Highly Fluorescent and Stable Quantum Dot-Polymer-Layered Double

Article pubs.acs.org/cm

Highly Fluorescent and Stable Quantum Dot-Polymer-Layered Double Hydroxide Composites Seungho Cho,† Jungheon Kwag,‡ Sanghwa Jeong,† Yeonggyeong Baek,‡ and Sungjee Kim*,†,‡ †

Department of Chemistry and ‡School of Interdisciplinary Bioscience and Bioengineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-Dong, Nam-Gu, Pohang, Gyungbuk, South Korea 790-784 S Supporting Information *

ABSTRACT: We report a designed strategy for a synthesis of highly luminescent and photostable composites by incorporating quantum dots (QDs) into layered double hydroxide (LDH) matrices without deterioration of a photoluminescence (PL) efficiency of the fluorophores during the entire processes of composite formations. The QDs synthesized in an organic solvent are encapsulated by polymers, poly(maleic acid-altoctadecene) to transfer them into water without altering the initial surface ligands. The polymer-encapsulated QDs with negative zeta potentials (−29.5 ± 2.2 mV) were electrostatically assembled with positively charged (24.9 ± 0.6 mV) LDH nanosheets to form QD-polymer-LDH composites (PL quantum yield: 74.1%). QD-polymer-LDH composite films are fabricated by a drop-casting of the solution on substrates. The PL properties of the films preserve those of the organic QD solutions. We also demonstrate that the formation of the QD-polymer-LDH composites affords enhanced photostabilities through multiple protections of QD surface by polymers and LDH nanosheets from the environment. KEYWORDS: quantum dot, photoluminescence quantum yield, amphiphilic polymer encapsulation, layered double hydroxide, photostability

1. INTRODUCTION Quantum dots (QDs) have potential in nanoscale device applications such as next-generation electronic and optoelectronic devices because of their unique physical properties (e.g., their tunable band gap energy derived by tuning the particle size) which arise due to the quantum confinement effect.1−4 Many electronic, optoelectronic, and optical devices require solid state QDs isolated from solutions. It is of paramount importance to guarantee the photoluminescence (PL) stability of QDs in solid state as incorporated in light emitting materials and devices. However, there are some challenges to fabricate efficient light emitting materials involving solid state QDs: (i) First, QD PL quantum yields (QYs) can be significantly reduced via a solvent transfer step which can be involved in fabrication procedures. Currently, procedures for syntheses of colloidal QDs using organic solvents (pyrolysis synthetic methods) have been widely used to prepare high quality QDs with few defects, well-controlled sizes, and high crystallinity. For various potential applications in biological labeling,5−7 medical treatments,8−10 chemical analysis,11,12 and electronic and optoelectronic device applications,13,14 the transfer of QDs synthesized in an organic solvent to aqueous solution can be required. Altering the initial ligands of QD surfaces in a ligand exchange step widely used for the solution transfer normally leads to a decreased QY, as this property is very sensitive to the surface chemistry of QDs.15 (ii) PL QYs can be rapidly © 2013 American Chemical Society

deteriorated upon continuous excitation in solid state as a sensitive function of moisture, oxygen, heat, and light unlike solution phase where there typically exist excess of surface ligands passivating the QD surface. (iii) Optical spectra can be red-shifted by QD aggregation during the solidification process. As nanoparticles, like many other materials, QDs show a strong tendency toward agglomeration upon their isolation from colloidal suspensions; this will lead to a red shift for both the optical absorption and emission.16−18 To solve the PL QY decrease problem via a solvent transfer, some strategies have been developed which, instead of exchanging the initial surface ligands, are based on the addition of a second layer.19,20 QDs have been encapsulated by amphiphilic molecules, such as phospholipids,21 polymers,19,22,23 and saccharides24 that can intercalate the first hydrophobic ligand layer with their hydrophobic portion and that ensure water solubility of the QDs with their hydrophilic groups.19 Meanwhile, one effective solution to the spectral red shift problem caused by solidification is the incorporation of nanocrystals into an appropriate host matrix such as polymers,25−32 carbon nanomaterials,33,34 and inorganic materials18,35−38 through in situ nanocrystal growth, electroReceived: December 18, 2012 Revised: March 15, 2013 Published: March 18, 2013 1071

dx.doi.org/10.1021/cm3040505 | Chem. Mater. 2013, 25, 1071−1077

Chemistry of Materials

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temperature, the mixture of cadmium and selenium precursors was quickly injected into the reaction flask and the temperature was maintained at 280 °C. The reaction mixture was kept stirred until CdSe nanocrystals of the desired size were obtained. Upon completion, the mixture was cooled to room temperature and diluted by hexanes. For purification, the product mixture was precipitated by adding excess methanol, collected by centrifugation, and redispersed in a small amount of hexanes. CdS and ZnS shells were deposited onto CdSe bare nanocrystals by the following procedure. To obtain the cadmium precursor, cadmium acetate (0.3 mmol) was dissolved in oleic acid (1.5 mmol) at 100 °C under vacuum. The solution was cooled to room temperature, then the cadmium precursor was mixed with sulfide precursor. The sulfide precursor was previously prepared by dissolving bis(trimethylsilyl)sulfide (45 μL) in TOP (3 mL) in a glovebox. For precursor of zinc and sulfide, diethylzinc (130 μL) and bis(trimethylsilyl)sulfide (240 μL) were dissolved in TOP (5 mL). ODE (45 mL) was placed in a four-neck flask. Under nitrogen gas flow, bare CdSe nanocrystals (9.0 × 10−4 mmol) were added to the reaction flask. When the temperature of the reaction flask reached 120 °C, the mixture of Cd and S precursors was slowly added using a syringe pump. After allowing 30 min for the CdS shell growth, the temperature was raised to 140 °C then the mixture of Zn and S precursors was added dropwise. The temperature was maintained with stirring for 30 min to allow ZnS shell growth. Upon completion, the mixture was cooled to room temperature and diluted by hexanes. For purification, the product mixture was precipitated by excess methanol, collected by centrifugation, and redispersed to a small amount of chloroform. Preparation of Water-Soluble Polymer-Encapsulated CdSe/ CdS/ZnS QDs. A 50 μM portion of aqueous poly(maleic acid-altoctadecene) (Mw: 30 000−50 000) solution was prepared at room temperature. The organic solution of 1 nmol QDs was added to 2 mL of the aqueous polymer solution (QD: polymer = 1:100). The solution was ultrasonicated at room temperature for 20 min, and then, chloroform was removed by elevating the mixture temperature to 80 °C with stirring. Preparation and Exfoliation of LDHs. An aqueous solution containing 12 mM zinc nitrate hexahydrate, 4 mM aluminum nitrate nonahydrate, and 0.875 M ammonia was prepared at room temperature and maintained at 95 °C for 24 h for preparing LDHs. After reaction, the solution was filtered through a polycarbonate membrane filter (ISOPORE). The filtered powders (LDHs) were washed several times with deionized (DI) water then dried in an oven at 60 °C for 12 h. The deintercalation of carbonate ions from the asprepared CO32−-intercalated LDH was conducted by the salt-acid method as described in refs 46 and 47. A 0.2 g portion of the LDHs was dispersed into 0.2 dm3 of an aqueous solution containing 1 M NaCl and 3.3 mM HCl. The flask was sealed after purging with nitrogen gas and, then, stirred for 12 h at room temperature under a nitrogen gas flow. The product was filtered, washed with DI water and ethanol, and finally air-dried at room temperature. A 0.1 g portion of the intercalated ion-exchanged powder was mixed with 100 cm3 of formamide in a flask, which was sealed after purging with nitrogen gas. Then, the mixture was stirred vigorously under a nitrogen gas flow for 2 days. To remove the unexfoliated particles, the suspension was centrifuged at 2000 rpm for 10 min and then the sediment was discarded. Formation of QD-Polymer-LDH Composites. The 20 mL colloidal LDH suspension was centrifuged at 14000 rpm for 30 min. The supernatant was discarded, and then, the sediment was mixed with 1 mL of the aqueous solution of polymer-encapsulated QDs (pH 8.2). The mixture was stirred for 3 h at room temperature. The stirred solution was centrifuged at 2000 rpm for 5 min. The supernatant was discarded, and the sediment was dispersed in 1 mL DI water. For preparing a QD-polymer-LDH composite film, the 0.5 mL QDpolymer-LDH composite-dispersed aqueous solution was dropped onto 1 cm × 1 cm area of a glass and air-dried at room temperature for 12 h and then placed in 60 °C oven for 12 h. Characterization. The morphology, crystallinity, crystalline nature, chemical composition, and functional groups of the

static interactions, and cross-linked processes. Layered double hydroxides (LDHs), hydrotalcitelike clays, are an important class of ionic lamellar solids.39 LDHs have the following noticeable features that allow the rational design of synthetic strategy for QD composites: (i) Two-dimensional and positively charged brucitelike metal hydroxide layers are balanced by intercalation of interlayer anions of choice, and consequently, LDHs have interlayer galleries. (ii) Various combinations of metal ion identities can be adopted to tune their optical and chemical properties. (iii) LDH layers possess a high charge density (∼300 mequiv/100 g).40,41 (iv) They are a cheap, easily available, and environmentally friendly material. Thus, LDH is a promising candidate for a QD host media improving the chemical and photostability of QDs and preventing their aggregation.36,42−45 However, most of these methods for fabricating QD-LDH composites have used QDs synthesized in aqueous solutions as building blocks or in situ growth procedures for QDs on LDH matrices. Therefore, these methods could not take advantage of the well-established pyrolysis synthetic methods that can yield nearly monodispersed QDs at the delicately demanded size for their optical properties and high PL QYs. In this paper, we report a designed strategy for fabricating highly fluorescent and stable QD composites. To overcome the PL QY reduction problem via a solvent transfer, the QDs were transferred to water by encapsulating them with poly(maleic acid-alt-octadecene), without exchanging initial ligands of QD surfaces. Their PL QY in water (73.0%) was enhanced in comparison with that in chloroform (56.7%). The polymerencapsulated QDs (QD-polymer particles) had carboxyl groups on their surfaces, resulting in negative zeta potentials under our assembly condition, which drove a successful electrostatic assembly of the QD-polymer particles and positively charged LDH nanosheets. The attachment of the QD-polymer particles to the LDH matrices could be a solution of the spectral redshift problem by QD aggregation during the solidification process. The fluorescent spectrum of the composite did not change compared to their colloidal form while the QDs or the polymer-encapsulated QDs without LDH composite formations were red-shifted by their isolation from the colloidal state. Moreover, the QDs were multiply protected by polymers and LDH nanosheets from the environment. We demonstrate that the formation of the QD-polymer-LDH composites afforded significantly enhanced photostabilities. This method has a strong point for delicate control of optical properties because QDs synthesized by well-established methods using organic solvent were used. In principle, this method allows composite fabrications using QDs covering a wide range of size and identity. These QD-polymer-LDH composites have a variety of potential application area such as lighting, display, and optical coating materials.

2. EXPERIMENTAL DETAILS Preparation of CdSe/CdS/ZnS (Core/Shell/Shell) Quantum Dots (QDs). All chemicals used in this study were of analytical grade and were used without further purification. CdSe bare nanocrystals were prepared by the following procedures. To obtain a cadmium precursor, cadmium acetate (1.2 mmol) was dissolved in oleic acid (6.0 mmol) at 100 °C under vacuum. The solution was cooled to room temperature, then the cadmium precursor solution was mixed with selenium precursor. The selenium precursor was previously prepared by dissolving selenium shots (6.0 mmol) in TOP (6 mL) in a glovebox. ODE (40 mL) and oleylamine (6 mmol) were placed in a three-neck flask and heated to 300 °C under nitrogen gas flow. At this 1072



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synthesized materials were determined using field-emission scanning electron microscopy (FESEM, JEOL JMS-7401F, operated at 10 keV), high-resolution transmission electron microscopy (HR-TEM, JEOL JEM-2100 with an energy-dispersive X-ray spectrometer operating at 200 kV), X-ray diffraction (XRD, RIGAKU, D/MAX-2500/PC), and Fourier transform infrared spectroscopy (FT-IR, SensIR Technologies, IlluminatIR). UV−vis absorption spectra were obtained using an Agilent 8453. Photoluminescence (PL) spectra were obtained using a HORIBA FluoroLog instrument. Zeta potentials were measured using a Malvern zetasizer S. PL QYs of the QDs were calculated by comparing their integrated emissions with that of rhodamine 101 ethanol solution with an identical optical density of 0.03 at 480 nm. Atomic force microscope (AFM) imaging was performed using a Nanoscope V system (Veeco). Photostabilities of the samples were conducted under light irradiation (InGaN LED, Lumex, spectrum peak position 405 nm, power 3 mW/cm2). PL spectra for the photostability tests were taken by an Ocean Optics USB4000-UV−vis spectrophotometer.

diameters of 6.09 ± 0.59 nm. Figure 1B shows absorption and PL spectra of the QDs in the organic solvent. The PL peak wavelength was 611 nm. Figures 1C-I and II show the QD solution under room light and UV light, respectively. The PL QY of the QDs in chloroform was 56.7%. The QDs were encapsulated by poly(maleic acid-alt-octadecene) to transfer them into water without altering the initial surface ligands (Figure 1D). Polymer-dispersed aqueous solution was mixed with the QD-dispersed chloroform solution. The mixture was ultrasonicated and then chloroform was removed by elevating the mixture temperature to 80 °C with stirring. During ultrasonication, the amphiphilic polymers intercalate the hydrophobic ligand layer of QDs with their hydrophobic portion and that ensure water solubility of the QDs with their hydrophilic groups. The QDs were encapsulated with poly(maleic acid-alt-octadecene) via hydrophobic interactions between their surface ligands and −CH2(CH2)14CH3 of the polymers and transferred to water.20 Zeta potential measurements were performed on the polymer-encapsulated QDs to investigate whether charges are present on the particles in the aqueous solution (pH 8.2). The result demonstrated their negatively charged state (−29.5 ± 2.2 mV). These negative zeta potentials resulted from the carboxyl groups of the polymers, which allowed a successful assembly of polymer-encapsulated QDs and LDH nanosheets by electrostatic interactions in the assembly step which will be discussed later. Figure 1E shows a TEM image of polymer-encapsulated QDs. The brighter part marked by the black arrow was a bare carbon layer coated on the grid, and the darker part marked by the white arrow was polymer-encapsulated QDs. The polymers which encapsulated QDs were observed. Fourier transform infrared spectra (Figure S1 in the Supporting Information) show that CO stretching of carboxylic groups of polymers appeared at 1570 and 1728 cm−1 after polymer-encapsulation.49 The PL QY of the polymer-encapsulated QDs in water was 73.0%. This PL QY enhancement likely stemmed from multiple QD surface passivation effects by initial surface ligands and the polymers.15,50,51 LDHs were synthesized and, then, exfoliated to form individual LDH nanosheets as host matrices for QD composites. Figure 2A shows an SEM image of LDH powders synthesized from the reaction of an aqueous solution containing 12 mM zinc nitrate, 4 mM aluminum nitrate, and 0.875 M ammonia at 95 °C for 2 h. They had hexagonal shapes with lateral dimensions from 300 nm to 2 μm. The XRD pattern obtained for the powders was typical of hydrotalcitelike materials, exhibiting sharp and symmetric (00l) reflections (Figure 2B). For exfoliation of the LDHs, an anion exchange process was conducted. The interlayer attractions are dependent on the affinity of anions to LDH layers. CO32− ions are known to have a much higher affinity to LDH layers than other anions do.47 Thus, Cl−-intercalated LDH was prepared by the additional anion exchange process, namely the salt-acid method reported previously.46 Due to the incorporation of Cl−, the basal spacing (d003 spacing) of the LDH increased from 0.762 to 0.782 nm after applying the salt-acid method. (Figure 3A). The interlayer expansion is consistent with that in previous works.46,47 The as-prepared Cl−-intercalated LDH powders were treated with formamide for exfoliation. Figure 3B shows the XRD pattern of exfoliated and dried LDH powders. A (003) diffraction peak of a LDH structure is related to the brucitelike layer stacking and the interlayer distance. On the other hand, (110) diffraction peak represents the arrangement

3. RESULTS AND DISCUSSION We designed a strategy for a synthesis of highly luminescent and photostable composites by incorporating QDs into LDH matrices without deterioration of PL efficiency of the fluorophores during the entire processes of composite formations. CdSe/CdS/ZnS (core/shell/shell) QDs as fluorophores were prepared and dispersed in chloroform by modified protocols of a previous report.48 Figure 1A is TEM images of the QDs. Single crystalline nanocrystals had

Figure 1. (A) TEM images of CdSe/CdS/ZnS (core/shell/shell) quantum dots (QDs). (B) Absorption and photoluminescence spectra of the QDs in chloroform. (C) Photographs of the QD solution under room light (I) and under UV light irradiation (II). (D) Approach for the fabrication of poly(maleic acid-alt-octadecene)-encapsulated QDs. (E) TEM image of polymer-encapsulated QDs. 1073



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observations further confirmed exfoliation of LDHs, visualizing the morphology of the obtained LDH nanosheets. Figure 3D shows an AFM image of the exfoliated LDH. Sheetlike objects with a similar lateral size as those detected by the SEM analysis were observed. The height analysis, carried out at steps between a nanosheet and the substrate surface, gives a value of ∼1 nm. If the exfoliated LDHs consisted of only two LDH layers, the height of the structures shown in Figure 3C would be no less than 1.6 nm (0.8 nm × 2).47,55 It can thus be reasonably concluded that the LDH nanosheets had monolayer structures. An exfoliated metal hydroxide nanosheet has a positively charged surface due to the existence of the trivalent metal ions (in our case, Al3+). The exfoliated LDH nanosheets had a positive zeta potential of 24.9 ± 0.6 mV. As discussed above, the polymer-encapsulated QDs had a negative zeta potential under our assembly condition. Exploiting the opposite surface charges possessed by the polymer encapsulated QDs and LDH nanosheets, the QD-polymer-LDH composites could be selfassembled via electrostatic interactions as illustrated in Figure 4. Figure 2. (A) SEM image and (B) XRD pattern of the structures synthesized by reaction of an aqueous solution containing 12 mM zinc nitrate hexahydrate, 4 mM aluminum nitrate nonahydrate, and 0.875 M ammonia at 95 °C for 2 h.

Figure 4. Schematic illustration of the assembly process for the formation of QD-polymer-LDH composites.

After completion of the assembly step, their PL properties were investigated. Figure 5A displays PL spectra of polymerencapsulated QD (QD-polymer) solution (I) and QDpolymer-LDH composite solution (II). The shapes (the intensities and line widths) and peak wavelengths of the PL spectra were nearly identical for both samples. The insets show photographs of the solutions under UV irradiation. Under 60 μW/cm2 power UV light irradiation, the QD-polymer-LDH composites were highly luminescent as shown in the left inset. The right inset is a photograph of the solutions for color recognition under lower power UV light. The PL QY of the QD-polymer-LDH composite solution was 74.1%. Figure 5B shows the solutions after the mild centrifugation (2000 rpm, 5 min). While the polymer-encapsulated QDs retained the well dispersibility in the solution after the centrifugation, QDpolymer-LDH composites were settled on the e-tube wall and the supernatant shows very weak PL and absorption (1% of that before the assembly), suggesting that the polymerencapsulated QDs were successfully captured by LDH matrices. Figure 6A shows a TEM image of the composites. Matrices and QDs were observed. The greater magnified image (Figure 6B) reveals that QDs were distributed in the LDH matrix. The matrix was highly crystalline, with a lattice spacing of about 0.35 nm, corresponding to the distance between the (012) planes in the layer of the zinc aluminum LDH. In addition, a lattice spacing of 0.22 nm corresponded to the distance between the (220) planes in the CdSe crystal lattice. The composition of the composite was investigated by energy dispersive X-ray

Figure 3. (A) XRD patterns of layered double hydroxide (LDH) powders before and after intercalated-anion exchange. (B) XRD pattern of the exfoliated and dried LDH nanosheets. (C) Photograph of a colloidal suspension of exfoliated LDH nanosheets under room light. The inset is a photograph of the suspension in the dark. (D) Atomic force microscope (AFM) image of exfoliated LDH nanosheets. The curve is the corresponding cross-sectional data along the white line in the image.

of ions along the metal hydroxide layer plane.52 Before the formamide treatment, the (003) peak was much stronger than the (110) peak as shown in Figure 2B. However, after the formamide treatment (Figure 3B), the (110) to (003) ratio was much higher than that before the treatment. Moreover, even this relatively small (003) peak might appear during the drying step for the XRD analysis.52−54 A clear Tyndall light scattering was observed by a side-incident light beam, indicating the presence of exfoliated LDH nanosheets dispersed in the solution (Figure 3C). The inset displays the Tyndall light scattering in the dark. The colloidal suspension was stable, and no sediment was observed upon long-term standing. AFM 1074



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QD-polymer-LDH composite films were fabricated by a simple drop-casting of the solution on substrates. Figure 7

Figure 7. (A) Photoluminescence spectra of (I) QD-polymer film and (II) QD-polymer-LDH composite film. (left inset) Photograph of the QD-polymer film and the QD-polymer-LDH composite film under 60 μW/cm2 power UV light irradiation. (right inset) Photograph of the films under lower power UV light irradiation for color recognition.

Figure 5. (A) Photoluminescence spectra of (I) QD-polymer solution and (II) QD-polymer-LDH composite solution. (left inset) Photograph of the QD-polymer solution and the QD-polymer-LDH composite solution under 60 μW/cm2 power UV light irradiation. (right inset) Photograph of the solutions under lower power UV light irradiation for color recognition. (B) Photographs of the QD-polymer solution and the QD-polymer-LDH composite solution under room light and UV light irradiation after the centrifugation (2000 rpm, 5 min).

shows PL spectra and photographs of QD-polymer (polymerencapsulated QD) film (I) and the QD-polymer-LDH composite film (II) under UV light irradiations. The QDpolymer-LDH composite film was highly luminescent under 60 μW/cm2 UV light (the left inset). The QD-polymer-LDH composite film was clearly brighter than the QD-polymer film. This PL QY reduction of QD-polymer film is attributable to a nonuniform distribution of fluorophores of the QD-polymer film without LDH matrices, and the PL QY reduction due to their exposure to oxygen and moisture during the solidification. It should be noted that the PL peak wavelengths of the QDpolymer film and the QD-polymer-LDH-composite film were 613 and 611 nm, respectively. In the case of the QD-polymer film, the emission spectrum was slightly red-shifted to that of the solution-phase (Figure 5). Without polymer-encapsulation, the emission spectrum of bare QDs was significantly red-shifted by solidification (PL peak wavelength: 628 nm) as shown in Figure S2 in the Supporting Information. In the case of the QD-polymer-LDH composite films, however, the fluorescence peak position was almost the same as that of the organic QD solution. The LDH can host polymer-encapsulated QDs in solid state without undesired energy transfers from the agglomeration. Photostabilities of the samples were examined under light irradiation (peak position 405 nm, power 3 mW/cm2). Figure 8A shows the fluorescence intensities at peak wavelengths, normalized by their initial values, as a function of the light irradiation time. The QD film fabricated by a drop-casting of the organic QD solution showed rapid PL decrease down to 63% of the initial intensity within as early as 5 h and kept deteriorating to 34% after 80 h. The polymer-encapsulated QD (denoted as QD-polymer in Figure 8A) composites exhibited 74% and 44% of their original intensity at 5 and 80 h, respectively. This enhanced photostability may be attributed to the QD surface passivation by the polymers. The QD-polymerLDH composites retained 65% of its original intensity after the 80 h light irradiation. This is likely attributable to multiple protections of the QDs by the polymers and the LDH matrices from the environment. When the QD-polymer-LDH composites were dried at 80 °C under vacuum, their photostabilities were further improved. The vacuum-dried QD-polymer-LDH composites displayed a high photostability (81% of their initial

Figure 6. (A and B) Low-magnified TEM image and HR-TEM image of the QD-polymer-LDH composites, respectively. (A inset) Energy dispersive X-ray spectroscopy (EDX) pattern of the areas indicated by the circle in part A.

spectroscopy (EDX). The EDX pattern (the inset of Figure 6A) of the area marked by the circle in Figure 6A shows the Cd, Se, S, Zn, Al, O, and C signals; the Cu signal is attributed to the copper mesh used for TEM imaging, which confirms that QDpolymer-LDH composites were successfully synthesized by this method. 1075



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formation of the QD-polymer-LDH composites afforded significantly enhanced photostabilities in comparison with the QDs and the polymer-encapsulated QDs. This method has a strong point for delicate control of optical properties because already-synthesized QDs were used. In principle, this method allows composite fabrications using QDs covering a wide range of size and identity. These QD-polymer-LDH composites have a variety of potential application area such as lighting, display, and optical coating materials. The present study also warrants efforts to extend this strategy to synthesize other LDH-QD hybrid composites, in general with different combinations of QDs and divalent and trivalent cations of various LDH nanostructures. Well-designed combinations can create unique properties of composites suitable to a wide range of applications.

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ASSOCIATED CONTENT

S Supporting Information *

Fourier transform infrared spectra of quantum dots (QDs) and polymer-encapsulated QDs (Figure S1). Photoluminescence spectrum of quantum dot film (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 8. (A) Photostability tests under light irradiation (peak position 405 nm, power 3 mW/cm2). (B) Green, yellow, and red color QDpolymer-LDH composite films fabricated using CdSe/CdS/ZnS QDs and the corresponding photoluminescence spectra of the composite films.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by KOSEF grant funded by MOST (20120006280, 20120005973), the Priority Research Center Program through NRF (2009-0094036), and by the Industrial Strategic technology development program (No. 10035274, Quantum dot phosphorus converted LED Module) funded by the Ministry of Knowledge Economy (MKE, Korea).

peak intensity) after 80 h, which is enhanced when compared to that without vacuum drying (65%). This photostability enhancement is ascribed to reductions in water content of the composites by vacuum drying. Unlike solution phase where there typically exist excess of surface ligands passivating the QD surface, PL QYs of QDs can be deteriorated upon continuous excitation in solid state as a sensitive function of moisture, oxygen, and light. Thus, the presence of water can expedite deterioration of PL intensity under continuous light irradiation. Composites with a variety of emission spectra can be also fabricated by tuning energy band gaps of CdSe/CdS/ZnS QDs as building blocks. Figure 8B displays a photograph of green, yellow, and red color composite films and their corresponding PL spectra. Their initial PL properties of the colloidal QD organic solutions were preserved. This method should be applied to prepare highly luminescent composites covering a wide spectral range using QDs with various identities and sizes.

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REFERENCES

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4. CONCLUSIONS In summary, highly luminescent and photostable composites were fabricated by incorporating QDs into LDH matrices without deterioration of a PL efficiency of the QDs during the entire processes of composite formations. The QDs were encapsulated by the amphiphilic polymers to transfer them into water without exchanging initial surface ligands (PL QY 73.0%). The negative zeta potentials of the polymerencapsulated QDs allowed a simple assembly of polymerencapsulated QDs and LDH nanosheets with the positive zeta potential by electrostatic interactions (PL QY 74.1%). QDpolymer-LDH composite films were fabricated by a simple drop-casting of the solution on substrates. The PL properties of QD-polymer-LDH composite film preserved those of the QDdispersed organic solution. We also demonstrated that the 1076



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Highly Fluorescent and Stable Quantum Dot-Polymer-Layered Double

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