbazole pendant groups were prepared by physical vapor deposition of 3-(N-carbazolyl)propyl acrylate (CPA). The conventional vacuum evaporation of CPA formed thin films that consist mainly of small molecules. The evaporated films had needlelike crystallites in the as-deposited state, and underwent severe coalescence upon annealing at 125°C. The deposition polymerization of CPA was achieved by initiating polymerization by electron irradiation onto the monomer vapor by ionization-assisted deposition (IAD). Infrared analysis showed that the polymerization reaction was enhanced by increasing the electron current used for irradiation. The deposition polymerization was effective in improving the uniformity and thermal stability of the film. Photoluminescence analysis suggested that excimer formation can be suppressed by the deposition-polymerization of CPA. The deposited film was found to function as the hole transport layer of the organic light-emitting diode.

Josephson effects at 77 K in grain boundary bridge made of thick films

Using the screen printing method, YBaCuO high Tc superconducting films were fabricated. The thickness of the films was ≃30 µm and maximum grain size was ≃70 µm. Tczero=90.7 K and Jc=20 A/cm2 at 77 K were obtained. With a razor edge, the films were cut and formed to grain boundary bridges having a few grain boundaries in the bridge region. The devices showed clear Shapiro steps and SQUID patterns at 77 K.

Liquefaction mechanism of Wandoan coal using tritium and 14C tracer methods: 1. Liquefaction in 3H and 14C labelled solvent

Abstract Liquefaction of Wandoan coal using a 3 H labelled tetralin solvent which contains a small amount of 14 C labelled naphthalene has been studied at 400 °C under an initial hydrogen pressure of 5.9 MPa, in the presence or absence of Niue5f8Moue5f8Al 2 O 3 catalyst. The amounts of 3 H and 14 C transferred from the solvent to the products were measured as liquefaction progressed. The reaction pathways in the presence and absence of the catalyst were discussed and their reaction rate constants were calculated. According to the mass balances of hydrogen and 3 H, in the absence of catalyst, tetralin provided coal with hydrogen atoms, and the degree of hydrogen exchange between coal and solvent was small. The catalyst decreased the hydrogen addition from solvent to coal and increased that from gas to coal.

Interface Control by Surface-Initiated Deposition Polymerization and Its Application to Organic Light-Emitting Devices

Surface-initiated deposition polymerization was applied to the preparation of a hole transport layer (HTL) on an indium–tin oxide (ITO) electrode. A silane-based self-assembled monolayer (SAM) that has an azo terminal group was prepared on an ITO surface on which a vinyl monomer of carbazole (CPA) or tetraphenyldiaminobiphenyl (vTPD) was deposited by physical vapor deposition. The polymerization of the HTL was achieved by electron-assisted deposition. UV irradiation, as well as conventional evaporation, was not capable of growing polymer films on the SAM. Organic light-emitting diodes (OLEDs) were prepared by depositing tris(8-quinolinolato) aluminum (Alq3) on the HTL. The surface-initiated deposition polymerization was effective in markedly increasing device current flow and reducing turn-on voltage. However, luminescence efficiency was not improved by this method owing to improper carrier balance under excessive hole injection. Nevertheless, surface-initiated deposition polymerization was effective in improving film morphology, stability, and hole injection characteristics.

Phosphorescent Organic Light Emitting Diode Using Vinyl Derivatives of Hole Transport and Dopant Materials

Organic light-emitting diodes (OLEDs) were prepared by a deposition polymerization method, in which vinyl monomers were evaporated and then annealed after deposition. N,N-diphenyl-N,N-bis(4-vinylphenyl)benzidine (DvTPD) was used both for a hole transport layer (HTL) and for the host material of an emissive layer (EML). bis(1-phenylisoquinoline)acetylacetonate iridium(III) [Ir(piq)2acac] and its styryl derivative bis(1-phenylisoquinolinate)-6-(4-vinylphenyl)acetylacetonate iridium(III) [Ir(piq)2acac-vb] were used as dopants. Ir(piq)2acac and Ir(piq)2acac-vb showed comparable emission characteristics. The device lifetime was improved by the thermal polymerization of DvTPD, and it was further improved by polymerizing both the host and dopant molecules to form a copolymer instead of a molecularly dispersed EML. In addition, the device with a codeposition-polymerized EML showed a small tendency of luminescence efficiency to decrease with increasing luminance. Deposition polymerization, achieved by evaporating vinyl monomers followed by thermal annealing, can be a promising method for constructing OLED structures.

Preparation of Carbazole Polymer Thin Films Chemically Bound to Substrate Surface by Physical Vapor Deposition Combined with Self-Assembled Monolayer

Vinyl polymer thin films having carbazole units were prepared by a new method combining physical vapor deposition and self-assembled monolayer (SAM) techniques. 3-(N-carbazolyl)propyl acrylate monomer was evaporated onto a gold substrate that had a VAZO 56 (DuPont) initiator attached as a SAM. The VAZO initiator was activated by irradiating ultraviolet light after depositing the monomer. Although the polymerization reaction can proceed even without the surface initiator, the SAM was effective in improving the surface smoothness, thermal stability, and film-substrate adhesion as a consequence of the formation of covalent chemical bonds between the film and the substrate. Thermal activation of the initiator was examined for the deposition polymerization of 9-H-carbazole-9-ethylmethacryrate. Substrate heating during the evaporation was not effective for accumulating thin films. On the other hand, performing postdeposition annealing on the film after deposition at room temperature resulted in the formation of a polymer thin film chemically bound to the substrate.

Liquid quenched superconductor Ba-Y-Cu-O with Tc,zero=88 K and AC Josephson effect at 77 K

A superconducting Ba-Y-Cu-O with Tc,zero=88 K was prepared by a liquid quenching method, in which the melt of nominal composition Ba2YCu3O7-δ was first quenched and then the sample was annealed at 900°C for 48 h in air. AC Josephson effect was observed at 77 K in point-contacts made of the present samples. This new method is applicable to the preparation of superconducting thin films, wires, tapes and superconduciting cavities.

Rf Power Dependence of AC Josephson Current in Point-Contacts of BaY(Tm)CuO Ceramics

Using BaYCuO and BaTmCuO ceramics, point-contact Josephson junctions were fabricated to observe ac Josephson effect. Clear Shapiro steps were observed at 4.2 K. The rf power dependence of Josephson currents were qualitatively fitted to Bessel functions.

AC Josephson effect in point-contacts of Ba-Y-Cu-O ceramics

Clear Shapiro-steps were observed in I-V curves of point-contact Josephson junctions fabricated by BaYCuO ceramics. The observed voltage step is just equal to applied frequency times (h/2e), where h and e are, respectively, the Planck constant and electron charge.

Current-Induced Spectrum Change of Phosphorescent Organic Light-Emitting Diode Constructed with Vinyl Compounds

Organic light-emitting diodes were prepared by coevaporating 4,4-N,N-dicarbazolylbiphenyl (CBP) host material, tris(2-phenylpyridinate) iridium(III) [Ir(ppy)3] dopant material, and their vinyl derivatives for the emissive layer. It was found that the emission spectrum changes with time during device operation when both the host and the dopant molecules have vinyl groups. This spectral change corresponded to the transition of emission from vinyl-modified Ir(ppy)3 to nonmodified Ir(ppy)3. The spectral transition was more prominent when an acrylate derivative was used in place of a vinyl derivative of CBP. A similar change in luminescence spectrum was observed when the film was polymerized by thermal annealing or electron-assisted deposition. It is speculated that the spectral transition is caused by the copolymerization of host and dopant molecules during device operation. Such a current-induced polymerization can be a novel and convenient method of preparing polymeric thin films by the physical vapor deposition of functional monomers.