1. Introduction

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⁺ [1] and SrS:Eu²⁺ [2] was located at 591nm and 620nm, respectively, and that of CaS:Eu²⁺ was reported to be at 652nm by Yamashita et al. [2], and 663nm by Van et al. [3]. Furthermore, the effect of partial replacement of the cation ion (Ca_1-xSr_xS:Eu²⁺ [3, 4]) or selenide ligand (CaSe_1-xS_x:Eu²⁺ [5], SrSe_1-xS_x:Eu²⁺ [6]) on emission position was investigated, and the peak wavelength shift of (Ca_1−xSr_x)(S_1−ySe_y):Eu²⁺ was also predicted by Nazarov and Yoon [7] 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 Sr_1-xCa_xSe: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 Sr_1-xCa_xSe:Eu²⁺ phosphors, and the device characteristics were analyzed by measuring CIE Chromaticity Coordinates, CRI, and correlated color temperatures (CCT).

2. Experimental

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, Sr_1-xCa_xSe:Eu²⁺ particles showed a uniform spherical in shape with diameters between 800 and 900nm. 5mol% Zn doped particles, Sr_0.95Zn_0.05Se:Eu²⁺, and Ca_0.95Zn_0.05Se: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.

 

Fig. 1 The SEM image of the as-prepared (a)SrSe:Eu²⁺, (b)CaSe:Eu²⁺, post heat-treated (c)SrSe:Eu²⁺, and (d)CaSe:Eu²⁺. Inset shows the size distribution of as-prepared particles. (e) Sr_0.8Ca_0.2Se:Eu²⁺, (f) Sr_0.6Ca_0.4Se:Eu²⁺, (g) Sr_0.4Ca_0.6Se:Eu²⁺, (h) Sr_0.2Ca_0.8Se:Eu²⁺, (i) Sr_0.95Zn_0.05Se:Eu²⁺, (j) Ca_0.95Ca_0.05Se:Eu^2+.

Download Full Size | PDF

Figure 2 shows the XRD patterns of Sr_1-xCa_xSe: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).

 

Fig. 2 (a) The XRD patterns of Sr_1-xCa_xSe:Eu²⁺ (X = 0, 0.2, 0.4, 0.6, 0.8, 1) phosphors, (b) calculated values of crystal planes spacing.

Download Full Size | PDF

Figure 3 shows the excitation and emission spectra of the Sr_1-xCa_xSe: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 [9]. All samples had a strong and broad excitation band between 420 and 460nm region regardless of the cation ion ratio, which demonstrates that the Sr_1-xCa_xSe: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 [2], and that of CaSe:Eu²⁺ was consistent with the 607nm reported by Su et al [8], and longer than 594nm derived by Yamashita et al [2] and Kim et al [5]. 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) [16];

 

Fig. 3 The photoluminescence of Sr_1-xCa_xSe:Eu²⁺ (X = 0, 0.2, 0.4, 0.6, 0.8, 1) phosphors: (a) excitation (b) emission spectra (λ_ex = 460nm).

Download Full Size | PDF

As shown in Fig. 4 , the luminescence intensity was largely enhanced by doping of the Zn ion. According to Kim and associates [17], 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 Sr_0.95Zn_0.05Se:Eu²⁺ and Ca_0.98Zn_0.02Se: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 Sr_0.95Zn_0.05Se:Eu²⁺ and Ca_0.98Zn_0.02Se: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.

 

Fig. 4 The relative emission intensity and variation of emission wavelength with different Zn concentration. (a) Sr_1-xZn_xSe:Eu²⁺ (b) Ca_1-xZn_xSe:Eu²⁺.

Download Full Size | PDF

 

Fig. 5 The excitation and emission intensity of Zn doped phosphors. (a) Sr_0.95Zn_0.05Se:Eu²⁺, (b) Ca_0.98Zn_0.02Se:Eu²⁺.

Download Full Size | PDF

The XRD patterns of Sr_1-xZn_xSe:Eu²⁺ and Ca_1-xZn_xSe: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%.

 

Fig. 6 The XRD patterns of Zn doped (a) Sr_1-xZn_xSe:Eu²⁺, (b) Ca_1-xZn_xSe:Eu²⁺ phosphors. (X = 0.01, 0.05, 0.1, 0.2, 0.3)

Download Full Size | PDF

The yellowish-orange emitting Sr_0.95Zn_0.05Se:Eu²⁺ and red emitting phosphor Ca_0.98Zn_0.02Se:Eu²⁺ was employed on the color convertor in white LED. The fabricated white LED containing 460nm InGaN LED chip and 575nm yellowish-orange emitting Sr_0.95Zn_0.05Se: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 Sr_0.95Zn_0.05Se: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 Ca_0.98Zn_0.02Se: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 Ca_0.98Zn_0.02Se:Eu²⁺ contributed to extension of red spectrum, and higher CRI (86.3) and warm white light CCT (4719.2K) was created.

 

Fig. 7 The emission spectra of fabricated white LED. (a) 460nm InGaN LED + Sr_0.95Zn_0.05Se:Eu²⁺ phosphor (b) 460nm InGaN LED + YAG and Ca_0.98Zn_0.02Se:Eu²⁺ blending phosphor with various applied currents (20-70mA).

Download Full Size | PDF

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.

4. Conclusion

Spherical and submicron size of Sr_1-xCa_xSe: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 Sr_0.95Zn_0.05Se: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 Ca_0.98Zn_0.02Se:Eu²⁺ and 565nm yellow emitting YAG. Thus, the color tunable Sr_1-xCa_xSe:Eu²⁺ holds great promise for use in fabricating and improving high quality white LED.