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A series of Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ (−0.4 ≤ x ≤ 0.6) phosphors were prepared by solid-state reaction method, and the photoluminescence properties were investigated in detail. By varying the relative contents of Ca2+ and Sr2+, the single-phase solid solution samples were obtained, which can be found from the corresponding XRD patterns. The excitation spectra of the as-prepared samples reveal that broad excitation bands appear and different excitation spectral shapes are observed by monitoring at 433 and 585 nm, which could be due to the various Eu2+ emission centers. Every emission spectrum includes two emission bands, and their relative intensities depend on the excitation wavelength and the Ca2+/Sr2+ concentration ratio. As a result, the tunable-emission has been realized. The spectra characteristics for different Ca2+/Sr2+ concentration ratio in Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ further verify that two Eu2+ emission centers exist and the emission bands at 433 and 585 nm are attributed to the Eu2+ ions in Sr2+ and Ca2+ sites, respectively. The temperature-dependent photoluminescence spectra reveal that the Eu2+ emission centers in the Sr2+ and Ca2+ sites have different thermal stabilities, which results in the emission color changes somewhat for different temperature.
© 2015 Optical Society of America
1. Introduction
Phosphor converted white light-emitting diode (LED) is regarded as a new lighting source for the next generation due to the advantages such as high brightness, energy saving, reliability, maintenance, and safety [1–4 ]. The most common and the simplest commercial strategy to achieve white LEDs is the combination of a blue LED chip and the yellow-emitting YAG:Ce3+ phosphor [5,6 ]. However, this type of white light suffers from poor color rendering index (CRI) and high correlated color temperature (CCT) due to the deficiency of red fluorescent component, which restricts their use in several important applications, e.g., residential and commercial lighting [7,8 ]. To overcome these drawbacks, one attractive approach by utilizing a near ultraviolet (NUV) LED chip coated with Red/Green/Blue tri-color phosphors has been proposed, and the appropriate warm white light can be achieved according to the trichromatic theory [7,9 ]. This type of white LEDs can produce an excellent color rendering index and easily controlled emission color properties [10,11 ]. On the other hand, it is also interesting to synthesize novel multicolor phosphors that can overcome the shortages of phosphors combination such as different degradation rates and re-absorption between phosphors [12]. Moreover, the performances of white LEDs strongly depend on the luminescence properties of phosphors used. Thus, it is essential to find novel phosphors with high conversion efficiency, appropriate emission colors, and high chemical stability.
Phosphors consist of the activators and hosts. Rare earth (RE) ions have been playing an important role in the activator ions of phosphors. Their unique electronic structures enable RE ions in solids to emit photons efficiently in the spectral region from UV to visible light [13,14 ]. Eu2+ ion is one of the most important luminescent activators for phosphors because it exhibits broad and intense excitation and emission bands which could be well matched with the NUV LED chip. The emission wavelength of Eu2+ mainly depends on the hosts. Strong ligand-activator bonding interactions will cause low energy difference between the 4f65d1 and 4f7 states, and thus, leading to a red shift of the emission band of the Eu2+ ions [7,15 ]. On the other hand, the host materials for phosphors should have high physical and chemical stability, easy preparation, and environmental friendliness. The silicate has been one of the most favorable host materials at present due to the above features and has been studied widely. To the best of our knowledge, the structure of Li4CaSr(SiO4)2 was first investigated by Akella and Keszler, which revealed the LCSS belongs to space group Pbcm and has orthorhombic structure [16]. The photoluminescence properties of Eu2+/Tb3+/Mn2+ activated Li4CaSr(SiO4)2 were reported subsequently [17–20 ]. It has been indicated that two Eu2+ emission centers exist in the Li4CaSr(SiO4)2:Eu2+ phosphor, which occupy the Ca2+ and Sr2+ sites [20]. However, tuning the emission color by changing the relative concentrations of Ca2+ and Sr2+ in the Li4CaSr(SiO4)2:Eu2+ has not been investigated in detail.
In this paper, to develop the emission-tunable phosphors, the Li4CaSr(SiO4)2:Eu2+ samples were synthesized by conventional solid-state reaction method, and their photoluminescence properties were studied. The spectra results for different Ca2+/Sr2+ concentrations also indicate two Eu2+ emission centers exist in the Eu2+ activated Li4CaSr(SiO4)2 phosphors.
2. Experimental
Powder samples of Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ (−0.4 ≤ x ≤ 0.6) were prepared by solid-state reaction. The starting materials included Li2CO3 (analytical reagent, AR), CaCO3 (AR), SrCO3 (AR), SiO2 (AR), and Eu2O3 (4N). Stoichiometric amounts of the starting reagents were thoroughly mixed and ground together by an agate mortar. The mixture was then calcined at 800°C for 8 h in a reduction atmosphere (N2: H2 = 95: 5).
The phase purity was determined by using an ARL X'TRA powder X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.5418 Å) operating at 40 kV and 35 mA. Diffuse reflection spectra (DRS) were obtained by a UV/visible spectrophotometer (UV-3600, SHIMADZU) using BaSO4 as a reference in the range of 200-700 nm. The morphology of the as-prepared sample was inspected by field emission scanning electron microscope (FESEM, FEI, Quanta FEG). The luminescence spectra and external quantum efficiencies (QE) were recorded on an FLS-920T fluorescence spectrophotometer with Xe 900 (450 W xenon arc lamp) as the light source.
3. Results and discussion
Figure 1(a) shows the XRD patterns of the Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ (−0.4 ≤ x ≤ 0.6) samples. All the diffraction peaks can be indexed to pure orthorhombic-structured Li4CaSr(SiO4)2 (JCPDS Card NO.83-0763). No obvious impurity phase was detected when changing the relative concentrations of Ca2+ and Sr2+. From the enlarged XRD patterns for the predominated peaks in Fig. 1(b), it can be found the diffraction peaks exhibit a continuous shift toward small angle direction with increasing Sr2+ content and a gradual shift toward large angle direction with increasing Ca2+ content. This observation is owing to the ionic radius of Sr2+ is larger than that of Ca2+, which also reveals the Sr2+/Ca2+ ions have entered the host lattices successfully.
Fig. 1 (a) XRD patterns of Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ (−0.4 ≤ x ≤ 0.6); (b) enlarged XRD patterns for their predominated diffraction peaks.
Figure 2 presents the SEM image of the typical Li4CaSr(SiO4)2 host. The relatively dispersed particles have been obtained by solid-state reaction method. But the particle size is not very uniform and somewhat large with the average scale around 25 μm. Besides, it is found that the surface of every particle is rough where some very small particles still exist.
To study the optical properties of the as-prepared phosphors, Fig. 3 shows the DRS of the typical Li4CaSr(SiO4)2 host and the typical Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ (x = 0, 0.3, and −0.2) samples. It can seen from the diffuse reflection spectrum of Li4CaSr(SiO4)2 in Fig. 3(a) that the high reflection in the visible range (400-700 nm) appears, so the daylight color of this sample is white. To determine the optical bandgap value of the Li4CaSr(SiO4)2 compound experimentally, the corresponding absorption spectrum (see the inset of Fig. 3(a)) was obtained from its reflection spectrum using the Kubelka-Munk (K-M) function [21]
where R, K and S are the reflection, absorption and scattering coefficient, respectively. By extrapolating the K-M function to K/S = 0, the optical bandgap value is determined to be about 5.1 eV. When the Eu2+ is introduced into the Li4Ca1-xSr0.96 + x(SiO4)2 (x = 0, 0.3, and −0.2) host, the optical behaviour changes immediately as shown in Fig. 3(b). New absorption band in the UV region appears which belongs to the Eu2+ f-d transition. It is worthwhile to note that the absorption band edge has extended to 530 nm for x = −0.2, indicating the introduction of Sr2+ ions into Ca2+ sites is helpful to the Eu2+ absorption in the visible region, which is in agreement with the excitation spectra characteristics below.Fig. 3 (a) Diffuse reflection spectrum of Li4CaSr(SiO4)2, the inset shows its absorption spectrum; (b) DRS of Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ (x = 0, 0.3, and −0.2).
The photoluminescence spectra of Eu2+ activated Li4CaSr(SiO4)2 have been investigated in [17,18,20 ]. The optimal Eu2+ doping concentration was determined to be 4 mol% [18], so in this work the Eu2+ content in all the Eu2+-doped Li4Ca1-xSr0.96 + x(SiO4)2 phosphors is also fixed at 4 mol%. The excitation and luminescence spectra reported in [20] show that the excitation band of Eu2+ activated Li4CaSr(SiO4)2 covers a wide range from 250 to 400 nm and a blue emission around 433 nm was found. To study the effect of different contents of Ca2+ and Sr2+ in Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ on the spectra, Fig. 4 presents the typical normalized excitation spectra of Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ (x = 0.3 and −0.2) monitored at 433 and 585 nm. By monitoring 433 nm, both the samples show broad excitation bands in the UV region with the strongest excitation position at 299 nm, and the spectral profiles are similar to that reported [20]. However, by monitoring 585 nm, the excitation spectra of the two samples exhibit three main excitation bands around 258, 299, and 385 nm, but the relative intensities are completely different. The predominated excitation peaks are located at 258 and 385 nm for x = −0.2 and 0.3, respectively, and the intensity of the excitation band at 299 nm is between the other two. According to [20], two crystallographic sites are available for Eu2+ in the Li4CaSr(SiO4)2:Eu2+, the Sr2+ site and the Ca2+ site; the emission band around 433 nm is derived from the Eu2+ emission center at Sr2+ site and the emission band at 585 nm belongs to the Eu2+ emission center at Ca2+ site. Thus, the above excitation spectra features indicate the excitation band at 299 nm for both the as-prepared samples is mainly responsible for the Eu2+ center at Sr2+ site and the other two are for the Eu2+ center at Ca2+ site. This conclusion could be also verified by the emission spectral features of Li4Ca0.8Sr1.16(SiO4)2:0.04Eu2+ below. In addition, a narrow excitation peak at 394 nm is also found which is attributed to the 7F0-5L6 transition of Eu3+ [12], indicating the doped Eu3+ ions were not reduced to Eu2+ completely in the preparation.
To study the influence of the excitation wavelength on the emission spectra, Fig. 5 shows the emission spectra of the typical Li4Ca0.8Sr1.16(SiO4)2:0.04Eu2+ sample excited at various wavelengths. Two kinds of emission bands are found at 433 and 585 nm, which could be ascribed to the d-f transition of Eu2+. For different excitation wavelengths, the relative intensities for the two emission bands are very different. This could be owing to the different responses of the Eu2+ emission centers in the Sr2+ and Ca2+ sites to the excitation wavelengths, which is in agreement with the excitation spectra characteristics discussed in Fig. 4. To further interpret this point, the intensity ratio of the blue emission to the orange emission as a function of the excitation wavelength is depicted in the inset of Fig. 5. It can be seen obviously that the intensity ratio demonstrates the maximum value under 300 nm excitation, and shows relatively low values when the excitation wavelength is close to 250 and 390 nm. In other words, the profile of this function curve is very similar to those of the excitation spectra by monitoring 433 nm in Fig. 4. This observation indicates the blue emission bands around 433 nm are effectively excited by the excitation wavelengths around 300 nm and the orange emission bands around 585 nm are effectively excited by the excitation wavelengths around 250 and 390 nm, which is completely accordant with the analysis of the excitation spectra in Fig. 4. Additionally, the narrow emission peaks at 589 and 611 nm are observed, which are assigned to the 5D0-7F1 and 5D0-7F2 transitions of Eu3+ [12]. To understand the emitting light colors of this sample under different excitation wavelengths, the corresponding Commission International del’Eclairage (CIE) chromaticity coordinates are shown in the CIE chromaticity diagram in Fig. 6 (see Points 1-7), whose values are (0.372, 0.264), (0.304, 0.201), (0.238, 0.143), (0.306, 0.219), (0.374, 0.291), (0.512, 0.437), and (0.535, 0.462), respectively. It can be seen the emission colors have ranged from blue-purple to yellow. Additionally, the blue emission bands include the wavelength region of 400-450 nm, to which the eye sensitivity is low. However, this wavelength part is useful to improve the color rendering and saturation of the final white light.
Fig. 5 Emission spectra of Li4Ca0.8Sr1.16(SiO4)2:0.04Eu2+ under various excitation wavelengths, the inset shows the intensity ratio of the blue emission to the orange emission as a function of the excitation wavelength
Fig. 6 CIE chromaticity diagram for Li4Ca0.8Sr1.16(SiO4)2:0.04Eu2+ (Points 1-7 are for excitation wavelengths of 250, 263, 300, 355, 365, 390, and 415 nm, respectively) and Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ (Points 8-15 are for x = −0.3, −0.1, 0, 0.1, 0.3, 0.4, 0.5, and 0.6, respectively) under 355 nm excitation, the insets (a-f) show the digital photographs of Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ (−0.3 ≤ x ≤ 0.6) under 365 nm ultraviolet lamp irradiation.
To investigate the effect of introducing Sr2+ ions into Ca2+ sites on the emission characteristics, Fig. 7 presents the emission spectra of the Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ (0 ≤ x ≤ 0.6) phosphors under 355 nm excitation. Both emission bands around 433 and 585 nm are observed for every spectrum. With increasing Sr2+ content, the intensity of the blue emission band decreases gradually, and that of the orange emission band increases until x = 0.3 and beyond this concentration the orange emission intensity starts to decay. As mentioned above, the emission bands around 433 and 585 nm are assigned to the Eu2+ emission centers in the Sr2+ and Ca2+ sites, respectively. Thus, when the concentration ratio of Sr2+ to Ca2+ in the Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ host increases, the substitution competition in the Sr2+ sites between Sr2+ and Eu2+ ions sources will be intensified, which urges the doped Eu2+ ions more enter the Ca2+ sites. In this case, with the Sr2+ content increased, the emission intensity of the Eu2+ center in the Sr2+ site will decreases accompanying with the increasing emission intensity of the Eu2+ center in the Ca2+ site. As for the intensity decrease of the emission band at 585 nm beyond x = 0.3, the possible reason could be related to the concentration quenching of Eu2+ in the Ca2+ site. Due to the above emission spectra characteristics, it can be found the tunable-emission has been obtained. To further understand this point, the normalized emission spectra (at 433 nm) are presented in the inset of Fig. 7. With increasing Sr2+ content, the emission at 585 nm is enhanced continuously. Thus, the emitting light color will be tuned toward the orange gradually. The CIE chromaticity coordinates of the Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ (0 ≤ x ≤ 0.6) samples were calculated from the emission spectra, and the values are (0.172, 0.060), (0.262, 0.169), (0.306, 0.219), (0.333, 0.249), (0.347, 0.258), (0.373,0 0.300), and (0.370, 0.324) for x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6, respectively. The corresponding CIE chromaticity diagram described in Fig. 6 also indicates the tunable-emission from blue to nearly white has been realized (Points 4 and 10-15). To observe the emission colors visually, the digital photographs of Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ (0 ≤ x ≤ 0.6) under 365 nm ultraviolet lamp irradiation are shown in the insets (b-f) of Fig. 6, which also reveal the tunable emission. The external QE of the typical Li4Ca0.8Sr1.16(SiO4)2:0.04Eu2+ sample was measured to be about 14.8%.
Fig. 7 Emission spectra of Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ (0 ≤ x ≤ 0.6), the inset shows their normalized emission spectra at 433 nm.
To investigate the effect of introducing Ca2+ ions into Sr2+ sites on the emission characteristics, Fig. 8 presents the emission spectra of the Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ (−0.4 ≤ x ≤ 0) phosphors under 355 and 410 nm excitation. With increasing Ca2+ content, the intensity of the emission band around 433 nm increases gradually until x = −0.2, and this emission intensity starts to decay beyond this Ca2+ content. This observation could be owing to the concentration quenching for Eu2+ in the Sr2+ site, which is similar to that of the emission band at 585 nm in Fig. 7. Under 355 nm excitation, the emission band around 585 nm exhibits a continuous decrease with the Ca2+ content increased, as can be seen in the enlarged figure in Fig. 8. This result could be derived from the decrease of the Eu2+ centers in Ca2+ sites, which has a similar interpretation to the emission characteristics in Fig. 7. To further understand the change of the relative emission intensities between 433 and 585 nm for the Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ (−0.4 ≤ x ≤ 0) samples, the inset of Fig. 8 presents their normalized emission spectra at 433 nm. As a result, the emission intensity for 585 nm show a gradual decrease. In addition, it can be found the weak emission band at 585 nm appears upon 410 nm excitation and the blue emission band doesn’t exist, indicating the long-wavelength emission band at 585 nm mainly corresponds to the long-wavelength excitation which is agreement with the above analysis. The CIE chromaticity coordinates of the typical Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ (x = −0.1 and −0.3) samples were calculated respectively to be (0.164, 0.050) and (0.159, 0.043), indicating blue emissions as can be seen from the CIE chromaticity diagram in Fig. 6 (Points 8 and 9). The external QE of the typical Li4Ca1.2Sr0.76(SiO4)2:0.04Eu2+ sample was measured to be about 22.1%.
Fig. 8 Emission spectra of Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ (−0.4 ≤ x ≤ 0), the inset shows their normalized emission spectra at 433 nm.
From the emission spectra features in Fig. 7 and 8 , it can be concluded that two Eu2+ emission centers exist in the Li4CaSr(SiO4)2:Eu2+ phosphors, and the emission band around 433 and 585 nm are respectively attributed to the Sr2+ and Ca2+ sites, which are accordant with the previous investigation results. The effect of different Ca2+ and Sr2+ contents in Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ on the relative intensities of the blue and yellow emissions for Eu2+ further supports this conclusion.
To evaluate the thermal stability of the as-prepared samples, Figs. 9 and 10 present the mission spectra of Li4Ca0.8Sr1.16(SiO4)2:0.04Eu2+ and Li4Ca1.2Sr0.76(SiO4)2:0.04Eu2+ at different temperatures, respectively. With increasing temperature, it can be seen that the intensity of the blue emission bands decrease obviously with increasing temperature, but those of the orange emission bands increase somewhat. To further understand this point, the temperature dependence of the predominated emission intensities are shown in the insets of Figs. 9 (for the blue and orange emissions) and 10 (for the blue emission). This observation indicates that the Eu2+ emission centers in the Sr2+ and Ca2+ sites have different thermal stabilities, which could be owing to their different chemical environments. As a result, the emitting light colors have been changed somewhat for different temperatures. The CIE chromaticity coordinates of the above two samples were calculated to be (0.306, 0.219), (0.306, 0.224), (0.310, 0.235), (0.318, 0.258), (0.353, 0.305), (0.405, 0.375) for Li4Ca0.8Sr1.16(SiO4)2:0.04Eu2+ and (0.160, 0.044), (0.160, 0.045), (0.161, 0.049), (0.170, 0.058), (0.188, 0.087), (0.236, 0.153) for Li4Ca1.2Sr0.76(SiO4)2:0.04Eu2+ for T = 20, 50, 80, 110, 140, and 170 °C, respectively. The corresponding CIE chromaticity diagram in Fig. 11 further demonstrates that the emitting light colors for both the samples shift toward the “warm color” region with increasing measure temperature. This spectral characteristic could be beneficial to the application in near-UV LEDs combined with several phosphors. In many cases, the emission band of a phosphor usually exhibits continuous blue-shift with increasing temperature, such as Ba2Ln(BO3)2Cl:Eu2+ [22], KCaY(PO4)2:Eu2+, Mn2+ [23], Ca2BO3Cl:Eu2+ [24], Ca2SiO4:Eu2+ and Ba2SiO4:Eu2+ [25]. Correspondingly, the emitting light colors for this kind of phosphors will change toward the “cold color” region. Thus, if this kind of phosphors are used with our Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ phosphors in a white LED, the changes in their emission colors at high temperatures will complement each other, which could stabilize the emitting light color of a white LED effectively with changing temperature.
Fig. 9 Emission spectra of Li4Ca0.8Sr1.16(SiO4)2:0.04Eu2+ at different temperatures, the inset shows the temperature dependence of the predominated emission intensities.
Fig. 10 Emission spectra of Li4Ca1.2Sr0.76(SiO4)2:0.04Eu2+ at different temperatures, the inset shows the temperature dependence of the predominated emission intensities.
Fig. 11 CIE chromaticity diagram for Li4Ca0.8Sr1.16(SiO4)2:0.04Eu2+ (Points 1-6 are for T = 20, 50, 80, 110, 140, and 170 °C, respectively) and Li4Ca1.2Sr0.76(SiO4)2:0.04Eu2+ (Points 7-12 are for T = 20, 50, 80, 110, 140, and 170 °C, respectively)
4. Conclusions
In this paper, to explore emission-tunable phosphors, a series of Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ (−0.4 ≤ x ≤ 0.6) were synthesized by solid-state reaction method. By changing the relative concentrations of Ca2+ and Sr2+, the single-phase samples could be all obtained successfully. The phosphors exhibit two main emission bands from 400 to 725 nm, peaking at 433 and 585 nm. The relative intensities of the above two emissions are different for different excitation wavelengths, and thus, tunable-emission is achieved. On the other hand, the emitting light color could be also tuned by changing the relative concentrations of Ca2+ and Sr2+. All the above emission features are owing to the various Eu2+ emission centers in the Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ phosphors. Correspondingly, the excitation spectra for the samples demonstrate different profiles depending on the monitoring wavelength and the relative contents of Ca2+ and Sr2+, but they can well match with the NUV LED chip. The emission spectra characteristics by varying the Ca2+ and Sr2+ concentrations further confirms the site-occupancy of the Eu2+ ions on Ca2+ and Sr2+ sites. The investigation of temperature-dependent spectra show that the Eu2+ emission centers in the Sr2+ and Ca2+ sites have different thermal stabilities, leading to the emission color also changes somewhat at different temperature, and this may be useful to stabilize the final emitting light color of a white LED with changing operating temperature.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 21403081), Natural Science Foundation of Jiangsu Province of China (No. BK20140456) and Natural Science Foundation of the Jiangsu Higher Education Institutions of China (No. 14KJD140002).
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