1. Introduction

The pioneering work by Nakamura and associates in solving the p-type doping and growths of high quality GaN materials revolutionized the development of III-Nitride light-emitting and lasers [1], and further development of LEDs has progressed at a rapid pace [2,3]. Recent progress has led to significant improvement in III-Nitride LEDs enabling the practical implementation of this technology serving as pump excitation sources in phosphor-converted white LEDs. Several key advances in InGaN-based LEDs were realized by engineering the active regions with improved radiative recombination rates [4–6], improvement in material quality [7,8], and nano/microphotonic structures for light extraction enhancement [9,10]. The progress in high performance sources in the deep/mid UV spectral regimes [11–14] is also important for UV-excitation phosphor-converted LEDs [15]. Currently, white LEDs are attracting a great deal of attention in terms of their varieties advantages, such as high efficiency, compactness, long operational lifetime, and resultant energy savings, and the commercialization is expanding to high-volume applications, including general lighting and displays [16,17]. In phosphor converted white LEDs, phosphors are one of the important materials in lighting technology and have been widely investigated. Currently, the most dominant phosphor converted white LEDs is by combining a GaN-based LED chip and a yellow-emitting yttrium aluminum garnet YAG: Ce³⁺ phosphor [18]. However, such generated light color is not true because this system lacks of a red emitting component, which restricts their use in more vivid applications [19,20]. In order to obtain higher efficiency white LEDs with appropriate color temperature and higher color-rendering index, a new approach using near-ultraviolet (n-UV) or ultraviolet (UV) LED chips coated with blue/green/red tricolor phosphors was introduced [21–25]. Therefore, there is an urgent need to develop new n-UV/UV excitable phosphors that can be effectively excited in the near ultraviolet range.

Apatite structure contains a large family of inorganic compounds and the oxyapatite structure compounds with a formula A_2+xB_8-x(SiO₄)_6-x(PO₄)_xO₂ have been demonstrated to be very suitable for luminescent host due to their excellent luminescence properties and potential applications in solid-state lighting [26], typically, Yb³⁺ doped Ca₈La₂(PO₄)₆O2, Ce³⁺, Mn²⁺ co-doped Ca₂Gd₈(SiO₄)₆O₂ and Ce³⁺, Mn²⁺, Tb³⁺ co-doped Mg₂Y₈(SiO₄)₆O₂ phosphors [27,28]. Till now, it has been recognized that Ce³⁺ are good activators for phosphors because their efficient luminescence due to the parity allowed 4f–5d transitions. However, the Ce³⁺ doped apatite phosphors and their relevant applications are confining and challenged although they have been widely investigated because the emission location of most Ce³⁺ doped apatite phosphors locates at shorter wavelength and emits either violet or blue light, such as Mg₂Y₈(SiO₄)₆O₂: Ce³⁺ (395 nm) [29], Ca₅(PO₄)₃F: Ce³⁺ (432 nm) [30], Ca₅La₅(SiO₄)₃(PO₄)₃O₂: Ce³⁺ (385 nm) [31], Ca₈La₂(PO₄)₆O₂:Ce³⁺ (415 nm) [32] and Ca₄Y₆(SiO₄)₆O: Ce³⁺ [33], Gd_9.33(SiO₄)₆O₂:Ce³⁺ (392 nm) [34], etc. In contrast, there has been rare report and study on the Ce³⁺-doped apatite phosphor with longer emission wavelength, as a result, information on their applications is mostly unavailable. Consequently, it is significative and desirable to develop new apatite phosphors with long wavelength Ce³⁺ emission.

In addition, as is well known, the position of the lowest 5d band of Ce³⁺ is strongly influenced by host lattice selection and it often varies in color from ultraviolet to yellow, depending on the different types of crystal fields around them [35]. So in principle, it is realizable to obtain long wavelength emission in Ce³⁺ doped apatite phosphor by designing a suitable crystal structure. For the structure of oxyapatite A_2+xB_8-x(SiO₄)_6-x(PO₄)_xO₂ compounds, there are two kinds of cationic sites labeled as M(I) and M(II), the six M(II) sites are preferentially occupied by the seven coordinated trivalent ions with C_s symmetry and next the bivalent ions; while the other four M(I) sites are usually occupied by bivalent ions or the remaining trivalent ions with nine coordination and C₃ symmetry [36]. In our previous work, we have demonstrate that in Ca₅La₅(SiO₄)₃(PO₄)₃O₂ oxyapatite phosphors, when Ce³⁺ occupy trivalent La³⁺ sites with seven coordination, it gives a blue emission and if Ce³⁺ ions enter the bivalent Ca²⁺ with nine or seven coordination, it can exhibit shorter (violet) or longer emission due to different crystal field environment [31]. Based on the above clues, in this work, we designed a new greening emitting phosphor Sr₄La₂Ca₄(PO₄)₆O₂:Ce³⁺ (SLCPO:Ce³⁺) with apatite structure. As expected, strong green emission is observed in it. We mainly report on its synthesis, crystal structure, the photoluminescence (PL) properties and thermal quenching properties as well as its potential application in white LEDs.

2. Experimental

2.1 Materials and synthesis

All the powder samples were synthesized using solid-state reaction. The starting materials were analytical-grade SrCO₃ (A.R. 99.9%), CaCO₃ (A.R. 99.9%), La₂O₃ (A.R. 99.9%), (NH₄)₂HPO₄ (A.R. 99.9%) and CeO₂ (A.R. 99.9%). The stoichiometric raw materials were ground thoroughly in an agate mortar for 1-2 h and subsequently heated to 1273K in a closed alumina crucible in air for 4h. Then the preheated mixture was ground again and fired to1623K for 6h in an alumina crucible under reducing atmosphere of N₂-H₂ (8%) in horizontal tube furnaces.

2.2 Measurements and characterization

The crystal structure of the synthesized samples was identified by using a Rigaku D/Max-2400 X-ray diffractometer (XRD) with Nifiltered Cu Kα radiation. The photoluminescence (PL) and PL excitation (PLE) spectra of the samples were measured by using an FLS-920T fluorescence spectrophotometer equipped with a 450W Xe light source and double excitation monochromators. Thermal quenching was tested using a heating apparatus (TAP-02) in combination with PL equipment. All of the measurements were performed at room temperature.

3. Results and discussion

3.1 XRD Refinement

Fig. 1(a) plots the experimental, calculated, background and difference results of the XRD reference of SLCPO host at room temperature, obtained using Materials Studio (MS) program [37,38]. All the observed peaks satisfy the reflection condition and the final parameters of crystallography and refinement are shown in Table 1 . The results of the final refinement data indicate that the powder sample is crystallized in hexagonal symmetry with space group P63/m and in the apatite structure. The lattice parameters are a = b = 9.54307 Å, c = 7.03085 Å. A series of XRD patterns of Ce³⁺ doped SLCPO phosphors with different doping contents are illustrated in Fig. 1(b). No detectable impurity phase is observed in the obtained samples even at high doping concentration. The XRD profiles are well fitted with the calculated XRD patterns, indicating that the obtained samples are of single phase and the Ce³⁺ ions have been successfully incorporated in the SLCPO host lattice without changing the crystal structure.

 

Fig. 1 (a) The experimental, calculated, background and difference results of the XRD reference of SLCPO host; (b) XRD patterns of SLCPO:Ce³⁺ phosphors with different Ce³⁺ doping contents.

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Table 1. The Final Parameters for Crystallography and Refinement

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3.2 Photoluminescence properties

Figures 2(a) and 2(b) illustrate the PLE and PL spectra of Ce³⁺ doped SLCPO phosphor. When monitored at 506 nm, the PLE spectrum shows a broad band of 260 to 400 nm and has strong intensity around 360-370 nm, matching well with the UV-LED chips. The unresolved broad band is assigned to the transition between the ground-state 4f⁷ and the crystal-field split 4f⁶5d configuration of Ce³⁺. Upon excitation of 352 nm, the SLCPO: Ce³⁺ phosphor exhibits an intense green luminescence and the corresponding emission spectrum consists of a broad emission band due to the electric-dipole-allowed transition from the 5d excited state to the 4f ground state of the Ce³⁺ [39]. In order to further investigated the luminescence properties, a series of Ce³⁺ with different doping content samples SLCPO:xCe³⁺ (0.04≤x≤0.14) are synthesized. As we can see from the inset of Fig. 2(b), with the increase of Ce³⁺ contents, the PL intensity was found to increase gradually until x˃0.08, reaching to the concentration quenching. That is, when the Ce³⁺ ions content increases, more and more Ce³⁺ ions pair or aggregate with others, efficient resonant energy transfer between Ce³⁺ ions and a fraction of migration to distant luminescent killer of quencher occur, leading to the luminescence quenching. With x increasing, the peak positions of the PL spectra were also found to move to the long wavelength, as shown in the inset. This can be explained as following: When La³⁺ ion is substituted and occupied by a smaller Ce³⁺ ion, the distance between Ce³⁺ and O^2- becomes shorter. Since crystal field splitting is proportional to 1/R⁵ [40], this shorter Ce³⁺-O^2- distance also increases the magnitude of crystal field, so that it results in lowering of the 5d band of Ce³⁺, which results in the red shift of the emission wavelengths. We also compared SLCPO: 0.08Ce³⁺ with the commercial green phosphor and the result shows that the integral intensity of SLCPO: 0.08Ce³⁺ is 78.6% of the commercial-green (com-green) phosphor (LMS520B). In addition, the full width at half maximum (FWHM) of SLCPO: 0.08Ce³⁺ is measured to be about 150 nm and the broad emission band starts from blue region and extends to green even yellow region in the spectrum, while that of the commercial green phosphor (LMS520B) is measured to be about 70 nm and the spectrum only covers the green region, centered at 524 nm. This means, to realize the fabrication of white light LEDs lamp, we only need to mix SLCPO: 0.08Ce³⁺ with a red phosphor instead of traditionally mixing three color phosphors. This cannot only effectively decrease the loss due to the reabsorption, but also can simplify the fabrication process greatly.

 

Fig. 2 (a) The PLE spectrum of SLCPO: 0.04Ce³⁺ phosphor monitored at 506 nm. (b) The PL spectra of SLCPO: xCe³⁺ (0.04≤x≤0.14) phosphors at 352 nm excitation; the inset shows the content dependent PL intensity and peak positions of SLCPO:xCe³⁺ phosphors.

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3.3 Thermal quenching properties

The thermal quenching property is one of the important technological parameters for phosphors used in solid-state lighting as it has considerable influence on the light output and color rendering index. The temperature dependent emission spectra for SLCPO: 0.08Ce³⁺ excited at 352 nm were measured and illustrated in Fig. 3(a) . As we can see, both the intensity of the SLCPO: 0.08Ce³⁺ phosphor and the com-green phosphor decrease gradually as the temperature increases and the thermo stability of SLCPO: 0.08Ce³⁺ is found to be inferior compared with the com-green phosphor. When the temperature increases to 150 °C, the PL intensity of SLCPO: 0.08Ce³⁺ dropped to 25% of its initial intensity while that of the com-green phosphor decreases to 60% (Fig. 3(b), upper left). The integral intensity (brightness) also decreases much for both of the two phosphors (Fig. 3(b), bottom right). The observation can be rationalized by the fact that increasing temperature has increased the population of higher vibration levels, the density of phonons and the probability of non-radiative transfer (energy migration to defects). For the application in high power solid-state lighting devices, the thermal quenching property has to be enhanced and this part work is now underway. In addition, a slight blueshift of the emission band is observed as the temperature increases. To account for this observation it should be considered that thermally active phonon-assisted excitation from the lower energy sublevel to the higher energy sublevel in excited states of Ce³⁺ in the configuration coordinate diagram occurs [41]. The temperature dependent Commission International de l'Eclairage (CIE) chromaticity coordinates (x, y) of SLCPO: 0.08Ce³⁺ and the com-green phosphor are also calculated and illustrated in Fig. 3(b). With increasing temperature, the CIE chromaticity coordinates of SLCPO: 0.08Ce³⁺ shift gradually from green region to greenish region and the corresponding CIE coordinates changed from (0.282, 0.392) at 20 °C to (0.249, 0.334) at 200 °C, as shown in Fig. 4 . The change of the CIE chromaticity coordinates is supported by the blueshift of emission spectra with increasing temperature.

 

Fig. 3 (a) The temperature dependent emission spectra for SLCPO:0.08Ce³⁺ under 352 nm excitation. (b) The temperature dependent PL intensity, integral intensity and CIE coordinates of Sr₄La₂Ca₄(PO₄)₆O₂:0.08Ce³⁺compared with the com-green phosphor (LMS520B).

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Fig. 4 The temperature dependent CIE coordinates of SLCPO: Ce³⁺ and the EL spectrum of the WLED lamp fabricated by coating SLCPO: Ce³⁺ phosphor and commercial red phosphor (ZYP630) on a GaN chip (365~370 nm).

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3.4 Electroluminescence (EL) of SLCPO: Ce³⁺ and fabrication of LED lamps

As is known to all, the CRI of WLEDs fabricated with blue InGaN chips and YAG:Ce phosphors is low due to the lack of a red component and it is reported that it could be improved by incorporating some other activators such as Pr³⁺, Tb³⁺ [42], or mixing other phosphors such as Sr₃SiO₅:Eu [43] and CdS:Cu/ZnS quantum dots [44]. The optical properties of various WLEDs are listed in Table 2 . In order to further investigate the potential application of SLCPO: Ce³⁺ in n-UV white LEDs, the electroluminescent spectrum of GaN (365~370 nm) chip with a green emitting SLCPO: Ce³⁺ phosphor and commercial red phosphor (ZYP630) was measured and shown in Fig. 4. The CIE coordinates (x, y), correlated color temperature (CCT) and color rendering index (CRI) of the generated white light were measured to be (0.347, 0.340), 4868 K and 84.6, respectively, and the luminous efficacy is 30.63 lm/W under a forward-bias current of 200 mA at room temperature. From Table 2 we can see, the current obtained data is better than not only those of the WLEDs fabricated by combining a yellow YAG:Ce³⁺ phosphor with a blue InGaN chip but also the improved WLEDs by incorporating Tb³⁺ or Pr³⁺. The inset displays the photographs of the fabricated white LED and the emission color of the white LED lamp. These results demonstrate that SLCPO: Ce³⁺ is a potential green phosphor for applications in white LEDs.

Table 2. Optical Properties of Various WLEDs

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4. Conclusion

In conclusion, a new green emitting phosphor Ce³⁺ doped Sr₄La₂Ca₄(PO₄)₆O₂ was synthesized via a solid state reaction and the crystal structure was investigated by XRD refinement with apatite structure. When excited by UV light, Sr₄La₂Ca₄(PO₄)₆O₂: Ce³⁺ phosphors show intense green light predominates at 506 nm, due to the 5d-4f transition of Ce³⁺. The integral intensity of the optimal Sr₄La₂Ca₄(PO₄)₆O₂: Ce³⁺ phosphor is 78.6% of the commercial green phosphor (LMS520B). Thermal quenching properties were also studied in detail. The white emitting UV LED lamp with mixed phosphors of Sr₄La₂Ca₄(PO₄)₆O₂: Ce³⁺ and the commercial red phosphor (ZYP630) was fabricated and shows excellent CIE (0.347, 0.340), low CCT (4868K) and high CRI (84.6), respectively. These results indicate that the present Sr₄La₂Ca₄(PO₄)₆O₂: Ce³⁺ enriches the emission color of Ce³⁺ doped apatite phosphors and can serve as a good candidate for UV white LEDs.