Potential red phosphor for UV-white LED device

Potential red phosphor for UV-white LED device

ARTICLE IN PRESS Journal of Luminescence 122–123 (2007) 964–966 www.elsevier.com/locate/jlumin Potential red phosphor for UV-white LED device Taehyu...

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ARTICLE IN PRESS

Journal of Luminescence 122–123 (2007) 964–966 www.elsevier.com/locate/jlumin

Potential red phosphor for UV-white LED device Taehyung Kim, Shinhoo Kang School of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea Available online 13 March 2006

Abstract Potassium-europium double tungstate and molybdate, KEu(WO4)2x(MoO4)x, has been synthesized by solid state reactions. As the molybdate content increased, the intensity of the 5D0-7F2 emission of Eu3+, activated by irradiation at 396 nm, increased and reached a maximum at a mole ratio of molybdenum to tungsten of 3. These changes were accompanied by changes in the shape of the spectra, which is associated with the crystal field splitting of the 5D0-7F2 transition. As the molybdenum content increased, the emission intensity at 615 nm increased and that at 620 nm decreased. This indicates the formation of a new crystal structure, rather than solid solution formation. In this study, the effects of flux, concentration of molybdate, and resulting crystal structure on the PL properties was discussed. r 2006 Elsevier B.V. All rights reserved. Keywords: Phosphor; UV-LED; Tungstate; Molybdate

1. Introduction Alkali rare-earth double tungstates and molybdates (ALn(MO4)2, where A ¼ Na, K; Ln ¼ lanthanide, Y; and M ¼ W, Mo) are known as promising host materials for laser applications [1–4]. Nd-doped potassium–gadoliniumdouble tungstate, Nd:KGd(WO4)2, which emits 1.06 mm light is a well known example. The crystal structures, optical properties and vibration spectra of these compounds have been studied since the 1970s and many systems have been investigated [5–9]. However, europiumbased double tungstates and molybdates have not been well studied compared to other rare-earth-based materials. This is probably because europium-based systems are not as promising as laser materials and their applications are also limited. However, recent interest in solid state lighting has attracted renewed interest in europium-based or -doped systems for use as phosphor materials [10,11]. For white LED applications, the 400 nm UV emission of LED, double tungstates are adequate materials for red phosphor because of their unusual optical properties: they show almost no concentration quenching. This weak concentra-

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E-mail address: [email protected] (S. Kang). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.339

tion quenching property is known to be due to the W–O covalent bond [12]. Do et al. recently reported another interesting property in the case of potassium-europium double tungstate (KEW). The luminescent intensity changed with its composition and showed the maximum value at a nonstoichiometric composition [10], i.e., the optimized composition was reported to be K5Eu2(WO4)5.5 rather than KEu(WO4)2. As an application for phosphor, molybdate systems are also considered to be good choices because it is known that double molybdates and tungstates are crystallized in the same structure in many cases. It has been reported that the luminescent intensity changes as molybdate and tungstate are mixed to form a solid solution [13]. In this paper, we report on the luminescent property changes in relation to changes in crystal structure in a KEW and molybdate (KEM) system. 2. Experimental Potassium carbonate (K2CO3), europium oxide (Eu2O3), molybdenum oxide (MoO3) and tungsten oxide (WO3) were used as raw materials. Stoichiometric amounts of raw materials were weighed and ball-milled in acetone to give K1xEux(WO4)2y(MoO4)y. The samples were dried and

ARTICLE IN PRESS T. Kim, S. Kang / Journal of Luminescence 122–123 (2007) 964–966

3. Results and discussion The crystal structures of KEW and KEM are known to be monoclinic and triclinic, respectively [6,7]. However, XRD patterns of KEW are not known and atomic coordinate information is not currently available for XRD patterns of KEM. Therefore XRD patterns of KGd(WO4)2 were used as reference for KEW, which were in a good agreement with the XRD patterns of the synthesized KEW. In the case of KEM, the XRD patterns of the synthesized powders corresponded to the JCPDS standard card (31-1006), although the crystal structure was not specified. It would be expected that mixed tungstate and molybdate would not form a solid solution, but rather, a mixture of separate phases would be formed because KEW and KEM have different crystal structures. Fig. 1 shows XRD patterns of the prepared powders having various mixed compositions. All samples were heated at 900 1C for 1 h. It can be seen from Fig. 1 that the XRD patterns of KEu(WO4)1.4(MoO4)0.6–KEu(WO4)0.6 (MoO4)1.4 are similar, but different from both KEW and KEM. Therefore, the mixture of tungstate and molybdate appears to give a new compound, rather than a solid solution or a simple mixture. To confirm this conclusion, the same experiments were done at 700 1C and a new pattern was observed to be the main phase only for the case of KEu(WO4)(MoO4). This suggests that the composition of the new compound is KEu(WO4)(MoO4) and that this phase develops gradually as the temperature is increased.

Fig. 1. XRD patterns of KEu(WO4)2x(MoO4)x. (a) x ¼ 0:2, (b) x ¼ 0:6, (c) x ¼ 1:0, (d) x ¼ 1:4, and (e) x ¼ 1:8.

x=0 x=0.6 x=1.0 x=1.4 x=2.0

Intensity (a.u)

heated at 700–900 1C for 1 h. The final powders were obtained after soaking the crucible in warm water. XRD powder patterns were obtained by MAC Science Co. M18XHF using CuKa radiation. Excitation and emission spectra were measured by a JASCO FP-5600 spectrofluorometer. All spectra were recorded at room temperature.

585

965

590

595

600

605 610 615 Wavelength (nm)

620

625

630

Fig. 2. Photoluminescence spectra of KEu(WO4)2x(MoO4)x (x ¼ 0, x ¼ 0:6, x ¼ 1:0, x ¼ 1:4, x ¼ 2:0, lex ¼ 396 nm).

Fig. 2 shows the emission (excited at 396 nm) spectra of KEu(WO4)2x(MoO4)x (x ¼ 0, 0.6, 1, 1.4, 2). Both KEW and KEM show a strong 5D0-7F2 emission and a weak 5 D0-7F1 emission, which is desirable in terms of the color purity for a red phosphor. This high I0-2/I0-1 ratio and the crystal field splitting patterns are consistent with reported results and have been proposed to be due to the Eu site symmetry of KEW and KEM [5]. Secondly, the emission spectrum of the new compound is different from KEW and KEM, which show no crystal field splitting in the 5D0-7F2 transition. These spectral changes are related to changes in the Eu site symmetry in the corresponding crystal structures. Considering the classifications reported by Vliet and Blasse [5], the emission spectra of this compound is similar to RbEuMo2O8 and CsEuMo2O8 which are both known to have D4 Eu site symmetry [14]. However, the XRD pattern is different from that of orthorhombic RbEuMo2O8 and CsEuMo2O8. Therefore, a determination of its crystal structure will be required and further study is going on concerning this compound. From the standpoint of luminescent intensity, KEM showed highest value and the intensity decreased as small amounts of KEW were added. In the case of KEW, the luminescent intensity was also decreased after the addition of KEM, although KEM showed a strong luminescence compared to KEW. However, the luminescent intensity increased as the amount of the other phase increased. This behavior may be related to the XRD patterns shown in Fig. 1. KEW and KEM appear to accommodate about 10 mol% of the other phase within their structure, the luminescent intensity is decreased. When a certain amount of KEM and KEW are mixed, a new compound is formed and the luminescent intensity is increased. As mentioned above, KEW was reported to show a higher luminescent intensity in the case of a potassium-rich non-stoichiometric composition. Therefore, we added a

ARTICLE IN PRESS T. Kim, S. Kang / Journal of Luminescence 122–123 (2007) 964–966

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increased in the case of a potassium- and tungstate-rich composition.

Intensity (a.u)

x=0 x=0.6 x=1.0 x=1.4 x=2.0 x=1.5

4. Conclusions The PL properties and crystal structures of K1xEux(WO4)2y(MoO4)y were investigated. The results are summarized as follows: (1) a new compound with composition around KEu(WO4) (MoO4) was formed after mixing KEW and KEM, and (2) the PL intensity of the new compound was greatly increased by the addition of 10 wt% K2WO4.

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605 610 615 Wavelength (nm)

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Fig. 3. Photoluminescence spectra of KEu(WO4)2x(MoO4)x after the addition of 10 wt% K2WO4 (x ¼ 0, x ¼ 0:6, x ¼ 1:0, x ¼ 1:4, x ¼ 1:5, x ¼ 2:0, lex ¼ 396 nm).

The new compound composed of KEW and KEM has the potential for use in applications as a red phosphor for UV-white LED. We are currently extending this work to identify the precise composition and the crystal structure. References

10 wt% excess of K2WO4 and investigated the changes in the XRD and optical spectra. The XRD pattern was changed negligibly compared to Fig. 1, but the peak intensity of the new compound was greatly increased for KEu(WO4)1.4(MoO4)0.6–KEu(WO4)0.6(MoO4)1.4. The emission spectra were similar to that shown in Fig. 2, but the emission intensity of the new compound was greatly increased and it was almost tripled, while KEW and KEM showed a 100 and 20% increase, respectively. Fig. 3 shows that the sample showing the highest 615 nm intensity is KEu(WO4)0.5(MoO4)1.5. This enhancement appears to be related to the changes in the XRD patterns. However, in our other experiments, the luminescent intensity was not improved, even though a new compound phase was formed if potassium carbonate or chloride was added. Therefore, this improvement in photoluminescence (PL) is thought to be related to the W–O bond’s blocking effect in concentration quenching as in the case of KEW, i.e., the PL intensity

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