- Nano Express
- Open Access
Tuning photoluminescence of organic rubrene nanoparticles through a hydrothermal process
© Kim et al; licensee Springer. 2011
- Received: 1 February 2011
- Accepted: 1 June 2011
- Published: 1 June 2011
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.
- Hydrothermally Treat
- Hydrothermal Temperature
- Blue Luminescence
- Blue Light Emission
Optical properties of metal nanoparticles (NPs) can be controlled by their size and shape, which have been studied with respect to the surface plasmon band of the metal nanostructures [1–4]. For advanced control of optical properties, metal NPs can be oxidized, incorporate dye, or use polymers for the surface passivation [5–10]. In semiconducting silicon NPs, photoluminescence (PL) characteristics depend on the thickness of the oxidation layer . Organic fluorescence particles have been intensively studied for fundamental research and applications to optoelectronics [12–15]. In organic semiconducting NPs, Nakanishi and coworkers reported that PL characteristics of perylene microcrystals were size dependent [16, 17]. Variations in PL of 1-phenyl-3-((dimethylamino)styryl)-5-((dimethylamino)phenyl)-2-pyrazoline NPs resulted from various size crystals treated with various organic solvents and temperatures .
The π-conjugated 5,6,11,12-tetraphenylnaphthacene (rubrene) crystals showed excellent hole mobility and light-emitting characteristics [19–21]. Therefore, rubrene crystals and nanostructures have been intensively studied for optoelectronics applications [22–24]. Electrical and optical properties of rubrene nanowires have been investigated for field-effect transistors and optical waveguides [25–27]. However, the luminescence characteristics and their tuning of rubrene NPs have not been studied thoroughly. In this study, we introduce a hydrothermal process for control of the PL characteristics of organic rubrene NPs. Hydrothermal processes have been used for crystallization of amorphous materials, fabrication of new materials, and easy tuning of intrinsic properties in aqueous solution [28–30]. For example, bulk MgO was converted to Mg(OH)2 nanoplates with a hydrothermal method involving a heterogeneous reaction in aqueous media above 100°C .
We fabricated pristine rubrene NPs using a simple reprecipitation method. The color of light emission of the rubrene NPs changed from yellow-orange to blue with increasing hydrothermal temperatures. The diameters of filtered-up rubrene NPs increased from 350 to 890 nm with increasing hydrothermal temperatures, while those of filtered-down rubrene NPs were almost unchanged at approximately 120 nm. Hydrothermally treated (HT) rubrene NPs have size-dependent PL characteristics. Luminescence color and relative dominance of PL peaks at 464 nm to approximately 516 and 560 nm varied, depending on the hydrothermal temperature. 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, which was supported by the optical absorption spectra. 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.
Pristine rubrene NPs were prepared by a conventional reprecipitation method . Rubrene powder was purchased from Sigma-Aldrich Co. and used without further purification. The pristine rubrene NPs were treated hydrothermally for 10 h using a hydrothermal autoclave (Parr Instrument Acid Digestion Bombs, 4744 General Purpose Bomb, Parr Instrument Company, Moline, IL, USA). Hydrothermal treatment occurred at 110°C, 130°C, 140°C, 150°C, 160°C, 170°C, and 180°C with samples denoted HT-110, HT-130, HT-140, HT-150, HT-160, HT-170, and HT-180, respectively. During the hydrothermal process, external pressure was applied to the rubrene NPs. After the hydrothermal treatment, the hydrothermal chamber was slowly cooled at room temperature (RT). Pristine and HT rubrene NPs were centrifugally filtered (low-binding durapore PVDF membrane, Millipore Corporation, Billerica, MA, USA), with a membrane pore size of approximately 220 nm. After filtration at 5,000 rpm for 2 min, the NPs were deposited in the upper and lower parts of the filter device. Filtered-down NPs were obtained directly from the lower part of the filter. For the filtered-up NPs, 1 ml of distilled water was dropped onto the upper part of the device, and then the NP solution was sonicated for 5 min. The rubrene NPs were dried on a glass substrate in a vacuum oven for 2 h at RT.
Formation of rubrene NPs was investigated using a field-emission scanning electron microscope (SEM; JEOL KSM-5200, JEOL Ltd., Tokyo, Japan) and a high-resolution transmission electron microscope (HR-TEM; JEOL JEM-3010, JEOL Ltd., Tokyo, Japan). Size distributions of the rubrene NPs, which were homogeneously dispersed in distilled water, were measured by dynamic light scattering (DLS; BI-200SM, Brookhaven Instruments Co., Holtsville, NY, USA). For the optical properties of the rubrene NPs, ultraviolet and visible absorption (UV/vis; Agilent HP-8453 UV/vis absorption spectrophotometer, Agilent Technologies, Santa Clara, CA, USA) and PL spectra (Hitachi F-7000, Hitachi High-Technologies Co., Tokyo, Japan) in solution were measured at RT. The confocal laser spectrum microscope (CLSM, LSM 5 Exciter, Carl-Zeiss, Göttingen, Germany) was used to investigate the red (R), green (G), and blue (B) color distribution of luminescence.
Unfiltered rubrene NPs
The inset of Figure 2b is the photographs of light emission for pristine and HT NPs. Luminescence color varied from orange-yellow for pristine rubrene NPs to blue for HT-180 rubrene NPs. For pristine rubrene NPs, PL characteristic peaks were observed at 464, 516, and 556 nm. The main PL peak of bulk rubrene single crystals was observed at 570 nm, due to the M-axis polarized band of a short tetracene backbone in the rubrene molecules . The main PL peak of the pristine rubrene NPs studied here was slightly blue shifted and 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) . These PL peaks at 464 and 516 nm were only observed for the NP structure, 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 sample. As hydrothermal temperatures increased, the relative 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
Pristine rubrene NPs prepared by reprecipitation were hydrothermally treated. The HT rubrene NPs have different size distributions depending on treatment temperature. The sizes of filtered-down rubrene NPs after the hydrothermal treatment were relatively homogeneous, with a mean diameter of approximately 120 nm. Diameters 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 filtered-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, supporting by the formation of H-aggregation. Above 160°C, the filtered-up rubrene NPs exhibited blue luminescence because of the decrease of excimer density with increasing size. Color distributions for the rubrene NPs in the CLSM images qualitatively agreed with PL characteristics. Hydrothermal processing is a promising post-manipulation technique to control PL characteristics of π-conjugated organic nanostructures.
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).
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