ZnS:Cu Quantum Dots for White

Feb 2, 2015 - Finally, high color rendition white light was generated from the CdS/ZnS:Cu QDs-assisted phosphor- converted white light-emitting diode ...
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Microwave-Assisted Synthesis of CdS/ZnS:Cu Quantum Dots for White Light-Emitting Diodes with High Color Rendition Tong-Tong Xuan,† Jia-Qing Liu,† Rong-Jun Xie,‡ Hui-Li Li,*,† and Zhuo Sun† †

Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of Education, Department of Physics, East China Normal University, Shanghai, 200062, China ‡ Sialon Group, National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan S Supporting Information *

ABSTRACT: High quality CdS/ZnS:Cu quantum dots (QDs) were first synthesized via a green microwave irradiation route. As-prepared core/shell doped QDs presented a strong absorption in the blue light region and highly efficient red to deep red emission with a maximum quantum yield of 40%. The composite formed by dispersing CdS/ZnS:Cu QDs into silicone resin showed an excellent photostability under blue illumination. Finally, high color rendition white light was generated from the CdS/ZnS:Cu QDs-assisted phosphorconverted white light-emitting diode (WLED) in which there was no reabsorption between quantum dots and phosphors. Under operation of 40 mA forward bias current, the fabricated WLED emitted bright natural white light with a high color rendering index of 90, a luminous efficiency of 46.5 lm/W, and the correlated color temperature of 6591 K. Simultaneously, the good color stability was accompanied by the CIE color coordinates of (0.3155, 0.3041) under different forward bias currents.

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tuning the electronic and optical properties of QDs, which usually exhibits a wide range of tunable emission in the whole visible−NIR window depending on the size and nature of the host QD.28,29 For example, CdS:Cu gives a broad emission turning window from 590 to 700 nm but a low absorption for wavelengths longer than 470 nm, which makes it an optimal red-emitting material to improve the color rendering index of YAG:Ce-based WLEDs.30 Wang et al. reported the synthesis of OA/TOP capped CdS:Cu/ZnS d-QDs via a hot-injection route and first applied them to YAG:Ce-based WLEDs for enhancing the Ra value without inducing the self-quenching effect.25 However, it is hard to apply the hot-injection process to largescale production due to complex manipulations and the difficulty in controlling the rate of precursor injection, batch transfer in a short time, and the reaction temperature. Additionally, the use of toxic and expensive TOP is unfriendly to the environment. As an alternative, Cu doped CdS QDs with tunable emission can be synthesized by a conventional noninjection route, which may be enlarged to gram-scale production, but the absorption band of the synthesized d-QDs located under 430 nm, which does not match well with the blue InGaN LED chip.31 In our previous work,32 CdS QDs with strong absorption at 450 nm were successfully synthesized by the one-pot microwave irradiation, which has been proved to be a great technique to prepare nanoparticles.8,32−38 Through this

hite light-emitting diodes (WLEDs) have drawn considerable attention in recent years due to their long lifetimes, low power consumption, fast response, high luminous efficiency, and so on.1−5 Currently, the most popular commercial WLEDs are based on a blue LED chip with a yellow-emitting Y3Al15O12:Ce3+ (YAG:Ce) phosphor, due to their simple structure, high luminous efficiency, and low cost.6 However, it is difficult for this type of WLEDs to achieve a high color rendering index (Ra) due to the lack of red components in the emission. In order to improve the color rendering properties, red-emitting inorganic phosphors and colloidal quantum dots (QDs) such as Ca2SiO4:Eu2+, Sr2Si5N8:Eu2+, CdSe, CdSe/ZnSe, and CuInS2 with high photoluminescence quantum yield (PLQY) have been developed for WLEDs with excellent Ra.2,7−15 Nonetheless, the reabsorption between multiple phosphors as well as the intrinsic small Stokes shift of QDs usually induces a strong self-quenching effect, which results in significant color altering and a decrease in the luminous efficiency of WLEDs.16,17 Recently, transition metal ion doped semiconductor quantum dots (d-QDs) have been widely investigated due to their unique optical properties.18−23 They not only retain advantages of intrinsic QDs but also have new properties, such as longer lifetimes, improved thermal and photochemical stability, and especially the minimization of the vexing selfabsorption owing to the enlarged Stokes shift.20,24−27 Therefore, in comparison with intrinsic QDs, d-QDs are more suitable to solve the low Ra problem of YAG:Ce-based WLEDs. Recently, Cu-doping is recognized as a versatile strategy for © XXXX American Chemical Society

Received: October 14, 2014 Revised: January 5, 2015

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DOI: 10.1021/cm503770w Chem. Mater. XXXX, XXX, XXX−XXX

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microwave route.32 In a typical synthesis, 4 mL of cadmium solution and 1 mL of sulfur solution were loaded into a 10 mL reaction vessel and heated to 260 °C for 10 min in the air. Then, it was rapidly cooled down to 60 °C. The resultant CdS QDs were precipitated by acetone and redispersed in n-hexane. Second, ZnS shell was grown onto the above CdS core QDs at 240 °C in the mixture solution of ODE and OAm with a volume ratio of 3:1. Typically, a hexane solution of asprepared CdS QDs was added into a mixture solution of 1.25 mL of OAm and 3.75 mL of ODE, and then, the hexane was removed by heating the solution to 120 °C under N2 flow for 20 min. Subsequently, the mixture was cooled to room temperature under N2 flow. The resultant solution together with 1 mL of 50 mM zinc− stearate solution and 1 mL sulfur solution were loaded into a microwave reaction vessel and heated to 240 °C for 10 min by microwave in the air. CdS/ZnS core/shell QDs were produced and precipitated by acetone and redispersed in n-hexane. Third, Cu doped CdS/ZnS core/shell QDs were synthesized via the microwave route in the Ar flow. In a typical experiment, a n-hexane solution of CdS/ZnS core/shell QDs (25 nmol) was added to a mixture solution of ODE and OAm (5 mL, ODE: OAm = 3:1), and then n-hexane was removed by heating to 120 °C under N2 flow for 20 min. Subsequently, the mixture was cooled to room temperature under N2 flow. After that, the resulting mixture solution and 15 μL of Cu(S2CNEt2)2 solution were loaded into a reaction vessel in an Ar glovebox and heated to 220 °C by microwave for 10 min. The resultant Cu doped core/shell QDs were purified by the standard precipitation method using acetone as a precipitant and redispersed in chloroform. Large Scale Synthesis of CdS/ZnS:Cu d-QDs. A mixture of 16 mL of Cd precursor and 4 mL of S precursor was added to a 35 mL microwave reaction vessel and heated to 260 °C for 10 min in the air. The resultant CdS core QDs were precipitated by acetone and redispersed in n-hexane. Subsequently, ZnS shell was grown and Cu was doped to synthesize CdS/ZnS:Cu d-QDs by simply repeating the above experimental manipulation, but the amount of reaction solutions was magnified 4 times. Also, for the present microwave system, 24 microwave reactors can be precisely manipulated in turn by the computer. After 4 h, about 1.8 g of CdS/ZnS:Cu QDs was obtained. For mass production, microwave reactors can be further amplified to a liter scale. Undoubtedly, gram-scale QDs can be easily prepared at one time by a multimode microwave heater. Thus, our method is highly automated and fully adaptable for batch manufacturing CdS/ZnS:Cu d-QDs. Fabrication of Doped Core/Shell QDs-Assisted High Color Rendition WLEDs. A total of 0.5 mL of chloroform solution of CdS/ ZnS:Cu QDs with a density of 0.12 g/mL was added to the mixture of silicone resin (YD65-5A) and anhydride curing agent (YD65-5B) with vigorous stirring, and the weight ratio of YD65-5A to YD65-5B was kept as 1:1. After the mixture was heated to 40 °C for 30 min under vacuum to evaporate chloroform, 8 wt % LuAG:Ce and YAG:Ce phosphors with a weight ratio of 1:5 synthesized by the conventional solid-state reaction method40 were added with vigorous stirring. The obtained mixture slurry was directly coated onto blue chips (λpeak = 455 nm) by dipping and thermally cured in a vacuum oven at 120 °C for 30 min to form a luminescent layer with the thickness of ∼2.5 mm. Finally, a white LED with high color rendering was fabricated. Characterization. The absorption spectra of as-prepared QDs in chloroform solution were measured by using a UV/vis spectrophotometer (Hitachi U-3900). Photoluminescence (PL) spectra and external quantum yield (QY) were measured by using a fluorescence spectrophotometer (Horiba Jobin Yvon, FluoroMax-4) containing an integrating sphere unit (Horiba Jobin Yvon, F3029). The optical density of doped QDs at the excitation wavelength was around 0.05 to avoid any significant reabsorption. The samples were placed in the integrating sphere and excited by a monochromatic light source with a wavelength of 445−455 nm. For the temperature-dependent photoluminescence measurement, the sample was bedded in a sample cavity and heated to the desired temperature by a high-temperature fluorescence controller (Tianjin Orient KOJI Co., Ltd., TAP-02). The sample was kept for 10 min to reach thermal equilibrium, which would guarantee a uniform temperature both on the surface and in the

route, gram-scale QDs can be easily achieved at one time by one multimode microwave system.32,38 When Cu dopants are incorporated into the above CdS QDs, orange-red emission coming from the recombination of electrons in the conduction band with holes in the Cu T2 states is expected. Thus, the onepot microwave irradiation should be an ideal mass production route to synthesize deep red CdS:Cu QDs. In this article, we first synthesized CdS/ZnS:Cu core/shell QDs having an efficient blue absorption and a strong red emission in the organic phase by a fast and facile microwave irradiation process (Figure 1). Cu dopant was introduced

Figure 1. Schematic representation of preparation for Cu doped CdS/ ZnS core/shell QDs.

during shelling of ZnS, which is called growth doping. This doping strategy can efficiently quench bandgap emission of the host and enhance the dopant emission greatly.31 As-prepared dQDs in the present work not only possess a high PLQY (∼40%) but also exhibit the excellent thermal and photochemical stability. The optical properties can be easily tailored by controlling the reaction temperature and the Cu concentration. Finally, the as-prepared CdS/ZnS:Cu d-QDs were combined with LuAG:Ce, YAG:Ce phosphors, and a blue LED chip to successfully fabricate a WLED, which shows high color rendering properties and no reabsorption between d-QDs and phosphors. It suggests that the as-prepared CdS/ZnS:Cu dQDs would be a promising red emitter for improving the Ra of WLEDs.

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EXPERIMENTAL SECTION

Chemicals. Cadmium oxide (CdO, 99.5%), sulfur powder (S, 99.99%), oleic acid (OA, >90%), zinc stearate (Zn(S2CNEt2)2, chemically pure), n-hexane (analytical regent), methanol (analytical regent), and acetone (analytical regent) were purchased from Sinopharm Chemical Reagent Co., Ltd. Octadecene (ODE, 90.0%), and oleylamine (OAm, >70%) were purchased from Alfa Aesar. Transparent silicone resin (YD65-5A, YD65-5B) used for WLED packaging was purchased from Raypower Optoelectronics (Suzhou) Co., Ltd. All reagents were used as received without further experimental purification. Precursor Preparation. CdO (26 mg) was dissolved in the mixture of OA (0.565 mL) and ODE (3.5 mL) at 160 °C for 10 min by the microwave irradiation (Explorer-48, CEM Co.) to yield a golden-yellow solution as Cd precursor. S powder (32 mg) was dissolved in ODE (10 mL) at 120 °C for 10 min by the microwave irradiation to yield a transparent solution as S precursor. Cu(S2CNEt2)2 was prepared as reported in the literature.39 Cu(S2CNEt2)2 (6.3 mg) was dissolved in OAm (8 mL) in a three-neck flask at 100 °C under N2 flow to yield a blue transparent solution as Cu precursor. Note that the Cu precursor should be freshly made before the synthesis. Zn(S2CNEt2)2 powder was added to a microwave reaction vessel with ODE and heated to 200 °C for 10 min. The solution was cooled to room temperature, and then the slurry was formed, which was directly used as Zn precursor for ZnS shell growth. Microwave-Assisted Synthesis of Cu Doped CdS/ZnS Core/ Shell QDs. First, zinc-blende CdS core QDs were prepared via a B

DOI: 10.1021/cm503770w Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials interior of sample, and then the PL spectrum was measured by using a fluorescence spectrophotometer. The phase evolution and morphology were characterized by a M21XVHF2Z (Mac Science Co. Ltd.) Xray diffractometer with Cu Kα radiation (λ = 1.5406 Å, V = 40 kV, and I = 40 mA) and a JEM-2100F transmission electron microscope (TEM), respectively. Electroluminescence (EL) properties such as emission spectrum, luminous efficiency, color rendering index (CRI), correlated color temperature (CCT), and Commission Internationale de l’Eclairage (CIE) color coordinates of as-fabricated WLEDs were investigated by a UV/vis/near IR spectrophoto-colorimeter (PMS-80) at room temperature.

absorption band to that of undoped CdS/ZnS, a large Stokes shift of 7050 cm−1 is acquired, suggesting that CdS/ZnS:Cu dQDs are very suitable for blue LED chip-based WLEDs to provide a deep red emission and avoid the reabsorption between multiple phosphors. To further confirm Cu doping into CdS/ZnS core/shell QDs, X-ray photoelectron spectroscopy (XPS) analysis of the constituent elements was carried out for the CdS/ZnS and CdS/ZnS:Cu QDs. As shown in Figure 3a, almost identical Cd

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RESULTS AND DISCUSSION Figure 2 displays the UV/vis absorption and photoluminescence spectra of CdS core QDs, CdS/ZnS core/shell QDs, and

Figure 3. (a) XPS spectra of CdS/ZnS and CdS/ZnS:Cu QDs. (b) Magnification of Cu 2p peaks for CdS/ZnS:Cu QDs.

3d, Zn 2p, S 2p, O 1s, and C 1s XPS spectra are observed for both QDs, whereas the XPS spectrum of Cu doped CdS/ZnS QDs exhibits two additional Cu 2p peaks, indicating that Cu ions are doped into the CdS/ZnS QDs. The binding energies located at 932.8 eV (2p3/2) and 952.3 eV (2p1/2) (Figure 3b) match well with the 2p signals of Cu2+ in CuS, concluding that the oxidation state of the Cu element in the CdS/ZnS:Cu QDs is +2. The signals of O 1s and C 1s in Figure 3a should come from organics wrapped in the QDs surface. To optimize the optical properties of Cu doped CdS/ZnS QDs, the d-QDs with different Cu molar concentrations were synthesized at the same reaction temperature. Figure 4 illustrates their UV/vis absorption and emission spectra. It can be seen that the absorption positions are independent of the Cu dopant concentration (Figure 4a), indicating that the size of d-QDs does not change with the dopant concentration of Cu, which is in accord with PL excitation (PLE) spectra shown in Supporting Information Figure S2a. However, the dopant Cu emission in the deep red region gives a significant shift to the longer wavelength from 657 to 678 nm with the Cu concentration increasing. Simultaneously, the Cu dopant emission intensity is enhanced, and the corresponding host bandgap PL intensity is gradually decreased (Figure 4b). The reason is attributed to an increase of dopant Cu ions on the surface of the CdS/ZnS host QDs providing more holes to recombine with electrons from the bottom of the conduction band. The PL intensity reaches a maximum value at 0.03% Cu concentration, and the corresponding PLQY still remains about 40% (Supporting Information Figure S3a), a slight decrease in comparison to that of undoped CdS/ZnS QDs. Above 0.03%, the fluorescence intensity of dopant emission is quenched by the nonradiative transition between the neighboring dopant ions.20,25,31

Figure 2. UV/vis absorption and PL spectra of CdS, CdS/ZnS, and CdS/ZnS:Cu QDs (λex = 365 nm).

CdS/ZnS:Cu doped core/shell QDs synthesized by the microwave irradiation. The CdS cores were synthesized according to the previous report.32 It shows a sharp excitonic absorption band peaked at 448 nm and a symmetric and narrow band-edge emission centered at 463 nm. The absorption wavelength of 448 nm is slightly shorter than the emission peak of a blue InGaN LED chip. After successfully growing a ZnS shell around the host CdS by the microwave irradiation, about 7 nm redshift of UV/vis absorption and PL spectra could be observed, which enables it to match well with the blue InGaN LED chips. Most importantly, the PLQY of core/shell QDs increases to 45%, doubling that of CdS cores (only ∼21%). Further introducing Cu into CdS/ZnS core/shell QDs, a very broad PL spectrum centered at 670 nm is presented. According to a theory proposed by Srivastava et al.,39 the emission in the deep red region should result from the recombination of the excited electrons located at the bottom of the conduction band of the host with the holes in the Cu2+ T2 states (Supporting Information Figure S1).39,41 In consideration of the similar C

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Figure 5. TEM (a−c) and HR-TEM images (d−f) of CdS, CdS/ZnS, and CdS/ZnS:Cu QDs, respectively. Figure 4. UV/vis absorption (a) and PL spectra (b) of CdS/ZnS:Cu QDs with different Cu doping concentrations synthesized at 220 °C for 10 min. UV/vis absorption (c) and PL spectra (d) of 0.03% Cu doped CdS/ZnS QDs at different reaction temperatures (λex = 365 nm).

It is worth noting that the absorption and dopant PL peak positions of d-QDs are found to be independent of the reaction temperature in our experiments, which is quite different from the results reported by Wang and co-workers.25 The absorption (Figure 4c) and emission peaks (Figure 4d) do not exhibit any shift with the reaction temperature increasing, indicating that dQDs do not grow, which can be further confirmed by PLE spectra shown in Supporting Information Figure S2b. An enhancement of the Cu dopant PL intensity is observed in Figure 4d when the reaction temperature rises from 180 to 220 °C, which is ascribed to more Cu dopants adsorbed on the surface of the nanocrystals accessing the inside of the d-QDs, that is to say, the transition of “lattice adsorption” to the process of “lattice diffusion”.31 It means that the critical temperature of the lattice diffusion for the CdS/ZnS:Cu d-QDs system would be about 220 °C. Additionally, a decrease of the surface defect states at higher temperature may be another reason for the increased PL intensity.38 The highest PL intensity of d-QDs is obtained at 220 °C with a PLQY of 40% (Supporting Information Figure S3b). Above this temperature, the dopant PL intensity starts to decrease, owing to the occurrence of Ostwald ripening in high temperature.20,31 Figure 5 shows the typical transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) imàges of CdS, CdS/ZnS, and CdS/ZnS:Cu QDs prepared via the microwave route. The QDs exhibit monodispersity and nearly spherical shape with a narrow size distribution (Figures 5a−c). The average crystalline size of as-prepared CdS, CdS/ZnS, and CdS/ZnS:Cu QDs is measured as 2.91, 3.40, and 3.36 nm (Supporting Information Figure S4), respectively, which corresponds well with the absorption spectra in Figure 2. The similar and clear lattice fringe images presented in Figure 5e−f imply high crystallinity and the identical crystal structures of CdS, CdS/ZnS, and CdS/ZnS:Cu QDs, which can be further demonstrated by the powder X-ray diffraction (XRD) patterns given in Figure 6. The XRD results show that the as-prepared CdS core, CdS/ ZnS core/shell, and CdS/ZnS:Cu d-QDs belong to the facecentered cubic phase with the zinc blende structure (JCPDS.

Figure 6. XRD patterns of CdS, CdS/ZnS, and CdS/ZnS:Cu QDs.

No. 89-0440). The growth of the ZnS shell leads to a slight shift of the diffraction peaks from CdS to ZnS because of the lattice shrinkage caused by different ion radii between Cd2+ (0.95 Å) and Zn2+ (0.75 Å).42 Subsequently, the Cu incorporation causes no significant change of the crystal structure of the CdS/ZnS host QDs, which is in coincidence well with the analysis of TEM. Good stability is a prerequisite for d-QDs to successfully apply to WLEDs. Therefore, the photostability and thermal stability of as-prepared CdS/ZnS:Cu QDs and CdS/ZnS:Cu QDs/silicone resin composite were investigated and are shown in Figure 7. It can be clearly observed that PL intensities of the d-QDs and d-QDs/silicone resin composite are almost identical to the initial one after they were continuously irradiated for 5 h by a 10 W blue lamp under 455 nm (Figure 7a,c), which suggests an excellent photostability. As temperature rises, PL intensities of d-QDs and d-QDs/silicone resin composite show a decrease, and at 65 °C, their emission intensities preserve about 80% of the initial one (Figure 7b,d). In the actual LED applications, the temperature inside the device can reach above 100 °C due to the chip interior self-heating. And thus, the dQDs/silicone resin composite was heated further up to 150 °C where the PL intensity decreases to 60% (Figure 7d), which is lower than 78% of the YAG:Ce3+ phosphor. Such thermal stability is acceptable for WLEDs application. To fabricate WLEDs with high color rendering properties, deep red-emitting CdS/ZnS:Cu QDs were coated onto a blue chip of 25 lm/W (λpeak = 455 nm) combining with the greenemitting LuAG:Ce and yellow-emitting YAG:Ce phosphors. Figure 8 presents the optical properties of as-fabricated WLEDs. The pale yellow phosphor layer is attributed to the D

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Figure 7. (a) Photostability of CdS/ZnS:Cu QDs under blue illumination; (b) thermal stability of CdS/ZnS:Cu QDs under air atmosphere; (c) photostability of CdS/ZnS:Cu QDs/silicone resin composite under blue illumination; (d) thermal stability of CdS/ ZnS:Cu QDs/silicone resin composite and YAG:Ce under air atmosphere.

integration of yellow YAG:Ce and aqua LuAG:Ce phosphors with yellow CdS/ZnS:Cu QDs (Figure 8a). When it was operated at 20 mA, a bright white emission could be obtained, as shown in Figure 8b. Figure 8c gives the PLE spectra of asprepared CdS/ZnS:Cu QDs and LuAG:Ce and YAG:Ce phosphors. It is found that the three phosphors can be well excited by 455 nm. Also, there is no overlap not only between PLE spectra of phosphors (Figure 8c) and PL spectrum of QDs (Figure 8d) but between PL spectra of phosphors (Figure 8d) and PLE spectrum of QDs (Figure 8c) due to the appropriate bandgap and large Stokes shift of CdS/ZnS:Cu d-QDs. This indicates that the d-QDs are very suitable for application in YAG:Ce and LuAG:Ce-based WLEDs as a red emitter to improve their color rending properties while avoiding reabsorption. Electroluminescence (EL) spectra of as-fabricated WLED obviously consist of four emission bands peaked at 455, 515, 570, and 725 nm, respectively, which correspond well with the PL spectra of the blue LED chip, LuAG:Ce, YAG:Ce phosphors, and CdS/ZnS:Cu QDs (Figure 8d). An obvious emission from the red and deep red region is enhanced markedly by the incorporation of CdS/ZnS:Cu QDs. When the forward current increases from 10 to 200 mA, the luminous intensity of WLED continuously increases without luminescence saturation (Figure 8e). However, the ratio of two main emission bands gradually alters. Red emitting band from d-QDs is predominant at low forward current. With increasing forward current, yellow and green parts of the spectrum resulting from YAG:Ce and LuAG:Ce phosphors become dominant. This is because the thermal quenching of d-QDs is stronger than that of YAG:Ce and LuAG:Ce phosphors (Figure 7d).40,43 Simultaneously, a higher Ra (Figure 8f) value between 86 and 90 is always accompanies during the whole forward current increasing in comparison with LuAG:Ce, YAG:Ce-based, and YAG:Ce d-QDs-based WLEDs with Ra values of 79 and 80, respectively, due to the deficiency of red or cyan emission. Especially, the values of R1, R8, and R9 (Figure 8h), which are related to red and deep red color rendering, are greatly improved by the addition of deep red-emitting CdS/ZnS:Cu

Figure 8. Photographs of (a) a white LED based on as-prepared Cu doped CdS/ZnS QDs, LuAG:Ce, and YAG:Ce phosphors and (b) the white LED operated at 20 mA. (c) The PLE spectra of Cu doped CdS/ZnS QDs, LuAG:Ce, and YAG:Ce phosphors. (d) The EL and PL spectra of white LED, blue LED chip, Cu doped CdS/ZnS QDs, LuAG:Ce, and YAG:Ce phosphors. The EL spectra (e), Ra values (f), and CIE 1931 color coordinates (g) of as-fabricated white LED operated under various forward bias currents. (h) CRI values from R1 to R9 of the white LED and a white LED based on solely LuAG:Ce and YAG:Ce phosphors.

QDs; for example, special R9 is enhanced from 60 to 93. WLEDs with a high value of R9 have rarely been reported because the emission wavelength of commercial red-emitting phosphors is commonly below 630 nm, which makes it difficult to improve the deep-red region color rendering. However, the as-prepared CdS/ZnS:Cu QDs present not only the emission wavelength above 630 nm but also a broad emission band, which leads to the enhancement of the color rendering in both the red and deep-red regions and also shows their potential biomedical and painting appreciation applications. It is also pointed out that CIE color coordinates of WLED exhibit little change under different forward bias current and locate around (0.3155, 0.3041) as shown in Figure 8g, suggesting the good color stability of WLED. Under the operation of 40 mA forward bias current, the fabricated WLED shows a Ra of 90, a luminous efficiency (ηL) of 46.5 lm/W, and the CCT of 6591 K. These properties are much better than those of previously E

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Chemistry of Materials reported CdSe/ZnSe QDs-assisted Sr3SiO5:Ce3+, Li+-based WLED (ηL = 26.8 lm/W, CCT = 6140 K, Ra = 85).2,13,25 Moreover, a luminous efficiency of 46.5 lm/W is much higher than 37.4 lm/W of OA/TOP capped CdS:Cu/ZnS QDsassisted YAG:Ce3+-based WLED, although the luminous efficiency of a blue LED chip used in the present work is only 25 lm/W, far lower than 50 lm/W of a blue LED chip adopted by Wang and co-workers.25 The excellent optical properties of WLED fabricated in the present work should be ascribed to CdS/ZnS:Cu QDs with high PLQY and large Stokes shift caused by the optimized synthesis route and different doping mechanism.

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CONCLUSIONS In summary, CdS/ZnS:Cu QDs with high photochemical stability were successfully synthesized by a facile and green microwave irradiation route in the organic phase. The asprepared CdS/ZnS:Cu d-QDs presented a strong absorption in the blue light region and high efficient (PLQY up to 40%) red to deep red emission. The optical properties of d-QDs can be controlled by varying the reaction temperature and Cu concentration. A WLED was fabricated by combining redemitting CdS/ZnS:Cu QDs with green LuAG:Ce and yellow YAG:Ce phosphors to avoid reabsorption between d-QDs and phosphors. The as-fabricated WLED covered the whole visible spectral range coming from the blue LED chip, LuAG:Ce, YAG:Ce phosphors, and CdS/ZnS:Cu QDs. Under the operation of 40 mA forward bias current, it emitted bright natural white light with a high color rendering index of 90, a luminous efficiency of 46.5 lm/W, and the CCT of 6591 K. Also, it showed the good color stability with the CIE color coordinates of (0.3155, 0.3041) under different forward bias currents.

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ASSOCIATED CONTENT

S Supporting Information *

Additional schematic energy diagram, PLE spectra, PLQY of CdS/ZnS:Cu QDs, and the size distribution of CdS, CdS/ZnS, CdS/ZnS:Cu2+ QDs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.-L. Li). 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. 51472087), Shanghai Municipal Natural Science Foundation (13ZR1412500), Innovation Program of Shanghai Municipal Education Commission (14ZZ050), the Fundamental Research Funds for the Central Universities (No. 78260022), the ECNU Reward for Excellent Doctoral Students in Academics (xrzz2014029), and Special Project for Nanotechnology of Shanghai (No. 12 nm0503801).

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DOI: 10.1021/cm503770w Chem. Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/cm503770w Chem. Mater. XXXX, XXX, XXX−XXX