1. Introduction

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 [1]. 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. [2] 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. [1] 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 Ca_xSr_1-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 [13]. This phosphor is evaluated for a possible application as a source of white light in solid state lighting devices.

2. Experiments

Ca_xSr_1-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.

Ca_xSr_1-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 Ca_xSr_1-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 $\overline{2}20$ 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 [18] 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.

 

Fig. 1 XRD spectra of Ca_xSr_1-xAl₂O₄:Tb³⁺,Eu³⁺ phosphors with x = 0, 0.3, 0.7, and 1.

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The photoluminescence (PL) excitation and emission spectra were recorded from all Ca_xSr_1-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 Ca_xSr_1-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⁺→O^2- charge transfer transitions resulting from transfer of electrons from O^2- (2p⁶) orbitals to the 4f⁷ and 4f⁶ states of Eu³⁺ respectively. These transitions are assigned according to the literature cited [19,20].

 

Fig. 2 PL excitation spectra of Ca_xSr_1-xAl₂O₄:Tb³⁺,Eu³⁺ phosphor.

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Figure 3 shows the PL emission spectra of Ca_xSr_1-xAl₂O₄: Tb³⁺,Eu³⁺ phosphor recorded when the phosphors were excited at 240 nm. The insets are the PL emission spectra of Ca_xSr_1-x Al₂O₄:Tb³⁺ and Ca_xSr_1-xAl₂O₄:Eu³⁺ phosphors also recorded when the phosphors were excited at 240 nm. As shown in the one inset, the emission spectrum of Ca_xSr_1-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→⁷F_J (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 Ca_xSr_1-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→⁷F_J (J = 1, 3, 4) transitions of Eu³⁺. The PL emission spectrum of Ca_xSr_1-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 Ca_xSr_1-xAl₂O₄:Tb³⁺ in the inset. In the same manner, the Eu³⁺ emission peaks resemble those observed from Ca_xSr_1-xAl₂O₄:Eu³⁺ in the other inset. That is, the PL emission spectra of Ca_xSr_1-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 O^2-→Eu³⁺ charge transfer transitions occurred simultaneously following excitation by photons of sufficiently high energy.

 

Fig. 3 PL emission spectra of Ca_xSr_1-xAl₂O₄:Tb³⁺;Eu³⁺ . The insets are the PL emission spectra of Ca_xSr_1-xAl₂O₄:Tb³⁺ and Ca_xSr_1-xAl₂O₄:Eu³⁺. All the phosphors were excited at 240 nm.

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Fig. 4 PL emission spectra of Ca_xSr_1-xAl₂O₄:Tb³⁺,Eu³⁺. The insets are the PL emission spectra of Ca_xSr_1-xAl₂O₄:Tb³⁺ and Ca_xSr_1-xAl₂O₄:Eu³⁺. All the phosphors were excited at 227 nm.

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Figure 4 shows the PL emission spectra of Ca_xSr_1-x Al₂O₄:Tb³⁺,Eu³⁺ phosphor recorded when the phosphor was excited at 227 nm. The insets are the emission spectra of Ca_xSr_1-x Al₂O₄:Tb³⁺ and Ca_xSr_1-xAl₂O₄:Eu³⁺ (x = 0.3) also recorded when the phosphors were excited at 227 nm. The emission spectrum of the Ca_xSr_1-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 Ca_xSr_1-xAl₂O₄:Tb³⁺ (in the other inset) consists of four major line emission peaks with maxima at 380, 416, 437 ascribed to the ⁵D₃→⁷F_J (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→⁷F_J 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→⁷F_J 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→⁷F_J transitions are preferentially favoured when Tb³⁺ ions occupy Ca²⁺ sites than Sr²⁺ sites. Zhu et al [22] 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 Ca_xSr_1-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 Ca_xSr_1-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 Ca_xSr_1-xAl₂O₄:Tb³⁺ and Ca_xSr_1-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 O^2-→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 Ca_xSr_1-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 O^2-→Eu³⁺ charge transfer are overlapping at this wavelength.

The calculated chromaticity coordinates of the white light emitting (A) Ca_xSr_1-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) [22]. Also shown in the CIE diagram are the coordinates of red (B) Ca_xSr_1-xAl₂O₄:Eu³⁺ and blue (C) Ca_xSr_1-xAl₂O₄:Tb³⁺.

 

Fig. 5 The CIE diagram showing coordinates of white (A) Ca_xSr_1-xAl₂O₄:Tb³⁺,Eu³⁺, red (B) Ca_xSr_1-xAl₂O₄:Eu³⁺ and blue (C) Ca_xSr_1-xAl₂O₄:Tb³⁺ phosphor.

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4. Conclusion

In conclusion, a white light emitting Ca_xSr_x-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 O^2-→Eu³⁺ charge transfer transitions. The f→d and O^2-→Eu³⁺ transitions occur simultaneously after excitation by photons of sufficiently high energy. The current study shows that the Ca_xSr_x-1 Al₂O₄:Tb³⁺,Eu³⁺ phosphor as a good material candidate for white LEDs and solid state lighting applications.