Pristine- down UP blue green red Down Kim et al. Nanoscale Research Letters 2011, 6:405 http://www.nanoscalereslett.com/content/6/1/405 Page 7 of 8 (e) Pristine down (f) HT-130 down (g) HT-150 down (h) HT-180 down (j)100 ) 80 % ( n o i t60 u b i r is40 t D B G20 R e v it 0 a l e R (a) Pristine up (b) HT-130 up (c) HT-150 up (d) HT-180 up blue green red (i) 100 ) %80 ( n o i t u60 b i r t s D40 i B G R20 e v i t le a 0 R 0 30 60 90 120 150 180 Hydrothermal Temperature (oC) 0 30 60 90 120 150 180 Hydrothermal Temperature (oC) Figure 5 CLSM images. (a)-(d) CLSM images of the filtered-up pristine and HT rubrene NPs. (e)-(h) CLSM images of the filtered-down pristine and HT rubrene NPs. (i) Color distribution of the filtered-up pristine and HT NPs as a function of hydrothermal temperature. (j) Color distribution of the filtered-down pristine and HT NPs as a function of hydrothermal temperatures. supporting by the formation of H-aggregation. Above 160°C, the filtered-up rubrene NPs exhibited blue lumi- nescence because of the decrease of excimer density with increasing size. Color distributions for the rubrene NPs in the CLSM images qualitatively agreed with PL charac- teristics. Hydrothermal processing is a promising post- manipulation technique to control PL characteristics of π-conjugated organic nanostructures. Acknowledgements This work was supported by a National Research Foundation (NRF) funded by the Korean government (MEST) (No. R0A-2007-000-20053-0 and No. 2009- 89501). Author details 1Department of Physics, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Korea 2Department of Chemical & Biological Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Korea Authors’ contributions MSK fabricated the pristine and HT rubrene NPs and performed the SEM, HR-TEM, and PL experiments. EHC and DHP supported the fabrication of the NPs and PL experiments. HJ and JB performed DLS experiments. JJ analyzed the results. All authors read and approved the final manuscript. Competing interests The authors declare that they have no competing interests. Received: 1 February 2011 Accepted: 1 June 2011 Published: 1 June 2011