pristine down (d) up HT-180 down Page 5 of 8 5 m 5 m HT-110 HT-150 5 m 5 m 5 m down 5 m down 1 m (e) 1000 ) 800 m (n 600 r e t em400 a i D200 0 5 m Filter-up Filter-down down up Kim et al. Nanoscale Research Letters 2011, 6:405 http://www.nanoscalereslett.com/content/6/1/405 (a) up (b) up (c) up 110 120 130 140 150 160 170 180 Hydrothermal Temperature (oC) Figure 3 SEM images. The filtered-up and filtered-down (a) pristine, (b) HT-110, (c) HT-150, and (d) HT-180 rubrene NPs. (e) Diameters of the filtered-up and filtered-down pristine and HT rubrene NPs as a function of hydrothermal temperature. PL peak at 487 nm due to the filtered-up samples has been dominated, as shown in Figure 4f. The evolution of PL characteristics of rubrene NPs through the hydrothermal process was confirmed by CLSM. Figure 5a-d and 5e-h are CLSM images for fil- tered-up and filtered-down rubrene NPs, respectively. For pristine NPs, the red (R), green (G), and blue (B) luminescence color distributions are 45.08%, 25.86%, and 29.06% for the filtered-up samples and 56.48%, 12.33%, and 31.19% for the filtered-down ones, respec- tively. Red luminescence dominated for both kinds of pristine NPs. These results are qualitatively consistent with PL characteristics shown in Figures 2b, 4a. The dis- tribution of green luminescence for all filtered-up and filtered-down rubrene NPs were 18% to approximately 34% and 15% to approximately 26%, respectively, as shown in Figure 5i. As shown in Figure 5i, the distribu- tions of red and blue luminescence abruptly changed for the HT-160 and HT-170 rubrene NPs, indicating the transition temperature for PL characteristics of HT rubrene NPs is 160°C to approximately 170°C. This transition temperature corresponds to the rapid varia- tion in diameter of HT rubrene NPs, shown in Figure 3e. For filtered-up HT-180 rubrene NPs, blue lumines- cence increased to 78%, while that of red decreased to 18%, as shown in Figure 5i. For filtered-down rubrene NPs, blue luminescence increased from 31% in the pris- tine samples to 85% for the HT-180 ones, while that of red decreased from 56% in the pristine samples to 0% in the HT-180 ones. For both filtered-up and filtered-down HT-180 rubrene NPs, the dominance of blue lumines- cence agreed with the PL properties shown in Figure 4f. Conclusions Pristine rubrene NPs prepared by reprecipitation were hydrothermally treated. The HT rubrene NPs have differ- ent size distributions depending on treatment tempera- ture. The sizes of filtered-down rubrene NPs after the hydrothermal treatment were relatively homogeneous, with a mean diameter of approximately 120 nm. Dia- meters of filtered-up rubrene NPs increased from 350 to 890 nm as hydrothermal temperatures increased from 110°C to 180°C. The PL peaks of the filtered-up and fil- tered-down rubrene NPs, at hydrothermal temperatures from 110°C to 160°C, were observed at 560 nm (yellow- green light emission) and 464 nm to approximately 516 nm (green-blue light emission), respectively. With increasing temperature from 110°C to 160°C, the green- blue light emission became dominant for the filtered- down NPs due to the H-aggregation. From the UV/vis absorption spectra, the HT-150 and HT-155 rubrene NPs have new absorption band at approximately 399 nm,