Fabrication and luminescence properties of Dy 3+ doped CaMoO 4 powders

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Materials Chemistry and Physics 126 (2011) 391–397

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

abrication and luminescence properties of Dy3+ doped CaMoO4 powders

aojiang Gao ∗, Yue Li, Xin Lai ∗, Yanyan Wei, Jian Bi, Yang Li, Mengjiao Liuollege of Chemistry and Materials Science, Sichuan Normal University, No. 5 Jing’an Road, Jinjiang District, Chengdu 610066, China

r t i c l e i n f o

rticle history:eceived 1 February 2010eceived in revised form 14 October 2010ccepted 21 October 2010

a b s t r a c t

Dy3+ doped CaMoO4 powders (CaMoO4:Dyx3+) have been fabricated via solid-state ceramic method;

the microstructures and room temperature photoluminescence properties of the as-fabricated micro-crystallines were investigated by through X-ray diffraction (XRD), scanning electron micrograph (SEM),Fourier transform infrared spectroscopy (FT-IR) and fluorescence analysis (FA); and the effect of the dop-

3+ 3+

eywords:ptical materialshemical synthesisicrostructure

uminescence

ing of Dy ions on the microstructures and luminescence properties of the CaMoO4:Dyx phosphors werealso investigated. Our results reveal that the obtained CaMoO4:Dyx

3+ phosphor powders are single-phasescheelite structure with tetragonal symmetry. The doping of Dy3+ ion inhibits the grain growth, decreasesthe intrinsic emission of MoO4

2− complex ions. With regard to CaMoO4:Dyx3+ phosphors, a bright fluores-

cent yellow emission at 574 nm (4F9/2 → 6H13/2) and blue emission at 487 nm (4F9/2 → 6H15/2) have beenobserved. The decay time of the two emission spectra (487 nm and 574 nm) of Dy3+ ion both decrease

centr

with increasing Dy3+ con

. Introduction

Metal tungstates and molybdates are two important families ofnorganic materials that have great potential applications in variouselds, such as phosphors, optical fibers, scintillators, magnets andatalysts [1–3]. Recently, various technologies were used for thereparation of these powders and films, including co-precipitation,icrowave-hydrothermal, microwave irradiation, spray pyrolysis,

lectrochemical technique, galvanic cell method and polymericrecursor method (PPM) [4–17]. CaMoO4 is an important membermong metal molybdate families that have potential applications inarious fields, such as in photoluminescence [18], microwave appli-ations [19], white light emitting diodes [20] and laser materials21]. For CaMoO4 phosphor, green emission appears under UV-ight excitation (250–310 nm), but the orange emission at 580 nm isbservable only if the excitation wavelength is longer than 320 nm22,23].

As known, most rare-earth elements are not only filled with thenner layer of 4f electron orbital, but also have different energyevels owing to different arrangement of the 4f electrons. Whenf electrons occur the transition among different energy levels,hey can produce a large number of absorption and fluorescence

pectra. So most rare-earth elements are often doped into manyight-emitting and laser materials to synthesis nature and non-toichiometric compounds used as excellent fluorescent materials,aser materials and electric light materials [24–26].

∗ Corresponding authors. Tel.: +86 28 84781772; fax: +86 28 84767868.E-mail addresses: [email protected] (D. Gao), [email protected] (X. Lai).

254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2010.10.053

ation.© 2010 Elsevier B.V. All rights reserved.

Oxide and fluoride lattices activated with Dy3+ present intenseluminescence bands in the blue and yellow spectral regions, whichmake them attractive for applications in the phosphor and solid-state laser technologies [27,28]. The device potentialities of thesematerials strongly depend on efficiencies of two involved emis-sion channels, both originating in the 4F9/2 excited level. In thisconnection, it has been observed that the relative intensity of theblue 4F9/2 → 6H15/2 and yellow 4F9/2 → 6H13/2 transition can be sig-nificantly different in various lattices. In some cases, it has beenpossible to modulate the emitted light by suitably varying thecomposition of the host, allowing, for example, the realization ofinteresting white-emitting phosphors [29,30].

In this paper, we reported the fabrication of CaMoO4:Dyx3+

phosphors with different Dy3+ dopant concentration by a conven-tional solid-state method, and the influences of doping level ofDy3+ on the microstructures and the luminescent properties of theobtained CaMoO4:Dyx

3+ phosphors were investigated in detail. Ourresults show that the as-fabricated CaMoO4:Dyx

3+ phosphors havegood photoluminescence properties.

2. Experimental

A series of Dy3+ doped CaMoO4 phosphors were obtained by a normal solid-state reaction method in air. The reactants including CaO (A.R.), MoO3 (99.99%),Dy2O3 (99.9%), which were weighted in an appropriate stoichiometric ratio withthe doping level at 0, 2.5, 5 and 10 mol%, respectively. These powders were blended

with absolute ethyl alcohol and grounded thoroughly in an agate mortar for 24 h.Then, the homogeneous mixture was filled into an alumina crucible and calcined ina muffle furnace at 600–800 ◦C for 3 h in air. Finally, the products were ground intofine powders and the efficient phosphors were obtained.

The powder X-ray diffraction (XRD) was performed with a Model XD-2 (BeijingPurkinje General Instrument Co. Ltd., China) with Cu K� radiation (� = 0.154056 nm),

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392 D. Gao et al. / Materials Chemistry and Physics 126 (2011) 391–397

Fig. 1. SEM micrographs of the CaMoO4:Dyx3+ powders with x = 0, 0.025, 0.05 and 0.1 fabricated by solid-state method at 600 ◦C for 3 h.

Fig. 2. SEM micrographs of the CaMoO4:Dyx3+ powders with x = 0, 0.025, 0.05 and 0.1 fabricated by solid-state method at 650 ◦C for 3 h.

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D. Gao et al. / Materials Chemistry and Physics 126 (2011) 391–397 393

Fig. 3. SEM micrographs of the CaMoO4:Dyx3+ powders with x = 0, 0.025, 0.05 and 0.1 fabricated by solid-state method at 700 ◦C for 3 h.

Fig. 4. SEM micrographs of the CaMoO4:Dyx3+ powders with x = 0, 0.025, 0.05 and 0.1 fabricated by solid-state method at 750 ◦C for 3 h.

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394 D. Gao et al. / Materials Chemistry and Physics 126 (2011) 391–397

, 0.02

amsoapew1aFsapw

3

dt6cCpiw8datgt

Fig. 5. SEM micrographs of the CaMoO4:Dyx3+ powders with x = 0

nd a 2� range from 10◦ to 90◦ was scanned at the rate of 4◦ min−1. Surfaceorphology and microstructure were observed by field-emission environmental

canning electron microscope (SEM) (JSM-5900, Japan, JEOL, accelerating voltagef 20 kV). Fourier transform infrared spectroscopy (FT-IR) data were collected onNEXUS670 FT-IR spectrophotometer in the range of 400–4000 cm−1 using KBr

ellets. The photoluminescence spectra were obtained by a Fluorescence Spectrom-ter (F-7000, Hitachi, Japan) with a 20 kW Xenon lamp as excitation source, theavelength of spectra was varied from 300 to 900 nm with a scanning speed of

200 nm min−1 (i.e. the excitation time is 30 s). The luminescence decay curves of thes-fabricated CaMoO4:Dyx

3+ phosphors were recorded using Edinburgh InstrumentLS920 equipped with 450 W Xenon arc lamp having Peltier element cooled red sen-itive Hamamatsu R955 PMT and microsecond flashlamp (100 W). When the XRDnd fluorescence characterization were carried out, the measured powders wereut in the powder sample cell equipped with the apparatus. All the measurementsere performed at room temperature.

. Results and discussion

Figs. 1–5 give the SEM micrographs of the CaMoO4:Dyx3+ pow-

ers with x = 0, 0.025, 0.05 and 0.1 sintered at different sinteringemperatures (600–800 ◦C). When the sintering temperature is00 ◦C, all the microcrystallines are not well sintered (Fig. 1). Onean see that the crystallinity and the grain size of the obtainedaMoO4:Dyx

3+ powders increase with increasing sintering tem-erature (Figs. 2–5). Especially when the sintering temperature

s up to 750 ◦C, all the microcrystallines are well sintered. Whileith the further increases of the sintering temperature (Fig. 5,

00 ◦C), the crystallinity and the uniformity of CaMoO4:Dyx3+ pow-

ers decreases. This indicates that the sintering temperature hasn important influence on the crystallinity and grain growth ofhe microcrystallines. Considering the nuclear formation and therain growth of CaMoO4:Dyx

3+ powders, the preferable sinteringemperature was optimized as 750 ◦C.

5, 0.05 and 0.1 fabricated by solid-state method at 800 ◦C for 3 h.

As shown in Fig. 4, after being sintered at the optimal sinter-ing temperature (750 ◦C) for 3 h, all the microcrystallines are wellsintered. The pure CaMoO4 is well crystallized, shaped of agglomer-ated spherical particles, having large grains of size ∼5 �m (Fig. 4A).After the doping of Dy3+, the CaMoO4:Dyx

3+ phosphors becomeinhomogeneous and the grain size decreases significantly (simi-lar results appeared in the other sintering temperatures, seen inFigs. 1–3 and 5). On average, the grain size for the phosphorswith x = 0.025, 0.05 and 0. 10 is in the range 0.5–2 �m (Fig. 4B–D).This suggests that the doping of Dy3+ will lead to an inhibitionof grain growth. The reduction in grain size could be attributedto the increased nucleation sites resulting from higher stackingfault energy due to Dy3+ addition to the pure material [31]. Similarresults have been reported for La-doped KNN ceramics [32], CeO2-doped KNN ceramics [33] and Dy3+-doped SnO2 nano-crystallines[34]. In general, donor doping will lead to an inhibition of graingrowth [35,36].

The XRD patterns of the obtained CaMoO4:Dyx3+ phosphors sin-

tered at 750 ◦C for 3 h are given in Fig. 6.It can be seen that all the as-fabricated CaMoO4:Dyx

3+ phos-phors are well consistent with the standard data of scheelitestructured CaMoO4 (Joint Committee for Powder Diffractions,JCPDS card 85-1267), especially for those strong diffraction peakssuch as (1 1 2), (1 0 1), (2 0 4), (2 0 0), (2 2 0), (0 0 4), (1 1 6) and(3 1 2), suggesting that all the obtained CaMoO4:Dyx

3+ phosphorsare single-phase scheelite structure and Dy3+ ion has diffused into

the lattice of CaMoO4 to form a solid solution. It also can be seenthat the diffraction peaks of the CaMoO4:Dyx

3+ phosphors slightlyshift to the higher 2� diffraction angle when the doping level ofDy3+ ion (i.e. x) changes from 0 to 0.10, this should be attributedto the fine distinction in radius of Dy3+ and Ca2+ ion. The radius of

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D. Gao et al. / Materials Chemistry and Physics 126 (2011) 391–397 395

10 20 30 40 50 60 70

CaMoO4 Dy

0.1

2 Theta (degree)

CaMoO4 Dy

0.05

CaMoO4 Dy

0.025

(224

)(3

12)

(116

)

(220

)

(204

)

(211

)

(200

)

(004

)(1

12)

(101

)

CaMoO4

Inte

nsi

ty (

a.u

.)

Fs

DtttbBtcotmwpbc

Cs

Fs

200 250 300 350 400 450 500 550

CaMoO4

CaMoO4:Dy

0.025

CaMoO4:Dy

0.05

CaMoO4:Dy

0.1

Inte

nsit

y (a

.u.)

Wavelength (nm)

ig. 6. . XRD patterns of the CaMoO4:Dyx3+ phosphor powders fabricated by solid-

tate method at 750 ◦C for 3 h.

y3+ (91.2 pm) is very close to that of Ca2+ (100 pm). According tohe principles of crystal chemistry, Dy3+ ions can easily enter intohe A sites for substituting the Ca2+ ions in ABO4 scheelite struc-ure CaMoO4 crystals and form a solid solution. Similar result haseen reported for the Eu3+ and Na+ codoped CaWO4 phosphors [9].ecause that the radius of Dy3+ is little smaller than that of Ca2+,he interplanar distance of the CaMoO4:Dyx

3+ solid solution micro-rystallines will slightly decreases with the increasing doping levelf Dy3+ ion (i.e. x). According to the Bragg equation (� = 2dh k l sin �),he corresponding diffraction peaks of the obtained CaMoO4:Dyx

3+

icrocrystallines slightly shift to the higher 2�. It is also noted thatith the increase of the doping level of Dy3+ ion, the diffractioneaks of the obtained CaMoO4:Dyx

3+ microcrystallines accordingly3+

roadened, suggests that the grain size of CaMoO4:Dyx micro-

rystallines decrease. And this is in accord with the SEM results.Fig. 7 shows, as an example, the FT-IR spectra of the obtained

aMoO4 and CaMoO4:Dy3+0.1 phosphors. It can be seen that the two

amples have approximate vibration modes. A strong absorption

1000 950 900 850 800 750 700 650 600 550 500 450 400

CaMoO4: Dy

0.1

CaMoO4

440

619

807

(b)

(a)

Inte

nsit

y (a

.u.)

Wavenumbers (cm-1)

ig. 7. FT-IR spectra of CaMoO4 and CaMoO4:Dy3+0.1 phosphors fabricated by solid-

tate method at 750 ◦C for 3 h.

Fig. 8. Excitation spectra of the CaMoO4:Dyx3+ phosphor powders with x = 0, 0.025,

0.05 and 0.1 fabricated by solid-state method at 750 ◦C for 3 h.

peak at 807 cm−1 can be assigned to �3 antisymmetric stretch-ing vibration originating from the Mo–O stretching vibration inMoO4

2− tetrahedron [37,38]. And the weak absorption peak at440 cm−1 can be assigned to �2 bending vibration of Mo–O. Theband at 619 cm−1 refers to the CO2 absorbed from the environment.The FT-IR results also confirm that the obtained CaMoO4:Dyx

3+

phosphors possess single-phase scheelite structure.The excitation spectra of the CaMoO4:Dyx

3+ phosphor powderssintered at 750 ◦C for monitoring 575 nm at room temperatureare given in Fig. 8. There is a broad O → Mo charge transfer (CT)band with a maximum centered at 298 nm and the band edgeat about 350 nm. The other sharp lines including the peaks from350 nm to 500 nm (353 nm, 367 nm, 389 nm, 428 nm and 454 nm)are attributed to the intra-configurational f–f transition of Dy3+ ionsin the host lattice (except for the pure CaMoO4 powder), in which

the strongest absorption is at about 353 nm. Therefore, this novelphosphor can be well excited upon ultraviolet and visible light.

Fig. 9 shows the emission spectra of the obtained CaMoO4:Dyx3+

powders with 280 nm excitation wavelength at room temperature.

300 400 500 600 700 800

6H13/2

4F9/2

6H15/2

4F9/2

(d)

(c)

(b)

(a)

(a) CaMoO4

(b) CaMoO4 : Dy

0.025

(c) CaMoO4 : Dy

0.05

(d) CaMoO4 : Dy

0.1

Inte

nsit

y (a

.u.)

Wavelength (nm)

Fig. 9. Emission spectra of the CaMoO4:Dyx3+ powders with x = 0, 0.025, 0.05 and

0.1 obtained at 750 ◦C under ultraviolet (280 nm) excitation at room temperature.

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396 D. Gao et al. / Materials Chemistry an

300 400 500 600 700 800

6H13/2

4F9/2

6H15/2

4F9/2

(e)(d)

(b)

(c)

(a)

(a) 600-CaMoO4:Dy

0.025

(b) 650-CaMoO4:Dy

0.025

(c) 700-CaMoO4:Dy

0.025

(d) 750-CaMoO4:Dy

0.025

(e) 800-CaMoO4:Dy

0.025

Inte

nsit

y (a

.u.)

Wavelength (nm)

Fs

Awaot

Fa

ig. 10. Emission spectra of the CaMoO4:Dy3+0.025 powders obtained at different

intering temperature (excitation by 280 nm ultraviolet at room temperature).

s shown in Fig. 9, the pure CaMoO4 powders emit a strong blueide band located at about 490 nm, which should be attributed to

n intrinsic emission of MoO42− complex ions. (For interpretation

f the references to color in this sentence, the reader is referred tohe web version of the article.) It is clearly seen that after doping

0 500 1000 1500 2000

(c) CaMoO4: Dy

0.025I = I

0exp(-t/ )

I0 = 24355

= 71.0 s

Red solid line: Fitted by

Blue circles: experimental data

6H

15/2

4F

9/2

em

= 487 nm

ex

= 298 nm

Decay time (s)

(b) CaMoO4: Dy

0.05

I = I0exp(-t/ )

I0 = 41600

= 54.5 s

Red solid line: Fitted by

Green circles: experimental data

6H

15/2

4F

9/2

em = 487 nm

ex

= 298 nm

Inte

nsit

y (a

.u.)

(a) CaMoO4: Dy

0.1

I = I0exp(-t/ )

I0 = 61565

= 46.9 s

Red solid line: Fitted by

Violet circles: experimental data

6H

15/2

4F

9/2

em

= 487 nm

ex

= 298 nm

ig. 11. Decay curves for the luminescence of Dy3+ in CaMoO4:Dyx3+ powders (x = 0.0025,

t 487 nm and 574 nm.

d Physics 126 (2011) 391–397

of Dy3+, the intrinsic emission peak is suppressed markedly, whilethe emission spectra of the CaMoO4:Dy3+ phosphors consist of twoband systems centered at about 487 nm and 574 nm, both charac-teristic of Dy3+ in a solid solution. The blue emission at 487 nm isascribed to the magnetic dipole transition of 4F9/2 → 6H15/2 and theyellow emission at 574 nm is related to the electric dipole tran-sition of 4F9/2 → 6H13/2. The distinct suppression of the intrinsicemission for MoO4

2− complex ions may be explained as follows.It should be noted that the strong emission band of Dy3+ ion atabout 487 nm, i.e. the 4F9/2 → 6H15/2 blue emission (487 nm) is veryclose to the intrinsic emission of MoO4

2− complex ions (about490 nm), so they can be effectively overlapped. As a whole, theintrinsic emission spectra of MoO4

2− complex ions (wide band)for the CaMoO4:Dyx

3+ phosphors seem to be suppressed obviously.Moreover, the 4F9/2 → 6H13/2 yellow emission (574 nm) gives byfar the most intense band, and the 4F9/2 → 6H15/2 blue emission(487 nm) is much less intense than the yellow one. It was wellknown that the Dy3+ 4F9/2 → 6H13/2 transitions are hypersensitiveelectronic dipole transitions with �J = 2, which are greatly affectedby the coordination environment. While 4F9/2 → 6H15/2 transitionsare magnetic dipole transitions with much less sensitive to thecoordination environment. The 4F9/2 → 6H13/2 transition is a forcedelectric dipole transition being allowed only at low symmetrieswith no inversion centre. When Dy3+ is located at low symme-

try local site (without an inversion centre), this emission is oftenprominent in its emission spectrum. As the radius of Dy3+ (91.2 pm)is very close to that of Ca2+ (100 pm), so Dy3+ can easily enter intothe four fold coordination Ca2+ sites (without an inversion centre).Therefore, situated at such low symmetry local sites for Dy3+ ions,

Decay time (s)

Inte

nsit

y (a

.u.)

0 500 1000 1500 2000

(f) CaMoO4: Dy

0.025

I = I0exp(-t/ )

I0 = 20470

= 127.0 s

Red solid line: Fitted by

Blue circles: experimental data

6H

13/2

4F

9/2

em

= 574 nm

ex

= 298 nm

Red solid line: Fitted by

Green circles: experimental data

6H

13/2

4F

9/2

em = 574 nm

ex

= 298 nm

(e) CaMoO4: Dy

0.05

I = I0exp(-t/ )

I0 = 34431

= 74.6 s

6H

13/2

4F

9/2

Red solid line: Fitted by

Violet circles: experimental data

em

= 574 nm

ex

= 298 nm

I = I0exp(-t/ )

I0 = 43370

= 58.4 s

(d) CaMoO4: Dy

0.1

0.05 and 0.1) obtained at 750 ◦C under an excitation of 298 nm with signals detected

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D. Gao et al. / Materials Chemis

he 4F9/2 → 6H13/2 transition emission is prominent in the emissionpectra.

Similar results have been reported for Dy3+-doped transpar-nt glass ceramics containing CaF2 nanocrystals [39], Dy3+-dopedY3F10 and LiLuF4 crystalline fibers [40] and NaLa(WO4)2:Dy3+ sin-le crystal [41]. It is also noted that the luminescent propertiestrongly depend on the doping concentration in the host lattice.

hen the concentration of Dy3+ increases to 2.5 mol%, the intensityhows the strongest and slowly decreases at higher Dy3+ concen-rations. This is typical property named concentration quenchingf lanthanide-doped system due to mutual Dy3+–Dy2+ interactions24]. The reason must be that when the concentration of Dy3+

ontinues to increase, the interaction increases and leads to self-uench. So, the emission intensity decreases. The concentrationuenching might be elucidated by the following two factors, (i)he excitation migration due to resonance between the activatorss enhanced when the doping concentration is increased, and thushe excitation energy reaches quenching centers, and (ii) the activa-ors are paired or coagulated and are changed to quenching centers42,43].

As an example, the emission spectra of the CaMoO4:Dy3+0.025

owders obtained at different temperature (excitation by 280 nmltraviolet at room temperature) are shown in Fig. 10. It can be seenhat all the samples exhibit the similar spectrum with various emis-ion intensity. The CaMoO4:Dy3+

0.025 powders fabricated at 750 ◦Cossess the strongest emission intensity, this should be attributedo the good crystallinity and morphology of the phosphor powdersbtained at this sintering temperature.

The typical decay curves for the luminescence of Dy3+ inaMoO4:Dyx

3+ powders (x = 0.0025, 0.05 and 0.1) obtained at 750 ◦Cnder an excitation of 298 nm with signals detected at 487 nmnd 574 nm are shown in Fig. 11. It can be seen that differ-nt concentrations of Dy3+ ion doping did not obviously changehe decay behavior, and all the decay curves for 4F9/2 → 6H15/2487 nm) and 4F9/2 → 6H13/2 (574 nm) emission peaks of Dy3+ cane well fitted into a single exponential function as I = I0 exp(−t/�)here I0 is the initial emission intensity at t = 0 and � is the 1/eifetime of the emission centre). For the 4F9/2 → 6H15/2 (487 nm)mission peak, the lifetime of CaMoO4:Dy3+

0.025, CaMoO4:Dy3+0.05

nd CaMoO4:Dy3+0.1 are 71.0 �s, 54.5 �s and 46.9 �s, respec-

ively (Fig. 11(a)–(c)). As for the 4F9/2 → 6H13/2 (574 nm) emissioneak, the lifetime of CaMoO4:Dy3+

0.025, CaMoO4:Dy3+0.05 and

aMoO4:Dy3+0.1 are 127.0 �s, 74.6 �s and 58.4 �s, respectively

Fig. 11(d)–(f)). It is noted that the decay time of the two emis-ion spectra (487 nm and 574 nm) both obviously decreased withncreasing Dy3+ concentration. This was caused by the effect ofnergy exchange between the Dy3+ ions as the distance betweenhem decreased with increasing Dy3+ ion concentrations, enhanc-ng the energy depletion rate and causing the decay time toecrease. Similar results have been reported for Dy3+-doped YVO4anoparticles [44], Dy3+-doped GAG nanophosphors [42] and Dy3+-oped YInGe2O7 phosphors [45].

. Conclusions

CaMoO4: Dyx3+ phosphor powders have been fabricated

ia solid-state ceramic method, the microstructures and roomemperature photoluminescence properties of the obtained micro-rystallines were investigated. The microcrystallines possess acheelite structure with tetragonal symmetry. After the doping

f Dy3+, the grain growth is inhibited; the intrinsic emission ofoO4

2− complex ions is suppressed. With regard to CaMoO4:Dyx3+

hosphors, a bright fluorescent yellow emission at 574 nm4F9/2 → 6H13/2) and blue emission at 487 nm (4F9/2 → 6H15/2) haveeen observed. The decay time of the two emission spectra (487 nm

[

[

[

d Physics 126 (2011) 391–397 397

and 574 nm) of Dy3+ ion both decrease with increasing Dy3+ con-centration.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (No. 50472103), Sichuan Youth Science & Tech-nology Foundation (No. 08ZQ026-054) and Scientific Research Fundof Sichuan Normal University (No. 08KYL02).

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