- Email: [email protected]
Comparison of up-converted emissions in Yb3+, Er3+ co-doped Gd2(WO4)3 and Gd2WO6 phosphors
Journal of Luminescence ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Comparison of up-converted emissions in Yb3 þ , Er3 þ co-doped Gd2(WO4)3 and Gd2WO6 phosphors M. Sun a,b,c, L. Ma b, B.J. Chen c, F. Stepongzi b, F. Liu d, Z.W. Pan d, M.K. Lei a,n, X.J. Wang b,n a
Surface Engineering Laboratory, School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China Department of Physics, Georgia Southern University, Statesboro, GA 30460, USA c Department of Physics, Dalian Maritime University, Dalian 116026, China d Department of Physics and Astronomy, The University of Georgia, Athens, GA 30602, USA b
art ic l e i nf o
Keywords: Yb3 þ Er3 þ Gadolinium tungstate Phosphor Up-conversion Infrared emission
a b s t r a c t Yb3 þ , Er3 þ co-doped Gd2(WO4)3 and Gd2WO6 phosphors have been prepared using co-precipitation method and characterized by X-ray diffraction and optical and infrared spectroscopy. Different processes of upconverted transitions have been observed in the two hosts. Strong green and red emissions have been respectively detected in Gd2(WO4)3:Yb3 þ , Er3 þ and Gd2WO6:Yb3 þ , Er3 þ upon excitation at 980 nm, suggesting that the excited state absorptions from 4I11/2 and 4I13/2 of Er3 þ dominate the excitation processes in the two systems, respectively. The different excitation processes of the upconversion are related to structural difference. The existence of near-neighbor rare earth ion-pair in Gd2WO6 may promote the depopulation of 4F7/2 through a cross relaxation of 4F7/2-4F9/2 and 4I11/2-4F9/ 2, yielding the dominating red emission in Gd2WO6. On the other hand, intense near infrared emissions have been observed from two Stark splitting groups of 4I13/2 to 4I15/2 at 1540 nm and 1490 nm, respectively, in both hosts. However, the infrared emission is dominated by the lower state at 1540 nm in Gd2WO6, resulting in an efficient ESA, due to the better energy difference matching, from the lower state to 4F9/2 that gives red emission. Concentration dependence of the intensity ratio of green to red emissions is also studied and energy transfer between Yb3 þ and Er3 þ ions analyzed. & 2013 Elsevier B.V. All rights reserved.
1. Introduction Rare earth (RE) ion doped inorganic phosphor is one of the most promising materials for a variety of applications in solidstate lasers, lighting, displays, and biolabels [1,2]. Some morphology and color controllable phosphor synthesis techniques have been reported [3–5]. Among all the luminescent materials, tungstates have attracted great attention because they are perfect hosts for accommodating RE luminescence centers and are self-activated luminescence systems [6]. Gadolinium tungstates offer low phonon energy, high photochemical stability and high quantum yield, making it one of the best candidates as host matrix for RE doped unconverted phosphors. In addition, the similar sizes of the codoped ions, Yb3 þ and Er3 þ , and host ions Gd3 þ make suitable substitutions without causing much distortion of the crystal structure. Some results in visible emissions of RE ion doped gadolinium tungstates have been recently reported [7–11]. Few reports, however, have mentioned infrared emissions. While our experiments show that infrared emissions play an important role in the upn
Corresponding authors. Tel.: þ 1 912 478 5503; fax: þ1 912 478 0471. E-mail addresses: [email protected], [email protected] (X.J. Wang).
converted processes, for better understanding of the up-converted mechanisms, it is necessary to take both visible and infrared emissions into consideration when up-converted processes are analyzed. In this paper, we report the preparation and the structural characterization of Yb3 þ , Er3 þ co-doped gadolinium tungstate Gd2(WO4)3 and Gd2WO6 phosphors, as well as up-converted luminescence properties. Different processes of up-converted transitions have been observed in the two hosts. Green and red emissions dominate the transitions in Gd2(WO4)3:Yb3 þ , Er3 þ and Gd2WO6:Yb3 þ , Er3 þ , respectively. The difference of the upconverted emissions is analyzed and attributed mainly to the difference of crystal structures. The existence of near-neighbor RE ion-pairs in Gd2WO6 promotes the depopulation of 4F7/2 through a cross relaxation of 4F7/2-4F9/2 and 4I11/2-4F9/2, yielding the dominating red emission in Gd2WO6. Infrared emissions from 4 I13/2 to 4I15/2 in both phosphors are observed and taken into consideration in analyzing the up-converted emission processes. Stark splittings of 4I13/2 state and their electron populations in the two systems also affect the green-to-red emission ratios. The dependence of the intensity ratios on excitation power and RE concentrations are investigated and their behaviors are consistent with the argument of the cross relaxation of Er3 þ pairs.
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.10.047
Please cite this article as: M. Sun, et al., J. Lumin. (2013), http://dx.doi.org/10.1016/j.jlumin.2013.10.047i
M. Sun et al. / Journal of Luminescence ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
2. Experimental Analytically pure grade reagents (99.99%) Na2WO4 2H2O, GaCl3 6H2O, Er2O3 and Yb2O3 are used to prepare samples. The starting materials ErCl3 and YbCl3 are obtained via a recrystallization process. First, the raw reagents Er2O3 and Yb2O3 are respectively dissolved in hot hydrochloric acid solutions (1 mol/L), followed by the evaporation of excessive hydrogen chloride and water. The product is then dissolved into ID water and heated to evaporate off extra water again to realize recrystallization. After the material has been re-crystallized for five times, ErCl3 6H2O and YbCl3 6H2O are obtained. Finally, they are stored in the form of aqueous resolution with the concentration of 0.4 mol/L. Yb3 þ , Er3 þ co-doped Gd2(WO4)3 phosphors with various codoped concentration are prepared using co-precipitation method. 10 ml pre-blended solution of GdCl3 6H2O, ErCl3 6H2O and YbCl3 6H2O are added into 30 ml Na2WO4 aqueous solution drop by drop with stirring. Stirring continues for 30 min after adding these drops to ensure completion of the reaction. The precipitate suspended in aqueous solution is separated from water with a centrifugal machine at the speed of 6000 rpm. After washing the precipitate for three times with deionized water, it is finally dried in an oven at 90 1C for 3 h. The co-doped Gd2(WO4)3:Yb3 þ , Er3 þ crystals are obtained after the precursor is sintered at 900 1C for 4 h. The process of the co-doped Gd2WO6:Yb3 þ , Er3 þ phosphor preparation is similar to that mentioned above except that a certain amount of sodium hydroxide (0.1 mol/L) is required for the reaction, according to the reactant ratio in the chemical equation. To verify either the amorphous or crystalline state of the samples, X-ray diffraction is performed on a Phillips PW3020 diffractometer (Voltage 40 kV, current 30 mA, Cu Ka) with a step width of 0.021. The XRD patterns of the samples are collected in the range of 101o 2θ o901. All upconversion luminescence measurements are performed using an SPEX FLuoroMax III spectrofluorometer. A JDS Uniphase 980 nm current controlled laser diode (device type 63-00342) with maximum power of 400 mW is used as an excitation source for up-converted emissions.
3. Results and discussion 3.1. Crystallization structure Fig. 1(a) and (c) present the X-ray diffraction patterns for Gd2(WO4)3 and Gd2WO6 phosphors doped with 5 mol% Er3 þ and 10 mol% Yb3 þ , whose diffraction peaks are clearly indexed to pure-phases Gd2(WO4)3 and Gd2WO6 and aligned with standard JCPDS card with NO. 23-1076 and NO. 78-1704, respectively, as shown in Fig. 1(b) and (d). It indicates that erbium and ytterbium ions replace the position of gadolinium without much distortion of crystal structure, which is due to the similar ion sizes of the co-doped ions, Yb3 þ and Er3 þ , to the host ion Gd3 þ . Both of them belong to the monoclinic crystal system with the space group of C12/c1, but the cell parameters are different, a, b, and c, are 7.660 Å, 11.42 Å, and 11.401 Å for Gd2(WO4)3, and 16.38 Å, 11.15 Å, and 5.420 Å for Gd2WO6, respectively. The different structures and chemical compositions yield different phonon distributions [9,12]. 3.2. Spectral analysis of up-converted emissions The excitation spectra of Gd2(WO4)3:Yb3 þ , Er3 þ and Gd2WO6: Yb , Er3 þ monitoring 1527 nm emission and the emission spectra under excitation at 980 nm are depicted in Fig. 2. For the excitation spectra in both hosts, the bands peaking at 370 nm and 3þ
Fig. 1. XRD patterns of the studied phosphor samples. (a) Gd2(WO4)3:Yb3 þ , Er3 þ , (c) Gd2WO6:Yb3 þ , Er3 þ and their corresponding comparison with standard JCPDS cards, as shown in (b) and (d), respectively.
520 nm are attributed to 4I15/2-4G11/2 and 4I15/2-2H11/2 transitions of Er3 þ ions, respectively. The strong excitations at 250 nm are the inter-band transitions of the hosts. Excitations of near defect excitons (NDE) around RE3 þ defects and O–Er charge transfer transitions are also observed as shoulders at 270 nm and 310 nm, respectively [13]. Dominant green and red emissions are respectively observed in Gd2(WO4)3 and Gd2WO6 as shown in Fig. 2(b). Green (521 nm and 545 nm) and red (667 nm) emissions correspond to 2H11/2, 4S3/2, and 4F9/2 to ground state 4I15/2 transitions, respectively. Infrared emissions are also detected in both hosts. The emissions centered at 1490 nm and 1540 nm are due to the relaxations from two Stark splitting groups of 4I13/2 state (levels I and II as labeled in Fig. 3) to the ground state 4I15/2, respectively. In host Gd2(WO4)3, the two emissions are of the same strength, however, 1540 nm emission is much stronger than the emission at 1490 nm in host Gd2WO6. It is generally accepted that the up-converted emissions of Er3 þ are mainly two photon processes. First, ground state absorption of Yb3 þ from 2F7/2-2F5/2 occurs, followed by an energy transfer from Yb3 þ to 4I11/2 of Er3 þ . The excited state absorption (ESA) of Er3 þ from 4I11/2-4S3/2, 2H11/2 yields the green up-converted emissions (4S3/2, 2H11/2-4I15/2 transitions) as shown in Fig. 3. For red emission, the non-radiative relaxation from 4I11/2-4I13/2 is an important step after Yb3 þ to Er3 þ energy transfer, followed by the ESA of Er3 þ from 4I13/2- 4F9/2. 4F9/2-4I15/2 transition results in the red emission. The intense infrared emissions from 4I13/2-4I15/2 were observed in both hosts, as shown in Fig. 2(b), indicating that the non-radiative relaxation 4I11/2-4I13/2 is not the key factor for the different emissions from the two hosts. The structural difference may determine the emission selectivity. Both hosts belong to the monoclinic crystal system, however, Gd cations in Gd2(WO4)3
Please cite this article as: M. Sun, et al., J. Lumin. (2013), http://dx.doi.org/10.1016/j.jlumin.2013.10.047i
M. Sun et al. / Journal of Luminescence ∎ (∎∎∎∎) ∎∎∎–∎∎∎
3
and (4I11/2-4F9/2) can significantly reduce the green emission but enhance the red emission in Gd2WO6:Yb3 þ , Er3 þ . This effect has been observed and discussed in NaRF4:Yb3 þ , Er3 þ (R¼ Y, Lu) systems [16]. In addition, different Stark splittings of 4I13/2 state in the two hosts may also contribute to the emission selectivity. As discussed earlier, the intensity of 1540 nm emission is much stronger than the emission at 1490 nm in Gd2WO6 host, suggesting a greater electron population at the lower level whose difference to 4 F9/2 matches the 980 nm excitation or the 2F5/2-2F7/2 transition of Yb3 þ better than the higher one ( 200 cm 1 difference). As a consequence, red emission in Gd2WO6:Yb3 þ , Er3 þ becomes stronger. 3.3. Dependence of intensity ratio of green to red on doped RE concentration
Fig. 2. (a) Excitation spectra monitoring at 1527 nm and (b) up-converted emission spectra under 980 nm excitation for both Gd2(WO4)3:Yb3 þ , Er3 þ and Gd2WO6: Yb3 þ , Er3 þ . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Intensity ratio of green to red emissions, RG/R, is observed to increase with increasing excitation power (or laser current as given in the figure) in both hosts for different concentration, as shown in Fig. 4. That is due to the fact that three photon process starts to be involved for green up-converted emission as excitation power increases [17]. Fig. 4 also indicates the difference of RE concentration dependence of the ratios from the two hosts. Fig. 4 (a) gives the effects of Er3 þ concentration on RG/R. For the fixed concentration of Yb3 þ (10%), the ratio remains nearly the same when Er3 þ increases from 5 mol% to 15 mol% for Gd2(WO4)3:Yb3 þ , Er3 þ since the cross relaxation, discussed in last section, is still weak when Er3 þ concentration increases 3 times (minimum distance of Er3 þ pairs just reduces by a factor of 1.44). In case of Gd2WO6:Yb3 þ , Er3 þ , increase of Er3 þ concentration enhances the cross relaxation of Er3 þ pairs, which benefits the red emission and substantially reduces the RG/R. Fig. 4(b) illustrates the effects of Yb3 þ concentration on RG/R with fixed concentration of Er3 þ (5%). Increase of Yb3 þ concentration, to a certain extent, is equivalent to the increase of excitation power for up-converted emission, which benefits the three photon process and green emission. As a result, RG/R increases substantially as Yb3 þ increases from 5 mol% to 20 mol% in Gd2(WO4)3:Yb3 þ , Er3 þ . For Gd2WO6:Yb3 þ , Er3 þ , since the average distance of RE ions is much smaller, concentration quenching and energy transfer from Er3 þ to Yb3 þ may take place [18] at higher concentration of Yb3 þ . As a consequence, RG/R decreases at higher Yb3 þ concentration and remains nearly unchanged as excitation power increases. It is also worth noting that RG/R can be as high as 360 in host Gd2(WO4)3 and as low as 0.14 in host Gd2WO6. The color control from green to red can be realized by adjusting the doping concentration of Yb3 þ and Er3 þ in the two phosphors.
4. Conclusions
Fig. 3. Simplified energy level diagram of Yb3 þ and Er3 þ and up and down conversion processes of Yb3 þ , Er3 þ co-doped gadolinium tungstate under 980 nm excitation. 4I13/2 (I) and 4I13/2 (II) indicate the two Stark splitting groups of 4I13/2 (the separation is not in scale). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
are coordinated by eight O atoms and exist only a single type of position and the closest distance between the cations is 40.3 nm [14]. In case of Gd2WO6, there are three types of positions occupied by the Gd cations with minimum Gd pair distance o0.1 nm [15]. This near-neighbor pair structure could promote cross relaxation process of substituting Er3 þ ions to enhance the red emission. As shown in Fig. 3, cross relaxation of (4F7/2-4F9/2)
Co-doped Gd2(WO4)3:Yb3 þ , Er3 þ and Gd2WO6:Yb3 þ , Er3 þ phosphors have been prepared and their optical and infrared properties investigated through up-converted emissions. Dominant green and red emissions are found in Gd2(WO4)3:Yb3 þ , Er3 þ and Gd2WO6:Yb3 þ , Er3 þ , respectively, upon excitation at 980 nm. The emission preference can be ascribed to the structural difference that gives the different average distance of Er3 þ pairs and their corresponding cross relaxations of the pairs. Intense infrared emissions from 4I13/2-4I15/2 have been detected in both phosphors and the different Stark splittings of 4I13/2 may affect the visible emissions due to the energy difference matching of the ESA from Stark splitting levels. The ratio RG/R in both hosts are measured and their dependence on excitation power and RE concentration supports the argument of the cross relaxation of Er3 þ pairs.
Please cite this article as: M. Sun, et al., J. Lumin. (2013), http://dx.doi.org/10.1016/j.jlumin.2013.10.047i
M. Sun et al. / Journal of Luminescence ∎ (∎∎∎∎) ∎∎∎–∎∎∎
4
support from the National Natural Science Foundation of China (No. 11374044 and No. 21276036), Scientific Research Fund of Liaoning Provincial Education Department (No. L2012712), Jilin Provincial Science and Technology Department Foundation Grant 201101038 and the Fundamental Research Funds for the Central Universities (No. 2012TD018 and No. 3132013337).
References
Fig. 4. RE concentration dependence of intensity ratio of green to red emissions, RG/R, in both Gd2(WO4)3:Yb3 þ , Er 3 þ and Gd2WO6:Yb3 þ , Er 3 þ upon 980 nm excitation. (a) Er3 þ concentration changes from 5 mol% to 15 mol% when the Yb3 þ is fixed at 10 mol% and (b) Yb3 þ concentration changes from 5 mol% to 20 mol% when the Er3 þ is fixed at 5 mol%. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
[1] W.O. Gordon, J.A. Carter, B.M. Tissue, J. Lumin. 108 (2004) 339. [2] G.K. Liu, X.Y. Chen, Spectroscopic properties of lanthanides in nanomaterials, in: K.A. Gschneidner Jr., J.C.G. Bunzli, V.K. Pecharsky (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 37, North-Holland, Amsterdam, 2007, p. 99. (Chapter 233). [3] Z.Y. Hou, Z.Y. Chen, G.G. Li, W.X. Wang, C. Peng, C.X. Li, P.A. Ma, D.M. Yang, X.J. Kang, J. Lin, Nanoscale 3 (2011) 1568. [4] Y.B. Zeng, Z.Q. Li, L.M. Wang, Y.J. Xiong, Cryst. Eng. Comm. 14 (2012) 7043. [5] H. Zheng, X. Wang, M.J. Dejneka, W.M. Yen, R.S. Meltzer, J. Lumin. 108 (2004) 395. [6] W.S. Wang, L. Zhen, C.Y. Xu, L. Yang, W.Z. Shao, J. Phys. Chem. C 112 (2008) 19390. [7] J.S. Sun, T.T. Yu, X.P. Li, J.S. Zhang, L.H. Cheng, H.Y. Zhong, Y. Tian, R.N. Hua, B. J. Chen, J. Phys. Chem. Solids 73 (2012) 465. [8] T.T. Yu, J.S. Sun, R.N. Hua, L.H. Cheng, H.Y. Zhong, X.P. Li, H.Q. Yu, B.J. Chen, J. Alloys Compd 509 (2011) 391. [9] F. Lei, B. Yan, H.H. Chen, J. Solid State Chem. 181 (2008) 2845. [10] E. Tomaszewicz, A. Worsztynowicz, S.M. Kaczmarek, Solid State Sci. 9 (2007) 43. [11] Z.Y. Hou, C.X. Li, J. Yang, H.Z. Lian, P.P. Yang, R.T. Chai, Z.Y. Cheng, J. Lin, J. Mater. Chem. 19 (2009) 2737. [12] B. Grobelna, M. Szabelski, K. Kledzik, A.M. Kłonkowski, J. Non-Cryst. Solids 353 (2007) 2861. [13] P. Dorenbos, E.V.D. van Loef, A.P. Vink, E. van der Kolk, C.W.E. van Eijk, K.W. Kramerc, H.U. Gudelc, W.M. Higginsb, K.S. Shah, J. Lumin 117 (2006) 147. [14] M. Weil, B. Stöger, L. Aleksandrov, Acta Crystallogr E65 (2009) i45. [15] G. Blasse, J. Chem. Phys. 45 (1966) 2356. [16] J. Wang, H.W. Song, W. Xu, B. Dong, S. Xu, B.T. Chen, W. Yu, S. Zhang, Nanoscale 5 (2013) 3412. [17] T.T. Yu, The Synthesis and Luminescence Properties of Rare Earth Ions Doped Gadolinium Tungstate Nano-Phosphors, Ph.D. Thesis, Dalian Maritime University, 2011. [18] F. Vetrone, J.C. Boyer, J.A. Capobianco, A. Speghini, M. Bettinelli, J. Phys. Chem. B 107 (2003) 1107.
Acknowledgements The authors thank the support from the department of physics, Georgia Southern University. MS is grateful for the financial
Please cite this article as: M. Sun, et al., J. Lumin. (2013), http://dx.doi.org/10.1016/j.jlumin.2013.10.047i
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Comparison of up-converted emissions in Yb3 þ , Er3 þ co-doped Gd2(WO4)3 and Gd2WO6 phosphors M. Sun a,b,c, L. Ma b, B.J. Chen c, F. Stepongzi b, F. Liu d, Z.W. Pan d, M.K. Lei a,n, X.J. Wang b,n a
Surface Engineering Laboratory, School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China Department of Physics, Georgia Southern University, Statesboro, GA 30460, USA c Department of Physics, Dalian Maritime University, Dalian 116026, China d Department of Physics and Astronomy, The University of Georgia, Athens, GA 30602, USA b
art ic l e i nf o
Keywords: Yb3 þ Er3 þ Gadolinium tungstate Phosphor Up-conversion Infrared emission
a b s t r a c t Yb3 þ , Er3 þ co-doped Gd2(WO4)3 and Gd2WO6 phosphors have been prepared using co-precipitation method and characterized by X-ray diffraction and optical and infrared spectroscopy. Different processes of upconverted transitions have been observed in the two hosts. Strong green and red emissions have been respectively detected in Gd2(WO4)3:Yb3 þ , Er3 þ and Gd2WO6:Yb3 þ , Er3 þ upon excitation at 980 nm, suggesting that the excited state absorptions from 4I11/2 and 4I13/2 of Er3 þ dominate the excitation processes in the two systems, respectively. The different excitation processes of the upconversion are related to structural difference. The existence of near-neighbor rare earth ion-pair in Gd2WO6 may promote the depopulation of 4F7/2 through a cross relaxation of 4F7/2-4F9/2 and 4I11/2-4F9/ 2, yielding the dominating red emission in Gd2WO6. On the other hand, intense near infrared emissions have been observed from two Stark splitting groups of 4I13/2 to 4I15/2 at 1540 nm and 1490 nm, respectively, in both hosts. However, the infrared emission is dominated by the lower state at 1540 nm in Gd2WO6, resulting in an efficient ESA, due to the better energy difference matching, from the lower state to 4F9/2 that gives red emission. Concentration dependence of the intensity ratio of green to red emissions is also studied and energy transfer between Yb3 þ and Er3 þ ions analyzed. & 2013 Elsevier B.V. All rights reserved.
1. Introduction Rare earth (RE) ion doped inorganic phosphor is one of the most promising materials for a variety of applications in solidstate lasers, lighting, displays, and biolabels [1,2]. Some morphology and color controllable phosphor synthesis techniques have been reported [3–5]. Among all the luminescent materials, tungstates have attracted great attention because they are perfect hosts for accommodating RE luminescence centers and are self-activated luminescence systems [6]. Gadolinium tungstates offer low phonon energy, high photochemical stability and high quantum yield, making it one of the best candidates as host matrix for RE doped unconverted phosphors. In addition, the similar sizes of the codoped ions, Yb3 þ and Er3 þ , and host ions Gd3 þ make suitable substitutions without causing much distortion of the crystal structure. Some results in visible emissions of RE ion doped gadolinium tungstates have been recently reported [7–11]. Few reports, however, have mentioned infrared emissions. While our experiments show that infrared emissions play an important role in the upn
Corresponding authors. Tel.: þ 1 912 478 5503; fax: þ1 912 478 0471. E-mail addresses: [email protected], [email protected] (X.J. Wang).
converted processes, for better understanding of the up-converted mechanisms, it is necessary to take both visible and infrared emissions into consideration when up-converted processes are analyzed. In this paper, we report the preparation and the structural characterization of Yb3 þ , Er3 þ co-doped gadolinium tungstate Gd2(WO4)3 and Gd2WO6 phosphors, as well as up-converted luminescence properties. Different processes of up-converted transitions have been observed in the two hosts. Green and red emissions dominate the transitions in Gd2(WO4)3:Yb3 þ , Er3 þ and Gd2WO6:Yb3 þ , Er3 þ , respectively. The difference of the upconverted emissions is analyzed and attributed mainly to the difference of crystal structures. The existence of near-neighbor RE ion-pairs in Gd2WO6 promotes the depopulation of 4F7/2 through a cross relaxation of 4F7/2-4F9/2 and 4I11/2-4F9/2, yielding the dominating red emission in Gd2WO6. Infrared emissions from 4 I13/2 to 4I15/2 in both phosphors are observed and taken into consideration in analyzing the up-converted emission processes. Stark splittings of 4I13/2 state and their electron populations in the two systems also affect the green-to-red emission ratios. The dependence of the intensity ratios on excitation power and RE concentrations are investigated and their behaviors are consistent with the argument of the cross relaxation of Er3 þ pairs.
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.10.047
Please cite this article as: M. Sun, et al., J. Lumin. (2013), http://dx.doi.org/10.1016/j.jlumin.2013.10.047i
M. Sun et al. / Journal of Luminescence ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
2. Experimental Analytically pure grade reagents (99.99%) Na2WO4 2H2O, GaCl3 6H2O, Er2O3 and Yb2O3 are used to prepare samples. The starting materials ErCl3 and YbCl3 are obtained via a recrystallization process. First, the raw reagents Er2O3 and Yb2O3 are respectively dissolved in hot hydrochloric acid solutions (1 mol/L), followed by the evaporation of excessive hydrogen chloride and water. The product is then dissolved into ID water and heated to evaporate off extra water again to realize recrystallization. After the material has been re-crystallized for five times, ErCl3 6H2O and YbCl3 6H2O are obtained. Finally, they are stored in the form of aqueous resolution with the concentration of 0.4 mol/L. Yb3 þ , Er3 þ co-doped Gd2(WO4)3 phosphors with various codoped concentration are prepared using co-precipitation method. 10 ml pre-blended solution of GdCl3 6H2O, ErCl3 6H2O and YbCl3 6H2O are added into 30 ml Na2WO4 aqueous solution drop by drop with stirring. Stirring continues for 30 min after adding these drops to ensure completion of the reaction. The precipitate suspended in aqueous solution is separated from water with a centrifugal machine at the speed of 6000 rpm. After washing the precipitate for three times with deionized water, it is finally dried in an oven at 90 1C for 3 h. The co-doped Gd2(WO4)3:Yb3 þ , Er3 þ crystals are obtained after the precursor is sintered at 900 1C for 4 h. The process of the co-doped Gd2WO6:Yb3 þ , Er3 þ phosphor preparation is similar to that mentioned above except that a certain amount of sodium hydroxide (0.1 mol/L) is required for the reaction, according to the reactant ratio in the chemical equation. To verify either the amorphous or crystalline state of the samples, X-ray diffraction is performed on a Phillips PW3020 diffractometer (Voltage 40 kV, current 30 mA, Cu Ka) with a step width of 0.021. The XRD patterns of the samples are collected in the range of 101o 2θ o901. All upconversion luminescence measurements are performed using an SPEX FLuoroMax III spectrofluorometer. A JDS Uniphase 980 nm current controlled laser diode (device type 63-00342) with maximum power of 400 mW is used as an excitation source for up-converted emissions.
3. Results and discussion 3.1. Crystallization structure Fig. 1(a) and (c) present the X-ray diffraction patterns for Gd2(WO4)3 and Gd2WO6 phosphors doped with 5 mol% Er3 þ and 10 mol% Yb3 þ , whose diffraction peaks are clearly indexed to pure-phases Gd2(WO4)3 and Gd2WO6 and aligned with standard JCPDS card with NO. 23-1076 and NO. 78-1704, respectively, as shown in Fig. 1(b) and (d). It indicates that erbium and ytterbium ions replace the position of gadolinium without much distortion of crystal structure, which is due to the similar ion sizes of the co-doped ions, Yb3 þ and Er3 þ , to the host ion Gd3 þ . Both of them belong to the monoclinic crystal system with the space group of C12/c1, but the cell parameters are different, a, b, and c, are 7.660 Å, 11.42 Å, and 11.401 Å for Gd2(WO4)3, and 16.38 Å, 11.15 Å, and 5.420 Å for Gd2WO6, respectively. The different structures and chemical compositions yield different phonon distributions [9,12]. 3.2. Spectral analysis of up-converted emissions The excitation spectra of Gd2(WO4)3:Yb3 þ , Er3 þ and Gd2WO6: Yb , Er3 þ monitoring 1527 nm emission and the emission spectra under excitation at 980 nm are depicted in Fig. 2. For the excitation spectra in both hosts, the bands peaking at 370 nm and 3þ
Fig. 1. XRD patterns of the studied phosphor samples. (a) Gd2(WO4)3:Yb3 þ , Er3 þ , (c) Gd2WO6:Yb3 þ , Er3 þ and their corresponding comparison with standard JCPDS cards, as shown in (b) and (d), respectively.
520 nm are attributed to 4I15/2-4G11/2 and 4I15/2-2H11/2 transitions of Er3 þ ions, respectively. The strong excitations at 250 nm are the inter-band transitions of the hosts. Excitations of near defect excitons (NDE) around RE3 þ defects and O–Er charge transfer transitions are also observed as shoulders at 270 nm and 310 nm, respectively [13]. Dominant green and red emissions are respectively observed in Gd2(WO4)3 and Gd2WO6 as shown in Fig. 2(b). Green (521 nm and 545 nm) and red (667 nm) emissions correspond to 2H11/2, 4S3/2, and 4F9/2 to ground state 4I15/2 transitions, respectively. Infrared emissions are also detected in both hosts. The emissions centered at 1490 nm and 1540 nm are due to the relaxations from two Stark splitting groups of 4I13/2 state (levels I and II as labeled in Fig. 3) to the ground state 4I15/2, respectively. In host Gd2(WO4)3, the two emissions are of the same strength, however, 1540 nm emission is much stronger than the emission at 1490 nm in host Gd2WO6. It is generally accepted that the up-converted emissions of Er3 þ are mainly two photon processes. First, ground state absorption of Yb3 þ from 2F7/2-2F5/2 occurs, followed by an energy transfer from Yb3 þ to 4I11/2 of Er3 þ . The excited state absorption (ESA) of Er3 þ from 4I11/2-4S3/2, 2H11/2 yields the green up-converted emissions (4S3/2, 2H11/2-4I15/2 transitions) as shown in Fig. 3. For red emission, the non-radiative relaxation from 4I11/2-4I13/2 is an important step after Yb3 þ to Er3 þ energy transfer, followed by the ESA of Er3 þ from 4I13/2- 4F9/2. 4F9/2-4I15/2 transition results in the red emission. The intense infrared emissions from 4I13/2-4I15/2 were observed in both hosts, as shown in Fig. 2(b), indicating that the non-radiative relaxation 4I11/2-4I13/2 is not the key factor for the different emissions from the two hosts. The structural difference may determine the emission selectivity. Both hosts belong to the monoclinic crystal system, however, Gd cations in Gd2(WO4)3
Please cite this article as: M. Sun, et al., J. Lumin. (2013), http://dx.doi.org/10.1016/j.jlumin.2013.10.047i
M. Sun et al. / Journal of Luminescence ∎ (∎∎∎∎) ∎∎∎–∎∎∎
3
and (4I11/2-4F9/2) can significantly reduce the green emission but enhance the red emission in Gd2WO6:Yb3 þ , Er3 þ . This effect has been observed and discussed in NaRF4:Yb3 þ , Er3 þ (R¼ Y, Lu) systems [16]. In addition, different Stark splittings of 4I13/2 state in the two hosts may also contribute to the emission selectivity. As discussed earlier, the intensity of 1540 nm emission is much stronger than the emission at 1490 nm in Gd2WO6 host, suggesting a greater electron population at the lower level whose difference to 4 F9/2 matches the 980 nm excitation or the 2F5/2-2F7/2 transition of Yb3 þ better than the higher one ( 200 cm 1 difference). As a consequence, red emission in Gd2WO6:Yb3 þ , Er3 þ becomes stronger. 3.3. Dependence of intensity ratio of green to red on doped RE concentration
Fig. 2. (a) Excitation spectra monitoring at 1527 nm and (b) up-converted emission spectra under 980 nm excitation for both Gd2(WO4)3:Yb3 þ , Er3 þ and Gd2WO6: Yb3 þ , Er3 þ . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Intensity ratio of green to red emissions, RG/R, is observed to increase with increasing excitation power (or laser current as given in the figure) in both hosts for different concentration, as shown in Fig. 4. That is due to the fact that three photon process starts to be involved for green up-converted emission as excitation power increases [17]. Fig. 4 also indicates the difference of RE concentration dependence of the ratios from the two hosts. Fig. 4 (a) gives the effects of Er3 þ concentration on RG/R. For the fixed concentration of Yb3 þ (10%), the ratio remains nearly the same when Er3 þ increases from 5 mol% to 15 mol% for Gd2(WO4)3:Yb3 þ , Er3 þ since the cross relaxation, discussed in last section, is still weak when Er3 þ concentration increases 3 times (minimum distance of Er3 þ pairs just reduces by a factor of 1.44). In case of Gd2WO6:Yb3 þ , Er3 þ , increase of Er3 þ concentration enhances the cross relaxation of Er3 þ pairs, which benefits the red emission and substantially reduces the RG/R. Fig. 4(b) illustrates the effects of Yb3 þ concentration on RG/R with fixed concentration of Er3 þ (5%). Increase of Yb3 þ concentration, to a certain extent, is equivalent to the increase of excitation power for up-converted emission, which benefits the three photon process and green emission. As a result, RG/R increases substantially as Yb3 þ increases from 5 mol% to 20 mol% in Gd2(WO4)3:Yb3 þ , Er3 þ . For Gd2WO6:Yb3 þ , Er3 þ , since the average distance of RE ions is much smaller, concentration quenching and energy transfer from Er3 þ to Yb3 þ may take place [18] at higher concentration of Yb3 þ . As a consequence, RG/R decreases at higher Yb3 þ concentration and remains nearly unchanged as excitation power increases. It is also worth noting that RG/R can be as high as 360 in host Gd2(WO4)3 and as low as 0.14 in host Gd2WO6. The color control from green to red can be realized by adjusting the doping concentration of Yb3 þ and Er3 þ in the two phosphors.
4. Conclusions
Fig. 3. Simplified energy level diagram of Yb3 þ and Er3 þ and up and down conversion processes of Yb3 þ , Er3 þ co-doped gadolinium tungstate under 980 nm excitation. 4I13/2 (I) and 4I13/2 (II) indicate the two Stark splitting groups of 4I13/2 (the separation is not in scale). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
are coordinated by eight O atoms and exist only a single type of position and the closest distance between the cations is 40.3 nm [14]. In case of Gd2WO6, there are three types of positions occupied by the Gd cations with minimum Gd pair distance o0.1 nm [15]. This near-neighbor pair structure could promote cross relaxation process of substituting Er3 þ ions to enhance the red emission. As shown in Fig. 3, cross relaxation of (4F7/2-4F9/2)
Co-doped Gd2(WO4)3:Yb3 þ , Er3 þ and Gd2WO6:Yb3 þ , Er3 þ phosphors have been prepared and their optical and infrared properties investigated through up-converted emissions. Dominant green and red emissions are found in Gd2(WO4)3:Yb3 þ , Er3 þ and Gd2WO6:Yb3 þ , Er3 þ , respectively, upon excitation at 980 nm. The emission preference can be ascribed to the structural difference that gives the different average distance of Er3 þ pairs and their corresponding cross relaxations of the pairs. Intense infrared emissions from 4I13/2-4I15/2 have been detected in both phosphors and the different Stark splittings of 4I13/2 may affect the visible emissions due to the energy difference matching of the ESA from Stark splitting levels. The ratio RG/R in both hosts are measured and their dependence on excitation power and RE concentration supports the argument of the cross relaxation of Er3 þ pairs.
Please cite this article as: M. Sun, et al., J. Lumin. (2013), http://dx.doi.org/10.1016/j.jlumin.2013.10.047i
M. Sun et al. / Journal of Luminescence ∎ (∎∎∎∎) ∎∎∎–∎∎∎
4
support from the National Natural Science Foundation of China (No. 11374044 and No. 21276036), Scientific Research Fund of Liaoning Provincial Education Department (No. L2012712), Jilin Provincial Science and Technology Department Foundation Grant 201101038 and the Fundamental Research Funds for the Central Universities (No. 2012TD018 and No. 3132013337).
References
Fig. 4. RE concentration dependence of intensity ratio of green to red emissions, RG/R, in both Gd2(WO4)3:Yb3 þ , Er 3 þ and Gd2WO6:Yb3 þ , Er 3 þ upon 980 nm excitation. (a) Er3 þ concentration changes from 5 mol% to 15 mol% when the Yb3 þ is fixed at 10 mol% and (b) Yb3 þ concentration changes from 5 mol% to 20 mol% when the Er3 þ is fixed at 5 mol%. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
[1] W.O. Gordon, J.A. Carter, B.M. Tissue, J. Lumin. 108 (2004) 339. [2] G.K. Liu, X.Y. Chen, Spectroscopic properties of lanthanides in nanomaterials, in: K.A. Gschneidner Jr., J.C.G. Bunzli, V.K. Pecharsky (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 37, North-Holland, Amsterdam, 2007, p. 99. (Chapter 233). [3] Z.Y. Hou, Z.Y. Chen, G.G. Li, W.X. Wang, C. Peng, C.X. Li, P.A. Ma, D.M. Yang, X.J. Kang, J. Lin, Nanoscale 3 (2011) 1568. [4] Y.B. Zeng, Z.Q. Li, L.M. Wang, Y.J. Xiong, Cryst. Eng. Comm. 14 (2012) 7043. [5] H. Zheng, X. Wang, M.J. Dejneka, W.M. Yen, R.S. Meltzer, J. Lumin. 108 (2004) 395. [6] W.S. Wang, L. Zhen, C.Y. Xu, L. Yang, W.Z. Shao, J. Phys. Chem. C 112 (2008) 19390. [7] J.S. Sun, T.T. Yu, X.P. Li, J.S. Zhang, L.H. Cheng, H.Y. Zhong, Y. Tian, R.N. Hua, B. J. Chen, J. Phys. Chem. Solids 73 (2012) 465. [8] T.T. Yu, J.S. Sun, R.N. Hua, L.H. Cheng, H.Y. Zhong, X.P. Li, H.Q. Yu, B.J. Chen, J. Alloys Compd 509 (2011) 391. [9] F. Lei, B. Yan, H.H. Chen, J. Solid State Chem. 181 (2008) 2845. [10] E. Tomaszewicz, A. Worsztynowicz, S.M. Kaczmarek, Solid State Sci. 9 (2007) 43. [11] Z.Y. Hou, C.X. Li, J. Yang, H.Z. Lian, P.P. Yang, R.T. Chai, Z.Y. Cheng, J. Lin, J. Mater. Chem. 19 (2009) 2737. [12] B. Grobelna, M. Szabelski, K. Kledzik, A.M. Kłonkowski, J. Non-Cryst. Solids 353 (2007) 2861. [13] P. Dorenbos, E.V.D. van Loef, A.P. Vink, E. van der Kolk, C.W.E. van Eijk, K.W. Kramerc, H.U. Gudelc, W.M. Higginsb, K.S. Shah, J. Lumin 117 (2006) 147. [14] M. Weil, B. Stöger, L. Aleksandrov, Acta Crystallogr E65 (2009) i45. [15] G. Blasse, J. Chem. Phys. 45 (1966) 2356. [16] J. Wang, H.W. Song, W. Xu, B. Dong, S. Xu, B.T. Chen, W. Yu, S. Zhang, Nanoscale 5 (2013) 3412. [17] T.T. Yu, The Synthesis and Luminescence Properties of Rare Earth Ions Doped Gadolinium Tungstate Nano-Phosphors, Ph.D. Thesis, Dalian Maritime University, 2011. [18] F. Vetrone, J.C. Boyer, J.A. Capobianco, A. Speghini, M. Bettinelli, J. Phys. Chem. B 107 (2003) 1107.
Acknowledgements The authors thank the support from the department of physics, Georgia Southern University. MS is grateful for the financial
Please cite this article as: M. Sun, et al., J. Lumin. (2013), http://dx.doi.org/10.1016/j.jlumin.2013.10.047i