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

The phosphors Sr 3 B 2 SiO 8 :Ce have been successfully synthesized via solid-state reaction process. Emission/excitation spectra and photoluminescence decay behaviorswere investigated in detail. Under the excitation of 340 nm, the emission spectrumpresented an asymmetry emission band extended from350 to 600 nm,whichwith themain peak at 425 nmcan be fitted in twopeaks (23940 cm and 21934 cm).The chromaticity coordinates of Sr 3−x B 2 SiO 8 :xCe are fixed in the blue region; when the intensity of Ce 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.


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:Ce 3+ to gain white light [3][4][5]; however, the type of white color varies with the input power and a poor color rendering index (  < 80).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][7][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.
Ce 3+ 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.Ce 3+ -activated phosphors commonly act as the blue-emitting phosphors as the result of their 4f 1 configuration in solids shows efficient broad band luminescence which due to the 4f → 5d parity allowed electric dipole transition.The 4f → 5d transitions of the Ce 3+ ion have been widely investigated and doped in the hosts such as Gd 3 (Ga,Al) 5 O 12 [9], NaY 0.6− Ce 0.1 Gd 0.3 EuF 4 [10], and Sr 3 Al 2 O 5 Cl 2 [11]; generally, the 5d → 4f emissions of the Ce 3+ 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 Sr 3 B 2 SiO 8 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 Sr 3 B 2 SiO 8 has been investigated by the researchers [12][13][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 Sr 3 B 2 SiO 8 as the substrate of luminescent material and doped trivalent rare earth Ce 3+ to analyze the luminescence properties under ultraviolet excitation conditions.Sr 3 B 2 SiO 8 :Ce 3+ phosphors were synthesized successfully by  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% N 2 + 5% H 2 ).Finally, corundum crucibles were cooled to room temperature and the phosphor samples were obtained.due to optically allowed 5d 0 4f 1 → 5d 1 4f 0 transitions.The excitation band split two characteristic peaks that are influenced by the crystal field environment of Ce 3+ 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 2 F 5/2 and 2 F 7/2 (about 2000∼2200 cm −1 ) of Ce 3+ [17].

The Emission Spectra of Series Samples of Sr
Ce 3+ ( = 0.0025∼0.05).Figure 3 shows the emission spectra of series samples of Sr 3− B 2 SiO 8 :Ce 3+ ( = 0.0025∼ 0.05) under the excitation of 340 nm.From Figure 3, it is shown that the emission intensities increase with the Ce 3+ 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 Ce 3+ concentration which attribute crystal field environment of Ce 3+ ions.Emission intensity increases significantly as the Ce 3+ concentration is increased and gradually decreases as the doping concentration becomes greater than  = 0.005.As a result, the distance between Ce 3+ ions becomes smaller with the adding of Ce 3+ ions, leading to high probability of energy transfer among the Ce 3+ ions.The loss of energy causes the emission intensity to be reduced, then leading to concentration quenching.Therefore, the optimum doping concentration of Ce 3+ is fixed at 0.5 mol%.

The Critical Distance of Energy Transfer between Ce 3+
Ions.With the concentration of Ce 3+ ions increasing, the average distance between Ce 3+ ions gradually decreased; then, the concentration quenching occurred; the concentration quenching may be induced by cross-relaxation processes in close Ce 3+ → Ce 3+ .Namely, as the Ce 3+ 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 Sr 3 B 2 SiO 8 host,  = 252.12(11) Å3 ,  = 4, 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 Ce 3+ and Ce 3+ in the system.
where  and  0 are the luminescence intensities at times  and 0,  is the time,  is the luminescence lifetime, and  is the value for different fittings.For Sr 3− B 2 SiO 8 :Ce 3+ ( = 0.0025∼0.05)samples, based on the decay curves and the  above-mentioned equation (2), the luminescence lifetime of Ce 3+ is 39.98, 40.03, 38.77, 36.95, and 35.80 ns.

Conclusions
In summary, a series of Sr 3 B 2 SiO 8 :Ce 3+ phosphors have been synthesized by traditional high temperature solid-state reaction.The phosphors Sr 3 B 2 SiO 8 :Ce 3+ can be excited under the UV region and show blue-emitting light extended from 350 to 600 nm.When the concentration of Ce 3+ is 0.5% mol, the chromaticity coordinates of Sr 2.995 B 2 SiO 8 :0.005Ce 3+ are (0.154, 0.088) which are more close to the standard CIE of blue light (0.140, 0.080).The results showed that Sr 3 B 2 SiO 8 :Ce 3+ phosphors may act as a blue component for white LEDs.

3. 5 .
Fluorescence Lifetime of Phosphors.The experiment tests the fluorescence lifetime of different concentrations of Ce 3+ in Sr 3 B 2 SiO 8 system.Figure 4 presented the fluorescence decay curves and simple orbit transition of Ce 3+ ; after fitting, the values can be well fitted by a single exponential function:
ties were investigated in detail; the CIE of phosphors were also calculated.The results suggest that Sr 3 B 2 SiO 8 :Ce 3+ may be used as potential blue phosphors for UV-based w-LEDs.
[16]of the diffraction peaks are in accordance with Sr 3 B 2 SiO 8 (JCPDS card number 32-1224).All these samples are single phase without any impurities.This indicates that doping Ce 3+ ions in Sr 3 B 2 SiO 8 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)Å3.When the replacement of Sr 2+ by Ce 3+ occurs, the lattice constant and lattice volume vary, the radii of Ce 3+ ion (0.1143 nm) are smaller than the radius of Sr 2+ ion (0.1260 nm), and the variety of the lattice constant and lattice volume decreases along with the different Cecontaining adding, as presented in Table 1[16].3.2.The PL Excitation and Emission Spectra of Ce3+ Doped Sr 3 B 2 SiO 8 Phosphor.Figure2shows the PL excitation and emission spectra of Ce 3+ doped Sr 3 B 2 SiO 8 phosphor samples with 0.5 mol% concentration.It can be observed clearly that the excitation spectrum of Ce 3+ 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 Sr 2.995 B 2 SiO 8 :0.005Ce 3+ 3.1.TheCrystalStructures of the Samples.The phase purities of the as-prepared powder samples were characterized by XRD at room temperature.The XRD patterns of Sr 3− B 2 SiO 8 :Ce 3+ ( = 0.0025∼0.05)phosphorsamples with Sr 2.95 B 2 SiO 8 :0.05Ce 3+ Sr 2.97 B 2 SiO 8 :0.03Ce 3+ Sr 2.99 B 2 SiO 8 :0.01Ce 3+ Sr 2.995 B 2 SiO 8 :0.005Ce 3+ Sr 2.9975 B 2 SiO 8 :0.0025Ce 3+Figure 1: Typical XRD patterns of Sr 3− B 2 SiO 8 :Ce 3+ .different Ce 3+ concentrations are shown in Figure 1.

Table 2 .
3−x B 2 SiO 8 :Ce 3+ .Figure 5 presented the chromaticity coordinates of different molar (0.0025∼0.05 mol) of Ce 3+ -activated Sr 3 B 2 SiO 8 phosphors.It can be observed that, with the Ce 3+ ions added (0.0025∼0.05 mol), the color moved to the blue region, and the chromaticity coordinates of Sr 3 B 2 SiO 8 :Ce When the intensity of Ce 3+ 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 BaMgAl 10 O 7 :Eu 2+ is (0.147, 0.067); from the CIE chromaticity diagram of Sr 3 B 2 SiO 8 :Ce 3+ , it is obviously seen that we have realized that the blue light emission could be excited by UV in Sr 3 B 2 SiO 8 :Ce 3+ phosphors, which may be promising for white LEDs under UV excitation.