Kim et al. Nanoscale Research Letters 2011, 6:405 http://www.nanoscalereslett.com/content/6/1/405 Page 4 of 8 Figure 2 (a) UV/vis absorption and (b) normalized PL spectra of the unfiltered pristine and HT NPs. Inset: Photographs of light emission for the pristine and HT rubrene NPs. experiments, using a syringe filter with pore size of 1 μm for the elimination of dust, as shown in Figure 3e. Filtered-down rubrene NPs had average diameters of 120 nm (± 110 nm), which were almost independent of hydrothermal temperature. Mean diameters of filtered- up rubrene NPs slightly increased from approximately 350 nm to approximately 450 nm as hydrothermal tem- peratures increased from 110°C to 160°C, and those of the filtered-up HT-170 and HT-180 rubrene NPs rapidly increased to 740 and 890 nm, respectively. The rapid increase in mean diameters for the filtered-up HT rubrene NPs above 160°C might correlate with the decrease of the PL peak at 560 nm shown in Figure 2b. PL spectra of the centrifugally filtered rubrene NPs are shown in Figure 4. The insets of Figure 4 shows photo- graphs of light emission from the filtered rubrene NPs. For the pristine NPs, the main PL peaks of both fil- tered-up and filtered-down NPs were at 556 nm with weak PL peaks at 464 and 516 nm, as shown in Figure 4a. For the filtered-up HT-110 rubrene NPs, the main PL peak was at 556 nm with shoulder peaks at 464, 516, and 610 nm. PL intensities of filtered-down HT-110 NPs were much weaker than those of filtered-up NPs, as shown in Figure 4b. For the HT-130, HT-150, and HT-160 rubrene NPs, contributions of the filtered-up and filtered-down NPs to the PL spectra were clearly divided into two wavelength regions, i.e., 464 nm to approximately 516 and 560 nm, as shown in Figure 4c, d, e. Filtered-up HT-130, HT-150, and HT-160 rubrene NPs had yellow luminescence, while the filtered-down samples were blue, as shown in the insets of Figure 4c, d, e. As hydrothermal temperatures increased from 110° C to 160°C, PL peaks at 464 nm to approximately 516 nm became dominant for the filtered-down rubrene NPs, as shown in Figure 4c, d, e. The enhancement of the PL peaks at 464 nm to approximately 516 nm for the filtered-down samples originated from molecular- level aggregation in the nano-size particles. Variation in optical properties of organic NPs has been reported in terms of H-type or J-type aggregation [31,36-39]. J-type aggregation, representing a head-to-tail molecular arrangement, induces red shift in PL by enhancement of fluorescence emission intensities [36,37]. H-type aggre- gation, representing a face-to-face packing (π-π stack- ing) molecular arrangement, induces blue fluorescence emission as a result of enhanced intermolecular interac- tions [38,39]. The degree of condensation and intermo- lecular interaction of rubrene molecules increased with increasing hydrothermal temperature, because external high pressure was applied to the NPs during the hydro- thermal process. This process leads to generate new optical absorption band at approximately 399 nm sup- ported by the UV/vis absorption spectra in Figure 2a, and increase the relative PL intensity at 464 nm to approximately 516 nm, which indicate the formation of H-aggregation [31,38,39]. Therefore, for the filtered- down rubrene NPs, the relative PL intensity at 464 nm to approximately 516 nm caused by the tetracene back- bone monomer in the rubrene molecules increased with increasing hydrothermal temperature, as a result of H- aggregation. In the filtered-up samples, PL peaks at 560 nm decreased as diameters of the HT rubrene NPs increased. The decrease in PL intensities of organic nanostructures at longer wavelengths (≥550 nm) can be interpreted in terms of the decrease of the density of excimers [40,41]. The decrease of specific surface area with increasing particle sizes reduced the density of intermolecular excimers [40]. With increasing hydro- thermal temperature for the filtered-up rubrene NPs, the diameters were increased, and the density of exci- mers due to the molecular packing was reduced, result- ing in a decrease in the main PL peak at the 560-nm wavelength. Therefore, for the HT-180 rubrene NPs, the