Enhanced luminescence intensity of novel red-emitting phosphor -Sr3Lu2(BO3)4:Bi3+,Eu3+ via energy transfer
© Maggay et al.; licensee Springer 2014
Received: 1 March 2014
Accepted: 1 July 2014
Published: 16 July 2014
Bi3+/Eu3+ co-activated Sr3Lu2(BO3)4 was successfully synthesized via a solid state reaction. The optimal concentration of Bi3+,Eu3+ and Bi3+/Eu3+ are 1 mol%, 60 mol% and 1 mol%/20 mol%, respectively. The emission spectra of Sr3Lu2(BO3)4:Bi3+, Eu3+ gives three peaks located at 405 nm, 489 nm which were attributed to Bi3+ S6 (blue) and C2 (green) site symmetry, respectively and 610 nm which was ascribed to Eu3+ (5D0 → 7 F2) transition. The emission intensity of Bi3+ decreases with increasing Eu3+ content which indicates that a efficient energy transfer occurred in the Sr3Lu2(BO3)4 host. The relative intensity of Sr3Lu1.79(BO3)4:0.01Bi3+,0.20Eu3+ excited at 327 nm and 370 nm was remarkably enhanced by 201% and 265%, respectively, via the energy transfer from Bi3+ to Eu3+. The results indicate that Sr3Lu2(BO3)4:Bi3+, Eu3+ is a potential novel red-emitting phosphor for UV LED applications.
Recently, there has been a rapid increase in the number of researches on white-light emitting diodes (W-LEDs) and it has begun replacing conventional lighting sources due to its advantages such as high brightness, high energy efficiency, low power consumption, longer working performance and low environmental risk -. The common fabrication of W-LEDs involves a blue-emitting InGaN chip and a yellow emitting phosphor Y3Al5O12:Ce3+ (YAG) ,,. However, this combination displays low color rendering index (Ra) of ~80 and high color temperature which is due to the insufficiency of red emission in the visible spectrum . Thus, it is very essential to search for new red phosphors that can be efficiently excited at around 400 nm .
Eu3+ rare earth ions have drawn much attention of scientists in obtaining a red-emitting due to its lowest excited level 5D0 of the 4f6 configuration which is located below the 4f65d configuration and it principally displays very sharp red emission lines at 5D0-7 F2 transition around 610 ~ 618 nm ,. Most of the red-emitting phosphors are efficiently excited at around 393 nm , originating from 7 F0-5 L6 transition which is parity forbidden therefore it exists as a sharp peak and cannot absorb the excitation energy efficiently. In order to improve and broaden the excitation spectrum of Eu3+ ions, one of the common strategies is by introducing Bi3+ as a sensitizer. Liu et al.  found that the luminescence intensity and quantum efficiency of ZnB2O4:Bi3+, Eu3+ phosphors were much higher than that of ZnB2O4:Eu3+ phosphors by co-doping Bi3+ into the host via an energy transfer process. The optimized-composition of ZnB2O4:Eu3+, Bi3+ was even superior to that of commercial phosphor, La2O2S:Eu3+. Zhou et al.  reported that the enhanced luminescence properties and energy transfer mechanism of Ca3SnSi2O9:Bi3+, Eu3+ phosphors by a solid state reaction. Zhu et al.  investigated the energy transfer phenomena of Bi3+/Eu3+ co-doped Ca10(PO4)6 F2 phosphors for UVLED applications. Park et al.  discovered that with increasing the content of Bi3+ on Eu3+/Bi3+ co-doping YVO4:Eu3+ phosphors, it demonstrate a shifting of the excitation band to a longer wavelength due to the energy transfer from Bi3+ to Eu3+. Single-phased phosphors have attracted much attention in the fabrication of white LED and borate compounds are good candidates because they can be easily synthesized and are chemically stable. To the best of our knowledge, spectral and laser properties of Er:Yb Sr3Lu2(BO3)4 have been reported and it have manifested good spectral properties. Moreover, a study was carried on the luminescent characteristics of Sr3(RE)2(BO3)4:Dy3+ (RE = Y, La, Gd) for white LED applications and provided a significant evidence.
To the best of our knowledge, there has been no reported study on the luminescence properties of Bi3+/Eu3+ co-doped Sr3Lu2(BO3)4. In this study, crystal structure, phase purities as well as luminescence and energy transfer mechanism of Sr3Lu2(BO3)4:Bi3+, Eu3+ phosphors were firstly investigated. The results demonstrate that Sr3Lu2(BO3)4:Bi3+, Eu3+ is a potential red-emitting phosphor for UVLED applications.
Polycrystalline powder samples were prepared via a solid state reaction. The starting materials used were SrCO3 (99.99%, Aldrich), H3BO3 (99.99%, Strem), Lu2O3 (99.99%Aldrich), Eu2O3 (99.99%, Aldrich) and Bi2O3 (99.99%Aldrich) were weighed in stoichiometric ratios and were homogeneously mixed and ground in agate mortar then transferred in alumina crucibles and sintered at 1200°C for 8 hours in air. The products were cooled down to ambient temperature and pulverized for further analysis.
The phase purity of the as-synthesized samples were characterized using X-ray diffraction (XRD) patterns with Cu Kα radiation (λ = 0.15418 Å) generated at 45 kV and 30 mA. Data were collected in the 2θ range of 10–80° with a scan speed of 5°/min. The PL/PLE spectra of Bi3+/Eu3+ co-doped Sr3Lu2(BO3)4 phosphors were measured at room temperature and recorded by a Spex Fluorolog-3 spectrophotometer equipped with 450 W Xenon light source. All the spectra were measured with a scan rate of 150 nm min−1. The Commission International de I’Eclairage (CIE) chromaticity coordinates were measured by a Laiko DT-101 color analyzer equipped with a CCD detector (Laiko Co., Tokyo, Japan).
Results and discussion
XRD and crystal structure analyses
Figure 1(e) displays the crystallographic structure. The Sr3Lu2(BO3)4. Lu3+ ions occupy two distinct crystallographic sites Lu1 and Lu2 and each Lu3+ has an eight-fold coordination that forms LuO8 polyhedra. The bond valence of Lu1 is stronger compared to Lu2, therefore Bi3+ and Eu3+ occupies the Lu3+ site. Figure 1. XRD patterns of Sr3Lu2(BO3)4 doped with (a) 1 mol% Bi3+, (b) 60 mol% Eu3+, (c) 1 mol% Bi3+, 20 mol% Eu3+, (d) ICSD 10213 and (e) crystal structure of Sr3Lu2(BO3)4.
Photoluminescence properties of Sr3Lu2(BO3)4:Bi3+phosphors
The excitation spectrum monitored at 492 nm (green emission) consists of two peaks at 326 nm and 342 nm respectively meanwhile the excitation spectrum monitored at 410 nm (blue emission) consists of a weak broad band at 334 nm and a strong broad band at 371 nm. If the Bi3+ ion is asymmetrically coordinated, the Stokes shift of Bi3+ is large. Therefore, the blue emission and green emission originates from S6 and C2 symmetry respectively .
Photoluminescence properties of Sr3Lu2(BO3)4:Eu3+phosphors
Photoluminescence properties of Sr3Lu1.99-y(BO3)4:0.01Bi3+, yEu3+ phosphors
CIE coordinates and relative intensity of SLBO:Bi 3+ ,Eu 3± phosphors
Excitation Wavelength (nm)
CIE chromaticity coordinates
5D0 → 7F2
Sr3Lu2(BO3)4:Bi3+, Eu3+ were effectively synthesized by a solid state reaction and pure phase phosphors were successfully synthesized. Changing the excitation wavelengths from 320 ~ 370 nm greatly affects the emission spectra of Sr3Lu2(BO3)4:Bi3+. Hence, the energy distribution of blue and green emission is highly dependent on the excitation wavelengths. The concentration quenching for Sr3Lu2-x(BO3)4:xBi3+, Sr3Lu2-y(BO3)4:yEu3+ and Sr3Lu2-x-y (BO3)4:xBi3+, yEu3+ are 1 mol%, 60 mol% and 20 mol%, respectively. The spectral overlap of the emission spectra of Bi3+ and the excitation band of Eu3+ depicts an efficient energy transfer from Bi3+ to Bi3+ Moreover, it was shown that the emission intensity of Bi3+ quenches as the concentration of Eu3+ increases hence, the energy absorbed by Bi3+ was transferred to Eu3+. The critical distance of Eu3+ was determined to be 10.7 Å. The relative intensity of Sr3Lu1.79(BO3)4:0.01Bi3+, 0.20Eu3+ at 327 nm and 370 nm were dramatically enhanced by 201% and 265%, respectively. The results indicate that Sr3Lu2(BO3)4:Bi3+, Eu3+ is a potential novel red-emitting UV LED phosphor for display application.
IVBM and PCL are both taking their Master’s Degree in Chung Yuan Christian University, ROC Taiwan. WRL has a Ph.D. degree and is currently an assistant professor in Chung Yuan Christian University and is affiliated with Industrial Technology Research Institute of Taiwan (ITRI), ROC Taiwan.
The work is financially supported from NSC under contracts no. of 102-2221-E-033-050-MY2 and 102-3011-P-033-003.
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