Photoinduced phase transition phenomena displayed by organic photochromic compounds have attracted a great deal of recent attention for their application to functional materials. Photoinduced ionic crystal-to-ionic liquid phase transition was demonstrated for ionic azobenzene derivatives, which allowed high-density storage of photon energy as conformational energy in the liquid cis-isomer. It was discharged in the course of cis-to-trans thermal isomerization and succeeding crystallization of the trans-isomer. Reversible, photoinduced liquefaction was observed for a nonionic, methoxy derivative of arylazopyrazole as well. Notably, this arylazopyrazole molecule spontaneously forms polar crystals, and its photoinduced phase transition allowed optical switching of second harmonic generation. Besides, macroscopically oriented, large-area thin films of the polar crystal were obtained at the air-water interface by photoillumination of the aqueous cis-isomer.
Nano/microcrystals of fullerenes such as C60 and C70 have attracted significant interest for their unique morphologies and properties. Solvated fullerene crystals with various morphologies called nanowhiskers, nanosheets, nanowires, nanorods, nanoballs, nanocubes etc. have been grown by various solution methods. A liquid-liquid interfacial precipitation (LLIP) method, using a mixture of solvents consisting of a good solvent and a poor solvent, shows great potential in the morphology control of the solvated crystals because of its facileness and versatility. The solvents play a crucial role in the control of the morphology and crystal structure with the physical properties. The subtle variation of the solvent type or solvent ratio can remarkably change the morphological and properties of the products. Recently, we found unique changes in the morphology and mechanical properties such as elastic properties in C60 nanocrystals grown by the LLIP method. In this review, we show the LLIP method and our recent results on the morphology transformation in C60 nanosheets and large elastic deformation in C60 nanowhiskers.
Vacuum deposition is expected to be a promising method for fabricating organometal halide perovskite, that has been attracting a lot of attention as next generation solar cells. The vaporization feature of CH3NH3I is a critical issue, which disturbs the deposition rate control for vacuum deposition. We have developed a laser deposition process to overcome the vaporization issue and the precise deposition-rate control was demonstrated. We also succeeded in the epitaxial growth of CH3NH3PbI3. In addition, precise crystallization mechanisms were analyzed using synchrotron radiation facility (SPring-8). In this paper, overview for the vacuum-deposition based perovskite fabrication was presented with our recent research results.
Metal−organic frameworks (MOFs) / Porous coordination polymers (PCPs) have attracted considerable interest for molecule separation, gas storage, and sensing because of their specific structural properties such as regulated surface areas and large pore volumes. A key aspect of MOF studies is control of MOF construction in terms of geometry, crystallinity, and crystal faces. Recently, integration with MOFs and other materials has been studied for thin-film device applications. This article focuses on the preparation methods of MOF as films especially on surface-mounted MOFs (SURMOFs). The SURMOFs were prepared on self-assembled monolayers (SAMs) in early studies whereas recent studies have been paving the way for MOF growth directly on amorphous polymer surface. The applications of these MOF films were also discussed in terms of optoelectronic applications to show the excellence of MOF films as organic-inorganic hybrid functional materials.
In situ measurement systems combining with synchrotron X-ray source have been developed for characterizations of organic thin film transistor (OTFT). We show an in situ measurement system for investigating the correlations of morphology on the organic semiconductor layers and charge transport properties during vacuum deposition. The coverage of each monolayer was estimated using the intensity of off-specular diffuse scattering and diffraction. We also show a bias-applied HAXPES measurement system for OTFT that makes it possible to observe the electric potential of the semiconductor and upper insulator in OTFT. The electric potential during OTFT operation was studied using this method.