The spherical and submicron size of Sr1-xCaxSe:Eu2+ phosphors were successfully prepared by ultrasonic spray pyrolysis. The phosphors adopted a cubic structure, and the replacement of Sr2+ with Ca2+ decreased the lattice parameter. The Sr1-xCaxSe:Eu2+ showed broad and strong excitation under 420-460nm blue light, and the emission band could be tuned from 565 to 607nm by increasing the Ca2+ ratio in the host lattice. In addition, the doping of Zn2+ into Sr2+ or Ca2+ enhanced the emission intensity with a small red shift due to the change in crystal field strength and nephelauxetic effects. The warm and high CRI of white LED was achieved using blue LED pumped with blending phosphors of 612nm emitting Ca0.98Zn0.02Se:Eu2+ and 565nm emitting YAG. The correlated color temperatures and CRI were 4719.2K, and 86.3, respectively, and an acceptable color variation was also observed at operating currents ranging from 20 to 70mA.
© 2012 OSA
Recently, white light emitting diodes (LED) have been regarded as a promising next generation solid state lighting source due to their high efficiency, compactness, and environmental-friendly. Conventional white LED is composed of blue InGaN LED and yellow emitting Y3Al5O12:Ce3+ phosphor. Although bi-color mixing white LED shows high luminous efficiency, the low color rendering index (CRI) and poor chromatic stability under different driving currents limits the expansion of their applications.
Sulfide-based compounds have been used to overcome the drawback of YAG phosphor. Due to the low electronegativity of S, the nephelauxetic effect of sulfide compounds is larger than that of oxide compounds, which can lower the 5d energy level of Ce3+ or Eu2+. Eu2+ doped alkaline earth sulfides, such as MgS, CaS, and SrS, have been extensively evaluated as red color convertors because of low cost, good linear behavior, and excitable by blue light (400-460nm). According to previous studies, the emission peak of MgS:Eu2+  and SrS:Eu2+  was located at 591nm and 620nm, respectively, and that of CaS:Eu2+ was reported to be at 652nm by Yamashita et al. , and 663nm by Van et al. . Furthermore, the effect of partial replacement of the cation ion (Ca1-xSrxS:Eu2+ [3, 4]) or selenide ligand (CaSe1-xSx:Eu2+ , SrSe1-xSx:Eu2+ ) on emission position was investigated, and the peak wavelength shift of (Ca1−xSrx)(S1−ySey):Eu2+ was also predicted by Nazarov and Yoon  using a mathematical diagram.
Several studies have already demonstrated the properties of sulfides based phosphors; however, only a few studies have examined the properties of Eu2+ doped alkaline earth selenide phosphors [8, 9]. In addition, most reported selenide phosphors were prepared by conventional solid state reactions, which require a high sintering temperature and results in poor homogeneity and irregular shapes. In contrast with this process, spray pyrolysis has been used to efficiently prepare homogeneous and submicron fine sized powders. When precursor droplets of the spray solution are carried into the hot reactor, fine particles are formed via pyrolytic decomposition of the solution. This process has been widely investigated for preparation of high purity nanosized particles [10–13] or formation of dense thin film layer [14, 15]. However, spray pyrolysis prepared alkaline earth selenide phosphors and their application on white LED color convertor have not yet been reported in the literature.
In this study, highly luminescent Sr1-xCaxSe:Eu2+ phosphors with spherical and submicron size were prepared via ultrasonic spray pyrolysis, and the structural and optical properties of the phosphors were investigated. To enhance the emission intensity, some of the Zn2+ ions were substituted for Sr2+ or Ca2+, and a change in luminescent properties was observed. Furthermore, white LED was fabricated using Sr1-xCaxSe:Eu2+ phosphors, and the device characteristics were analyzed by measuring CIE Chromaticity Coordinates, CRI, and correlated color temperatures (CCT).
Sr(NO3)2 (Aldrich, 99.995%), Ca(NO3)2 (Aldrich, 99.997%), Zn(NO3)2 (Aldrich, 99.999%), SeO2 (Aldrich, 99.999%), Eu(NO3)3·5H2O (Aldrich, 99.99%) were used as starting materials. The precursors were dissolved in D.I. water with total concentration of 0.1M. The solutions were atomized by 1.7 MHz of 6 ultrasonic nebulizers to generate liquid droplets. The produced droplets were carried into a quartz tube reactor (36mm diameter and 700mm length) using 40L/min of nitrogen gas, and residence time was about 0.8s. The phosphor particles were formed at 650°C, and obtained particles were collected in a bag filter. The as-prepared particles were post-heated at 950°C for 4hr under H/N (5/95) reduction atmosphere to increase the crystallinity and reduce Eu3+ to Eu2+.
The morphologies of the particles were observed by FE-SEM (Hitachi S-4700) with Pt coating, and the crystal phase was characterized by powder X-ray diffraction (XRD, Philips XPERT MPD) using Cu Kα irradiation under 40Kv and 30mA. The sizes of as-prepared phosphors were estimated from laser particle size analyzer (CILAS 1090) by liquid state mode. The photoluminescence (PL) was measured under the powder condition at room temperature (PerkinElmer LS-55) using Xenon lamp source, and the characteristics of fabricated white LED were analyzed using an integrating sphere (Lab sphere).
3. Results and discussion
Figure 1 shows a SEM image of the as-prepared and post heat treated phosphors. The morphology of the particles formed by ultrasonic spray pyrolysis can be adjusted by controlling the precursor type, solution concentration, reaction temperature, and residence time. In this experiment, all samples were prepared under the same operating conditions, therefore, the obtained particles had a similar morphology. Based on the SEM images, Sr1-xCaxSe:Eu2+ particles showed a uniform spherical in shape with diameters between 800 and 900nm. 5mol% Zn doped particles, Sr0.95Zn0.05Se:Eu2+, and Ca0.95Zn0.05Se:Eu2+, also exhibited the spherical with submicron size. However, some hollow or cracked particles were observed and the size distribution was broad. The as-prepared particles were dispersed into D.I water, and particle size was measured by Particle Size Analyzer (PSA) using Fraunhofer method. The mean diameter of SrSe:Eu2+ and CaSe:Eu2+ was 866.7 and 858.1nm, with polydispersity index of 1.22, and 1.36, respectively, which was similar to diameter estimated from SEM image analysis. After heat treatment at high temperature, the spherical shape of the particles disappeared and changed to irregular shapes due to the collapsing and agglomeration of hollow particles. However, some dense particles retained their spherical shape.
Figure 2 shows the XRD patterns of Sr1-xCaxSe:0.04Eu2+ (X = 0, 0.2, 0.4, 0.6, 0.8, 1) phosphors after heat treatment at 950°C. According to the XRD patterns, both SrSe:Eu2+ and CaSe:Eu2+ had a cubic structure with a space group of Fm3m (225), and the diffraction patterns were well matched with JCPDS references of 75-0260 and 77-2012, respectively. The small amount of Eu2+ dopant had no significant effect on host structure, and impurity phases such as SrO or CaO were not detected. Due to the same crystal structure of SrSe:Eu2+ and CaSe:Eu2+, SrSe-CaSe alloy was be completely miscible, and the cubic crystals structure was maintained regardless of Sr:Ca ratio. The atomic radius of Ca2+ (0.112nm) is smaller than Sr2+ (0.127nm), therefore, replacement of the Ca2+ ions for Sr2+ ions site resulted in a contraction of the d-spacing in the crystals lattice. When the Ca2+ content in the lattice was increased, the 3 strong phases of (2 0 0), (2 2 0), and (2 2 2) were slightly shifted to a higher angle. Based on Bragg’s law, calculated d-spacing of (2 0 0), (2 2 0), and (2 2 2) crystals planes were plotted in Fig. 2(b) with different Sr:Ca ratio. As the Ca ratio was increased, the calculated lattice constant decreased from a = 0.6245nm (X = 0), 0.6230nm (X = 0.2), 0.6213 (X = 0.4), 0.6145 (X = 0.6), 0.6025 (X = 0.8) to a = 0.5911nm (X = 1).
Figure 3 shows the excitation and emission spectra of the Sr1-xCaxSe:Eu2+ (X = 0, 0.2, 0.4, 0.6, 0.8, 1) phosphors at room temperature. The concentration of Eu2+ was fixed to 0.04mol% relative to [Sr2+] + [Ca2+], which value was obtained from our previous work . All samples had a strong and broad excitation band between 420 and 460nm region regardless of the cation ion ratio, which demonstrates that the Sr1-xCaxSe:Eu2+ phosphors can be well excited using a commercial blue LED chip. The emission spectra using a 460nm excitation source exhibited the typical characteristic of Eu2+ emission. The broad band emission that originated from the allowed 4f65d1-4f7 transition of Eu2+ ion was observed and the FWHM of the samples were approximately 54 nm (X = 0), 51 nm (X = 0.2), 76 nm (X = 0.4), 78nm (X = 0.6), 53 nm (X = 0.8), and 51 nm (X = 1). The emission peaks of the SrSe:Eu2+ and CaSe:Eu2+ phosphors prepared by spray pyrolysis were 565 and 607nm, respectively. The emission center of SrSe:Eu2+ was closed to 571nm, as reported by Yamashita et al , and that of CaSe:Eu2+ was consistent with the 607nm reported by Su et al , and longer than 594nm derived by Yamashita et al  and Kim et al . As shown in Fig. 3(b), when the Ca ratio was increased, the emission peak moved to longer wavelengths linearly (565nm (X = 0), 571nm (X = 0.2), 574nm (X = 0.4), 585nm (X = 0.6), 593nm (X = 0.8) to 607nm (X = 1)). The red shift of the emission peak was strongly associated with change of the crystal field strength. This correlation between the crystal field strength and bond length can be described using Eq. (1) ;Fig. 2(a), an increase of the Ca2+ ratio in the host structure decreased the lattice parameter, and the distance between Eu2+ (central ion) and Se2- (ligands) became shorter. Thus, the crystals field strength became stronger, indicating that the lowest 4f65d configuration of Eu2+ ion was lowered resulting in a red shift.
As shown in Fig. 4 , the luminescence intensity was largely enhanced by doping of the Zn ion. According to Kim and associates , these results can be explained by the nephelauxetic effect. The Zn2+ ion can contribute to an interaction between the unshielded 5d electron of Eu2+ and the ligand, hence, the electron cloud of the 5d orbital of Eu2+ is extended. This nephelauxetic effect can increase the covalency of Eu2+ and improve the emission intensity. The maximum intensity was observed at 5 mol% Zn doped SrSe:Eu2+, and 2 mol% Zn doped CaSe:Eu2+ phosphor. The emission intensity of Sr0.95Zn0.05Se:Eu2+ and Ca0.98Zn0.02Se:Eu2+ was 53.8 and 44.9% higher than that of SrSe Eu2+ and CaSe:Eu2+, respectively. At higher Zn2+ concentrations, the emission intensity decreased, and only a weak emission was detected when the doping concentration of Zn ion was up to 30%. Due to the small ionic radius of Zn2+ (0.083nm), the Eu2+ (0.120nm) ion can readily occupy the Sr2+ (0.127nm) or Ca2+ (0.112nm) sites rather than the Zn2+ site. Therefore, the addition of high Zn2+ concentrations could hinder the emission of Eu2+. As described above, the emission peak is strongly dependent on crystals field strength. Doping of Zn2+ ions, which have a much smaller radius, for Sr2+ or Ca2+ led to a decrease in the size of the lattice structure and a decrease in the crystals field strength. Thus, a corresponding red shift occurred when Zn was substituted into the SrSe:Eu2+ and CaSe:Eu2+ phosphor. However, unlike the emission intensity, Zn2+ had little effect on the position of the emission peak. When 15 and 25mol% of Zn2+ was doped in SrSe:Eu2+ and CaSe:Eu2+, the emission peak was shifted to 12 and 6nm longer wavelength, respectively, and no further red shift was observed until the concentration of Zn ion was increased to 30mol%. As shown in Fig. 5 , both Sr0.95Zn0.05Se:Eu2+ and Ca0.98Zn0.02Se:Eu2+ had strong excitation level at around 430-460nm which was suitable for combining 460nm blue LED, and emission center was located at 575, and 612nm, with FWHM of 52, and 51nm respectively.
The XRD patterns of Sr1-xZnxSe:Eu2+ and Ca1-xZnxSe:Eu2+ phosphors were presented in Fig. 6 . Although Zn2+ ions were doped in host lattice, notable shift of 2θ value was not observed, and the cubic structure of SrSe and CaSe remained as dominant crystal phase. Also, the strongest (1 1 1) phase of cubic ZnSe located at 2θ = 27.47° (JCPDS 80-0021) was not appeared until Zn content was reached at 30mol%.
The yellowish-orange emitting Sr0.95Zn0.05Se:Eu2+ and red emitting phosphor Ca0.98Zn0.02Se:Eu2+ was employed on the color convertor in white LED. The fabricated white LED containing 460nm InGaN LED chip and 575nm yellowish-orange emitting Sr0.95Zn0.05Se:Eu2+ exhibited CIE chromaticity coordinates of (0.3618, 0.3207) with a warm white region of 4133.2K at 20mA. Due to the longer emission center of Sr0.95Zn0.05Se:Eu2+ compared to YAG, warm white light was created; however, the CRI was only about 41.5 at 20m due to the narrow FWHM of phosphor as well as the lack of a red component. As shown in Fig. 7(a) , when different operating currents were injected, the CIE chromaticity coordinates changed from (0.3618, 0.3207) at 20mA to (0.3478, 0.3034) at 70mA. These unstable color variations are a general problem of bi-color white LED. To achieve the high quality white LED, 612nm red emitting Ca0.98Zn0.02Se:Eu2+ was blended with commercial yellow emitting YAG, and coated onto LED chip. As presented in Fig. 7(b), the emission band around 460nm corresponded to InGaN LED, and the other emission band was from blending phosphors excited by the LED chip. The improved white LED showed acceptable color stability against different driven currents. The CIE chromaticity coordinates varied from (0.3189, 0.2737) at 20mA to (0.3115, 0.2672) at 70mA. In comparison with the YAG (CRI = 67.1, CCT = 19231.9K), the addition of Ca0.98Zn0.02Se:Eu2+ contributed to extension of red spectrum, and higher CRI (86.3) and warm white light CCT (4719.2K) was created.
Based on these results, ultrasonic spray pyrolysis technique can use for preparation of precisely controlled composition and morphology of selenide phosphors. In addition, color tunable alkaline earth selenide phosphors hold great promise for use in the development and controlling the white color parameters such as correlated color temperature, CIE and CRI.
Spherical and submicron size of Sr1-xCaxSe:Eu2+ (X = 0, 0.2, 0.4, 0.6, 0.8, 1) phosphors were successfully prepared via ultrasonic spray pyrolysis. The obtained particles had a cubic structure, and the lattice parameter decreased with an increase in the Ca2+ ratio in the host structure. The excitation spectra showed a strong and broad band between 400 and 460nm, which was suitable for commercial blue LED applications. A broad emission band was also observed due to the d-f transition of the Eu2+ ion and the emission peak was shifted from 565nm yellow to 607nm red emitting at different Sr2+/Ca2+ ion ratios due to a change in the crystal field strength. In addition, the doping Zn2+ ions iton SrSe or CaSe with remarkably enhanced the emission intensity and caused a red shift, which could be explained by changes in the crystal field and nephelauxetic effects. Although a low CRI of white light was generated from the Sr0.95Zn0.05Se:Eu2+ based white LED, a warm (CCT = 4719.2K) with excellent CRI (86.3) of white LED could be achieved by blending the 612nm red emitting Ca0.98Zn0.02Se:Eu2+ and 565nm yellow emitting YAG. Thus, the color tunable Sr1-xCaxSe:Eu2+ holds great promise for use in fabricating and improving high quality white LED.
This work was supported by the Human Resources Development the Korea Institute of Energy Technology Evaluation and Planning (20114010203050) grant funded by the Korea government Ministry of Knowledge Economy
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