A facile synthesis of SiO₂-based nanocomposites containing multiple quantum dots at high concentration for LED applications

Abstract

Facile synthesis of highly luminescent SiO₂-based nanocomposites containing CdSe–ZnS quantum dots at extremely high concentration is reported.

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Light emitting diodes (LEDs) are extensively used as alternatives to commonly used incandescent bulbs and fluorescent lamps which lack efficiency or contain mercury.¹ Phosphors were usually applied to down-convert blue light from LED chips to realize white light. Currently, white LEDs based on down conversion of blue InGaN LEDs by Y₃Al₅O₁₂:Ce (YAG:Ce)-based yellow phosphors emit a “cold” white light with low color rendering index (CRI) and high correlated color temperature (CCT). The demand for a new lighting source with high CRI and low CCT to replace incandescent lamps emitting “warm” white color has driven phosphors beyond the conventional YAG:Ce. Blending red emitting phosphors with yellow phosphors can improve color quality, but it reduces the power efficiency of LEDs significantly.² Then, there should be a trade-off between light quality and power efficiency of white LED.²

Recently, semiconductor nanocrystals or quantum dots (QDs) were introduced as possible candidates for light emitters to correct the emission spectra of conventional white LEDs with minimal loss of power efficiency.³ QDs are efficient emitters of visible light providing very narrow emission spectra with finely tunable emission wavelengths.² Hence, highly efficient white LEDs with high CRI are expected to be realized with QDs. However, QDs require uniform mixing with a silicone polymer in the packaging process of white LEDs and surface ligands on QDs that can possibly disturb polymerization or reduce the mechanical stability of packaging materials.⁴ Furthermore, the long chain surface ligands with hydrophobicity reduce the compatibility with polar silicone polymers, which is preferred in LED fabrication. Ziegler et al. suggested QDs encapsulated with silica to overcome these obstacles and to provide improved emission spectra of LEDs with high a CRI.⁴

During past decades, inorganic nanocrystal-based nanocomposites have been of extensive interest due to their tunability of physical and chemical properties for various applications such as light-emitting diodes (LED), catalysis, biological imaging, etc.^5–9 Among these, silica-based nanocomposites have been the most extensively studied with quantum dots (QDs).^8,9 Although several routes to silica-based nanocomposites with finely dispersed QDs have been tried, one of the more popular routes is based on the process consisting of a ligand exchange step for the hydrophilic surface of semiconductor nanocrystals and subsequent growth of silica shell layers using the Stöber method.^10,11 Alternatively, a reverse micelle approach was also popularly applied to synthesize SiO₂-nanocrystal composite particles.^12–15 Surfactants were added to induce reverse micelle formation and hydrolysis/condensation of the silica precursors (e.g., tetraethylorthosilicate, TEOS) occurs at the water–oil interface. A smooth silica surface with good monodispersity was obtained with this approach. Recently, Koole et al. suggested that hydrolyzed TEOS can replace the hydrophobic ligands on the surface of QDs where silica growth takes place.¹⁴ However, the number of nanocrystals incorporated in each nanocomposite particle is usually limited¹⁵ and it needs SiO₂ nanohybrids with multiple nanocrystals at high concentration to synthesize highly luminescent phosphors for various applications including LEDs.¹⁶

We have developed a facile two-step processing route for hydrophobic CdSe–ZnS core–shell QDs to be encapsulated with SiO₂. Silica-nanocomposites containing multiple QDs at extremely high concentration were developed with this approach. As a first step, we prepared assembled QDs by spray-drying. The spray-drying method was often applied to the evaporation-assisted self-assembly of a colloidal nanoparticles.¹⁷ Evaporation of the solvent during spray-drying process induces shrinkage of droplets containing nanoparticles and nanoparticles inside droplets are assembled due to capillary force. In particular, the aerosol spray technique maximizes the evaporation of solvent and restricts the self-assembly of nanoparticles within the droplet. In fact, various nanoparticles were self-assembled to form mesospheres using the aerosol-based spray process.^17,18

In this study, QDs dispersed in toluene were ultrasonically sprayed at 250 °C to evaporate solvents and to assemble hydrophobic QDs. The sprayed QDs formed highly porous aggregates with complex shapes which were finally dispersed into hydrophilic ethanol (Fig. 1(a and b)). Although the QDs were heated at 250 °C for a few seconds, they conserved the hydrophobic nature of the surface and could be easily dispersed in toluene. Subsequently, additional SiO₂ layers were formed around spray-dispersed QD aggregates by addition of TEOS and aqueous ammonia solution. After hydrolytic reaction of TEOS and ammonia, the SiO₂ layers were directly formed on QDs. In a reverse micelle synthesis of silica spheres containing QDs, Koole et al. demonstrated that rapid ligand exchange took place by adding hydrolyzed TEOS due to its high affinity to the QD surface.¹⁴ The transmission electron microscopy (TEM) shown in Fig. 1(c–f) illustrats the detailed structure of the QD–silica nanocomposite during the growth of SiO₂ layers. In the initial stage of encapsulation, only thin silica layers were formed on the self-assembled ensembles of QDs. The aggregates of QDs were ca. 50 nm in diameter and linked to each other. TEM images of final products displayed in Fig. 1(c and f) showed that uniform silica layers were formed conserving the original complex shape of assembled QDs even after further progress of encapsulation. The thickness of the outer silica layer was estimated to be about 15 nm after reaction for 2 h. Continued reaction increased the size of QD–silica nanocomposite particles up to a few micrometers as shown in Fig. S3 (see ESI†). Fig. 2(a) shows the emporal evolution of photoluminescence (PL) spectra during the silica growth step. The emission intensity of the QD–silica nanocomposite was highly minimized just after addition of TEOS/NH₄OH. Many groups have reported that the emission intensity of QDs dropped significantly after formation of QD–silica nanocomposite particles.^11–14 The degradation of PL intensity after silica encapsulation were usually interpreted as the formation of non-radiative recombination channels.¹³ The possible origin of these non-radiative recombination channels were attributed to increase of surface defects during ligand exchange of silane primer or hydrolyzed TEOS in the synthesis of QD–silica nanocomposite particles.^13,14 In particular, the low emission intensity of silica-capped QDs were attributed to disordered arrangement of TEOS on the QD surface due to rapid ligand exchange with partially hydrolyzed TEOS.^14,19 In addition, OH⁻ ions in aqueous ammonia solution for condensation of TEOS can remove surface organic ligands of QDs, presumably by forming hydroxide complexes and incomplete passivation of the surface.²⁰ So, direct reaction of aqueous ammonia solution should be avoided for high PL intensity. Since, however, we utilized aqueous ammonia solution to hydrolyze TEOS, a rapid decrease in emission intensity seems inevitable as reported by many researchers.^11–19

Fig. 1 TEM of CdSe–ZnS after addition of TEOS/NH₄OH at different time intervals; (a and b) spray dispersed QD ensemble, (c and d) after 1 h, (e and f) after 2 h. The thickness of the SiO₂ shell layers were estimated to be ∼15 nm

Fig. 2 Emission spectra of spray-dispersed CdSe–ZnS QDs at various stages of encapsulation (λ_ex = 395 nm). TEOS and NH₄OH were added to QDs for silica encapsulation. In panel (b), APS was added to spray-dispersed QDs prior to TEOS/NH₄OH addition. The percentages represent the relative integrated intensity of emission spectra and the hours indicate time intervals after addition of TEOS/NH₄OH. In panel (c and d) TEM of CdSe–ZnS after sequential addition of APS and TEOS/NH₄OH and reaction for 2 h. Thickness of SiO₂ shell layers were estimated to be ∼10 nm.

In order to minimize deterioration in PL intensity of QDs during SiO₂ shell formation, several routes such as initial silanization,²¹ and formation of multiple shells on QDs²² have been suggested. In particular, the initial silanization approach was proven to be effective for conservation of the quantum yield of initial quantum dots after silica encapsulation. In this study, we applied an initial silanization step to minimize the significant reduction in PL intensity after SiO₂ layer formation. We added 3-aminopropyl-triethoxysilane (APS) to a spray-dispersed QD solution and reacted it for 10 min under stirring to silanize the QDs. In sequence, we added TEOS and aqueous ammonia solution to thicken the SiO₂ layers. Here, the PL intensity of spray-dispersed QD solution was conserved after APS addition as shown in Fig. 2(b). Even after TEOS/NH₄OH addition, the PL intensity showed only slight decrease up to reaction time of 1 h. As the reaction proceeded further, the photoluminescence was weakened by 15–20%, which is much smaller compared to APS-free synthesis. The reduction in PL intensity after TEOS/NH₄OH addition presumably indicates incomplete passivation of the QD surface by APS.

It is known that the photoluminescence from assembled solids of QDs was usually deteriorated compared with QDs in solution owing to re-absorption of emitted light or interdot energy transfer via electrostatic coupling.²² In order to minimize those adverse effects on emission intensity, and to widen interdot distance, the number of QDs inside silica matrices needs to be controlled. Here, we added APS to QDs dispersed in toluene and sprayed at the same conditions described above. In addition, we added TEOS/NH₄OH for further growth of silica layers in sequence. As shown in Fig. 3, QD–SiO₂ nanocomposites containing a lower number of QDs were synthesized with this approach. That is, simply by dispersing QDs with APS into toluene prior to spray processing as illustrated in Fig. S3(c) (in the ESI†), the number of QDs in the QD–SiO₂ nanocomposites were highly reduced and the interdot distance was increased to be ∼20 nm.

Fig. 3 TEM of QD–SiO₂ nanocomposites after spray processing. Here, the solution for spraying was prepared by dispersing QDs and APS to toluene. In panel (b), a few QDs were observed inside SiO₂ particles as indicated by arrows.

In order to check the possibility of using QD–SiO₂ nanocomposites to fabricate highly efficient white LEDs with high CRI and low CCT, we combined a conventional white LED based on a yellow emitting YAG:Ce phosphor with a red emitting QD–SiO₂ nanocomposite by packaging with silicone-based LED encapsulation resin. We enhanced the color quality of conventional “cold” white-emitting LEDs with QD–SiO₂ nanocomposites containing multiple QDs at extremely high concentration prepared by the synthetic route illustrated in Fig. S3(a) (see the ESI†). Fig. 4(a) shows the full emission spectra of the white LED with QD–SiO₂ nanocomposites. For comparison, an emission spectrum of an LED lamp with only the YAG:Ce-loaded blue LED chips operated under the same conditions at an operating current of 16 mA is also presented. As the operating current increased, the emission of the white LED with QD–SiO₂ nanocomposites was uniformly intensified with a small variation in the CIE color coordinates which were less than 5% of the original coordinates. The CRI of white LEDs was highly improved from the original to 90% by applying QD–SiO₂ nanocomposites. At an operating current of 30 mA, the measured luminous efficacy (η) of the white LED with QD–SiO₂ nanocomposites was 27 lm W⁻¹ which is comparable to the YAG:Ce-loaded blue LED with η = 29 lm W⁻¹ at 28 mA. Compared with previously published results,⁴ the present QD–SiO₂ nanocomposites containing CdSe–ZnS quantum dots at high concentration displayed a much smaller decrease in the luminous efficacy of white LEDs.

Fig. 4 (a) Emission spectra of white LED with QD–SiO₂ nanocomposite operated at various current from 16 to 96 mA. Dotted line indicates an emission spectrum of original white LED at 16 mA. Inset is a photograph of QD–SiO₂ nanocomposite assisted white LED operating at 30 mA; (b) luminous efficiency and color rendering index for a conventional white LED (squares) and a white LED with QD–SiO₂ nanocomposite (circles) operated at various currents.

In summary, we have synthesized SiO₂–based nanocomposites containing CdSe–ZnS quantum dots at high concentration by spray-dispersion of QDs and further encapsulation through hydrolysis and condensation of TEOS. Addition of APS after spray dispersion of QDs suppressed the rapid decrease in emission intensity of the nanocomposites after TEOS/ammonia addition to form SiO₂ layers. In addition, it was shown that the number of QDs incorporated in the nanocomposites was controlled by addition of APS prior to spray processing. To test the effect of QD–SiO₂ nanocomposites on the emission spectra of white LEDs, we have constructed a white-emitting LED with QD–SiO₂ nanocomposites with a high loading of QDs. The white LED with QD–SiO₂ nanocomposites showed luminous efficacy comparable to the YAG:Ce-loaded blue LED and displayed high CRI and small decrease in luminous efficacy.

This work was partially supported by the basic research program in the Korea Institute of Materials Science (KIMS).

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Footnote

† Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c2ra20780j

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