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Industrial fabrication of Mn-doped CdS/ZnS core/shell nanocrystals (NCs) was conducted using a one-pot method, and the NCs’ fluorescence peak could be varied from 564 to 616 nm by changing the Mn2+ doping process and ZnS shell thickness. The synthesis parameters were optimized to increase the NCs’ output, and ~10 g of NCs could be yielded in 1.5 L of solution. A reverse microemulsion method was used to encapsulate these CdS:Mn/ZnS NCs with SiO2 aerosols to improve their photostability in white-light-emitting diodes (WLEDs). A yield of ~3 × 106 1 W WLEDs was obtained by enveloping blue chips with CdS:Mn/ZnS@SiO2 NCs and YAG:Ce3+ is helpful to industrialization; the WLEDs’ color rendering index and luminous efficiency were 88 and 108 lm/W, respectively.
© 2015 Optical Society of America
1. Introduction
Commercial white-light-emitting diodes (WLEDs) often consist of two major components: blue-emitting GaN-based chips and yellow YAG:Ce3+ phosphors. There are already published alternatives where using green- and red-emitting CdSe/CdS/ZnS NCs instead of YAG:Ce3+ to obtain WLEDs [1]. In comparsion, Mn-doped II–VI group nanocrystals (NCs) are good choices for WLEDs owing to their large light absorption cross sections, large Stokes shifts, and almost complete lack of self-absorption, and their emission wavelength could be tuned with the Mn ion’s environment inside the NCs [2,3].
A colloidal preparation method is a common way to synthesize Mn-doped NCs [3,4]. This method, however, requires a high concentration of precursors to increase the production of NCs during the synthesis process, which often results in rapid growth of NCs after their nucleation, resulting in not only inhomogeneous NC sizes but also low particle density. To date, many research groups have successfully prepared NCs with high particle density by optimizing the synthesis process, e.g., the reaction temperature and ligand. For instance, in 2008, Liu et al. reported a method of synthesizing high-quality gram-scale CdSe NCs by increasing the precursor concentration and reducing the growth temperature [5]. In 2010, Srivastava et al. synthesized gram-scale Mn-doped ZnS core/shell NCs using an improved growth doping technique and a reasonable ratio of Zn to S [6].
WLEDs, which are made using a type of phosphor and a blue-emitting GaN-based chip, often have a low color rendering index (CRI) [7,8]. To address this issue, the synthesis of tunable Mn-doped CdS NCs can be incorporatd to provide an improved red component, and its CRI could be theoretically improved towards 89. To date, several methods have been reported to effectively tune the wavelength of Mn-doped NCs, although the tunability mechanism is still unclear. In Mn-doped CdS/ZnS NCs, a red-shift in the Mn emission from 621 to 633 nm was found as the ZnS shell was increased to 7.5 monolayers (MLs) owing to the stress on the ZnS shell when Mn2+ ions were at the interface of the CdS core and ZnS shell [9]. Moreover, a photoluminescence (PL) red-shift from 580 to 668 nm could also be achieved by controlling the location of Mn2+ ions within similar Mn-doped CdS NCs [10]. Accordingly, the luminescent color of Mn emission in Mn-doped CdS/ZnS NCs is tunable, ranging from green to red, because of changes in the local ligand field strength around the Mn site [11].
In this paper, we successfully used a one-pot method [6] to obtain ~10 g of Mn-doped CdS/ZnS core/shell NCs in 1.5 L of solvents by optimizing the process parameters, especially the ligand (stearic acid, SA) concentration and the order of surfactant addition (oleylamine, OAm). Because the absorption edge of NCs is located at a wavelength of ~450 nm, it is well matched with current commercial blue LEDs. Furthermore, their fluorescence peak can be varied from 564 to 616 nm by changing the Mn2+ doping process and ZnS shell thickness to keep the absorption edge above 440 nm. Further, to efficiently improve the photostability of WLEDs, the NCs were coated with SiO2 shells and then packaged with SiO2 aerosols, yielding inorganic WLEDs.
2. Results and discussion
We successfully prepared ~10 g of CdS:Mn/ZnS NCs powder-coated with SA in 1.5 L of solvents by a one-pot method. In our experiment, SA, which has longer carbon chains, is used as the ligand, and the best molar ratio of SA to Cd is 6, as shown in Fig. 1(a). According to this figure, CdO cannot be dissolved completely in less SA, leading to an inadequate reaction; conversely, more SA will greatly restrain the nucleation of NCs, resulting in a small number of large NCs [12]. Further, Fig. 1(b) shows that an appropriate molar ratio of 5S to 1Cd is helpful to promote the reaction effectively, and the obtained small CdS:Mn nanoclusters can also exist stably in the reaction solution [13]. If too little S monomer is present, the Cd2+ cannot be consumed quickly, and nucleation is slow [12], leading to a small number of large NCs. In contrast, a large amount of S monomer will significantly increase the speed of growth, and the mean size of the obtained NCs is relatively large. In addition, nucleation alone may be prevented by adding OAm in the coating process because OAm can inhibit nucleation [14]. The corresponding concentration and size of CdS:Mn cores with different amounts of OAm are also illustrated in Fig. 1(c).
Fig. 1 Concentration and size of the CdS:Mn cores obtained using different molar ratios of (a) SA to Cd and (b) S to Cd, and (c) different amounts of OAm. TEM (High-Resolution TEM) images of CdS:Mn “core” (d) with and (e) without an added mass of OAm.
Figure 1(d) shows the agglomerated CdS:Mn NCs with an added mass of OAm. In contrast, Fig. 1(e) shows the CdS:Mn NCs without added OAm; their structures are close to spherical with a good distribution, and their mean size is 4.45 nm. To achieve a mass of Mn-doped CdS core/shell NCs, we optimize the precursor concentration, ligand (SA) concentration, and order of surfactant (OAm) addition.
Industrial fabrication of Mn-doped CdS core/shell NCs was realized, and their fluorescence peak can be varied from 564 to 616 nm by changing the Mn doping concentration and core/shell structure. Figure 2(a) shows the PL spectra of samples I–IV, which have peaks at 564, 584, 593, and 616 nm, respectively. The inset shows a typical transmission electron microscopy (TEM) image of NCs in sample II. From this, the obtained NCs are close to spherical with a good distribution, and the average diameter is 5.7 nm. The peak at ~564 nm is caused by the energy transition of Mn2+ between the 6A1 state and the lowest 4T1 state, which is lower than the energy level (410 nm) between its first excited state and the ground state, and the luminescence probability is high in this position [13]. Moreover, green emission can be generated in a tetrahedral coordination environment because the luminescence location of Mn2+ is very sensitive to the crystal field strength [15]. The luminescence location of Mn2+ generally depends on both the electron–phonon coupling strength and the crystal field strength; here, however, the average diameter of samples I–IV varies from 5.15 to 8.17 nm, making the electron–phonon coupling unclear [16]. As a result, the variation in the crystal field of Mn2+ dominates the shift of the fluorescence peak of the NCs. With increasing shell thickness, the stress at the NCs’ surface is also increased, decreasing the space enveloping the Mn2+. Then the crystal field strength related to the transition energy grows, decreasing the energy difference between the energy states of 6A1 and 4T1, which corresponds to the red shift of the fluorescence peak, as shown in Fig. 2(c) [9,15]. Further, the crystal lattice of CdS will also become more symmetrical, which decreases the energy level splitting of Mn2+, and the fluorescence becomes red [13]. A PL red-shift was also observed with increasing concentration of Mn impurity is shown in Fig. 2(b), but it is little compared to the influence of shell structure.
Fig. 2 (a) PL spectra of four samples I-IV. Inset: TEM and HRTEM images of sample II. The dependency of the emission wavelength on Mn doping concentration (b) and shell structure (c).
Figure 3 shows the x-ray diffraction (XRD) patterns for the CdS:Mn core and NCs in samples I–IV. The CdS:Mn core is in the zinc-blende phase, and the XRD patterns of samples I–IV are roughly the same as that of the bare CdS:Mn core, but their diffraction angles (2θ) are gradually shifted to large values as the shell grows [17]. According to further calculation of peak (111), their lattice constant decreases with shell growth, which might result from the presence of lattice contraction in the CdS:Mn core [18], and the red shift of the PL peak is also affected by this lattice strain. Compared with sample I, the calculated red shifts of the PL peaks of samples II–IV induced from the lattice strain are 91.50, 127.68, and 145.01 meV [9], respectively, and their corresponding measured experimental results are 75.42, 107.69, and 185 meV, respectively. The non-igorable difference between the calculation and measurement values indicates that the PL red shift is not from the lattice strain only. The change of bondage symmetry may induce the PL red-shift also [3].
Fig. 3 XRD patterns for the CdS:Mn core and NCs in samples I–IV. Vertical lines indicate zinc-blende CdS (bottom) and ZnS (top) bulk reflections.
Figure 4 shows the typical lifetime decay curves of the four samples in toluene solution at room temperature. All the PL decay curves could be double-exponentially fitted well. The PL lifetimes of samples I–IV are within the millisecond scale, which is consistent with those in previous reports [19,20], and their values are 1.64, 1.01, 0.81, and 0.45 ms, respectively, indicating that the NCs’ interior is indeed doped with Mn2+. Mn2+ diffusion occurs in NCs during overcoating of the ZnS shell, and Mn2+ ions are probably aggregated, enhancing the exchange coupling effect of Mn2+. Further, the forbidden transition is partially allowed, decreasing the PL lifetimes of Mn2+ [21], which corresponds to the shift in the PL peaks [22].
Figure 5 shows the electron paramagnetic resonance (EPR) spectra of NC samples I–IV. We can clearly identify a six-line spectrum with hyperfine coupling constants (A) of about 67.96, 68.12, 68.47, and 69.64 G for the Mn-doped samples I–IV, respectively, revealing that the interior of the NCs rather than the surface is doped with Mn. Additionally, the A values of surface-bound Mn in the NCs are reportedly about 90 G or more [23], which is much larger than the A values in this work. Another significant parameter in the EPR spectra is the g factor, which is 2.0030, 2.0021, 2.0019, and 2.0014 for samples I–IV, respectively. The increase in A and the decrease in g reflect weakening of the covalence between the Mn2+ ions and their anions, which corresponds to the enhancement of the crystal field strength [3]. The EPR line widths of samples I–IV are 2.93, 4.39, 4.41, and 7.33 G, respectively, which explains well why the PL lifetime is decreased in the sequence of samples I-IV [24].
The sample II NCs are well coated with silica, which can be clearly observed in Figs. 6(a) and 6(b). Figure 6(c) shows the absorption and PL spectra of the sample II NCs before and after coating with silica. Note that the absorption peak of the silica-coated NCs becomes unclear because of enhancement of the surface scattering signal [25]. Further, a red shift of 5 nm in the PL peak indicates that the surface strain of the sample II NCs is increased and that they are well coated with silica. Figure 6(d) illustrates the variation in the PL intensity of CdS:Mn/ZnS and CdS:Mn/ZnS@SiO2 NCs under strong UV light (365 nm, 10 W/mm2). The PL intensity of the CdS:Mn/ZnS NCs drops very rapidly and tends to 0 after 8 h of irradiation owing to the existence of residual oxygen [26]. In the case of CdS:Mn/ZnS@SiO2, the NC’s PL intensity is slightly decreased in the initial stage of SiO2 encapsulating, which may be resulted from the further chemical reaction from SiO2 aerosol to SiO2 film under UV irradiation. After the formation of the pyknotic SiO2 shell, the PL intensity becomes stable [27]. A comparison of these two cases reveals that the photostability of the silica-coated NCs is obviously greater.
Fig. 6 TEM images of sample II (a) before and (b) after coating with silica. (c) Absorption (left) and PL spectra (right) of sample II NCs before and after coating with silica. (d) PL intensity trends under strong UV light (365 nm, 10 W/mm2) of sample II before and after coating with silica.
The red part in the EL spectrum of the WLED was compensated after Mn-doped NCs were added, as shown in Fig. 7, and the CRI of the YAG:Ce3+-based WLED was improved significantly. Moreover, the device is packaged with SiO2 aerosols, and the measured CRI is 88, which is larger than that of the YAG:Ce3+-based device (CRI ~70) and suitable for indoor lighting, which requires a CRI of >80. Further, the corresponding luminous efficiency, color temperature (Tc), and (x, y) values are 108 lm/W, 4884 K, and (0.34, 0.29), respectively. Therefore, the prepared tunable Mn-doped NCs are very effective for improving the CRI of YAG:Ce3+-based WLEDs.
Fig. 7 Electroluminescence (EL) spectra (left) and corresponding Commission Internationale de l’Éclairage (CIE) color coordinates of the WLED with CdS:Mn/ZnS@SiO2 and YAG:Ce3+ (λNCs = 610 nm) (right). Insets show the WLED before and after the application of a forward current of 350 mA.
3. Experimental
3.1 Chemicals
Cadmium oxide (CdO, 99.99%), Zinc Stearate (ZnSt2, 12.5%–14% ZnO), 1-octadecene (ODE, tech. 90%), stearic acid (SA, 99%), tetraethyl orthosilicate (TEOS, 98%), and poly(5)oxyethylene-4-nonylpheny-ether (Igepal CO-520) were purchased from Alfa Aesar. Manganese (II) stearate (MnSt2, >95%) was purchased from Wako. Oleylamine (OAm, 70%) and sulfur (S, 99.98%) were purchased from Aldrich. All chemicals were used directly without any further purification. Pure water was obtained from a Milli-Q synthesis system.
3.2 Synthesis of Cadmium Stearate (CdSt2)
CdO (0.03 mol, 3.9 g) and 0.18 mol of SA (51.21 g) were mixed in a 2 L three-necked flask and heated to 120°C under argon with vigorous stirring to obtain a clear solution. The reaction mixture was cooled to room temperature, and a white precipitate of cadmium stearate flocculated and was collected.
3.3 Synthesis of ~10 g of Mn-doped NCs
MnSt2 (1.5 mmol), 0.15 mol of S powder, and 1 L of ODE were added to the above three-necked flask filled with CdSt2 and heated under argon with vigorous stirring. OAm (0.1 L) was injected when the reaction mixture was at 260°C, and then overcoating by Cd(OA)2 or ZnSt2/ODE was performed using a dropwise method. The shell overcoating reaction lasted for 20 min. Next, the reaction solution was cooled to room temperature, and ~10 g of free-flowing powder was obtained with a quantum yield (QY) of ~68%.
We synthesized four samples, CdS:Mn/CdS (1 ML) (sample I), CdS:Mn/ZnS (2 ML) (sample II), CdS:Mn/ZnS (4 ML) (sample III), and CdS:Mn/ZnS (6 ML) (sample IV), overcoated with 0.15 L of Cd(OA)2 (0.1 M), 0.24 L of ZnSt2/ODE (0.2 M), 0.3 L of ZnSt2/ODE (0.2 M), and 0.5 L of ZnSt2/ODE (0.2 M).
The amount of Cadmium and Zinc precursor required for each monolayer was determined by the volume increment of every shell and NC's particle concentration. Cadmium and Zinc precursor was injected with layer-by-layer method and every shell overcoating reaction lasted for 20 min. The definition of a monolayer is a CdS and ZnS shell of 0.35 nm and 0.31 nm along the major axis of a single dot. According to the TEM measurement, the shell thickness of the obtained NCs fluctuate within 10% compared to the design thickness, so the performance of the WLEDs containing the Mn-doped NCs exhibits good repetition.
3.4 Coating of Mn-doped NCs with SiO2
The method is adopted from an improved reverse microemulsion method [28]. NCs were coated with SiO2 shells in two steps: silanization of NCs and a reverse micelle process. For step 1, 0.01 mmol of the above as-prepared CdS:Mn core/shell NCs was dispersed in 0.3 mL of anhydrous toluene solution and ultrasonicated for 30 min. A total of 1.5 μL of TEOS was added with stirring for 20 h to obtain a silanized NC solution. Anhydrous toluene is crucial for retaining a high PL efficiency because it might result in slow hydrolysis of TEOS. For step 2, Igepal CO-520 (1.0 g) was added to cyclohexane (10 mL) under stirring until the solution became clear to obtain the stock solution. Silanized NCs (0.3 mL) were added, and ammonia solution (25–28 wt%, 0.15 mL) was then added to the stock solution with stirring for 10 min. Next, 1.5 μL of TEOS was injected, and the solution was reacted for 24 h. SiO2-coated NCs were separated at 20,000 rpm for 15 min, washed three times with ethanol, and then dispersed in deionized water with a QY of ~60%, and that of NCs before coating with SiO2 is ~68%.
3.5 Photostability under UV Light
In a N2 purged glove box, 3 mL of a suspension of uncoated NCs and SiO2-coated NCs, with the same concentration were transferred to quartz cuvettes. The cuvettes were tightly closed and sealed with Teflon tape to avoid any contact with O2 and to prevent evaporation of the solvent. Subsequently, the two samples were taken outside the glove box and kept for 14 h under constant UV irradiation (365 nm, 10 W/mm2). At set intervals, at the same time, the illumination was interrupted and the absorption and PL spectra of each sample were recorded. The PL intensity of the samples was then obtained.
3.6 Fabrication of inorganic WLEDs with NCs and YAG:Ce3+
CdS:Mn/ZnS@SiO2-NC–YAG:Ce3+-based inorganic WLEDs were fabricated by dispensing the mixture of CdS:Mn/ZnS@SiO2 NCs, YAG:Ce3+ phosphors and a silicone resin onto a 38 × 38 mil2 surface-mounted InGaN-based blue-emitting LED (λpeak = 445 nm, Cree, USA). First, 1 mL of thermo-curable silicone resin was homogeneously mixed with CdS:Mn/ZnS@SiO2 NCs and YAG:Ce3+ phosphors in ethanol solution (1 mg of the mixture of CdS:Mn/ZnS@SiO2 NCs and YAG:Ce3+ phosphors was dispersed in 0.1 mL of ethanol), and the ethanol in the mixture was completely evaporated by heating at 60°C for 1 h. This final NC mixture was dispensed into the mold of a blue LED chip and thermally cured at 80°C for 2 h on a hot plate.
3.7 Measurement
The absorption and PL spectra were measured with a UV3600 spectrophotometer (Shimazu) and a FLS920 F900 luminescence spectrometer (Edinburgh). A Tecnai G2 transmission electron microscope was used to analyze the size distributions. XRD patterns were obtained with a Rigaku D/max 2500VL/PC diffractometer with Cu Kα radiation (λ = 0.15418 nm). EPR spectra were taken by an X-band EMX-10/12 spectrometer (Bruker). The lifetime decays were recorded by a two-channel color digital phosphor oscillograph (Tektronix TDS 3052) using excitation light from a Powerlite Precision II 8010 (Continuum) laser with a wavelength of 355 nm. The EL spectra, correlated color temperatures (Tc), CIE color coordinates, and CRI of the WLEDs were measured in an integrating sphere with a high-accuracy array rapid spectroradiometer (PMS-80, Everfine). All the characterizations were done at room temperature.
4. Conclusion
In summary, we successfully obtained industrial fabrication of high-quality Mn-doped CdS core/shell NCs with a simple and scalable synthetic approach. The NCs’ fluorescence peak can be tuned from 564 to 616 nm by changing the core/shell structure, which is helpful for improving the CRI of YAG:Ce3+-based WLEDs. Moreover, to improve efficiently the photostability of the WLEDs, we enveloped NCs with SiO2 by a reverse microemulsion method and packaged the WLEDs with SiO2 aerosols. According to the experimental results, the prepared device with a CRI of 88, luminous efficiency of 108 lm/W, Tc of 4884 K, and (x, y) values of (0.34, 0.29) is very suitable for indoor and outdoor lighting and even for use in display backlights owing to its high performance.
Acknowledgments
This work is supported by the National Basic Research Program of China (Grant No. 2012CB921801) and the Science and Technology Department of JiangSu Province, China (Grant No. BE2012163).
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