Er3+-doped Sr2ScF7

Journal of Luminescence 192 (2017) 385–387

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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Spectroscopic and pump power dependent color-tunable upconversion studies of Yb3+/Er3+-doped Sr2ScF7 ⁎

Yuanyuan Zhang, Lefu Mei , Haikun Liu, Zhaohui Huang

MARK

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Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Pump power Luminescence properties Phosphors Upconversion Sr2ScF7

Er3+ and/or Yb3+ doped Sr2ScF7 up-conversion (UC) phosphors were synthesized, and the optimum doping concentrations of Yb3+ and Er3+ in the Sr2ScF7 host were found to be 20% mol and 7% mol, respectively. Under excitation of 980 nm laser, the UC spectra of the samples is composed of three green emission bands from 510 to 570 nm centered at 525, 543 and 551 nm and two red emission band from 640 to 690 nm with two peaks at 657 and 671 nm, which is attributed to the 2H11/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+, respectively. The UC emission color of Er3+ can be tuned by adjusting the intensity ratio of red to green emission through manipulating the population of red and green emitting states.

1. Introduction In recent years, rare earth (RE) doped up-conversion(UC) luminescence materials have drawn a lot of attentions owing to its significant potential application such as solid-state lasers, solar cells, optical communications, and biological fluorescence labels [1–3]. As a spectral modification material, UC phosphors show importance for converting photons with low energy to those of high energy by “merging” low energy photons [4–7]. Generally, oxides, fluorides, chlorides, and bromides are used as the host crystal for UC luminescence materials. Small phonon energies and refractive indices make fluorides attractive hosts for a variety of luminescent processes [8–13]. It is well known that, as the host for the luminescent RE, fluoride is preferable over other compounds mainly for the low phonon energy to avoid non-radiative transition of RE ions. The UC emission intensity is not only influenced by the host lattice but also depends on the interaction between host lattice and RE3+ activator ions. Therefore, mostly of RE3+ ions, Sc3+, Y3+ and Lu3+ ions are selected as host lattice materials to get efficient UC emission due to the absence of unpaired 4f electrons [14–18]. As part of our interests in studying the optical and structural characteristics of compounds containing the ion Sc3+, the inorganic compound Sr2ScF7 was used to study as host material owing to its stable physical and chemical properties. In this structure, it belongs to the monoclinic symmetrical system with a space group of P21/c and cell parameters a = 5.450 Å, b = 12.190 Å and c = 8.236 Å,β = 89.530, Z = 4 [19]. Basically, this structure type contains two inequivalent larger cations. The cation Sr(1) occupies a 10-coordination environment, and

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Corresponding authors. E-mail addresses: [email protected] (L. Mei), [email protected] (Z. Huang).

http://dx.doi.org/10.1016/j.jlumin.2017.07.013 Received 30 March 2017; Received in revised form 8 July 2017; Accepted 10 July 2017 Available online 11 July 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.

other Sr(2) occupies a 9-coordinate site; the smaller cation Sc occupies a 7-coordinate site [19,20]. In this paper, the synthesis and upconversion luminescence properties investigation on Yb3+/Er3+ co-doped Sr2ScF7 materials were demonstrated, and the interesting enhanced red upconversion emission was observed, and the possible upconversion luminescence mechanism was also discussed detail. 2. Experimental section A series of Sr2ScF7:Yb3+,Er3+ phosphors were synthesized by a solid-state technique. The raw materials were selected from the following materials, SrCO3 (A.R.), Sc2O3 (99.99%), NH4HF2 (A.R.), Yb2O3 (99.99%) and Er2O3(99.99%). After mixing and thoroughly grinding, the stoichiometric mixture was placed into an alumina crucible and annealed at 850 °C for 3 h. After that, the samples were furnace-cooled to room temperature, and ground again into powders. Thus, the Sr2ScF7:Yb3+,Er3+ phosphors were finally obtained. Powder X-ray diffraction (XRD) data were checked by the X-ray diffractometer ((SHIMADZU, XRD-6000, Kyoto, Japan) with Cu-Kα radiation (λ = 0.15406 nm) operated at 40 kV and 30 mA. The continuous scanning XRD data were collected in a 2θ ranging from 10° to 70°. The UC emission spectra were measured by using a 980 nm optical fiber laser (BWT KS3-11312, Beijing Kaipulin Co., China) as the excitation source, and the data were recorded on a fluorescence spectrophotometer (Hitachi F-4600, Japan). All the measurements were performed at room temperature.

Journal of Luminescence 192 (2017) 385–387

Y. Zhang et al.

Fig. 3. Up spectra of Sr2Sc93%-yF7:7%Er3+,yYb3+(y = 0, 4%, 8%, 15%, 20%, 30%, 40%) under laser excitation of 980 nm. The insert shows the dependences of green and red up conversion emission intensities on Yb3+ concentration.

Fig. 1. The XRD patterns of the as-prepared Sr2ScF7 host, Sr2Sc93%F7:7%Er3+, Sr2Sc73%F7:20%Yb3+,7%Er3+ and the standard data for Sr2ScF7 (ICSD no. 74360).

The phase purity of as-prepared samples was checked by XRD patterns. Fig. 1 shows the XRD patterns of the Sr2ScF7 host, Sr2Sc93%F7:7% Er3+ and Sr2Sc73%F7: 20%Yb3+, 7%Er3+ samples. It can be seen that the diffraction patterns of the synthesized samples are all well indexed to the standard ICSD card no. 74360 of Sr2ScF7 crystals, and no second phase was detected. This indicates the Sr2ScF7 single phase has been formed, Yb3+ and Er3+ ions have been successfully incorporated in the host structure and the doping did not cause the structural variation. Fig. 2 displays the room-temperature UC spectra of Sr2Sc90%3+ , xEr3+ (x = 3%, 5%, 7%, 10%, 15%, 20%, 30%) phosxF7:10%Yb phors under the 980 nm laser excitation and the relationship between the variation in UC luminescence emission intensity and Er3+ concentrations. As shown in Fig. 2, the UC spectra of the samples is composed of two green emission bands from 510 to 570 nm centered at 525, 543 and 551 nm and a red emission band from 640 to 690 nm with two peaks at 657 and 671 nm in the visible region, which is attributed to the 2 H11/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+, respectively. As is shown in the inset of Fig. 2, the UC luminescence emission intensity firstly increased with the increase of Er3+ concentration, and reached the maximum when the concentration of Er3+ is 7%, and then the emission intensity decreased with further increasing concentration. The UC spectra of Sr2Sc93%-yF7:7%Er3+, yYb3+ (y = 0, 4%, 8%, 15%, 20%, 30%, 40%) is given in Fig. 3. One can see that the UC spectrum of Er3+ singly doped sample includes very weak UC emission,

while the green and red emissions were improved greatly by the doping of Yb3+, and reaches its optimum value when the concentration is 20%. As we know, Yb3+ ion has large absorption cross section at 980 nm and the energy can be easily transferred to Er3+ ions. Therefore the UC emission properties were enhanced. However, the UC emission intensity showed a downward trend with further increasing of Yb3+ ion concentration mainly because of the energy back transfer (EBT) from Er3+ to Yb3+ ions. It is concluded that energy transfer between Yb3+ and Er3+ promoted the UC emissions [21]. To further analyze the physical mechanism of the UC process, the UC spectra of Sr2Sc73%F7:20%Yb3+, 7%Er3+ under laser excitation of 980 nm at different pumping powers was checked and given in Fig. 4. And the UC luminescence intensity increases with the increase of laser power. Generally, UC emission intensity (Iem) depends on the pumping laser power (Ppump) follows the relation of Iem∝ (Ppump)n, where n (slope of log Iem versus log Ppump) is the required number of pump photons for the transition from ground state to the upper emitting state [22,23]. In the sample of single Er3+ doped Sr2ScF7 phosphor, the slope values for green and red UC emissions were fitted to be 2.168 and 2.296 as can be seen in the insert, which were very close to 2, meaning that both the green and red UC emissions were the two-photon process. Energy level diagram of Er3+, Yb3+ ions and possible upconversion mechanisms for green and red light emitting under 980 nm laser excitation is shown in Fig. 5. In the case of Er3+ doped Sr2ScF7 phosphor, green and red upconversion emissions produced by an excited state absorption (ESA) and energy transfer upconversion mechanisms. In

Fig. 2. Up spectra of Sr2Sc90%-xF7:10%Yb3+, xEr3+(x = 3%, 5%, 7%, 10%, 15%, 20%, 30%) under laser excitation of 980 nm. The insert shows the dependences of green and red UC emission intensities on Er3+ concentration.

Fig. 4. Up conversion spectra of Sr2Sc73%F7:7%Er3+,20%Yb3+ under laser excitation of 980 nm at different laser power. The insert shows the Log-log plot of the UC emission intensity as a function of laser power.

3. Results and discussion

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References [1] H. Suo, C.F. Guo, L. Li, Host sensitized spherical up-conversion phosphor Yb2O3:Er3+, Ceram. Int. 41 (2015) 7017–7020. [2] D.Q. Chen, Y. Zhou, Z.Y. Wan, H. Yu, H.W. Lu, Z.G. Ji, P. Huang, Tunable upconversion luminescence in self-crystallized Er3+:K(Y1−xYbx)3F10 nano-glass ceramics, Phys. Chem. Chem. Phys. 17 (2015) 7100–7103. [3] L. Feng, L. Bian, W. Ren, X.Y. Zhang, H.L. Li, Cooperative upconversion of Tb3+/ Yb3+-codoped oxyfluoride glasses, Mater. Res. Bull. 89 (2017) 263–266. [4] D.M. Yang, P.A. Ma, Z.Y. Hou, Z.Y. Cheng, C.X. Li, J. Lin, Current advances in lanthanide ion (Ln3+)-based upconversion nanomaterials for drug delivery, Chem. Res. Soc. 44 (2015) 1416–1448. [5] D.T. Klier, M.U. Kumke, Analysing the effect of the crystal structure on upconversion luminescence in Yb3+,Er3+-co-doped NaYF4 nanomaterials, J. Mater. Chem. C 3 (2015) 11228–11238. [6] C. Shi, S. Soltani, A.M. Armani, Gold nanorod plasmonic upconversion microlaser, Nano Lett. 13 (2013) 5827–5831. [7] T.V. Gavrilović, D.J. Jovanović, K. Smits, M.D. Dramićanin, Multicolor upconversion luminescence of GdVO4:Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+, Ho3+/Er3+/ Tm3+) nanorods, Dyes Pigments 126 (2016) 1–7. [8] L.N. Guo, Y.H. Wang, Z.H. Zou, B. Wang, X.X. Guo, L.L. Han, W. Zeng, Facile synthesis and enhancement upconversion luminescence of ErF3 nano/microstructures via Li+ doping, J. Mater. Chem. C 2 (2014) 2765–2772. [9] Y. Li, S.F. Zhou, Y.Y. Li, K. Sharafudeen, Z.J. Ma, G.P. Dong, M.Y. Peng, J.R. Qiu, Long persistent and photo-stimulated luminescence in Cr3+-doped Zn-Ga-Sn-O phosphors for deep and reproducible tissue imaging, J. Mater. Chem. C 2 (2014) 2657–2663. [10] L. Li, H.H. Lin, X.Q. Zhao, Y.J. Wang, X.J. Zhou, C.G. Ma, X.T. Wei, Effect of Yb3+ concentration on upconversion luminescence in Yb3+, Tm3+ co-doped Lu2O3 nanophosphors, J. Alloy. Compd. 586 (2014) 555–560. [11] J.K. Li, J.G. Li, X.D. Li, X.D. Sun, Up-conversion luminescence of new phosphors of Gd3Al5O12:Yb/Er stabilized with Lu3+, Ceram. Inter 42 (2016) 3268–3274. [12] X.J. Zhu, Q.Q. Su, W. Feng, F.Y. Li, Anti-Stokes shift luminescent materials for bioapplications, Chem. Soc. Rev. 46 (2017) 1025–1039. [13] J.L. Zhuang, X.F. Yang, J. Wang, B.F. Lei, Y.L. Liu, M.M. Wu, Additives and solventsinduced phase and morphology modification of NaYF4 for improving up-conversion emission, J. Solid. State Chem. 233 (2016) 178–185. [14] B.Y. Lai, L. Feng, J. Wang, Q. Su, Optical transition and upconversion luminescence in Er3+ doped and Er3+–Yb3+ co-doped fluorophosphate glasses, Opt. Mater. 9 (2010) 1154–1160. [15] A. Suchocki, B. Koziarska, A. Brenier, C. Pedrini, G. Boulon, Energy transfer in GGG:Yb3+,Ho3+ crystals, J. Alloy. Compd. 225 (1995) 559–563. [16] L.P. Tu, X.M. Liu, F. Wu, H. Zhang, Excitation energy migration dynamics in upconversion nanomaterials, Chem. Soc. Rev. 44 (2015) 1331–1345. [17] J.D. Fidelus, Ya Zhydachevskii, W. Paszkowicz, A. Reszka, P. Dłużewski, A. Suchocki, Enhancement of luminescence of nanocrystalline TiO2:Yb3+ nanopowders due to co-doping with Nd3+ ions, Opt. Mater. 47 (2015) 361–365. [18] A.A. Savina, V.V. Atuchin, S.F. Solodovnikov, Z.A. Solodovnikova, A.S. Krylov, E.A. Maximovskiy, M.S. Molokeev, A.S. Oreshonkov, A.M. Pugachev, E.G. Khaikina, Synthesis, structural and spectroscopic properties of acentric triple molybdate Cs2NaBi(MoO4)3, J. Solid. State Chem. 225 (2015) 53–58. [19] Y.B. Yin, D.A. Keszler, Structure of distrontium scandium heptafluoride and chromium(III) luminescence, Mater. Res. Bull. 28 (1993) 931–938. [20] S.A. Cotton, Recent advances in the chemistry of scandium, Polyhedron 18 (1999) 1691–1715. [21] S. Das, A. Amarnath Reddy, S. Surendra Babu, G. Vijaya Prakash, Tunable visible upconversion emission in Er3+/Yb3+-codoped KCaBO3 phosphors by introducing Ho3+ ions, Mater. Lett. 120 (2014) 232–235. [22] Y. Yang, C. Mi, F. Yu, X. Su, C. Guo, G. Li, J. Zhang, L. Liu, Y. Liu, X. Li, Optical thermometry based on the upconversion fluorescence from Yb3+/Er3+ codoped La2O2S phosphor, Ceram. Int. 40 (2014) 9875–9880. [23] M. Guan, H. Zheng, L. Mei, M.S. Molokeev, J. Xie, T. Yang, X. Wu, S. Huang, Z. Huang, A. Setlur, Preparation, structure, and up-conversion luminescence of Yb3+/Er3+ codoped SrIn2O4 Phosphors, J. Am. Ceram. Soc. 98 (2015) 1182–1187.

Fig. 5. Energy level diagram of Yb3+ and Er3+ ions and the proposed UC mechanisms in Yb3+/Er3+ co-doped Sr2ScF7 under 980 nm excitation.

case of excited state absorption (ESA), the Er3+ ion is excited from ground level (4I15/2) to the 4I11/2 excited level through ground state absorption (GSA). Subsequently, the nonradiative (NR) transition occurs and the (2H11/2, 4S3/2) levels are populated. Then the excited ion at 4 I11/2 level absorb a second photon of same energy and promoting to 4 F7/2 level. In case of energy transfer upconversion, two Er3+ ions excited to 4I11/2 state. One of them decays to ground state and other is excited to 4I7/2 state [23]. After transfer of Er3+ ion to the 4F7/2 level relaxes and decays non-radiatively through multi-phonon process to lower lying 2H11/2, 4S3/2 and 4F9/2 levels. Finally, the radiative transition takes place and the UC emission is observed. 4. Conclusions Single Er3+ doped and Er3+/Yb3+ co-doped Sr2ScF7 phosphors were synthesized by a solid-state reaction method. All the prepared phosphors showed excellent UC luminescence in which Sc3+ ions can be substituted by Er3+ and Yb3+. In the light of UC spectra, Yb3+ ions strengthened the nonradiative relaxation process of Er3+ ions between 4 S3/2 and 4F9/2 levels, bringing about a dominated 665 nm emission in Er3+/Yb3+ co-doped phosphors. The pumping powers study showed that the UC luminescent process in either single Er3+ or Er3+/Yb3+ codoped Sr2ScF7 phosphors mainly are the two-photon process. Acknowledgements This present work was supported by the National Natural Science Foundations of China (Grant nos. 41672044 and 51672257), the Fundamental Research Funds for the Central Universities (Grant no. 2652016050), the State Scholarship Fund of China Scholarship Council (CSC). The authors are deeply grateful to Dr. Yanmin Yang for his help in testing.

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