Facile Access to White Fluorescent Carbon Dots toward Light-Emitting

Mar 21, 2014 - Surface-passivated photoluminescent carbon dots (CDs) were prepared via a facile one-step pyrolysis of poly(acrylic acid) (PAA) in the ...
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Facile Access to White Fluorescent Carbon Dots toward LightEmitting Devices Li-Hua Mao, Wen-Qi Tang, Zheng-Yan Deng, Si-Si Liu, Cai-Feng Wang,* and Su Chen* State Key Laboratory of Materials-Oriented Chemical Engineering and College of Chemistry and Chemical Engineering, Nanjing Tech University, 5 Xin Mofan Road, Nanjing 210009, People’s Republic of China S Supporting Information *

ABSTRACT: Surface-passivated photoluminescent carbon dots (CDs) were prepared via a facile one-step pyrolysis of poly(acrylic acid) (PAA) in the presence of glycerol. In the formation process of CDs, glycerol not only acts as solvent, but also promotes the carbonization of PAA (carbon source) and passivates the surface of CDs. The as-prepared CDs can emit brightwhite fluorescence under ultraviolet (UV) illumination. The structure and optical properties of the CDs were thoroughly investigated. The CDs show excellent solubility in water and high photoluminescence stability in UV-radiation, salty, oxidic, or reductive environments, suggesting their great promise as white-light-emitting materials. For their practical applications, a white light-emitting diode (LED) with CDs as white-light converters was demonstrated. Moreover, a backlight with use of CDs as coatings was successfully constructed for the first time. This research would offer a promising new way to obtain white fluorescent CDs and suggests their strong potential for solid-state lighting systems.

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practical applications in diverse fields. Therefore, CDs with other emission colors are quite desirable, especially the white fluorescent CDs (WCDs), which are of great significance to the development of WLEDs. Unfortunately, there are still few reports about WCDs. Liu et al. reported the first fabrication of WCDs, which was achieved via the thermal oxidation of citric acid in molten LiNO3, followed by surface functionalization with poly(ethylene glycol).31 Our group reported that WCDs could be obtained from poly(styrene-co-glycidylmethacrylate) photonic crystals via a one-step pyrolysis process,32 but a complicated precursor was needed. Therefore, more facile and economic approaches to WCDs are still in great demand. In this work, we present an easy operating and low-cost method to prepare WCDs. These WCDs were synthesized via a facile one-step pyrolysis of easily available poly(acrylic acid) (PAA) mixed in glycerol. The roles of PAA and glycerol in the formation of WCDs were thoroughly investigated. PAA is the carbon source, and glycerol acts as the solvent as well as the carbonization promoter and the surface passivation agent. The as-prepared WCDs emit bright-white fluorescence directly with UV illumination and exhibit excellent water-solubility and highly stable fluorescence. To suggest the great potential uses of these WCDs in the optoelectronic devices, we applied WCDs as a single white-light converter to successfully construct a backlight for the first time, as well as a white LED.

INTRODUCTION Solid-state white light-emitting devices (WLEDs) are considered to be a promising illumination source and have attracted extensive research interest owing to their long working life, low power consumptions, fast response times, flexibility in the final products, and so forth.1 Recently, WLEDs based on semiconductor quantum dots (QDs or nanocrystals (NCs)) have been the focus of extensive interest because of many superior performances derived from NCs, such as broad excitation range, size-dependent emission, and high chemical and optical stability.2−4 However, NC-based WLEDs are usually constructed by mixing different color-emitting NCs5,6 or combining NCs with traditional phosphors,7,8 which may involve self-quenching and reabsorption problems due to the intrinsic small ensemble Stokes shift of the NCs,7,9 as well as complicated package processes. White fluorescent NCs, like magic-sized CdSe,10 organics-capped ZnSe,11 trap-rich CdS,12 and onion-like CdSe/ZnS/CdSe/ZnS,12 however, have been successfully prepared to bypass these problems, the biological toxicity of the heavy metals in NCs, especially the II−VI NCs containing Cd and Hg, is still an embarrassing obstacle for their practical applications. Photoluminescent carbon dots (CDs) as novel quantum dots13 with distinct fluorescence, photostability, chemical inertness, low toxicity, and excellent biocompatibility, are comparable or even alternative materials to NCs in a wide range of technologies, like bioimaging,14,15 photocatalysis,16 sensing,17 optoelectronic devices,18 and coding.19 Until now, various CDs have been prepared from graphite,20 multiwalled carbon nanotubes,21 activated carbon,22 graphene,23 carbon soot,24 natural plants,19 and molecular precursors25−30 via a variety of methods, including laser ablation,20 electrochemical routes,21 thermal routes,26−28,30 microwave routes,25 and plasma routes.29 Nevertheless, most CDs just emit dull blue fluorescence under UV excitation, seriously limiting their © 2014 American Chemical Society

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EXPERIMENTAL METHODS Materials. Acrylic acid, isopropanol, ammonium persulfate (APS), glycerol, sodium chloride (NaCl), potassium persulfate (KPS), N,N-dimethylformamide (DMF), N-methyl pyrrolidone Received: Revised: Accepted: Published: 6417

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on a Rigaku Corporation D/max-rC rotating anode X-ray powder diffractometer using a copper target. Raman study was performed using a Horiba HR 800 Raman system equipped with a 514.5 nm laser. Ultraviolet−visible (UV−vis) absorption spectra were taken with a Perkin-Elmer Lambda 850 UV−vis spectrometer. Photoluminescence (PL) measurements were carried out on a Varian Cary Eclipse spectrophotometer, and the quantum yield was calculated by comparison using quinine sulfate in 0.10 M H2SO4 solution as the standard (Φ = 0.54). Time-correlated single-photon counting (TCSPC) data were performed on an Edinburgh FL 900 photocounting system. All the spectra were obtained at room temperature.

(NMP), and dimethylsulfoxide (DMSO) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Ascorbic acid was purchased from Aladdin Chemistry Co., Ltd. All the chemicals were of analytical grade and used as received. High-purity water with the resistivity of greater than 18 MΩ· cm−1 was used in the experiments. Preparation of PAA. First, 60 g isopropanol and 10 mL water were mixed in a four-neck flask. Then, 30 g acrylic acid and 1.5 g APS dissolved in 30 mL water were injected dropwise into the flask when the reaction temperature reached 90 °C, respectively. After 4 h, the product was poured out, and the reduced pressure evaporation was employed to remove solvent and unreacted monomer. Preparation of WCDs. A total of 0.5 g PAA and 10 mL glycerol was placed into a 50 mL three-neck flask, degassed with N2 for 30 min. As the temperature of the mixture increased to 230 °C, the reaction time started to be counted. The reaction was kept for 2 h at this temperature, and then the system was cooled down to ambient temperature naturally. The product was first diluted by water, followed by high-speed centrifugation (12 000 rpm, 20 min) to remove large particles, and at last purified by dialyzing against water with a cellulose ester membrane bag (Mw = 3500) to remove excess glycerol. Preparation of Blue-Luminescent CDs (BCDs) and Treated BCDs (t-BCDs). A total of 0.5 g PAA was dissolved in 30 mL water and degassed with N2 for 30 min. Then the solution was transferred to a poly(tetrafluoroethylene) (Teflon)-lined autoclave (50 mL) and heated at 230 °C for 2 h. After the above reaction, the reactor was cooled down to room temperature naturally. The obtained brown solution was centrifuged at 12 000 rpm for 20 min to remove large particles. The upper solution was vacuum-dried to obtain the solid BCDs. For t-BCDs, the solid BCDs were further refluxed in 10 mL glycerol at 230 °C for 2 h. The process was similar to the preparation of WCDs. Preparation of White LED. For the fabrication of white LED, a UV-LED chip with the peak wavelength centered at ∼380 nm was attached on the bottom of the LED base. The two leads on LED were prepared to connect to the power supply. Afterward, the thermocurable resin (silicone, DowCorning Co.) was mixed with WCDs phosphor (WCDs/ silicone = 1/10 wt/wt) and put in a vacuum chamber to remove the bubbles. The WCD/silicone mixtures were dispensed on the LED chip and thermally cured at 150 °C for 1 h. All the optical performances were measured using a ZWL-600 instrument with integral sphere. Preparation of Backlight. The water solution of WCDs was first concentrated by solvent evaporation. Then, the concentrated solution was directly cast onto a preprepared sheet (glass or polypropylene), and the sheet was kept at room temperature for a period to remove the residual solvent. Finally, an image we wanted to illuminate was covered upon the sheet, which could be lit up with UV excitation from the bottom. Characterization. High-resolution transmission electron microscope (HRTEM) observation was performed with a Tecnai G2 F30 S-TWIN transmission electron microscope. The element analyses were measured on an Elementar Vario EL cube (Germany). Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet 6700 FT-IR spectrometer. 1 H nuclear magnetic resonance (1H NMR) spectrum was obtained from a Bruker AVANCE 600. The sample was dissolved in water-d2. X-ray diffraction (XRD) was performed

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RESULTS AND DISCUSSION Synthesis and Characterization of WCDs. In this work, carbon dots with white fluorescence, WCDs, were prepared by refluxing PAA in glycerol under N2 flow at 230 °C for 2 h without any catalysts or further surface passivation, and their applications in backlight and white LED were demonstrated, as shown in Figure 1. For comparison, the CDs derived from the pyrolysis of PAA in water were also prepared, to exhibit blue fluorescence.

Figure 1. Illustration of the route for the synthesis of WCDs and their applications in backlight and white LED.

Glycerol was found to play a great role in the formation of WCDs. Previous reports indicated that glycerol could not produce any CDs in the absence of oxygen at high temperature.26,33 Therefore, intuitively, PAA is the only carbon source, and glycerol is the solvent. However, when PAA was subjected to pyrolysis in water, only dull blue-luminescent CDs (BCDs) dispersed in water with a poor quantum yield of 1.6% were obtained, as seen in the inset of Figure 2a. The quantum yield of WCDs dispersed in water is 9.0%, nearly 5 times higher than that of BCDs. Besides, from the UV−vis absorption spectra (Figure 2a), a weak absorption peak at ca. 270 nm assigned to the π−π* transition of aromatic sp2 domains is observed for WCDs, while no obvious absorption peaks are observed in the UV−vis absorption of BCDs. The enhancement of quantum yield and the variation of UV−vis absorption for WCDs all illustrate that glycerol acts as more than just solvent at elevated temperature. Moreover, when the asprepared BCDs were further treated by glycerol at 230 °C for 2 h, UV−vis absorption and PL spectrum of the obtained product (t-BCDs) were very close to those of WCDs, and the quantum 6418

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Figure 2. (a) UV−vis absorption spectra and (b) PL emission spectra (λex = 380 nm) of BCDs, WCDs, and t-BCDs dispersed in water. Inset: photographs of BCDs dispersed in water under sunlight and UV (365 nm, center) illumination, respectively.

yield of t-BCDs increased up to 9.7%, revealing the key role of glycerol in the formation of WCDs (Figure 2). From the elemental analyses (Table 1), the carbon content of WCDs (53.4%) and t-BCDs (52.2%) is obviously higher

and red-shift of PL peak of CDs. Indeed, Figure 2b obviously shows a red-shift and broadening of the emission peaks of WCDs and t-BCDs in comparison with that of BCDs. FT-IR analyses (Figure 3A) were used to identify the surface chemistry of the as-prepared CDs. The FT-IR spectrum of BCDs is almost the same as that of PAA. This result is very similar to the polymer dots reported by Yang’s group,34 which means BCDs may keep abundant PAA chains on the surface of BCDs. However, the characteristic peaks (2700−2500, 1244, 1172, 1105, 908, and 804 cm−1) of PAA39 are absent in the spectra of WCDs and t-BCDs (Supporting Information, SI, Figure S1), further confirming a higher carbonization degree of WCDs and t-BCDs with the help of glycerol during the hightemperature treatment process. In the FT-IR spectrum of WCDs, the broad and intense peak around 3426 cm−1 and the vibration band at 1055 cm−1 can be ascribed to the stretching vibrations of O−H and C−O of alcohol, respectively, implying the existence of residual hydroxyl groups of glycerol on the surface of WCDs. These functional groups confer hydrophilicity and excellent water-solubility to WCDs. The solubility of WCDs in water is over 190 mg/mL. The stretching vibration band of CO (1733 cm−1) combined with the asymmetric and symmetric stretching vibration bands of C−O−C (1273 and 1130 cm−1) demonstrates the existence of the carboxylate groups,25,40 which results from the surface passivation of WCDs by glycerol. The passivation process might be similar to that of CDs passivated by poly(ethylene glycol).25,26,31,40−42 The surface passivation of glycerol may give rise to the increase in quantum yield like previous reports.31,41,42

Table 1. Elemental Analyses of WCDs, BCDs, and t-BCDs element

WCDs (%)

BCDs (%)

t-BCDs (%)

C H

53.4 7.6

48.1 6.1

52.2 7.6

than that of BCDs (48.1%), indicating a higher carbonization degree of WCDs and t-BCDs. This may be caused by two main reasons. First, the abundant hydroxyl groups of glycerol could take part in the dehydration reaction with the hydrogen atoms of PAA and accelerate the carbonization process.34 Second, the high water-absorbing ability of glycerol may further promote the dehydration reaction.35 Hence, glycerol could expedite the carbonization of PAA in the formation process of WCDs. An increase of carbonization degree may make WCDs own higher sp2 fraction (more and larger sp2 nanodomains) than BCDs, which is supported by the more obvious π−π* transition absorption of WCDs (Figure 2a).36 The photoluminescence produced by CDs is proposed to be closely related to the nanosized carbon domains (aromatic structures) and the surface localized states of CDs, and the emission band gaps are determined by the sizes of sp2 nanodomains within CDs.37,38 Therefore, a broader size distribution of nanodomains and a higher fraction of large ones could cause the broadening

Figure 3. (A) FT-IR spectra of (a) PAA, (b) BCDs, and (c) WCDs; (B) 1H NMR spectra of WCDs in D2O. 6419

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Figure 4. (a) TEM images of WCDs. (b) Size histogram of WCDs measured from TEM images for over 100 particles. (c,d) High-resolution TEM images of WCDs (scale bars = 2 nm). Lines and arrows indicate the lattice plane distances.

Figure 5. XRD pattern (a) and Raman spectrum (b) of WCDs. 1

H NMR spectrum shown in Figure 3B is also meant to reveal the surface chemistry of WCDs,43 which further confirms the surface passivation of WCDs by glycerol. The 4.16 ppm and 5.14 ppm peaks are assigned to the hydrogen atoms adjacent to the oxygen atoms of the carboxylate groups that result from surface passivation. The peaks around 3.6 ppm are the hydrogen atoms adjacent to hydroxyl groups, indicating that the unreacted hydroxyl groups of glycerol may be dangled on the surface of WCDs. Transmission electron microscope (TEM) images of WCDs are shown in Figure 4a. The as-prepared WCDs are relatively uniform and well-dispersed, showing near-spherical morphology. The average size of the WCDs was measured to be about 3.4 nm, according to the size distribution histogram (Figure 4b). High-resolution TEM (HRTEM) images (Figure 4c,d) reveal that WCDs have well-resolved lattice fringes. The lattice spacings of 0.25 nm (Figure 4c) and 0.34 nm (Figure 4d) agree well with the (100) and (002) diffraction facets of graphite, respectively. TEM images of BCDs (SI Figure S2a) show a similar size distribution, ranging from 2 to 5 nm. The lattice spacings of BCDs (SI Figure S2b,c) were determined to be 0.27 and 0.36 nm, corresponding to the (020) and (002) diffraction facets of graphite carbon, respectively.32 The increased (002) interlayer spacing may be attributed to the lower carbonization level of BCDs compared to WCDs, and more oxygencontaining groups involved can increase the interlayer spacing.44

XRD analysis (Figure 5a) was conducted to detect the crystallinity of WCDs. Like many reported CDs,23,44,45 the pattern shows a broad (002) peak around 2θ = 24°, further confirming the graphite structure of WCDs. The Raman spectrum of WCDs (Figure 5b) shows a weak D band at 1386 cm−1 and an apparent G band at 1554 cm−1, indicating there are mainly sp2 carbons with some sp3 hybrid carbons in WCDs. Therefore, the as-prepared WCDs are mainly composed of sp2 graphitic carbons with sp3 carbon defects. Optical Properties of WCDs. WCDs are strongly photoluminescent in the suspension state. The PL spectra of WCDs in water excited by various incident lights are shown in Figure 6a. The PL emission wavelength of the WCDs is excitation-dependent. As the excitation wavelength increases from 300 to 500 nm, the emission peak position red-shifts from ∼476 nm to ∼550 nm (SI Figure S3) and the PL intensity reaches a maximum when excited at 420 nm. The mechanism underlying the optical behavior however remains elusive, which may be due to the effects of nanoparticles of different sizes within the sample and a distribution of different emissive sites on each nanoparticle.22 The quantum yield of the as-prepared WCDs (347 nm excitation) was determined to be 9.0%, comparable to the WCDs prepared by Liu et al.31 and even higher than those of many CDs previously reported.21,24,25,29,40 Here, it should be pointed out that both PAA and glycerol in water exhibit negligible fluorescence. Thus, the fluorescence in the products is attributed to the obtained nanoparticles. Due to 6420

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Figure 6. (a) PL emission spectra of WCDs in water at different excitation wavelengths; (b) PL emission spectra of WCDs dispersed in different solvent (λex = 365 nm); (c) PL emission spectra under different UV light of WCDs in water, inset: photographs of WCDs in water with sunlight and UV (365 nm, center) illumination, respectively; and (d) time-resolved fluorescence decay curves for BCDs and WCDs in water (λex = 380 nm).

the abundant hydrophilic groups on the surface of WCDs, such nanomaterials can also be dispersed in other polar solvents, like DMF, NMP, and DMSO, to exhibit solvent-dependent fluorescence (Figure 6b). As shown in Figure 6c, the aqueous solution of WCDs, which is brownish yellow under sunlight, displays white photoluminescence under 365 nm UV light. The PL spectra of WCDs in water excited by a series of UV illuminations are all broad and span the entire width of the visible spectrum. Commission Internationale d’Eclairage (CIE) chromaticity coordinates can be calculated from those emission curves.31,46 The color coordinates are (0.24, 0.29), (0.24, 0.29), (0.24, 0.30), and (0.24, 0.31) corresponding to the excitation wavelength of 320 , 340 , 360, and 380 nm, all belonging to the white gamut (SI Figure S4). Therefore, besides the UV light of 365 nm, a wide range of UV from 320 to 380 nm can excite the water solution of WCDs to emit white light. To acquire further insight into the photoluminescence, the time-resolved fluorescence of BCDs and WCDs were performed under the excitation of 380 nm. As seen in Figure 6d, both CDs exhibit double exponential decay, and the fitting parameters were summarized in Table 2. The average lifetime could be concluded from eq 1. τ̅ =

α1τ12 + α2τ22 α1τ1 + α2τ2

Table 2. Biexponential Fit Values of BCDs and WCDs in Water BCDs WCDs

α1 (%)

τ1

α2 (%)

τ2

χ2

98.5 90.3

5.00 5.50

1.5 9.7

7.70 20.00

1.031 1.012

associated with the glycerol surface passivation33,47 and the higher carbonization of WCDs.37 The PL stability of the as-prepared WCDs was also investigated, which is of great significance to their practical applications. The PL intensity almost does not change after high power UV radiation (∼1 W, 365 nm) for 30 h and storage for 3 months (Figure 7a,b). In addition, when the water solution of WCDs contains NaCl at concentrations even up to 350 mM, no obvious change of PL intensity is observed (Figure 7c). Moreover, the strong oxidant K2S2O8 and the strong reductant ascorbic acid were also used to test the PL stability of WCDs. The PL intensity changes little with the concentrations of ascorbic acid up to 100 mM. For K2S2O8, with the content increasing up to 60 mM, the PL intensity decreases slightly (Figure 7d). This means that the as-prepared WCDs can even keep its PL intensity in an oxidic or reductive environment. All these results reveal the excellent PL stability of WCDs. Applications of WCDs in White Light-Emitting Devices. By virtue of their white-emitting ability, WCDs show enormous practical applications in the lighting and display field. Here, we fabricated a white LED based on WCDs, where WCD/silicone mixtures were coated on a 380 nm UV chip and acted as white-light converters (SI Figure S5a). The UV light emitted from the UV chip can be transformed into white light when passing through the WCD/silicone mixtures.

(1)

The calculated average lifetimes for BCDs and WCDs are 5.06 and 9.56 ns, respectively. Both results are comparable to the reported values.29,32 Relative to BCDs, the significantly increased lifetime of WCDs implies decreased nonradiative traps and increased fluorophore density, which may be closely 6421

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Figure 7. (a) Dependence of PL intensity on radiation time for WCDs under 365 nm in water; (b) PL spectra of fresh and 3-month-stored solutions of WCDs (λex = 380 nm); (c) PL intensity of WCDs in NaCl aqueous solution against the ionic strength; and (d) effect of the content of K2S2O8 and ascorbic acid on the PL intensity of WCDs.

Figure 8. (a) EL spectrum of the white LED based on WCDs operated at 350 mA, inset: photographs of the white LED under daylight and operation, respectively. (b) The photograph of the white LED in the dark.

When illuminated by UV from the bottom, the image can be well displayed in the dark by the bright-white light emitted from the sheet (Figure 9a). Besides the glass sheet, flexible plastic sheets can also be selected as the substrate. The WCD coated plastic sheet also emits bright-white light upon UV (365 nm) illumination and is flexible and easily bent. Similar to the water solution of WCDs, the PL spectrum of the WCD coatings is also broad and spans the entire width of the visible spectrum (Figure 9b). The CIE chromaticity coordinate calculated from the PL spectrum is (0.27, 0.32) (Figure 9c), which means that WCDs can be promising white-light-emitting coatings and may find potential applications in illumination and display backlights.

The as-fabricated white LED produces bright-white light with a color rendering index (CRI) of 78.2 at 350 mA, which is comparable to commercial white LEDs. The emission spectrum for this device is broad and spreads almost the entire visible range (from 400 to 700 nm) (Figure 8a), which is out of the broad emission of WCDs under UV excitation. The coordinate of the white LED is located at (0.26, 0.30), belonging to the white gamut (SI Figure S5b). For practical applications, the white LED can illuminate an image in the dark (Figure 8b), suggesting the heavy metal-free WCDs with stable optical properties are promising materials in the optoelectronic applications. To further broaden their applications, the water solution of WCDs was used as white photoluminescent ink and cast onto a glass sheet, which could then emit bright-white fluorescence upon illumination with 365 nm radiation. Furthermore, we constructed a backlight by covering an image on the sheet. 6422

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Figure 9. (a) Illustration of the route for the preparation of the backlight. (b) PL emission spectra of the WCD coatings. Inset: photographs of the white-light-emitting flexible plastic sheet with UV (365 nm, center) illumination. (c) CIE 1931 chromaticity diagram for the WCDs coatings.

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CONCLUSIONS

Corresponding Author

In summary, we successfully synthesized surface-passivated white-fluorescence carbon dots (WCDs) via a facile one-step pyrolysis process from PAA and glycerol. Compared with blueemitting CDs pyrolyzed from PAA in water, WCDs show a higher carbonization degree and greatly improved quantum yield due to the participation of glycerol in the reaction. Glycerol expedites the carbonization of PAA and also passivates the surface of WCDs. The obtained WCDs can produce brightwhite fluorescence under UV illumination. The excellent watersolubility and the high photoluminescent stability in various environments make WCDs promising white-light-emitting materials for light-emitting devices. To realize their potential, we exploited WCDs as a single white-light converter to construct a backlight for the first time. A white LED based on WCDs was also demonstrated. Both devices can produce bright-white light for lighting, exhibiting great potential use of these WCDs in the solid-state lighting systems.

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AUTHOR INFORMATION

*Tel.: 86-25-83172258. Fax: 86-25-83172258. E-mail: chensu@ njtech.edu.cn (S.C.); [email protected] (C.-F.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National High Technology Research and Development Program of China (863 Program) (2012AA030313), National Natural Science Foundation of China (21076103), Specialized Research Fund for the Doctoral Program of Higher Education of China (20103221110001), Industrial Project in the Science and Technology Pillar Program of Jiangsu Province (BE2012181), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

(1) Schubert, E. F.; Kim, J. K. Solid-State Light Sources Getting Smart. Science 2005, 308, 1274. (2) Wang, X.; Li, W.; Sun, K. Stable Efficient CdSe/CdS/ZnS Core/ Multi-Shell Nanophosphors Fabricated through a Phosphine-Free Route for White Light-Emitting-Diodes with High Color Rendering Properties. J. Mater. Chem. 2011, 21, 8558. (3) Jang, H. S.; Yang, H.; Kim, S. W.; Han, J. Y.; Lee, S. G.; Jeon, D. Y. White Light-Emitting Diodes with Excellent Color Rendering Based

ASSOCIATED CONTENT

S Supporting Information *

FT-IR, TEM images, PL emission spectra, CIE 1931 chromaticity diagrams, and schematic view. This material is available free of charge via the Internet at http://pubs.acs.org. 6423

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