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Materials Chemistry and Physics 133 (2012) 617– 620

Contents lists available at SciVerse ScienceDirect

Materials Chemistry and Physics

j ourna l ho me pag e: www.elsev ier .com/ locate /matchemphys

he effect of excitation power density on frequency upconversion in Yb3+/Er3+

odoped Gd6WO12 nanoparticles

ue Tiana,b, Ruinian Huaa,∗, Jicheng Yua, Jiashi Sunb, Baojiu Chenb,∗∗

College of Life Science, Dalian Nationalities University, Dalian 116600, ChinaDepartment of Physics, Dalian Maritime University, Dalian 116026, China

r t i c l e i n f o

rticle history:eceived 5 April 2011eceived in revised form 4 January 2012

a b s t r a c t

Yb3+/Er3+ co-doped Gd6WO12 nanoparticles were prepared via a coprecipitation technique and charac-terized by means of XRD and FE-SEM. The effect of excitation power density in different regions on theupconversion mechanisms was investigated. It was found that the green upconversion emissions is a two

ccepted 11 January 2012

eywords:. Optical materials. Nanostructures. Luminescence

photons process when the excitation power density is low, but a three photons process occurs when theexcitation power density is high. However, the red upconversion emission is still a two photons process.A possible mechanism for three photons green upconversion luminescence was proposed and discussed.

© 2012 Elsevier B.V. All rights reserved.

. Optical properties

. Introduction

Over the past several decades, the upconversion of infrared toisible light by materials doped with rare earth (RE3+) ions hasttracted considerable attention due to their potential applicationsn the areas of color displays, lasers and biomedicine [1–8]. Amongll the RE3+ ions, the Er3+ ion, which has several metastable energyevels and can restore the excitation energy for up-convertingear infrared (NIR) light into short wavelength radiation, is a veryxcellent candidate of luminescence centers for up-conversionuminescence [9–12]. Therefore, the Er3+ ion has been studiedxtensively in various hosts. However, the absorption across sec-ion of Er3+ is small in the infrared region, thus leading to relativelyow upconversion luminescent efficiency while pumped at thesenfrared wavelengths. In order to increase upconversion lumines-ent efficiency, the Yb3+ ion is usually introduced as a sensitizerwing to its large absorption across section of 980 nm pumpingight [11,13,14].

The gadolinium tungstates are very interesting inorganicompounds existing in different chemical formulations such asd6WO12, Gd2WO6, Gd2W2O9 and Gd2(WO4)3. Gd2(WO4)3 is aell known phosphor host and has been extensively studied

15,16]. However, other forms of gadolinium tungstates were rarelytudied. In this paper, we reported on the preparation, struc-ural and morphological characterization of Gd6WO12:Yb3+, Er3+

∗ Corresponding author. Tel.: +86 411 87656220.∗∗ Corresponding author. Tel.: +86 411 84728909.

E-mail addresses: rnhua@dlnu.edu.cn (R. Hua), chenmbj@sohu.com (B. Chen).

254-0584/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2012.01.025

nanoparticles, as well as upconversion luminescence properties.It was found that the red upconversion emission is a two pho-tons process, but the green upconversion emission is, respectively,two or three photons processes when the sample is pumped withlaser power density in low and high range. The possible three pho-tons process for green upconversion emission was proposed andanalyzed.

2. Experimental

2.1. Chemical reagents

All reagents used in this work are analytical grade except for thespectrographically pure Er2O3, Yb2O3 and Gd2O3·Er(NO3)3·6H2O,Yb(NO3)3·6H2O and Gd(NO3)3·6H2O were prepared via a recrystal-lization technique. Firstly, Er2O3 (Yb2O3 or Gd2O3) was dissolvedin nitric acid solution with the 1:1 volume ratio of nitric acid towater. Secondly, the solution was heated with an electric furnaceto evaporate off the extra-water, and then the obtained productwas dissolved in water and heated again, after three times recrys-tallization the final rare earth nitrate with six crystal waters wasobtained after dried in vacuum at 90 ◦C for 12 h.

2.2. Synthesis

Yb3+/Er3+ codoped Gd6WO12 nanoparticles were prepared viaa coprecipitation process. In order to obtain the precursors, an15 ml aqueous solution containing Gd(NO3)3 (0.001 mol), Yb(NO3)3(0.5 mol%) and Er(NO3)3 (0.5 mol%) was dropped slowly in Na2WO4

6 try and Physics 133 (2012) 617– 620

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18 Y. Tian et al. / Materials Chemis

0.001 mol) aqueous solution under stirring, then the stirring wasept for 30 min to ensure the reaction complete, after that theuspension was centrifuged at 4500 rpm for 15 min to obtain therecursor. The precursor was washed with distilled water and driedt 90 ◦C for 10 h. Finally, the Yb3+/Er3+ codoped Gd6WO12 nanopar-icles were obtained after the precursor was sintered at 900 ◦C for

h.

.3. Characterization

The crystal structure of the prepared sample was examinedy using X-ray diffraction (XRD-6000, Shimadzu, Japan) under Cu�1 radiation (� = 0.15406 nm). The XRD data in the 2� range from0 to 70◦ for index and cell-parameter calculation were collected

n a scanning mode with a step of 0.02◦ and a scanning rate of.0◦ min−1. In the XRD measurement a single crystal silicon sampleas used as an internal standard. The particle size and morphol-

gy were inspected by using a field emission scanning electronicroscopy (FE-SEM Hitachi S-4800, Japan) working at accelerat-

ng voltage of 15 kV. The upconversion spectra were recorded with fluorescence spectrophotometer (Hitachi F-4600, Japan) and a80 nm optical fiber laser (KS3-11312, BWT, Beijing Kaipulin Co.,TD, China) with continuous output maximum power of 4.0 W wassed as excitation source. The output power of the 980 nm LD waseasured with a laser power meter (VLP-2000, Xi’an Hirsh Laser

ech. Co., LTD). In order to investigate the excitation power depen-ence of upconversion luminescence, the area of laser beam spotLBS) was chosen, respectively, as 0.8 and 20.0 mm2 by focusing theaser output light beam, and in the measurements for each case ofonstant LBS area the laser power was changed.

. Results and discussion

.1. Crystal structure characterization

Fig. 1 shows the XRD pattern for final Yb3+/Er3+ codopedd6WO12 nanoparticles, where the diffraction peak positions areonsistent well with those reported in JCPDS Card No. 24-0434,nd that there are no extra diffraction peaks corresponding tother compounds in the obtained pattern. This means that the pre-ared nanoparticles exist in a tetragonal phase, and codoping withb3+/Er3+ does not influence the crystal structure. The crystallo-

raphic size of nanoparticles can be estimated from XRD pattern inerms of Debye–Scherrer equation [17,18]. The calculation resultisplays that the crystallographic size of nanoparticles is about0 nm.

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20 25 30 35 40 45 50 55 60 65 70

JCPDS card No. 24-0434

2 Theta (degree)

ig. 1. XRD pattern of Gd6WO12:Yb3+/Er3+ nanoparticles (top part); diffraction pat-ern plotted by using the data taken from JCPDS Card No. 24-0434 (bottom part).

Fig. 2. (a) FE-SEM image of Gd6WO12:Yb3+/Er3+ nanoparticles; (b) histograph ofparticle size distribution for the studied Gd6WO12:Yb3+/Er3+ nanoparticles.

3.2. Field emission scanning electron microscopy of product

The FE-SEM image for the obtained product is shown in Fig. 2(a).It can be found that the product is composed of many sphere-shaped particles, and some of them are agglomerated together.By counting 100 clearly identifiable particles, the average particlesize was estimated to be 53 nm. The statistic histogram is shownin Fig. 2(b), where the Gaussian’s fitting was also carried out. Thedifference between the average size confirmed from FE-SEM andthe crystallographic size derived from XRD is probably due to thereasons that, on the one hand the crystallographic size calculationbased on the Debye–Scherrer equation in the case of larger sizeparticles may introduce larger error, on the other hand the crys-tallographic size reflects the size of well-crystallized part of theparticles, but the average size observed from FE-SEM presents theapparent size of the particles, thus they may be different.

3.3. Dependence of frequency upconversion on excitation powerdensity

In order to study on the effect of the excitation power densityon the upconversion luminescence, the excitation-power-density-dependent upconversion emission spectra were measured in both

Y. Tian et al. / Materials Chemistry and Physics 133 (2012) 617– 620 619

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Fig. 5. Simplified energy level diagram of Yb3+ and Er3+ and upconversion processesof Yb3+/Er3+ codoped Gd6WO12 nanoparticles under 980 nm excitation. (For inter-pretation of the references to color in text, the reader is referred to the web versionof this article.)

ig. 3. Upconversion emission spectra of Gd6WO12:Yb3+/Er3+ nanoparticles underhe 980 nm excitation with excitation power density.

he cases of 0.8 and 20.0 mm2 laser beam spot areas as mentionedn Section 2.3. Fig. 3 shows the upconversion emission spectra ofd6WO12:Yb3+/Er3+ nanoparticles under the 980 nm pump withifferent maximum excitation power densities for above men-ioned two cases. It can be found that each upconversion emissionpectrum consists of green and red band ranging, respectively, from00 to 580 nm and from 640 to 690 nm, which correspond to 2H11/2,S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions [1,19].

Fig. 4 shows the dependent relationships between integratedpconversion emission intensities and the excitation power values.he integrated upconversion emission intensities were obtainedrom the upconversion emission spectra via simple numerical cal-ulations, and the spectral measurements were carried out in theases of two different laser beam spot areas (0.8 and 20.0 mm2), andhat for each case of constant laser beam spot area the excitationower of 980 nm laser was changed. From Fig. 4 it can be seen thathe integrated emission intensity evolution with excitation poweror the same transition (green color square and triangle) is differ-nt in the two cases, which means the upconversion luminescenceechanisms are different.

.4. Upconversion mechanism

It is well known that the dependence of integrated emissionntensity on the excitation power follows the exponential function

2.90 2.95 3.00 3.05 3.10 3.15 3.20

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Red n = 2.10 ± 0.25

Green n = 2.10 ± 0.08

Red n = 1.80 ± 0.19

ig. 4. Log–log plots of integrated upconversion emission intensity versus laserower for the cases of different laser beam spot areas. (For interpretation of theeferences to color in text, the reader is referred to the web version of this article.)

and the index is an indicator for the number of needed infrared pho-tons of generating one visible photon [20], thus in the double logcoordinates system the dependence displays linear relationship,and the slope has the same meaning as the index of exponentialfunction. The slopes for green and red emissions were obtainedthrough linear fittings to be 2.10 and 1.80 when the laser beamspot area is 20.0 mm2, and 2.70 and 2.10 when the laser beam spotarea is 0.8 mm2. This fact means that the green upconversion mayoriginate more than two photons upconversion process when thelaser power density falls into higher region, but the red upconver-sion emission still follows the two photons process without changewith excitation power density.

The two photons processes for green and red upconversionemissions were widely observed, and the commonly acceptablemechanisms can be expressed as below [21,22].

For red upconversion emission (see Fig. 5):

(1) 4I15/2 (Er3+) + 2F5/2 (Yb3+) → 4I11/2 (Er3+) + 2F7/2 (Yb3+) (Energytransfer)

(2) 4I11/2 (Er3+) → 4I13/2 (Er3+) (Nonradiative relaxation)(3) 4I13/2 (Er3+) + 2F5/2 (Yb3+) → 4F9/2 (Er3+) + 2F7/2 (Yb3+) (Energy

transfer)

For green upconversion emission (see Fig. 5):

(1) 4I15/2 (Er3+) + 2F5/2 (Yb3+) → 4I11/2 (Er3+) + 2F7/2 (Yb3+) (Energytransfer)

(2) 4I11/2 (Er3+) + 2F5/2 (Yb3+) → 2H11/2 (Er3+) + 2F7/2 (Yb3+)(Energy transfer)

(3) 2H11/2 (Er3+) → 4S3/2 (Er3+) (Nonradiative relaxation)

When excitation power density is higher, the population of4F9/2 level can greatly increase, thus the probability of cross-relaxation 4I13/2 + 4F9/2 → 4F3/2 + 4I15/2 between Er3+ ions mayincreases greatly, which causes that Er3+ ions are excited from the4I level to the 4F level. Then, these ions return to the 2H

13/2 5/2 11/2and the 4S3/2 levels by nonradiative relaxation. Therefore, the greenupconversion emission is a three photons process, which can beexpressed as follows:

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20 Y. Tian et al. / Materials Chemis

1) 4I15/2 (Er3+) + 2F5/2 (Yb3+) → 4I11/2 (Er3+) + 2F7/2 (Yb3+) (Energytransfer)

2) 4I11/2 (Er3+) → 4I13/2 (Er3+) (Nonradiative relaxation)3) 4I13/2 (Er3+) + 4F9/2 (Er3+) → 4F3/2 (Er3+) + 4I15/2 (Er3+) (Cross

relaxation)4) 4F3/2 (Er3+) → 2H11/2 (Er3+) → 4S3/2 (Er3+) (Nonradiative relax-

ation)

It can be sure that the cross-relaxation processI13/2 + 4F9/2 → 4F3/2 + 4I15/2 plays a very important role in thehree photons upconversion process [4]. And if it really happens,he ratio of red to green upconversion emissions would be theneduced. As evidence, it was observed in Fig. 3 that the intensityatio of red to green does decrease when the excitation powerensity is high, which supports the conclusion that the cross relax-tion 4I13/2 + 4F9/2 → 4F3/2 + 4I15/2 between Er3+ ions is existent.hus, the three photons process may occur when the excitationower density is high. It should be mentioned that in this case

t does not mean that there is no two photons process for greenpconversion. In fact, the two and three photons processes maye co-existent, which is also the reason why the slope value wasonfirmed to be 2.70 for green upconversion emission but not 3or the case of small laser beam spot area.

. Conclusion

Yb3+/Er3+ codoped Gd6WO12 nanoparticles were prepared andheir crystal structure and morphology were characterized by

eans of XRD and FE-SEM. The effect of excitation power densityn the upconversion luminescence process was studied, and it wasound that the red upconversion is not affected by the excitationower density, but the green upconversion process depends on thexcitation power density: when the excitation power density is lowhe two photons process is dominant, however, the three photons

rocess is involved when the excitation power density is high. Theossible three photons process was assigned to be relevant with anssential cross relaxation 4I13/2 + 4F9/2 → 4F3/2 + 4I15/2 between Er3+

ons.

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d Physics 133 (2012) 617– 620

Acknowledgments

This work was partially supported by the Fundamental ResearchFunds for the Central Universities (Grant Nos. DC10020105,017004, 2011ZD032, 2011QN152 and 2011ZD033), Foundation ofEducation Department of Liaoning Province (Grant Nos. 2009A148and L2010056), NSFC (National Natural Science Foundation ofChina, Grant Nos. 61078061, 11104024 and 11104023), and Tech-nology Services Pre-research project fund of Dalian NationalitiesUniversity (2009B301).

References

[1] Y.Q. Lei, H.W. Song, L.M. Yang, L.X. Yu, Z.X. Liu, G.H. Pan, X. Bai, L.B. Fan, J. Chem.Phys. 123 (2005) 174710.

[2] M. Shojiya, M. Takahashi, R. Kanno, Y. Kawamoto, K. Kadono, Appl. Phys. Lett.65 (1994) 1874.

[3] T. Catunda, L.A.O. Nunes, A. Florez, Y. Messaddeq, M.A. Aegerter, Phys. Rev. B53 (1996) 6065.

[4] J. Wang, J. Hao, P.A. Tanner, Opt. Express 19 (2011) 11753.[5] H.J. Liang, Y.D. Zheng, L. Wu, L.X. Liu, Z.G. Zhang, W.W. Cao, J. Lumin. 131 (2011)

1802.[6] P.S. Golding, S.D. Jackson, T.A. King, M. Pollnau, Phys. Rev. B 62 (2000) 856.[7] C. Joshi, K. Kumar, S.B. Rai, Spectrochim. Acta A 79 (2011) 127.[8] L. Li, Z.G. Zhou, L. Feng, H. Li, Y. Wu, J. Alloy. Compd. 509 (2011) 6457.[9] K.Y. Wu, J.B. Cui, X.X. Kong, Y.J. Wang, J. Appl. Phys. 110 (2011) 053510.10] J.A. Capobianco, G. Prevost, P.P. Proulx, P. Kabro, M. Bettinelli, Opt. Mater. 6

(1996) 175.11] D. Solis, E. De la Rosa, O. Meza, L.A. Diaz-Torres, P. Salas, C. Angeles-Chavez, J.

Appl. Phys. 108 (2010) 023103.12] J. Koetke, G. Hubber, Appl. Phys. B 61 (1995) 151.13] R.H. Page, K.I. Schaffers, P.A. Waide, J.B. Tassano, S.A. Payne, W.F. Krupke, W.K.

Bischel, J. Opt. Soc. Am. B 15 (1998) 996.14] L.A. Gomez, L. de S. Menezes, B.B. de Araujo, R.R. Goncalves, S.J.L. Ribeiro, Y.

Messaddeq, J. Appl. Phys. 107 (2010) 113508.15] M.L. Pang, J. Lin, M. Yu, J. Solid State Chem. 177 (2004) 2237.16] P.Y. Jia, X.M. Liu, M. Yu, Y. Luo, J. Fang, J. Lin, Chem. Phys. Lett. 424 (2006) 358.17] T. Yu, J. Sun, R. Hua, L. Cheng, H. Zhong, X. Li, H. Yu, B. Chen, J. Alloys Compd.

509 (2011) 39.18] R.C. Kambale, K.M. Song, Y.S. Koo, N. Nur, J. Appl. Phys. 110 (2011) 053910.19] X. Bai, H.W. Song, G.H. Pan, Y.Q. Lei, T. Wang, X.G. Ren, S.Z. Lu, B. Dong, Q.L. Dai,

L.B. Fan, J. Phys. Chem. C 111 (2007) 13611.

20] F. Vetrone, J.C. Boyer, J.A. Capobianco, J. Appl. Phys. 96 (2004) 661.21] F. Song, G.Y. Zhang, M.R. Shang, H. Tan, J. Yang, F.Z. Meng, Appl. Phys. Lett. 79

(2001) 1748.22] H.W. Song, B.J. Sun, T. Wang, S.Z. Lu, L.M. Yang, B.J. Chen, X.J. Wang, X.G. Kong,

Solid State Commun. 132 (2004) 409.