Hidetoshi Oikawa

A Novel Preparation Method of Organic Microcrystals

Organic microcrystals ranging from several tens nm to µm in size of several chromophores were successfully prepared by simply dispersing ethanol solutions of compounds into stirred water, i.e. by a reprecipitation method. The size of microcrystals was found to depend on concentration of ethanol solutions, dispersing conditions, temperature and so on.

Size-Dependent Colors and Luminescences of Organic Microcrystals

Perylene microcrystals with different sizes from about 50 nm to about 200 nm were prepared using the reprecipitation method. Their excitonic absorption peaks were found to shift to the high energy side with decreasing crystal size, that is to say, the shifts from that of the bulk crystal were about 500 cm-1 and 1420 cm-1 for 200 nm and 50 nm microcrystals, respectively. Moreover, strong fluorescence from the free-exciton energy level in the microcrystals was observed even at room temperature, which is not usually observed in bulk crystals.

Size Control of Polydiacetylene Microcrystals

Aiming at the preparation of nanometer-size polydiacetylene microcrystals, the crystallization process during reprecipitation method was investigated using electron micrographs and light-scattering technique. We found that an amorphous nanoparticle was formed initially and subsequently crystallized. We conclude that amorphous nanoparticles must be sufficiently small in order to achieve nanometer-size crystals.

Fullerene Fine Crystals with Unique Shapes and Controlled Size

The preparation of fullerene fine crystals with uniform size and shape would permit the control of their specific electronic energy levels and the fabrication of materials with completely new properties. To this end, we have successfully fabricated, for the first time, shape- and size-controlled C60 fine crystals using a reprecipitation method developed in our laboratory. The C60 fine crystals obtained were clearly monodisperse and came in an interesting diversity of shapes such as spherical, rodlike, fibrous, disk, and octahedral. We were able to selectively control these sizes and shapes by simply changing the combination of solvents used and the reprecipitation conditions.

Preparation and Characterization of Poly-Diacetylene Microcrystals

The microcrystals formation process of diacetylene (DCHD) in the reprecipitation method has been investigated by SEM and light scattering. Amorphous DCHD particles were confirmed to be formed first, and then to crystallize. The size control of DCHD monomer microcrystals could be achieved by controlling the size of such amorphous DCHD particles, which depended on the concentration of the injected solution. The solid-state polymerization of DCHD monomer microcrystals was carried out by UV-irradiation, and the size of resulting poly-DCHD microcrystals were 50 nm to 150 nm. The size was unchanged during the solid-state polymerization. By adding SDS in the reprecipitation process, poly-DCHD microcrystal size was further reduced to ca. 15 nm, whose dispersion showed extremely low scattering loss, and expected to be the promising nonlinear optical material.

Multibranched C60 Micro/Nanocrystals Fabricated by Reprecipitation Method

We found that monodispersed C60 micro/nanocrystals (M/NCs) with unique multibranched structures can be fabricated by reprecipitation method, using m-xylene and 2-propanol as good and poor solvents, respectively. The resulting C60 M/NCs had a hexagonal crystal structure and was found to be a kind of crystal solvates in which the molar ratio of C60 to m-xylene was 3:2. C60 M/NCs seem to be important nanoparts in an integrated device.

Fabrication of organic nanocrystals using microwave irradiation and their optical properties

1,1,4,4-Tetraphenyl-1,3-butadiene nanocrysytals were fabricated by the microwave method, whose procedure is the conventional reprecipitation followed by the microwave irradiation. Using this method, monodispersive nanocrystals possessing the crystal structure different from that of bulk crystals were produced. Change in optical properties depending on the crystal size is also reported.

Light propagation within colloidal crystal wire fabricated by a dewetting process.

We present a colloidal crystal wire composed of thousands of connected microspheres that is fabricated by a simple dewetting process utilizing a drain phenomenon, and we directly observe the light propagation within the wire by near-field scanning optical microscopy. The optical properties of propagation light suggest that the propagation mechanism was attributed mainly to nanojet-induced mode coupling for the straight propagation component and partly to whispering-gallery mode coupling within the colloidal crystal wire.

Optical Properties of Perylene Microcrystals

Abstract The optical properties of perylene microcrystals in wide range of crystal size from 50 ran to 1 μm were measured. The interesting behavior of exciton in microcrystals with less than 200nm was found. For example, with decreasing the crystal size, self-trapped exciton states in microcrystals shifted to the high energy side. Therefore, the strong luminescence from free-exciton was observed, and life time of self-trapped exciton became shorter .

Size and Form Control of Titanylphthalocyanine Microcrystals by Supercritical Fluid Crystallization Method

Abstract Supercritical fluid crystallization (SCFC) method is proposed for preparing microcrystals of π-conjugated organic compounds as a development of conventional reprecipitation method. In SCFC method, both size and form of titanylphthalocyanine microcrystals could be controlled by changing experimental conditions such as temperature and type of solvent. We have been able to fabricate γ-form of titanylphthalocyanine microcrystals with 50 nm in size, which is expected to be promising material for organic photoconductors.