A white light emitting CaxSr1-xAl2O4:Tb3+;Eu3+ phosphor was synthesized by a combustion method using metal nitrates as precursors and urea as a fuel. The X-ray diffraction patterns from the samples showed phases associated with monoclinic structures of CaAl2O4 and SrAl2O4. White photoluminescence with the CIE coordinates (x = 0.343, y = 0.325) was observed when the phosphor was optically-excited at 227 nm using a monochromatized xenon lamp. The white photoluminescence was a result of the combination of blue and green line emissions from Tb3+, and red line emission from Eu3+. The structure and photoluminescence properties of this phosphor are reported.
©2012 Optical Society of America
Today, many researchers are making efforts to develop white light emitting phosphors that can be used in solid state lighting applications such as phosphor lamps and light emitting diodes (LEDs). In traditional white light LEDs, white light is generated by combining a lnGaN-based blue diode with a yellow phosphor such as YAG:Ce3+ or by combining a UV chip with a three converter system of red, green and blue phosphors. However, YAG:Ce3+ has been reported to show high thermal quenching and poor colour rendition. In addition, the blue emission efficiency is often affected by re-absorption by the red or green phosphor in the three converter system . One way of addressing these inefficiencies is to develop a single phosphor that emits white light constituted by simultaneous emission of blue, green, and red light when excited by ultraviolet (UV) radiation. For example, Kim et al.  were able to generate white light from Sr3MgSi2O8:Eu2+,Mn2+ based on broadband emissions with maxima at 470, 570 and 680 nm while Song et al.  reported white light from a tunable Sr3Al2O5Cl2:Ce3+,Eu2+ based on energy transfer from Ce3+ to Eu2+ by a down-conversion process. It has been demonstrated that the development of white LEDs depends on the advances in the visible InGaN-based LEDs [3–8], which are key technologies for pump excitation for white LEDs. The advances in nitride LEDs have also led to high-performance sources emitting in the green spectral regimes [3–5], and several approaches based on novel QWs have also been used to extend the emission wavelengths to yellow/red spectral regimes [6–8]. Specifically, related to the development of LEDs-based pump sources applicable for the current white phosphor discussed here, the importance of having high performance deep UV LEDs [9–12] is of great importance for enabling the proposed white light-emitting phosphor material to be practical. Recently, significant works have been pursued to achieve improved understanding on the physics of high Al-content AlGaN quantum wells [9–12] for achieving high performance deep- and mid-UV LEDs/lasers, which will be applicable as pump excitation source for the white phosphor. In this study, white photoluminescence was observed from CaxSr1-xAl2O4:Tb3+,Eu3+ phosphor that was prepared by the combustion method. The white photoluminescence was a result of the simultaneous emission of blue and green light from Tb3+, and red light from Eu3+ when the phosphor was excited at 227 nm using a monochromatized xenon lamp. This result is unique because the generated white was a combination of blue, green and red narrow line emissions compared to broadband (or a combination of broadband and narrow line) emissions reported from a variety of phosphors used in white LEDs [13–15]. Furthermore, the blue, green and red line emissions occurred simultaneously following excitation by photons of sufficiently high energy rather than by a UV down-conversion process, i.e. capturing of the UV excitation energy by one luminescent centre and a subsequent transfer to the other luminescent centre as reported in ref . This phosphor is evaluated for a possible application as a source of white light in solid state lighting devices.
CaxSr1-xAl2O4:Tb3+,Eu3+ (x = 0, 1, 0.3 and 0.7) phosphors were prepared by a combustion method. In a typical preparation, stoichiometric amounts of metal nitrates: Ca(NO3)2.4H2O, Sr(NO3)2, Al(NO3)3.9H2O, Tb(NO3)3.6H2O and Eu(NO3)3.5H2O; and urea (CO(NH2)2) were mixed and dissolved in distilled water. A homogeneous solution was obtained after stirring vigorously for 20 minutes. The solution was transferred to a muffle furnace pre-heated to and maintained at a temperature of 500 ± 10°C. After all the liquid had evaporated, the reagents decomposed and released large amounts of gases. A large amount of heat released (due to the exothermic nature of this process) resulted in a flame that decomposed the reagents further and released more gases. The flame lasted for ~60 seconds and the combustion process was completed within 5 minutes. The resulting combustion products (powders) were cooled down to room temperature and were ground gently using a pestle and mortar.
CaxSr1-xAl2O4:Tb3+,Eu3+ (x = 0, 1, 0.3 and 0.7) phosphors with different concentrations of Ca2+ and Sr2+ were produced. The concentration of Tb3+ and Eu3+ were fixed at 0.6 and 0.4 mol% respectively in all samples. The powders were characterized without any further post-preparation treatment. The crystalline structure of the phosphor was analyzed using a Bruker D8 Advanced powder diffractometer and the photoluminescence (excitation and emission) data were recorded using a Cary Eclipse fluorescence spectrophotometer.
3. Results and discussions
Figure 1 shows the X –ray diffraction (XRD) patterns of CaxSr1-xAl2O4:Tb3+,Eu3+ (x = 0, 0.3, 1 and 0.7). For x = 0, the patterns are consistent with the standard monoclinic structure of SrAl2O4 referenced in JCPDS file No. 70-134. Except for a marginal increase in the diffraction peak intensities, this structure did not change when x = 0.3. For x = 1, the patterns resembles the standard monoclinic structure of CaAl2O4 referenced in JCPDS file No. 34-379. When x = 0.7, the peak shifted slightly to the left. The shifting can be attributed to the expansion of the CaAl2O4 crystal lattice due to the substitution of Ca2+ by Sr2+ whose ionic radius (0.123 nm) is greater than that of Ca2+ (0.114 nm) [16,17]. The Tb3+ and Eu3+ ions with ionic radii of 0.109 nm and 0.112 nm  respectively had no effect on the crystalline structure of the host due to their relatively lower concentrations. These ions are expected to preferentially occupy the Ca2+ and Sr2+ sites due to similarities in their ionic radii. The average particle size of the phosphors estimated using the Debye-Scherrer equation was ~30 nm.
The photoluminescence (PL) excitation and emission spectra were recorded from all CaxSr1-xAl2O4:Tb3+,Eu3+ powders with x = 0, 0.3, 0.7, and 1; but white light was only observed from the powder with x = 0.3. Therefore we present only the spectra recorded from the powder with x = 0.3. Figure 2 shows the photoluminescence (PL) excitation spectrum of CaxSr1-x Al2O4:Tb3+,Eu3+ recorded when monitoring major emission peaks at 380, 416, 437 and 543 nm, all coming from Tb3+; and at 617 nm coming from Eu3+. The excitations peaking at ~227 nm are assigned to direct excitation of Tb3+through f→d transitions while the excitation peak at 240 nm is assigned to Eu3+→O2- charge transfer transitions resulting from transfer of electrons from O2- (2p6) orbitals to the 4f7 and 4f6 states of Eu3+ respectively. These transitions are assigned according to the literature cited [19,20].
Figure 3 shows the PL emission spectra of CaxSr1-xAl2O4: Tb3+,Eu3+ phosphor recorded when the phosphors were excited at 240 nm. The insets are the PL emission spectra of CaxSr1-x Al2O4:Tb3+ and CaxSr1-xAl2O4:Eu3+ phosphors also recorded when the phosphors were excited at 240 nm. As shown in the one inset, the emission spectrum of CaxSr1-xAl2O4:Tb3+ consists of major green emission at 543 nm ascribed to the 5D4→7F5 transition of Tb3+ and minor emission peaks at 380 (violet), 416 (blue), 437 (blue) ascribed to the 5D3→7FJ (J = 6, 5, 4) transitions of Tb3+ and 484 nm (bluish-green) ascribed to the transitions of 5D4→7F6 transition of Tb3+. In the other inset, the PL emission spectrum of CaxSr1-xAl2O4:Eu3+ consists of major red emission at 617 nm (red) ascribed to the 5D0→7F2 transition of Eu3+ and minor emissions at 590, 653 and 702 nm ascribed to the 5D0→7FJ (J = 1, 3, 4) transitions of Eu3+. The PL emission spectrum of CaxSr1-x Al2O4:Tb3,Eu3+ consists of emission peaks from both Eu3+ and Tb3+ with major emissions at 617 nm coming from Eu3+. Notice that all the Tb3+ peaks in Fig. 4 resemble those observed from CaxSr1-xAl2O4:Tb3+ in the inset. In the same manner, the Eu3+ emission peaks resemble those observed from CaxSr1-xAl2O4:Eu3+ in the other inset. That is, the PL emission spectra of CaxSr1-x Al2O4:Tb3+,Eu3+ is a combination of the spectra shown in the two insets suggesting that there was no energy transfer between Tb3+ and Eu3+. It is therefore most likely that in this particular host the f→d transitions of Tb3+ and O2-→Eu3+ charge transfer transitions occurred simultaneously following excitation by photons of sufficiently high energy.
Figure 4 shows the PL emission spectra of CaxSr1-x Al2O4:Tb3+,Eu3+ phosphor recorded when the phosphor was excited at 227 nm. The insets are the emission spectra of CaxSr1-x Al2O4:Tb3+ and CaxSr1-xAl2O4:Eu3+ (x = 0.3) also recorded when the phosphors were excited at 227 nm. The emission spectrum of the CaxSr1-x Al2O4:Eu3+ in inset of Fig. 4 is the same as that observed when the same sample was excited at 240 nm as shown in the inset of Fig. 3. The emission spectrum of CaxSr1-xAl2O4:Tb3+ (in the other inset) consists of four major line emission peaks with maxima at 380, 416, 437 ascribed to the 5D3→7FJ (J = 6,5,4) transition of Tb3+ and 543 nm ascribed to the 5D4→7F5 transition of Tb3+. Notice that the violet-blue line emissions at 380, 416, 437 nm are more intense than the well known green emission from Tb3+ at 543 nm, which is the opposite of the peak intensities of the same sample in the inset of Fig. 3. The increase in the intensity of the blue emission due to the 5D3→7FJ of Tb3+ (compared to the well known green emission due to 5D4→7F5 transitions) has been reported to be due to suppressed cross relaxation (non-radiative transitions) from the 5D3 to 5D4 state at relatively lower concentrations (<< 1 mol%) of Tb3+ [18,21]. At higher concentrations (>> 1 mol%) of Tb3+, the well known green emission due to 5D4→7F5 has been found to be much more intense than all other Tb3+ emissions. Consistent with these reports, we observed increased blue emission due to the 5D3→7FJ transitions at a relatively lower concentration (0.6 mol%) of Tb3+. In addition, we observed increased blue emission from 0.6 mol% of Tb3+ in CaAl2O4:Tb3+ while the green emission was prominent in SrAl2O4:Tb3+ (PL spectra not shown), suggesting that 5D3→7FJ transitions are preferentially favoured when Tb3+ ions occupy Ca2+ sites than Sr2+ sites. Zhu et al  have demonstrated that Eu2+ can occupy three different sites in SiO2 coated BaMgAl10O17 giving different emission intensities when excited at different wavelengths. It is therefore most likely that in our CaxSr1-xAl2O4:Tb3+,Eu3+ system, the blue emission was mainly due to Tb3+ ions occupying the Ca2+ sites while the green emission was mainly coming from those occupying the Sr2+ sites. On the other hand, the red emission from Eu3+ at 617 nm was consistent and was not dependent on whether or not Eu3+ ions occupy the Ca2+ or Sr2+ sites. In order to balance the blue, green and red emissions, the relative intensities of these emissions were tuned by adjusting the mol ratio of Ca2+ to Sr2+ and the right combination was found to be 0.3:0.7 (i.e. when x = 0.3). The reason why the balance was obtained when x = 0.3 is not known yet but this study is in progress and our findings will be reported in future publications. As shown in Fig. 4, the PL emission spectrum of CaxSr1-x Al2O4:Tb3+,Eu3+ consists of blue and green line emissions from Tb3+ and red line emissions from Eu3+ and their combination constituted white light. Notice that all the Tb3+ and Eu3+ peaks in Fig. 4 resemble the peaks observed when CaxSr1-xAl2O4:Tb3+ and CaxSr1-xAl2O4:Eu3+ were excited at 227 nm as shown in the insets except that the 543 nm peak is marginally more intense than all other peaks. Again, these emissions are most probably a result of simultaneous occurrence of f→d and O2-→Eu3+ charge transfer transitions after excitation by photons of sufficiently high energy and a subsequent emission of blue, green and red photons. After testing different phosphors with different mol ratios of Ca2+:Sr2+ we concluded that there are two important conditions to produce white photoluminescence from our material, namely the mol ratio of Ca2+:Sr2+ and the excitation wavelength. For example, we were only able to produce white light when we excited CaxSr1-x Al2O4:Tb3+,Eu3+ at 227 nm when the mol ratio of Ca2+ to Sr2+ was 0.3:0.7. This is probably due to the fact the f→d direct excitation of Tb3+ and the O2-→Eu3+ charge transfer are overlapping at this wavelength.
The calculated chromaticity coordinates of the white light emitting (A) CaxSr1-x Al2O4:Eu3+ are shown in Fig. 5 . The chromaticity coordinates of the white light are x = 0.343, y = 0.325 and are very close to the chromaticity coordinates of standard white light (x = 0.333, y = 0.333) . Also shown in the CIE diagram are the coordinates of red (B) CaxSr1-xAl2O4:Eu3+ and blue (C) CaxSr1-xAl2O4:Tb3+.
In conclusion, a white light emitting CaxSrx-1Al2O4:Tb3+,Eu3+ phosphor was prepared using the combustion method. The white light generated was a combination of simultaneous blue and green narrow line emission from Tb3+ and the red emission from Eu3+. The production of the white light was shown to depend on the molar ratio of Ca2+ to Sr2+ in the host lattice, as well as the pump excitation wavelength. For example, the white light emission was observed when the sample with 0.3:0.7 mol ratio of Ca2+ to Sr2+ was excited at 227 nm. Our studies demonstrated that the material was excited through f→d transitions of Tb3+ and O2-→Eu3+ charge transfer transitions. The f→d and O2-→Eu3+ transitions occur simultaneously after excitation by photons of sufficiently high energy. The current study shows that the CaxSrx-1 Al2O4:Tb3+,Eu3+ phosphor as a good material candidate for white LEDs and solid state lighting applications.
The authors would like to acknowledge the financial support from the cluster funds of the University of the Free State and the South African National Research Foundation.
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