Yoshiki Hidaka

Role of surrounding oxygen on oxide nanowire growth

The ability to control oxide nanowires via vapor-liquid-solid (VLS) mechanism remains an important challenge to explore various applications of oxide nanowires. Here we demonstrate the crucial roles of surrounding oxygen on VLS grown SnO2 nanowires. When the partial pressure of surrounding oxygen is relatively low, the oxygen mainly acts as an oxygen source, promoting VLS growth. While for relatively high oxygen partial pressures, vapor-solid (VS) growth emerges, suppressing nanowire growth. The findings as to the roles of surrounding oxygen on the inherent competition between VLS and VS growths are in principle rather universal for VLS grown nanowires of various oxides.

On an Isomorphous Transition

A phase diagram and several physical quantities related to an isomorphous transition are studied on the basis of a Landau-type free energy function. Relationship with a two-sublattice model including an asymmetric double minimum potential is discussed.

Removal of metal ions from aqueous solutions using carboxymethyl cellulose/sodium styrene sulfonate gels prepared by radiation grafting

Sodium Carboxymethyl Cellulose (CMCNa)/Sodium Styrene Sulfonate (SSS) hydrogels with grafted and crosslinked polymeric networks were prepared by γ-radiation at atmosphere condition. The obtained hydrogels were characterized by gel fraction, swelling ratio, TGA and FTIR spectroscopy. The results showed the ratio of CMC and SSS 1:0 gave the highest gel fraction, compared with other ratios. The swelling capacity increased by increasing SSS content due to the presence of SO3Na, OH groups in gel structure. The FTIR spectrum of CMC/SSS gel showed the new absorption peaks at 1034 and 1012cm-1 corresponds to SO3Na group. The metal ion adsorption capacity of CMC/SSS gel was investigated. The grafted gel effectively removed metal ions, especially Cr and Pb. The effects of hydrogel composition, contact time, and initial concentration on the adsorption capacity of the grafted hydrogels were studied. The adsorption kinetics and equilibrium isotherms were investigated using pseudo-second-order model and Langmuir model.

Prewavy Pattern: A Director-Modulation Structure in Nematic Liquid Crystals

Abstract A periodic structure (the prewavy pattern) found below the onset of electroconvection in a homeotropically aligned nematic liquid crystal is reported. It is characterized by a periodic modulation of the director in the xy-plane, which can be distinguished from electroconvection patterns with modulation in the xz-plane. The prewavy instability is investigated in detail; a phase diagram in the frequency-voltage plane, the voltage dependence of the azimuthal rotation angle, the voltage and frequency dependence of the wavelength of the prewavy pattern, and the director field in the prewavy pattern are provided. This prewavy pattern always evolves into the so-called chevrons above onset of convection. The wavelength of the chevrons and the orientation of their alternating zig and zag rolls depend on the director structure of the prewavy pattern.

Study on transport pathway in oxide nanowire growth by using spacing-controlled regular array

Metal oxide nanowires formed via vapor-liquid-solid (VLS) process are promising nanoscale building blocks. Although understanding material transport pathways across three phases is crucial to realize well-defined oxide nanowires, such knowledge is unfortunately far from comprehensive. Here we investigate the material transport pathway in VLS grown MgO nanowires by utilizing spacing-controlled regular array. Defining the regular spacing of catalysts allows us to extract the information of transport pathway for each catalyst. We found the significant contribution of vapor phase transport pathway rather than the surface diffusion transport. This result highlights the critical role of re-evaporation process on VLS oxide nanowires.

Formation Scenarios for Nonlinear Patterns in Electroconvection under Controlling Goldstone Modes in Magnetic Field

We report novel scenarios for the transitions from a stationary periodic state to spatio-temporal chaos (STC) for electroconvection in homeotropically aligned nematics, in which a constant magnetic...

Dynamics of One-Dimensional Periodic Pattern Formation on the Electro-Hydrodynamic Convection of a Nematic Liquid Crystal

The formation processes of a one-dimensional periodic pattern were investigated on the electro-hydrodynamic convection of a nematic liquid crystal. Two kinds of processes for different initial conditions were observed. In the first initial condition the system was set to be the no-flow state in the absence of the applied field, and in the second the turbulent state under a strong field. The temporal change of the one-dimensional image intensity was measured after the amplitude of an ac electric field was jumped from these initial values to a value slightly above the critical one. The power spectrum function and the locally varying phase were calculated from the image intensity. The obtained results were compared with the numerical simulations of the diffusion equation.

Transition Properties of the Soft-Mode Turbulence in the Homeotropic Electroconvection Superimposing Magnetic Fields

The experimental study on transition processes of the spatio-temporal chaos (STC) as well as periodic patterns in homeotropically aligned nematic liquid crystals is reported under superimposing magnetic fields with electric fields. The strong dependence of the threshold for STC on the strength of magnetic fields is observed above certain normalized voltage e th , where the two types of STC are found out below and beyond the characteristic value of the magnetic field H * = 620 G. The periodic patterns were observed for the normalized voltage e below e th . Below H * the soft-mode turbulence appears due to the Goldstone mode related to a continuous rotational-symmetry which can be controlled by the application of H . Beyond H * , the conventional STC called defect turbulence, which is similar to STC in the planar system, is observed.

Dopant homogeneity and transport properties of impurity-doped oxide nanowires

Controlling and understanding an impurity doping on semiconductor oxide nanowires grown by the vapor-liquid-solid (VLS) method remains an important challenge. Homogeneous dopant distribution within oxide nanowires has been assumed without the direct evaluations to interpret the transport properties. Here we report the direct measurements of dopant distributions for Ta-doped SnO2 nanowires. We find that differences in dopant incorporations between VLS and vapor-solid growth processes give rise to a heavily doped shell surrounding an underdoped core. Thus, understanding the dopant incorporation pathways is essential to designing and controlling impurity doping on VLS grown oxide nanowires.

Anomalous diffusion and Lévy distribution of particle velocity in soft-mode turbulence in electroconvection

We study the complex behavior in soft-mode turbulence (SMT), a recently discovered type of spatiotemporal chaos observed in electrohydrodynamic instability (EHD) of a nematic liquid crystal with homeotropic alignment. A particle, small compared to the characteristic length of the macroscopic flow, injected in SMT travels with random velocity such as an active Brownian motion. Tracking the particle trajectory induces changes in the flow velocity as a function of space and time (the Lagrange picture). The mean amplitude of the velocity is linearly proportional to the control parameter e, normalized voltage in EHD. The probability density distribution of the particle velocity changes from Levy for small e to Gaussian distribution for large e through intermediate distributions. The different regimes of anomalous diffusions are also observed. The stochastic properties of SMT are discussed based on these results.