ZnS Quantum Dots Embedded in Polyvinylpyrrolidone (PVP

Oct 24, 2016 - Cu−In−S/ZnS Quantum Dots Embedded in Polyvinylpyrrolidone. (PVP) Solids for White Light-Emitting Diodes (LEDs). Wen-Qing Ji,. ‡...
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Cu−In−S/ZnS Quantum Dots Embedded in Polyvinylpyrrolidone (PVP) Solids for White Light-Emitting Diodes (LEDs) Wen-Qing Ji,‡ Qiu-Hong Zhang,‡ Cai-Feng Wang,* and Su Chen* State Key Laboratory of Materials-Oriented Chemical Engineering and College of Chemical Engineering, Nanjing Tech University, 5 Xin Mofan Road, Nanjing 210009, People’s Republic of China S Supporting Information *

ABSTRACT: Here, we report a facile and green route to prepare Cu−In−S (CIS)/ZnS core/shell quantum dots (QDs) embedded in polyvinylpyrrolidone (PVP) solids and their application as phosphors for white light-emitting diodes (WLEDs). CIS/ZnS core−shell QDs were easily synthesized in the aqueous phase, assisted by microwave irradiation within 20 min at 95 °C. The emission wavelength of CIS/ZnS QDs could be tunable between a large range of 543−700 nm by adjusting the [Cu]/[In] ratios, and the photoluminescence (PL) quantum yield could be up to 43% by coating a ZnS layer. QD-embedded PVP solids then were facilely fabricated by mixing CIS/ZnS QDs and PVP in water, followed by drying, to show improved PL intensity, higher photostability, and higher thermostability. Finally, we demonstrated their potential application as a white light source by using green- and redemitting CIS/ZnS/PVP nanocomposite solids as color converters, in combination with a blue light-emitting diode (LED) chip.

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INTRODUCTION Solid-state white light-emitting diodes (WLEDs) have great prospects in commercial lighting technologies, because they show higher luminous efficiency than traditional incandescent bulbs and fluorescent lamps.1 Current WLEDs are generally constructed from InGaN blue chips with broadband yellowemitting color converter phosphors such as Y3Al5O12:Ce3+ (YAG:Ce).2−5 However, these WLEDs have some drawbacks, including lack of red spectral region, low color-rendering indexes, and expensive raw materials. Colloidal quantum dots (QDs), especially type II-VI QDs, such as CdSe- and CdTebased QDs, are considered to be a promising alternative to conventional phosphors for WLEDs, because of their merits of size-tunable emission wavelength, high color purity, and high luminescence efficiency. When QDs are condensed into solid state, however, photoluminescence (PL) quenching is commonly observed, mainly due to QD aggregation and energy transfer between neighboring QDs.6,7 While inert matrices have been introduced to partly solve the aggregation and energy transfer issues,8,9 self-quenching still occurs, because of reabsorption derived from the intrinsic small Stokes shift of type II-VI QDs.10,11 Also, the presence of cadmium in QDs evokes environmental concern, which limits their applications. Recently, type I-III-VI QDs, such as CuInS2 (CIS), have received much attention, because of their low toxicity and relatively large Stokes shifts, which might be able to replace traditional cadmium-containing QDs.12−15 As reported previously, CIS QDs could be prepared by hot-injection methods or heat-up synthesis16 via a one-pot synthesis strategy. The © 2016 American Chemical Society

synthesis of CIS QDs usually requires a large number of organic solvents and relative high reaction temperatures,17−20 which are not friendly in green synthesis. Compared to the organic-phase synthesis, the preparation of QDs in water shows more exciting prospects, since water is inexpensive and green. Thus, some water-soluble CIS QDs has been explored, including several microwave-assisted procedure examples.21,22 Meanwhile, highbandgap ZnS shell has been developed to passivate CIS QDs to improve their quantum yield (QY).12,15 On the other hand, to address aggregation issue, CIS/ZnS QDs embedded in silica have been realized via a high-temperature organic-phase synthesis, ligand exchange, and silanization process.23,24 Despite great advances in this field, the preparation of CIS-based QDs in polymer matrix via aqueous synthetic strategies still remains sparse, and it is highly desirable to develop a facile and green route to fabricate CIS QDs/polymer nanocomposites. Herein, we present the green and rapid aqueous synthesis of CIS/ZnS/PVP (where PVP = polyvinylpyrrolidone) nanocomposites and their potential application in WLEDs (see Scheme 1). Water-soluble CIS/ZnS core/shell QDs were facilely fabricated by using dual stabilizing agents of Lglutathione (GSH) and sodium citrate under microwave irradiation within 20 min. The CIS/ZnS QDs with a tunable emission from 543 nm to 700 nm and PL QY up to 43% were Received: Revised: Accepted: Published: 11700

July 15, 2016 September 23, 2016 October 24, 2016 October 24, 2016 DOI: 10.1021/acs.iecr.6b02698 Ind. Eng. Chem. Res. 2016, 55, 11700−11705

Article

Industrial & Engineering Chemistry Research

washing by ethanol and then drying, the CIS core QDs powders for characterization were obtained. Synthesis of CIS/ZnS Core/Shell QDs. After 5 min, the appropriate amount of Zn(OAc)2·2H2O, thiourea, and Lglutathione (molar ratio of 1:1:2) dissolved in 5 mL of water were added into the original solution to grow ZnS shell around the CIS cores and the reaction would last for 15 min under microwave irradiation. CIS/ZnS core/shell QDs were precipitated by the addition of ethanol. Finally, CIS/ZnS core/ shell QDs powders were obtained by several circles of dispersion−precipitation with ethanol and then drying for further use. Preparation of CIS/ZnS/PVP Phosphors. Twenty milliliters (20 mL) of aqueous solution of CIS/ZnS QD and PVP (0.7 g) were treated by ultrasonic for 30 min, and then the mixture was dried in an oven at 60 °C for 12 h under air atmosphere. Dried blocks were subjected to grinding in agate mortar and screening out by mesh sieve to obtain finely dispersed CIS/ZnS/PVP powders. Preparation of WLEDs. To fabricate a WLED, a blue-lightemitting LED chip (emission wavelength at 460 nm) was fixed on an LED pedestal. Next, green (540 nm) and red (655 nm) CIS/ZnS/PVP QD powders [green/red phosphors = 3/1 (w/ w)] were mixed in equal amounts with the heated curable silicone, and then placed under vacuum, to remove the bubbles in the mixture. Subsequently, the mixture was loaded on the LED chip and treated under 150 °C for 30 min. Characterizations. PL was performed with Varian Cary Eclipse spectrophotometer and ultraviolet−visible (UV-vis) absorption were measured on PerkinElmer Lambda 900 UV-vis spectrometer. Transmission electron microscopy (TEM) images of QDs were achieved using a JEOL Model JEM2100 electron microscope. Fourier transform infrared (FT-IR) spectrometry was carried out on a Nicolet 6700 FT-IR spectrometer, over a range from 400 cm−1 to 4000 cm−1. Xray diffraction (XRD) was examined with a Rigaku Corp. D/ max-rC rotating-anode powder X-ray diffractometer, using a copper target. Energy-dispersive X-ray (EDX) spectra were investigated using a Hitachi S-4800 field-emission scanning electron microscopy (FESEM) system, coupled with an energydispersive X-ray spectrometer. Time-resolved fluorescence decay curves were explored with Edinburgh Model FL 900 photocounting system under 405 nm excitation. The PL QY measurements were referenced to Rhodamine 6G in ethanol (literature QY = 0.95 at 488 nm) as the standard (specific details are given in the Supporting Information). Electroluminescence (EL) spectra of WLEDs were performed with a

Scheme 1. Microwave Synthetic Illustration for the Preparation of CIS/ZnS QDs and CIS/ZnS/PVP Nanocomposites for WLEDs

obtained by varying the molar ratio of Cu/In. Subsequently, the CIS/ZnS QDs were dispersed into a highly transparent PVP matrix by simply mixing them in water, followed by drying directly instead of ethanol sedimentary procedure, to afford solid-state down-conversion materials with good stability. Finally, we employed green- and red-emitting CIS/ZnS/PVP nanocomposite solids as color converters in combination with blue light-emitting diodes to demonstrate their potential in WLEDs.

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EXPERIMENTAL METHODS Materials. Zinc acetate [Zn(OAc)2·2H2O, 99.0%], indium chloride (InCl3, 99.9%), copper chloride (CuCl2·2H2O, 99.0%) and L-glutathione were purchased from Aldrich. Sodium citrate (C6H3Na3O7·2H2O, analytical reagent grade), sodium sulfide (Na2S·9H2O, 98%), thiourea (99.0%), sodium hydroxide (NaOH, 96%) and PVP were acquired from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were used without further purification. Clean water with a resistance of >18 MΩ cm was used in all experiments. Synthesis of CIS Core QDs. CIS core QDs were prepared according to a modified literature method,15 by replacing the conventional heating procedure with a microwave-assisted procedure. In a typical procedure for the synthesis of CIS core QDs with a Cu/In ratio of 1:4, 0.01 mmol of CuCl2, 0.04 mmol of InCl3, 0.16 mmol of sodium citrate, 0.06 mmol of Lglutathione, and 20 mL of water were loaded in a 50 mL flask. Afterward, the mixture solution was degassed by N2 under magnetic stirring for 20 min. Subsequently, the solution was heated to 95 °C by microwave irradiation and the reaction was kept at this temperature for 5 min. CIS core QDs were precipitated by the addition of absolute ethanol. By repeated

Figure 1. (a) UV-vis absorption spectra and (b) PL emission spectra of CIS/ZnS core/shell QDs with different Cu/In ratios in the cores. 11701

DOI: 10.1021/acs.iecr.6b02698 Ind. Eng. Chem. Res. 2016, 55, 11700−11705

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Industrial & Engineering Chemistry Research

CIS cores QDs have a mean diameter of 3.2 nm, while CIS/ ZnS core/shell QDs show a mean diameter of 4.8 nm. The XRD patterns of CIS cores and CIS/ZnS core/shell QDs are shown in Figure 2c, which display broad diffraction peaks for both types of QDs. Notably, the peaks of CIS/ZnS core/shell QDs shift to higher angles after coating of the ZnS shell, which is consistent with a previous report.26 The lowactivity thiourea as a precursor might be a key factor to preventing the independent nucleation of ZnS.27 Combined with the TEM images and XRD pattern, it can be concluded that there is a successful coating of the ZnS shells on CIS cores to form well-defined CIS/ZnS QDs. The wide diffraction peaks also imply the small particle size of the QDs. Time-resolved fluorescence of the CIS core QDs and CIS/ ZnS core/shell QDs was performed with excitation at 405 nm to further investigate the dynamical features, as shown in Figure 2d. The PL decay lifetime was achieved by a multidimensional time-correlated single photon counting (TCSPC) and decay curves of as-prepared QDs exhibited biexponential decay, using eq 1:

spectral analysis system (Hangzhou Zhongwei Photoelectricity Co., Ltd., Model ZWL-600).

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RESULTS AND DISCUSSION In this work, we employed a microwave-assisted synthetic strategy to replace the conventional heating procedure, to quickly fabricate CIS/ZnS QDs, according to a modified literature method.15 Two stabilizing agents, L-glutathione and sodium citrate, were used as the ligands to balance the reactivity of different precursors to avoid the phase separation during the QD synthesis process. CIS/ZnS core−shell QDs were easily synthesized in the aqueous phase assisted by microwave irradiation within 20 min at 95 °C. High reactive Na2S as the sulfur source is the key factor in preparing subsize CIS core QDs at low temperature. ZnS was chosen as the shell to prepare CIS/ZnS QDs that wide-band-gap ZnS could provide further passivation to CIS cores and efficiently improve PL QY of CIS QDs.25 The band gap of the CIS QDs is dominated by copper content, as shown in Figure 1. Different Cu/In ratios were used to prepare CIS/ZnS core/shell QDs. The UV-vis absorption and PL spectra of a series of CIS/ZnS QDs prepared from Cu/ In ratios of 1:15, 1:10, 1:8, and 1:4, respectively, are demonstrated in Figures 1a and 1b. The absorption of CIS/ ZnS QDs red-shifts with the increase of Cu/In ratio, because of the decrease of band gap of the QDs. Meanwhile, the PL peak shifts to longer wavelengths, from 542 nm to 700 nm (Figure 1b). Therefore, by adjusting Cu/In ratios, we could obtain CIS/ZnS QDs with large-range tunable emission varying from the visible region to the near-infrared (NIR) spectral region. The PL QYs of the CIS/ZnS QDs were determined to be 22%−43% by using Rhodamine 6G as a standard [see detailed calculation in the Supporting Information (Table S1)]. The morphology of both CIS and CIS/ZnS QDs were investigated by transmission electron microscopy (TEM). Both types of QDs were prepared under optimal condition with Cu/ In ratio of 1/4. As seen in Figures 2a and 2b, both the QDs are spherical and show homogeneous dispersion without distinct aggregation. The inset graphs show size distribution of QDs.

⎛ t ⎞ ⎛ t⎞ Y (t ) = α1 exp⎜ − ⎟ + α2 exp⎜ − ⎟ ⎝ τ2 ⎠ ⎝ τ1 ⎠

(1)

In this equation, τ1 and τ2 represent the attenuation lifetimes, and α1 and α2 indicate the fractional contributions. The mean lifetime (τ)̅ was determined according to eq 2: τ̅ =

α1τ12 + α2τ22 α1τ1 + α2τ2

(2)

The decay curve is shown in Figure 2d with specific fitting parameters in the table. The τ1 values of two types of QDs is 80.4 and 101.0 ns, respectively, and the τ2 values are 202.3 and 350.1 ns, respectively. The mean PL decay lifetime of the CIS core QDs is calculated to be 196 ns. The average lifetime of CIS/ZnS core/shell QDs increases to 286 ns due to the surface passivation function of the ZnS shell. As described previously, long decay components arise from recombination of donor− acceptor and conduction band-to-acceptor, which relates to internal defect structures that play as donor or acceptor sites.28 To investigate the chemical structure and composition of CIS core and CIS/ZnS core/shell QDs, energy-dispersive spectroscopy (EDX) was carried out to analyze chemical elements. Table 1 shows the EDX results of CIS core QDs and CIS/ZnS Table 1. EDX Results of CIS and CIS/ZnS QDs with Cu/In Ratio of 1/4 Cu (at. %) S (at. %) In (at. %) Zn (at. %)

CIS

CIS/ZnS

9.78 58.62 31.60

6.67 48.72 7.40 37.21

core/shell QDs with Cu/In ratio of 1:4. We could find that the real Cu/In ratio of CIS core QDs is 0.31, which is close to the ratio of 0.25 in original ratio. Obviously, after coating the ZnS shell, the content of In3+ decreased from 31.60 to 7.40, suggesting that In3+ would be partially displaced by Zn2+. In addition, FT-IR spectra were performed to investigate the surface chemistry of the as-prepared CIS/ZnS core/shell QDs (Figure S1 in the Supporting Information). The FT-IR spectrum of the CIS/ZnS core/shell QDs is quite similar to

Figure 2. TEM images of (a) CIS and (b) CIS/ZnS QDs with a Cu/ In ratio of 1/4. The scale bar is 10 nm. The inset graphs are the histogram of size distributions for the corresponding QDs. (c) XRD patterns and (d) Time-resolved fluorescence decay curves of CIS core and CIS/ZnS core/shell QDs. 11702

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decreased abruptly to only the half of the initial intensity. Whereas, the emission of the CIS/ZnS/PVP sample could be maintained at ∼75% of its initial intensity after irradiating for 18 h. In addition, thermostability of the QDs should be considered, since they would be subjected to high temperatures during LED packaging processes. We treated the CIS/ZnS and CIS/ZnS/PVP powders under 150 °C for 1 h. Interestingly, the PL emission intensity of the CIS/ZnS powder decreased to ∼70% of the initial PL intensity (Figure 3c), while there was slight enhancement in the PL intensity of the CIS/ZnS/PVP powder (Figure 3d). Meanwhile, a red shift was observed for the PL emission of CIS/ZnS QDs after thermal annealing, because of the further growth and aggregation of QDs. Whereas, the PVP matrix could provide further protection environment for QDs, leading to a smaller red shift of PL peak for CIS/ZnS/PVP nanocomposites. Both the photostability and thermostability of CIS/ZnS/PVP powder had been improved, because the PVP matrix effectively protected the properties of CIS/ZnS QDs. Therefore, the CIS/ZnS/PVP nanocomposite solids show improved PL and stability, which are useful for LED applications. With merit of intrinsic large Stokes shifts, CIS/ZnS QDs show promising potentials for solid-state down-conversion materials.29−36 Here, taking advantage of their excellent PL properties, good stability and low toxicity, the CIS/ZnS/PVP nanocomposite solids with green and red fluorescence were applied to fabricate WLED devices, in combination with blue LEDs, based on three additive primary color principals (blue, green, and red). A blue chip with an EL peak centered at 455 nm, and two types of CIS/ZnS/PVP light-conversion materials with green and red emission at 540 and 655 nm, respectively, were utilized to construct WLED devices (see Figure 4a). The

that of L-glutathione and sodium citrate. The peak at 2530 cm−1, assigned to the −SH stretching, disappear in the spectrum of CIS/ZnS core/shell QDs, indicating the formation of chemical bonds between ligands and QDs. Solid-state CIS/ZnS/PVP nanocomposites phosphors with bright and stable emission were then fabricated by simply mixing CIS/ZnS QDs and PVP in water and then drying. It is innovative to evaporate the water directly instead of traditional ethanol sedimentary methods, avoiding the use of organic solvents. As examples, three types of CIS/ZnS/PVP powders with emission wavelengths of 540, 586, and 695 nm are shown in Figure S2 in the Supporting Information, which exhibit bright green, orange, and red fluorescence under UV irradiation, respectively. Significantly, the CIS/ZnS/PVP nanocomposite solids show superior optical properties. As demonstrated in Figure 3a,

Figure 3. (a) PL spectra of the solid powders of CIS/ZnS QDs and CIS/ZnS/PVP nanocomposites. (b) Dependence of PL intensity on radiation time for the solid powder of CIS/ZnS/PVP powders under 365 nm UV irradiation. PL spectra of the solid powders of (c) CIS/ ZnS QDs and (d) CIS/ZnS/PVP nanocomposites before and after thermal treatment for 1 h.

compared with the PL peak of the CIS/ZnS QD powders, there is an ∼5 nm blue-shift for the PL peak of CIS/ZnS/PVP nanocomposite solids, suggesting good dispersion of QDs in PVP matrix. As we know, when the dilute solution of QDs is concentrated into solid-state powders or film, red shift of PL peak is commonly observed, because of QD aggregation and interdot energy transfer.6,7 This red-shift phenomenon could be effectively suppressed in CIS/ZnS/PVP nanocomposite powders as the PVP matrix well-isolates QDs to prevent QD aggregation and energy transfer among QDs, and, thus, the PL peak of CIS/ZnS/PVP powders is centered at lower wavelength, in comparison with that of CIS/ZnS QD powders. Furthermore, the PL intensity of CIS/ZnS/PVP powders is stronger than that of CIS/ZnS powders, which confirms the effective suppression of luminescence quenching by introducing polymer matrix to separate QDs.8 Moreover, for practical application in photoelectronic devices such as WLEDs, photostability of QDs is a significant index. We investigated the optical properties of CIS/ZnS QDs and CIS/ZnS/PVP nanocomposites under UV irradiation (Figure 3b). As the CIS/ ZnS QD sample was irradiated under UV light (UV LED, 365 nm, ∼1 W) for 6 h, the PL intensity of CIS/ZnS powders

Figure 4. (a) EL spectrum of a LED chip, and PL spectra of greenand red-emitting CIS/ZnS/PVP fluorescent powders. (b) EL spectra of the as-prepared WLED device. (c, d) Photographs of the WLED device under daylight with power off (panel c) and on (panel d). (e) CIE color coordinate graph of the WLED device.

EL spectrum of the as-prepared WLED device is shown in Figure 4b, which covers the full visible-spectrum area, and presents distinct emission in the region of 500−700 nm benefiting from the incorporation of green and red CIS/ZnS/ PVP nanocomposites. Figures 4c and 4d show the photographs of the constructed WLED with power off and on at daylight, respectively, suggesting the realization for white illumination. 11703

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(4) Zhang, K.; Liu, H. Z.; Wu, Y. T.; Hu, W. B. Synthesis of (Y,Gd)3Al5O12:Ce Nanophosphor by Co-Precipitation Method and its Luminescence Behavior. J. Mater. Sci. 2007, 42, 9200. (5) Dang, H.; Huang, L. K.; Zhang, Y.; Wang, C. F.; Chen, S. LargeScale Ultrasonic Fabrication of White Fluorescent Carbon Dots. Ind. Eng. Chem. Res. 2016, 55, 5335. (6) Pal, B. N.; Ghosh, Y.; Brovelli, S.; Laocharoensuk, R.; Klimov, V. I.; Hollingsworth, J. A.; Htoon, H. “Giant” CdSe/CdS core/shell Nanocrystal Quantum Dots as Efficient Electroluminescent Materials: Strong Influence of Shell Thickness on Light-Emitting Diode Performance. Nano Lett. 2012, 12, 331. (7) Lunz, M.; Bradley, A. L.; Chen, W. Y.; Gerard, V. A.; Byrne, S. J.; Gun’ko, Y. K.; Lesnyak, V.; Gaponik, N. Influence of Quantum Dot Concentration on Förster Resonant Energy Transfer in Monodispersed Nanocrystal Quantum Dot Monolayers. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 205316. (8) Cao, X.; Li, C. M.; Bao, H.; Bao, Q.; Dong, H. Fabrication of Strongly Fluorescent Quantum Dot-Polymer Composite in Aqueous Solution. Chem. Mater. 2007, 19, 3773. (9) Yang, S. Y.; Wang, C. F.; Chen, S. Interface-Directed Assembly of One-Dimensional Ordered Architecture from Quantum Dots Guest and Polymer Host. J. Am. Chem. Soc. 2011, 133, 8412. (10) Woo, J. Y.; Kim, K.; Jeong, S.; Han, C. S. Enhanced Photoluminance of Layered Quantum Dot-Phosphor Nanocomposites as Converting Materials for Light Emitting Diodes. J. Phys. Chem. C 2011, 115, 20945. (11) Wang, X.; Yan, X.; Li, W.; Sun, K. Doped Quantum Dots for White-Light-Emitting Diodes without Reabsorption of Multiphase Phosphors. Adv. Mater. 2012, 24, 2742. (12) Xie, R. G.; Rutherford, M.; Peng, X. G. Formation of highquality I-III-VI semiconductor nanocrystals by tuning relative reactivity of cationic precursors. J. Am. Chem. Soc. 2009, 131, 5691. (13) Feng, J.; Sun, M.; Yang, F.; Yang, X. A Facile Approach to Synthesize High-Quality ZnxCuyInS1.5+x+0.5y Nanocrystal Emitters. Chem. Commun. 2011, 47, 6422. (14) Zhang, W.; Zhong, X. Facile Synthesis of ZnS−CuInS2-Alloyed Nanocrystals for a Color-Tunable Fluorchrome and Photocatalyst. Inorg. Chem. 2011, 50, 4065. (15) Chen, Y. Y.; Li, S. H.; Huang, L. J.; Pan, D. C. Green and Facile Synthesis of Water-Soluble Cu−In−S/ZnS Core/Shell Quantum Dots. Inorg. Chem. 2013, 52, 7819. (16) Sarkar, S.; Karan, N. S.; Pradhan, N. Ultrasmall Color-Tunable Copper-Doped Ternary Semiconductor Nanocrystal Emitters. Angew. Chem., Int. Ed. 2011, 50, 6065. (17) Xie, R. G.; Rutherford, M.; Peng, X. G. Formation of HighQuality I-III-VI Semiconductor Nanocrystals by Tuning Relative Reactivity of Cationic Precursors. J. Am. Chem. Soc. 2009, 131, 5691. (18) Zhang, J.; Xie, R. G.; Yang, W. A Simple Route for Highly Luminescent Quaternary Cu−Zn−In−S Nanocrystal Emitters. Chem. Mater. 2011, 23, 3357. (19) Zhang, W.; Zhong, X. Facile Synthesis of ZnS−CuInS2-Alloyed Nanocrystals for a Color-Tunable Fluorchrome and Photocatalyst. Inorg. Chem. 2011, 50, 4065. (20) Park, J.; Kim, S. W. CuInS2/ZnS Core/Shell Quantum Dots by Cation Exchange and Their Blue-Shifted Photoluminescence. J. Mater. Chem. 2011, 21, 3745. (21) Pein, A.; Baghbanzadeh, M.; Rath, T.; Haas, T.; Maier, E.; Amenitsch, H.; Hofer, F.; Kappe, C. O.; Trimmel, G. Investigation of the Formation of CuInS2 Nanoparticles by the Oleylamine Route: Comparison of Microwave-Assisted and Conventional Syntheses. Inorg. Chem. 2011, 50, 193. (22) Mange, Y. J.; Dewi, M. R.; Macdonald, T. J.; Skinner, W. M.; Nann, T. Rapid Microwave Assisted Synthesis of Nearly Monodisperse Aqueous CuInS2/ZnS Nanocrystals. CrystEngComm 2015, 17, 7820. (23) Sohn, I. S.; Unithrattil, S.; Im, W. B. Stacked Quantum Dot Embedded Silica Film on a Phosphor Plate for Superior Performance of White Light-Emitting Diodes. ACS Appl. Mater. Interfaces 2014, 6, 5744.

Figure 4e demonstrates the CIE color coordinate of (0.30, 0.33) for the WLED, falling in the white gamut, and approaching to the coordinate of pure white-light emission (0.33, 0.33). The development of LEDs should still be given attention, because of potential applications in lighting and display. For practice, CIS/ZnS/PVP nanocomposite solids could be promising materials with low-toxic and stable properties in a photoelectronic field.

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CONCLUSIONS In summary, we have presented a rapid, green, and environmentally friendly route to prepare Cu−In−S/ZnS quantum dots (QDs) embedded in polyvinylpyrrolidone (PVP) solids for white light-emitting diodes (WLEDs). Water-dispersed Cu−In−S/ZnS core/shell QDs were synthesized within 20 min under microwave irradiation, to exhibit tunable photoluminescence (PL) emission from 543 nm to 700 nm by adjusting the molar ratio of Cu/In, as well as PL quantum yield up to 43%. The evaporation of the aqueous solution of CIS/ZnS QDs and PVP then afforded QD-embedded PVP solids with excellent fluorescence and higher photostability/thermostability. Finally, we demonstrated the potential for WLEDs with use of greenand red-emitting CIS/ZnS/PVP solids as color conversion materials. This work might contribute a facile and green route to fabricate low-toxic QD solids with high performance, showing wide potentials in the fields of optoelectronics and optical display.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02698. Quantum yield measurements, FT-IR, photographs of CIS/ZnS/PVP powders (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.-F. Wang). *E-mail: [email protected] (S. Chen). Author Contributions ‡

These authors have contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (No. 21474052), National Key Research and Development Program of China (No. 2016YFB0401700) and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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REFERENCES

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DOI: 10.1021/acs.iecr.6b02698 Ind. Eng. Chem. Res. 2016, 55, 11700−11705

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DOI: 10.1021/acs.iecr.6b02698 Ind. Eng. Chem. Res. 2016, 55, 11700−11705