Kenji Sakurai

Instrumentation for X-ray reflectivity in micro area: present status and future outlook

X-ray reflectivity is sensitive to slight structural changes along the depth of layered materials in the order of sub-nanometers or even smaller. This property is extremely promising for the observation of buried interfaces, but the conventional X-ray reflectivity technique unfortunately lacks spatial resolution. The method looks at quite a large area, typically mm2~ cm2of the sample, and this often makes it difficult to analyze realistic problems in modern nano sciences and technologies. The present article discusses instrumentation for upgrading the X-ray reflectivity technique to give it a much higher spatial resolution. Recent preliminary results with high-energy white X-rays are reported.

Hydrophobic switching nature of methylcellulose ultra-thin films: thickness and annealing effects

We have studied the thermosensitive property of methylcellulose (MC) thin films supported on Si substrate by static sessile drop contact angle measurements, and their surface properties and thin film structure by x-ray reflectivity (XRR) and atomic force microscopy (AFM) techniques. From the static sessile drop contact angle measurements, the MC thin films showed the characteristic hydrophilic-to-hydrophobic transition at ∼70u2009°C, which is the lower critical solution temperature of the bulk solution volume phase separation transition. For films with thickness d ≤ R(g), the onset of such a transition is affected by the film thickness while very thick films, d ≫ R(g), yielded higher contact angles. Annealing the MC thin films with thicknesses ∼200 Å (near the radius of gyration, R(g), of the polymer) below the bulk glass transition temperature (T(g) ∼ 195u2009° C) would not change the hydrophobic switch nature of the film but annealing at and above the bulk T(g) would change its surface property. From surface topography images by AFM, there were no significant changes in either the roughness or the film texture before and after annealing. With XRR data, we were able to determine that such changes in the surface properties are highly correlated to the film thickness changes after the annealing process. This study, we believe, is the first to examine the thermal annealing affects on the thermal response function of a thermoresponsive polymer and is important for researching how to tailor the hydrophobic switching property of MC thin films for future sensing applications.

Quick X-ray reflectivity of spherical samples

X-ray reflectivity is a non-destructive testing technique used to investigate the structure of surfaces, thin films (multi-layers), or buried interfaces (depth profiling), and for studying the processes occurring at surfaces and interfaces such as adsorption, adhesion and interdiffusion. Classical X-ray reflectivity is a relatively slow technique, with a typical time for one scan on the order of hours. Recently, a new experimental setup for quick x-ray reflectivity was proposed, which is based on parallel recording of the x-ray reflectivity curve over all angles of interest. The new setup for quick x-ray reflectometry will allow measurements to be done within seconds, thus permitting studies of the time evolution of chemical, thermal, and mechanical changes at the surface and interface of different materials.

Optically Reconfigurable Monolayer of Azobenzene Donor Molecules on Oxide Surfaces

The structural configuration of molecules assembled at organic-inorganic interfaces within electronic materials strongly influences the functional electronic and vibrational properties relevant to applications ranging from energy storage to photovoltaics. Controlling and characterizing the structural state of an interface and its evolution under external stimuli is crucial both for the fundamental understanding of the factors influenced by molecular structure and for the development of methods for material synthesis. It has been challenging to create complete molecular monolayers that exhibit external reversible control of the structure and electronic configuration. We report a monolayer/inorganic interface consisting of an organic monolayer assembled on an oxide surface, exhibiting structural and electronic reconfiguration under ultraviolet illumination. The molecular monolayer is linked to the surface through a carboxylate link, with the backbone bearing an azobenzene functional group and the head group consisting of a rhenium-bipyridine group. Optical spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and X-ray reflectivity show that closely packed monolayers are formed from these molecules via the Langmuir-Blodgett technique. Reversible photoisomerization is observed in solution and in monolayers assembled on Si and quartz substrates. The reconfiguration of these monolayers provides additional means to control excitation and charge transfer processes that are important in applications in catalysis, molecular electronics, and solar energy conversion.

Micro-imaging of buried layers and interfaces in ultrathin films by X-ray reflectivity

X-ray nreflectivity is a promising technique for characterizing buried layers and interfaces in ultrathin films because of its ability to probe the electron density profile along the depth in a non-destructive manner. While routine X-ray nreflectivity assumes the in-plane uniformity of the sample to be measured, it is also quite important to see buried inhomogeneous/patterned layers and interfaces. The present paper describes the addition of spatial resolution and imaging capability to an X-ray nreflectivity technique to visualize surfaces and buried interfaces. To visualize quite wide viewing area size quickly, the image reconstruction scheme has been employed instead of the scanning of microbeam. Though the mathematics is quite close to X-ray computer tomography, the technique gives the image contrast caused by the difference in reflectivity at each in-plane point in the thin film sample. By choosing a grazing angle, the image gives inhomogeneity of X-ray nreflectivity at the specific wavevector transfer. With a collimated monochromatic synchrotron X-ray beam of 0.05u2009mm (H)u2009×u20098u2009mm (V), the intensity profiles of X-ray reflection projections have been taken at many different in-plane rotation angles, from 0° to 180°. We have succeeded in visualizing buried layers and interfaces of the 8u2009mm dia area with the spatial resolution of better than 20 μm. Because of the brilliance of synchrotron radiation, the typical measuring time is shorter than 1u2009min. Three analytical cases have been discussed: (i) imaging of a buried layer and an interface covered by a protection layer, (ii) distinguishing different local parts of different thicknesses in an ultrathin film, and (iii) selective imaging of a specific metal in the thin film form.

Interface-sensitive imaging by an image reconstruction aided X-ray reflectivity technique

This article describes interface-sensitive imaging of heterogeneous thin films by an image reconstruction aided X-ray reflectivity technique with an 8u2005mm-wide parallel beam; the possibility of extracting micro-X-ray reflectivity profiles from the same data collection is discussed.

Formation of Ultrathin Liesegang Patterns.

For many years, it has been believed that self-organized periodic ring structures known by the name of Liesegang patterns (LPs) are formed only in quite thick media, typically thicker than at least several micrometers. Actually growing LPs in ultrathin films is extremely difficult because of the drying of film and susceptibility to rapid capillary wetting. The present work reports how we obtain successful LPs in ultrathin films of 65 nm thick. The key parameters are temperature control and the introduction of equilibrium water vapor in the sample environment. Atomic force microscope images clearly showed that the LPs are composed of 300-600 nm laterally coagulated particles. We have also evaluated the densities and thicknesses of the ultrathin films by X-ray reflectivity. During the present research, new patterns, which are different from ordinary LPs, have been discovered for the first time in the outermost part of the whole pattern. Studying LPs in ultrathin films may help to forge a better understanding of the mechanism underlying the intriguing phenomenon. Because of nanoscale scale thicknesses, self-organized periodic structures including so-called LPs will open up new opportunities in nanotechnologies.

Optimization of the design of a multilayer X-ray mirror for Cu-Kα energy

A method for optimizing the design of a multilayer X-ray mirror to obtain a high X-ray reflectivity at a specific angle and for a specific energy, based on quarter-wave layer thickness, is described. The quarter-wave design method is widely used for designing optical multilayers, and there is extensive experience in applying this method, which can be useful when designing multilayer X-ray mirrors. The purpose of this paper is to investigate if the quarter-wave design method can be adapted to designing X-ray multilayers. The method is demonstrated for the case of reflectivity from a multilayer structure, of the Cu Kα line (8.04 keV) at 5.5°. Theoretical reflectivity higher than 50% can be achieved with the proposed design by using 500 layers.

Uniaxial negative thermal expansion of polyvinyl acetate thin film

The present paper reports some experimental observations of reproducible uniaxial negative thermal expansion (u-NTE) in an amorphous polyvinyl acetate (PVAc) ultrathin film. It has been found that the mechanism of the phenomena is different from latest reports on so-called NTE in crystal or other topological materials. It is known that PVAc exhibits glass transition at around 31 °C. During cooling from the high-temperature side, one can observe the decrease of the thickness by monitoring interference fringes in the X-ray reflectivity curve as a function of temperature. Across the glass transition, however, the thickness starts to increase, instead of reducing. In the heating process, the thickness decreases as long as the temperature is lower than that for glass transition ( Tg). In the present research, such changes in thickness during repeated heating/cooling cycles have been studied systematically. To discuss the mechanism, dependence on film thickness has been investigated as well. It has been found that the present phenomena are well explained as u-NTE, which induces reduction and increase of thickness ( z-direction) just by thermal expansion and shrinking in x- y directions, respectively. This would be caused and enhanced by the growth of a mechanically hard, high-density layer near the interface to the surface of hydrophilic silicon dioxide. The structural change during heating/cooling cycles is discussed in detail.