Synthesis and Luminescent Characteristics of Ce³⁺-Activated Borosilicate Blue-Emitting Phosphors for LEDs

Abstract

The phosphors Sr₃B₂SiO₈:Ce³⁺ have been successfully synthesized via solid-state reaction process. Emission/excitation spectra and photoluminescence decay behaviors were investigated in detail. Under the excitation of 340 nm, the emission spectrum presented an asymmetry emission band extended from 350 to 600 nm, which with the main peak at 425 nm can be fitted in two peaks (23940 cm⁻¹ and 21934 cm⁻¹). The chromaticity coordinates of :Ce³⁺ are fixed in the blue region; when the intensity of Ce³⁺ reached the maximum, the chromaticity coordinate is (0.154, 0.088) which is more close to the standard CIE of blue light (0.140, 0.080). The results showed the kind of phosphor may have potential applications in the fields of UV-excited white LEDs.

1. Introduction

White-light-emitting diodes (w-LEDs) attract more attention as the result of their advantages such as longer lifetime, higher rendering index, higher luminosity efficiency, and lower energy consumption [1, 2]. Commonly, we combine the blue light of GaN chips and the yellow emission of YAG:Ce³⁺ to gain white light [3–5]; however, the type of white color varies with the input power and a poor color rendering index (). Researchers made efforts to overcome the disadvantages mentioned above; novel phosphors can be effectively excited by ultraviolet or blue light and emit strong blue, green, and red light [6–8]. Tricolor phosphors with higher stability and intense absorption in UV spectral region are just in demand to meet the optimum requirements of w-LEDs.

Ce³⁺ ions play a significant role in the rare earth ions; commonly, they act as the blue-emitting phosphors due to the parity allowed electric dipole transition of 4f → 5d. Ce³⁺-activated phosphors commonly act as the blue-emitting phosphors as the result of their 4f¹ configuration in solids shows efficient broad band luminescence which due to the parity allowed electric dipole transition. The transitions of the Ce³⁺ ion have been widely investigated and doped in the hosts such as Gd₃(Ga,Al)₅O₁₂ [9], [10], and Sr₃Al₂O₅Cl₂ [11]; generally, the emissions of the Ce³⁺ ion shift slightly to longer wavelengths which depend on the host composition, the crystal structure, or the lattice symmetry. In all of the hosts, borates have attracted extensive attention attributed to their stable physical and chemical properties, excellent thermal stability, and better absorption in UV region, especially borosilicate host Sr₃B₂SiO₈ as one kind of borate hosts, due to the adding of the boric acid; it can reduce the synthesis temperature of preparation of silicate. Recently, borosilicate host Sr₃B₂SiO₈ has been investigated by the researchers [12–14]; it indicated that the materials could emit intense visible light and may act as promising phosphors for practical application. However, the PL properties of borosilicate host materials have not been investigated widely; this prompted us to study the fluorescence properties of rare earth ions in these borates.

In this paper, we utilize the advantages of borosilicate and choose Sr₃B₂SiO₈ as the substrate of luminescent material and doped trivalent rare earth Ce³⁺ to analyze the luminescence properties under ultraviolet excitation conditions. Sr₃B₂SiO₈:Ce³⁺ phosphors were synthesized successfully by the solid-state reaction; corresponding luminescent properties were investigated in detail; the CIE of phosphors were also calculated. The results suggest that Sr₃B₂SiO₈:Ce³⁺ may be used as potential blue phosphors for UV-based w-LEDs.

2. Experimental Section

2.1. Synthesis of Sr_3−xB₂SiO₈:Ce³⁺  ~ Phosphors

:Ce³⁺ ( = 0.0025~0.05) phosphors were prepared by a solid-state reaction technique [15]. Reactants SrCO₃, H₃BO₃, and SiO₂ are analytical reagent grade (99.90%), and CeO₂ is spectrographic grade (99.99%). Reactant samples were first quantified by the stoichiometric ratio and then thoroughly mixed by grinding them in an agate mortar for 2 hours; then, samples were transferred into the corundum crucible and placed in a muff furnace at 600°C for 1 h; then, the samples were got out from the muff furnace and ground for 1 h again, subsequently, firing at 1000°C for 3 h in the reducing atmosphere (95% N₂ + 5% H₂). Finally, corundum crucibles were cooled to room temperature and the phosphor samples were obtained.

2.2. Characterization of Sr_3−xB₂SiO₈:Ce³⁺  ~ Phosphors

The powder X-ray diffraction (XRD) patterns were recorded with a Bruker D8-advance X-ray powder diffractometer with Cu Kα radiation ( Å); the operation voltage and current were maintained at 40 kV and 40 mA, respectively; a scan rate of 2°/min was applied to record the patterns in the range of 2θ = 10~60°. The excitation and emission spectra were measured by a Spectrofluorophotometer RF-5301PC series equipped with a 150 W Xenon lamp. The luminescence decay curves were obtained from a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz). All the experiments were performed at room temperature.

3. Result and Discussion

3.1. The Crystal Structures of the Samples

The phase purities of the as-prepared powder samples were characterized by XRD at room temperature. The XRD patterns of :Ce³⁺ ( = 0.0025~0.05) phosphor samples with different Ce³⁺ concentrations are shown in Figure 1. All of the diffraction peaks are in accordance with Sr₃B₂SiO₈ (JCPDS card number 32-1224). All these samples are single phase without any impurities. This indicates that doping Ce³⁺ ions in Sr₃B₂SiO₈ host with such a small concentration has no other phase specific changes. The crystal system is Orthorhombic, the space group is Pnma, and lattice parameters of a = 12.355 (2), b = 3.916 (1), c = 5.405 (4), and V = 261.50 (56) ų. When the replacement of Sr²⁺ by Ce³⁺ occurs, the lattice constant and lattice volume vary, the radii of Ce³⁺ ion (0.1143 nm) are smaller than the radius of Sr²⁺ ion (0.1260 nm), and the variety of the lattice constant and lattice volume decreases along with the different Ce-containing adding, as presented in Table 1 [16].

Table 1 

Crystal data of B₂SiO₈: ( = 0~0.05) phosphors.

Figure 1 

Typical XRD patterns of :Ce³⁺.

3.2. The PL Excitation and Emission Spectra of Ce³⁺ Doped Sr₃B₂SiO₈ Phosphor

Figure 2 shows the PL excitation and emission spectra of Ce³⁺ doped Sr₃B₂SiO₈ phosphor samples with 0.5 mol% concentration. It can be observed clearly that the excitation spectrum of Ce³⁺ covers the range from 220 to 400 nm that shows two absorption bands, one between 220 and 300 nm and one around 340 nm. Both bands are due to optically allowed transitions. The excitation band split two characteristic peaks that are influenced by the crystal field environment of Ce³⁺ ions and the strongest excitation peak centered at 340 nm. Under the excitation of 340 nm, the emission spectrum presents an asymmetry emission band with the main peak at 425 nm; it also can be fitted in two peaks (23940 cm⁻¹ and 21934 cm⁻¹); the deviation of the peaks is 2006 cm⁻¹ which is in accordance with the theoretical energy difference of ²F_5/2 and ²F_7/2 (about 2000~2200 cm⁻¹) of Ce³⁺ [17].

Figure 2 

Photoluminescence excitation ((a) = 425 nm) and emission ((b) = 340 nm) spectra of Sr_2.995B₂SiO₈:0.005Ce³⁺.

3.3. The Emission Spectra of Series Samples of Sr_3−xB₂SiO₈:Ce³⁺  ~

Figure 3 shows the emission spectra of series samples of :Ce³⁺ ( = 0.0025~0.05) under the excitation of 340 nm. From Figure 3, it is shown that the emission intensities increase with the Ce³⁺ concentration adding and then gradually decrease above 0.5 mol%. The figure also shows that the emission maximum shifts slightly to longer wavelengths with the increase of Ce³⁺ concentration which attribute crystal field environment of Ce³⁺ ions. Emission intensity increases significantly as the Ce³⁺ concentration is increased and gradually decreases as the doping concentration becomes greater than . As a result, the distance between Ce³⁺ ions becomes smaller with the adding of Ce³⁺ ions, leading to high probability of energy transfer among the Ce³⁺ ions. The loss of energy causes the emission intensity to be reduced, then leading to concentration quenching. Therefore, the optimum doping concentration of Ce³⁺ is fixed at 0.5 mol%.

Figure 3 

Photoluminescence emission spectra of :Ce³⁺ with different Ce³⁺ concentration under the excitation of 340 nm.

3.4. The Critical Distance of Energy Transfer between Ce³⁺ Ions

With the concentration of Ce³⁺ ions increasing, the average distance between Ce³⁺ ions gradually decreased; then, the concentration quenching occurred; the concentration quenching may be induced by cross-relaxation processes in close . Namely, as the Ce³⁺ concentration increases, the possibility of energy transfer increases. According to the report of Blasse, we can roughly estimate the critical distance of energy transfer () and calculate it as follows [18]:

Here is the unit cell volume, is the critical concentration of dopant ions, and is the number of host cations in the ions in a unit cell. For Sr₃B₂SiO₈ host, = 252.12 (11) ų, , and the critical concentration is about 0.005 in our system.

The value is about 28.8 Å by using (1). The value obtained above indicates the possibility of exchange interaction of ions. In general, there are three mechanisms for nonradiate energy transfer including exchange interaction, radiation reabsorption, and electric multipolar interactions. The exchange interaction is only for 5 Å, and the radiation reabsorption needs the emission and excitation spectra has widely overlapping; therefore, it can be inferred that electric multipolar interactions would be the energy transfer mechanism between Ce³⁺ and Ce³⁺ in the system.

3.5. Fluorescence Lifetime of Phosphors

The experiment tests the fluorescence lifetime of different concentrations of Ce³⁺ in Sr₃B₂SiO₈ system. Figure 4 presented the fluorescence decay curves and simple orbit transition of Ce³⁺; after fitting, the values can be well fitted by a single exponential function: where and are the luminescence intensities at times and 0, is the time, is the luminescence lifetime, and is the value for different fittings. For :Ce³⁺ ( = 0.0025~0.05) samples, based on the decay curves and the above-mentioned equation (2), the luminescence lifetime of Ce³⁺ is 39.98, 40.03, 38.77, 36.95, and 35.80 ns.

Figure 4 

Photoluminescence decay curves of Ce³⁺ in :Ce³⁺ ( = 0.0025~0.05) phosphors.

3.6. The Chromaticity Coordinates of Sr_3−xB₂SiO₈:Ce³⁺

Figure 5 presented the chromaticity coordinates of different molar (0.0025~0.05 mol) of Ce³⁺-activated Sr₃B₂SiO₈ phosphors. It can be observed that, with the Ce³⁺ ions added (0.0025~0.05 mol), the color moved to the blue region, and the chromaticity coordinates of Sr₃B₂SiO₈:Ce³⁺ are (1) (0.175, 0.099), (2) (0.154, 0.088), (3) (0.194, 0.116), (4) (0.210, 0.135), and (5) (0.223, 0.155), respectively, as shown in Table 2. When the intensity of Ce³⁺ reaches the maximum, it is more close to the standard CIE of blue light (0.140, 0.080) and the CIE of the commercial blue phosphor BaMgAl₁₀O₇:Eu²⁺ is (0.147, 0.067); from the CIE chromaticity diagram of Sr₃B₂SiO₈:Ce³⁺, it is obviously seen that we have realized that the blue light emission could be excited by UV in Sr₃B₂SiO₈:Ce³⁺ phosphors, which may be promising for white LEDs under UV excitation.

Table 2 

Corresponding variations of CIE chromaticity coordinate for Sr₃B₂SiO₈:Ce³⁺ ( = 0.0025~0.05) phosphor.

Figure 5 

Chromaticity coordinates of :Ce³⁺.

4. Conclusions

In summary, a series of Sr₃B₂SiO₈:Ce³⁺ phosphors have been synthesized by traditional high temperature solid-state reaction. The phosphors Sr₃B₂SiO₈:Ce³⁺ can be excited under the UV region and show blue-emitting light extended from 350 to 600 nm. When the concentration of Ce³⁺ is 0.5% mol, the chromaticity coordinates of Sr_2.995B₂SiO₈:0.005Ce³⁺ are (0.154, 0.088) which are more close to the standard CIE of blue light (0.140, 0.080). The results showed that Sr₃B₂SiO₈:Ce³⁺ phosphors may act as a blue component for white LEDs.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This present work has been supported by the Foundation of Chongqing University of Arts and Sciences (R2013CJ09, R2015CH11); Mineral and Ore Resources Comprehensive Utilization of Advanced Technology Popularization and Practical Research (MORCUATPPR) and China Geological Survey (Grant no. 12120113088300).