Kim et al. Nanoscale Research Letters 2011, 6:405 http://www.nanoscalereslett.com/content/6/1/405 approximately 516 nm from filtered-down NPs was enhanced by H-type aggregation, which was supported by the optical absorption spectra. Filtered-up rubrene NPs treated at 170°C and 180°C exhibited blue lumines- cence due to the decrease of intermolecular excimer densities with the rapid increase in size. Experiment section Pristine rubrene NPs were prepared by a conventional reprecipitation method [5]. Rubrene powder was pur- chased from Sigma-Aldrich Co. and used without further purification. The pristine rubrene NPs were trea- ted hydrothermally for 10 h using a hydrothermal auto- clave (Parr Instrument Acid Digestion Bombs, 4744 General Purpose Bomb, Parr Instrument Company, Moline, IL, USA). Hydrothermal treatment occurred at 110°C, 130°C, 140°C, 150°C, 160°C, 170°C, and 180°C with samples denoted HT-110, HT-130, HT-140, HT- 150, HT-160, HT-170, and HT-180, respectively. During the hydrothermal process, external pressure was applied to the rubrene NPs. After the hydrothermal treatment, the hydrothermal chamber was slowly cooled at room temperature (RT). Pristine and HT rubrene NPs were centrifugally filtered (low-binding durapore PVDF mem- brane, Millipore Corporation, Billerica, MA, USA), with a membrane pore size of approximately 220 nm. After filtration at 5,000 rpm for 2 min, the NPs were depos- ited in the upper and lower parts of the filter device. Fil- tered-down NPs were obtained directly from the lower part of the filter. For the filtered-up NPs, 1 ml of dis- tilled water was dropped onto the upper part of the device, and then the NP solution was sonicated for 5 min. The rubrene NPs were dried on a glass substrate in a vacuum oven for 2 h at RT. Formation of rubrene NPs was investigated using a field-emission scanning electron microscope (SEM; JEOL KSM-5200, JEOL Ltd., Tokyo, Japan) and a high- resolution transmission electron microscope (HR-TEM; JEOL JEM-3010, JEOL Ltd., Tokyo, Japan). Size distribu- tions of the rubrene NPs, which were homogeneously dispersed in distilled water, were measured by dynamic light scattering (DLS; BI-200SM, Brookhaven Instru- ments Co., Holtsville, NY, USA). For the optical proper- ties of the rubrene NPs, ultraviolet and visible absorption (UV/vis; Agilent HP-8453 UV/vis absorption spectrophotometer, Agilent Technologies, Santa Clara, CA, USA) and PL spectra (Hitachi F-7000, Hitachi High-Technologies Co., Tokyo, Japan) in solution were measured at RT. The confocal laser spectrum micro- scope (CLSM, LSM 5 Exciter, Carl-Zeiss, Göttingen, Germany) was used to investigate the red (R), green (G), and blue (B) color distribution of luminescence. Page 2 of 8 Results and discussion Unfiltered rubrene NPs Figure 1a, b, c, d, e, f and their insets show the SEM and HR-TEM images of the unfiltered pristine and HT rubrene NPs, respectively. Pristine NPs were spherical with diameters of 100 nm to approximately 200 nm (Figure 1a). The diameters of HT-110 rubrene NPs were 100 nm to approximately 250 nm (Figure 1b), and some had a nanohole of ≤20 nm on the surface. The inset of Figure 1b shows an HR-TEM image of HT-110 rubrene NPs with nanoholes. As shown in Figure 1c, HT-130 rubrene NPs have diameters of 100 nm to approxi- mately 500 nm, some with nanoholes on the surface. We can suggest that the formation of nanoholes on the rubrene NPs might be due to the aggregation of the pristine NPs during the hydrothermal process, in which the empty spaces between the NPs could be existed and induced the nanoholes [30]. Diameters of the HT-150 NPs were 100 nm to approximately 900 nm (Figure 1d). The shapes of HT-160 and HT-180 rubrene NPs were similar to those of HT-150 NPs, and their diameters increased to 100 nm to approximately 900 and 200 nm to approximately 2 μm, respectively (Figure 1e, f). The average diameters of the unfiltered HT rubrene NPs were increased with increasing hydrothermal temperatures. Figure 2a, b shows UV/vis absorption and normalized PL spectra, repectively, of the unfiltered pristine and HT rubrene NPs. The UV/vis absorption peaks of pristine NPs were observed at the 438, 465, 496, and 531 nm, as shown in Figure 2a. In the case of the HT-150 and HT- 155 NPs, the absorption peaks were observed at 435, 463, 500, and 547 nm, and new broad absorption band was appeared at approximately 399 nm (Figure 2a). The absorption peaks at 438 and 465 nm were slightly blue shifted to 435, 463 nm, respectively. The blue-shift of the absorption peaks and new absorption band at approximately 399 nm might be due to the H-aggrega- tion [31], which will be discussed more detail in PL properties of the filtered rubrene NPs. For the HT-160 rubrene NPs, the UV/vis absorption characteristic peaks were disappeared. The inset of Figure 2b is the photographs of light emission for pristine and HT NPs. Luminescence color varied from orange-yellow for pristine rubrene NPs to blue for HT-180 rubrene NPs. For pristine rubrene NPs, PL characteristic peaks were observed at 464, 516, and 556 nm. The main PL peak of bulk rubrene single crys- tals was observed at 570 nm, due to the M-axis polar- ized band of a short tetracene backbone in the rubrene molecules [25]. The main PL peak of the pristine rubrene NPs studied here was slightly blue shifted and