Photoresponsive LB Films under Light Irradiation Observed by Time-Resolved XRR

Langmuir-Blodgett (LB) films containing azobenzene undergo reversible structural change under light irradiation because of trans-to-cis or cis-to-trans transformations of azobenzene molecules. Such films are a candidate for molecular machines. Time resolved measurements of specular X-ray reflectivity (XRR) curves were carried out for polymer specimens of azobenzene-containing polyvinil alcohol (6Az10-PVA) monolayer LB films on quartz substrates during light (365 nm or 436 nm) irradiation (1 mW/cm2). Measurements were performed with a time-resolution of 10 s using an X-ray reflectometer [1, 2], which can simultaneously measure the whole XRR curves with no need to rotate the specimen, detector or monochromator crystal. Profiles of XRR curves changed as a function of the elapsed time after initiation of the light irradiation reflecting the structural change of the film. Despite of a common belief that the photo-induced structural change occurs directly between the initial and final states, we found an evidence of the existence of the intermediate third structure. We also found that the time needed for changes in XRR curves was several times longer than for optical absorption spectroscopy (OAS) spectra reported with the same irradiation power. Details of such changes of XRR curves and structures of the film will be discussed and compared with the changes of OAS spectra. An XRR curve for the intermediate state of the 6Az10-PVA monolayer LB film specimen separated from the XRR curves measured under 365 nm light irradiation is shown in the figure together with curves for the initial and final states of the same specimen.

A quick laboratory X-ray reflectometer for time-resolved observations

The measurement of the X-ray reflectivity curve is a widely used method to obtain structural information about thin films, surfaces and interfaces. With conventional instruments, the reflectivity curve is measured sequentially for a range of incident angles, which takes a long time. A recently developed method using white synchrotron radiation can measure the whole curve at once [1, 2]. In this contribution, the adaption of this method to a laboratory characteristic X-ray source is presented. This will make it possible to do time-resolved or high-throughput measurements using standard laboratory sources. The basic idea of our method is to focus the divergent X-ray beam emitted from a point source with either a doubly-curved Si crystal monochromator or a bent-twisted Si crystal monochromator [1]. Instead of using the whole beam, however, only the fan-shaped beam from a diagonal line on the monochromator is focused onto the sample. This is realized by placing an inclined slit before the monochoromator. The beam reflected from the sample forms a line on a two-dimensional pixel array detector. For each horizontal position on the detector, the incident angle onto the sample, and therefore the momentum transfer, is different. The reflectivity curve for a range of momentum transfers can therefore be measured with a single detector exposure without moving the sample, monochromator or detector. Reflectivity curves from a silicon wafer sample measured by our method are compared with the conventional angle scan method in the figure. The reflectivity down to 10 to the -7th power can be obtained, because the background can be subtracted from the measured intensity. We will show an example of time-resolved (10 s) measurements of specular X-ray reflectivity curves. We will also discuss the momentum transfer range that can be measured simultaneously and factors limiting the resolution of the method.

Structural change of the TiO₂(110) surface in the hydrophilic reaction

Photocatalysis of titanium dioxides has been extensively studied in the past. Especially, after the discovery of the UV-light induced hydrophobic-hydrophilic transition of the rutile-TiO2(110) surface in the late of 1990's [1], the number of photochemistry-related publications increased dramatically over the last decade to the extent that ~2400 related papers were published in 2008, in which ~80% of the papers involve the TiO2-related materials. The remarkable research activity arises from the potential applications of the photo-induced wettability control such as anti-fog coatings or self-cleaning coatings. However, despite the intensive study, the mechanism of the hydrophilic reaction is not completely clarified yet, mainly due to the lack of the detailed information of the atomic-scale surface structure. We have studied the surface structural change by means of surface X-ray diffraction. By using the recently developed time-resolved x-ray crystal truncation rod (CTR) scattering measurement [2] and the static measurement for the hydrophobic and hydrophilic surfaces, we confirmed that (i) the surface roughness increases during the reaction probably due to the desorption of the surface oxygen atoms and (ii) an ordered water molecular layer formed on the hydrophobic surface disappears in the hydrophilic surface. Considering the previous reports which show the increase of hydrogen bond density in the hydrophilic surface, we suggest that the ill-ordered surface of the hydrophilic phase allows a larger number of water molecules to adsorb by making a hydrogen-bond network.

Gemstones and Salts as Light Emitters for Learning X-ray Detectors

The scintillation counter is a widely-used X-ray detector. It contains a scintillator as a luminescent material that converts X-rays into visible light, which is detected with a sensor. A well-known scintillator in the X-ray region is sodium iodide, NaI, an ionic crystal. Before use, it is important to understand how the detector works. For students, the material name and the chemical formula of the scintillator are not familiar, however. In addition, students cannot watch or touch the key element in the detector, because the scintillator is installed inside the housing. Many jewels emit visible light or change their colors under ultraviolet light irradiation. Under X-ray irradiation, the same jewels exhibit similar responses as well. If popular jewels instead of special ionic crystals were used as scintillators, students might show interest in these materials. We propose that photographs of beautiful, brightly shining gemstones and salts could be used as visual educational materials for students to learn the principles of X-ray detectors. Different gemstones and salts were irradiated by intense white synchrotron X-ray radiation at beamline NE7A1 of the PF-AR synchrotron radiation facility at KEK, Japan. Photographs of fluorescence and phosphorescence from the gemstones, and of color changes due to the irradiation, were taken with a remote controlled digital camera. It should be noted that the experimental setup of this study is an easily understood handmade X-ray detector. We will present photographs of exciting gemstones such as Fluorite from the US, Hackmanite from Afghanistan, Mangano Calcite from China, Ruby from Brazil, Selenite from Canada, and Black Opal from Australia. We also irradiated different kinds of colored Himalayan Rock Salt from India or Pakistan, shown in Fig. 1. We will explain basic concepts of X-ray detectors, such as photon counting, dead time, recording, and quantum efficiency, with these photographs.