Journal of Luminescence 130 (2010) 2223–2225

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Journal of Luminescence

0022-23

doi:10.1

n Corr

E-m

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

Green photoluminescence, but blue afterglow of Tb3+ activated Sr4Al14O25

Su Zhang a,c, Ran Pang a, Chengyu Li a,n, Qiang Su a,b

a State Key Laboratory of Application of Rare Earth Resources, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun 130022, Chinab State Key Laboratory of Optoelectronic Materials and Technology, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, Chinac Graduate School of the Chinese Academy of Sciences, 100039, China

a r t i c l e i n f o

Article history:

Received 24 November 2009

Received in revised form

10 March 2010

Accepted 28 June 2010

Keywords:

Cross relaxation

Afterglow

Photoluminescence

Tb3+

13/$ - see front matter & 2010 Elsevier B.V. A

016/j.jlumin.2010.06.024

esponding author. Tel.: +86 431 85262208; f

ail addresses: cyli@ciac.jl.cn, Ichengyu2001@y

a b s t r a c t

Tb3 + activated Sr4Al14O25 phosphors were synthesized by the high temperature solid-state reaction. For

the sample, the color of the photoluminescence (PL) was green, but that of the afterglow was blue. The

spectral results indicated that the photoluminescence was mainly due to the transitions from 5D4 to the

ground energy levels of Tb3 + and obeyed the cross-relaxation mechanism; however, the afterglow was

derived from the transitions from 5D3 and independent with the concentration of Tb3 +. This difference

was attributed to the reason that the energy transfer process of cross-relaxation was halted by the traps

during the period of afterglow.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Tb3 + has been widely used as green-emitting activator forluminescent materials, and one character of which is the cross-relaxation represented by the quenching of the transitions from5D3 to 7FJ and the predominating of those from 5D4 with theincreasing of the concentration of Tb3 + [1–3]. Moreover someauthors have reported the green afterglow of Tb3 + activatedphosphor [4,5]. However, when we studied the luminescentproperties of Tb3 + activated Sr4Al14O25 phosphor, to our surprise,the color of the photoluminescence (PL) is distinguished greatlyfrom that of the afterglow. For the PL of the samples, the greenluminescence was mainly due to the transitions coming fromthe 5D4 level with the strongest emission at about 542 nm and thecross-relaxation phenomenon was also observed, namely, thequenching of 5D3 with the increasing of the concentration of Tb3 +.However, the afterglow spectra of the samples were mainlycomprised of the 5D3–7FJ (J¼3, 4, 5, 6) transitions of Tb3 + with amaximum intensity at about 380 nm. And the afterglow spectrumwas independent with the concentration of Tb3 +. It means thatthe cross-relaxation is hindered during the afterglow period. Thisphenomenon is very interesting, because it is seldom reportedthat the spectrum of the afterglow and PL of same luminescention e.g. Eu2 +and Tb3 + in the same material exhibits such a bigdifference [4–6].

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ahoo.com (C. Li).

Sr4Al14O25 and its structure were firstly reported by Nadzhinaet al. [7]. It was found that Sr ions have two sites in Sr4Al14O25,which were coordinated by six and eight oxygen atoms. Where-after, the luminescent properties of Eu2 + in Sr4Al14O25 werestudied, in which Eu2 + is believed to occupy the sites of Sr. Thisphosphor emits the blue–green PL emission peaking at about490 nm and the weak emission at about 400 nm, which are bothdue to the d–f transition of Eu2 + [8]. Recently, the afterglowphenomenon was observed in Eu2 + doped, Eu2 +/Dy3 + co-dopedand Eu2 +/Dy3 +/Cr3 + tri-doped Sr4Al14O25 phosphors [9–11].

Although exhibiting the characteristic emission of the sameluminescent ion, there is a great difference between the mechan-ism of the PL and that of the afterglow. For the former, the generalcase is that in the excitation process, the electrons are pumpedfrom the ground level to the excitation levels, and subsequentlyreturn to the ground level to result in the radiative transitions. Forthe realization of afterglow, it consists of three steps: firstly, thematerials store the energy from the excitation light in the form ofcaptured charges (holes or electrons), secondly, the stored energyeffectively transfers to the activator and lastly, the energy is setfree as the radiative transitions of activator rather than non-radiative ones. Of course, for PL, there exists the case that theenergy of the excitation light is transferred from the host or ionsto the activator, but all those processes take place in the period ofexcitation and the luminescence disappears with the removal ofthe excitation source. However, for afterglow, the emission can beobserved for a long time after the excitation source is shut off.Therefore, it is reasonable to consider that the difference betweenthe afterglow and PL of Tb3 + doped Sr4Al14O25 is centered on therespective energy transfer processes.

S. Zhang et al. / Journal of Luminescence 130 (2010) 2223–22252224

We consider that the study of this phenomenon is helpful topromote the understanding of the mechanism of afterglow, whichis unclear till now and, furthermore, provide a new view toexplore electron-trapping material, which can store the energy ofan incident photon ranging from high energetic ray to visible lightand usually used in the field of dosimeter and so on. For example,according to McKeever [12], it is best for the emission wavelengthof thermoluminescence dosimeteric material to be blue to satisfythe maximum sensitivity of photomultiplier tubes adopted indosimeteric instruments, thus that Tb3 + is not a good choice forthe activator of this kind of material for its strong green emission.However, the phenomenon of present case reveals that Tb3 + is anoptional luminescent ion of dosimeteric material with anappropriate structure.

Fig. 2. The afterglow and PL spectra of Sr4Al14O25:0.001Tb3+ phosphor. lex¼254

nm.

2. Experimental

The SrCO3 (A.R.), Al2O3 (A.R.) and Tb4O7 (99.99%) were used asstarting materials, and the adopted concentration of Tb ion is 0.1and 4 mol%. The thoroughly mixed raw materials were pre-firedin alumina crucible at 700 1C for 2 h in an ambient atmosphere,then ground again and sintered at 1400 oC for 8 h in a thermal-carbon reducing atmosphere. The X-ray diffraction patterns weremeasured with a Rigaku D/max-II B X-Ray Diffractometer usingCu Ka (l¼1.5405 A) radiation. For the spectral measurement atroom temperature, a Hitachi F-4500 Fluorescence Spectrofluo-rometer with 150 W Xe lamp was used. For the afterglow, thesamples were firstly irradiated by the 254 nm UV light obtainedvia the excitation channel of the spectrofluorometer for 2 min,and then the excitation channel was shut off during themeasurement. For the low temperature measurements at 10 K,Edinburgh FLS920 spectrometer equipped with 450 W Xe lampwas used, and the sample was fixed in a cryostat and a closedcycle cooler with gas helium coolant was used. The thermo-luminescence (TL) glow curve was measured with a FJ-427A TLmeter (Beijing Nuclear Instrument Factory, China).

3. Results and discussion

Fig. 1 shows the XRD patterns of Sr4Al14O25: 0.04Tb3 +. Theresults show that the co-doping of Tb3 + does not cause anysignificant change in the host structure.

Fig. 1. XRD patterns of Sr4Al14O25:0.04Tb3+.

Fig. 3. The afterglow and PL spectra of Sr4Al14O25:0.04Tb3 + phosphor. lex¼254

nm.

The afterglow and PL spectra with different concentration ofTb3 + are shown in Figs. 2 and 3. It is evident that the PL of boththe samples show the maximum emission at about 542 nm,due to the transition from 5D4 to 7F5 and the weak emissionoriginating from 5D3 of Tb3 +in the range 350–480 nm.When theconcentration of Tb3 +is 0.1 mol%, the intensity of transitionsoriginating from 5D3 has a considerable ratio to those from 5D4,but when the concentration of Tb3 + is 4 mol%, the transitionsfrom 5D3 are obviously quenched due to the well-known cross-relaxation mechanism. However, for the afterglow, the spectra ofboth samples show a maximum emission at about 380 nm, due tothe 5D3–7F6 transition, as well as the weak transitions originatingfrom 5D4 ranging 480–600 nm.

Because the afterglow is closely related to the traps, thethermoluminescence (TL) curve of Sr4Al14O25:0.04Tb3 + wasmeasured and shown in Fig. 4. The TL glow curve consists ofthree peaks at 338, 460 and 600 K, which means that the traps inthe sample are complicated and, unfortunately, only by the TL

Fig. 4. The TL curve of Sr4Al14O25:0.04Tb3+ phosphor. The heating rate is 2 K/s.

Fig. 5. The afterglow and PL (in the inset) spectra of Sr4Al14O25:0.04Tb3+ phosphor

at 10 K.

S. Zhang et al. / Journal of Luminescence 130 (2010) 2223–2225 2225

curve it is difficult to know the details of the traps. The generalmechanism of the afterglow of the sample can be deduced. Duringthe excitation process, the traps store the light energy, and thenthe captured charges transfer to Tb3 + ions after the excitation.In this respect, there are two possible mechanisms for afterglow,one is the captured charges transfer from the traps to the activatorby conduction or valence band, and the other is by the tunnellingbetween the traps and activator. The latter is testified by theobservation of afterglow at low temperature [13,14]. The after-glow and PL spectrums of Sr4Al14O25:0.04Tb3 + were measured at10 K and shown in Fig. 5. Based on the results discussed above, itis clear that the afterglow of the sample is mainly due to thetransitions from 5D3 level; however, the PL chiefly originated fromthe transitions of 5D4, similar with the results at room tempera-ture. Therefore, we consider that the afterglow of Tb3 + activated

Sr4Al14O25 is due to the tunnelling process. But what puzzles us iswhy the cross-relaxation disappears for afterglow.

The cross-relaxation phenomenon of Tb3 + has been observedin many Tb3 +-activated luminescent materials and received lotsof attention. Brandstadter et al. observed the absence of cross-relaxation in Tb3 + activated glasses under the excitation ofcathode rays, and considered that the energy transfer, i.e.,(5D3-

5D4)-(7F6-7F0), was hindered by the electric field caused

by the injected electrons, or by the slow formation of some longlived species which prevented the depopulation of the 5D3 level ofTb3 + [15]. Likewise, we deduced that the traps in Tb3 +-activatedSr4Al14O25 halted the energy transfer of the cross-relaxationduring the afterglow. It is reasonable because the capturedcharges will transfer from the traps to the excitation levels ofTb3 +, according to the mechanism of the afterglow, which meansthat those traps should have an effect on the energy levels of Tb3 +.Moreover for the PL spectrum, because the electrons are directlypumped from the ground to excitation levels by the excitationlight, the obstructive effect of traps on the cross-relaxation isweak. For other Tb3 + activated phosphors, of which the afterglowis similar with the PL [4,5], it may be attributed to the differencebetween the traps and those in Tb3 +-activated Sr4Al14O25.

4. Conclusion

In conclusion, we observed the blue afterglow and greenphotoluminescence phenomena for Tb3 +-activated Sr4Al14O25

phosphor. It was suggested that the blue afterglow was due tothe interference of the traps on the cross-relaxation process. Thestudies were helpful in promoting the understanding of themechanism of afterglow and provided a further view of findingnew electron-trapping materials.

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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