The novel red phosphor of Eu3+-Bi3+ co-activated ZnB2O4 was prepared by a solid-state reaction. The composition-optimized (Zn0.9Eu0.1)B2O4 phosphor exhibits a dominant emission peak at 610 nm (5D0–7F2) with CIE coordinates of (0.63, 0.36) under the excitation at 393 nm. By co-doping Bi3+ ions in ZnB2O4:Eu3+, the emission intensity and quantum efficiency can be efficiently enhanced by an increment of 14% and 6%, respectively. The luminescence performance and thermal stability of (Zn0.8Bi0.1Eu0.1)B2O4 phosphor were found to be superior to that of the commodity phosphor, La2O2S:Eu3+. The red-emitting borate phosphor may be potentially useful in the fabrication of white light-emitting diodes (LEDs).
©2010 Optical Society of America
In the last decade, there has been a dramatic increase in the number of research on white light-emitting diodes (LEDs) as a new light source for general lighting and displays . The conversional white light illumination are mostly composed of blue-emitting InGaN chip and yellow phosphor, typically Y3Al5O12:Ce3+ (YAG) [2,3], which exhibits high luminescence efficiency and chemical stability. The combination of blue chip and YAG, however, show a lower color rendering index (Ra) of ~80 due to the lack of red color contribution. Thus, developing new red phosphors, which can be efficiently excited by either 460 or 405 nm LEDs is very crucial issue. For seeking suitable red phosphors, sulfides and oxysulfides, such as CaS:Eu2+, SrS:Eu2+, La2O2S:Eu3+, and Y2O2S:Eu3+, were reported to be efficiently excited under 460 or 405 nm. Nevertheless, the drawbacks of these compounds are sensitive to moisture, giving a poor chemical stability. Recently, nitrides and oxynitrides [4–8] demonstrate good potential as red phosphors due to their good thermal stability. Hence, it is urgent to search for new red phosphor with low cost and high chemical stability. From the points of view, the oxide-based hosts with Eu3+ have been widely investigated [9–12]. It has been well known that in some hosts, Bi3+ or Sm3+ was proven to be a very good sensitizer for Eu3+ with not only enhancing the luminescent performance but also broadening the excitation spectrum. Datta  investigated the role of Bi3+ in YVO4:Eu3+ and showed an increase in luminescence intensity of almost 200% as Bi3+ is incorporated into the (Y0.95Eu0.05)VO4 lattice. Neeraj et al.  reported the excitation band of BixLn1-xVO4 system doped with Eu3+ or Sm3+ can be broaden by choosing suitable sensitizers. Park et al.  investigated the effect of Bi3+, Eu3+ codoping on the excitation of Eu3+ in YVO4:Bi3+,Eu3+ and concluded that with increasing Bi3+ dopant content, the excitation range was found to shift to longer wavelength. Wang et al.  also demonstrated that the incorporation of Bi3+ and Sm3+ into NaEu(MoO4)2 host can both broadened the excitation band and enhance the emission intensity of Eu3+ under 395/405nm. Chi et al.  found that Bi3+-doped Y2O3:Eu is used as red phosphors with very high efficiency. Recently, Li et al  reported photoluminescence (PL) and thermoluminescence (TL) of Zn(BO2)2:Tb3+ and concluded that the phosphor exhibited potential application in gamma-rays TL dosimeter. To the best of our knowledge, the luminescence properties of Bi3+/Eu3+ – co-activated ZnB2O4 have not been reported yet. Hence, the aim of this study is to investigate and examine the luminescence properties of a new red phosphor, ZnB2O4:Bi3+,Eu3+. The improvement of luminescence intensity through energy transfer from co-doped sensitizer Bi3+ to activator Eu3+ in ZnB2O4 host will also be discussed in this work.
Polycrystalline samples of ZnB2O4:Eu3+ and ZnB2O4:Bi3+,Eu3+ powders were synthesized by conventional solid state reaction, starting from ZnO, H3BO3, Eu2O3, and Bi2O3. The raw materials were weighed out in stoichiometric proportions and the mixtures were then fired at 850°C for 10 h under ambient atmosphere. The detailed measurements of photoluminescence (PL), photoluminescence excitation (PLE), Commission International de I’Eclairage (CIE) chromaticity, and diffuse reflectance (DR) spectra were carried out under ambient atmosphere and described in our previous work .
3. Results and discussion
3.1. XRD patterns and atom structure of synthesized ZnB2O4
As reported in the JCPDS card No. 39-1126, ZnB2O4, crystallizes in a cubic (space group: Im-3m (229)) structure with lattice constant: a = 7.473 Å and Z = 6. Figure 1 shows the XRD patterns of the as-synthesized ZnB2O4, Zn0.9Eu0.1B2O4, and Zn0.8Eu0.1Bi0.1B2O4 samples. These XRD patterns were found to be consistent with that reported in JCPDS card No. 39-1126 and no peak shifting was observed in the XRD patterns of ZnB2O4:Eu or ZnB2O4:Bi,Eu samples.
The decrease in crystallinity of ZnB2O4:Eu,Bi with increasing Eu3+ and Bi3+ dopant content was observed from the XRD patterns, which can be attributed to the variation of charges of dopants that results in the defect formation in the lattice. Little impurity phases, such as H3BO3 and B2O3 were observed at 2θ = 29.7°, 30.2°, 30.9°, 31.3°, 31.6° and 32.1° in Fig. 1b and 1c, respectively. The intensity of impurity phases are too small that the effect on luminescence of ZnB2O4:Eu or ZnB2O4:Bi,Eu could be neglected. The XRD data shown in Fig. 1 indicated the as-prepared ZnB2O4 samples were almost pure phase with highly crystallinity. It is known that the ionic radii (r) of Zn2+ (CN = 4) and B3+ (CN = 3) are 0.60 Å and 0.21 Å , respectively. As a result of the ionic radius of B3+ is too small, it is difficult for these elements to replace B3+ in the ZnB2O4. Hence, in this study, it is believed that the Zn2+ sites are substituted of Eu3+ and Bi3+ in the lattice.
3.2. The UV excitation spectrum under Bi3+/Eu3+ activators
Figure 2 shows the reflectance spectra of pure ZnB2O4 and tri-valance ions (such as Bi, Eu or Bi-Eu) activated–ZnB2O4. The spectrum of pure ZnB2O4 host (Fig. 2(a)) exhibited the fundamental absorption edge at ~380 nm. For doping 10 mol% Eu3+ in ZnB2O4, two absorption bands peaks at ~280 nm and 393 nm are observed in the spectrum. The former is attributed to the O2--Eu3+ charge transfer band (CTB); the latter is resulting from f-f transition of Eu3+ (7F0→5L6). While solely doping 1 mol % Bi3+, a very broad absorption band between 250 nm to 390 nm is observed, which could be responsible for the 6s6p excitation state of Bi3+ .
For Zn0.89B2O4:Bi0.01Eu0.1 and Zn0.8B2O4:Bi0.1Eu0.1, the spectra show that with increasing the Bi3+ content, the absorption band shifts to longer wavelength. It was found that the absorption edges were almost overlapping in ZnB2O4, ZnB2O4:1%Eu, ZnB2O4:3%Eu and ZnB2O4:5%Eu. The unobserved absorption edge for ZnB2O4:10%Eu may be due to its relatively strong luminescence. The detector of reflector received higher luminescence while excitation wavelength in the range of 350~400 nm.
The PL and PLE spectra of ZnB2O4:0.1Eu3+ are shown in Fig. 3 . A broad excitation band observed at 250~280 nm can be attributed to the O2--Eu3+ charge transfer (CT) transition. The sharp excitation peaks between 300 and 550 nm are due to the typical f-f transition of Eu3+. The strongest line absorption in the excitation spectrum is located at 393 nm, which is resulting from the 7F0→5L6 transition. The PL spectrum exhibits typically Eu3+ line emission at 578 nm (5D0→7F0), 585, 590, 600 nm (5D0→7F1), 610, 621 nm (5D0→7F2), 651 nm (5D0→7F3) and 689 nm (5D0→7F4). It has been reported that the highly intense line at 5D0→7F1 is due to magnetic dipole transition, while strong emission at 5D0→7F2 is attributed to the electric dipole transition which is observed at 610 nm (5D0→5F2) indicating that Eu3+ ion occupied the site of non-inversion symmetry . The 5D0→7F0 transition at 578 nm is observed in (Zn0.9Eu0.1)B2O4 revealing that Eu3+ occupied a site with Cv, Cnv or Cs symmetry . The inset displays that the PL intensity of Eu3+ excited at 393 nm as a function of Eu3+ concentration. It can be seen that the optimal dopant concentration of Eu3+ is 10 mol %. Below 10 mol%, the PL intensity of the emission peaks were found to increase with increasing Eu3+ content, while above 10 mol%, it decreased with the increasing Eu3+ content. The former observation could be attributed to the distance between Eu3+ ions is too far away that the intensity is proportional to the content of Eu3+. The latter observation is presumably due to the concentration quenching of Eu3+ ions.
Figure 4 shows the PL spectra of ZnB2O4 with different content of Eu3+. The inset in Fig. 4 displayed the intensity ratio of 583 nm and 609 nm. In this study, the dominant emission peak was varied with the content of Eu3+. The ratio of 5D0 – 7F1/5D0 – 7F2 was different with the increase of Eu3+ concentrations. The results indicate that there are more than one Eu sites in the lattice. The excitation and emission wavelengths of Bi3+ observed at 380 nm and 485 nm are corresponded to the 1S0→1P1 and 3P1→1S0 transitions . The emission band of Bi3+ was found to overlap with the excitation bands of Eu3+ ions, especially at 393 nm (7F0→5L6 transition) and 464 nm (7F0→5D2 transition), respectively. It is well known that for effective energy transfer to occur, an overlap of the emission region of the sensitizer, said Bi3+ in this study, and the absorption region of the activator, namely Eu3+ was necessary. We also investigated the emission intensity of (Zn0.9Eu0.1)B2O4 doped with 1, 3, 5, 10, 15 and 20 mol % of Bi3+. Similar to Eu3+ doping in ZnB2O4, an optimal doping concentration of Bi3+ was also found to be 10 mol %. With Eu3+/Bi3+ codoping in ZnB2O4, the optimal composition was determined to be (Zn0.8Bi0.1Eu0.1)B2O4. Thus, it can be concluded that in ZnB2O4, Bi3+ is a sensitizer for the luminescence of Eu3+ and with the Bi3+ codoped in ZnB2O4:Eu3+, the PL intensity and brightness can be significantly improved by an increment of 16% and 52%, respectively. As compared to the red-emitting phosphor La2O2S:Eu3+ the intensity and integrated area of ZnB2O4:Eu3+,Bi3+ were found to be 95% and 104% of those of the commodity, respectively.
3.3 PL spectra and relative emission intensity dependence of temperature effect
For the application of high power LEDs, the thermal stability tests of phosphors are shown in Fig. 5 . In general, the luminescence intensity decreases with increasing temperature. The thermal quenching phenomena observed in phosphors are generally attributed to (a) the degradation of host structure, (b) the oxidation of activators, and (c) the interruption of energy transfer processes between host and activator. Figure 5 displays the PL spectra of (Zn0.8Bi0.1Eu0.1)B2O4 excited at 393 nm with varied temperatures ranging from 25°C to 300°C. The PL spectra of (Zn0.8Bi0.1Eu0.1)B2O4 were found to show little change with increasing temperature except the intensity.
In comparison, the luminescence of La2O2S:Eu3+ commodity was also carried out under the same condition. The inset in Fig. 5 shows the comparison of the temperature-dependent PL intensity or thermal quenching behavior for (Zn0.8Bi0.1Eu0.1)B2O4 and La2O2S:Eu3+. The PL intensity of La2O2S:Eu3+ was firstly observed to increase before reaching its maximum value at 100°C and dramatically decreased with increasing temperature. The thermal behavior of (Zn0.8Bi0.1Eu0.1)B2O4 is a “irreversible” process, meaning that while the temperature returns to room temperature, the PL intensity cannot go back to its original value. The increase of PL intensity at lower temperature was probably due to the red shift of excitation band of La2O2S:Eu3+ at ~395 nm. As for (Zn0.8Bi0.1Eu0.1)B2O4, the inset of Fig. 5 shows a linear decay in PL intensity with increasing temperature, which may result from the interruption of energy transfer processes between host and activator. The study on the thermal quenching of luminescence indicates that ZnB2O4:Eu,Bi can serve as a potential candidate as a red-emitting phosphor in the application high power phosphor-converted LEDs.
A new red-emitting phosphor, ZnB2O4:Eu3+,Bi3+, was synthesized by solid state reactions for the first time. The optimized compositions of ZnB2O4 co-doping Eu3+ and Eu3+/Bi3+ are ZnB2O4:10%Eu and ZnB2O4:10%Eu,10%Bi, respectively. The introduction of Bi3+ ions successfully enhances both emission intensity and quantum efficiency by 14% and 6%, respectively. The present results indicate that the novel red emitting phosphor is a suitable candidate for the application on NUV white LEDs.
The work is financially supported from ITRI under contract no. 8301XS1751, the NSC (contract no. 97-2113-M-002-012-MY3 and 97-3114-M-002-005), and the Economic Affair (contract no. 97-EC-17-A-07-S1-043).
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