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

Authors: Hong Yu; Jinlei Chen; Shucai Gan
Journal: Journal of Spectroscopy
 (2016)

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## 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 Sr3-xB2SiO8:xCe3+ 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.


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## Body

## 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 (Ra<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 [6–8]. 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 4f→5d parity allowed electric dipole transition. The 4f→5d transitions of the Ce3+ ion have been widely investigated and doped in the hosts such as Gd3(Ga,Al)5O12 [9], NaY0.6-xCe0.1Gd0.3EuxF4 [10], and Sr3Al2O5Cl2 [11]; generally, the 5d→4f 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 Sr3−xB2SiO8:xCe3+  (x=0.0025~0.05) Phosphors

S
r
3
-
x
B
2
S
i
O
8:xCe3+ (x = 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 Sr3−xB2SiO8:xCe3+  (x=0.0025~0.05) Phosphors

The powder X-ray diffraction (XRD) patterns were recorded with a Bruker D8-advance X-ray powder diffractometer with Cu Kα radiation (λ=1.5406 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.


## 2.1. Synthesis of Sr3−xB2SiO8:xCe3+  (x=0.0025~0.05) Phosphors

S
r
3
-
x
B
2
S
i
O
8:xCe3+ (x = 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 Sr3−xB2SiO8:xCe3+  (x=0.0025~0.05) Phosphors

The powder X-ray diffraction (XRD) patterns were recorded with a Bruker D8-advance X-ray powder diffractometer with Cu Kα radiation (λ=1.5406 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 ofSr3-xB2SiO8:xCe3+ (x = 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 ofa = 12.355 (2),b = 3.916 (1),c = 5.405 (4), andV = 261.50 (56) Å3. 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 ofSr3-xB2SiO8:xCe3+ (x = 0~0.05) phosphors.
x
C
e
3
+ (mol)
0
0.0025
0.005
0.01
0.03
0.05
Radiation type
Cu-Kα
Cu-Kα
Cu-Kα
Cu-Kα
Cu-Kα
Cu-Kα
Wavelength (Å)
1.5406
1.5406
1.5406
1.5406
1.5406
1.5406
Profile range (°)
10–60
10–60
10–60
10–60
10–60
10–60
Crystal system
Orthorhombic
Orthorhombic
Orthorhombic
Orthorhombic
Orthorhombic
Orthorhombic
Space group
Pnma
Pnma
Pnma
Pnma
Pnma
Pnma
Z
4
4
4
4
4
4
a
12.355 (2)
12.350 (2)
12.234 (1)
12.018 (3)
11.871 (0)
11.684 (1)
b
3.916 (1)
3.894 (4)
3.873 (1)
3.849 (0)
3.827 (2)
3.805 (2)
c
5.405 (4)
5.393 (3)
5.321 (0)
5.315 (0)
5.302 (2)
5.294 (3)
V (Å3)
261.50 (56)
259.35 (42)
252.12 (11)
245.85 (74)
240.87 (15)
235.35 (86)Figure 1
Typical XRD patterns ofSr3-xB2SiO8:xCe3+.


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

Figure2 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 5d04f1→5d14f0 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 2F5/2 and 2F7/2 (about 2000~2200 cm−1) of Ce3+ [17].Figure 2
Photoluminescence excitation ((a)λem = 425 nm) and emission ((b) λex = 340 nm) spectra of Sr2.995B2SiO8:0.005Ce3+.


### 3.3. The Emission Spectra of Series Samples of Sr3−xB2SiO8:xCe3+  (x=0.0025~0.05)

Figure3 shows the emission spectra of series samples of Sr3-xB2SiO8:xCe3+ (x = 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 x=0.005. 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 ofSr3-xB2SiO8:xCe3+ 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 Ce3+→Ce3+. 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 (Rc) and calculate it as follows [18]:(1)Rc≈23V4πXcN1/3.HereV is the unit cell volume, Xc is the critical concentration of dopant ions, and N is the number of host cations in the ions in a unit cell. For Sr3B2SiO8 host, V = 252.12 (11) Å3, N=4, and the critical concentration Xc is about 0.005 in our system.TheRc value is about 28.8 Å by using (1). The Rc 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: (2)I=I0exp⁡-tτ+A,where I and I0 are the luminescence intensities at times t and 0, t is the time, τ is the luminescence lifetime, and A is the value for different fittings. For Sr3-xB2SiO8:xCe3+ (x = 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 Sr3-xB2SiO8:xCe3+ (x = 0.0025~0.05) phosphors.


### 3.6. The Chromaticity Coordinates of Sr3−xB2SiO8:xCe3+

Figure5 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:xCe3+ (x = 0.0025~0.05) phosphor.
White LEDs site
CIE coordinates (x,y)
λ
e
x  range  (nm)
λ
e
m  (nm)
1
0.175, 0.099
220–400
422
2
0.154, 0.088
220–400
425
3
0.194, 0.116
220–400
430
4
0.210, 0.135
220–400
439
5
0.223, 0.155
220–400
442
Standard CIE of blue light
0.140, 0.080
—
—Figure 5
Chromaticity coordinates ofSr3-xB2SiO8:xCe3+.


## 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 ofSr3-xB2SiO8:xCe3+ (x = 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 ofa = 12.355 (2),b = 3.916 (1),c = 5.405 (4), andV = 261.50 (56) Å3. 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 ofSr3-xB2SiO8:xCe3+ (x = 0~0.05) phosphors.
x
C
e
3
+ (mol)
0
0.0025
0.005
0.01
0.03
0.05
Radiation type
Cu-Kα
Cu-Kα
Cu-Kα
Cu-Kα
Cu-Kα
Cu-Kα
Wavelength (Å)
1.5406
1.5406
1.5406
1.5406
1.5406
1.5406
Profile range (°)
10–60
10–60
10–60
10–60
10–60
10–60
Crystal system
Orthorhombic
Orthorhombic
Orthorhombic
Orthorhombic
Orthorhombic
Orthorhombic
Space group
Pnma
Pnma
Pnma
Pnma
Pnma
Pnma
Z
4
4
4
4
4
4
a
12.355 (2)
12.350 (2)
12.234 (1)
12.018 (3)
11.871 (0)
11.684 (1)
b
3.916 (1)
3.894 (4)
3.873 (1)
3.849 (0)
3.827 (2)
3.805 (2)
c
5.405 (4)
5.393 (3)
5.321 (0)
5.315 (0)
5.302 (2)
5.294 (3)
V (Å3)
261.50 (56)
259.35 (42)
252.12 (11)
245.85 (74)
240.87 (15)
235.35 (86)Figure 1
Typical XRD patterns ofSr3-xB2SiO8:xCe3+.


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

Figure2 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 5d04f1→5d14f0 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 2F5/2 and 2F7/2 (about 2000~2200 cm−1) of Ce3+ [17].Figure 2
Photoluminescence excitation ((a)λem = 425 nm) and emission ((b) λex = 340 nm) spectra of Sr2.995B2SiO8:0.005Ce3+.


## 3.3. The Emission Spectra of Series Samples of Sr3−xB2SiO8:xCe3+  (x=0.0025~0.05)

Figure3 shows the emission spectra of series samples of Sr3-xB2SiO8:xCe3+ (x = 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 x=0.005. 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 ofSr3-xB2SiO8:xCe3+ 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 Ce3+→Ce3+. 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 (Rc) and calculate it as follows [18]:(1)Rc≈23V4πXcN1/3.HereV is the unit cell volume, Xc is the critical concentration of dopant ions, and N is the number of host cations in the ions in a unit cell. For Sr3B2SiO8 host, V = 252.12 (11) Å3, N=4, and the critical concentration Xc is about 0.005 in our system.TheRc value is about 28.8 Å by using (1). The Rc 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: (2)I=I0exp⁡-tτ+A,where I and I0 are the luminescence intensities at times t and 0, t is the time, τ is the luminescence lifetime, and A is the value for different fittings. For Sr3-xB2SiO8:xCe3+ (x = 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 Sr3-xB2SiO8:xCe3+ (x = 0.0025~0.05) phosphors.


## 3.6. The Chromaticity Coordinates of Sr3−xB2SiO8:xCe3+

Figure5 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:xCe3+ (x = 0.0025~0.05) phosphor.
White LEDs site
CIE coordinates (x,y)
λ
e
x  range  (nm)
λ
e
m  (nm)
1
0.175, 0.099
220–400
422
2
0.154, 0.088
220–400
425
3
0.194, 0.116
220–400
430
4
0.210, 0.135
220–400
439
5
0.223, 0.155
220–400
442
Standard CIE of blue light
0.140, 0.080
—
—Figure 5
Chromaticity coordinates ofSr3-xB2SiO8:xCe3+.


## 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 Sr2.995B2SiO8:0.005Ce3+ 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.



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*Source: 9346503-2016-02-01.xml*