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In this study, non-toxic and highly stable silica coated ZnCuInS NCs were synthesized by a reverse microemulsion method. The single NCs were uniformly encapsulated in a silica shell with a diameter of ~30nm. Although hydrolyzed TEOS caused a QY reduction, and a 12.5nm red shift occurred after silica coating, the photo and thermal stabilities were extremely improved. For LED application, the silica coated ZnCuInS NCs phosphor layer was arrayed on the InGaN LED surface by layer-by-layer deposition utilizing electrostatic attraction. When the ZnCuInS/SiO2 NCs single monolayer was fabricated, 6.73% high color conversion efficiency was achieved.
© 2014 Optical Society of America
1. Introduction
Semiconductor nanocrystals (NCs) have been widely studied for applications in optoelectronic devices [1–4], and biological labeling [5,6] because of their tunable emission wavelength, and broad absorption. In particular, the broad absorption property allows for the simultaneous excitation of various emissive NCs by a single excitation wavelength, and thus, NCs become promising candidates for color converter in white LED. Since Klimov et al. demonstrated the non-radiative energy transfer from InGaN/GaN quantum wells to NCs [1], many studies have reported the creation of a high color rendering index with warm white light using CdSe based NCs [1,2,7–10].
Despite the high quantum yield (QY) of CdSe NCs, the intrinsic toxicity of Cd restricts their practical application. To overcome this problem, CuInS2 has been regarded as potential alternative to Cd based NCs, and has been extensively synthesized via hot-injection [11–13], solvothermal [14], and thermolysis [15,16] methods. Peng et al. reported a 30% QY of CuInS2/ZnS NCs by controlling the reactivity of precursors [11], and Kim et al. enhanced the QY to 65% by cation exchange [12]. Recently, Xie’s group obtained a QY of Zn-Cu-In-S NCs over 70%, without ZnS shell coating [13]. Accompanying with developments of synthetic strategy, CuInS2 based color conversion white LEDs also have been reported. Yang et al. fabricated white LEDs with over 60 lm/W, and with CRI of 72-75 using CuInS2/ZnS NCs [17,18], and Kim et al. enhanced the CRI value up to 80 by combination of yellow and red emissive ZnCuInS2 NCs [19]. Moreover, integration of commercial phosphor and red emissive CuInS2 NCs showed an excellent CRI of over 90 with warm white light [20].
In spite of previous successful results, the stability of NCs, and the formation of a phosphor layer have not yet been fully investigated. The encapsulation of silica is a facile method to improve the stability of NCs due to their chemical inertness and optical transparency. Additionally, silica coating offers functionalized binding sites and water soluble properties for further uses.
On the other hand, conventional phosphor layers are prepared by randomly dispersing NCs in a polymer matrix, and therefore, this method easily causes the re-absorption and aggregation of NCs in the matrix. Ideally, a well ordered multi NCs layer can prevent the re-absorption and control the energy transfer within different emissive NCs, and consequently, can increase the color conversion efficiency by minimizing several intermediate energy-loss steps [21]. Layer-by-Layer (LbL) self-assembly is suitable for fabricating a uniformly ordered layer, utilizing electrostatic attractions between oppositely-charged materials, hydrogen bonding, covalent bonding, and other specific interaction. This technique has been widely applied to fabricate organic-inorganic hybrid thin films on quartz, glass, or silicon wafers [22–25]. To achieve both the stability of NCs and uniformity of phosphor layers, fabrication of silica coated NCs phosphor layer via LbL deposition is a promising method. However, this approach has never been applied for the fabrication of a phosphor layer on a GaN substrate.
In this study, non-toxic ZnCuInS NCs were passivated by silica shell via a reverse microemulsion method to improve the photo and thermal stability, and their optical properties were investigated. Furthermore, the LbL self-assembly of ZnCuInS/SiO2 NCs monolayer was fabricated onto the GaN LED chip surface using oppositely-charged materials, and showed the possibilities for applying a white LED color converter.
2. Experimental
ZnCuInS NCs were synthesized according to published procedure [13]. Copper(I) acetate (0.2 mmol), zinc acetate (0.5 mmol), indium acetate (0.2 mmol), 1-dodecanethiol (DDT, 4 mmol), and oleic acid (0.2 ml) were mixed with octadecene (ODE, 6 mL) in a flask, and heated to 100°C until the solution became clear. DDT acts as stabilizing ligand as well as sulfur source forming thiolates with metal salts, and could lead to nearly complete consumption of the cationic precursors. Then sulfur dissolved in oleyamine (1mL) and ODE (2mL) was injected, and heated at 180 °C for 20 min. The sufficient sulfur lead to the burst nucleation of quaternary NCs, and additionally increased the quantum yield.
The silica shell was coated by a reverse microemulsion method, using Igepal CO-520 (polyoxyethylene(5)nonylphenyl ether), ammonia aqueous solution (28wt%), and TEOS (tetraethyl orthosilicate) as nonionic surfactant, base catalyst, and silanization precursors, respectively [26, 27]. The Igepal CO-520 (1.1 mL) was dispersed in cyclohexane (10 mL), and the prepared ZCIS NCs solution (30 μL, 10 mg/mL of cyclohexane) was added. The mixture was stirred for 30 min, and ammonia aqueous solution (150 μL) was introduced to form the microemulsion system. Lastly, TEOS (100 μL) was added, and stirred at room temperature for 24 hr.
Prior to fabricating the LbL self-assembly phosphor layer, GaN surface (1x1 mm) was treated using reactive ion etching with O2 plasma to form a negative charge. The substrate was alternately dipped into an aqueous solution of PDDA (Poly(diallyldimethylammonium chloride), Mw = 100,000-200,000, 20wt% in H2O) and ZnCuInS/SiO2 NCs (1 mg/mL) for 20 min, and washed with D.I water.
3. Results and discussion
Figure 1(a) and (b) shows the HR-TEM (Tecnai G2 F30) image of bare and silica coated ZnCuInS NCs. The bare ZnCuInS NCs were almost monodispersed with a spherical shape, and their size was ~5nm. After silica coating, the single ZnCuInS NCs were uniformly encapsulated in silica shell, and their thickness was approximately 25nm. From the XRD patterns (Philips XPERT MPD), as shown in Figs. 1(c) and 1(d), silica coated ZnCuInS NCs had a zinc blend structure, and no other peaks of shift were observed, such as those observed for bare ZnCuInS NCs, indicating that the silica shell was amorphous, and had no effect on the crystal structure.
Fig. 1 The HR-TEM image of (a) ZnCuInS NCs, and (b) silica coated ZnCuInS NCs. (c) The X-ray diffraction of ZnCuInS NCs, and silica coated ZnCuInS NCs.
Figure 2 shows the absorption (Perkin Elmer Lambda 35) and emission (Perkin Elmer LS55) spectra of uncoated and silica-coated ZnCuInS NCs. The emission wavelength of ZnCuInS NCs was 577nm with FWHM of 87.5nm, and QY was 63% compared to Rhodamine 6G. The dominant emission of II-VI CdSe is excitonic recombination, while that of I-III-VI CuInS2 NCs is donor-acceptor pair (DAP) recombination mechanism related to intrinsic donor-acceptor defects and surface defects, which act as radiative and non-radiative process, respectively. Due to the distribution of intrinsic defects, ZnCuInS NCs showed a broad FWHM and large stokes shift compared to II-VI CdSe NCs [25]. We expected that wide band-gap of silica shell eliminates the surface trap states by passivation of dangling bonds on NCs surface, and enhances the QY. Conversely, FWHM was broadened to 92.5nm, and the QY was reduced from 63% (ZnCuInS NCs in cyclohexane) to 29% (ZnCuInS/SiO2 NCs in ethanol) relative to Rhodamine 6G (standard, Q.Y. = 94% in ethanol) using following equation.
Where A, n, and I are absorbance, refractive index, and integrated emission areas, respectively.As shown in Fig. 2(b), the emission intensity decreased to 90.8%, and 80.1%, compared to initial intensity, after addition of Igepal CO-520 and ammonia, respectively, followed by a drastic decrease to 56.7%, after the addition of TEOS. From the results, when the hydrolyzed TEOS reacted with the surface of NCs, the surface of ZnCuInS NCs could be oxidized, and unexpected surface defects might appear at the interface between ZnCuInS NCs and the silica shell. Moreover, the incomplete transparency and absorption of UV excitation by the silica shell were also attributed to the decrease of QY. Generally, the surface charge on NCs can generate an electric field, influencing luminescent properties. In this case, a 10nm red shift was found when ammonia solution was introduced, and a further 7.5nm red shift was observed after silica coating. This suggested that the negative surface charges, such as those of hydroxyl ions (-OH) from ionic ammonia solution, and Si-O- and/or Si-OH groups from hydrolyzed TEOS, caused the red shift. The aggregation or energy transfer from higher energy NCs to lower energy NCs in silica shell contributed to the red shift of the emission wavelength.
The stability was compared by fabricating ZnCuInS NCs and ZnCuInS/SiO2 NCs film on quartz. Photo-stability was examined by continuous radiation using a 365nm UV lamp (2200 μW/cm2) under ambient conditions. As shown in Fig. 3(a), uncoated ZnCuInS NCs were vulnerable to UV irradiation. The emission intensity was rapidly reduced to 55% compared to initial intensity within 10 h. Whereas a slight degradation of emission intensity was observed in silica coated ZnCuInS NCs. Over 70% of initial intensity was maintained even after 25 h. Thermal stability was measured by annealing at a high temperature for 1 h, and the emission intensity was compared in Fig. 3(b). No significant difference of emission intensity was observed under 100°C. After annealing at 150°C, the emission intensity of ZCIS NCs decreased to below 50%, with an 8nm blue shift, while that of ZnCuInS/SiO2 NCsdecreased to 84% without a wavelength shift. The blue shift was attributed to Zn diffusion into CuInS2 lattice at high temperature. Furthermore, when NCs films were annealed over 250°C, the emission of ZnCuInS NCs was completely quenched, however, weak emission was observed in ZnCuInS/SiO2 NCs. This results indicates that the silica shell successfully protected the ZnCuInS NCs core from the high temperature.
The ZnCuInS/SiO2 NCs phosphor layer was fabricated onto InGaN blue LED substrate via LbL self-assembly. The phosphor layer consisted of positive polyelectrolytes (PDDA) and negative ZnCuInS/SiO2 NCs (−24.56 mV, measured by zeta potential). Although the particles were sparsely distributed on the GaN surface due to the repulsive electrostatic interaction within particles, aggregation and multilayer structure were not observed. The strong polyelectrolytes of PDDA chains tend to orientate themselves linearly in the absence of salt, due to the strong repulsion between polyelectrolytes. However, at high salt concentrations, polymer chains change to loop formation due to the decrease of interaction between polyelectrolytes layers, and thus the surface will have a higher charge density. Accordingly, as shown in Figs. 4(a)-4(c) (FE-SEM, Hitachi S-4700), ZnCuInS/SiO2 NCs were more densely arrayed as the NaCl concentration was increased.
Fig. 4 FE-SEM image of ZnCuInS/SiO2 NCs monolayer with different NaCl concentration. (a) 0M NaCl; (b) 0.1M NaCl; (c) 1M NaCl.
Figure 5 shows the emission spectrum (Lab Sphere) of a color conversion LED consisting of 430nm InGaN LED chip and ZnCuInS/SiO2 NCs monolayer. A thin ZnCuInS/SiO2 NCs phosphor monolayer absorbed the high energy photons from an InGaN LED chip, and efficiently re-emitted the lower energy photons. The color conversion efficiency is defined as the ratio of the integrated NCs emission to the integrated spent blue emission from the InGaN LED chip. When monolayer NCs were deposited on the LED surface, the color conversion efficiency was about 6.73%, which was a comparable result to using CdSe NCs [21]. The CIE was shifted from (0.2154, 0.1091) to (0.2428, 0.1440) at 20mA, and could emit white light by using subsequent adsorption step.
Fig. 5 The emission spectrum of fabricated color conversion LEDs. 430nm InGaN LED pumped with ZnCuInS/SiO2 NCs monolayer phosphor.
The LbL assembly of the NCs layer is a promising approach towards the fabrication of a well ordered phosphor structure, and for efficiently controlling the energy transfer when multicolor phosphors was employed. Also, this method can be expanded to applications on flexible and patterned substrate.
4. Conclusion
Strong luminescent non-toxic ZCIS NCs were synthesized, and photo and thermal stabilities were improved by ~30nm silica shell coating. Although silica coating caused a decrease in QY due to the generation of an oxide defect on the NCs surface, photo and thermal stabilities were largely enhanced. The ZnCuInS/SiO2 NCs phosphor layer was fabricated via LbL self-assembly using a PDDA polycation. When a monolayer of ZnCuInS/SiO2 NCs was arrayed on a GaN LED chip, a 6.73% high color conversion efficiency was obtained. These results indicate that the stable silica coated ZnCuInS NCs are promising materials for color converting in solid state light sources. Furthermore, layer-by-layer deposition is suitable for fabricating a uniformly ordered phosphor layer, and for high color conversion efficiency.
Acknowledgment
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (2012R1A1A2007908).
This work was supported by the Human Resources Development the Korea Institute of Energy Technology Evaluation and Planning (20114010203050) grant funded by the Korean government Ministry of Knowledge Economy.
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