Luminescence characteristics of Ca4YO(BO3)3 doped with Bi3+, Dy3+ and Pr3+ ions

JOURNAL OF RARE EARTHS, Vol. 28, No. 1, Feb. 2010, p. 37

Luminescence characteristics of Ca4YO(BO3)3 doped with Bi3+, Dy3+ and Pr3+ ions TIAN Lianhua (田莲花) (Department of Physics, Yanbian University, Yanji 133002, China) Received 3 March 2009; revised 10 September 2009

Abstract: The photoluminescence (PL) properties of Ca4YO(BO3)3 doped with Bi3+, Dy3+, and Pr3+ ions were investigated. These compounds were prepared using a typical solid-state reaction. The excitation and emission spectra were measured using a spectrofluorometer. For Ca4YO(BO3)3:Bi3+, the excitation spectrum showed the bands at about 228, 309, and 370 nm which correspond to the 1S0→1P1 transition and the 1S0→3P1 transition of Bi3+ ions. The emission band at 390 nm corresponded to the 3P1→1S0 transition of Bi3+ ions. For Ca4YO(BO3)3: Bi3+,Dy3+, energy transfer occurred from Bi3+ to Dy3+ somewhat. In Ca4YO(BO3)3:Bi3+,Dy3+,Pr3+, the excitation band at 367 nm was enhanced obviously due to the energy migration from Bi3+ to Pr3+, which converted efficiently the emission of semiconductor InGaN based light-emitting diode (LED). Therefore, the emission of Dy3+ ions was enhanced due to the energy migration from the process of Bi3+→Pr3+→Dy3+. It resulted in the good color rendering. Keywords: Ca4YO(BO3)3: Bi3+,Dy3+,Pr3+; photoluminescence; light-emitting diode; energy transfer; rare earths

Recently, white light-emitting diodes (LEDs) are of great importance in lighting applications[1–8]. High-efficiency blue and violet light-emitting indium-gallium nitride (InGaN) diodes have been developed[1,2]. It has been possible to tune the InGaN semiconductor emission to 370 nm, and the AlGaN-based LEDs are also developed to operate at a short wavelength UV region (346 nm)[3,4]. It is desired to develop the new phosphors to convert the UV pump light of the III-N semiconductor LED into the visible wavelength bands. Borates and oxyborates have been extensively investigated as host lattices for luminescent materials because of their stability[8–10]. One of the oxyborates, Ca4RO(BO3)3 (R=Gd, La or Y), is of interest as a host material for phosphor applications. The crystal structure of calcium-rare earth oxyborates with the composition Ca4RO(BO3)3 is well known[11]. The space group is monoclinic noncentrosymmetric Cm, and the rare- earth ion site has a Cs symmetry in a distorted octahedral coordination. Four oxygen ions belong to the borate groups, and the other two are free oxygen ions. The free oxygen ions are not bound to boron and are coordinated in a way similar to that in the simple R2O3 oxides, such as Y2O3. Hence, the luminescence properties of activators such as Eu3+ or Tb3+ in Ca4YO(BO3)3 are similar to those in oxides, not to those in borates[10]. In this work, the other activators such as Bi3+, Dy3+ and Pr3+ codoped in Ca4YO(BO3)3 were investigated. It is possible to obtain white light emission in this compound due to the energy migration from the process of Bi3+→Pr3+→Dy3+ ions.

1 Experimental The powder compounds of Ca4YO(BO3)3:Dy3+,Bi3+,Pr3+

were prepared using a typical solid-state reaction. Stoichiometric amounts of CaCO3 (99.9%, Aldrich), Y2O3 (99.9%, Aldrich), and the activator Dy2O3 (99.9%, Aldrich), Bi2O3 (99.9%, Aldrich) or Pr6O11 (99.9%, Aldrich) were well mixed with 110% of the stoichiometric amount of H3BO3 (99.8%, Aldrich). The mixtures were heated in an alumina crucible at 176 °C, the melting point of boric acid, for 10 min before heating at 1250 °C for 2 h. The PL spectra were measured using a spectrofluorometer (Kontron SFM25), which is composed of a 150 W Xe high-pressure arc lamp, two monochromators (1200 line/mm; f.l.=100 mm, D−1=8 nm/mm), and a photomultiplier tube (PMT Hamamatsu R928).

2 Results and discussion The space group of Ca4YO(BO3)3 belongs to a monoclinic crystallographic system. Y atom is coordinated with 6 oxygen atoms. Two shorter Y–O bonds are formed to the nonborate oxygen atoms O. Thus, the Y atoms are located closer to one edge of the distorted octahedral and the coordination can be considered to be of 2+4 type[10]. Unlike in the borates, in the oxyborates, two shorter Y–O bonds are formed to the oxygen ions that are not bound to boron and are coordinated in a way similar to the oxygen in simple oxides such as Y2O3. The Bi3+ activator, which substitutes for the Y3+ ion, shows a very efficient luminescence in Ca4YO(BO3)3. The excitation spectrum of Ca4YO(BO3)3:Bi3+(1%) is shown in Fig. 1(a). The broad excitation band of Bi3+ appears strongly from 250 to 350 nm with monitoring wavelength at 433 nm. Two very weak excitations appear at 228 and 370 nm. It is speculated that the excitation at 228 nm corresponds to the transition of

Foundation item: Project supported by Scientific Research Foundation for Returned Scholars of Ministry of Education of China (20071108) Corresponding author: TIAN Lianhua (E-mail: [email protected]) DOI: 10.1016/S1002-0721(09)60046-6

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S0→1P1. The excitation bands at 309 and 370 nm are the two components of the 1S0→3P1 excitation band, which are caused by the crystal-field splitting of the 3P1 excited state. The excitation bands are at lower energy than other borates such as MBO3:Bi3+ (M=Sc, In, Lu, Ga)[12]. It is speculated that the molecular (BiO6)9– clusters are embedded in Ca4YO(BO3)3 in a way similar to Y2O3[13]. The Bi3+ ions are coordinated with 6 oxygens. The two of six oxygen ions are not bound to borons. It results in the formation of the molecular (BiO6)9– embedded cluster in Ca4YO(BO3)3:Bi3+. The emission spectrum of Ca4YO(BO3)3:Bi3+(1%) is shown in Fig. 1(b). The strong emission band is observed at 390 nm with a shoulder at about 370 nm, which corresponds to the 3 P1→1S0 transitions. The luminescence intensity is increased with Bi3+ concentration till 1%, and then is decreased due to concentration quenching. The excitation and emission spectra of Ca4YO(BO3)3: Dy3+(1%) are shown in Fig. 2. The excitation bands in the range of 300–400 nm correspond to the f–f transition of Dy3+ ions. The emission spectrum exhibits two peaks at 486 and 582 nm that originate from transition of 4F9/2→6H15/2 and 4 F9/2→ 6H13/2 of Dy3+ ions, respectively. In order to obtain white phosphors, Dy3+ ions are codoped

Fig. 1 Excitation spectrum of Ca4YO(BO3)3:Bi3+(1%) with monitoring wavelength at 433 nm (a) and emission spectrum of Ca4YO(BO3)3:Bi3+(1%) under excitation of 309 nm (b)

Fig. 2 Excitation spectrum of Ca4YO(BO3)3:Dy3+(1%) with monitoring wavelength at 486 nm (a) and emission spectrum of Ca4YO(BO3)3:Dy3+(1%) under excitation of 309 nm (b)

JOURNAL OF RARE EARTHS, Vol. 28, No. 1, Feb. 2010

in Ca4YO(BO3)3:Bi3+. With monitoring either at 433 nm which corresponds to the emission of Bi3+ or at 486 nm which corresponds to the emission of Dy3+, excitation spectra are observed in these compounds similar to Ca4YO(BO3)3: Bi3+(1%) as shown in Fig. 3(a). In contrast to Ca4YO(BO3)3: Dy3+(1%), there are no transitions of 4f lines of Dy3+ ion. The emission spectra are shown in Fig. 3(b). When the samples are excited with a wavelength of 309 nm which is the absorption of Bi3+ ions, there are three emission peaks in the emission spectra of Ca4YO(BO3)3:Bi3+,Dy3+, i.e. 390, 486 and 582 nm, originating from transition of 3P1→1S0 of Bi3+ ions, 4F9/2→6H15/2, and 4F9/2→6H13/2 of Dy3+ ions, respectively. The presence of the strong Bi3+ absorption band at 309 nm for the Dy3+ emission at 486 and 582 nm in Ca4YO(BO3)3:Bi3+,Dy3+ indicates the Bi3+→Dy3+ energy transfer. The Bi3+ ions act as sensitizer for Dy3+ ions in this compound. The decrease of emission intensity of Bi3+ with the increase of Dy3+ concentration also proves the energy transition. The Pr3+ ions are incorporated into Ca4YO(BO3)3:Bi3+, Dy3+ to improve the emission color. Two main excitation bands appear at 309 and 367 nm as shown in Fig. 4. The former

Fig. 3 Excitation spectra of Ca4YO(BO3)3:Bi3+(1%),Dy3+(x) (x=1%, and x=5%) with monitoring wavelength at 433 nm, and with monitoring wavelength at 486 nm (▲) for Ca4YO(BO3)3: Bi3+(1%),Dy3+(5%) (a) and emission spectrum of Ca4YO(BO3)3:Bi3+(1%),Dy3+(x) (x=1%, and x=5%) under excitation of 309 nm (b)

Fig. 4 Excitation spectra of Ca4YO(BO3)3:Bi3+(1%), Dy3+(5%), Pr3+ (1%) with monitoring wavelength at 486 nm (a) and 433 nm (b), respectively

TIAN Lianhua, Luminescence characteristics of Ca4YO(BO3)3 doped with Bi3+, Dy3+ and Pr3+ ions

corresponds to 1S0→3P1 transition of Bi3+, and the latter is an overlap of the 1S0→3P1 transition of Bi3+ and the 4f5d transition of Pr3+. The excitation band at 367 nm is enhanced twice in this compound compared with that in Ca4YO(BO3)3:Bi3+, Dy3+ as shown in Figs. 3(a) and 4(a). It means that Ca4YO(BO3)3:Bi3+,Dy3+,Pr3+ can convert efficiently the 370 nm emission of semiconductor InGaN in LED. However, the intensity of excitation and emission are very weak in Ca4YO(BO3)3:Pr3+. The excitation band at 367 nm is enhanced due to the energy migration of Bi3+→ Pr3+. The ratio of the excitation intensity at 367 and 309 nm is varied with monitoring wavelength as shown in Fig. 4. The I367/I309 is changed from 0.61 to 1.93 with monitoring wavelength 433 nm to 486 nm which are the 3P1→1S0 transition of Bi3+ and the 4F9/2→6H15/2 of Dy3+, respectively. It is speculated that the energy transfer occurs from Pr3+ to Dy3+ according to the excitation spectrum. As shown in Fig. 5(a), the emission of Dy3+ ions (transitions of 4F9/2→6H15/2 and 4F9/2→6H13/2 at 486 and 582 nm respectively) is enhanced, while the emission of Bi3+ ions (transition of 3P0→1S0 at 390 nm) is decreased with excitation of 367 nm compared to that of Ca4YO(BO3)3:Bi3+(1%),Dy3+(x%). However, the emission spectrum is similar to that of Ca4YO(BO3)3:Bi3+ (1%), Dy3+(x%) when excited at 309 nm as shown in Fig. 5(b). It confirms that the energy migration occurs in the process of Bi3+→Pr3+→Dy3+. The emissions of Pr3+ do not appear in Ca4YO(BO3)3:Bi3+(1%),Dy3+(5%),Pr3+(1%) because of nonradiation of Pr3+ to Dy3+. The ratio of 3P0→1S0 and 4 F9/2→6H15/2 transition is almost 1:1 as shown in Fig. 5(a). As a result, it exhibits white color with a chromaticity coordinate (0.23, 0.27).

Fig. 5 Emission spectra of Ca4YO(BO3)3:Bi3+(1%), Dy3+(5%), Pr3+ (1%) with excited wavelength at 367 nm (a) and 309 nm (b), respectively

3 Conclusions The activators Bi3+, Dy3+ and Pr3+ were codoped in the host Ca4YO(BO3)3. For Ca4YO(BO3)3:Bi3+, the excitation bands at about 228, 309 and 370 nm corresponded to the 1 S0→ 1P1 transition and the 1S0→ 3P1 transition of Bi3+ ions, respectively. The emission band at 390 nm corresponds to the 3P1→ 1S0 transition of Bi3+ ions. For Ca4YO(BO3)3:Bi3+,

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Dy3+, with monitoring at 486 nm which corresponded to the 4 F9/2→ 6H15/2 transition of Dy3+ ions, the excitation spectrum was similar to Ca4YO(BO3)3:Bi3+. However, the emission spectrum showed two weak emissions of Dy3+ at 486 and 582 nm which correspond to the 4F9/2→6H15/2 and 4 F9/2→ 6H13/2 transitions of Dy3+ ions, respectively. Energy transfer occurred from Bi3+ to Dy3+ somewhat. But it is not efficient. In Ca4YO(BO3)3:Bi3+,Dy3+,Pr3+, the excitation band at 367 nm with monitoring at 486 nm, was enhanced due to the energy migration from Bi3+ to Pr3+. It resulted in the enhancement of the emission bands of Dy3+ in Ca4YO(BO3)3:Bi3+,Dy3+,Pr3+ compared with that in Ca4YO(BO3)3:Bi3+,Dy3+ due to the energy migration of process Bi3+→Pr3+→Dy3+.

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