Nd3+-codoped tellurite glasses

Nd3+-codoped tellurite glasses

ARTICLE IN PRESS Journal of Luminescence 126 (2007) 677–681 www.elsevier.com/locate/jlumin Up-conversion luminescence of Er3+/Yb3+/Nd3+-codoped tell...

186KB Sizes 0 Downloads 16 Views

ARTICLE IN PRESS

Journal of Luminescence 126 (2007) 677–681 www.elsevier.com/locate/jlumin

Up-conversion luminescence of Er3+/Yb3+/Nd3+-codoped tellurite glasses Longjun Lua,, Qiuhua Niea, Tiefeng Xua, Shixun Daia, Xiang Shena, Xianghua Zhanga,b a

Faculty of Information Science and Engineering, Ningbo University, The State Key Laboratory Base of Novel Functional Materials and Preparation Science, Ningbo, Zhejiang 315211, PR China b Laboratoire des Verres et ceramiques, Universite de Rennes I, 35042 Rennes Cedex, France Received 19 May 2006; received in revised form 14 October 2006; accepted 18 October 2006 Available online 4 December 2006

Abstract Up-conversion luminescence and energy transfer (ET) processes in Nd3+–Yb3+–Er3+ triply doped TeO2–ZnO–Na2O glasses have been studied under 800 nm excitation. Intense green up-conversion emissions around 549 nm, which can be attributed to the Er3+: 4S3/2 -4I15/2 transition, are observed in triply doped samples. In contrast, the green emissions are hardly observed in Er3+ singly doped and Er3+–Yb3+ codoped samples under the same condition. Up-conversion luminescence intensity exhibits dependence of Yb2O3concentration and Nd2O3-concentration. Up-conversion mechanism in the triply doped glasses under 800 nm pump is discussed by analyzing the ET among Nd3+, Yb3+ and Er3+. And a possible up-conversion mechanism based on sequential ET from Nd3+ to Er3+ through Yb3+ is proposed for green and red up-conversion emission processes. r 2006 Elsevier B.V. All rights reserved. Keywords: Rare-earth; Up-conversion; Energy transfer; Triply doped

1. Introduction With the development of up-conversion visible or ultraviolet photonic devices, rare earth (RE) ions-doped glasses have been investigated extensively, which may be the candidate materials to be applied in areas of highdensity optical storage, color displays, optoelectronics and medical diagnostics [1–4]. Many trivalent RE ions, such as Er3+ [4–7], Tm3+ [7,8], Ho3+ [9,10], and Tb3+ [11,12], sensitized with ytterbium, have been widely studied in various host materials and their up-conversion properties were researched for developing the glasses with the intense frequency up-conversion emission. In certain situations, one takes advantage of the strong absorption cross-section of Yb3+ around 980 nm and the efficient energy transfer (ET) mechanism between Yb3+ ions and those RE Corresponding author. Tel.: +86 574 8760 0358; fax: +86 574 8760 0946. E-mail address: [email protected] (L. Lu).

0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.10.023

acceptors. For a lot of applications, it is desirable to access the visible wavelength emitting levels of such RE ions employing an excitation source in the wavelength region of 800 nm [13]. Nd3+ is a good candidate for improving the pumping efficiency of 800 nm LD, since it exhibits an intense absorption cross-section around 800 nm in conjunction with efficient ET mechanism involving neodymium and ytterbium ions [13,14,19]. Until now, a specific up-conversion mechanism has been reported under 800 nm excitation for three kinds of RE ions, Nd3+–Yb3+–Ho3+, Nd3+–Yb3+–Tm3+ and Nd3+–Yb3+–Tb3+ codoped ArF4-based glasses [14–16]. In these particular situations, however, Yb3+ ions can play a role of an energy-transfer bridging ions between an energy donor (Nd3+) ion and an energy acceptor ion (Ho3+, Tb3+ and Tm3+). In this work, we report for the first time the spectral properties of Nd3+–Yb3+–Er3+ codoped tellurite glasses. Optical absorption, and up-conversion under 800 nm excitation have been measured and the results are discussed finally.

ARTICLE IN PRESS L. Lu et al. / Journal of Luminescence 126 (2007) 677–681

678

2. Experimental Glasses composition employed were (mol%)70TeO2– 15ZnO–15Na2O doped with 0.5 wt% Er2O3, 0.5 wt% Nd2O3 and X wt% Yb2O3 (ENYX, X ¼ 0, 0.5, 1.0, 1.5, 2.0, 3.0), (mol%)70TeO2–15ZnO–15Na2O doped with 0.5 wt% Er2O3, 2 wt% Yb2O3 and X wt% Nd2O3 (EYNX, X ¼ 0, 0.2, 0.5, 0.8, 1) and (mol%)70TeO2–15ZnO– 15Na2O–0.5 wt% Er2O3 (TZN-Er). The starting materials were reagent grade TeO2, ZnO, and Na2CO3. About 10 g batches of starting materials were fully mixed and then melted in the Pt crucibles at 850 1C in an electronic furnace. When the melting was completed, the liquid was cast into stainless steel plate. The obtained glasses were annealed to room temperature at a rate of 10 1C/h, and then were cut and polished carefully in order to meet the requirements for optical measurements. The absorption spectra were recorded between 400 and 2000 nm with a Perkin–Elmer Lambda 950 UV–VIS–NIR spectrophotometer. Up-conversion luminescence spectra under 800 nm LD excitation were measured in the wavelength rang of 500–700 nm with a TRIAX550 spectrofluorimeter. Near-infrared fluorescence spectra in the wavelength rang from 850 to 1450 nm were also measured with a TRIAX550 spectrofluorimeter. All the spectral measurements were performed at room temperature. 3. Results and discussion Fig. 1 illustrates typical absorption spectrum of an Er3+–Nd3+–Yb3+ triply doped sample in the visible and near-infrared region, in which one indicates the major transitions associated to the three RE ions presented in the host matrix. It is well known that the Yb3+ has only one excited state of 2F5/2 which locates at about 10,000 cm1

above the ground state 2F7/2. Therefore, Yb3+ cannot be directly excited to 2F5/2 level by an 800 nm excitation. The absorption of Nd3+ for the 4I9/2-(4F5/2, 2H9/2) transition overlaps that of Er3+ for the 4I15/2-4I9/2 transition. Since the absorption of Er3+ at 800 nm is very weak, the absorption of Nd3+ around 800 nm is predominant in this triply doped sample. As an example, an up-conversion luminescence spectrum of ENY2.0 glass in the 500–700 nm wavelength range under 800 nm LD excitation is shown in Fig. 2. Three emission bands centered at 533, 549, and 662 nm were observed. The emission band around 533 nm attributed to the Er3+: 2H11/2-4I15/2 and Nd3+: 4G7/2-4I9/2 transitions, and the 650 nm radiation corresponded to the Er3+: 4 F9/2-4I15/2 transition. On the other hand, a strong emission band at 549 nm (green) corresponded to the Er3+: 4S3/2-4I15/2 transition. It is important to point out that the green (549 nm) emission is bright enough to be observed by naked eye even under a low excitation power 50 mW. In the Er3+ single doped glass, however, the upconversion luminescence intensity is very weak compare to that of Er3+–Nd3+–Yb3+ triply doped sample, as shown in Fig. 2. Fig. 3 shows the Yb2O3-concentration dependence of Er3+ green up-conversion luminescence intensities in Er3+–Nd3+–Yb3+ triply doped glasses. As can be observed, there exists green (549 and 533 nm) emission for the glass with no Yb3+ content and its intensity gradually increases with increasing Yb2O3-concentration, exhibiting a maximum value at approximately 2 wt% Yb2O3 and then rapidly decreases with further Yb2O3 addition. This phenomenon indicates that Yb3+ ions play major role in contributing to the excitation mechanism of Er3+ up-conversion luminescence in Er3+–Nd3+–Yb3+ triply doped glasses under 800 nm excitation.

Er:4G11/2

optical density(a.u)

0.8

Er:2H11/2+Nd:4G7/2

0.6

Er:4I9/2 + Nd: (4F5/2+2H9/2)

Nd:4G5/2+2G7/2

0.4

Nd:4F7/2+4S3/2

Er:4I11/2+Yb:2F5/2

Er:4F

5/2

Nd:4F3/2

Nd:4F9/2 0.2

Er:4I13/2

Nd:2P1/2 Er:4F7/2

Er:4S3/2

Er:4F9/2

0.0 400

600

800

1000

wavelength (nm) Fig. 1. Absorption spectrum of the ENY2.0 sample.

1400

1600

ARTICLE IN PRESS L. Lu et al. / Journal of Luminescence 126 (2007) 677–681

4

Er:4S3/2

I5/2 0.5Er3+/0.5Nd3+ 0.5Er3+(wt%)

Nd3+:4G7/2

4I 9/2

Upconversion intensity (a.u.)

/2.0Yb3+(wt%)

Er:2H11/2

4

I15/2 4

Er: F9/2

500

550

600

4I 15/2

650

700

wavelength /nm Fig. 2. Up-conversion spectra of TZN-Er and ENY2.0 glasses under 800 nm excitation.

6

1.5

1.2

4 0.9

0.6 2 0.3

Emission intensity (a.u.)

Emission intensity (a.u.)

549nm 533nm

679

contracted with the added Yb3+ ions. As shown in Fig. 3, the up-conversion luminescence increases at first, and then slightly decreases with the increase of Yb3+ content, which might be mainly caused by the backward ET from Er3+ to Yb3+ (4I11/2(Er3+)+2F7/2(Yb3+)4 I15/2(Er3+)+2F5/2(Yb3+)) and/or Nd3+. On the other hand, the distance between Yb3+ ions becomes close with increasing Yb2O3-concentration. When Yb2O3-concentration is above a certain content (e.g. the this content is 3 mol% in 60ZrF4–30BaF2–(10x) LaF3–xYbF3 [14]), the distance between two Yb3+ ions reaches a critical distance for a cooperative phenomenon which occurs by two excited Yb3+ ions [14,18]. This leads to a situation in which ET from Nd3+ ions to Er3+ ions through Yb3+ ions is reduced. This fact is also one of the reasons why, in rang above 2 wt% Yb2O3, the up-conversion luminescence intensities of 533 and 549 nm bands decrease with increasing Yb2O3-concentration. The effect of Nd3+ content in the visible up-conversion emission of erbium ions under 800 nm excitation was also studied and result was presented in Fig. 4. The result indicates that an optimum Nd2O3 concentration occurs around 0.5 wt% and the emission intensity slightly decreases when the Nd2O3 content is more than 0.5 wt%. This might attribute to the backward ET from Yb3+ to Nd3+ which increased with increasing of Nd2O3 content. It can be assumed that the up-conversion excitation progress of Er3+ could be accomplished by two paths of ET between/among RE ions: (a) direct ET from Nd3+ to Er3+, and (b) sequential ET from Nd3+ to Er3+ via Yb3+. If the ET is mainly caused through the former path there should be little effect of Yb3+ concentration on the emission intensity of Er3+ up-conversion luminescence. Therefore, the latter path is expected to be dominant for 7000

0

0.0 0.0

1.5

6000

3.0

Fig. 3. Dependence of up-conversion luminescence intensity around 549 and 533 nm upon Yb2O3-concentration in ENYX (X ¼ 0, 0.5, 1.0, 1.5, 2.0, 3.0) samples under 800nm excitation. (The line is drawn as a guide to the eye.)

It has been well known that the probability of ET due to multipolar interactions increases when the distance R between the sensitizer and the acceptor decreases [14]. According to Dexter’s theory [17], the ET probability is proportional to R6 (dipole–dipole interaction), R8 (dipole–quadrupole interaction), R10 (quadrupole–quadrupole interaction) [14]. When Yb3+ ions are added to the Nd3+–Er3+ codoped glass, they play as ET bridging ions, and the energies could be transfer from Nd3+ ions to Er3+ ions through Yb3+ ions. In this case, it seems that the distance between Nd3+ ions and Er3+ ions becomes

Emission intensity (a.u.)

Yb2O3 concentration / wt%

5000 4000 3000 2000 1000 0 0.0

0.2

0.4

0.6

0.8

1.0

Nd2O3 concentration / wt% Fig. 4. Dependence of green (549 nm) up-conversion luminescence intensity upon Nd2O3-concentration in EYNX (X ¼ 0, 0.2, 0.8, 1.0) glasses under 800 nm excitation. (The line is drawn as a guide to the eye.)

ARTICLE IN PRESS L. Lu et al. / Journal of Luminescence 126 (2007) 677–681

680

800 nm excitation. The insert shows the integrated areas ratio between Yb3+: 2F5/2-2F7/2 (980 nm) and Nd3+: 4F3/2-4I112 (1062 nm) transitions. It can be observed that the addition of Nd3+ increases the intensity of the emission arising from the Yb3+: 2F5/2 level. The integrated areas ratio between Yb3+: 2F5/2-2F7/2(980 nm) and Nd3+: 4F3/2-4I112 (1062 nm) transitions steeply decrease with increasing Nd2O3 concentration. This result proves that, under 800 nm excitation, the efficiency of ET from Nd3+ to Yb3+ is very high. According to Refs. [19,20], the ET between Nd3+ and Yb3+ is attributed to the following processes: (1) 2F7/2 (Yb3+)+4F3/2 (Nd3+)4 I9/2 (Nd3+)+2F5/2 (Yb3+), (2) 2F7/2 (Yb3+)+4F3/2 (Nd3+)-4I11/2 (Nd3+)+2F5/2 (Yb3+). These processes are illustrated in Fig. 6. Consequently, the path (b) may be concluded to be essential for the excitation mechanism in the Er3+–Nd3+–Yb3+ triply doped glasses. An up-conversion mechanism under 800 nm excitation propounded for the Er3+–Nd3+–Yb3+ co-doped tellurite glass is depicted in Fig. 6 [4–7,14–16]. First, both Nd3+ and Er3+ are excited by 800 nm pump photons corresponds to the Nd3+: 4I9/2-(4F5/2, 2H9/2), and Er3+: 4I15/2-4I9/2 transitions. The Nd3+: (4F5/2, 2H9/2) excited states relax quickly to the next lower level 4F3/2 by the multi-phonon relaxation. And then, an ET between Nd3+ and Yb3+ ions takes place with considerably high efficiency as aforementioned. Subsequently the energies on the Yb3+ ions are transferred to the Er3+ ions by the following process: 2F5/2 (Yb3+)+4I15/2 (Er3+)-2F7/2 (Yb3+)+4I11/2 (Er3+). In

1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.6 0.4 0.8 1.0 Nd2O3 conventration / wt%

Nd3+:4F3/2

4

I11/2

x=1 x=0.5 x=0.2 x=0

900

1000

1100

1200

1300

wavelength /nm Fig. 5. Fluorescence spectra of EYNX (X ¼ 0, 0.2, 0.5, 0.8, 1) glasses under 800 nm excitation. The inset shows the integrated areas ratio between Yb3+: 2F5/2-2I7/2 and Nd3+: 4F3/2-4I11/2 transitions.

25

2

D5/2 P3/2

4F 3/2

2

2K 4 15/2 G11/2

4G 7/2

2H

4F 5/2 4F 3/2

9/2,

10

2

4I 9/2

ET

CR

F5/2

4I 11/2

533 nm

4F 7/2

800 nm

NR 4I 13/2

4I 13/2 4I 11/2

0

2F 7/2

4I 9/2

Nd3+

Yb3+

662 nm

5

549 nm

4I 15/2

533 nm

Energy (103cm-1)

4S , 3/2

4F 7/2 2H 11/2 4S 3/2

4F 9/2

4F 9/2

15

ESA

2K 13/2 4G , 9/2

F5/2

2G 9/2

ESA

20

4

800 nm

F7/2

4I 9/2

2

Nd3+:4F3/2

Intensity (a.u.)

Yb3+: 2F5/2

Integrated areas ratio

the excitation mechanism in Er3+–Nd3+–Yb3+ triply doped glasses. Before discussing the Er3+ up-conversion luminescence mechanism in Er3+–Nd3+–Yb3+ triply doped glasses, ET from Nd3+ to Yb3+ must be examined at first. Fig. 5 shows the fluorescence spectra of EYNX (X ¼ 0, 0.2, 0.5, 0.8, 1) glasses in the 850–1300 nm wavelength range under

4I 15/2

Er3+

Fig. 6. Simplified energy-level diagram for the Er3+–Nd3+–Yb3+-triply doped glasses.

ARTICLE IN PRESS L. Lu et al. / Journal of Luminescence 126 (2007) 677–681

addition, most of Er3+ ions at 4I9/2 level relaxe rapidly and nonradiatively to the next lower levels, 4I11/2 and 4I13/2. For green emission, the next step involves the excitation processes based on the long lived Er3+: 4I11/2 level, ET, 2 F5/2 (Yb3+)+4I11/2 (Er3+)-2F7/2 (Yb3+)+4F7/2 (Er3+); cross-relaxation (CR), 4I11/2 (Er3+)+4I11/2 (Er3+)-4F7/2 (Er3+)+4I15/2 (Er3+); excited stated absorption (ESA), 4 I11/2 (Er3+)+a photon-4F3/2 (Er3+), 4I13/2 (Er3+)+a photon-4H11/2 (Er3+) CR between Er3+ and Nd3+ ions, 4 I11/2 (Er3+)+4F3/2 (Nd3+)-2H11/2 (Er3+)+4I11/2 (Nd3+), and 4I13/2 (Er3+)+4G7/2 (Nd3+)-2H11/2 (Er3+)+4I15/2 (Nd3+). The Er3+: 4I13/2 level is populated owing to the nonradiative relaxation (NR) from the upper Er3+: 4I11/2, 4I9/2 levels. And the Nd3+: 4G7/2 level population can be attributed to the combination of excited state absorption (ESA) from Nd3+: 4F3/2 and NR, as shown in Fig. 6. The populated 4F7/2 and 4F5/2 levels of Er3+ then relaxe rapidly and nonradiatively to the next lower levels, 2H11/2 and 4S3/2, resulting from the small energy gap between them. Erbium ions at the 2H11/2 level can also decay to the 4S3/2 level due to multiphonon relaxation process. The estimated energy gap between the 2 H11/2 level and the next lower level 4S3/2 is about 800 cm1 [21]. Thus, multiphonon relaxation rate is very large and the 533 nm emission intensity will be reduced. The above processes then produce the two transitions Er3+: 2H11/2 -4I15/2 and Er3+: 4S3/2-4I15/2, which centered at 533 and 549 nm, respectively. In addition, the green emission at 533 nm may partly due to the Nd3+: 4G7/2-4I9/2 transition [20]. The red emission at 662 nm is originated from the Er3+: 4F9/2-4I15/2 transition, and the population of the 4 F9/2 level are based on the following processes, ET from Yb3+, 2F5/2 (Yb3+)+4I13/2 (Er3+)-2F7/2 (Yb3+)+4F9/2 (Er3+), and CR between Er3+ ions, 4I13/2 (Er3+)+4I11/2 (Er3+)-4I15/2 (Er3+)+4F9/2 (Er3+). In addition, the nonradiative process from the Er3+: 4S3/2 level, which is populated by the processes described previously, to the Er3+: 4F9/2 level also contributes to the red emission. 4. Conclusions Up-conversion emission properties of TeO2–ZnO–Na2O glasses codoped with Nd3+, Yb3+ and Er3+ were examined under 800 nm excitation. Highly efficient green up-conversion luminescence at around 549 nm, which corresponded to Er3+: 4S3/2-4I15/2 transition, was observed in these glasses. ET from Nd3+ to Yb3+, which plays a important role in these glasses, was discussed for

681

the further analysis. The green (549 nm) emission intensity remarkably depended on the Yb2O3 concentration in Nd3+–Er3+ codoped glass. This indicated that energy could be transferred from Nd3+ to Er3+ through Yb3+, because the Yb3+ ions shortened the ET distance between Nd3+ and Er3+ ions and consequently the ET probability from Nd3+ to Er3+ was increased. Based on various spectral-analysis results, a most possible up-conversion mechanism was proposed for Nd3+–Yb3+–Er3+ triply doped tellurite glasses. Acknowledgments The work was financially supported by the Project of the Natural Science Foundation of Zhejiang Province (Grant No. Y104498), the science and technology department of Zhejiang Province (Grant Nos. 2005C31014 and 2006C21082). References [1] S.Q. Man, E.Y.B. Pun, P.S. Chung, Appl. Phys. Lett. 77 (2000) 483. [2] A.S. Oliverira, M.T. De Araujo, A.S. Gouveia Neto, A.S.B. Sombra, J.A. Medeiros Nneto, N. Aranha, J. Appl. Phys. 83 (1998) 604. [3] L.H. Huang, X.R. Liu, W. Xu, B.J. Chen, J.L. Lin, J. Appl. Phys. 90 (2001) 5550. [4] S. Xu, Z. Yang, J. Zhang, G. Wang, S. Dai, L. Hu, Z. Jiang, Chem. Phys. Lett. 385 (2004) 263. [5] Q.H. Nie, C. Jiang, X.S. Wang, T.F. Xu, H.Q. Li, Mater. Res. Bull. 41 (2006) 1496. [6] X. Shen, Q.H. Nie, T.F. Xu, Y. Gao, Spectrochim. Acta Part A 61 (2005) 2827. [7] S.-Q. Xu, J.-J. Zhang, G.-N. Wang, S.-X. Dai, L.-L. Hu, Z.-H. Jiang, Chin. Phys. Lett. 21 (5) (2004) 927. [8] X. Shen, Q. Nie, T. Xu, T. Peng, Y. Gao, Phys. Lett. A 332 (2004) 101. [9] X. Zhang, J.P. Jouart, G. Mary, J. Phys.: Condens. Matter 10 (1998) 493. [10] A. Brenier, L.C. Courrol, C. Pedrini, C. Madej, G. Boulon, Opt. Mater. 3 (1994) 25. [11] M. Yamazaki, K. Kojima, Solid State Commun. 130 (2004) 637. [12] G.M. Salley, R. Valiente, H.U. Guedel, J. Lumin. 94 (2001) 305. [13] A.S. Gouveia-Neto, E.B. da Costa, L.A. Bueno, S.J.L. Ribeiro, Y. Messaddeq, J. Lumin. 116 (2006) 52. [14] J. Qiu, M. Shojiya, Y. Kawamoto, J. Appl. Phys. 86 (1999) 909. [15] J. Qiu, Y. Kawamoto, J. Appl. Phys. 91 (2002) 954. [16] J. Qiu, M. Shojiya, Y. Kawamoto, K. Kadono, J. Lumin. 86 (2000) 23. [17] D.L. Dexter, J. Chem. Phys. 21 (1953) 836. [18] E. Nakazawa, S. Shionya, Phys. Rev. Lett. 25 (1970) 1710. [19] M.J. Weber, Phys. Rev. B 4 (1971) 3153. [20] J. Fernandez, I. Iparraguirre, R. Balda, et al., Opt. Mater. 25 (2004) 185. [21] F. Auzel, J. Lumin. 45 (1990) 341.