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Bubble cloud cavitation plays an important role in sonoporation, which produces microhollows on the cell membrane using a microjet. However, it has complicated mechanisms and the optimization of the ultrasonic wave irradiation sequence is difficult. The optimization of an ultrasonic wave irradiation sequence, which allows for effective bubble cloud cavitation, is very important for producing high-quality microhollows. Thus, it is necessary to clarify the mechanism of cavitation for different-frequency ultrasonic waves. In this study, we evaluate bubble cloud cavitation for ultrasonic wave irradiations of different frequencies. The observation of bubble cloud dynamics using a high-speed camera and of microhollows using a confocal laser microscope are performed in this study. From these results, a novel ultrasonic wave irradiation sequence that uses two ultrasonic waves with different frequencies is proposed.

Sonoporation, which forms small pores through the cell membrane, is a useful method in ultrasonic-wave-mediated drug delivery systems. However, if microbubbles aggregate by acoustic radiation force and form bubble clouds, the mechanism of sonoporation becomes complicated. In this paper, bubble cloud cavitation is evaluated by simultaneous observation of bubble cloud motion and microhollows, which are produced on a flow channel wall. To observe the bubble cloud motion, three optical methods are adopted. Three-dimensional position measurement of bubble clouds using two cameras with different focal lengths, which is newly developed in this study, shows the three-dimensional motion of bubble clouds during cavitation. Microhollows on the flow channel wall are evaluated by confocal laser microscopy. It is found that bubble cloud cavitation can be classified into three stages. Among them, the first and second stages, which are characterized by both bubble cloud movement in the vicinity of the wall and the formation of large bubble clouds, play important roles in microhollow production.

The crystal quality of pentacene thin films was found to be markedly improved by the use of atomically flat sapphire substrates. In general, the crystallinity of the thin film depends strongly on the surface condition of the substrate or the underlying layer. However, there are few reports on the flatness effect on the underlying layer to organic thin films grown on dielectric materials such as SiO2 and Al2O3, where it is necessary to fabricate organic thin films for the practical configuration of bottom-gated organic field-effect transistors. The surface of heat-treated sapphire shows step structures with atomically flat terraces, and is an ideal model of the surface of dielectric layers. The grain size and the X-ray diffraction intensity of a pentacene film deposited on this atomically flat substrate increases markedly in comparison with the films deposited on as-polished sapphires. In addition, the photoconductivity of pentacene films was also improved using atomically flat sapphire substrates. This result suggests that the surface roughness of the substrate and the underlying layer plays a important role in the crystallinity and electrical property of pentacene thin films.

We have investigated the fabrication of liquid crystal polymer (LCP) thin films by thermal evaporation, and pulsed laser deposition (PLD). Due to the carbonization of LCP, no films were fabricated by thermal evaporation. The laser desorption time-of-flight mass spectra of a LCP target exhibited that the main ablation species were the fragments of the LCP in the case of low laser fluence, indicating the possibility of film fabrication by PLD. Thin films fabricated by PLD using a KrF excimer laser have a pale yellow color and a surface roughness of 1 nm, as measured by atomic force microscopy. The spectra obtained by Fourier transform infrared spectroscopy exhibit a peak corresponding to carbonyl bond at approximately 1730 cm-1, indicating a polyester structure similar to that of the original LCP. The breakdown field of the films was 1 MV/cm. We demonstrate that this film acts as a passivation films for organic field-effect transistors.

We have designed and set up a fabrication and characterization system for organic devices which enables us to assemble all components of devices and to characterize the device properties without breaking the vacuum. Using this system, top and bottom contact C60 field effect transistors (FETs) were fabricated and their performance was characterized. The top contact FET exhibited a mobility as high as 1.4 cm2/(V·s), which was higher than the bottom contact FET.

We report the remarkable effects of physical and chemical treatments of substrate surface on the physical vapor deposition of rubrene thin films. Highly c-axis oriented rubrene thin films were fabricated by combinatorial molecular beam epitaxy on atomically flat α-Al2O3 (0001) substrates, the surface of which was partially modified with pentacene buffer film. Rubrene thin films grown at room temperature on a sapphire substrate without pentacene buffer layer did not exhibit any X-ray diffraction pattern, whereas films deposited on a pentacene buffer layer exhibited peaks of c-axis orientation. Atomic force microscope images of the crystalline films show the steps of 1.3 nm height which correspond to the half c-axis length of the rubrene crystal. Preliminary, p-type operation was observed in bottom-gate field effect transistors using this rubrene film deposited on a pentacene buffer.