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The color tunability and mechanism of upconversion emissions in Yb3+ and Er3+ co-doped Y2Ce2O7 phosphors
Optik 154 (2018) 15–21
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Optik journal homepage: www.elsevier.de/ijleo
Full length article
The color tunability and mechanism of upconversion emissions in Yb3+ and Er3+ co-doped Y2 Ce2 O7 phosphors Weixiong You a,∗ , Jiapeng Li a , Youfusheng Wu a , Fengqin Lai b , Xiufang Yang a a b
School of Material Science and Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, PR China School of Mechanical Science and Electrical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, PR China
a r t i c l e
i n f o
Article history: Received 29 June 2017 Accepted 4 October 2017 Keywords: Upconversion Color tunability Mechanism Y2 Ce2 O7 :Yb3+ ,Er3+
a b s t r a c t Y2 Ce2 O7 powders doped with different Yb3+ and Er3+ concentrations were synthesized via the sol-gel method. The structure and upconversion properties were investigated. The results indicated that a single phase was obtained and that the structure was cubic. The green and red upconversion emissions were detected with excitation at 980 nm, and the emission intensities could be tuned by the Yb3+ and Er3+ concentrations. The mechanism of the upconversion process was evaluated on the basis of the emission intensities and lifetimes of the 4 S3/2 , 4 F9/2 and 4 I13/2 levels. The results demonstrated that cross-relaxation was responsible for the dependence of green and red upconversion emissions on the dopant concentration. © 2017 Published by Elsevier GmbH.
1. Introduction Upconversion is an anti-Stokes process in which a substance absorbs two or more low-energy photons and emits a higherenergy photon [1]. The upconversion of luminescent materials can be used in many fields, including optical temperature sensing, laser cooling and cell imaging [2–4]. Recently, explorations of the application of upconversion in solar cells and white-light emission have attracted much attention [5–7]. Lanthanide rare-earth (RE) ions are suitable candidates for the upconversion process because of their abundant energy levels and narrow emission spectral lines [8]; The Er3+ ion is the most efficient and popular RE ion for this application. To enhance the efficiency of upconversion emission of Er3+ ions, the Yb3+ ions are often co-doped into materials doped with Er3+ as a sensitizer. Furthermore, the fluorescent color of upconversion can be tuned by the concentration of the Yb3+ ion [9,10], which is useful for fluorescent labels or biological images. Host materials with low phonon energy are necessary to achieve efficient upconversion emissions. Such materials enable the multi-phonon relaxation rate to be reduced and the lifetime of excited levels to be increased [11]. Fortunately, Re2 Ce2 O7 with a pyrochlore structure or a defect fluorite-type structure has a low phonon energy (approximately 464 cm−1 [12]) and excellent chemical stability [13] and is therefore a potential upconversion material. However, the literature contains few reports of the photoluminescence properties of RE-doped Re2 Ce2 O7 compounds. In this paper, we focus on the upconversion properties of Y2 Ce2 O7 phosphors co-doped with Yb3+ and Er3+ . The mechanism of upconversion emission and the effects of the dopant concentrations on the color tunability are studied.
∗ Corresponding author. E-mail address: you [email protected] (W. You). https://doi.org/10.1016/j.ijleo.2017.10.021 0030-4026/© 2017 Published by Elsevier GmbH.
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Fig. 1. (a) The X-ray diffraction patterns of Y2 Ce2 O7 :xYb, 2at%Er samples with different Yb3+ concentration; (b) (111) diffraction peaks of Y2 Ce2 O7 :xYb, 2at%Er samples with different Yb3+ concentration; (c) The X-ray diffraction patterns of Y2 Ce2 O7 :4atYb, yEr samples with different Er3+ concentration; (d) (111) diffraction peaks of Y2 Ce2 O7 :4at%Yb, yEr samples with different Er3+ concentration;.
2. Experiment Y2 Ce2 O7 samples co-doped with Yb3+ and Er3+ were prepared by the sol-gel method. Y2 O3 (99.99%), Yb2 O3 (99.99%), Er2 O3 (99.99%) and CeO2 (99.99%) were used as starting materials. According to the formula, (Ybx Ery Y1-x-y )2 Ce2 O7 (y = 0.02, x = 0, 0.02, 0.04, 0.06, 0.08, 0.1 and x = 0.04, y = 0.02, 0.04, 0.06, 0.08, 0.1), stoichiometric amounts of the starting materials were dissolved in nitric acid, and citric acid was then added into the solution as the chelating agent. The molar ratio of metal ions and citric acid was 1:2.5. H2 O2 was added when CeO2 is dissolved using nitric acid. The mixture was stirred at approximately 90 ◦ C for 4 h until a sol was formed. The sol was transformed into a sticky gel by evaporating the sol over several hours. The gel was dried at approximately 100 ◦ C in air. After being fully ground, the dried powders were sintered at 1200 ◦ C in a muffle furnace in air atmosphere for approximately 4 h, and white powder samples were obtained. X-ray diffraction (XRD) patterns of the samples in the range of 10◦ ≤ 2 ≤ 90◦ were recorded on a Bruker D8 Advance X-ray diffractometer with high-intensity CuK˛ radiation ( = 1.54178 Å). The upconversion emission spectra were detected using a spectrophotometer (FL980, Edinburgh) with a laser diode (LD) as the pump source; the excitement wavelength was 980 nm. The fluorescence decay curves, at a wavelength of 547, 677 and 1535 nm, were also excited at 980 nm by an optical parametric oscillator (OPO) in the same instrument. The signal was detected using an NIR PMT (R5509, Hamamatsu). All measurements were carried out at room temperature, and the spectra were corrected. 3. Results and discussion Fig. 1(a)–(d) shows the XRD patterns of samples doped with different concentrations of Yb3+ and Er3+ . From the figure, the diffraction peaks of Y2 Ce2 O7 agree with those of the standard card (JCPDS 09-0286), which indicates that the crystal
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Table 1 The cell parameters and CIE chromaticity coordinates of different Yb3+ and Er3+ concentration doped Y2 Ce2 O7. sample 3+
Yb 0 2 4 6 8 10 4 4 4 4
concentration (at%)
3+
Er
2 2 2 2 2 2 4 6 8 10
a (Å)
V (Å3 )
CIE x
CIE y
5.3835(8) 5.3833(9) 5.3818(7) 5.3802(5) 5.3782(3) 5.3782(2) 5.3765(1) 5.3794(5) 5.3796(8) 5.3799(6)
156.0319 156.0154 155.8833 155.7426 155.5672 155.5664 155.4180 155.6731 155.6931 155.7174
0.328 0.405 0.478 0.560 0.614 0.628 0.481 0.502 0.524 0.532
0.662 0.586 0.515 0.435 0.382 0.369 0.513 0.492 0.471 0.463
concentration (at%)
structure belongs to the cubic system with space group Fm3m. No diffraction peaks from impurities were observed in the XRD patterns, indicating that pure, single-phase Y2 Ce2 O7 was obtained and that Yb3+ and Er3+ ions were incorporated into the Y2 Ce2 O7 lattice. Fig. 1(b) and (d) shows the 2 range from 28.5◦ to 29.1◦ in the XRD patterns. With increasing concentrations of Yb3+ and Er3+ , the (111) diffraction peaks at approximately 28.696◦ shift slightly toward higher angles, indicating that the lattice parameters of Y2 Ce2 O7 :Yb,Er decrease. The unit-cell parameters were calculated on the basis of the XRD patterns; the results are summarized in Table 1. The decrease in lattice parameters is attributed to the radii of the Er3+ (8.81 nm) and Yb3+ (8.58 nm) ions being smaller than the radius of the Y3+ ions (8.93 nm) they replace [14]. These results further demonstrate that Yb3+ and Er3+ ions had entered the host lattice. Furthermore, in the case of the Y2 Ce2 O7 sample doped with 4 at% Yb3+ and 4 at% Er3+ , the (111) diffraction peak shifts to a higher angle than the corresponding peak in the XRD patterns of the other samples, which may be due to the random distribution of Yb3+ and Er3+ ions in the host. Fig. 2(a) demonstrates the dependence of upconversion emissions on the Yb3+ concentration when excited by 980 nm light. Two upconversion emission bands are observed in the range 500–750 nm: a green one at approximately 547 nm and a red one at approximately 677 nm, corresponding to the 2 H11/2 /4 S3/2 → 4 I15/2 and 4 F9/2 → 4 I15/2 transitions of the Er3+ ion, respectively. The upconversion emission intensities (green and red) in samples doped only with Er3+ can be neglected in comparison to those in samples doped with both Yb3+ and Er3+ . The green emission intensities decrease with increasing Yb3+ concentration, whereas the red emission intensities increase with increasing Yb3+ concentration. Fig. 2(b) demonstrates the CIE chromaticity coordinates of samples doped with different Yb3+ concentrations; the values are listed in Table 1. The upconversion emissions range from yellowish-green to red, depending on the Yb3+ concentration, when excited with 980-nm light, which indicates that the color of emissions can be tuned by the Yb3+ concentration. However, the color cannot be efficiently tuned via the Er3+ concentration. Fig. 2(c) demonstrates the dependence of upconversion emissions on the Er3+ concentration under excitation at 980 nm. The green and red emission intensities decrease with increasing Er3+ concentrations. The color coordinates are located at the yellow region for samples doped with different concentrations of Er3+ , as shown in Fig. 2(d) and Table 1. Our previous work demonstrates that the mechanism for green and red upconversion emissions is a two-photon process [15]. Because of the large absorption cross-section of the Yb3+ ion at 980 nm and the resonance between the 2 F5/2 → 2 F7/2 transition of Yb3+ ion and the 4 I15/2 → 4 I11/2 transition of Er3+ ion, the energy-transfer (ET) process dominates samples co-doped with Yb3+ and Er3+ . The Er3+ ion can be excited to the 4 I11/2 level from the 4 I15/2 level via the ET process from Yb3+ to Er3+ (ET process in Fig. 3) or via direct excitation. The proportion of Er3+ ions at the 4 I11/2 level will be excited to the 4 F7/2 level after absorbing another pump photon. The Er3+ ion at the 4 F7/2 level will nonradiatively decay to the 4 S3/2 and then the 4 F9/2 level; the green and red emissions can be derived when the Er3+ ions on these two levels relax to the ground 4 I15/2 level, respectively. Another portion of Er3+ ions on the 4 I11/2 level will nonradiatively relax to the 4 I13/2 level and then be populated to the 4 F9/2 level by the cross-relaxation (CR) process, 2 F5/2 (Yb) + 4 I13/2 (Er) → 2 F7/2 (Yb) +4 F9/2 (Er); thus, the red emission can be derived from the 4 F9/2 → 4 I15/2 transition. Therefore, the 4 F9/2 level can be populated via two channels: nonradiative transition process (4 S3/2 → 4 F9/2 ) and CR process (4 I13/2 → 4 F9/2 ). The CR process is dominant for the upconversion red emission. Because the radiative and nonradiative transitions are intrinsic transitions, they are independent of the dopant concentration. If the nonradiative transition process is dominant, the ratio of upconversion of red-to-green emission (R) cannot be changed by varying the Yb3+ concentration. Actually, the ratio R increases from 0.38 to 46.67 with increasing Yb3+ concentration, as shown in Fig. 4. Therefore, upconversion green emission competes with the red emission. When the Yb3+ concentration is increased, the distance between the Yb3+ and Er3+ ions is shortened, and the CR process becomes more efficient, resulting in a decrease in the upconversion green emissions and enhancement of the upconversion red emissions. This process can be confirmed by the various lifetimes of the 4 I13/2 level of the Er3+ ion, as shown in Fig. 5. The lifetime decreases with increasing Yb3+ concentration in samples doped with both Yb3+ and Er3+ , which indicates that an additional decay channel is introduced by Yb3+ . The distance between the Yb3+ and Er3+ ions decreases with increasing Yb3+ concentration, and the probability of the CR process increases. Thus, the lifetime of the 4 I13/2 level decreases with increasing Yb3+ concentration.
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Fig. 2. (a) The upconversion emissions of Y2 Ce2 O7 :xYb, 2at%Er samples excited at 980 nm; The inset shows the dependence of peak emission intensities (547 and 677 nm) on the Yb3+ concentration. (b) The CIE chromaticity coordinates of Y2 Ce2 O7 :xYb, 2at%Er samples. (c) The upconversion emissions of Y2 Ce2 O7 :4atYb, yEr samples excited at 980 nm; The inset shows the dependence of peak emission intensities (547 and 677 nm) on the Er3+ concentration. (d) The CIE chromaticity coordinates of Y2 Ce2 O7 :4atYb, yEr samples.
Fig. 3. The energy levels diagram of Yb3+ and Er3+ for the up-conversion emissions under 980 nm excitation.
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Fig. 4. The ratio of upconversion red to green emission (R) in different Yb3+ concentration doped samples.
Fig. 5. The lifetime of 4 I13/2 level of Er3+ in different Yb3+ concentration doped samples.
Fig. 6. The emissions of Y2 Ce2 O7 :xYb, 2at%Er samples excited at 980 nm; The inset shows the dependence of peak emission intensities (1535 nm) on the Yb3+ concentration.
In contrast to the declining tendency of lifetimes of the 4 I13/2 level with increasing Yb3+ concentration, the emission intensity of the 4 I13/2 → 4 I15/2 transition increases and then decreases with increasing Yb3+ concentration, as shown in Fig. 6. This behavior may be due to the following effects: (1) The distance between the Yb3+ and Er3+ ions decreases with increasing Yb3+ concentration, and the ET becomes more efficient; thus, the population of the 4 I13/2 level increases. Although the CR process can depopulate the 4 I13/2 level, the net population still increases and the emission intensity increases. (2) Another CR occurs, 4 S3/2 (Er) + 2 F7/2 (Yb) → 4 I13/2 (Er) + 2 F5/2 (Yb) (marked as CR1 in Fig. 3) [16]. This CR1 process can be confirmed by the variety of lifetimes of the4 S3/2 level of the Er3+ ion shown in Fig. 7. In samples doped with higher Yb3+ concentrations, the CR process is dominant, and the emission intensity of the 4 I13 /2 → 4 I15/2 transition decreases. In addition, the lifetimes of the 4 F9/2 level of Er3+ in samples doped with different Yb3+ concentrations are also shown in Fig. 7. These lifetimes increase with increasing Yb3+ concentration when the Yb3+ concentration is not higher than 4
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Fig. 7. The lifetime of green and red upconversion emissions of Er3+ in different Yb3+ concentration doped samples. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. The lifetime of green and red upconversion emissions of Er3+ in different Er3+ concentration doped samples. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
at%, which is caused by the greater probability of the CR process because of the shorter distance between Yb3+ and Er3+ as Yb3+ concentration increases. If the Yb3+ concentration is higher than 4 at%, the lifetime will decrease. Some authors have speculated that the Er3+ ion will be excited to the 2 H9/2 level with phonon assistance in samples doped with high Yb3+ concentrations [17,18]. However, in our work, the upconversion emissions in the range from 400 to 500 nm were not observed. Therefore, the pairing or aggregation of Yb3+ ions may turn some of those ions into quenchers [19]. However, the reverse CR process, 2 F7/2 (Yb) + 4 F9/2 (Er) → 2 F5/2 (Yb) + 4 I13/2 (Er), may be responsible for this result. Higher Er3+ concentrations adversely affect the upconversion emission. Fig. 2(b) demonstrates that the green and red upconversion emissions, along with the lifetimes of the 4 S3/2 and 4 F9/2 levels, decrease with increasing Er3+ concentration (shown in Fig. 8). The results demonstrate that interactions between Er3+ ions become stronger with decreasing distance between Er3+ ions due to increasing Er3+ concentrations. Therefore, the CR process among Er3+ ions cannot be neglected. The CR2 process, 4 S3/2 (Er) + 4 I15/2 (Er) → 4 I9/2 (Er) + 4 I13/2 (Er), which depends on the Er3+ concentration, reduces the population at the 4 S3/2 level and the lifetime of this level in samples doped with high Er3+ concentrations [20]. Furthermore, the back energy transfer (BET) from Er3+ to Yb3+ ions will induce reduced intensities of upconversion emission [21]. The reduced lifetimes of the 4 F9/2 level are due to the formation of clusters of Er3+ ions [22]. 4. Conclusions Single phases of Y2 Ce2 O7 phosphors doped with different Yb3+ and Er3+ concentrations were obtained in this work. Green and red upconversion emissions were observed under excitation by 980 nm light. The intensities of the green emissions decrease with increasing Yb3+ concentration, whereas the red emission intensities increase with increasing Yb3+ concentration. However, the green and red upconversion emissions decrease with increasing Er3+ concentration. The color of emissions can be tuned from yellowish to red by changing the Yb3+ concentration, whereas the color is in the yellow region for samples doped with different Er3+ concentrations. The green upconversion emission competed with that of the red. The mechanisms for the green and red upconversion were investigated and confirmed by the emission intensities and lifetimes. In samples doped with different concentrations of Yb3+ or Er3+ , the CR processes (CR, CR1, and CR2) between dopants were
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dominant. In brief, the upconversion process in Y2 Ce2 O7 phosphors doped with Yb3+ and Er3+ is very complicated and should be investigated further. Acknowledgements This work has been supported by the National Natural Science Foundation of China (11464017), the Science and Technology Landing Plan for Colleges of Jiangxi Province(KJLD14045), Foundation of Science and Technology Pillar Program in Industrial Field of Jiangxi Province (20123BBE50075), and the Program of Qingjiang Excellent Young Talents of Jiangxi University of Science and Technology. References [1] W. Peng, S. Zou, G. Liu, Q. Xiao, J. Meng, R. Zhang, Combustion synthesis and upconversion luminescence of CaSc2 O4 :Yb3+ ,Er3+ nanopowders, J. Rare Earths 29 (2011) 330–334. [2] X. Chai, J. Li, X. Wang, Y. Li, X. Yao, Color-tunable upconversion photoluminescence and highly performed optical temperature sensing in Er3+ /Yb3+ co-doped ZnWO4 , Opt. Express 24 (2016) 22438–22447. [3] H.T. Wong, M.K. Tsang, C.F. Chan, K.L. Wong, B. Fei, J. Hao, In vitro cell imaging using multifunctional small sized KGdF4 :Yb3+ , Er3+ upconverting nanoparticles synthesized by a one-pot solvothermal process, Nanoscale 5 (2013) 3465–3473. [4] A.J. Garcia-Adeva, R. Balda, J. Fernandez, Upconversion cooling of Er-doped low-phonon fluorescent solids, Phys. Rev. B 79 (2009) 033110. [5] H. Yang, F. Peng, Q. Zhang, W. Liu, D. Sun, Y. Zhao, X. Wei, Strong upconversion luminescence in LiYMo2 O8 :Er, Yb towards efficiency enhancement of dye-sensitized solar cells, Opt. Mater. 35 (2013) 2338–2342. [6] M.A. Hernández-Rodríguez, M.H. Imanieh, L.L. Martín, I.R. Martín, Experimental enhancement of the photocurrent in a solar cell using upconversion process in fluoroindate glasses exciting at 1480 nm, Sol. Energy Mater. Sol. Cells 116 (2013) 171–175. [7] T. Passuello, F. Piccinelli, M. Pedroni, M. Bettinelli, F. Mangiarini, R. Naccache, F. Vetrone, J.A. Capobianco, A. Speghini, White light upconversion of nanocrystalline Er/Tm/Yb doped tetragonal Gd4 O3 F6 , Opt. Mater. 33 (2011) 643–646. [8] M.K. Mahata, T. Koppe, T. Mondal, C. Brusewitz, K. Kumar, V. Kumar Rai, H. Hofsass, U. Vetter, Incorporation of Zn2+ ions into BaTiO3 :Er3+ /Yb3+ nanophosphor: an effective way to enhance upconversion, defect luminescence and temperature sensing, Phys. Chem. Chem. Phys. 17 (2015) 20741–20753. [9] Q. Lu, Y. Hou, A. Tang, H. Wu, F. Teng, Upconversion multicolor tuning: red to green emission from Y2 O3 :Er Yb nanoparticles by calcination, Appl. Phys. Lett. 102 (2013) 233103. [10] Y. Wei, C. Su, H. Zhang, J. Shao, Z. Fu, Color-tunable up-conversion emission from Yb3+ /Er3+ /Tm3+ /Ho3+ codoped KY(MoO4 )2 microcrystals based on energy transfer, Ceram. Int. 42 (2016) 4642–4647. [11] A.J. Garcia-Adeva, Spectroscopy upconversion dynamics, and applications of Er3+ -doped low-phonon materials, J. Lumin. 128 (2008) 697–702. [12] C. Wang, Y. Wang, A. Zhang, Y. Cheng, F. Chi, Z. Yu, The influence of ionic radii on the grain growth and sintering-resistance of Ln2 Ce2 O7 (Ln = La, Nd Sm, Gd), J. Mater. Sci. 48 (2013) 8133–8139. [13] H.S. Zhang, Y. Wei, G. Li, X.G. Chen, X.L. Wang, Investigation about thermal conductivities of La2 Ce2 O7 doped with calcium or magnesium for thermal barrier coatings, J. Alloys Compd. 537 (2012) 141–146. [14] W. You, F. Lai, H. Jiang, J. Liao, The up-conversion properties of Yb3+ and Er3+ doped Y4 Al2 O9 phosphors, Phys. B 407 (2012) 1094–1098. [15] H. Jiang, W. You, X. Liu, J. Liao, P. Wang, B. Yang, Yb3+ and Er3+ co-doped Y2 Ce2 O7 nanoparticles: synthesis and spectroscopic properties, Bull. Mater. Sci. 36 (2013) 1147–1151. [16] G. Chen, G. Somesfalean, Y. Liu, Z. Zhang, Q. Sun, F. Wang, Upconversion mechanism for two-color emission in rare-earth-ion-doped ZrO2 nanocrystals, Phys. Rev. B 75 (2007) 195204. [17] G.Y. Chen, Y. Liu, Z.G. Zhang, B. Aghahadi, G. Somesfalean, Q. Sun, F.P. Wang, Four-photon upconversion induced by infrared diode laser excitation in rare-earth-ion-doped Y2 O3 nanocrystals, Chem. Phys. Lett. 448 (2007) 127–131. [18] V. Lojpur, L. Mancic, P. Vulic, M.D. Dramicanin, M.E. Rabanal, O. Milosevic, Structural, morphological and up-converting luminescence characteristics of nanocrystalline Y2 O3 :Yb/Er powders obtained via spray pyrolysis, Ceram. Int. 40 (2014) 3089–3095. [19] Y. Dwivedi, K. Mishra, S.B. Rai, Synthesis of bright multicolor down and upconversion emitting Y2 Te4 O11 :Er:Yb nanocrystals, J. Alloys Compd. 572 (2013) 90–96. [20] J. Sokolnicki, Upconversion luminescence from Er3+ in nanocrystalline Y2 Si2 O7 :Er3+ and Y2 Si2 O7 :Yb3+ ,Er3+ phosphors, Mater. Chem. Phys. 131 (2011) 306–312. [21] H. Liu, L. Wang, S. Chen, Effect of Yb3+ concentration on the upconversion of Er3+ ion doped La2 O3 nanocrystals under 980 nm excitation, Mater. Lett. 61 (2007) 3629–3631. [22] D. Solis, E. De la Rosa, O. Meza, L.A. Diaz-Torres, P. Salas, C. 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Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Full length article
The color tunability and mechanism of upconversion emissions in Yb3+ and Er3+ co-doped Y2 Ce2 O7 phosphors Weixiong You a,∗ , Jiapeng Li a , Youfusheng Wu a , Fengqin Lai b , Xiufang Yang a a b
School of Material Science and Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, PR China School of Mechanical Science and Electrical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, PR China
a r t i c l e
i n f o
Article history: Received 29 June 2017 Accepted 4 October 2017 Keywords: Upconversion Color tunability Mechanism Y2 Ce2 O7 :Yb3+ ,Er3+
a b s t r a c t Y2 Ce2 O7 powders doped with different Yb3+ and Er3+ concentrations were synthesized via the sol-gel method. The structure and upconversion properties were investigated. The results indicated that a single phase was obtained and that the structure was cubic. The green and red upconversion emissions were detected with excitation at 980 nm, and the emission intensities could be tuned by the Yb3+ and Er3+ concentrations. The mechanism of the upconversion process was evaluated on the basis of the emission intensities and lifetimes of the 4 S3/2 , 4 F9/2 and 4 I13/2 levels. The results demonstrated that cross-relaxation was responsible for the dependence of green and red upconversion emissions on the dopant concentration. © 2017 Published by Elsevier GmbH.
1. Introduction Upconversion is an anti-Stokes process in which a substance absorbs two or more low-energy photons and emits a higherenergy photon [1]. The upconversion of luminescent materials can be used in many fields, including optical temperature sensing, laser cooling and cell imaging [2–4]. Recently, explorations of the application of upconversion in solar cells and white-light emission have attracted much attention [5–7]. Lanthanide rare-earth (RE) ions are suitable candidates for the upconversion process because of their abundant energy levels and narrow emission spectral lines [8]; The Er3+ ion is the most efficient and popular RE ion for this application. To enhance the efficiency of upconversion emission of Er3+ ions, the Yb3+ ions are often co-doped into materials doped with Er3+ as a sensitizer. Furthermore, the fluorescent color of upconversion can be tuned by the concentration of the Yb3+ ion [9,10], which is useful for fluorescent labels or biological images. Host materials with low phonon energy are necessary to achieve efficient upconversion emissions. Such materials enable the multi-phonon relaxation rate to be reduced and the lifetime of excited levels to be increased [11]. Fortunately, Re2 Ce2 O7 with a pyrochlore structure or a defect fluorite-type structure has a low phonon energy (approximately 464 cm−1 [12]) and excellent chemical stability [13] and is therefore a potential upconversion material. However, the literature contains few reports of the photoluminescence properties of RE-doped Re2 Ce2 O7 compounds. In this paper, we focus on the upconversion properties of Y2 Ce2 O7 phosphors co-doped with Yb3+ and Er3+ . The mechanism of upconversion emission and the effects of the dopant concentrations on the color tunability are studied.
∗ Corresponding author. E-mail address: you [email protected] (W. You). https://doi.org/10.1016/j.ijleo.2017.10.021 0030-4026/© 2017 Published by Elsevier GmbH.
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Fig. 1. (a) The X-ray diffraction patterns of Y2 Ce2 O7 :xYb, 2at%Er samples with different Yb3+ concentration; (b) (111) diffraction peaks of Y2 Ce2 O7 :xYb, 2at%Er samples with different Yb3+ concentration; (c) The X-ray diffraction patterns of Y2 Ce2 O7 :4atYb, yEr samples with different Er3+ concentration; (d) (111) diffraction peaks of Y2 Ce2 O7 :4at%Yb, yEr samples with different Er3+ concentration;.
2. Experiment Y2 Ce2 O7 samples co-doped with Yb3+ and Er3+ were prepared by the sol-gel method. Y2 O3 (99.99%), Yb2 O3 (99.99%), Er2 O3 (99.99%) and CeO2 (99.99%) were used as starting materials. According to the formula, (Ybx Ery Y1-x-y )2 Ce2 O7 (y = 0.02, x = 0, 0.02, 0.04, 0.06, 0.08, 0.1 and x = 0.04, y = 0.02, 0.04, 0.06, 0.08, 0.1), stoichiometric amounts of the starting materials were dissolved in nitric acid, and citric acid was then added into the solution as the chelating agent. The molar ratio of metal ions and citric acid was 1:2.5. H2 O2 was added when CeO2 is dissolved using nitric acid. The mixture was stirred at approximately 90 ◦ C for 4 h until a sol was formed. The sol was transformed into a sticky gel by evaporating the sol over several hours. The gel was dried at approximately 100 ◦ C in air. After being fully ground, the dried powders were sintered at 1200 ◦ C in a muffle furnace in air atmosphere for approximately 4 h, and white powder samples were obtained. X-ray diffraction (XRD) patterns of the samples in the range of 10◦ ≤ 2 ≤ 90◦ were recorded on a Bruker D8 Advance X-ray diffractometer with high-intensity CuK˛ radiation ( = 1.54178 Å). The upconversion emission spectra were detected using a spectrophotometer (FL980, Edinburgh) with a laser diode (LD) as the pump source; the excitement wavelength was 980 nm. The fluorescence decay curves, at a wavelength of 547, 677 and 1535 nm, were also excited at 980 nm by an optical parametric oscillator (OPO) in the same instrument. The signal was detected using an NIR PMT (R5509, Hamamatsu). All measurements were carried out at room temperature, and the spectra were corrected. 3. Results and discussion Fig. 1(a)–(d) shows the XRD patterns of samples doped with different concentrations of Yb3+ and Er3+ . From the figure, the diffraction peaks of Y2 Ce2 O7 agree with those of the standard card (JCPDS 09-0286), which indicates that the crystal
W. You et al. / Optik 154 (2018) 15–21
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Table 1 The cell parameters and CIE chromaticity coordinates of different Yb3+ and Er3+ concentration doped Y2 Ce2 O7. sample 3+
Yb 0 2 4 6 8 10 4 4 4 4
concentration (at%)
3+
Er
2 2 2 2 2 2 4 6 8 10
a (Å)
V (Å3 )
CIE x
CIE y
5.3835(8) 5.3833(9) 5.3818(7) 5.3802(5) 5.3782(3) 5.3782(2) 5.3765(1) 5.3794(5) 5.3796(8) 5.3799(6)
156.0319 156.0154 155.8833 155.7426 155.5672 155.5664 155.4180 155.6731 155.6931 155.7174
0.328 0.405 0.478 0.560 0.614 0.628 0.481 0.502 0.524 0.532
0.662 0.586 0.515 0.435 0.382 0.369 0.513 0.492 0.471 0.463
concentration (at%)
structure belongs to the cubic system with space group Fm3m. No diffraction peaks from impurities were observed in the XRD patterns, indicating that pure, single-phase Y2 Ce2 O7 was obtained and that Yb3+ and Er3+ ions were incorporated into the Y2 Ce2 O7 lattice. Fig. 1(b) and (d) shows the 2 range from 28.5◦ to 29.1◦ in the XRD patterns. With increasing concentrations of Yb3+ and Er3+ , the (111) diffraction peaks at approximately 28.696◦ shift slightly toward higher angles, indicating that the lattice parameters of Y2 Ce2 O7 :Yb,Er decrease. The unit-cell parameters were calculated on the basis of the XRD patterns; the results are summarized in Table 1. The decrease in lattice parameters is attributed to the radii of the Er3+ (8.81 nm) and Yb3+ (8.58 nm) ions being smaller than the radius of the Y3+ ions (8.93 nm) they replace [14]. These results further demonstrate that Yb3+ and Er3+ ions had entered the host lattice. Furthermore, in the case of the Y2 Ce2 O7 sample doped with 4 at% Yb3+ and 4 at% Er3+ , the (111) diffraction peak shifts to a higher angle than the corresponding peak in the XRD patterns of the other samples, which may be due to the random distribution of Yb3+ and Er3+ ions in the host. Fig. 2(a) demonstrates the dependence of upconversion emissions on the Yb3+ concentration when excited by 980 nm light. Two upconversion emission bands are observed in the range 500–750 nm: a green one at approximately 547 nm and a red one at approximately 677 nm, corresponding to the 2 H11/2 /4 S3/2 → 4 I15/2 and 4 F9/2 → 4 I15/2 transitions of the Er3+ ion, respectively. The upconversion emission intensities (green and red) in samples doped only with Er3+ can be neglected in comparison to those in samples doped with both Yb3+ and Er3+ . The green emission intensities decrease with increasing Yb3+ concentration, whereas the red emission intensities increase with increasing Yb3+ concentration. Fig. 2(b) demonstrates the CIE chromaticity coordinates of samples doped with different Yb3+ concentrations; the values are listed in Table 1. The upconversion emissions range from yellowish-green to red, depending on the Yb3+ concentration, when excited with 980-nm light, which indicates that the color of emissions can be tuned by the Yb3+ concentration. However, the color cannot be efficiently tuned via the Er3+ concentration. Fig. 2(c) demonstrates the dependence of upconversion emissions on the Er3+ concentration under excitation at 980 nm. The green and red emission intensities decrease with increasing Er3+ concentrations. The color coordinates are located at the yellow region for samples doped with different concentrations of Er3+ , as shown in Fig. 2(d) and Table 1. Our previous work demonstrates that the mechanism for green and red upconversion emissions is a two-photon process [15]. Because of the large absorption cross-section of the Yb3+ ion at 980 nm and the resonance between the 2 F5/2 → 2 F7/2 transition of Yb3+ ion and the 4 I15/2 → 4 I11/2 transition of Er3+ ion, the energy-transfer (ET) process dominates samples co-doped with Yb3+ and Er3+ . The Er3+ ion can be excited to the 4 I11/2 level from the 4 I15/2 level via the ET process from Yb3+ to Er3+ (ET process in Fig. 3) or via direct excitation. The proportion of Er3+ ions at the 4 I11/2 level will be excited to the 4 F7/2 level after absorbing another pump photon. The Er3+ ion at the 4 F7/2 level will nonradiatively decay to the 4 S3/2 and then the 4 F9/2 level; the green and red emissions can be derived when the Er3+ ions on these two levels relax to the ground 4 I15/2 level, respectively. Another portion of Er3+ ions on the 4 I11/2 level will nonradiatively relax to the 4 I13/2 level and then be populated to the 4 F9/2 level by the cross-relaxation (CR) process, 2 F5/2 (Yb) + 4 I13/2 (Er) → 2 F7/2 (Yb) +4 F9/2 (Er); thus, the red emission can be derived from the 4 F9/2 → 4 I15/2 transition. Therefore, the 4 F9/2 level can be populated via two channels: nonradiative transition process (4 S3/2 → 4 F9/2 ) and CR process (4 I13/2 → 4 F9/2 ). The CR process is dominant for the upconversion red emission. Because the radiative and nonradiative transitions are intrinsic transitions, they are independent of the dopant concentration. If the nonradiative transition process is dominant, the ratio of upconversion of red-to-green emission (R) cannot be changed by varying the Yb3+ concentration. Actually, the ratio R increases from 0.38 to 46.67 with increasing Yb3+ concentration, as shown in Fig. 4. Therefore, upconversion green emission competes with the red emission. When the Yb3+ concentration is increased, the distance between the Yb3+ and Er3+ ions is shortened, and the CR process becomes more efficient, resulting in a decrease in the upconversion green emissions and enhancement of the upconversion red emissions. This process can be confirmed by the various lifetimes of the 4 I13/2 level of the Er3+ ion, as shown in Fig. 5. The lifetime decreases with increasing Yb3+ concentration in samples doped with both Yb3+ and Er3+ , which indicates that an additional decay channel is introduced by Yb3+ . The distance between the Yb3+ and Er3+ ions decreases with increasing Yb3+ concentration, and the probability of the CR process increases. Thus, the lifetime of the 4 I13/2 level decreases with increasing Yb3+ concentration.
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Fig. 2. (a) The upconversion emissions of Y2 Ce2 O7 :xYb, 2at%Er samples excited at 980 nm; The inset shows the dependence of peak emission intensities (547 and 677 nm) on the Yb3+ concentration. (b) The CIE chromaticity coordinates of Y2 Ce2 O7 :xYb, 2at%Er samples. (c) The upconversion emissions of Y2 Ce2 O7 :4atYb, yEr samples excited at 980 nm; The inset shows the dependence of peak emission intensities (547 and 677 nm) on the Er3+ concentration. (d) The CIE chromaticity coordinates of Y2 Ce2 O7 :4atYb, yEr samples.
Fig. 3. The energy levels diagram of Yb3+ and Er3+ for the up-conversion emissions under 980 nm excitation.
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Fig. 4. The ratio of upconversion red to green emission (R) in different Yb3+ concentration doped samples.
Fig. 5. The lifetime of 4 I13/2 level of Er3+ in different Yb3+ concentration doped samples.
Fig. 6. The emissions of Y2 Ce2 O7 :xYb, 2at%Er samples excited at 980 nm; The inset shows the dependence of peak emission intensities (1535 nm) on the Yb3+ concentration.
In contrast to the declining tendency of lifetimes of the 4 I13/2 level with increasing Yb3+ concentration, the emission intensity of the 4 I13/2 → 4 I15/2 transition increases and then decreases with increasing Yb3+ concentration, as shown in Fig. 6. This behavior may be due to the following effects: (1) The distance between the Yb3+ and Er3+ ions decreases with increasing Yb3+ concentration, and the ET becomes more efficient; thus, the population of the 4 I13/2 level increases. Although the CR process can depopulate the 4 I13/2 level, the net population still increases and the emission intensity increases. (2) Another CR occurs, 4 S3/2 (Er) + 2 F7/2 (Yb) → 4 I13/2 (Er) + 2 F5/2 (Yb) (marked as CR1 in Fig. 3) [16]. This CR1 process can be confirmed by the variety of lifetimes of the4 S3/2 level of the Er3+ ion shown in Fig. 7. In samples doped with higher Yb3+ concentrations, the CR process is dominant, and the emission intensity of the 4 I13 /2 → 4 I15/2 transition decreases. In addition, the lifetimes of the 4 F9/2 level of Er3+ in samples doped with different Yb3+ concentrations are also shown in Fig. 7. These lifetimes increase with increasing Yb3+ concentration when the Yb3+ concentration is not higher than 4
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Fig. 7. The lifetime of green and red upconversion emissions of Er3+ in different Yb3+ concentration doped samples. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. The lifetime of green and red upconversion emissions of Er3+ in different Er3+ concentration doped samples. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
at%, which is caused by the greater probability of the CR process because of the shorter distance between Yb3+ and Er3+ as Yb3+ concentration increases. If the Yb3+ concentration is higher than 4 at%, the lifetime will decrease. Some authors have speculated that the Er3+ ion will be excited to the 2 H9/2 level with phonon assistance in samples doped with high Yb3+ concentrations [17,18]. However, in our work, the upconversion emissions in the range from 400 to 500 nm were not observed. Therefore, the pairing or aggregation of Yb3+ ions may turn some of those ions into quenchers [19]. However, the reverse CR process, 2 F7/2 (Yb) + 4 F9/2 (Er) → 2 F5/2 (Yb) + 4 I13/2 (Er), may be responsible for this result. Higher Er3+ concentrations adversely affect the upconversion emission. Fig. 2(b) demonstrates that the green and red upconversion emissions, along with the lifetimes of the 4 S3/2 and 4 F9/2 levels, decrease with increasing Er3+ concentration (shown in Fig. 8). The results demonstrate that interactions between Er3+ ions become stronger with decreasing distance between Er3+ ions due to increasing Er3+ concentrations. Therefore, the CR process among Er3+ ions cannot be neglected. The CR2 process, 4 S3/2 (Er) + 4 I15/2 (Er) → 4 I9/2 (Er) + 4 I13/2 (Er), which depends on the Er3+ concentration, reduces the population at the 4 S3/2 level and the lifetime of this level in samples doped with high Er3+ concentrations [20]. Furthermore, the back energy transfer (BET) from Er3+ to Yb3+ ions will induce reduced intensities of upconversion emission [21]. The reduced lifetimes of the 4 F9/2 level are due to the formation of clusters of Er3+ ions [22]. 4. Conclusions Single phases of Y2 Ce2 O7 phosphors doped with different Yb3+ and Er3+ concentrations were obtained in this work. Green and red upconversion emissions were observed under excitation by 980 nm light. The intensities of the green emissions decrease with increasing Yb3+ concentration, whereas the red emission intensities increase with increasing Yb3+ concentration. However, the green and red upconversion emissions decrease with increasing Er3+ concentration. The color of emissions can be tuned from yellowish to red by changing the Yb3+ concentration, whereas the color is in the yellow region for samples doped with different Er3+ concentrations. The green upconversion emission competed with that of the red. The mechanisms for the green and red upconversion were investigated and confirmed by the emission intensities and lifetimes. In samples doped with different concentrations of Yb3+ or Er3+ , the CR processes (CR, CR1, and CR2) between dopants were
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dominant. In brief, the upconversion process in Y2 Ce2 O7 phosphors doped with Yb3+ and Er3+ is very complicated and should be investigated further. Acknowledgements This work has been supported by the National Natural Science Foundation of China (11464017), the Science and Technology Landing Plan for Colleges of Jiangxi Province(KJLD14045), Foundation of Science and Technology Pillar Program in Industrial Field of Jiangxi Province (20123BBE50075), and the Program of Qingjiang Excellent Young Talents of Jiangxi University of Science and Technology. References [1] W. Peng, S. Zou, G. Liu, Q. Xiao, J. Meng, R. Zhang, Combustion synthesis and upconversion luminescence of CaSc2 O4 :Yb3+ ,Er3+ nanopowders, J. Rare Earths 29 (2011) 330–334. [2] X. Chai, J. Li, X. Wang, Y. Li, X. Yao, Color-tunable upconversion photoluminescence and highly performed optical temperature sensing in Er3+ /Yb3+ co-doped ZnWO4 , Opt. Express 24 (2016) 22438–22447. [3] H.T. Wong, M.K. Tsang, C.F. Chan, K.L. 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