an intense near-ultraviolet converting blue phosphor

PAPER

www.rsc.org/materials | Journal of Materials Chemistry

Ca2PO4Cl : Eu2+: an intense near-ultraviolet converting blue phosphor for white light-emitting diodes Yi-Chen Chiu,a Wei-Ren Liu,*a Chun-Kuei Chang,b Cheng-Chun Liao,c Yao-Tsung Yeh,a Shyue-Ming Janga and Teng-Ming Chen*b Received 2nd October 2009, Accepted 30th November 2009 First published as an Advance Article on the web 14th January 2010 DOI: 10.1039/b920610h A blue phosphor Ca2PO4Cl : Eu2+ was synthesized by solid state reaction and evaluated as a candidate for white LEDs. The luminescent intensity of Ca2PO4Cl : Eu2+ was found to be 128% under excitation at 380 nm, 149% under 400 nm, and 247% under 420 nm, as high as that of BaMgAl10O17 : Eu2+. Furthermore, Ca2PO4Cl : Eu2+ reveals high quantum efficiency and excellent thermal stability. By utilizing a mixture of blue-emitting Ca2PO4Cl : Eu2+, green-emitting (Ba,Sr)2SiO4 : Eu2+ and redemitting CaAlSiN3 : Eu2+ as light converters, an intense white GaN-based n-UV-LED (400 nm) was fabricated to exhibit an excellent color-rendering index Ra of 93.4 at a correlated color temperature of 4590 K. Based on the results, we are currently evaluating the potential application of Ca2PO4Cl : Eu2+ as a blue-emitting near-UV convertible phosphor.

1 Introduction In recent years, there has been growing importance focused on research in light emitting diodes (LEDs) because of their long operation lifetime, energy-saving feature and high material stability.1–3 Hence, white LEDs are promising candidate to replace conventional incandescent and fluorescent lamps. White light can be generated by blue LED chips and yellow phosphor YAG:Ce3+ (YAG).4 However, the disadvantages of this method are low color-rendering index and high color temperature due to the deficiency of red emission in the visible spectrum. During the past few years, white LEDs fabricated using a near ultraviolet (n-UV) LED (380–420 nm) coupled with red, green, and blue phosphors have attracted much attention. The most frequently used blue phosphor for n-UV LEDs is BaMgAl10O17 : Eu2+ (BAM). However, the absorption of BAM for wavelengths above 400 nm is quite poor. Accordingly, it is urgent to develop new blue phosphors that could be effectively excited in the near ultraviolet range especially for wavelengths of 400 nm. Halidecontaining oxide-type hosts are good candidates as host structures due to several merits, such as low synthesis temperature, high chemical and physical stability, which exhibit interesting luminescence properties when activated with Eu2+, such as Ba5SiO4X6 (X ¼ Cl, F),5,6 Ca3SiO4Cl2,7 Sr2LiSiO4F,8 Sr8Si4O12Cl8,9 M2B5O9Cl (M ¼ Ca, Sr, Ba),10–13 Ca2BO3Cl,14 Sr3.5Mg0.5Si3O8Cl4,15 M5(PO4)3Cl (M ¼ Ca, Sr, Ba),16,17 and Ca8Mg(SiO4)4Cl2.18,19 The calcium chlorophosphate Ca2PO4Cl (CAP) was first reported by Greenblatt et al.20 In the 1990s, Blasse et al. reported the photoluminescence (PL) at 4.2 K and thermoluminescence properties of Ca2PO4Cl : Eu2+.21 a Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu, 300, Taiwan. E-mail: [email protected]; [email protected]; Tel: + 886-3-5732438 b Phosphors Research Laboratory and Department of Applied Chemistry, National Chiao Tung University, Hsinchu, 300, Taiwan c Electronics and Optoelectronics Research Laboratories, Industrial Technology Research Institute, Hsinchu, 310, Taiwan

This journal is ª The Royal Society of Chemistry 2010

Nevertheless, little attention in literature has been drawn towards Ca2PO4Cl : Eu2+ in the applications of n-UV LEDs for solid state lighting. In this paper, the luminescence properties, thermal stability and applications in n-UV LED of blue-emitting Ca2PO4Cl : Eu2+ (CAP:Eu2+) phosphors are reported. The present results demonstrate that Ca2PO4Cl : Eu2+ has good thermal stability and white LEDs fabricated based on Ca2PO4Cl : Eu2+ exhibit high color-rendering index and high luminous efficiency. Ca2PO4Cl : Eu2+ is therefore a promising blue-emitting phosphor for n-UV LEDs.

2

Experimental

2.1 Materials and synthesis Polycrystalline phosphors with compositions of (Ca1xEux)2PO4Cl described in this work were prepared by a solid-state reaction. Briefly, the constituent raw materials CaO (99.99%), CaCl2 (99.99%), NH4H2PO4 (99.9%) and Eu2O3 (99.9%) (all from Aldrich Chemicals, Milwaukee, WI, USA) were weighed in stoichiometric proportions (except excess CaCl2 was used) and intimately ground in a glove box, and were then sintered under a reducing atmosphere at 800–900 C for 8 h with one intermittent regrinding to avoid possible incomplete reaction. The obtained product was washed with D.I. water several times to remove excess CaCl2 and dried at 100 C in an oven for further measurements. 2.2 Materials characterization The phase purity of the as-prepared samples was checked by powder X-ray diffraction (XRD) analysis with a Bruker AXS D8 advanced automatic diffractometer with Cu-Ka radiation The measurements of PL and photoluminescence (l ¼ 1.5418 A). excitation (PLE) spectra were performed by using a Spex Fluorolog-3 spectrofluorometer (Instruments S.A., NJ, USA) J. Mater. Chem., 2010, 20, 1755–1758 | 1755

equipped with a 450 W Xe light source and double excitation monochromators. The quantum efficiency (QE) was measured by an integrating sphere whose inner face was coated with Spectralon equipped with a spectrofluorometer (Horiba JobinYvon Fluorolog 3–22 Tau-3). The measurement procedures and correlation theorem were described previously by Liu et al.22

3 Results and discussion 3.1 Crystal structure of orthorhombic Ca2PO4Cl Fig. 1 shows the XRD pattern of Ca2PO4Cl:11%Eu2+ which agrees well with that of JCPDS file No. 72-0010. These results indicate that doping of Eu2+ into CAP does not generate any impurity phase. Greenblatt et al.20 first reported the X-ray single crystal structure of Ca2PO4Cl, which crystallizes in the orthorhombic system with a space group of Pbcm and with four formula units per unit cell. The dimensions of the unit cell are a ¼ b ¼ 6.983 A, and c ¼ 10.816 A. The crystal structure 6.1850 A, consists of discrete and distorted PO43 tetrahedra, which were held together primarily by Ca2+ ions. Two different crystallographic sites are available for the divalent Ca2+ ions, one with site symmetry C2 and the other with site symmetry CS. In both sites the Ca2+ is coordinated by six oxide ions and two chloride ions. and For the larger CS site, the average Ca–O distance is 2.50 A the average Ca–Cl distance is 2.89 A. For the smaller C2 site, and 2.81 A, respectively. Based on the these distances are 2.46 A effective ionic radii (r) of cations with different coordination numbers (CN) reported by Shannon,12 the ionic radius of Eu2+ when CN ¼ 8) is close to that of Ca2+ (r ¼ 1.120 A (r ¼ 1.25 A site is when CN ¼ 8). Since the four-coordinated P5+ (r ¼ 0.17 A) too small for Eu2+ to occupy, the Eu2+ was supposed to occupy the Ca2+ sites due to size considerations. 3.2 Photoluminescence properties of Ca2PO4Cl : Eu2+ Fig. 2 shows the comparison between the PL/PLE spectra of CAP:Eu2+ phosphors and those of the blue-emitting BAM commodity phosphor under the same measurement conditions. The samples presented a blue-emitting band peaking at 454 nm under optimal excitation at 370 nm and the Stokes shift was estimated to be 5 000 cm1. The excitation band was observed to

Fig. 1 XRD patterns of Ca2PO4Cl (JCPDS 72-0010) and Ca2PO4Cl : Eu2+ samples. Inset: the crystal structure of Ca2PO4Cl.

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Fig. 2 The PL and PLE spectra of Ca2PO4Cl:11%Eu2+ compared with those of the commercial phosphor BAM. Inset: PL intensity of (Ca1xEux)2PO4Cl as a function of Eu2+ content.

mainly consist of unresolved band due to the 4f65d1 multiplets of the Eu2+ excited states. The emission spectrum exhibited a broad and asymmetric band in the wavelength range of 400–550 nm, which corresponds to the allowed 4f65d / 4f7 electronic transitions of Eu2+. Owing to the fact that Eu2+ ions occupy two different Ca2+ sites, the emission band was deconvoluted into two individual emission bands centered at 447 nm and 466 nm, respectively. When the Eu2+ ion occupies the lattice sites with C2 or CS symmetry in Ca2PO4Cl, the fivefold degeneracy of the 5d levels is expected in the excitation spectra. Nevertheless, the dominating bands in the PLE spectra were difficult to resolve because of serious overlap between 5d levels. The broad excitation band was ascribed to the high covalency of CaEu–Cl bonding and large crystal-field splitting. The crystal-field splitting of Eu2+ was estimated to be 10 000 cm1 for Ca2PO4Cl. Fig. 2 shows the PL intensity of (Ca1xEux)2PO4Cl as a function of doped Eu2+ content. The optimal doping concentration was observed to be at 11 mol%, the PL intensity was found to decline dramatically when the content of Eu2+ exceeds 11 mol% due to concentration quenching. The PL/PLE spectra exhibited no significant change in the wavelength and band shape with increasing Eu2+ concentration, which indicated that the crystal-field strength experienced by the activator does not change. In other words, doping into the lattice site does not cause the expansion or shrinkage of the unit cell, as revealed by the XRD data. As represented in Fig. 2, the emission wavelength of CAP:Eu2+ (454 nm) is close that (i.e., 453 nm) of BaMgAl10O17 : Eu2+ (BAM). However, the emission intensity of CAP:Eu2+ was found to be 128% under excitation at 380 nm, 149% under 400 nm, and 247% under 420 nm as high as that of BAM and the full width at half maximum (FWHM) of CAP:Eu2+ is about 37 nm, which is much narrower than that for BAM (61 nm). These observations indicate that CAP:Eu2+ phosphor exhibits superior color purity to that of the blue-emitting commodity phosphor. The internal (hi) and external (h0) quantum efficiencies (QEs) were calculated based on the equations reported by Hirosaki et al. previously.23 The internal quantum efficiencies of CAP:Eu2+ and BAM phosphor were found to be 84.8% and 87.2% and the corresponding external quantum efficiencies are 61% and 41%, respectively, at the excitation wavelength of 400 nm. These This journal is ª The Royal Society of Chemistry 2010

results indicate that CAP:Eu2+ may serve as a potential candidate for near-UV light-emitting diodes. 3.3 Thermal quenching properties of Ca2PO4Cl : Eu2+ For the application of high power LEDs, the thermal stability of phosphor is one of important issues to be considered. Temperature dependence of PL spectra for CAP:Eu2+ under excitation at 370 nm is shown in Fig. 3. The activation energy (Ea) can be expressed by I E ln o ¼ ln A a (1) I kT where Io and I are the luminescence intensity of CAP:Eu2+ at room temperature and the testing temperature, respectively; A is a constant; k is the Boltzmann constant (8.617 105 eV K1). Ea was obtained to be 0.0114 eV K1. Fig. 3 inset displays and compares the thermal quenching properties of CAP:Eu2+ and BAM. As shown in Fig. 3, we observed only 5% decay at 100 C for CAP:Eu2+, which reveals that CAP:Eu2+ exhibited thermal quenching as good as that of BAM shown in the inset of Fig. 3. The results also indicate that for high-power LED application, CAP:Eu2+ phosphor could be a promising phosphor. 3.4 Electroluminescence properties of Ca2PO4Cl : Eu2+ As revealed by chromaticity simulations, Ca2PO4Cl : Eu2+, (Ba,Sr)2SiO4 : Eu2+ and CaAlSiN3 : Eu2+ were selected with 400 nm n-UV GaN as the pumping light source for white light LED package demonstration. An emitter-type LED package was chosen for its high light extraction efficiency, the resulting luminous efficiency hence was found to reach as high as 75.3 lm W1 under 20 mA driving current. Its electroluminescence (EL) spectrum is shown in Fig. 4. The CIE color coordinates and correlated color temperature (Tc) of the white LED were found to be (0.363, 0.380) and 4590 K, respectively. The average color-rendering index Ra was determined to be 93.4, which was considered to be excellent for lighting applications.

Fig. 4 EL spectrum of a white-emitting GaN-based n-UV-LED (400 nm) comprising of Ca2PO4Cl : Eu2+ (blue), (Ba,Sr)2SiO4 : Eu2+ (green) and CaAlSiN3 : Eu2+ (red) phosphors driven by a 20 mA current.

The inset of Fig. 4 shows the appearance of a well-packaged three-phosphor-converted-LED lamp in operation. These results demonstrate that in addition to BAM, CAP:Eu2+ is a potential blue phosphor for applications of display and illumination because of its high quantum efficiency and excellent color purity.

4

Conclusions

In summary, an intense blue-emitting CaPO4Cl2 : Eu2+ phosphor has been reported. The phosphor shows a broad absorption band, high quantum efficiency and good thermal stability. The white LEDs fabricated with an n-UV chip, green/red-emitting phosphors and blue-emitting CaPO4Cl2 : Eu2+ generate white light with high color rendering index (Ra ¼ 93.4). The results indicate that CaPO4Cl2 : Eu2+ is a promising blue phosphor for application in n-UV white LEDs.

Acknowledgements This research was financially supported by Industrial Technology Research Institute under contract No. 8301XS1751 and by National Science Council of Taiwan under contract No. NSC98-2113-M-009-005-MY3 (T.-M.C.).

References

Fig. 3 Temperature-dependent PL spectra of CAP phosphor excited at 370 nm. Inset: normalized PL intensity as a function of temperature. For comparison, thermal quenching data of BAM excited at 370 nm were also measured as a reference.


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