In this paper, we present the VUV-vis spectroscopic properties of Na2GdF2PO4:Tb3+ phosphors prepared at 700 °C. The phosphors exhibit some favorable luminescence characteristics such as intensive and broad absorption near 147/172 nm, high bright emission in whole doping concentration, and tunable chromaticity coordinates from blue to whitish and further to yellowish-green range by changing the doping concentration of Tb3+. As a result, this series of phosphors Na2Gd1-xTbxF2PO4 can be considered as promising candidates for plasma display panels (PDPs) and Hg-free fluorescent tubes application.
©2009 Optical Society of America
Phosphors for excitation in the vacuum ultraviolet region, especially the Xe resonance emission line (147 nm) and/or the Xe2 molecular emission band (172 nm), are required for application in plasma display panels (PDPs) and Hg-free fluorescent tubes. Nowadays, (Y,Gd)BO3:Eu3+, Zn2SiO4:Mn2+ (ZSM), and BaMgAl10O17:Eu2+ (BAM) with red/green/blue-emitting are usually used as tricolor phosphors in these devices [1-3]. But there are some shortcomings for each phosphor. In detail for green and blue phosphors, ZSM has a long decay time and BAM exhibits serious degradation under heating and VUV irradiation. In recent years, much work was devoted to decrease the degradation of BAM, such as atomic substitution with Nd3+ and Er3+ for reducing the damage caused by VUV-irradiation [4,5], coating the phosphor surface with MgO [6,7], SiO2  for improving thermal degradation. Another way to solve the degradation problem is to develop a new blue phosphor for PDP application, such as Si3Al10SiO20:Eu2+ , CaMgSi2O6:Eu2+ . It is the feature of these approaches that blue emission is obtained by Eu2+.
In this paper, we prepared a novel blue-emitting VUV phosphor at 700 °C, which is activated by Tb3+ instead of Eu2+. Moreover, when we change the doping concentration of Tb3+ in the samples, the chromaticity coordinates of these Tb3+ doped Na2GdF2PO4 phosphors are tunable from blue to whitish and further to yellowish-green range. By a single host compound and with a sole doping ion, the intensive emission with a large color gamut is fulfilled.
The powder samples were prepared using a high-temperature solid-state reaction technique. The reactants are (NH4)2HPO4 (A.R.), NaF (A.R.), Tb4O7 (99.9%), and Gd2O3 (99.9%). After a good mixing as stoichiometric raw materials, mixtures were pre-heated at 350 °C, and fired at 700 °C in CO atmosphere for several hours.
The structure of the samples was examined by powder X-ray diffraction using Cu Kα radiation (λ= 1.5405 Å) on a Rigaku D/max 2200 vpc X-ray diffractometer.
The luminescence decay curves were measured at an EDINBURGH FLS 920 combined fluorescence lifetime and steady-state spectrometer, which is equipped with a time-correlated single photon counting (TCSPC) card.
The VUV spectra at room temperature were measured at the VUV spectroscopy experimental station on beam line U24 of National Synchrotron Radiation Laboratory (NSRL, Hefei, China). The electron energy of the storage ring is 800 MeV, and the beam current is about 150–250 mA. A Seya-Namioka monochromator (1200 g·mm-1, 100–400 nm) is used for the synchrotron radiation excitation photons, while an Acton-275 monochromator (1200 g·mm-1, 330–700 nm) for the emission photons and the signal is detected by a Hamamatsu H5920-01 photomultiplier (PMT). The system has been corrected for the monochromators but not for the PMT recently. Therefore the systematic error may occur when we evaluate spectral efficiency by intensity integration. However, we think the result can be considered as a reference to evaluate relative luminance of phosphor in the same spectral range. The resolution of the instruments is about 0.2 nm. The pressure in the sample chamber is about 1×10-3 Pa. The relative VUV excitation intensities of the samples are corrected by dividing the measured excitation intensities of the samples with that of sodium salicylate (o-C6H4OHCOONa) in the same excitation conditions.
3. Results and discussion
3. 1. The VUV-UV excitation spectrum and emission spectra under VUV-UV excitation
VUV-UV excitation spectrum and emission spectra for phosphor Na2Gd0.999Tb0.001F2PO4 at RT are exhibited in Fig. 1. Fig. 1(a) shows two broad absorption bands (A and B) peaking at 224 and 174 nm, respectively. The band A should correspond to the lowest spin-allowed (LSA) f-d transition of Tb3+ in the host Na2GdF2PO4, which is near that in NaGdFPO4 . A series of sharp lines around 194, 202, 252, 273, 311 nm in Fig. 1(a) corresponds to the 8S7/2→6GJ, 6DJ, 6IJ, 6PJ transitions of Gd3+ ion, respectively. The occurrence of the intense peaks indicates efficient energy transfer from Gd3+ to Tb3+. That is, 6GJ, 6DJ, 6IJ, 6PJ states of Gd3+ play an intermediate role and can sensitize Tb3+ luminescence.
The absorptions below wavelength 190 nm which include the band B, are mainly the host-related absorption. At the same time, we do not exclude the probable occurrence of f-d transitions of Tb3+ and Gd3+←O2- charge transfer band (CTB) in this spectral range. It is known that the host-related absorption of most phosphates appears in the range of 140~180 nm [12, 13]. The host absorption edge ~190 nm in the present case moves to longer wavelength region than that in GdPO4 (~165 nm) and NaGdFPO4 (~180 nm) [14, 15]. We think that the presence of F- in the crystal may show important influence on this change, more F- ions in crystal lattice of Na2GdF2PO4 than in that of NaGdFPO4 and GdPO4 might incline to increase covalency of P-O bond for PO4 3- groups . Therefore, the host absorption edge of Na2GdF2PO4 moves to long wavelength. The f-d transitions of Tb3+ are complicated in excitation spectrum, which is in relation with two factors. First, when one electron is promoted from the ground states 4f8 (7F6) to 4f75d1 excitation levels, it results in two types of f-d transitions: the spin-allowed 7F6-7DJ transitions (with higher energies and intensities) and the spin-forbidden 7F6-9DJ transitions (with lower energies and intensities); Second, the crystal field will split both 7DJ and 9DJ levels into many sublevels, and each transition from ground state to CFS (crystal field splitting) 7DJ and 9DJ sublevels will probably occur in the excitation spectrum. To elucidate the probable overlapping of Tb3+ 5d states to the host absorption, we can evaluate the highest 5d level of Tb3+ in Na2GdF2PO4 by the magnitude of 5d CFS of Ce3+ (around 17 430 cm-1) in the same host site. If Ce3+ and Tb3+ keep nearly same CFS values in same host lattice, the highest 5d level of Tb3+ in Na2GdF2PO4 can be evaluated to be about 161 nm. For the CTB of Gd3+←O2-, its energy can only be roughly estimated by the Jørgensen empirical formula : E CT = [χ opt(X) - χ opt(M)] × 30 × 103 (cm-1) [χ opt(X) and χ opt(M) are the optical electronegativities of the anion X and central metal cation M, respectively]. Using χ opt(O2-)=3.2 and χ opt(Gd3+)= 0.91 , the CTB energy of Gd3+ in most oxides can be approximately estimated to be 68 700 cm-1 (146 nm).
Upon exciting Tb3+ 5d states at 226 nm, strong blue emission from Tb3+ 5D3 to 7F6,5,4,3,2 transition appears in the emission spectrum [Fig. 1(b)]. In addition, weak emission around 311 nm from Gd3+ 6PJ→8S7/2 transitions appears whose intensity is 4% of total Tb3+ emission, exhibiting weak energy transfer from Tb3+ to Gd3+. With excitation at 172 nm, much stronger emission from Gd3+ 6PJ→8S7/2 transitions appears, whose intensity is 47% of total Tb3+ emission, as shown in Fig. 1(d). This situation is the same for 147 nm excitation [Fig. 1(c)], which shows efficiently energy transfer from host to both Gd3+ and Tb3+ ions.
3.2. The concentration quenching and tunable CIE chromaticity coordinates
The dependence of emission intensities of 5D3 and 5D4 levels on the Tb3+ doping content in Na2Gd1-xTbxF2PO4 upon 147, 172, and 226 nm excitation are shown in Fig. 2, respectively. At relative low Tb3+ content (below about 1.5 mol %), emission intensity of 5D3 is stronger than that of 5D4. 5D3 emission shows maximum intensity around 0.3 mol %, and then rapidly decreases due to cross-relaxation between 5D3, 7F0→5D4, 7F6 states. Above 20 mol % Tb3+ content, the 5D3 emission is so weak as to nearly disappear. At the same time, 5D4 emission intensity fast increases, and reach to luminescence saturation above 25 mol %, this phenomenon is especially obvious for 172 nm excitation. The concentration quenching has not been evidently observed up to 100 mol % of Tb3+ substituted for Gd3+, showing the interaction between Tb3+ ion and host lattice is very weak .
When Tb3+ content increases, blue emission from 5D3 levels decreases and green emission from 5D4 levels increases. Therefore, samples Na2Gd1-xTbxF2PO4 show tunable emission from blue to whitish and further to yellowish-green depending on Tb3+ content, as shown in the CIE chromaticity diagram of Fig. 3 in which the dots indicate the CIE chromaticity coordinate positions.
It is worth pointing out that this series of phosphors shows high efficient luminescence. In order to evaluate the possibility of using the phosphors as potential components in PDPs and Hg-free fluorescent tubes, here we give comparisons of three samples Na2Gd1-xTbxF2PO4 (x = 0.003, 0.01, 0.1) with two commercially available PDP phosphors Zn2SiO4:Mn2+ (ZSM, with green emission) and BaMgAl10O17:Eu2+ (BAM, with blue emission). To compare, we normalized all parameters including slit width, integrated time, beam intensity and relative intensity of energy at excitation wavelength. That is, all spectra are thought to be measured with nearly the same condition. The Spectroscopic curves of five phosphors, Na2Gd1-xTbxF2PO4 (x = 0.003, 0.01, 0.1) and commercial phosphors BAM and ZSM, are shown in Fig. 4(a). Table 1 lists peaking emission wavelength λ peak and relative luminance of emission spectra of those five samples under λ ex= 147/172 nm excitation.
Here, we used the standard visual spectral efficiency (integrated intensities in the whole 380 ~ 730 nm visible range) curves to calculate relative luminance. For sample Na2Gd0.997Tb0.003F2PO4, strong blue emission originates from Tb3+ 5D3 levels and its relative luminance is calculated to be about 42% (for 147 nm excitation) and 80% (for 172 nm excitation) of that commercially available BAM phosphor according to the values in Table 1. On the other hand, a good thermal stability for luminescence is an important requirement for a phosphor in PDPs. For Na2Gd0.997Tb0.003F2PO4 phosphor, its thermal quenching effects under 273 nm excitation are not clearly observed in temperature range from 273 K to 420 K (see Fig. 4(b)), showing that the luminescence exhibits a good thermal stability. In addition, the sample Na2Gd0.997Tb0.003F2PO4 exhibits a shorter 10% decay time which is about 3.2 ms (see Fig. 4(c)) and therefore has no any effect on the quality of rapid-moving pictures of plasma display panels. Moreover, the reprocessing of the expensive Tb3+ from the phosphate is easier than that of Eu2+ from aluminate phosphor (BAM), which can effectively reduce the consumption of rare earths, and low preparation temperature can further save energy.
The Sample Na2Gd0.9Tb0.1F2PO4 shows strong green emission under 147/172 nm. According to the values in Table 1, its relative luminance is estimated to be about 104% (for 147 nm excitation) and 126% (for 172 nm excitation) in comparison with that of commercially available ZSM phosphor, respectively. Furthermore, the 10% decay time of Na2Gd0.9Tb0.1F2PO4 phosphor (~ 7.4 ms, see Fig. 4(c)) is shorter than that of commercial ZSM phosphors.
As for medial content sample Na2Gd0.99Tb0.01F2PO4, its relative luminance is evaluated about 53% of that commercial blue phosphor BAM and 104% of that commercial green phosphor ZSM in the same conditions under 147 nm excitation, and these values are respectively about 85% and 148% for 172 nm excitation. The CIE color coordinate of Na2Gd0.99Tb0.01F2PO4 (0.23, 0.28) is near cold-white light region.
In summary, VUV-vis spectroscopic properties of phosphors Na2GdF2PO4:Tb3+ under 147/172 nm excitation are investigated, the emitted color of Na2GdF2PO4:Tb3+ can be easily tailored from blue to whitish and further to yellowish-green by varying the Tb3+ content, especially for three high efficient phosphors Na2Gd1-xTbxF2PO4 (x = 0.003, 0.01, 0.1) under 147/172 nm excitation: (1) Phosphor Na2Gd0.997Tb0.003F2PO4 exhibits strong blue emission and good thermal stability for luminescence performance. A very low Tb3+ content, easier recovery and a lower preparation temperature tend to decrease the consumption of rare earth resource and save energy. (2) Green-emitting phosphor Na2Gd0.9Tb0.1F2PO4 shows a stronger luminescence and a shorter decay time than the commercially available ZSM phosphor. (3) The emission of phosphor Na2Gd0.99Tb0.01F2PO4 exhibits chromaticity coordinates (0.23, 0.28) in cold-white light region. These results demonstrate that this series of phosphors is expected to be promising candidates for application in PDPs and Hg-free fluorescent tubes.
The work is financially supported by the National Basic Research Program of China (973 Program) (Grant No. 2007CB935502), and by the National Natural Science Foundation of China (Grant Nos. 20571088, 20871121).
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