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Formation of Nanoparticles of Organic Molecules by Liquid Laser Ablation Ikuko Akimoto1, Masahiro Ohata1, Nobuhiko Ozaki1, and Gu Ping2 1 Faculty of Systems Engineering, Wakayama University, Sakaedani 930, Wakayama 640-8510, Japan 2 Faculty of Education, Wakayama University, Sakaedani 930, Wakayama 640-8510, Japan ABSTRACT We carried out laser ablation of three organic molecules, rubrene (Rb), Oralith Brilliant Pink R (BP) and quinacridonequinone (QQ) in a poor solvent, water. As a result, nanoparticles of BP and QQ were formed, but those of Rb were not formed because of photodissociation. For a rigid molecule, QQ, optical properties of colloidal solutions were investigated in relation to the size of the included nanoparticles. A linear correlation between the blue shift of the absorption peak energy and the decrease in the diameter of the nanoparticles was found, indicating that the nanoparticle diameter can be easily estimated from the absorption spectrum of a colloidal solution. From the solution, a nanoparticle film was fabricated on an electrode by the electrophoretic deposition method. INTRODUCTION Organic materials have been utilized in the production of optical and electronic devices in recent years [1]. The fabrication of devices such as organic light-emitting diodes (OLED) and organic field-effect transistors (OFET) requires high-cost vacuum sublimation processes for small organic molecules. If aqueous solutions of simple small organic molecules could be obtained without the addition of hydrophilic moieties on the molecule or any surfactants, various new applications of organic molecules would be expected: for example, wet-process device fabrication by application of an ink-print technique, a drag delivery system, and cosmetic application. A promising technique for the preparation of dispersions of small organic molecules involves size modification from microcrystals to nanoparticles. As one break-down method to form organic nanoparticles, laser ablation in solvents has been developed by Masuhara and co-workers [2–13]. Formation of stable colloidal solutions consisting of organic nanoparticles has been demonstrated by several groups for various organic systems [2–15]. However, there are few reports on the use of the colloidal solutions, and the characteristic optical properties of such solutions have not been clarified. In this study, we investigated the laser ablation method for three organic molecules, rubrene (Rb), Oralith Brilliant Pink R (BP), and quinacridonequinone (QQ), in a poor solvent, water. Rb is promising as a fluorescent and hole-transfer material, while BP and QQ are coloring pigments that are used for their tolerance to light exposure. The aims of our research were to seek organic materials that were suitable for laser ablation treatment and to find an easy way to fabricate organic devices by the wet process. With a series of colloidal solutions of yellow pigment QQ, optical properties were investigated in relation to the size of the included nanoparticles. Moreover, the possibility of dev