Preparation of SiO₂/dye luminescent nanoparticles and their application in light-converting films

Abstract

Highly luminescent and photostable dye-h-silica (dye-hybrid-silica) nanoparticles (NPs) were prepared using a base-catalyzed reverse microemulsion method. We obtained dye-h-silica nanoparticles with ∼65 nm radius, a narrow particle size distribution, high fluorescence intensity, and a controlled internal architecture. The luminescence intensity of the dye-h-SiO₂ is 2.4 times higher than that of the pure dye in aqueous solution. The incorporation of dye molecules into the silica NPs protects the dye from the surrounding environment, as the silica layer is a good shield against oxygen and moisture, so that the dye-h-silica monoliths (SMs) maintain their initial luminescence after the encapsulation curing process. Green and red light-emitting dye-h-SMs are successfully applied to the preparation of remote trigger LEDs.

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1 Introduction

In response to the ever-increasing energy demands coupled with the serious concern for global warming, there has been a great deal of interest in new light sources that can save on the electrical energy consumption. Solid-state light emitting diodes (LEDs) are thus receiving great attention as energy-saving light sources and are intensely explored in this context.¹ Phosphor-converted LEDs are highly promising because they possess high luminescence efficiency with only a single LED chip.² Quantum dots (QDs) have attracted great interest in this area because of their broad excitation spectra and tunable emission wavelengths. However, QDs are relatively difficult to prepare in a reproducible manner, and their surface modification chemistry is still under investigation. Organic dyes have advantages of various sorts and abundant fluorescent colours. However, organic dyes have a relatively bad stability and the hybridized NPs fall short of abundant colour combinations, which limits their use in white LEDs. Commercially available fluorescent polymer or latex NPs and microspheres have been utilized in various chemical applications. Recently, Wang reported the usage of non-blinking, bright, and photo-stable silica NPs and dye-h-silica (dye-hybrid-silica) NPs for multiplexed signaling.³ Compared to polymer NPs, dye-h-silica NPs possess several advantages: (1) silica NPs provide a small enough cell, in which the dye molecules show weak scattering, a more effective energy transfer and high quantum efficiency. (2) The incorporation of dye molecules into the silica NPs protects the dye from the surrounding environment. (3) The silica NPs present excellent dispersion in the matrix. (4) The matrix made by the dye-h-silica NPs presents high transparency, because the dye-h-silica NPs show excellent dispersion in the matrix and the small cell of SiO₂ NPs benefits the refraction of the dye. (5) To enhance the mechanical performance of the composites, the silica NPs require a small size which could avoid sedimentation effects and phase separation.⁴ In our article, the passivated dyes with new multi-layered structures show high luminescence intensity and improved stability in a solid-state device. In the dye-doped silica NP system, two types of organic dyes, Ru(bpy)₃²⁺ (tris(2,2′-bipyridyl)dichlororuthenium(II) chloride hexahydrate) and FITC (fluorescein isothiocyanate), were encapsulated into silica NPs using the reverse microemulsion system. Dye-h-silica NPs optically functionalized in this manner may thus be utilized for many potential applications, e.g. in fiber amplification and lasing, in safety marking (both emergency and anti-counterfeit), as down converters in light emitting diodes (LEDs), as light-converting films in remote-phosphor LEDs and last but not least for decorative or advertising purposes.

Different to the dye-h-silica mentioned above, a traditional dye-silica composite with moderate emission efficiency was prepared using phenyltriethoxysilane (PTES) which can produce both hydrophobic and hydrophilic sites inside the silica matrix.⁵ Thereby, organic dye could be found inside the silica matrix by adjusting the amount of PTES. A high quantity of PTES was found to make the particles hydrophobic and caused difficulties in the surface modification by dispersion, and the particles formed using this approach have a broad size distribution. There are some problems when using the Stöber process, for example, (1) the diameters of the SiO₂ nanoparticles are nonuniform, (2) the SiO₂ nanoparticles showed severely reduced photoluminescence (PL), (3) serious dye leakage occurs in aqueous solution.

A reverse microemulsion method, taking place between the water and the oil phase, has been applied to prepare dye-h-silica. Several encapsulating methods have been reported for nanoparticles.^6–13 In the following, we present in detail methods for preparing homogeneously doped dye-h-SMs, while retaining the original PL efficiency, and show their application for LEDs. Currently, remote-phosphor LEDs are receiving a great deal of attention and their commercialization is expanding to high-volume applications, including general lighting and displays. Especially, compared to traditional white LEDs, remote-phosphor LEDs have many advantages in terms of high efficiency and a high-temperature environment with a more stable color point system. However, remote-phosphor LEDs need to be more cost-effective and more color-uniform. At present, there are several combinations for remote trigger LEDs.

The mixed light from a blue InGaN LED with a yellow phosphor (cerium-doped yttrium aluminum garnet: YGA) makes a cool remote-phosphor LEDs.¹⁴ This is the most efficient and economical structure found so far, while the pale white light from the blue and yellow hues cannot express the natural colors of an object faithfully in general circumstances. However, until now, few red phosphors can be excited by low energy wavelengths (blue light), and RG dye-h-silicas display a much enhanced luminescence intensity.

2 Experimental

2.1 Chemicals

All chemicals were used as received without further purification. Trition X-100 (99.99%) was purchased from Acros. Hexanol (C₅H₁₁CH₂OH) (99.99%), cyclohexane (C₆H₁₂) (99.99%), ammonium hydroxide (NH₃·H₂O) (25%), tetraethylorthosilicate (CH₃CH₂OSi(OCH₂CH₃)₃) (99.99%), acetone (CH₃COCH₃) (99.99%), ethanol (CH₃CH₂OH) (99.99%), epoxy (EP-400A and B) (99.99%), Ru(bpy)₃²⁺ and FITC were purchased from Aladin Reagents Corporation. Blue InGaN LEDs with a peak emission at 460 nm were purchased from Seoul Semiconductor Inc. Silicone (EG6301) was purchased from Dow Corning.

2.2 Synthesis of dye-h-SiO₂

For the preparation of the silica nanoparticles using reverse microemulsion with Triton X-100 as the surfactant, the procedure consisted of mixing 1.77 mL of Triton X-100, 1.80 mL of hexanol and 7.5 mL of cyclohexane, followed by addition of 500 μL of water and 0.0015 g of Ru(bpy)₃²⁺ dye. The mixture was shaken for 24 h (speed: 180 r min⁻¹, amplitude: 20 mm, temperature: 26 °C). Ammonium hydroxide and TEOS were added and the mixture was shaken for another 24 h. After the reaction, 12 mL of acetone was added to break the microemulsion and to recover the nanoparticles. The nanoparticles were washed two times with ethanol and deionized water using centrifugation and decantation several times to remove the unreacted chemicals.

2.3 Preparation of the dye-h-SMs

Transparent dye-h-SiO₂/epoxy super-nanocomposites were prepared using the in situ polymerization method with an ultrasound technique. The as-prepared dye-h-SiO₂ nanocomposite particles and dyes were dispersed in an anhydride curing agent (EP-400B), using the ultrasound technique for 10 min, the resulting mixture was then mixed with bis-phenol. The epoxy (EP-400A) and the curing agent (EP-400B) were mixed with the weight ratio 1 : 1 and were stirred well until a homogeneous mixture was obtained. The homogeneous mixture was poured into a glass tube mold and heated in a vacuum oven at 60 °C for 4 h.

2.4 Preparation of LED with dye-h-SM

Blue InGaN LEDs with a peak emission at 460 nm were used. The LED package consisted of a 300 μm × 300 μm chip, and the average efficacy of the blue LED was 8 IM/W when it operated with 60 mA at 3.3 V. The dye-h-SMs, which were molded into disks of 4 mm diameter, were placed on top of the LED chip, and the empty space in the package was encapsulated with silicone at 80 °C for 3 h in air.

2.5 Characterization

The field-emission scanning electron microscopy (FESEM) images were obtained using a JEOL JSM-7500F microscope operated at an acceleration voltage of 15 kV. A JEOL JEM-200CX microscope operating at 160 kV in the bright-field mode was used for transmission electron microscopy (TEM). Fourier transformation IR (FT-IR) spectra for the dendrimers and metal particles were recorded on a Nicolet Avatar 370 FT-IR spectrometer. The photoluminescence excitation (PLE) and photoluminescence (PL) spectra were measured using a Varian spectrometer at room temperature. The transmittance curves of the nanoporous films were measured using a UV-vis-NIR spectrophotometer (Varian, Cary 500).

3 Results and discussion

To design dye–polymer nanocomposites with desired integrated properties and application potential, normally, the dye nanofillers would have to match the following requirements: (1) high stability of structure and properties, (2) good dispersion in the matrix, (3) small size to avoid sedimentation effects and phase separation.

To obtain stable dyes, packaging with polymers and inorganic shells such as silica is generally employed to avoid a direct contact between the dye and its surroundings. However, the large-scale preparation of such core–shell nanomaterials in a cost-effective way is yet to be investigated. Therefore, we developed a one-pot strategy for the scalable fabrication of silica-hybridized dyes in aqueous solution. The structure of the dye-h-SiO₂ is not an exact core–shell structure with single dye molecules being solely in the middle of silica as previously reported,¹⁵ but in the form of a hybrid in which the dyes are embedded in the silica. The ultrafine dye-h-SiO₂ can be readily obtained on a sub-kilogram-scale in one batch, and the dyes have been shown to be highly stable in the silica resin, encouraging the industrial application of dyes. Hence, we chose this kind of dye-h-SiO₂ as the critical raw material for the fabrication of dye–polymer composites.

Fig. 1(a₁) and (a₂) show the scanning electron microscopy (SEM) images of the Ru(bpy)-h-SiO₂ nanoparticles. The average diameter of the dye-h-SiO₂ nanoparticles is 65 ± 3 nm. The functionalized NPs were dispersed very well in aqueous solution, and no aggregation was observed due to the electrostatic repulsion force between the NPs. (The analysis results of the FITC-h-SiO₂ nanoparticles are similar to those of the Ru(bpy)-h-SiO₂ nanoparticles.) The dye-h-SiO₂ nanoparticles are so small that they could avoid sedimentation and phase separation. Silica particles with a large size may cause phase segregation at the interface of SiO₂ and the polymer matrix. The internal pore structure of the NPs could be observed using transmission electron microscopy (TEM, Fig. 1(b₁) and (b₂)). Many disordered holes could be found in the SiO₂ NPs and the dye molecules are dwelling in these holes steadily. Upon introduction of the dye to the silica NPs, it has been found that the dyes maintain their luminescence efficiency or even exceed those of the pure dyes, and the stability of the dyes has improved significantly. As we all know, it is difficult to maintain the initial optical properties of pure dyes during the device fabrication and long-term operation, while the silica layer is a good shield against oxygen and moisture so that the dye-h-SMs could maintain their initial luminescence after high-power UV irradiation and the 150 °C LED encapsulation curing process.

Fig. 1 Scanning electron microscopy images of Ru(bpy)₃²⁺-doped silica nanoparticles (a₁, a₂). Transmission electron micrographs of Ru(bpy)₃²⁺-doped silica nanoparticles (b₁, b₂).

The fluorescence of dye-h-SiO₂ NPs was compared with that of the corresponding pure dye molecules (Fig. 2). By matching the absorption of the free dyes and the particle solutions, we determined the intensity ratios of the luminescence emission of Ru(bpy)₃²⁺ and FITC in our silica particles versus the free dyes in aqueous solution to be 2.4 and 1.6, respectively. In other words, the luminescence intensities of the nanoparticles are 2.4 or 1.6 times higher than those of the pure dyes, and the nanoparticles do not undergo photobleaching as the pure dyes do over a long period of continuous intense light exposure. The encapsulation in silica can improve the luminescence intensity because of an increase in quantum efficiency of the dyes.^16,17 The emission maximum of the nanoparticles shifts by 10 nm towards longer wavelengths compared with the pure Ru(bpy)₃²⁺ dye due to aggregation of the dye molecules inside the nanoparticles.¹⁸ For the Ru(bpy)-doped nanoparticles, this results in a redshift with the emission at 614 nm while that of the pure Ru(bpy)₃²⁺ dye is at 604 nm. For the FITC-doped nanoparticles, there also exists a redshift with the emission being at 528 nm while that of the pure FITC dye is at 522 nm. These superior properties suggest that the dye-h-SiO₂ nanoparticles are excellent fluorescent powders for LEDs.

Fig. 2 Panels (a) and (b) display the fluorescence spectra of the free dye molecules and the dye-h-SiO₂ nanoparticles.

Ru(bpy)₃²⁺ is well-known to be a good oxygen-sensing reagent, because the luminescence of the dye can be quenched by oxygen. Oxygen exercises a great influence on the emission intensity of the dye in the solid state.¹⁹ Recently, Santra and co-workers have reported that the dyes are well-protected inside the silica network in normal atmosphere.¹⁸ There was no evident change in the emission intensity for the dye-h-SiO₂ NPs when the air pressure was increased from 1 to 8 psi. Only when the pressure was further increased could a decrease in emission intensity be observed. There are two possibilities for this photobleaching in the solid state. The first one is that a minute amount of oxygen penetrates into the silica network, and the second one is that some of the dye remains close to the surface of the silica particles, because the surface area/volume ratio of those dye composites is quite high. In both cases, external silica coating on the dye surface will be a reasonable shield to minimize photobleaching of the dyes and there was no photobleaching observed over a long period of intensive laser excitation. This observation, therefore, suggests that external silica coating completely isolates the dyes from the outside environment, thereby preventing any oxygen molecule penetration. In conclusion, the dyes have been passivated by the novel multi-layered structure and the passivated dyes show a high luminescence intensity and improved stability in solid-state devices.

Fig. 3(a) shows the PL spectra of the dye-h-SMs with different concentrations after the thermal treatment using an excitation at 460 nm. The luminescence emission intensity of the dye-h-SMs with 0.016 to 0.047 wt% doping content has been enhanced with the increased addition of the dye molecule. However, the luminescence emission intensity decreased with a further increase in dye content (0.094 wt%). The dye-h-SMs are highly transparent, and the transparency comes to more than 86% from the visible to the infrared region, as show in Fig. 3(b). The inset displays that the transparency of the dye-h-SMs decreases with increasing dye-h-SiO₂ doping content, but the dye-h-SMs still maintain high transparency in the visible range. The decrease in transmittance of the dye-h-SMs for the 0.094 wt% doping content, compared to the 0.016 to 0.047 wt% dye contents, would reduce the luminescence emission intensity of the dye-h-SMs. The increase in luminescence emission intensity due to the increase in doping content is not enough to compensate the decrease in transmittance, which leads to the observed overall decrease of the luminescence emission intensity, as showed in Fig. 3(a). It can be observed from Fig. 3(c) that the dye-h-SMs exhibit a high visible light transparency and Fig. 3(d) depicts the visual effect of the samples under irradiation with a handheld broadband 365 nm UV lamp, from left to right the doping dyes are Ru(bpy)₃²⁺ and FITC. There are two reasons for the high transparency of the dye-h-SMs. The first one is that the presence of silanol groups on the silica surface makes the silica nanoparticles hydrophilic, thus the dye-h-silica NPs could basically homogenously disperse in the epoxy matrix,¹² so it is easy to disperse them into epoxy resin. The second one is that the small cell of the SiO₂ NPs benefits the refraction of the dye.

Fig. 3 (a) PL spectra of the dye-h-SMs with different dye mass fractions after 60 °C thermal treatment. (b) UV-vis spectra of the Ru(bpy)-h-SiO₂/epoxy nanocomposites with different filler contents. (c) Photographs of the dye-h-SMs under sunlight. (d) Photographs of the dye-h-SMs under 365 nm UV light.

Green and red light-emitting dye-h-SMs containing different dye concentrations were prepared and applied to blue LEDs. The dye-h-SMs were molded as a disk with a diameter of 4 mm and placed on the LED chip, followed by silicone encapsulation, as shown in Fig. 4(a). The blue LED and the dye-LED were operated using a 60 mA current at 3.3 V and both spectra are shown in Fig. 4. Traditionally, the phosphor glue was dropped on the LED chip and packaged by the transparent epoxy resin. The heat dissipation is very bad, and the working temperature is as high as 250 °C. By placing the phosphor layer far from the chip, heat would escape from the device rather than being trapped where it would cause excessive yellowing of the epoxy encapsulant, and then the lifetime of the LEDs could be significantly improved.²⁰ When doing a comparison between remote phosphor and traditional white LEDs, it can be shown that the new technology enables the thin remote phosphor sheet to be separated from the LED chip. The good heat dissipation can lower the phosphor’s operating temperature and improve both efficiency and longevity.

Fig. 4 (a) Schematic diagram of the LED using dye-h-SM as the color-converting layer. (b) Spectra of a blue LED and a green color converting dye-LED operated using a 60 mA current at 3.3 V. (c) The same as (b) except for red dye-LED. (d) Spectra of white LEDs made with mixed green and red dye-h-SM.

To achieve efficient dye-LEDs, therefore, it is essential to prepare high-quality dye-h-SiO₂ NPs with a high luminescence intensity, to protect the dyes against fluorescence quenching caused by high temperatures.

The color coordinates of the green spectrum, red spectrum and the commercial blue LED (460 nm) emission spectrum are marked at (0.2866, 0.6402), (0.6194, 0.3795) and (0.14, 0.12) in the CIE 1931 color space (Fig. 5). sRGB is a standard RGB color space created cooperatively by HP and Microsoft in 1996 to be used on monitors, printers and the Internet. The color coordinates of the sRGB color triangle are (xR, yR) = (0.64, 0.33), (xG, yG) = (0.30, 0.60), (xB, yB) = (0.15, 0.06), as shown in Fig. 5.²¹ The result indicates that an adjustable white light and highly color-saturated remote-phosphor LED could be realized by adjusting the ratio of the green and the red dye.

Fig. 5 Color coordinates of the dye-LED (red spot and green spot) and the sRGB-LED (white line) on the CIE 1931 color space.

4 Conclusions

In summary, herein we reported the bulk preparation of novel transparent and light-emitting dye-h-SMs using the reverse microemulsion technique. The stability of the dye has been improved after being packaged by SiO₂ in a transparent epoxy matrix. The dye-h-SiO₂/epoxy super-nanocomposites show a high luminescence intensity and an adjustable spectrum. The as-prepared novel transparent and light-emitting dye-h-SiO₂/epoxy super-nanocomposites have been successfully applied as encapsulating materials for remote-phosphor LEDs with no need of using conventional phosphors. The dye-LEDs show highly effective and adjustable white light. The dye-h-SiO₂/epoxy super-nanocomposites are very promising as novel encapsulating materials in remote-phosphor LED devices for general illuminations in offices, houses and traffics etc, due to their advantages of high luminescence intensity, high colour-saturation, good transparency and easy encapsulation process.

Acknowledgements

This work is supported by the Shanghai Science & Technology Committee (12521102501), Shanghai Educational Committee (11ZR1426500), Innovation Program of Shanghai Municipal Education Commission (14ZZ127), PCSIRT (IRT1269), and the Program of Shanghai Normal University (DZL124), Academic Leaders Training Program of Pudong Health Bureau of Shanghai (Grant no. PWRd2011-09).

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