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Tuning photoluminescence of organic rubrene nanoparticles through a hydrothermal process.

Kim MS, Cho EH, Park DH, Jung H, Bang J, Joo J - Nanoscale Res Lett (2011)

Bottom Line: The light-emitting color distribution of the NPs was confirmed using confocal laser spectrum microscope.Filtered-up rubrene NPs treated at 170°C and 180°C exhibited blue luminescence due to the decrease of intermolecular excimer densities with the rapid increase in size.Variations in PL of hydrothermally treated rubrene NPs resulted from different size distributions of the NPs.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Physics, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Korea. jjoo@korea.ac.kr.

ABSTRACT
Light-emitting 5,6,11,12-tetraphenylnaphthacene (rubrene) nanoparticles (NPs) prepared by a reprecipitation method were treated hydrothermally. The diameters of hydrothermally treated rubrene NPs were changed from 100 nm to 2 μm, depending on hydrothermal temperature. Photoluminescence (PL) characteristics of rubrene NPs varied with hydrothermal temperatures. Luminescence of pristine rubrene NPs was yellow-orange, and it changed to blue as the hydrothermal temperature increased to 180°C. The light-emitting color distribution of the NPs was confirmed using confocal laser spectrum microscope. As the hydrothermal temperature increased from 110°C to 160°C, the blue light emission at 464 to approximately 516 nm from filtered-down NPs was enhanced by H-type aggregation. Filtered-up rubrene NPs treated at 170°C and 180°C exhibited blue luminescence due to the decrease of intermolecular excimer densities with the rapid increase in size. Variations in PL of hydrothermally treated rubrene NPs resulted from different size distributions of the NPs.

No MeSH data available.


PL spectra. The filtered-up and filtered-down (a) pristine rubrene NPs and (b) HT-110, (c) HT-130, (d) HT-150, (e) HT-160, and (f) HT-180 rubrene NPs. Insets: Photographs of light emission for the corresponding rubrene NPs.
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Figure 4: PL spectra. The filtered-up and filtered-down (a) pristine rubrene NPs and (b) HT-110, (c) HT-130, (d) HT-150, (e) HT-160, and (f) HT-180 rubrene NPs. Insets: Photographs of light emission for the corresponding rubrene NPs.

Mentions: PL spectra of the centrifugally filtered rubrene NPs are shown in Figure 4. The insets of Figure 4 shows photographs of light emission from the filtered rubrene NPs. For the pristine NPs, the main PL peaks of both filtered-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 aggregation, representing a face-to-face packing (π-π stacking) molecular arrangement, induces blue fluorescence emission as a result of enhanced intermolecular interactions [38,39]. The degree of condensation and intermolecular interaction of rubrene molecules increased with increasing hydrothermal temperature, because external high pressure was applied to the NPs during the hydrothermal process. This process leads to generate new optical absorption band at approximately 399 nm supported 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 backbone 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 hydrothermal temperature for the filtered-up rubrene NPs, the diameters were increased, and the density of excimers due to the molecular packing was reduced, resulting in a decrease in the main PL peak at the 560-nm wavelength. Therefore, for the HT-180 rubrene NPs, the PL peak at 487 nm due to the filtered-up samples has been dominated, as shown in Figure 4f.


Tuning photoluminescence of organic rubrene nanoparticles through a hydrothermal process.

Kim MS, Cho EH, Park DH, Jung H, Bang J, Joo J - Nanoscale Res Lett (2011)

PL spectra. The filtered-up and filtered-down (a) pristine rubrene NPs and (b) HT-110, (c) HT-130, (d) HT-150, (e) HT-160, and (f) HT-180 rubrene NPs. Insets: Photographs of light emission for the corresponding rubrene NPs.
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Figure 4: PL spectra. The filtered-up and filtered-down (a) pristine rubrene NPs and (b) HT-110, (c) HT-130, (d) HT-150, (e) HT-160, and (f) HT-180 rubrene NPs. Insets: Photographs of light emission for the corresponding rubrene NPs.
Mentions: PL spectra of the centrifugally filtered rubrene NPs are shown in Figure 4. The insets of Figure 4 shows photographs of light emission from the filtered rubrene NPs. For the pristine NPs, the main PL peaks of both filtered-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 aggregation, representing a face-to-face packing (π-π stacking) molecular arrangement, induces blue fluorescence emission as a result of enhanced intermolecular interactions [38,39]. The degree of condensation and intermolecular interaction of rubrene molecules increased with increasing hydrothermal temperature, because external high pressure was applied to the NPs during the hydrothermal process. This process leads to generate new optical absorption band at approximately 399 nm supported 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 backbone 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 hydrothermal temperature for the filtered-up rubrene NPs, the diameters were increased, and the density of excimers due to the molecular packing was reduced, resulting in a decrease in the main PL peak at the 560-nm wavelength. Therefore, for the HT-180 rubrene NPs, the PL peak at 487 nm due to the filtered-up samples has been dominated, as shown in Figure 4f.

Bottom Line: The light-emitting color distribution of the NPs was confirmed using confocal laser spectrum microscope.Filtered-up rubrene NPs treated at 170°C and 180°C exhibited blue luminescence due to the decrease of intermolecular excimer densities with the rapid increase in size.Variations in PL of hydrothermally treated rubrene NPs resulted from different size distributions of the NPs.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Physics, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Korea. jjoo@korea.ac.kr.

ABSTRACT
Light-emitting 5,6,11,12-tetraphenylnaphthacene (rubrene) nanoparticles (NPs) prepared by a reprecipitation method were treated hydrothermally. The diameters of hydrothermally treated rubrene NPs were changed from 100 nm to 2 μm, depending on hydrothermal temperature. Photoluminescence (PL) characteristics of rubrene NPs varied with hydrothermal temperatures. Luminescence of pristine rubrene NPs was yellow-orange, and it changed to blue as the hydrothermal temperature increased to 180°C. The light-emitting color distribution of the NPs was confirmed using confocal laser spectrum microscope. As the hydrothermal temperature increased from 110°C to 160°C, the blue light emission at 464 to approximately 516 nm from filtered-down NPs was enhanced by H-type aggregation. Filtered-up rubrene NPs treated at 170°C and 180°C exhibited blue luminescence due to the decrease of intermolecular excimer densities with the rapid increase in size. Variations in PL of hydrothermally treated rubrene NPs resulted from different size distributions of the NPs.

No MeSH data available.