Sakae Tanemura

Preparation and characterization of polycrystalline anatase and rutile TiO2 thin films by rf magnetron sputtering

Titanium dioxide films with the anatase and rutile single phase were formed on Si substrates by rf sputtering through a precise control of critical parameters. The structure of the films was studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM), and the optical properties were evaluated with spectroscopic ellipsometry (SE). Lattice distortion was found in both anatase and rutile films from TEM observation. The obtained refractive indices n exhibit higher values than those reported for thin films due presumably to the density structure of the sputtered films. Optical band gaps were calculated by Tauc plot using the obtained extinction coefficient separately for anatase and rutile, with values larger than those reported for bulk materials. The reasons for the larger band gap might be due to the strain from lattice distortion.

Niobium Oxide Electrochromic Thin Films Prepared by Reactive DC Magnetron Sputtering

Nickel oxide electrochromic thin films were prepared by reactive DC magnetron sputtering. The as-deposited optical property and electrochromic behavior strongly depended on the target operation mode and the substrate temperature. The sample sputtered with low oxygen flow rate, high power and substrate temperature of 200-300° C showed a wide transmittance modulation range, and the deposition rate was as high as 30 nm/min. Its integrated luminous transmittance could be controlled from 6.6 to 82.3%. X-ray diffraction measurements of this sample in a colored state and bleached state suggested that the boundary and surface of NiO microcrystallites played an important role in the electrochromic reaction of sputtered nickel oxide films.

Formation and thermochromism of VO2 films deposited by RF magnetron sputtering at low substrate temperature

Thermochromic VO2 films were deposited on substrates of silicon and Pyrex glass by reactive rf magnetron sputtering and characterized by thin-film X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), atomic force microscopy (AFM) and spectrophotometry. Films with a VO2 single phase were formed above a fairly low temperature of 300° C by controlling precisely the oxygen flow ratio. The use of a nucleated substrate improved the crystallinity of the VO2 films deposited at low temperature. A negative substrate bias appeared to have an effect of favoring the formation of a VO2 single phase at a substrate temperature as low as 250° C.

Fabrication and characterization of anatase/rutile–TiO2 thin films by magnetron sputtering: a review

Abstract Ceramic-based nanocomposites were reviewed, emphasizing the newly developed concept of material design for ceramics. First, characteristics of the nanocomposites observed by previous researchers were summarized as, significant or moderate improvement in strength, drastic change of the fracture mode from intergranular fracture of monolithic ceramics to transgranular fracture of nanocomposites, moderate enhancement of fracture toughness, improvement of other mechanical properties, and observations of dislocations. Second, several mechanisms proposed previously to explain these characteristics were reviewed. Third, our strengthening and toughening mechanisms of nanocomposites on the basis of dislocation activities were explained. In nanocomposites, the highly localized residual stresses in the matrix grains are generated by the mismatch of thermal expansion coefficients between the matrix and the dispersed particles, and the dislocations are yielded during the cooling process after sintering. These dislocations then release the tensile residual stresses intrinsically existing in the matrix grains of sintered ceramics and improve the strength of the materials. In addition, as these dislocations cannot move at room temperature the sessile dislocations in the matrix operate as nano-crack nuclei in a frontal process zone (FPZ) ahead of the crack tip when the tip of a propagating crack approaches this area. Therefore, the size of the FPZ is expanded and as a result the fracture toughness is improved. Finally, estimation of the critical FPZ size was explained in order to clarify its toughening mechanism in nanocomposites.

Low resistivity p-ZnO films fabricated by sol-gel spin coating

N-doped and In-N-codoped ZnO films were fabricated on quartz glass substrate by sol-gel spin coating. Their p-type conductivities were characterized by the Hall measurements, revealing low resistivities of the order of 10−1Ωcm. Thin-film junctions comprising an undoped ZnO layer and a N-doped ZnO layer displayed the typical rectifying characteristics, suggesting formation of p-n homojunctions at the interfaces.

Relationship between transition temperature and x in V1−xWxO2 films deposited by dual-target magnetron sputtering

Thin films of V1-x Wx O2 were deposited on glass and Si by dual-target sputtering and characterized by thin film X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS) and spectrophotometry. The relationship between x (0-0.026) and the phase transition temperature τ c (0-67° C) in the most applicable range for advanced window coatings was clarified by formation of a single-phase film and precise determination of the amount of doping. Tungsten doping linearly decreases τ c by 23° C/at.%W and also significantly reduces the thermal hysteresis loop width in optical transmittance.

Exciton radiative lifetime in ZnO nanorods fabricated by vapor phase transport method

The exciton radiative lifetime in ZnO nanorods is studied. It is found that the exciton radiative lifetime increases with temperature as T2. Furthermore, the spectral linewidth of the photoluminescence of the ZnO nanorods also increases with temperature as T2, suggesting a linear dependence of exciton radiative lifetime on the spectral linewidth. The physics behind is that the oscillator strength of excitons at k=0 is shared equally among all the states within the spectral linewidth and the coherence extension of an exciton decreases with temperature due to the scattering by phonons, defects, or impurities.

Field electron emission from sputter-induced carbon nanofibers grown at room temperature

Graphite, carbon-coated silicon, and carbon-coated nickel surfaces were bombarded with obliquely incident Ar+ ions at room temperature. The sputtered surfaces were covered with conical protrusions, ∼2.5×105mm−2 or higher in numerical density, and partially aligned single carbon nanofibers (CNFs), ∼20nm in diameter and 0.3‐2μm in length, grew on the tips. They were characterized by the amorphous nature and the boundaryless structure between the CNF and the conical base. The field electron emission measurements for the CNFs thus grown on the carbon-coated silicon substrate showed the threshold field of 1.8V∕μm with a current density of 1μA∕cm2, and the field enhancement factor was estimated to be 1951 from the Fowler-Nordheim plot assuming the work function of 4.6eV for graphite. The morphological structure of CNFs grown on conical bases was thought to be effective to reduce the screening effect due to sufficient distance between adjacent CNFs. Thus, the sputter-induced CNFs were concluded to be quite promi...

Optical constants of V 1-x W x O 2 films

The spectral complex optical constants in the visible and the near-infrared region of VO(2) and V(1-x)W(x)O(2) films deposited on glass substrates were determined from observed reflectance and transmittance spectra for which the least-squares method was used. In the metallic phase, the optical properties were characterized by the Drude model in wavelength regions longer than 750 nm.

Room-temperature growth of a carbon nanofiber on the tip of conical carbon protrusions

Glassy carbon was Ar + -ion bombarded with a simultaneous Mo supply under ultrahigh vacuum conditions using a microprotrusion fabrication system that consists of a differentially pumped ion gun and a seed-material supply source. Conical protrusions were formed by sputtering with a seed supply, and carbon nanofibers (CNFs) grew on the tips even at room temperature. The length of CNFs reached up to ∼10 μm, and their diameter was almost uniform (50 nm) in the growth direction. The short CNFs aligned in the ion beam direction, whereas the long ones were non-aligned. The CNF growth on a glassy carbon surface was ascribed to the enhanced surface texturing and to the massive redeposition of C atoms onto cones, both of which are specific to the oblique ion bombardment: The former would lead to an increase in the number of possible nucleation sites for the CNF growth, and the C atoms arising from the latter process would migrate toward the conical tips, thus forming CNFs.