S Yamamoto

Preparation of MoO3 nanostructures and their optical properties

In this letter we report the synthesis of nanostructures of molybdenum trioxides by directly oxidizing a spiral coil of molybdenum, at ambient atmosphere, by passing a current through the coil. The advantage of this approach is that the temperature of the substrate is low (normally below 200?C), and that the nanostructures to be formed could be chosen, via controlling the current or, equivalently, the temperature of the coil. We show that, by adjusting the current through the coil, ?-MoO3 lamellas with a thickness of ~20?50?nm, and ?-MoO3 spheres of diameters down to nanometre scale can be synthesized at ambient atmosphere. These nanostructures exhibit a large optical band gap of ~3.05?eV, and room-temperature photoluminescence at ~395?nm. This study provides a simple, controllable way of fabricating metal oxide nanostructures of interest.

Single-crystalline epitaxy and twinned structure of vanadium dioxide thin film on (0001) sapphire

thin films prepared by pulsed-laser ablation have been investigated by high-resolution x-ray diffraction, x-ray pole-figure, Rutherford backscattering/channelling and electrical measurements. The results show that films deposited on (0001) sapphire substrates grow with well determined orientations in the plane: and out of the plane: . The aligned-to-random ratio of backscattered yield of the spectra can be as low as 5%, and the electrical resistivity changes by a factor of during the phase transformation with a hysteresis loop width of , which implies that both the structure and the properties of the film are very close to those of bulk single-crystal . The (010)-oriented film `prefers to form twinned structure with domain walls. Molybdenum substitutional doping up to the level of 1.5 at.% does not degrade the crystal quality.

Rutherford backscattering/channelling, XRD, XRD pole figure and AES characterization of thin films prepared in different ambients by laser ablation

In this work, the stoichiometry, epitaxial relationship, crystalline quality and surface of thin films prepared in oxygen, argon, vacuum and oxygen-argon mixture as ambients by pulsed laser deposition (PLD) have been characterized by Rutherford backscattering (RBS)/ion channelling, x-ray diffraction (XRD), XRD pole figures and Auger electron spectrometry (AES). We have found the films prepared in pure are oxygen rich even when the gas pressure is down to 0.001 mbar. Although nearly stoichiometric and crystalline films can be prepared in vacuum or pure argon environment, RBS and AES analyses show the films prepared in pure argon are titanium deficient and the films prepared in vacuum can be easily contaminated by carbon and other impurities. XRD and XRD pole figures clearly show the films prepared in the gas mixture have a parallel relationship with the substrate at the main plane and other planes. The films prepared in oxygen and argon also have epitaxial relationship to the substrate at the main plane, but the films prepared in vacuum have an epitaxial relationship to the substrate . The in-plane pole figures show films prepared in argon, oxygen and vacuum have much poorer crystal quality compared to the film deposited in the gas mixture both in the orientation relationship and the ratio of background to signal. An epitaxial film with high quality and clean surface can easily be prepared in the mixed gases of argon with 5% oxygen, and it has almost single crystal structure with a small amount of twins (less than 10%). The results show that a gas mixture of Ar and 5% can not only keep the stoichiometry of and protect the film from contamination of carbon, but also enhance the crystalline quality of the film, especially developing the tendency of film growth in a parallel direction to the substrate.

X-ray characterization of growth and structural modification of Si by MeV ion implantation

A combination of low-temperature MeV carbon ion implantation and post high- temperature annealing was used to produce a -SiC buried layer in Si(001). The growth of the -SiC layer and re-growth of the front Si layer upon annealing was monitored by x-ray diffraction and x-ray pole figure measurement. In the samples annealed at a temperature of or higher, the buried -SiC layer has a near-perfect orientation relationship with the substrate. The structure of the front Si layer, interestingly, was modified after annealing, i.e., the forbidden Si(002) diffraction was observed. The orientation relationships among the three layers, i.e., the front Si layer, the -SiC buried layer and the bulk substrate, were also investigated by the x-ray pole figure measurement.

Preparation and characterization of C60 films on ionic substrates

Epitaxial (111)-oriented C60 films have been grown on the (100) surfaces of a few ionic substrates, such as KCl, KBr, NaCl and LiF, by physical vapour deposition, and have been characterized by SEM (scanning electron microscopy), XRD (x-ray diffraction), pole figures of XRD, Raman and Rutherford backscattering spectrometry (RBS). We have found that continuously and entirely (111)-oriented epitaxial C60 films can be grown on ionic substrates in a wide temperature range, 40 to 120 °C, and at very different deposition rates, from 1.5 to 35 A min-1. Single crystal and entirely (111)-oriented C60 films with grain size of 1 to 3 µm could also be grown at a relatively high temperature and low deposition rate, 120 °C, and 1.5 A min-1, respectively. Raman and RBS measurements show that the films have high quality without significant impurities. The optical property has also been measured in the UV-visible region, and the optical band gap has been determined to be 1.74 eV.

The effect of sputtering ions on the structure and properties of diamond-like carbon films

Carbon films have been deposited on Al2O3(0001) substrates by sputtering a graphite target with 3 keV CH4+, N+, and Ar+ ions. The structure and properties of the films were examined by Auger electron spectrometry, micro-Raman spectroscopy, and optical absorption measurement, and were found to be dependent on which sputtering ions were used. The stability of the carbon films was also examined using Fourier-transformed infra-red spectroscopy by annealing the films at temperatures from 25 to 200 °C.Abstract:We study the influence of an aperiodic extended surface perturbation on the surface critical behaviour of the two-dimensional Ising model in the extreme anisotropic limit. The perturbation decays as a power of the distance l from the free surface with an oscillating amplitude where follows some aperiodic sequence with an asymptotic density equal to 1/2 so that the mean amplitude vanishes. The relevance of the perturbation is discussed by combining scaling arguments of Cordery and Burkhardt for the Hilhorst-van Leeuwen model and Luck for aperiodic perturbations. The relevance-irrelevance criterion involves the decay exponent , the wandering exponent which governs the fluctuation of the sequence and the bulk correlation length exponent . Analytical results are obtained for the surface magnetization which displays a rich variety of critical behaviours in the -plane. The results are checked through a numerical finite-size-scaling study. They show that second-order effects must be taken into account in the discussion of the relevance-irrelevance criterion. The scaling behaviours of the first gap and the surface energy are also discussed.

Structure and electrical characterization of prepared by reactive pulsed laser deposition

Epitaxial and stoichiometric films were prepared by reactive pulsed laser deposition (RPLD) with YAG:Nd laser on sapphire substrates. The film stoichiometry and structure were characterized by AES, XRD, RBS/channelling and XRD pole figures. It was found that epitaxially crystalline and nearly stoichiometric films with x of 0.96 on (0001) had been prepared by YAG:Nd laser at with the mixed gas of nitrogen and 3% hydrogen at 2 mbar. Three methods, XRD, AES and RBS/channelling, surely confirm the nearly stoichiometric crystalline characteristics of the films. The XRD pole figures show the films have twinned structure. The electrical property relationship with the temperature in the range of 16 to 300 K has been measured by conventional four probe methods.