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Yb3+ co-doped cubic NaYF4 single crystals by vertical Bridgman method with KF flux
Chinese Journal of Physics xxx (2016) 1e7
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Efficient upconversion luminescence in Er3þ/Yb3þ co-doped cubic NaYF4 single crystals by vertical Bridgman method with KF flux Shinan He a, Haiping Xia a, *, Qingyang Tang a, Qiguo Sheng a, Cheng Wang a, Zhigang Feng a, Dongsheng Jiang a, Xuemei Gu a, Jianli Zhang a, Haochuan Jiang b, **, Baojiu Chen c a b c
Key Laboratory of Photo-electronic Materials, Ningbo University, Ningbo, Zhejiang, 315211, China Ningbo Institute of Materials Technology and Engineering, The Chinese Academy of Sciences, Ningbo, Zhejiang, 315211, China Department of Physics, Dalian Maritime University, Dalian, Liaoning Province, 116026, China
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Er3þ/Yb3þ co-doped a-NaYF4 single crystals with various Yb3þ concentrations and ~0.50 mol% Er3þ were grown by Bridgman method using Potassium fluoride as flux. The up-conversion (UC) luminescence and its pump power dependence on UC emission of the crystals under excitation of 980 nm laser diode were investigated. The UC emissions at green ~522 nm, ~541 nm and red ~668 nm, which are attributed to 2H11/2 / 4I15/2, 4S3/ 4 4 4 3þ 3þ doped a-NaYF4 2 / I15/2 and F9/2 / I15/2, respectively, were obtained in Er /Yb single crystals. The quadratic dependence of pump power on the UC emission suggests that a two-photon process is responsible for the transition from the excited Yb3þ to the lower state of Er3þ ions in a-NaYF4 single crystals. A favorable performance of yellowish green light can be achieved with certain Yb3þ concentrations and 0.52 mol% Er3þ co-doped samples that makes further research valuable in developing UC displays for electro-optical devices. © 2016 Published by Elsevier B.V. on behalf of The Physical Society of the Republic of China (Taiwan).
1. Introduction During the past several decades, much effort has been devoted to the investigation on upconversion (UC) in rare earth doped materials because of both the scientific interest and the potential applications in short wavelength lasers, biological fluorescence, telecommunications, sensors and volumetric displays [1e3]. Photon UC is an anti-Stokes emission process in which the long wavelength photons are converted into the short wavelength photons. Among all the rare-earth ions, Er3þ is employed as a center of up-conversion luminescence due to its longer lifetimes of metastable energy levels and more homogeneous energy level array. The up-conversion emissions at blue (4F7/2 / 4I15/2, ~488 nm), green (2H11/2/4S3/2 / 4I15/2, ~520/550 nm) and red (4F9/2 / 4I15/2, ~660 nm) have been observed when pumped
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Xia), [email protected] (H. Jiang). http://dx.doi.org/10.1016/j.cjph.2016.04.011 0577-9073/© 2016 Published by Elsevier B.V. on behalf of The Physical Society of the Republic of China (Taiwan).
Please cite this article in press as: S. He et al., Efficient upconversion luminescence in Er3þ/Yb3þ co-doped cubic NaYF4 single crystals by vertical Bridgman method with KF flux, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/ j.cjph.2016.04.011
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with proper lights. By introducing Yb3þ ions as sensitizer into Er3þ doped materials, the performance of UC luminescence can be improved because of a larger absorption cross section at near infrared (NIR) of Yb3þ and the high-efficiency energy transfer from Yb3þ to Er3þ ions [4e7]. In recent years, most studies on Er3þ/Yb3þ co-doped systems have been focused on exploring the UC mechanisms and developing novel hosts [8e10]. The upconversion luminescence of rare-earth ions is also dependent on the host material. Many host materials such as fluorides, chlorides, sulfides and oxides have been widely studied [8,10]. It has been demonstrated that NaYF4 is an excellent host for up-conversion for its low phonon energy [11e13]. The crystal has two structures, one being cubic a-NaYF4 and the other hexagonal b-NaYF4 [14]. Er3þ and Yb3þ co-doped b-NaYF4 optical materials are of most efficient in UC among the reported UC materials [15]. However, the studied materials of NaYF4 mainly focus on nano-crystals and powders. There have scarcely been reports on NaYF4 single crystals because of the difficulty in the crystal growth. Compared with the nano-crystal of NaYF4, the single crystal, which is single-phased phosphors, provides high transmission for lights, good physical and mechanical performance, as well as good chemical durability. Thus, the growth of Er3þ/Yb3þ co-doped NaYF4 single crystal with high UC effect became a challenge to the researchers. Generally, there exists a problem of the inhomogeneous distribution of rare earth ions in single crystal resulted from crystal growth. In this study, NaYF4 single crystal was chosen as a matrix of Er3þ and Yb3þ ions in view of the same valence state and the very near ionic radii between Y3þ (0.88 Å) and Er3þ(0.881 Å), Y3þ and Yb3þ(0.858 Å), which resulted in relatively homogeneous concentration of Er3þ and Yb3þ in NaYF4 crystal. The NaYF4 single crystal with homogeneous rare earths is extremely important for the practical application in optical device. When Potassium fluoride (KF) is added into the starting components of NaFeYF3 system, the phase equilibrium between NaF and YF3 is significantly changed. With this change it becomes possible to crystallize only the composition of a-NaYF4 phase from incongruent melt. Some single crystals with high quality such as ZnO and YPO4 can be grown successfully by additional assistant flux into precursor melt [16,17]. The present paper describes the preparation of Er3þ and Yb3þ co-doped aNaYF4 single crystals grown with flux Bridgman method. High efficient UC luminescence of the obtained single crystal is presented under 980 nm excitation and the UC mechanisms are investigated from the measured pump power dependence. 2. Experimental The Er3þ/Yb3þ co-doped samples according to the formula 30NaFe18KFe(52-c-g)YF3ecErF3egYbF3 (c ¼ 0.4, g ¼ 0, 1.5, 2.5, 4), respectively, with 99.99% purity raw materials of NaF, KF, YF3, ErF3, and YbF3 were prepared. Then those mixture samples were ground thoroughly in a mortar for about 0.5 h. In order to remove the moisture and the oxide in the raw materials, the mixtures were sintered with anhydrous HF at 750 C for 8 h. The seed crystals were obtained from previous work by spontaneous nucleation from the seed wells and oriented along the a-axis. They were put in the bottom of seed well and then the sintered polycrystalline powders were filled in Pt crucibles of F10 mm 150 mm size. The crucibles were completely sealed in order to avoid contamination from water and oxygen in the air and to prevent the volatilization of the melt during crystal growth. The a-NaYF4 single crystals were grown in a resistively heated vertical Bridgman furnace. The furnace was gradually heated and held to 950 C. The seeding temperature was about 770e820 C and the temperature gradient cross solid-liquid interface was 70e90 C/cm. The growth process was carried out by lowering the crucible at a rate of 0.05e0.06 mm/h. After the growth was finished and the furnace cooled to room temperature slowly, the crystal was obtained by stripping off the Pt crucible. The X-ray diffraction (XRD) of the samples was measured using a XD-98X diffractometer (XD-3, Beijing). The absorption spectra were recorded with a Cary 5000 UV/VIS/NIR spectrophotometer (Agilent Co., America). The emission spectra were obtained with a FLSP 920 type spectrometer (Edinburgh Co., England). The Er3þ and Yb3þ concentrations in a-NaYF4 single crystals were measured by the inductive coupled plasma atomic emission spectroscopy (ICP-AES, PerkinElmer Inc., Optima 3000). All the measurements were performed at room temperature. Table 1 shows the concentrations of Er3þ and Yb3þ in the samples (mol%). 3. Results and discussion The phase equilibrium of NaFeYF3 is completely changed as introducing of KF into this system. The composition of aNaYF4 phase begins possibly to crystallize only from the incongruent melt. The suitable composition of raw materials had been demonstrated to be 30NaFe18KFe52YF3 through abundant of experiments. As shown in Fig. 1, there is a white region at the top of the boule, several centimeters in length, corresponding to the final portion of the melt to freeze. Examination using a polarized microscope showed the crystal to be free from macroscopic defects. The melting temperature of congruent crystal
Table 1 Concentration of Er3þ and Yb3þ ions in the crystals (mol%). Samples
1
2
3
4
Er3þ(c) Yb3þ(g)
0.51 0
0.52 2.97
0.51 4.99
0.52 7.97
Please cite this article in press as: S. He et al., Efficient upconversion luminescence in Er3þ/Yb3þ co-doped cubic NaYF4 single crystals by vertical Bridgman method with KF flux, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/ j.cjph.2016.04.011
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Fig. 1. X-ray diffraction patterns of Er3þ/Yb3þ co-doped a-NaYF4 single crystal of different part. Inset is the photo of a-NaYF4 single crystal.
is reduced by more than 100 C due to the introduction of KF. In order to seed successfully, a steep thermal gradient of the solid-liquid interface is also required. A slow growth speed of 0.05e0.06 mm/h was employed in order to keep KF away from the a-NaYF4 crystal structure completely. To understand the structures of the as-grown crystals, the XRD of different parts were tested and shown in Fig. 1. The boule of grown crystal is shown in the inset of Fig. 1. The initial and middle parts which are designated as A and B, respectively, appear light pink and lucid; however, the final section of the boule designated as C is white opaque. As compared with JCPD card (77-2042), one can confirm that the crystal of parts A and B are pure cubic a-NaYF4 phase and the current doping level does not cause any obvious peak shift or second phase. It can be confirmed that the crystals are a-NaYF4 single crystal since they are highly transparent with about 90% transmittance in the visible region, and no grain boundary is observed. In order to understand the structures of the white part C, which corresponds to the final stage of the growth process, the XRD pattern for part C is also recorded and shown in Fig. 1. One can see from Fig. 1 that the white part C is composed of KYF4, K2NaYF6, aNaYF4, and b-NaYF4, etc. The above XRD results suggest that the introduction of KF into NaFeYF3 system does not change the structure of a-NaYF4 single crystals. The KF is excluded from melt as the crystal grows and accumulates in the final portion of the boule. The absorption spectra in the 300e1100 nm range of the grown single crystal samples were measured and are shown in Fig. 2. The absorption peaks corresponding to transitions from the 4I15/2 ground state to the excited states of the Er3þ ions are also assigned in Fig. 2. By comparing the Er3þ doped sample with the Er3þ/Yb3þ co-doped samples, there is nearly no intensity and position changes for the peaks in the visible region, where only Er3þ ions can absorb light. For the absorption around 980 nm, where absorptions from 4I15/2 / 4I11/2 of Er3þ and 2F7/2 / 2F5/2 of Yb3þ exist, the absorption intensity shows a linear dependence on the Yb3þ concentration. The UC luminescence spectra of Er3þ/Yb3þ co-doped a-NaYF4 single crystal samples measured at room temperature with 980 nm laser are shown in Fig. 3, from which three obvious bands in the range of 450e750 nm can be observed. The inset of Fig. 3 is the dependence of integrated intensities of 522 nm, 541 nm, and 668 nm emissions and integrated total intensity on the Yb3þ concentration. In the measurements of these UC spectra, the same experimental conditions were held for all samples in order to get comparable results. Green emissions ranging from 509 to 534 nm and from 534 to 570 nm are due to the radiative transition 2H11/2 / 4I15/2 and 4S3/2 / 4I15/2 of Er3þ ion, respectively; whereas the red emission from 638 to 688 nm is attributed to the 4F9/2 / 4I15/2 transition. The above three emission peaks show energy Stark splitting of Er3þ under the action of a-NaYF4 single crystal field. The total UC intensity reaches its maximum when Yb3þ is ~7.97 mol%. By comparing the relative intensity of 541 nm and 668 nm, one can identify the difference. For the 541 nm emission the intensity increases rapidly as the Yb3þ concentration increases and reaches its maximal value when Yb3þ concentration is 7.97 mol%, while the maximum intensity of 668 nm emission is obtained when Yb3þ concentration is 4.99 mol%. Compared with reference [18], in which red emission is more efficient, this Er3þ/Yb3þ co-doped a-NaYF4 single crystal is highly beneficial for green UC emission. Since the observed UC emissions are located in visible wavelength range, the exact UC luminescence CIE chromaticity coordinates of all the Er3þ/Yb3þ co-doped a-NaYF4 single crystal samples are shown in Fig. 4 to obtain the true color of the UC emissions. The CIE coordinates (x, y) are (0.2871, 0.6711), (0.2943, 0.6898), (0.3371, 0.6482), and (0.2995, 0.6843), and color temperature Tc is 6288 K, 6155 K, 5529 K and 6082 K, respectively, for Er3þ/Yb3þ co-doped samples with 0.52Er/0Yb, 0.52Er/ 2.97Yb, 0.51Er/4.99Yb, and 0.52Er/7.97Yb doping concentrations. From the CIE chromaticity coordinates, it is clear that the combination of the UC emission is yellowish green color to which human eyes are sensitive. This characteristic is favorable for applications in UC displays for electro-optical devices. Fig. 5 illustrates the decay curves at 541 nm of a-NaYF4 single crystal co-doped with various Er3þ/Yb3þ concentrations under 980 nm excitation. The lifetime for 0.5 mol% Er3þ, 0.5 mol% Er3þ and 3 mol%Yb3þ, 0.5 mol% Er3þ and 5 mol%Yb3þ, Please cite this article in press as: S. He et al., Efficient upconversion luminescence in Er3þ/Yb3þ co-doped cubic NaYF4 single crystals by vertical Bridgman method with KF flux, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/ j.cjph.2016.04.011
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Fig. 2. Absorption spectra of Er3þ/Yb3þ co-doped a-NaYF4 single crystals.
Fig. 3. Comparison of the up-conversion luminescence spectra of Er3þ/Yb3þ doped a-NaYF4 single crystals under 980 nm excitation at room temperature.
0.5 mol% Er3þ and 8 mol%Yb3þ doped a-NaYF4 single crystals are tm ¼ 525.4 ms, tm ¼ 1640.2 ms, tm ¼ 1687.4 ms, and tm ¼ 1614.4 ms, respectively. As we can see, the lifetime for Er3þ singly doped a-NaYF4 single crystal is the least one. However, the other three Er3þ/Yb3þ doped samples show the similar value of ~1650 ms for the lifetimes at 541 nm. It suggests that there is no concentration quenching occurring in present doping concentration. The a-NaYF4 single crystal shows high incorporating concentrations for Er3þ and Yb3þ. Because of the similar radii size and same valence state among the rare earth ions, when the Er3þ and Yb3þ are doped in a-NaYF4 single crystal, they naturally replace the position of Y3þ. It is known that the ionic radius for Y3þ, Er3þ and Yb3þ are 0.893 Å, 0.881 Å and 0.858 Å, respectively. One can see that the differences between Y3þ and Er3þ, and Y3þ and Yb3þ are only 1.34% and 3.91%. An effective segregation coefficient near 1 for both Er3þ and Yb3þ in a-NaYF4 single crystals can be obtained due to the very close radii among the above ions. The Er3þ and Yb3þ ions tend to homogeneously distribute in a-NaYF4 single crystal that results in a high incorporating concentrations and high luminous effect of Er3þ and Yb3þ. In order to understand the UC dynamics of Er3þ/Yb3þ co-doped a-NaYF4 single crystal, the pump-power dependence of luminescence intensities is needed and shown in Fig. 6. In frequency up-conversion process the relation between emission intensity Iem and the NIR excitation power: IemfPn has been confirmed [19], where n is the number of pump photons required to excite to the emitting state. From the logelog dependence of the integrated green (~522 and ~541 nm) and red emission (~668 nm) intensities on the excitation power at 980 nm shown in Fig. 5, one can find n ¼ 1.968, 1.852, 1.721, respectively. The near-quadratic dependence indicates that two photons are involved in the up-conversion process. The mechanism of energy transfer (ET) from Yb3þ to Er3þ has been reported in the previously works [20,21]. In those papers the researchers attempted to quantitatively investigate the ET progress in the co-doped samples. Fig. 7 shows the Please cite this article in press as: S. He et al., Efficient upconversion luminescence in Er3þ/Yb3þ co-doped cubic NaYF4 single crystals by vertical Bridgman method with KF flux, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/ j.cjph.2016.04.011
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Fig. 4. CIE chromaticity coordinates of Er3þ/Yb3þ co-doped a-NaYF4 single crystals.
energy level diagram of Er3þ/Yb3þ and the energy transfer process. The 4I11/2 level of Er3þ ion can be directly excited by 980 nm pumping light from the ground state 4I15/2 or via the energy transfer (ET) process from 2F5/2 level of Yb3þ to Er3þ: 2F5/2 (Yb3þ) þ 4I15/2 (Er3þ) / 2F7/2 (Yb3þ) þ 4I11/2 (Er3þ). Since the absorption cross section of Yb3þ at ~980 nm is much larger than that of Er3þ: 4I11/2, which can be seen from Fig. 2, the ET process from Yb3þ to Er3þ is dominant in the excitation of (Er3þ) level. As if the 4I11/2 level is a long-life level, it is undoubtedly accelerating other ET processes that contribute to the excitation to 4F7/ 3þ 3þ in 4I11/2 level can be excited to the 4F7/2 level by means of cross relaxation 4I11/2 (Er3þ) þ 4I11/2 2 of Er . As a result, the Er (Er3þ) / 4I15/2 (Er3þ) þ 4F7/2 (Er3þ), or by absorbing another photon, or by the Yb3þ to Er3þ ET process from 2F5/2 (Yb3þ) þ 4I11/ 3þ 2 3þ 4 3þ 3þ ions can transfer their energy to an Er3þ ion and excite the ground2 (Er ) / F7/2 (Yb ) þ F7/2 (Er ). Two excited Yb 4 4 state I15/2 to the F7/2 state. This is the main two photon process in the Er3þ/Yb3þ co-doped a-NaYF4. The populated 4F7/2 level of Er3þ then relaxes to the next lower 2H11/2 and 4S3/2 levels nonradiatively due to the small energy gap between them. The above process produces two 2H11/2 / 4I15/2 and 4S3/2 / 4I15/2 green emissions centered at 522 and 541 nm, respectively. The
Fig. 5. Decay curves of the Er3þ/Yb3þ co-doped a-NaYF4 single crystals surveyed at 541 nm under 980 nm excitation.
Please cite this article in press as: S. He et al., Efficient upconversion luminescence in Er3þ/Yb3þ co-doped cubic NaYF4 single crystals by vertical Bridgman method with KF flux, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/ j.cjph.2016.04.011
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Fig. 6. Logelog dependence of the up-conversion intensities of 0.52 mol%Er3þ and 7.97 mol% Yb3þ co-doped a-NaYF4 single crystal at 522,541, 668 nm emissions as a function of the excitation power at 980 nm.
excited Er3þ ions can further relax nonradiatively to the 4I9/2 state; meanwhile, the Er3þ ions in the 4I11/2 state can relax nonradiatively to the 4I13/2 state and then populate to the 4F9/2 state via absorbing a phonon of 980 nm pumping light or energy transfer from Yb3þ ions, and leads to the red 668 nm 4F9/2 / 4I15/2 emission. When the concentration of Yb3þ increases to about 7.97 mol%, probably due to concentration quenching between the Er3þ and Yb3þ ions, the energy transfer process of Er3þ (4I13/2) þ Yb3þ (2F5/2) / Er3þ (4F9/2) þ Yb3þ (2F7/2) is reduced, and this leads to the reduction of the population of 4F9/2 of Er3þ ions and results in the decrease of 668 nm emission. Thus the strongest emission at 688 nm in this study is observed when the Yb3þ is about 4.99 mol%. However, the process of two photon UC process 22F5/2 (Yb3þ) þ 4I15/2 (Er3þ) / 22F7/2 (Yb3þ) þ 4F7/2 (Er3þ) continues to intensify as the Yb3þ concentration increases. The concentration quenching between Yb3þ and Yb3þ ions have not been observed in this study. 4. Conclusion The Er3þ/Yb3þ co-doped a-NaYF4 single crystal can be grown by a vertical Bridgman method using KF as flux and an enhanced up-conversion green and red lights can be obtained under excitation by a 980 nm diode laser. Study on the pump power dependent UC spectra shows that the UC emissions of the green and red lights arise from two-photon process from the
Fig. 7. Energy level diagram of Er3þ/Yb3þ and energy transfer progress.
Please cite this article in press as: S. He et al., Efficient upconversion luminescence in Er3þ/Yb3þ co-doped cubic NaYF4 single crystals by vertical Bridgman method with KF flux, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/ j.cjph.2016.04.011
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excited Yb3þ to Er3þ energy transfer. The combination of the UC green and red lights forms a yellowish green light to which human eyes are very sensitive. Such a Er3þ/Yb3þ co-doped a-NaYF4 single crystal may have potential applications in the fields of biosensors, infrared pumped visible eye-safe lasers, optical telecommunication and displaying devices due to its low phonon energy, high luminant efficiency, high transmittance for light and stability in chemical-physical properties. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51472125 and 51272109), and K.C. Wong Magna Fund in Ningbo University. References [1] N. Bloembergen, Phys. Rev. Lett. 15 (1965) 6e8, http://dx.doi.org/10.1142/9789812795793_0011. €mer, H.U. Güdell, J. Lumin. 94 (2001) 337e341, http://dx.doi.org/10.1016/S0022-2313(01)00395-7. [2] S. Heer, M. Wermuth, K. Kra [3] J.C. Boyer, F. Vetrone, J.A. Capobianco, A. Speghini, M. Zambelli, M. Bettinelli, J. Lumin. 106 (2004) 263e268, http://dx.doi.org/10.1016/j.jlumin.2003.11. 001. [4] M.J. Treadaway, R.C. Powel, J. Chem. Phys. 61 (1974) 4003e4011, http://dx.doi.org/10.1063/1.1681693. [5] J. Liu, J. Ma, B. Lin, Y. Ren, X. Jiang, J. Tao, X. Zhu, Ceram. Int. 34 (2008) 1557e1560, http://dx.doi.org/10.1016/j.ceramint.2007.03.025. [6] L.S. Cavalcante, J.C. Sczancoski, J.W.M. Espinosa, J.A. Varela, P.S. Pizani, E. Longo, J. Alloys Compd. 474 (2009) 195e200, http://dx.doi.org/10.1016/j. jallcom.2008.06.049. [7] H. Wang, F.D. Medina, Y.D. Zhou, Q.N. Zhang, Phys. Rev. B 45 (1992) 10356, http://dx.doi.org/10.1103/PhysRevB.45.10356. [8] Y. Onodera, T. Nunokawa, O. Odawara, H. Wada, J. Lumin. 137 (2013) 220e224, http://dx.doi.org/10.7567/jjap.53.05fk04. [9] V. Mahalingam, C. Hazra, R. Naccache, F. Vetrone, J.A. Capobianco, J. Mater. Chem. C 1 (2013) 6536e6540, http://dx.doi.org/10.1039/C5CP01083G. [10] Y. Cui, S. Zhao, Z. Liang, M. Han, Z. Xu, J. Alloys Compd. 593 (2014) 30e33, http://dx.doi.org/10.1016/j.jallcom.2014.01.051. [11] J.C. Boyer, L.A. Cuccia, J.A. Capobianco, J. Am. Chem. Soc. 128 (2006) 7444e7445, http://dx.doi.org/10.1021/ja061848b. [12] V. Ryzhii, A.A. Dubinov, V. Ya. Aleshkin, M. Ryzhii, T. Otsuji, IEEE Photonics J. 5 (2013), http://dx.doi.org/10.1063/1.4826113, 8400209-8400209. [13] A. Shalav, B.S. Richards, T. Trupke, K.W. Kr€ amer, H.U. Güdel, Appl. Phys. Lett. 86 (2005) 013505, http://dx.doi.org/10.1063/1.1844592. €lsa €, T. Laihinen, T. Laamanen, M. Lastusaari, L. Pihlgren, L.C. Rodrigues, T. Soukka, Physica B 439 (2014) 20e23, http://dx.doi.org/10.1016/j.physb. [14] J. Ho 2013.11.020. [15] H. Liang, G. Chen, L. Li, Y. Liu, F. Qin, Z. Zhang, Opt. Commun. 282 (2009) 3028e3031, http://dx.doi.org/10.1016/j.optcom.2009.04.006. [16] X.H. Li, J.Y. Xu, M. Jin, H. Shen, X.M. Li, Chin. Phys. Lett. 23 (2006) 3356, http://dx.doi.org/10.1088/0256-307X/23/12/065. [17] P. Vergeer, T. Vlugt, M. Kox, M. Hertog, J. Eerden, A.B. Meijerink, Phys. Rev. B Condens. Matter 71 (2005) 014119-1e014119-11, http://dx.doi.org/10.1103/ PhysRevB.52.3122. , J.A. Capobianco, Nanoscale 2 [18] F. Vetrone, R. Naccache, A. Juarranz de la Fuente, F. Sanz-Rodríguez, A. Blazquez-Castro, E.M. Rodriguez, D. Jaque, J.G. Sole (2010) 495e498, http://dx.doi.org/10.1039/b9nr00236g. [19] F.E. Auzel, Proc. IEEE 61 (1973) 758e786, http://dx.doi.org/10.1109/PROC.1973.9155. [20] J.P. Wittke, I. Ladany, P.N. Yocom, J. Appl. Phys. 43 (1972) 595, http://dx.doi.org/10.1063/1.1661372. , Jna. Gavald , F. Díaz, Opt. Mater. 27 (2004) 475, http://dx.doi.org/10.1016/j.optmat.2004. [21] X. Mateos, M.C. Pujol, F. Güell, R. Sole a, J. Massons, M. Aguilo 03.030.
Please cite this article in press as: S. He et al., Efficient upconversion luminescence in Er3þ/Yb3þ co-doped cubic NaYF4 single crystals by vertical Bridgman method with KF flux, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/ j.cjph.2016.04.011
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Chinese Journal of Physics journal homepage: http://www.journals.elsevier.com/ chinese-journal-of-physics/
Efficient upconversion luminescence in Er3þ/Yb3þ co-doped cubic NaYF4 single crystals by vertical Bridgman method with KF flux Shinan He a, Haiping Xia a, *, Qingyang Tang a, Qiguo Sheng a, Cheng Wang a, Zhigang Feng a, Dongsheng Jiang a, Xuemei Gu a, Jianli Zhang a, Haochuan Jiang b, **, Baojiu Chen c a b c
Key Laboratory of Photo-electronic Materials, Ningbo University, Ningbo, Zhejiang, 315211, China Ningbo Institute of Materials Technology and Engineering, The Chinese Academy of Sciences, Ningbo, Zhejiang, 315211, China Department of Physics, Dalian Maritime University, Dalian, Liaoning Province, 116026, China
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Er3þ/Yb3þ co-doped a-NaYF4 single crystals with various Yb3þ concentrations and ~0.50 mol% Er3þ were grown by Bridgman method using Potassium fluoride as flux. The up-conversion (UC) luminescence and its pump power dependence on UC emission of the crystals under excitation of 980 nm laser diode were investigated. The UC emissions at green ~522 nm, ~541 nm and red ~668 nm, which are attributed to 2H11/2 / 4I15/2, 4S3/ 4 4 4 3þ 3þ doped a-NaYF4 2 / I15/2 and F9/2 / I15/2, respectively, were obtained in Er /Yb single crystals. The quadratic dependence of pump power on the UC emission suggests that a two-photon process is responsible for the transition from the excited Yb3þ to the lower state of Er3þ ions in a-NaYF4 single crystals. A favorable performance of yellowish green light can be achieved with certain Yb3þ concentrations and 0.52 mol% Er3þ co-doped samples that makes further research valuable in developing UC displays for electro-optical devices. © 2016 Published by Elsevier B.V. on behalf of The Physical Society of the Republic of China (Taiwan).
1. Introduction During the past several decades, much effort has been devoted to the investigation on upconversion (UC) in rare earth doped materials because of both the scientific interest and the potential applications in short wavelength lasers, biological fluorescence, telecommunications, sensors and volumetric displays [1e3]. Photon UC is an anti-Stokes emission process in which the long wavelength photons are converted into the short wavelength photons. Among all the rare-earth ions, Er3þ is employed as a center of up-conversion luminescence due to its longer lifetimes of metastable energy levels and more homogeneous energy level array. The up-conversion emissions at blue (4F7/2 / 4I15/2, ~488 nm), green (2H11/2/4S3/2 / 4I15/2, ~520/550 nm) and red (4F9/2 / 4I15/2, ~660 nm) have been observed when pumped
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Xia), [email protected] (H. Jiang). http://dx.doi.org/10.1016/j.cjph.2016.04.011 0577-9073/© 2016 Published by Elsevier B.V. on behalf of The Physical Society of the Republic of China (Taiwan).
Please cite this article in press as: S. He et al., Efficient upconversion luminescence in Er3þ/Yb3þ co-doped cubic NaYF4 single crystals by vertical Bridgman method with KF flux, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/ j.cjph.2016.04.011
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with proper lights. By introducing Yb3þ ions as sensitizer into Er3þ doped materials, the performance of UC luminescence can be improved because of a larger absorption cross section at near infrared (NIR) of Yb3þ and the high-efficiency energy transfer from Yb3þ to Er3þ ions [4e7]. In recent years, most studies on Er3þ/Yb3þ co-doped systems have been focused on exploring the UC mechanisms and developing novel hosts [8e10]. The upconversion luminescence of rare-earth ions is also dependent on the host material. Many host materials such as fluorides, chlorides, sulfides and oxides have been widely studied [8,10]. It has been demonstrated that NaYF4 is an excellent host for up-conversion for its low phonon energy [11e13]. The crystal has two structures, one being cubic a-NaYF4 and the other hexagonal b-NaYF4 [14]. Er3þ and Yb3þ co-doped b-NaYF4 optical materials are of most efficient in UC among the reported UC materials [15]. However, the studied materials of NaYF4 mainly focus on nano-crystals and powders. There have scarcely been reports on NaYF4 single crystals because of the difficulty in the crystal growth. Compared with the nano-crystal of NaYF4, the single crystal, which is single-phased phosphors, provides high transmission for lights, good physical and mechanical performance, as well as good chemical durability. Thus, the growth of Er3þ/Yb3þ co-doped NaYF4 single crystal with high UC effect became a challenge to the researchers. Generally, there exists a problem of the inhomogeneous distribution of rare earth ions in single crystal resulted from crystal growth. In this study, NaYF4 single crystal was chosen as a matrix of Er3þ and Yb3þ ions in view of the same valence state and the very near ionic radii between Y3þ (0.88 Å) and Er3þ(0.881 Å), Y3þ and Yb3þ(0.858 Å), which resulted in relatively homogeneous concentration of Er3þ and Yb3þ in NaYF4 crystal. The NaYF4 single crystal with homogeneous rare earths is extremely important for the practical application in optical device. When Potassium fluoride (KF) is added into the starting components of NaFeYF3 system, the phase equilibrium between NaF and YF3 is significantly changed. With this change it becomes possible to crystallize only the composition of a-NaYF4 phase from incongruent melt. Some single crystals with high quality such as ZnO and YPO4 can be grown successfully by additional assistant flux into precursor melt [16,17]. The present paper describes the preparation of Er3þ and Yb3þ co-doped aNaYF4 single crystals grown with flux Bridgman method. High efficient UC luminescence of the obtained single crystal is presented under 980 nm excitation and the UC mechanisms are investigated from the measured pump power dependence. 2. Experimental The Er3þ/Yb3þ co-doped samples according to the formula 30NaFe18KFe(52-c-g)YF3ecErF3egYbF3 (c ¼ 0.4, g ¼ 0, 1.5, 2.5, 4), respectively, with 99.99% purity raw materials of NaF, KF, YF3, ErF3, and YbF3 were prepared. Then those mixture samples were ground thoroughly in a mortar for about 0.5 h. In order to remove the moisture and the oxide in the raw materials, the mixtures were sintered with anhydrous HF at 750 C for 8 h. The seed crystals were obtained from previous work by spontaneous nucleation from the seed wells and oriented along the a-axis. They were put in the bottom of seed well and then the sintered polycrystalline powders were filled in Pt crucibles of F10 mm 150 mm size. The crucibles were completely sealed in order to avoid contamination from water and oxygen in the air and to prevent the volatilization of the melt during crystal growth. The a-NaYF4 single crystals were grown in a resistively heated vertical Bridgman furnace. The furnace was gradually heated and held to 950 C. The seeding temperature was about 770e820 C and the temperature gradient cross solid-liquid interface was 70e90 C/cm. The growth process was carried out by lowering the crucible at a rate of 0.05e0.06 mm/h. After the growth was finished and the furnace cooled to room temperature slowly, the crystal was obtained by stripping off the Pt crucible. The X-ray diffraction (XRD) of the samples was measured using a XD-98X diffractometer (XD-3, Beijing). The absorption spectra were recorded with a Cary 5000 UV/VIS/NIR spectrophotometer (Agilent Co., America). The emission spectra were obtained with a FLSP 920 type spectrometer (Edinburgh Co., England). The Er3þ and Yb3þ concentrations in a-NaYF4 single crystals were measured by the inductive coupled plasma atomic emission spectroscopy (ICP-AES, PerkinElmer Inc., Optima 3000). All the measurements were performed at room temperature. Table 1 shows the concentrations of Er3þ and Yb3þ in the samples (mol%). 3. Results and discussion The phase equilibrium of NaFeYF3 is completely changed as introducing of KF into this system. The composition of aNaYF4 phase begins possibly to crystallize only from the incongruent melt. The suitable composition of raw materials had been demonstrated to be 30NaFe18KFe52YF3 through abundant of experiments. As shown in Fig. 1, there is a white region at the top of the boule, several centimeters in length, corresponding to the final portion of the melt to freeze. Examination using a polarized microscope showed the crystal to be free from macroscopic defects. The melting temperature of congruent crystal
Table 1 Concentration of Er3þ and Yb3þ ions in the crystals (mol%). Samples
1
2
3
4
Er3þ(c) Yb3þ(g)
0.51 0
0.52 2.97
0.51 4.99
0.52 7.97
Please cite this article in press as: S. He et al., Efficient upconversion luminescence in Er3þ/Yb3þ co-doped cubic NaYF4 single crystals by vertical Bridgman method with KF flux, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/ j.cjph.2016.04.011
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Fig. 1. X-ray diffraction patterns of Er3þ/Yb3þ co-doped a-NaYF4 single crystal of different part. Inset is the photo of a-NaYF4 single crystal.
is reduced by more than 100 C due to the introduction of KF. In order to seed successfully, a steep thermal gradient of the solid-liquid interface is also required. A slow growth speed of 0.05e0.06 mm/h was employed in order to keep KF away from the a-NaYF4 crystal structure completely. To understand the structures of the as-grown crystals, the XRD of different parts were tested and shown in Fig. 1. The boule of grown crystal is shown in the inset of Fig. 1. The initial and middle parts which are designated as A and B, respectively, appear light pink and lucid; however, the final section of the boule designated as C is white opaque. As compared with JCPD card (77-2042), one can confirm that the crystal of parts A and B are pure cubic a-NaYF4 phase and the current doping level does not cause any obvious peak shift or second phase. It can be confirmed that the crystals are a-NaYF4 single crystal since they are highly transparent with about 90% transmittance in the visible region, and no grain boundary is observed. In order to understand the structures of the white part C, which corresponds to the final stage of the growth process, the XRD pattern for part C is also recorded and shown in Fig. 1. One can see from Fig. 1 that the white part C is composed of KYF4, K2NaYF6, aNaYF4, and b-NaYF4, etc. The above XRD results suggest that the introduction of KF into NaFeYF3 system does not change the structure of a-NaYF4 single crystals. The KF is excluded from melt as the crystal grows and accumulates in the final portion of the boule. The absorption spectra in the 300e1100 nm range of the grown single crystal samples were measured and are shown in Fig. 2. The absorption peaks corresponding to transitions from the 4I15/2 ground state to the excited states of the Er3þ ions are also assigned in Fig. 2. By comparing the Er3þ doped sample with the Er3þ/Yb3þ co-doped samples, there is nearly no intensity and position changes for the peaks in the visible region, where only Er3þ ions can absorb light. For the absorption around 980 nm, where absorptions from 4I15/2 / 4I11/2 of Er3þ and 2F7/2 / 2F5/2 of Yb3þ exist, the absorption intensity shows a linear dependence on the Yb3þ concentration. The UC luminescence spectra of Er3þ/Yb3þ co-doped a-NaYF4 single crystal samples measured at room temperature with 980 nm laser are shown in Fig. 3, from which three obvious bands in the range of 450e750 nm can be observed. The inset of Fig. 3 is the dependence of integrated intensities of 522 nm, 541 nm, and 668 nm emissions and integrated total intensity on the Yb3þ concentration. In the measurements of these UC spectra, the same experimental conditions were held for all samples in order to get comparable results. Green emissions ranging from 509 to 534 nm and from 534 to 570 nm are due to the radiative transition 2H11/2 / 4I15/2 and 4S3/2 / 4I15/2 of Er3þ ion, respectively; whereas the red emission from 638 to 688 nm is attributed to the 4F9/2 / 4I15/2 transition. The above three emission peaks show energy Stark splitting of Er3þ under the action of a-NaYF4 single crystal field. The total UC intensity reaches its maximum when Yb3þ is ~7.97 mol%. By comparing the relative intensity of 541 nm and 668 nm, one can identify the difference. For the 541 nm emission the intensity increases rapidly as the Yb3þ concentration increases and reaches its maximal value when Yb3þ concentration is 7.97 mol%, while the maximum intensity of 668 nm emission is obtained when Yb3þ concentration is 4.99 mol%. Compared with reference [18], in which red emission is more efficient, this Er3þ/Yb3þ co-doped a-NaYF4 single crystal is highly beneficial for green UC emission. Since the observed UC emissions are located in visible wavelength range, the exact UC luminescence CIE chromaticity coordinates of all the Er3þ/Yb3þ co-doped a-NaYF4 single crystal samples are shown in Fig. 4 to obtain the true color of the UC emissions. The CIE coordinates (x, y) are (0.2871, 0.6711), (0.2943, 0.6898), (0.3371, 0.6482), and (0.2995, 0.6843), and color temperature Tc is 6288 K, 6155 K, 5529 K and 6082 K, respectively, for Er3þ/Yb3þ co-doped samples with 0.52Er/0Yb, 0.52Er/ 2.97Yb, 0.51Er/4.99Yb, and 0.52Er/7.97Yb doping concentrations. From the CIE chromaticity coordinates, it is clear that the combination of the UC emission is yellowish green color to which human eyes are sensitive. This characteristic is favorable for applications in UC displays for electro-optical devices. Fig. 5 illustrates the decay curves at 541 nm of a-NaYF4 single crystal co-doped with various Er3þ/Yb3þ concentrations under 980 nm excitation. The lifetime for 0.5 mol% Er3þ, 0.5 mol% Er3þ and 3 mol%Yb3þ, 0.5 mol% Er3þ and 5 mol%Yb3þ, Please cite this article in press as: S. He et al., Efficient upconversion luminescence in Er3þ/Yb3þ co-doped cubic NaYF4 single crystals by vertical Bridgman method with KF flux, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/ j.cjph.2016.04.011
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Fig. 2. Absorption spectra of Er3þ/Yb3þ co-doped a-NaYF4 single crystals.
Fig. 3. Comparison of the up-conversion luminescence spectra of Er3þ/Yb3þ doped a-NaYF4 single crystals under 980 nm excitation at room temperature.
0.5 mol% Er3þ and 8 mol%Yb3þ doped a-NaYF4 single crystals are tm ¼ 525.4 ms, tm ¼ 1640.2 ms, tm ¼ 1687.4 ms, and tm ¼ 1614.4 ms, respectively. As we can see, the lifetime for Er3þ singly doped a-NaYF4 single crystal is the least one. However, the other three Er3þ/Yb3þ doped samples show the similar value of ~1650 ms for the lifetimes at 541 nm. It suggests that there is no concentration quenching occurring in present doping concentration. The a-NaYF4 single crystal shows high incorporating concentrations for Er3þ and Yb3þ. Because of the similar radii size and same valence state among the rare earth ions, when the Er3þ and Yb3þ are doped in a-NaYF4 single crystal, they naturally replace the position of Y3þ. It is known that the ionic radius for Y3þ, Er3þ and Yb3þ are 0.893 Å, 0.881 Å and 0.858 Å, respectively. One can see that the differences between Y3þ and Er3þ, and Y3þ and Yb3þ are only 1.34% and 3.91%. An effective segregation coefficient near 1 for both Er3þ and Yb3þ in a-NaYF4 single crystals can be obtained due to the very close radii among the above ions. The Er3þ and Yb3þ ions tend to homogeneously distribute in a-NaYF4 single crystal that results in a high incorporating concentrations and high luminous effect of Er3þ and Yb3þ. In order to understand the UC dynamics of Er3þ/Yb3þ co-doped a-NaYF4 single crystal, the pump-power dependence of luminescence intensities is needed and shown in Fig. 6. In frequency up-conversion process the relation between emission intensity Iem and the NIR excitation power: IemfPn has been confirmed [19], where n is the number of pump photons required to excite to the emitting state. From the logelog dependence of the integrated green (~522 and ~541 nm) and red emission (~668 nm) intensities on the excitation power at 980 nm shown in Fig. 5, one can find n ¼ 1.968, 1.852, 1.721, respectively. The near-quadratic dependence indicates that two photons are involved in the up-conversion process. The mechanism of energy transfer (ET) from Yb3þ to Er3þ has been reported in the previously works [20,21]. In those papers the researchers attempted to quantitatively investigate the ET progress in the co-doped samples. Fig. 7 shows the Please cite this article in press as: S. He et al., Efficient upconversion luminescence in Er3þ/Yb3þ co-doped cubic NaYF4 single crystals by vertical Bridgman method with KF flux, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/ j.cjph.2016.04.011
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Fig. 4. CIE chromaticity coordinates of Er3þ/Yb3þ co-doped a-NaYF4 single crystals.
energy level diagram of Er3þ/Yb3þ and the energy transfer process. The 4I11/2 level of Er3þ ion can be directly excited by 980 nm pumping light from the ground state 4I15/2 or via the energy transfer (ET) process from 2F5/2 level of Yb3þ to Er3þ: 2F5/2 (Yb3þ) þ 4I15/2 (Er3þ) / 2F7/2 (Yb3þ) þ 4I11/2 (Er3þ). Since the absorption cross section of Yb3þ at ~980 nm is much larger than that of Er3þ: 4I11/2, which can be seen from Fig. 2, the ET process from Yb3þ to Er3þ is dominant in the excitation of (Er3þ) level. As if the 4I11/2 level is a long-life level, it is undoubtedly accelerating other ET processes that contribute to the excitation to 4F7/ 3þ 3þ in 4I11/2 level can be excited to the 4F7/2 level by means of cross relaxation 4I11/2 (Er3þ) þ 4I11/2 2 of Er . As a result, the Er (Er3þ) / 4I15/2 (Er3þ) þ 4F7/2 (Er3þ), or by absorbing another photon, or by the Yb3þ to Er3þ ET process from 2F5/2 (Yb3þ) þ 4I11/ 3þ 2 3þ 4 3þ 3þ ions can transfer their energy to an Er3þ ion and excite the ground2 (Er ) / F7/2 (Yb ) þ F7/2 (Er ). Two excited Yb 4 4 state I15/2 to the F7/2 state. This is the main two photon process in the Er3þ/Yb3þ co-doped a-NaYF4. The populated 4F7/2 level of Er3þ then relaxes to the next lower 2H11/2 and 4S3/2 levels nonradiatively due to the small energy gap between them. The above process produces two 2H11/2 / 4I15/2 and 4S3/2 / 4I15/2 green emissions centered at 522 and 541 nm, respectively. The
Fig. 5. Decay curves of the Er3þ/Yb3þ co-doped a-NaYF4 single crystals surveyed at 541 nm under 980 nm excitation.
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Fig. 6. Logelog dependence of the up-conversion intensities of 0.52 mol%Er3þ and 7.97 mol% Yb3þ co-doped a-NaYF4 single crystal at 522,541, 668 nm emissions as a function of the excitation power at 980 nm.
excited Er3þ ions can further relax nonradiatively to the 4I9/2 state; meanwhile, the Er3þ ions in the 4I11/2 state can relax nonradiatively to the 4I13/2 state and then populate to the 4F9/2 state via absorbing a phonon of 980 nm pumping light or energy transfer from Yb3þ ions, and leads to the red 668 nm 4F9/2 / 4I15/2 emission. When the concentration of Yb3þ increases to about 7.97 mol%, probably due to concentration quenching between the Er3þ and Yb3þ ions, the energy transfer process of Er3þ (4I13/2) þ Yb3þ (2F5/2) / Er3þ (4F9/2) þ Yb3þ (2F7/2) is reduced, and this leads to the reduction of the population of 4F9/2 of Er3þ ions and results in the decrease of 668 nm emission. Thus the strongest emission at 688 nm in this study is observed when the Yb3þ is about 4.99 mol%. However, the process of two photon UC process 22F5/2 (Yb3þ) þ 4I15/2 (Er3þ) / 22F7/2 (Yb3þ) þ 4F7/2 (Er3þ) continues to intensify as the Yb3þ concentration increases. The concentration quenching between Yb3þ and Yb3þ ions have not been observed in this study. 4. Conclusion The Er3þ/Yb3þ co-doped a-NaYF4 single crystal can be grown by a vertical Bridgman method using KF as flux and an enhanced up-conversion green and red lights can be obtained under excitation by a 980 nm diode laser. Study on the pump power dependent UC spectra shows that the UC emissions of the green and red lights arise from two-photon process from the
Fig. 7. Energy level diagram of Er3þ/Yb3þ and energy transfer progress.
Please cite this article in press as: S. He et al., Efficient upconversion luminescence in Er3þ/Yb3þ co-doped cubic NaYF4 single crystals by vertical Bridgman method with KF flux, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/ j.cjph.2016.04.011
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excited Yb3þ to Er3þ energy transfer. The combination of the UC green and red lights forms a yellowish green light to which human eyes are very sensitive. Such a Er3þ/Yb3þ co-doped a-NaYF4 single crystal may have potential applications in the fields of biosensors, infrared pumped visible eye-safe lasers, optical telecommunication and displaying devices due to its low phonon energy, high luminant efficiency, high transmittance for light and stability in chemical-physical properties. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51472125 and 51272109), and K.C. Wong Magna Fund in Ningbo University. References [1] N. Bloembergen, Phys. Rev. Lett. 15 (1965) 6e8, http://dx.doi.org/10.1142/9789812795793_0011. €mer, H.U. Güdell, J. Lumin. 94 (2001) 337e341, http://dx.doi.org/10.1016/S0022-2313(01)00395-7. [2] S. Heer, M. Wermuth, K. Kra [3] J.C. Boyer, F. Vetrone, J.A. Capobianco, A. Speghini, M. Zambelli, M. Bettinelli, J. Lumin. 106 (2004) 263e268, http://dx.doi.org/10.1016/j.jlumin.2003.11. 001. [4] M.J. Treadaway, R.C. Powel, J. Chem. Phys. 61 (1974) 4003e4011, http://dx.doi.org/10.1063/1.1681693. [5] J. Liu, J. Ma, B. Lin, Y. Ren, X. Jiang, J. Tao, X. Zhu, Ceram. Int. 34 (2008) 1557e1560, http://dx.doi.org/10.1016/j.ceramint.2007.03.025. [6] L.S. Cavalcante, J.C. Sczancoski, J.W.M. Espinosa, J.A. Varela, P.S. Pizani, E. Longo, J. Alloys Compd. 474 (2009) 195e200, http://dx.doi.org/10.1016/j. jallcom.2008.06.049. [7] H. Wang, F.D. Medina, Y.D. Zhou, Q.N. Zhang, Phys. Rev. B 45 (1992) 10356, http://dx.doi.org/10.1103/PhysRevB.45.10356. [8] Y. Onodera, T. Nunokawa, O. Odawara, H. Wada, J. Lumin. 137 (2013) 220e224, http://dx.doi.org/10.7567/jjap.53.05fk04. [9] V. Mahalingam, C. Hazra, R. Naccache, F. Vetrone, J.A. Capobianco, J. Mater. Chem. C 1 (2013) 6536e6540, http://dx.doi.org/10.1039/C5CP01083G. [10] Y. Cui, S. Zhao, Z. Liang, M. Han, Z. Xu, J. Alloys Compd. 593 (2014) 30e33, http://dx.doi.org/10.1016/j.jallcom.2014.01.051. [11] J.C. Boyer, L.A. Cuccia, J.A. Capobianco, J. Am. Chem. Soc. 128 (2006) 7444e7445, http://dx.doi.org/10.1021/ja061848b. [12] V. Ryzhii, A.A. Dubinov, V. Ya. Aleshkin, M. Ryzhii, T. Otsuji, IEEE Photonics J. 5 (2013), http://dx.doi.org/10.1063/1.4826113, 8400209-8400209. [13] A. Shalav, B.S. Richards, T. Trupke, K.W. Kr€ amer, H.U. Güdel, Appl. Phys. Lett. 86 (2005) 013505, http://dx.doi.org/10.1063/1.1844592. €lsa €, T. Laihinen, T. Laamanen, M. Lastusaari, L. Pihlgren, L.C. Rodrigues, T. Soukka, Physica B 439 (2014) 20e23, http://dx.doi.org/10.1016/j.physb. [14] J. Ho 2013.11.020. [15] H. Liang, G. Chen, L. Li, Y. Liu, F. Qin, Z. Zhang, Opt. Commun. 282 (2009) 3028e3031, http://dx.doi.org/10.1016/j.optcom.2009.04.006. [16] X.H. Li, J.Y. Xu, M. Jin, H. Shen, X.M. Li, Chin. Phys. Lett. 23 (2006) 3356, http://dx.doi.org/10.1088/0256-307X/23/12/065. [17] P. Vergeer, T. Vlugt, M. Kox, M. Hertog, J. Eerden, A.B. Meijerink, Phys. Rev. B Condens. Matter 71 (2005) 014119-1e014119-11, http://dx.doi.org/10.1103/ PhysRevB.52.3122. , J.A. Capobianco, Nanoscale 2 [18] F. Vetrone, R. Naccache, A. Juarranz de la Fuente, F. Sanz-Rodríguez, A. Blazquez-Castro, E.M. Rodriguez, D. Jaque, J.G. Sole (2010) 495e498, http://dx.doi.org/10.1039/b9nr00236g. [19] F.E. Auzel, Proc. IEEE 61 (1973) 758e786, http://dx.doi.org/10.1109/PROC.1973.9155. [20] J.P. Wittke, I. Ladany, P.N. Yocom, J. Appl. Phys. 43 (1972) 595, http://dx.doi.org/10.1063/1.1661372. , Jna. Gavald , F. Díaz, Opt. Mater. 27 (2004) 475, http://dx.doi.org/10.1016/j.optmat.2004. [21] X. Mateos, M.C. Pujol, F. Güell, R. Sole a, J. Massons, M. Aguilo 03.030.
Please cite this article in press as: S. He et al., Efficient upconversion luminescence in Er3þ/Yb3þ co-doped cubic NaYF4 single crystals by vertical Bridgman method with KF flux, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/ j.cjph.2016.04.011