Kim et al. Nanoscale Research Letters 2011, 6:405 http://www.nanoscalereslett.com/content/6/1/405 Page 3 of 8 (a) pristine (b) HT-110 (c) HT-130 (e) HT-160 50 nm 50 nm 2 m 2 m 50 nm (d) HT-150 5 m 5 m 50 nm (f) HT-180 50 nm 5 m 5 m Figure 1 SEM images. (a) Unfiltered pristine and (b) HT-110, (c) HT-130, (d) HT-150, (e) HT-160, and (f) HT-180 rubrene NPs. Inset of Figure 1a: Schematic chemical structure of rubrene molecule. Insets of Figure 1b, d, e, and f: HR-TEM images of corresponding HT rubrene NPs. observed at 556 nm, which has been also observed other NPs [32-34]. The weak PL peaks of the pristine rubrene NPs were observed at 464 and 516 nm, resulting from the PL peaks of tetracene monomers in the rubrene molecules (inset of Figure 1a) [35]. These PL peaks at 464 and 516 nm were only observed for the NP struc- ture, not detected for bulk rubrene crystals or thin films. The PL characteristics and their relative intensities of HT-110 rubrene NPs were similar to the pristine sam- ple. As hydrothermal temperatures increased, the rela- tive dominance of the PL peaks at 464 and 516 nm gradually increased and broadened for HT-140, HT-150, and HT-160 rubrene NPs, as shown in Figure 2b. The main PL peak at 556 nm for pristine rubrene NPs was blue shifted to 563, 560, and 557 nm for the HT-140, HT-150, and HT-160 samples, respectively. For the HT- 170 NPs, the PL peak at 560 nm decreased, while that at 464 nm to approximately 516 nm was considerably enhanced (Figure 2b). The dominant PL peak of the HT-170 rubrene NPs was observed at 464 nm to approximately 516 nm. Eventually, for the HT-180 rubrene NPs, the PL peak at 556 nm disappeared and the broad main PL peak was observed at 487 nm, as shown in Figure 2b. We infer that PL characteristics of rubrene NPs are related to size distributions that can be controlled by hydrothermal treatment temperature. The characteristic crystalline peaks of rubrene were not observed for the pristine and HT rubrene NPs under X- ray diffraction (not shown here) patterns, indicating the amorphous phase of all rubrene NPs studied here. The results of the PL spectra of the unfiltered rubrene NPs suggest the tuning of luminescence color through the hydrothermal process. Filtered rubrene NPs Figure 3a, b, c, d shows SEM images of the centrifugally filtered pristine and HT rubrene NPs. Filtered-up rubrene NPs have varying diameters depending on hydrothermal temperatures. The average diameters of the filtered-up and filtered-down pristine rubrene NPs were about 170 and 120 nm, respectively. Filtered-down rubrene NPs had homogeneous size distributions. Dia- meters of the filtered rubrene NPs after the hydrother- mal treatment were precisely measured by DLS