Kim et al. Nanoscale Research Letters 2011, 6:405 http://www.nanoscalereslett.com/content/6/1/405 N A N O E X P R E S S Open Access Tuning photoluminescence of organic rubrene nanoparticles through a hydrothermal process Mi Suk Kim1, Eun Hei Cho1, Dong Hyuk Park1,2, Hyunjung Jung2, Joona Bang2 and Jinsoo Joo1* Abstract Light-emitting 5,6,11,12-tetraphenylnaphthacene (rubrene) nanoparticles (NPs) prepared by a reprecipitation method were treated hydrothermally. The diameters of hydrothermally treated rubrene NPs were changed from 100 nm to 2 μm, depending on hydrothermal temperature. Photoluminescence (PL) characteristics of rubrene NPs varied with hydrothermal temperatures. Luminescence of pristine rubrene NPs was yellow-orange, and it changed to blue as the hydrothermal temperature increased to 180°C. The light-emitting color distribution of the NPs was confirmed using confocal laser spectrum microscope. As the hydrothermal temperature increased from 110°C to 160°C, the blue light emission at 464 to approximately 516 nm from filtered-down NPs was enhanced by H-type aggregation. Filtered-up rubrene NPs treated at 170°C and 180°C exhibited blue luminescence due to the decrease of intermolecular excimer densities with the rapid increase in size. Variations in PL of hydrothermally treated rubrene NPs resulted from different size distributions of the NPs. Introduction Optical properties of metal nanoparticles (NPs) can be controlled by their size and shape, which have been stu- died with respect to the surface plasmon band of the metal nanostructures [1-4]. For advanced control of optical properties, metal NPs can be oxidized, incorpo- rate dye, or use polymers for the surface passivation [5-10]. In semiconducting silicon NPs, photolumines- cence (PL) characteristics depend on the thickness of the oxidation layer [11]. Organic fluorescence particles have been intensively studied for fundamental research and applications to optoelectronics [12-15]. In organic semiconducting NPs, Nakanishi and coworkers reported that PL characteristics of perylene microcrystals were size dependent [16,17]. Variations in PL of 1-phenyl-3- ((dimethylamino)styryl)-5-((dimethylamino)phenyl)-2- pyrazoline NPs resulted from various size crystals trea- ted with various organic solvents and temperatures [18]. The π-conjugated 5,6,11,12-tetraphenylnaphthacene (rubrene) crystals showed excellent hole mobility and light-emitting characteristics [19-21]. Therefore, rubrene crystals and nanostructures have been intensively stu- died for optoelectronics applications [22-24]. Electrical * Correspondence: jjoo@korea.ac.kr 1Department of Physics, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Korea Full list of author information is available at the end of the article and optical properties of rubrene nanowires have been investigated for field-effect transistors and optical wave- guides [25-27]. However, the luminescence characteris- tics and their tuning of rubrene NPs have not been studied thoroughly. In this study, we introduce a hydro- thermal process for control of the PL characteristics of organic rubrene NPs. Hydrothermal processes have been used for crystallization of amorphous materials, fabrica- tion of new materials, and easy tuning of intrinsic prop- erties in aqueous solution [28-30]. For example, bulk MgO was converted to Mg(OH)2 nanoplates with a hydrothermal method involving a heterogeneous reac- tion in aqueous media above 100°C [30]. We fabricated pristine rubrene NPs using a simple reprecipitation method. The color of light emission of the rubrene NPs changed from yellow-orange to blue with increasing hydrothermal temperatures. The dia- meters of filtered-up rubrene NPs increased from 350 to 890 nm with increasing hydrothermal temperatures, while those of filtered-down rubrene NPs were almost unchanged at approximately 120 nm. Hydrothermally treated (HT) rubrene NPs have size-dependent PL char- acteristics. Luminescence color and relative dominance of PL peaks at 464 nm to approximately 516 and 560 nm varied, depending on the hydrothermal tem- perature. As the hydrothermal temperature increased from 110°C to 160°C, the blue light emission at 464 to © 2011 Kim et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.