Yasuhisa Fujita

Hydrogen related defect complexes in ZnO nanoparticles

Hydrogen related local vibrational modes (LVMs) of ZnO nanoparticles have been studied using Fourier transform infrared spectroscopy and Raman spectroscopy in as prepared and high temperature annealed samples. The obtained experimental results confirm the presence of cationic vacancies (VZn) in addition to unintentional hydrogen doping and their complex defects such as VZn–Hi and VZn–HO. After high temperature annealing, hydrogen related LVMs and multiphonon modes disappear. The presence of these complex defects determines the nonradiative and multiphonon recombination processes in the band gap of ZnO due to carrier trapping at deep levels.

Optimization of nonlinear optical properties of ZnO micro and nanocrystals for biophotonics

The defect and impurity states in ZnO nanocrystals synthesized using the plasma arc technique can be modified to optimize the nonlinear optical properties for optoelectronic and biophotonic applications. Highly efficient second harmonic signals over a wide range of near-infrared wavelengths, spanning from 735 nm-980 nm, has been observed and can be used in biological imaging. The use of further high energy excitation ranging from 700 nm-755 nm leads to two-photon absorption and yields broadband two photon emission extending from the 370 nm-450 nm wavelength regime which can be useful for therapeutic applications.

Ga-Doped ZnO Film as a Transparent Electrode for Phthalocyanine/Perylene Heterojunction Solar Cell

The photovoltaic properties of Zn–phthalocyanine (ZnPc)/3,4,9,10-perylene-tetracarboxyl-bis-benzimidazole (PTCBI) heterojunction solar cells using Ga-doped ZnO (GZO) film as a transparent electrode are studied. When GZO is the electrode in contact with the donor layer, i.e., ZnPc, the energy conversion efficiency η is only 1/4 of that for the cell using indium–tin-oxide (ITO) as the electrode. When GZO is the electrode in contact with the acceptor layer, i.e., PTCBI, on the other hand, the cell has a three times larger η than the cell using ITO. These results are explained by the work function of GZO being lower than that of ITO. When GZO, or ITO, is used as an electrode for PTCBI, an interesting aging effect is observed. For example, by keeping the GZO-based cell in open air for 6 days in the dark, the energy conversion efficiency and the short-circuit current are increased by factors of 1.67 and 1.36, respectively.

New Deep-Level Photoluminescence Bands of Homoepitaxial CdTe Films Grown by Metalorganic Chemical Vapor Deposition

Photoluminescence spectra of CdTe homoepitaxial layers on (100)- and (111)-oriented substrates grown by metalorganic chemical vapor deposition were measured at 4.2 K. Sharp and intense emission peaks at 1.47 and 1.36 eV have been observed in the films grown on (111) substrates. These sharp peaks seem to be structural defect-related emissions and are similar to the Y and Z bands observed in ZnSe.

Photo-assisted MOCVD of CdTe using an excimer laser

Abstract Low temperature homoepitaxial growth of CdTe has been achieved by the use of a KrF excimer laser introducing the in-situ measurement of photolysis in the MOCVD reactor. Specular surfaces were obtained between 100 and 150°C. Photoluminescence spectra of samples grown at 150°C had the intense peak at 1.5896 eV (A 0 , X).

Second harmonic imaging of plants tissues and cell implosion using two-photon process in ZnO nanoparticles

The optical properties of colloidal ZnO nanoparticle (NP) solutions, with size ranging from several nm to around 200 nm, have been tailored to have high optical nonlinearity for bioimaging with no auto-fluorescence above 750 nm and minimal auto-fluorescence below 750 nm. The high second harmonic conversion efficiency enables selective tissue imaging and cell tracking using tunable near-infrared femtosecond laser source ranging from 750-980 nm. For laser energies exceeding the two-photon energy of the bandgap of ZnO (half of 3.34 eV), the SHG signal greatly decreases and the two-photon emission becomes the dominant signal. The heat generated due to two-photon absorption within the ZnO NPs enable selective cell or localized tissue destruction using excitation wavelength ranging from 710-750 nm.

Growth of nitrogen-doped ZnSe and inhibition of hydrogen passivation of nitrogen acceptor by photoassisted metal-organic chemical vapor deposition

Growth of nitrogen doped ZnSe was carried out at the growth temperature of 330 to 390° C by photoassisted metal-organic chemical vapor deposition (MOCVD). Nitrogen concentration of more than 1018 cm-3 was obtained at growth temperatures lower than 350° C using ammonia and t-butylamine as nitrogen dopant sources. Furthermore, hydrogen passivation of nitrogen, which is regarded as a possible cause of acceptor compensation in the layer doped using ammonia, was drastically decreased and p-type layer with hole concentration of 8.3×1017 cm-3 was obtained using t-butylamine.

Bioimaging Using the Optimized Nonlinear Optical Properties of ZnO Nanoparticles

Nonlinear imaging using nanocrystals has advantages over fluorescent dyes. Highly nonlinear optically susceptible ZnO was synthesized for the purpose of bioimaging. ZnO nanoparticles with enhanced second and third order nolinearity have been utilized for nonlinear in vitro imaging of living cells. Highly efficient nonlinear processes in noncentrosymmetric ZnO nanoparticles was used to generate second harmonic signal within nanoparticles at extremely low-input light intensities in order to track and image live cells and thrombocytes infiltrated with ZnO.

Rapid measurement of deoxyribonuclease I activity with the use of microchip electrophoresis based on DNA degradation

Deoxyribonuclease I (DNase I) activity in serum has been shown to be a novel diagnostic marker for the early detection of acute myocardial infarction (AMI). However, the conventional method to measure DNase I activity is time-consuming. In the current study, to develop a rapid assay method for DNase I activity for clinical purposes, a microchip electrophoresis device was used to measure DNase I activity. Because DNase I is an endonuclease that degrades double-stranded DNA endo-nucleolytically to produce oligonucleotides, degradation of the DNA standard caused by DNase I action was detected using microchip electrophoresis. We detected DNase I activity within 10 min. This is the first study to apply microchip electrophoresis for the detection of DNase I activity; furthermore, it seems plausible that reduction of analysis time for DNase I activity could make this novel assay method using microchip electrophoresis applicable in clinical use.

Organic thin-film solar cells with a Cu anode: Improvement of the photovoltaic properties on aging in air

The photovoltaic properties of a Ga-doped ZnO (GZO)/3,4,9,10-perylene-tetracarboxyl-bis-benzimidazole (PTCBI)/Zn-phthalocyanine (ZnPc)/Cu heterojunction cell (cell A) and a GZO/ZnPc/Cu Schottky-barrier cell (cell B) were investigated. The energy conversion efficiency η of cell A was only 0.02% immediately after the device preparation but improved to 0.46% after aging for 24 days in air. To elucidate the mechanism of this aging effect, photocurrent action and electro-absorption spectra were measured for cell B. The results reveal that a Schottky barrier exists at the ZnPc/Cu interface which blocks the transport of photogenerated holes to the Cu electrode, and the barrier height is reduced by a white-light illumination of the device after aging. The change in barrier height is attributed to the formation of electron traps at the surface of the ZnPc layer on aging that trap photogenerated electrons.