Journal of Luminescence 131 (2011) 2730–2734

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Journal of Luminescence

0022-23

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jlumin

Energy transfer and excitation wavelength dependent long-lastingphosphorescence in Pr3þ activated Y3Al5O12

Su Zhang a,b, Chengyu Li a,n, Ran Pang a, Lihong Jiang a, Lili Shi a,b, Qiang Su a

a State Key Laboratory of Application of Rare Earth Resources, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,

5625 Renmin Street, Changchun 130022, Chinab Graduate University of the Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e i n f o

Article history:

Received 28 December 2010

Received in revised form

27 June 2011

Accepted 4 July 2011Available online 8 July 2011

Keywords:

YAG

Pr

Long-lasting phosphorescence

Energy transfer

13/$ - see front matter & 2011 Elsevier B.V. A

016/j.jlumin.2011.07.001

esponding author. Tel.: þ86 431 85262208;

ail addresses: cyli@ciac.jl.cn, lchengyu2001@y

a b s t r a c t

Long-lasting phosphorescence (LLP) was observed in Pr3þ-doped Y3Al5O12 (YAG:Pr) after it was excited

by 240 or 290 nm light. The photoluminescence (PL) and LLP properties were studied. It is interesting

that the PL and LLP spectra were different. In the PL emission spectra both the emissions of d–f and f–f

transitions of Pr3þ ions were observed. However, in the LLP spectra of YAG:Pr the emissions of d–f

transition were absent. It is deduced that the differences were due to the energy transfer process

between traps and emission centers. On the other hand, significant differences were observed between

the two LLP spectra after the sample was excited by 240 and 290 nm lights, respectively. The

thermoluminescence (TL) properties were also studied. It is suggested that these studies will be

significant for understanding the mechanism of LLP phenomenon.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Among the rare-earth ions, trivalent praseodymium ion (Pr3þ)presents an intricate energy level scheme, which can emit multi-color lights. And the complex spectral output of Pr3þ makes it agood candidate for various applications. On one hand, enablingred f–f emission, Pr3þ ion doped or co-doped phosphors areconsidered as promising candidates in phosphor based white LEDfor high color-rendering index [1–3]. On the other hand, fast5d1–4f radiative transition can be obtained in some host matriceswith strong crystal field, in which the lowest 5d1 state of Pr3þ isshifted below the 1S0 level. Therefore Pr3þ is considered as apromising activator for tunable ultraviolet (UV) solid-state lasersand scintillation detectors [4–6], such as Pr3þ-doped Y3Al5O12

(YAG:Pr) crystal [7–9]. Although many reports have beenpublished concerning the luminescence and scintillation propertiesof YAG:Pr, the long-lasting phosphorescence (LLP) phenomena areseldom concerned.

LLP phosphor is a kind of material, which can emit light for a longtime in darkness after excitation. Since Eu2þ-doped alkaline earthaluminates were synthesized; rare-earth doped LLP materials haveattracted more and more attentions [10]. However, the most widelystudied and used LLP materials are still Eu2þ doped phosphors. Themechanism of LLP is still indeterminate. And the LLP materials, which

ll rights reserved.

fax: þ86 431 85262005.

ahoo.com (C. Li).

can emit red light are limited. Therefore more studies are needed toimprove both the understanding and variety of LLP materials.

In this paper red LLP is observed in Pr3þ-doped YAG that wassynthesized in reducing atmosphere. The room temperaturepersistent luminescence of the Y3Al5O12:Pr can be seen by nakedeye in complete darkness for about 20 min. Despite the fact thatsome LLP materials were activated by Pr3þ [11–13], YAG:Pr hasits own characteristics: (a) The photoluminescence (PL) emissionspectra and the LLP spectra are different. Both the emissions ofthe 4f5d–4f and f–f transitions are demonstrated in PL spectra;however, the d–f emissions are absent in the LLP spectra. (b) TheLLP spectra of YAG:Pr have significant differences, when it wasirradiated by different wavelength light. The possible reasons areinterpreted in this paper. We hope that this study will be ofbenefit for understanding the mechanism of the LLP and theluminescent properties of Pr3þ doped YAG.

2. Experimental methods

Y3Al5O12 (YAG) and 0.6 mol% Pr3þ doped YAG were prepared.The raw materials were Y2O3 (99.999%), Al2O3(99.999%) andPr6O11(99.999%), which were thoroughly ground and mixed inan agate mortar. The mixtures were placed in alumina crucibles,and then sintered at 1550 1C for 4 h in reducing atmosphere (CO).

PL excitation and emission spectra were measured with a HitachiF-4500 fluorescence spectrofluorometer at room temperature. TheLLP emission spectra were also measured using the same instrument

S. Zhang et al. / Journal of Luminescence 131 (2011) 2730–2734 2731

after the sample was irradiated under 240 and 290 nm light for1 min. Thermoluminescence (TL) spectra were carried out on a TLmeter (FJ-427A1, Beijing Nuclear Instrument Factory). The excitationlight was obtained from the excitation channel of the Hitachi F-4500fluorescence spectrofluorometer.

3. Results and discussion

The space group of Y3Al5O12 is Ia3d with cubic symmetry. Thedoped Pr3þ ion substitutes the dodecahedral sites of Y with D2

point group symmetry. Because of the strong crystal field, thelowest 5d1 state of Pr3þ is shifted below the 1S0 level, and the d–ftransition of Pr3þ is expected. Furthermore, there is only on Y sitein the YAG lattice; therefore, only one d–f transition is antici-pated. Fig. 1 shows the room temperature PL emission spectra ofY3Al5O12: 0.6 mol% Pr3þ . When the sample is excited by 240 and290 nm light, both of the PL spectra are composed by thewell-known Pr3þ emissions: 5d–4f bands located between 300and 460 nm and 4f–4f lines located around 490 and 610 nm[9,14]. However Fig. 1 shows that when excited at 240 nm, therelative intensity between the 380 and 320 nm peak is increased.The increase is due to the intrinsic defect emission in the YAGhost. Many works were reported about the intrinsic defectemissions in YAG lattice (see [15–18] and references therein).Among them, an emission band located at about 380 nm was wellstudied, which was considered in relation to F-type centers.

Pujats et al. reported the F-type centers in the YAG lattice; aluminescence band at 400 nm is observed and ascribed to the Fþ

center, which has two absorption bands at 235 and 370 nm [17].Babin et al. [19] also reported the UV emission in undoped YAG,which is proposed to be raised from the defect. Wong et al. [20]reported a broad emission band located from 300 to 450 nm atroom temperature when excited with UV light, and ascribed thebroad emission band to the defect emission. Additionally, Yanget al. and Springis et al. also reported the emissions from Fþ

center [21,22]. Fig. 1 also shows the PL emission spectrum of YAG.A broad band emission located at about 380 nm was observed.Therefore it is reasonable to postulate that there is an overlappingof defect emission located around 380 nm with the d–f transitionof Pr3þ . The 240 nm light can generate both the defect emissionand the Pr3þ ion emission resulting in the increase of the relativeintensity between the 380 and 320 nm emission.

Fig. 1. Room temperature PL emission spectra of Y3Al5O12: 0.6 mol% Pr3þ excited

at 240 and 290 nm and undoped Y3Al5O12 excited at 240 nm.

Fig. 2 shows the room temperature PL excitation spectra ofPr:YAG monitored at 612 and 380 nm. When monitored at612 nm, the f–f transitions from 3H4 to 3P0, 3P1, 1I6 and 3P2

(E 460 nm) can be seen as lines. And the allowed 4f2 to 4f5dbands are located at 237 and 288 nm [7]. Therefore, whenmonitored at 380 nm, the two excitation bands peaked at about240 and 290 nm are due to the f–d transitions. Obviously, whenmonitored at 380 nm, the ratio of the intensity between the 240and 290 nm peak was increased. It is also because of the existenceof Fþ center in the YAG lattice, which has an absorption band at235 nm [17,22]. The room temperature excitation spectrum ofYAG is shown in the insert of Fig. 2. An excitation peak located atabout 240 nm was observed. It is consistent with our aboveopinion that the 240 nm light can excite the defect resulting inthe stronger 380 nm emission. Thus it is confirmed that the380 nm defect emission is corresponding to the excitation peaklocated at 240 nm. Similarly, Pawlak et al. [23] reported two absorp-tion bands in the region 200–300 nm of YAG:Pr. It is found that thetwo bands are sensitive to thermal treatment and proposed that thereis a chance of an accidental overlap of the defect absorption and thef–d transitions. Additional studies are still necessary to explain theorigin of the bands observed in this spectral region.

Fig. 3 shows the LLP spectra of YAG:Pr. It is interesting that inthe LLP spectra the emission from the transition of 5d–4f at about320 nm were not observed. For the origin of the 320 nm emission,Dorenbos et al. [14] reported that the 316 nm emission bandis due to the transitions from 5d to the 3H4 ground state.The 4f15d1-4f2 (3H4, 3H5, and 3H6) emission of Pr3þ:YAG extendfrom 400 to 312 nm with peaks approximately locating at 318,340 and 369 nm [24]. Additionally, Ganem et al. [25] alsoreported a UV emission band peaking in the vicinity of 314 nm,which is due to the transition from the lowest 5d level to 3HJ. Theenergy level scheme of the Pr3þ ion can be found elsewhere[4,26]; therefore, it is clear that the emission at about 320 nm isdue to the transition of 5d to 3HJ. Here, it is obvious that no peaksappeared in the LLP spectra between 300 and 350 nm, no matter,which excitation wavelength was chosen (as shown in Fig. 3).Therefore, because of the absence of the transition from 5d to 3HJ,it is unlikely that the LLP emission at 380 nm is due to thetransition from 5d to 3FJ. It is more likely that the broad bandemission in the LLP spectra is generated from the intrinsic defectin the YAG lattice. Therefore, it is deduced that the d–f transitionis absent in the LLP spectra of YAG:Pr.

Fig. 2. Room temperature PL excitation spectra of YAG: 0.6 mol% Pr3þ . Insert:

room temperature PL excitation spectrum of undoped YAG monitored at 380 nm.

Fig. 4. TL glow curve of YAG:Pr measured after excited by 240 nm (a) and 290 nm

(b). The excitation light was obtained from the excitation channel of the

spectrophotometer.

Fig. 3. LLP spectra of YAG:Pr measured after the sample was excited at 240 and

290 nm for 1 min. Insert: LLP spectrum of undoped YAG measured after excited by

240 nm light for 1 min.

S. Zhang et al. / Journal of Luminescence 131 (2011) 2730–27342732

On the other hand, it is important to notice that the peakshapes in the LLP spectra were very different after the sample wasexcited by different wavelength lights. The LLP spectrum ispredominated by f–f emission of Pr3þ , after the sample wasexcited by a 290 nm light. However, the LLP spectrum shows astronger broad band emission located between 300 and 460 nm,when it is excited at 240 nm.

The absence of the emission of the d–f transition in the LLPspectra indicates that there are different luminescent processesbetween PL and LLP emission. For the PL, the general case is thatduring irradiation the electrons are pumped from the groundlevel to the excitation level and subsequently return to theground level to result in the radiative transitions. However, forthe LLP, it is a more trap-related process. A generally acceptedview is that LLP consists of three steps: firstly, the traps store theenergy of excitation light in the form of captured carrier (hole orelectron), secondly, the stored energy effectively transfers to theactivator and lastly, the energy is set free as the radiativetransitions of activator [10,27].

For the second step, there are two common possible ways forthe energy to transfer from trap to emission center [27,28]. First,the energy transfer occurs by means of the detrapping and therecombination of the carriers through valence band (for hole trap)or conduction band (for electron trap), and secondly thermal-assisted tunneling may occur. According to the photoconductivitystudy in YAG:Pr, Ref. [7] reported that the 4fn�15d level of Pr3þ

lies in the conduction band of the host. The photoconductivity canbe influenced by subgap processes [29]. Ref. [30] reported that atabove room temperature there is no thermal ionization of 5d1level of Pr3þ in YAG host. Thus subgap processes, namely thetunneling between the trap and Pr3þ may occur. Therefore, it isprobable that the captured carrier can only be efficiently trans-ferred to the 4f level of Pr3þ , but not to the 5d level, which resultsin the absence of d–f emission in the LLP spectra.

Moreover, for the difference between the two LLP spectra, thepossible way is that when the sample was irradiated by differentwavelength lights, the emission center was selectively excited.Namely, the 290 nm light cannot excite the defect center. Ref. [31]reported the PL characteristics of Pr3þ . The 5d position of Pr3þ

has been discussed in Refs. [30] and [32]. Because the funda-mental absorption edge of Y3Al5O12 begins at E52,000 cm�1

[33], using the 240 or 290 nm UV light cannot produce directband to band excitation. Hence, when Pr:YAG was excited, the

excited electrons were generated from Pr3þ ions or other loca-lized level, such as defects. Furthermore, the intrinsic defect canbe excited by the 240 nm light rather than 290 nm. Therefore, if itis irradiated by 290 nm light, the LLP spectrum is predominatedby the f–f emission of Pr3þ (as shown in Fig. 3). In addition, theinsert of Fig. 3 shows the LLP spectrum of undoped YAG after itwas excited by 240 nm light. A broad band LLP emission peakedat about 380 nm was predominated. The YAG sample cannot raisethe LLP after excited by 290 nm light.

Fig. 4 shows the TL glow curves of Pr3þ-doped YAG measuredafter it was excited by the 240 and 290 nm lights. It can be seenthat the shapes of the two curves are similar. Both of the curveshave two broad glow peaks. And the peak located at lowtemperature (called peak1) is stronger than the peak located athigh temperature (called peak2). According to our previousresearch, the depth of traps corresponding to TL peaks rangingfrom room temperature to 120 1C is suitable for the appearance ofLLP [34]. The TL results indicated that the carrier may leave itstrap through a temperature-stimulated tunnel process [28]. If it isthe only possibility or if there is other possible ways to illustratethe phenomena, more experimental means are needed. In addi-tion Fig. 4(a) shows that a peak at about 100 1C appeared. Itmeans that when excited by different wavelength light, differenttraps may be involved and the persistent luminescence may occurby the recombination of the carries originated from differenttraps, which also need further studies.

S. Zhang et al. / Journal of Luminescence 131 (2011) 2730–2734 2733

In this article, to intuitively describe our suggestions, a schematicrepresentation of the band structure of YAG:Pr are proposed in Fig. 5.The energy levels of Pr3þ in YAG are presented according to theresults of Ref. [25]. However, the exact positions of the levels in theforbidden gap need to be further studied. And the energy levels ofthe traps and the defects in Fig. 5 are postulated. Furthermore,abundant papers, which focus on the defects in garnet are helpful[15–18,35,36], it is different to confirm the defect structures of thetrap and the defect emission center, though. However, it is reasonableto consider that the trap is related to the oxygen vacancy [37,38] andthe defect emission center is originated from the color centers[19–22]. These ambiguities should be removed by further analysis

Fig. 5. Possible process for the origin of the LLP in YAG:Pr3þ . The positions of the

levels in the forbidden gap are postulated and need to be further studied.

Fig. 6. Luminescence decay kinetics of 488 nm (solid line) and 380 nm (broken

line) emission at room temperature measured after the sample was excited by

290 nm (left) and 240 nm (right) UV light.

of the spectra of these materials. Fig. 5 shows that the stored energycannot transfer to the 4f5d(1) level of Pr3þ , therefore in the LLPspectra the emissions of d–f transitions are absent.

The phosphorescence intensity decay monitored at wave-length of 488 and 380 nm were recorded after the sample wasexcited by the 240 and 290 nm UV light and shown in Fig. 6. It isshown that when excited by 240 nm light, the intensity of theemission at 380 nm is always stronger than that of the emissionat 488 nm. When excited by 290 nm light the situation isreversed. This is coincident with the LLP spectra shown in Fig. 3.

4. Conclusions

There are two emission centers in the YAG:Pr lattice, the Pr3þ ionand defect emission center. When the YAG:Pr sample was excitedselectively by different irradiation wavelength, the LLP spectra arevery different. The result means that the two emission centers in theYAG:Pr lattice are relatively independent. It gives us an inspirationthat various LLP colors might be obtained in only one material byselectively exciting different emission centers. For the mechanism ofLLP in Pr3þ-doped YAG, the absence of the d–f transition in LLPspectrum indicates that the energy captured by the traps cannot beefficiently transferred to the 5d level of Pr3þ ions. Thermal assistedtunnel process was a possibility. We hope this finding will be ofbenefit for understanding the mechanism of LLP and the luminescentproperties of the intrinsic defects and Pr3þ ion in YAG.

Acknowledgment

The work was financially supported by National Basic ResearchProgram of China (2007CB935502) and National Natural ScienceFoundation of China (Grant no. 20921002).

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