Hisao Yanagi

Self-waveguided blue light emission in p-sexiphenyl crystals epitaxially grown by mask-shadowing vapor deposition

Highly polarized and self-waveguided blue light emission was observed in oriented organic crystals of p-sexiphenyl (6P). The 6P molecules were epitaxially grown in needlelike crystals with a submillimeter length by mask-shadowing vapor deposition on the KCl (001) surface. In the crystals, the molecular axes are oriented parallel to the [110] direction of the KCl substrate and perpendicular to the needle axis of the crystal. Under ultraviolet excitation, the blue light emission was confined within the crystal and self-waveguided along the needle axis with the transverse-electric mode, then radiated from the tips of the needles. This self-waveguided emission in the crystal was based on the uniaxially oriented transition dipoles which lay on the KCl surface and were aligned perpendicular to the needle axis in the crystal.

Orientation-controlled organic electroluminescence of p-sexiphenyl films

A multilayered electroluminescent device was constructed with epitaxially oriented films of p-sexiphenyl with its molecular axis lying or standing next to the substrate surface. The films with the lying and standing orientations were prepared by vapor deposition onto the KCl (001) surface kept at 20 and 150 °C, respectively. After successive depositions of electron-transport layer and Al cathode, the films were removed from the KCl substrate and transferred on an indium tin oxide coated glass anode. The cell with lying molecules emitted a higher electroluminescence with a narrowed spectrum at remarkably low driving voltages, as compared to one with standing molecules. This electroluminescent behaviors depend upon anisotropic distribution of the polarized emission light as well as efficiency of the carrier transport in the ordered molecular arrays with different orientation modes.

Ambipolar organic light emitting field effect transistors with modified asymmetric electrodes

The authors report on gate-controlled light emission from an organic field effect transistor composed of a vapor-deposited thin film of α,ω-bis(biphenyl-4-yl)-terthiophene (BP3T) with an electron injection layer of pentacene for the drain electrode. A n-triacontane thin film vapor deposited on a Si∕SiO2 wafer was used as a buffer layer for the gate dielectric. The location of emission zones within the channel where both injected carriers recombine was controlled by the gate voltage. The insertion of the pentacene and n-triacontane layers improved the threshold voltage and mobility for electrons, resulting in balanced ambipolar carrier injection and transport.

Epitaxial structuring of blue light-emitting p-phenylene oligomers

Epitaxial structuring of blue light-emitting molecules, p-phenylene oligomers (p-nP, n=3,4,5,6), was investigated in thin crystals vapor deposited on the (001) surface of a KCl single crystal. Their molecular orientation and morphology changed depending on the molecular length and substrate temperatures during deposition. As the length of p-nP was longer and the substrate temperature was lower, the orientation of the molecular axis changed from standing to lying on the KCl surface due to an increase of molecule–substrate interactions. p-terphenyl (3P) hardly adsorbed on the KCl (001) surface, but a portion of the standing molecules stuck along the step edge of KCl and formed a walllike structure. Long needlelike crystals of p-hexaphenyl (6P) epitaxially grew in two orthogonal KCl [110] directions taking lying orientation even at a higher substrate temperature. In these epitaxial needles, the transition dipoles parallel to the molecular axis gave rise to blue fluorescence sharply polarized along the KCl [1...

A naphthalocyanine-based EPR probe for localized measurements of tissue oxygenation.

A new electron paramagnetic resonance (EPR) oximetry probe, based on a naphthalocyanine macrocycle, is reported to exhibit high oxygen sensitivity and favorable EPR characteristics for biological applications. The free radical probe, lithium naphthalocyanine (LiNc), is synthesized as fine microcrystalline powder with particle size less than 1 microm and high spin density. It exhibits a single sharp EPR peak, whose width varies linearly with oxygen partial pressure (pO2). The EPR spectrum is nonsaturable at typical microwave power levels (< 25 mW at X-band). These unique characteristics make this probe ideal for measuring oxygen concentration in biological tissues, in vivo. The peak-to-peak width under anoxic conditions is 0.51 G (at X-band), and it increases linearly with increase in oxygen partial pressure and reaches 26.0 G for 100% oxygen (760 mmHg), showing an oxygen sensitivity of 34 mG/mmHg. The probe responds to changes in pO2 quickly and reproducibly, thus enabling dynamic measurements of regional oxygenation in real time. The application of this probe for oximetry is demonstrated in an in vivo biological system. The changes in pO2 were monitored in the leg muscle tissue of a living mouse breathing room air and carbogen (95% oxygen + 5% CO2), alternatively. The mean pO2 measured with this probe in muscle tissues was consistent with values reported previously using other methods. Overall, the probe shows very desirable characteristics for localized measurements of tissue oxygenation.

Fabrication of gold nanoparticles and their influence on optical properties of dye-doped sol-gel films

Abstract Au nanoparticles were generated in glass films by the sol–gel method. Au ions doped in silica/titania films were reduced by UV light or electron-beam irradiations resulting in Au nanoparticles with a diameter smaller than 20 nm. This method enabled us to produce any patterns of Au nanoparticles in thin films. Using two-beam interference of a UV laser and electron-beam lithography, periodic structures of Au nanoparticles with submicrometer intervals were fabricated. Furthermore, doping of fluorescent dyes into the films containing Au nanoparticles exhibited selective reactivation or quenching of the photoluminescence. By generation of Au nanoparticles, the fluorescence of Rhodamine B was reactivated, whereas that of Coumarin 152 was quenched, depending on their electronic energy configurations with respect to the plasmon absorption band of the Au nanoparticles. This selective phenomenon also enabled us to make fluorescence patterning in the glass films.

Characterization of dye-doped TiO2 films prepared by spray-pyrolysis

Thin films of TiO2 were prepared by spray-pyrolysis of titanium(IV)-oxy-acetylacetonate onto indium-tin-oxide glass electrodes. Depending upon the substrate temperature, morphology of the deposited TiO2 films changed from irregular aggregates at 200°C to homogeneous particles with a diameter of 50–100 nm above 400°C. Dye-doping was achieved at the substrate temperature below 260°C by dissolving phthalocyanine or rhodamine molecules into the oxy-acetylacetonate solution. Photoelectrochemical measurements indicated that anodic photocurrents at the wavelength range below 400 nm due to direct excitation of TiO2 increased by formation of homogeneous fine particles at higher temperature. The electrode doped with phthalocyanine exhibited photocurrents in its visible absorption band at 600–700 nm whereas that doped with rhodamine yielded no sensitized photocurrent.

Lasing from epitaxially oriented needle crystals of a thiophene/phenylene co-oligomer.

Optically pumped lasing of epitaxially oriented molecular crystals is presented. Needle-like crystals of a thiophene/phenylene co-oligomer is grown along the [110] direction of a KCl substrate. The fluorescence light is one-dimensionally confined and amplified by the self-waveguiding effect in the elongated needles. The Fabry-Pérot resonation by the terminated ends results in laser oscillations corresponding to the needle length.

Self-orientation of short single-walled carbon nanotubes deposited on graphite

In view of practical handling of single-walled carbon nanotubes (SWNTs), here we present cutting procedures and spontaneous ordering of SWNTs deposited on highly oriented pyrolytic graphite (HOPG). Purified SWNTs were cut by ultrasonication in H2SO4/HNO3 mixture and further in de-ionized water. Thus, prepared short SWNTs were resuspended in methanol and dropped on a freshly cleaved HOPG surface. Scanning tunneling microscopy under a vacuum revealed that the short tubes of 20–100 nm in length were aligned along the basal axis of the HOPG lattice. Tunneling spectroscopy obtained from the oriented tubes indicated a semiconducting behavior, which was characterized in relation to their helical structure. The observed orientation of SWNTs was attributed to their one-dimensional electronic nature along the tube axis with the HOPG lattice.

Electroluminescence of epitaxial perylene films

A multilayered electroluminescent device was constructed with ordered perylene thin films as an emission layer. The epitaxial perylene film was prepared by vapor deposition onto the KCl (001) surface, then removed from the KCl substrate and sandwiched between the hole‐transport layer coated on an indium tin oxide electrode and Al top electrode. In this epitaxial layer, the perylene molecules oriented their molecular planes perpendicular to the electrode surfaces. This epitaxial device emitted an electroluminescence in the longer wavelengths of 560–580 nm due to the ordered dimeric structure. On the other hand, the device with polycrystalline perylene films exhibited an electroluminescence mainly below 500 nm.