Genji Okada

NO2 sensitive LaFeO3 thin films prepared by r.f. sputtering

Abstract LeFeO 3 thin films with different thicknesses have been fabricated by the r.f. magnetron sputtering method on Al 2 O 3 substrates with comb-type Au electrodes. The influence of annealing temperatures and times on the NO 2 sensitivity of the thin films has been investigated. The thin films are annealed at temperatures from 600 to 1000 °C in flowing air for 30 or 60 min. The conductivity of thin films increases noticeably when NO 2 gas is introduced into the measuring chamber, and returns to its original level after NO 2 is removed. The NO 2 response curve of LaFeO 3 thin films shows two distinct phases: an initial fast and large conductivity change, followed by a much slower change. These phases are attributed to the surface chemisorption process of NO 2 and the oxidation process of the bulk, respectively. The materials structure plays a fundamental role in its NO 2 response, while the influence of film thickness is less important. The annealing temperature and time of LaFeO 3 sputtered thin films determine their NO 2 sensitivity. The thinner films show higher resistivities, slightly greater NO 2 sensitivities and less stable results.

Electrical and Optical Properties of Ca1-xSrxCuO2 Films Prepared by Organometallic Chemical Vapor Deposition Method

The as-deposited Ca1-xSrxCuO2 film is a semi-insulator, and the resistivity of the film is reduced by a factor of about one hundred after postannealing in 1 atm O2. The temperature dependence of the resistivities of these films is semiconductive. The positive Seebeck coefficients of these films indicate that holes in the valence band are generated by excess oxygen. The charge-transfer absorption is observed at a wavelength shorter than 800 nm.

Preparation of ZnO films on sapphire (01\(\bar 1\)2) substrates by low-pressure organometallic chemical vapour deposition

We have proposed a new photochromic device in which two kinds of transition metal ions are alternatively doped into a host material with a period of a few tens of nanometres [1]. Such a device with superlattice structure has been fabricated by means of low-pressure organometallic chemical vapour deposition (LPOMCVD) [2, 3]. Using LPOMCVD, we succeeded in the preparation of (11 20) orientated ZnO films [4, 5]. Nevertheless, the resultant ZnO fihn exhibited a fairly low resistivity, because of formation of oxygen vacancies. Insulating ZnO films are required as photochromic host materials. Thus, in the work reported in this letter, oxygen gas was introduced into a reaction chamber during the preparation of a ZnO film. The properties of the resultant ZnO films are discussed on the basis of data of X-ray diffraction (XRD) patterns, electric resistivity, absorption spectra and photoluminescence (PL) spectra. Zinc acetate dihydrate (ZnAc) was used as a precursor of Zn source. The LPOMCVD apparatus was the same system used in our previous letter [4]. A reaction chamber was evacuated to 1.0 x 10 -4 Pa then H20 vapour was introduced into the reaction chamber up to 2.7 × 10 -2 or 6.7 × 10 -3 Pa. Furthermore, oxygen gas was introduced into the chamber using a mass flow meter. The total pressure of the reaction chamber was almost unchanged after oxygen gas was introduced into it. ZnO films were deposited on sapphire (01 ]-2) substrates heated at 350°C. The preparation of the ZnO films was carried out at a low sublimation rate, because both (11 20) and ZnO (0002) reflections were observed in an XRD pattern of a ZnO film prepared at a sublimation rate higher than 3 m g h -1 [5]. The preferential orientation of the ZnO film was analysed from rocking curves for (11 20) ZnO reflection; in XRD measurements (CuK~), the angle (2 0) of the Xray detector was fixed to 56.5 °, then the angle (0) of the sample was independently scanned. The thickhess of the film was evaluated by a surface roughness meter (Kosaka SE-30D). Electric resistivity was measured by the Van der Pauw technique. PL spectra were measured using a 50 cm focal length grating monochromator, a lock-in-amplifier and a cw He-Cd laser (325 rim, 10 roW) as an excitor. The conditions for the preparation of ZnO films are listed in Table I. As shown in the results of samples 1 and 4, the deposition rate is not proportional to the sublimation rate alone. The partial pressure of H20 is another factor controlling the deposition rate. As a general tendency, addition of oxygen gas leads to a decrease in the deposition rate, irrespective of the H20 partial pressure. Fig. 1 shows XRD patterns of the ZnO films. Only (11 20) reflections were seen in the XRD patterns for all samples, so that the preferential orientation is not remarkably disturbed by the addition o f oxygen gas. Nevertheless, the intensity of the (11 20) reflection decreases with the flow rate of oxygen gas. As for the samples 1, 2 and 3, the intensity of the (11 20) reflection normalized by the film thickness is decreased by the addition of oxygen gas. In general, orientation of the ZnO film has an influence on the intensity of the XRD patterns. To clarify the relation between orientation and intensity of the ZnO (11 20) reflection, the orientations of these ZnO films were evaluated from rocking curves for the ZnO (11 2_-0) reflections. The rocking curves of the ZnO (11 20) reflections, which are not shown here, have full width at half maximum (FWHM) of 2.8, 3.2, 3.0, 4.2 and 6.1 ° for samples 1, 2, 3, 4 and 5, respectively. The inclination angles of ZnO (1120) plane with respect to A1203 (0224) plane are 2.3, 1.9, 2.4, 2.6 and 1.7 ° for samples 1, 2, 3, 4 and 5, respectively. Except for sample 5, the preferential orientation is not significantly affected by the addition of oxygen gas. Accordingly, the decrease in the intensities of the (1 t 20) reflections caused by introducing oxygen gas is not attributable to degradation of the

Mechanism of Photoinduced Charge Transfer in Co(Li)-Doped ZnO Film

A three-layer film consisting of In-doped ZnO, Co(Li)-doped ZnO and Li-doped NiO has been fabricated by means of a sputtering technique. The photocurrent spectrum of the Co(Li)-doped ZnO has been measured by applying a bias voltage between the In-doped ZnO and Li-doped NiO electrodes. A broad peak around 640 nm in the photocurrent spectrum is assigned to photothermal ionization of Co2+ ions. The time dependence of photocurrents indicates that the concentration of Co2+ ions is decreased by the irradiation of 500 nm and is recovered to the initial value by turning off the bias voltage.

EPR study on NO introduced into lithium ion-exchanged LTA zeolites

Nitrogen monoxide (NO) introduced into partially and fully lithium ion (Li+)-exchanged A-type zeolite was studied by X-band EPR measurements. Two types of NO species, NO monoradical and NO–NO biradical, were detected for the partially ion-exchanged zeolite, while the latter was less detectable for the fully ion-exchanged one. The EPR parameters of the NO monoradical provided evidence that the electrostatic field associated with Li+ ions in A-type zeolites is weaker than that with Na+ ions. It was found that the molecular distance of NO biradical formed in partially Li+-exchanged zeolite was shorter than that in sodium ion-exchanged A-type zeolite.

Optical and electronic properties of cobalt-doped zinc oxide films prepared by the sputtering method

In-doped ZnO, Co-doped ZnO and Li-doped NiO are successively deposited on quartz by the sputtering method. A current versus voltage curve of the three-layer device, in which the Indoped ZnO and Li-doped NiO are used as electrodes, reveals that the In-doped ZnO is an ohmic electrode and the Li-doped NiO is a blocking electrode with respect to electron injection. In photocurrent spectra of the three-layer device, there are two distinct peaks around 410 and 640 nm. The former is ascribed to the photoionization caused by the electric-dipole transition from the ground states,4A2(F), to the conduction band, and the latter to the thermal emission from electronic excited states of Co2+,4T1(P).

Conductivity of zeolite/poly(tetrafluoroethylene) composite membrane in the presence of water vapor

The conductivities of zeolite/poly(tetrafluoroethylene) composite membranes were measured in the presence of water vapor at 75–150 °C. The conductivity of composite membranes depended on zeolite structure, zeolite content, and exchangeable cation in zeolite; the Li ion-exchanged Y-type zeolite/poly(tetrafluoroethylene) composite membrane with 80 wt.% of zeolite powder showed the highest conductivity among the composite membranes tested in the present study. It was found that the conductivity increased with an increase in the amount of water adsorbed on the composite membrane.

New Preparation Method of CdS Clusters Encapsulated in Y-Type Zeolites

CdS clusters encapsulated in Y-type zeolite were prepared by a novel technique with non-aqueous systems. The mechanical mixture of CdCl2· 2.5H2O crystal with Y-type zeolite was heated at 773 K, followed by the H2S treatment at 373 K, resulting in the formation of CdS clusters with the size <2.3 nm.

Preparation of superconducting Bi−Sr−Ca−Cu−O films by plasma-assisted organometallic chemical vapor deposition

The Bi2Sr2CuOx and Bi2Sr2CaCu2Ox films are prepared on single-crystalline SrTiO3 (100) at 650°C by the plasma assisted organometallic chemical vapor deposition method. There are only (00l) reflections in the X-ray diffraction patterns of these films. This indicates that the c axes of these films are oriented preferentially perpendicular to the substrate surface. The Bi2Sr2CaCu2Ox films prepared in 1 Pa are semiconductive, while the films prepared in 100 Pa exhibit onset Tc at 63 K.

Switching in polystyrene and polymethyl methacrylate thin films: effect of preparation conditions of the polymers

Switching without any break in the electrodes was observed in polystyrene and polymethyl methacrylate films by controlling the current using a high resistance connected in series with the sample. The effect of the method of polymerization on the switching characteristics were studies for both polymers. Polymers were prepared by γ-ray irradiation or by catalytic or noncatalytic thermal polymerization. The switching characteristics gave poor reproducibility for the catalytically polymerized polystyrene, but no difference was observed for the three types of polymethyl methacrylate from different origins. In the case of polystyrene, the lower the molecule weight, the higher was the threshold voltage. With polymethyl methacrylate such a dependence was not observed. In both polymers the on-state is relatively stable. When the samples in the on-state were heated above 100° C, they were converted to the offstate. No dots or breaks on the sample were detected by scanning electron microscopy (SEM) after switching has occurred.