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Ce3+ and Eu2+ activated Li2SrSiO4 phosphors were prepared using a high temperature solid-state reaction technique. The VUV-UV-vis photoluminescence (PL) of Ce3+-doped phosphors Li2-xSr1-xCexSiO4 and Eu2+-doped phosphors Li2Sr1-xEuxSiO4, the low-voltage cathodoluminescence (CL) of Eu2+-doped phosphor Li2Sr0.991Eu0.009SiO4 and Ce3+-Eu2+ co-doped phosphors Li2-xSr0.991-xEu0.009CexSiO4 were measured and discussed in consideration of their potential application in field emission displays (FEDs). The intense emission of phosphors Li2-xSr0.991-xEu0.009CexSiO4 is tunable in yellow to white color gamut under low-voltage cathode ray excitation.
©2012 Optical Society of America
1. Introduction
Field emission display (FED) is a promising flat panel display technology [1, 2], in which the phosphors are key materials, because they are responsible for converting high current density low-voltage cathode ray to visible emission in FEDs. Traditionally, sulfide-based phosphors usually have high luminance under low current density high-voltage cathode ray excitation and they have been commercial phosphors in cathode-ray tubes (CRTs) or FED [3–6]. However, sulfide-based phosphors are easy to degrade under high current density electron-beam bombardment, and emit sulfide gases leading to cathode poisoning [7]. Though the main problem for oxide-based phosphors is their relative low light output because their high phonon frequency usually causes high nonradiative relaxation energy loss, they are more stable and environmentally friendly in comparison with sulfides [7, 8]. So it is necessary to extensively study low-voltage cathodoluminescence of different oxide-based luminescent materials to get potential phosphors for FEDs [1, 2, 7, 9–12].
Due to the importance for lighting, display and detecting, much attention has been addressed to f-d transitions of Ce3+ and Eu2+ in different host compounds [13–20]. Recently, luminescence of Ce3+ and Eu2+ doped Li2SrSiO4 phosphors has been reported [21–26], but most of these work focused on the LEDs (light-emitting diodes) application. In this paper, we investigate the VUV-UV-vis photoluminescence (PL) and low-voltage cathodoluminescence (CL) properties of Ce3+ and Eu2+ doped Li2SrSiO4 phosphors, showing that intense emission of Li2-xSr0.991-xEu0.009CexSiO4 is tuneable in yellow to white range under low-voltage electron-beam excitation.
2. Experimental
The samples Li2-xSr1-xCexSiO4 and Li2Sr1-xEuxSiO4 were synthesized using a solid-state reaction technique at high temperature using the following reactions.
The starting materials Li2CO3 (analytical reagent, A.R.), SrCO3 (A.R.), SiO2 (A.R.), Eu2O3 (99.99%), and CeO2 (99.99%) were weighed according to the chemical formulas of the target products Li2-xSr1-xCexSiO4 and Li2Sr1-xEuxSiO4 for both non-doped and doped materials and ground thoroughly in an agate mortar. Then the mixtures were heated at 1073 K for 6 hours under a reductive atmosphere (N2/H2 = 3:1). The final products were cooled down to room temperature (RT) and ground again.To check the phase purity, powder X-ray diffraction (PXRD) was performed on a Bruker D8 advance X-ray diffractometer with Cu Kα (λ = 1.5405 Å) radiation at 40 kV and 40 mA. The CL measurements were carried out on a homemade CL measurement system, in which six samples can be put in the vacuum chamber (~2 × 10−3 Pa) simultaneously, where the phosphors were excited by an electron-beam (~0.7 cm2) in the voltage range of 0.5-5 kV and different beam currents (<0.2 mA), and the emission spectra were recorded by a fiber spectrometer (Ocean Optics QEB0388) with a charge-coupled device camera through an optical fiber. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded on an Edinburgh FLS 920 spectrophotometer, and a 450 W xenon lamp was used as the excitation source. The VUV excitation spectra and corresponding emission spectra were measured at the VUV spectroscopy experimental station on beam line 4B8 of Beijing Synchrotron Radiation Facility (BSRF). The measurement details have been described elsewhere [27].
3. Results and discussion
3.1 XRD measurement
The diffractogram of phosphors Li1.991Sr0.991Ce0.009SiO4 (curve a) and Li2Sr0.991Eu0.009SiO4 (curve b) are shown in Fig. 1 as examples. Li2SrSiO4 has a hexagonal crystal structure with the P3121 space group, which is isomorphic with Li2EuSiO4 [28]. The doping of Ce3+ and Eu2+ ions does not cause significant change in the host structure. The XRD patterns of samples Li1.991Sr0.991Ce0.009SiO4 (curve a) and Li2Sr0.991Eu0.009SiO4 (curve b) agree well with JCPDS (Joint Committee on Powder Diffraction Standards) Powder Diffraction File (PDF) 470-120 of Li2EuSiO4, except for the weak diffraction peaks at about 31.3° and 32.7° in curve a. See discussion below, such a small amount of additional peaks in Li1.991Sr0.991Ce0.009SiO4 do not influence the luminescence spectra of Ce3+ in the host.
Fig. 1 The XRD patterns of samples Li1.991Sr1.991Ce0.009SiO4 (a), Li2Sr0.991Eu0.009SiO4 (b) and standard data of Li2EuSiO4 (PDF 470-120).
3.2 VUV-vis spectra of Li2-xSr1-xCexSiO4
VUV-UV excitation spectra of Ce3+ emission at RT are shown in Fig. 2(a, b) . In the excitation range below ~198 nm we found an intense absorption band H with a maximum at about 178 nm, which comes from host-related absorptions of SiO44- group, as other silicates also show host-related absorptions near this range [26, 29]. Another five bands labeled A (~208 nm), B (~222 nm), C (~250 nm), D (~278 nm) and E (~357 nm) are clearly observed in spectral range above 198 nm, which are due to 4f→5d transitions of Ce3+ ions in Li2SrSiO4 host [26]. Therefore, the total crystal field splitting between the highest and lowest band amounts ~20.1 × 103 cm−1, the centroid energy of the five 5d levels is ~39.4 × 103 cm−1.
Fig. 2 The excitation (λem = 420 nm) and emission (λex = 178 nm) spectra of sample Li1.991Sr0.991Ce0.009SiO4. The inset curve d is the dependence of PL intensity on Ce3+ concentration.
The emission spectrum of Li1.991Sr0.991Ce0.009SiO4 at RT is presented in Fig. 2(c). A broad emission can be observed, which is due to transitions from the relaxed lowest 5d excited state of Ce3+ to the 4f ground state levels 2F5/2 and 2F7/2. So the emission spectrum c was fitted with a sum of two Gaussian profiles. The peaks of two Gaussian bands are at about 392 and 422 nm, respectively and the energy difference of which is near the expected value 2000 cm−1. Hence the Stokes shift of the Ce3+ emission is ~2500 cm−1. Figure 2(d) shows the dependence of the integrated Ce3+ emission in 360–500 nm range on its doping concentration (x) in samples Li2-xSr1-xCexSiO4 (x = 0.001, 0.003, 0.005, 0.007, 0.009, 0.01, 0.012) under 357 nm excitation. The luminescence intensity increases first with increasing of Ce3+ concentration, and reaches the highest value at x = 0.009, then the intensity decreases due to concentration quenching.
3.3 VUV-vis spectra of Li2Sr1-xEuxSiO4
The normalized excitation and emission spectra of sample Li2Sr0.991Eu0.009SiO4 in VUV-UV-vis range are given in Fig. 3 . The host-related absorption band H with a maximum at about 175 nm is observed in the VUV-UV excitation spectrum (curve a), which is in accordance with Fig. 2(a). Additionally, three absorption bands A, B and C at about 250, 280 and 310 nm can be seen in this curve, and whose positions are in agreement with those in Ref. [25]. To obtain the excitation spectrum in long-wavelength range, we measured the UV-vis excitation spectrum in curve b. Another evident broad band D with a maximum at about 398 nm and a shoulder band around 475 nm can be observed. These bands are assigned to the parity allowed 4f7(8S7/2)→4f65d transitions of Eu2+ ions.
Fig. 3 The normalized excitation and emission spectra of sample Li2Sr0.991Eu0.009SiO4 . The inset curve e shows the dependence of PL intensity on Eu2+ concentration.
Upon 172 and 310 nm excitation, the normalized emission spectra (curves c and d in Fig. 3) exhibit a similar emission band with a peak at ~564 nm which originates from 4f65d1-4f7 transition of Eu2+ ions. The emission intensity of Eu2+ as function of Eu2+ concentration (x) in samples Li2Sr1-xEuxSiO4 is shown in inset curve e of Fig. 3. It is found that the concentration quenching occurs when Eu2+ concentration is at x = 0.009.
3.4 CL spectra of Li2Sr0.991Eu0.009SiO4
The cathodoluminescence (CL) spectrum of optimum phosphor Li2Sr0.991Eu0.009SiO4 under a low voltage electron-beam (100 μA, 4 kV) excitation was measured at RT and shown in Fig. 4(a) . The emission spectrum exhibits a similar broad band with curves c, d of Fig. 3. However, the emission peak (~575 nm) shifts about 10 nm to long-wavelength side in comparison with those in Fig. 3(c, d) due to different responses of the instrumental setups, because The CL measurement system was not corrected for the fiber spectrometer for the time being. So we think 565 nm may be the true emission peak for the phosphor Li2Sr0.991Eu0.009SiO4. The relevant luminescence photograph with spot size ~0.7 cm2 is displayed in the inset of Fig. 4(a), which show intensive yellow emission.
Fig. 4 CL spectrum (a, 100 μA, 4 kV) and relevant luminescent photograph (inset, spot size ~0.7 cm2) of Li2Sr0.991Eu0.009SiO4 and CL intensity as a function of accelerating voltages (b) and beam currents (c).
Figure 4 also displays the CL intensity of Li2Sr0.991Eu0.009SiO4 phosphor as a function of accelerating voltages (curve b) and beam currents (curve c), respectively. When the beam current is fixed at 100 μA (dot line), with the increase of accelerating voltage from 1 to 4.5 kV, the cathodoluminescence intensity almost linearly increases. The same phenomena occur for the dependence of beam currents. Under 2 kV cathode ray excitation (solid line), the luminescence intensity gradually increases with increasing of beam current from 25 to 225 μA. These properties indicate that the phosphor Li2Sr0.991Eu0.009SiO4 is resistant to the current saturation, which is in favor of FED application.
3.5 CL spectra of Li2-xSr0.991-xEu0.009CexSiO4
The Ce3+ → Eu2+ energy transfer under UV excitation was investigated recently [21, 24, 30–33]. Zhang [21] and He [24] found that co-doping of Ce3+ in Li2SrSiO4: Eu2+ enhances the emission intensity of Eu2+, and they attributed this effect the result of Ce3+ → Eu2+ energy transfer through electric dipole–dipole interaction. At the same time, Kim [22] proposed that the energy transfer from Ce3+ to Eu2+ might not be related to the luminescent improvement, and they suggested that Ce3+ ions could stabilize the Li+ vacancies, inhibit the oxidization of Eu2+ to Eu3+, and consequently increase Eu2+ emission intensity. No matter which reason, it is a fact that adding Ce3+ ions can enhance the Eu2+ emission in Li2SrSiO4 under UV excitation. Can this Eu2+-emitting enhancement effect occur under low-voltage electron-beam excitation? In order to answer this question, we measured the cathodoluminescence spectra of Ce3+–Eu2+ co-doped samples Li2-xSr0.991-xEu0.009CexSiO4 (x = 0, 0.001, 0.003, 0.005).
The emission spectra of Li2-xSr0.991-xEu0.009CexSiO4 under electron-beam excitation (4 kV, 100 μA) are plotted in Fig. 5 . As Ce3+ doping concentration increases, the intensity of Eu2+ emission increases. Clearly, the emission of Eu2+ can be enhanced by adding Ce3+ in this concentration range. In addition, Ce3+ emission is observed in these samples and its intensity gradually increases with the x value. For the two reasons of the Eu2+-emitting enhancement mentioned above, we believe that the nonradiative energy transfer arises from multipolar interaction between Ce3+ and Eu2+ ions may be dominant. This energy transfer process may be explained as follow: under low-voltage electron beams excitation, both Ce3+ and Eu2+ ions can be excited by the plasma produced by the incident electrons and emit visible light. Energy emitting from Ce3+ is absorbed by Eu2+ originated from the spectral overlap of the emission spectrum of Ce3+ (Fig. 2(c)) and excitation spectrum of Eu2+ (Fig. 3(b)). Due to the partial energy transfer from Ce3+ to Eu2+, emission from both Eu2+ and Ce3+ increases with the increase of Ce3+ concentration (x value) in Li2-xSr0.991-xEu0.009CexSiO4. At the same time, we cannot exclude the effect of stabilization of the Li+ vacancies by Ce3+ ions. To clarify this issue, maybe more experimental data are needed.
Fig. 5 The low-voltage CL spectra and luminescence photographs of Li2-xSr0.991-xEu0.009CexSiO4 (excitation voltage = 4 kV, beam current = 100 μA).
In Fig. 5, we can see that phosphors Li2-xSr0.991-xEu0.009CexSiO4 show intense blue emission of Ce3+ (420 nm) and yellow emission of Eu2+ (575 nm) under the electron-beam excitation. So it is possible to obtain white emission by co-doping with Eu2+ and Ce3+ in Li2SrSiO4 in an appropriate ratio. The insets of Fig. 5 exhibit the corresponding luminescence photographs of Li2-xSr0.991-xEu0.009CexSiO4 (x = 0, 0.001, 0.003, 0.005), demonstrating that the phosphors show intense emission under low-voltage electron-beam excitation, and the emission color is tunable in yellow-white color gamut at different Ce3+ doping concentrations. This series of phosphors could find potential application in field emission backlight units for LCDs (liquid display displays) [34].
4. Conclusion
By VUV-UV-vis photoluminescence of Ce3+ and Eu2+ doped Li2SrSiO4 phosphors, the 4f-5d excitation and emission bands of Ce3+ and Eu2+ in Li2SrSiO4 were discerned. Under low voltage electron-beam excitation, Li2Sr0.991Eu0.009SiO4 shows intense yellow-emission and high current saturation, the incorporation of Ce3+ in Li2SrSiO4:Eu2+ can further enhance the luminescence intensity of Eu2+. At the same time, the blue emission of Ce3+ can also be seen with different intensity ratios in Ce3+ and Eu2+ co-doped phosphors Li2-xSr0.991-xEu0.009CexSiO4. As a consequence, the luminescence color is tunable from yellow to white under low-voltage electron-beam excitation, demonstrating the potential application in FEDs and field emission backlights.
Acknowledgments
The work is financially supported by National Natural Science Foundation of China (Grant Nos. 21171176, 10979027 and 20871121).
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