Journal of Physics and Chemistry of Solids 72 (2011) 233–235

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Journal of Physics and Chemistry of Solids

0022-36

doi:10.1

n Corr

E-m

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

Green emission from Tb-doped SrSi2O2N2 phosphors under ultravioletlight irradiation

Ran Li, Renli Fu n, Xiufeng Song, Hong He, Xiaodong Yu, Bingbing He, Zhixiang Shi

College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China

a r t i c l e i n f o

Article history:

Received 8 June 2010

Received in revised form

24 November 2010

Accepted 13 January 2011Available online 21 January 2011

Keywords:

D. Luminescence

97/$ - see front matter & 2011 Elsevier Ltd. A

016/j.jpcs.2011.01.009

esponding author. Tel.: +86 25 52112906 845

ail address: renlifu@nuaa.edu.cn (R. Fu).

a b s t r a c t

Tb-doped SrSi2O2N2 phosphors with promising luminescent properties were synthesized by the

conventional solid-state reaction method, characterized by powder X-ray diffraction and studied by

photoluminescence excitation and emission spectra. The synthesized materials exhibited a weak blue

emission and a strong green emission in the region of 400–470 nm and 480–650 nm, which are

attributed to 5D3-7Fj (j¼5, 4, 3) and 5D4-

7Fj (j¼6, 5, 4, 3) transitions of Tb3 +, respectively. The green

emission from 5D4-7F5 at 543 nm showed the highest intensity under the optimized concentration of

0.1 mol, after which the quenching concentration became relevant. The quenching behavior of the

emission of Tb3 + was explained by the cross-relaxation of its excited state.

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1. Introduction

Rare earth (RE) elements have been considered as the mostimportant and promising activators for phosphors due to theirplentiful electronic structures. Since the application of Eu3 +

doped oxide red phosphors such as Y2O3:Eu3 + in fluorescentlamp and color TV from 1960s [1], various RE elements dopedphosphors have been investigated [2–11]. The terbium ion isconsidered as an excellent luminescent center for green phos-phors due to its characteristic green emission originated fromits 5D4-

7F5 electronic transition. Tb3 + doped green phosphors,such as Tb3 +:a-SiAlON [10], Tb3 +:YPO4 [13] and Tb3 +:MgLaLi-Si2O7 [14], have been used as trichromatic fluorescence materials.

As the rare earth doped luminescent host materials, the(oxy)nitrides, such as (Ca,Sr,Ba)2Si5N8 [3], CaSiAlN3 [4], (Ca,Sr,Ba)-Si2O2N2 [5–9] and SiAlON [10], have attracted great interest dueto their excellent thermal and chemical stabilities. Among them,SrSi2O2N2 is a potential host material in these (oxy)nitride basedphosphors due to its lower synthetic temperature. And thephotoluminescent properties of the Eu2 + and/or Ce3 + SrSi2O2N2

had been practically examined [7,12]. To our best knowledge, theluminescent properties of the Tb3 + in SrSi2O2N2 have not beenreported up to now. As Tb3 + coordinates to nitrogen in SrSi2O2N2

by covalent bonds between Tb3 + and (O, N), the spectra of Tb3 + inSrSi2O2N2 may be different from what has been reported else-where. Therefore, it seems to be interesting to investigate theluminescence spectra of Tb3 + in this nitrogen-rich material.

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In this work, Tb3+ doped SrSi2O2N2 phosphors were preparedthrough the solid-state reaction method. Results of the X-raydiffraction (XRD) were explained. Photoluminescence (PL) andphotoluminescence excitation (PLE) spectra of the phosphors acti-vated by Tb3+ were investigated and the decay behavior of the5D4-

7F5 emission of Tb3+ and the cross-relaxation phenomenon atdifferent Tb3+ activator concentrations were also discussed.

2. Experimental procedure

Terbium (Tb3 +) ions doped Sr1�2xTbxLixSi2O2N2 (x¼0.01, 0.02,0.05, 0.1 and 0.2 mol) phosphor samples were prepared using aconventional solid-state reaction method using stoichometricamounts of high purity SrCO3 (99.9% Sinopharm ChemicalReagent Co. Ltd., Shanghai, PR China), Li2CO3 (99.9% SinopharmChemical Reagent Co. Ltd., Shanghai, PR China), SiO2 (99.9%Sinopharm Chemical Reagent Co. Ltd., Shanghai, PR China), a-Si3N4 (99.9% Qingpeng Fine Ceramics Co. Ltd., Wuhan, PR China),Tb4O7 (99.999% Yuelong Rare Earth, Co. Ltd., Shanghai, PR China)as the raw materials. In order to compensate for the chargedefects resulting from divalent Sr2 + ions substituted by trivalentTb3 + ions, Li2CO3 was used to provide Li+ ions.

The starting materials were thoroughly mixed in alcoholmedia with the aid of a planetary ball–mill in a nylon jar withagate balls for 4 h and then dried in an oven (70 1C, 12 h). Allpowder samples were first calcined at 1200 1C for 1 h to com-pletely decompose SrCO3 and Li2CO3 and then the temperaturewas increased up to 1400 1C, where it was kept a constant for 6 h(the heating rate was 10 K/min). The whole process was under areducing atmosphere (N2:H2¼9:1).

R. Li et al. / Journal of Physics and Chemistry of Solids 72 (2011) 233–235234

The crystal structure of the synthesized powders was identi-fied using X-ray diffraction (XRD, Advance D8, Bruker, CuKa,l¼0.15418 nm). PLE and PL spectra of the phosphors weremeasured by a fluorescence spectrophotometer (Cary EclipseEL06043604, VARIAN, USA). The decay curve of samples wasmeasured using the same instrument. All of the measurementswere carried at room temperature.

Fig. 2. Excitation and emission spectra with lem¼543 nm and lex¼232 nm of

Sr0.8Tb0.1Li0.1Si2O2N2.

3. Results and discussion

3.1. Crystalline structure

The crystalline structure of the Sr1�2xTbxLixSi2O2N2 phosphorsamples with different Tb3+ concentrations (x¼0.01, 0.02, 0.05,0.1 and 0.2 mol) are shown in Fig. 1. Most XRD peaks of samplesfit well with those of SrSi2O2N2 (ICSD 17-2877). Traces of theunknown phase(s) (whose peaks were indexed with (n)) were alsoregistered. This agrees well with literatures that have also reportedthat a controversial unknown minor phase always exists whenpreparing SrSi2O2N2 by the solid-state reaction method [7,8,15].The amount of the unknown phase was increased obviously asTb3+ activator concentrations increased up to 0.2 mol. It could beattributed to excess doping ions, which are beyond the solidsolubility of SrSi2O2N2. And therefore, superfluous Tb3+ ions aggre-gate at the boundaries of the base phase leading to the generation ofmore contaminated phase(s).

The crystal structure of SrSi2O2N2 is similar to that of CaSi2O2N2,which are both identified to be a new class of layered materials withlayers of (Si2O2N2)2� consisting of exclusive SiON3� tetrahedrons.The N atom bridges three Si atoms, while the O atom is boundterminally to the Si atom. There are four similar Sr2+ ions sites thatare all surrounded by six oxygen atoms in a distorted trigonalprismatic manner [15]. This constructs the host crystal environmentwith strong crystal field strength for doping activators. The Tb3+

ionic radius (0.0923 nm, 6-coordination number, CN) is closer to Sr2+

radius (0.118 nm, 6-CN) than to the Si4+ radius (0.026 nm, 4-CN). So,under a geometric perspective, Tb3+ should preferably occupy theSr2+-sites rather than the Si4+-sites in the lattice of SrSi2O2N2.

3.2. Luminescence of Sr1�2xTbxLixSi2O2N2

Luminescence properties of the Sr1�2xTbxLixSi2O2N2 (x¼0.01,0.02, 0.05, 0.1 and 0.2 mol) samples were studied under UV

Fig. 1. XRD pattern of powder phosphors of Sr1�2xTbxLixSi2O2N2 (x¼0.01, 0.02,

0.05, 0.1 and 0.2 mol) (* peaks of an unknown phase).

excitation. PLE and PL spectra of Sr0.96Tb0.02Li0.02Si2O2N2 are pre-sented in Fig. 2. The excitation spectra show intense bands at 226and 232 nm. The peaks at 226 and 232 nm should be attributed tothe 4 f8-4 f75d (f–d) transitions of Tb3+, which are caused by thedipolar electric parity allowed transitions [16]. As it can be seen, theemission spectra of Sr0.8Tb0.1Li0.1Si2O2N2 phosphors could be dividedinto two parts: a weak blue emission in the region 400–470 nm anda strong green emission between 480 and 600 nm. The former is dueto the transitions from 5D3 excited states, and the latter is caused bythe transitions from 5D4 excited states of Tb3+ ions. The 5D3-

7Fj

(j¼5, 4, 3) characteristic emissions of Tb3+ activators, at 414, 437and 457 nm, correspond to the transitions of 5D3 to 7F5, 7F4 and 7F3,respectively. While, the emission bands at 489, 543, 586 and 622 nmare ascribed to the transitions of 5D4-

7F6, 5D4-7F5, 5D4-

7F4 and5D4-

7F3, respectively, among which the green-emission transition5D4-

7F5 at 543 nm is stronger for the nature of the dopant Tb3+ ionin the host matrix.

The effect of the Tb3+ concentration on the PL properties of Sr1–

2xTbxLixSi2O2N2 is illustrated in Fig. 3. It is found by comparing theemission intensity of the samples doped with different concentra-tions of Tb3+ that the emission intensities originated from the5D3-

7Fj (j¼5, 4, 3) and 5D4-7Fj (j¼6, 5, 4, 3) transitions show a

completely different tendency. As shown in the inset of Fig. 3, theluminescent intensity of the emission at 414 nm from 5D3 to 7F5

decreases with an increase in Tb3+ concentration, whereas theintensity of the emission at 543 nm from 5D4 to 7F4 increases firstand up to the maximum at 0.1 mol% of Tb3+. Therefore, 0.1 mol%should be considered as the optimized concentration since theemission is highest. As doping content exceeds this critical concen-tration (x¼0.1 mol%), emission intensities at 5D4-

7F5 transitionsdecrease dramatically. This is the so-called concentration quenchingbehavior that has already been studied [17], and could be attributedto the cross-relaxation process during the emission process of Tb3+.When Tb3+ content increases over the critical value, distancebetween Tb3+ ions in the host lattice decreases and results in pairsor clustering of Tb3+ ions, and then concentration quenching takesplace. This process can be described by the formula (1) and the cross-relaxation diagram of Tb3+ as presented by Fig. 4.

Tb3 +(5D3)+Tb3 +(7F6)-Tb3 +(5D4)+Tb3 +(7F0) (1)

The decay behavior of the 5D4-7F5 transition of Tb3 + ions

under excitation at 232 nm in the Sr0.8Tb0.1Li0.1Si2O2N2 host are

Fig. 4. Cross-relaxation diagram of Tb3+ ions.

Fig. 5. Decay curve of the green emission at 543 nm of Sr0.8Tb0.1Li0.1Si2O2N2

phosphor (lex¼232 nm).Fig. 3. Effect of Tb3+ concentration on P properties of Sr1�2xTbxLixSi2O2N2

(x¼0.01, 0.02, 0.05, 0.1 and 0.2 mol); inset of Fig. 3. Effect of Tb3+ concentration

on emission intensities of 5D3-7F5 and 5D4-

7F5 transitions (lex¼232 nm).

R. Li et al. / Journal of Physics and Chemistry of Solids 72 (2011) 233–235 235

monitored by measuring its decay curve, as shown in Fig. 5. Thedecay curve can be well fitted by a single-exponential function asI¼ I0 exp(�t/t). The fluorescence decay lifetime, defined as thetime required for the fluorescence intensity to decay down to 1/eof its initial value, is presented in the equation by t. The lifetimeof the green emission peak at 543 nm obtained by fitting thedecay data is 1.2 ms.

4. Conclusions

Sr1–2xTbxLixSi2O2N2 (x¼0.01, 0.02, 0.05, 0.1 and 0.2 mol) phos-phors have been successfully synthesized by the conventionalsolid-state reaction method. Luminescence properties of the

prepared phosphors were studied in detail. The PL spectra showeda strong green emission between 480 and 600 nm, and theemission transition of 5D4-

7F5 at 543 nm has been more intensefor the nature of the Tb3 + activators in the host matrix. Theluminescent emission intensities are strongly dependent on con-centrations of Tb3 + ions, and the maximum green emissionintensities are obtained at 0.1 mol of Tb3 + ions, which might beconsidered as the quenching concentration for this potentialluminescent material. The concentration quenching studied bythe luminescent emission was confirmed to be a cross-relaxationprocess.

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