Er3+ codoped single LiYF4 microparticle

Er3+ codoped single LiYF4 microparticle

Journal of Luminescence 152 (2014) 44–48 Contents lists available at ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/locate...

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Journal of Luminescence 152 (2014) 44–48

Contents lists available at ScienceDirect

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

Luminescence investigation of Yb3 þ /Er3 þ codoped single LiYF4 microparticle Wei Gao, Hairong Zheng n, Enjie He, Ying Lu, Fangqi Gao School of Physics and Information Technology, Shaanxi Normal University, Xi'an 710062, PR China

art ic l e i nf o

a b s t r a c t

Available online 29 October 2013

Tetragonal phase LiYF4:Yb3 þ /Er3 þ microparticles are synthesized via facile hydrothermal method. Single LiYF4 microparticle is excited with IR laser at 980 nm in a confocal setup, and strong green and weak red emissions are observed. It is found that single LiYF4:Yb3 þ /Er3 þ microparticle with sub-structure presents stronger upconversion luminescence emission and smaller intensity ratio of red to green emission than that from LiYF4:Yb3 þ /Er3 þ microparticle with no sub-structure. The possible mechanism, the influence of particle size and the existence of EDTA on the upconversion luminescence emission are investigated. The current study suggests that the luminescence observation with single micropaticle can effectively avoid the influence of environment and neighbor particles, which is important for investigating the luminescence properties of micro- or nano-crystals and for extending their application. & 2013 Elsevier B.V. All rights reserved.

Keywords: Single microparticle LiYF4:Yb3 þ /Er3 þ Upconversion luminescence emission

1. Introduction Lanthanide-doped fluoride has been regarded as an ideal upconversion (UC) fluorescence material. It has been used as phosphor in lighting and display, the development of new laser crystal, nanoscale biolabel and solar cell [1–5]. Up to now, a great deal of attention has been paid on the rare-earth doped cubic and hexagonal phase NaYF4 nano- and micro-crystals. However, like hexagonal phase NaYF4 crystal, the tetragonal phase LiYF4 crystal is also considered as an ideal matrix for trivalent rare earth ions [6,7]. Comparing with rare earth doped nanomaterials, microsized luminescence materials usually present stronger UC emission due to their smaller surface-to-volume ratio and less surface quenching centers [8,9], which is essential for extending the application in display and micro optoelectronic devices. There are some reports on fabricating rare-earth doped octahedral LiYF4 nano- and micro-structures, and on the observation of superior UC fluorescence of Yb3 þ /(Er3 þ , Tm3 þ , Ho3 þ ) doped LiYF4 nanocrystals [10–13]. Our group has studied the synthesization of tetragonal phase LiY/YbF4:Yb3 þ /Er3 þ microparticles, and we have found that LiY/YbF4:Yb3 þ /Er3 þ microparticles display strong green UC luminescence emission [14]. In this paper, We report the preparation of tetragonal phase LiYF4:Yb3 þ /Er3 þ microparticles with different morphologies via facile hydrothermal method in the presence of EDTA and in the absence of EDTA. Since the EDTA has strong thermal stability

n

Corresponding author. E-mail address: [email protected] (H. Zheng).

0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.10.046

under hydrothermal condition, it can be used to control the particle size and morphology [15]. The up- and down-conversion luminescence emission of single LiYF4:Yb3 þ /Er3 þ microparticle with different size and structure are studied with a confocal setup. It is found that the presence of EDTA during the synthesization process has a critical influence on particle morphology and UC luminescence emission. 2. Experimental details 2.1. Sample preparation LiYF4:Yb3 þ /Er3 þ microparticles are synthesized by hydrothermal method for which the detailed process is given in Ref. [14]. All chemicals used in the current study are graded and are analytical used without further purification. Y(NO3)3, Yb(NO3)3, Er(NO3)3 and Eu(NO3)3 are obtained by dissolving Y2O3, Yb2O3, Er2O3 and Eu2O3 (99.99%. Sigma-Aldrich) with nitric acid, respectively. The solution is stirred at 60 1C for several hours to remove excess nitric acid. Then it is dissolved in deionized water to form rare-earth nitrate solution. NH4HF2(98.0%), LiF(98.0%), and EDTA (ethylenediamine tetraacetic acid, 99.0%) with analytical grade are supplied by the Tianjin chemical reagent factory. The preparation process is presented as follow if we take the synthesization of LiYF4: 20.0%Yb3 þ /2.0%Er3 þ as an example. First, 0.48 g LiF and 0.17 g NH4HF2 are added into 10.0 ml deionized water under vigorous stirring to form solution A. Second, 0.78 ml 0.5 mol/l Y(NO3)3, 0.20 ml 0.5 mol/l Yb(NO3)3, 0.02 ml 0.5 mol/l Er (NO3)3 and 10.0 ml 0.05 mol/l EDTA are mixed to form a chelate

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complex solution, which is added into solution A afterwards. Then one need to stir it for about 20 min until it completely becomes white liquid. Finally, The white liquid is slowly transferred into a 40 ml Teflon-lined autoclave and is heated at 220 1C for 24 h to get LiYF4: 20.0%Yb3 þ /2.0%Er3 þ microparticles. The LiYF4: 20.0%Yb3 þ / 2.0%Er3 þ microparticles with EDTA in the reaction system are obtained by centrifuging and washing with deionized water and ethanol for several times. The collected samples are finally dried at 60 1C for 12 h. LiYF4: 20.0%Yb3 þ /2.0%Er3 þ microparticles with no

Fig. 1. XRD patterns of the sample A (a) and the sample B (b).

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EDTA in the reaction system are prepared by following the similar procedure. For simplicity, LiYF4: 20.0%Yb3 þ /2.0%Er3 þ microparticles with EDTA in the reaction process will be called sample A, and the LiYF4: 20.0%Yb3 þ /2.0%Er3 þ microparticles without EDTA in the reaction system will be called sample B. 2.2. Sample characterization and spectroscopic measurements The powder x-ray diffraction (XRD) patterns are recorded by D/ Max2550VBþ/PCx-ray diffraction meter with Cu Kα (40 kV, 40 mA) irradiation (λ¼0.15406 nm). The 2θ angle of the XRD spectra is from 101 to 701 and recorded at a scanning rate of 81 min  1. The morphology of the particles is characterized by the scanning electron microscope (SEM, Quanta 200) operating at voltage of 20 kV. Fourier transform infrared spectroscopy (FTIR) is measured with a Brucher EQUINX55 spectrometer using KBr pellets. For spectroscopic measurements, YAG:Nd3 þ (Quanta Ray Lab-170) pulse laser and Ti sapphire femtosecond laser (Mira-900) are employed as excitation sources. The spectrometer (SP2750i) with a spectral resolution of 0.008 nm is used for luminescence collection and detection. The optical microscope (OLYMPUS-BX51) is used in the confocal setup, and the corresponding magnifications are 100, 500 and 1000. Proper notch filters are placed in front of the entrance of the monochromator to block the scattering light. All of the spectroscopic measurements are carried out at room temperature.

Fig. 2. SEM of the sample A (a) and the sample B (b)–(d) EDS spectrum of the sample A and the sample B.

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3. Results and discussion 3.1. Crystal structure and morphology of LiYF4 microparticles The typical XRD patterns of the sample A and the sample B are shown in Fig. 1(a) and (b), respectively. The diffraction peaks could be readily indexed to the pure tetragonal phase, which is consistent with the standard card JCPDS card 81-2254. Fig. 2 presents SEM images of the sample A and the sample B. It is found that the sample A and the sample B are octahedral in shape and the average size of the particles is around 12 μm. Interestingly, the surface and structure of the sample A and B are very different. The surface of the sample A is smooth and no any obvious fine structure in it. While for the sample B, clear substructure is presented and its surface is less smooth. Fig. 2c and d are the EDS of the sample A and the sample B, in which elements of Y, Yb, F and Er are clearly indicated. 3.2. Photoluminescence study Investigation of UC luminescence emission and its properties are carried with the confocal setup. Fig. 3 shows UC fluorescence emission spectra and luminescence pictures of LiYF4:Yb3 þ /Er3 þ single microparticle under 980 nm excitation. The strong green (520–575 nm) and red (640–675 nm) emissions are assigned to (2H11/2, 4S3/2)-4H15/2 and 4F9/2-4I15/2 transitions of Er3 þ in the

single particle of the sample A and B [16–20]. The UC emission intensity is enhanced with increase of the microparticle size. This result indicates that bigger particle has stronger UC emission, which is probably due to less surface quenching centers and more the Yb3 þ ions number than that small size particle [9,16]. Fig. 3C shows UC emission spectra of a single particle of the sample A and B with the same size of 10 mm. The single particle of the sample B presents stronger UC luminescence emission and smaller intensity ratio of red to green emissions than that from the single particle of the sample A. To investigate the green (2H11/2, 4S3/2-4H15/2) and red (4H9/24 I15/2) UC mechanism, the pump power dependence of luminescence intensity is performed and the experimental observation is plotted in Fig. 4. Slopes of 1.80 and 1.69 of the sample A are yielded by fitting the experimental data in Fig. 4(a), and the slopes of 1.96 and 1.78 of the sample B are yielded by fitting the experimental data in Fig. 4(b). The values of the slope indicate that the green and red emissions from the particle of the sample A and sample B are two photo excitation process [17]. Possible UC process occurred in LiYF4:Yb3 þ /Er3 þ microparticle is demonstrated in Fig. 5. Since Yb3 þ has large absorption cross section for infrared light and long excited state lifetime, it is reasonable to believe that the main pathway to populate upper emitting states is energy transfer from Yb3 þ to Er3 þ . Under 980 nm excitation, Yb3 þ transfers its energy to the nearby Er3 þ through three successive energy transfers from Yb3 þ to Er3 þ

Fig. 3. UC fluorescence spectra of single particle of the sample A (A) and the sample B (B) with different particle sizes ((a) 8 μm, (b) 10 μm, (c) 12 μm and (d) 15 μm). (C) UC fluorescence spectra of a single particle of the sample A (a) and the sample B (b) with size of 10 μm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. Pump power dependence of the green and red UC intensity of the sample A (a) and the sample B (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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populating the 4I11/2, 4F9/2 and 4F7/2 levels of Er3 þ , leading to the green (2H11/2, 4S3/2-4H15/2) and red (4H9/2-4I15/2) emissions of Er3 þ . Generally, the UC emission efficiency of doped ions depends on the site symmetry of the crystal field, the surface defects and organic ligands. The site symmetry of the matrix crystal field can be explored by introducing proper probe ions. Eu3 þ ion is regarded as a good probe for detecting the site symmetry due to the fact that 5D0-7F1 (591 nm) transition is magnetic dipole nature being insensitive to the site symmetry, while 5D0-7F2 (615 nm) transition is electric dipole nature being very sensitive to the site symmetry. Therefore, the asymmetric ratio of emission intensities from 5D0-7F1 and 5D0-7F2 are often used as a fingerprint of local symmetry for luminescent centers [21,22]. Fig. 6A is emission spectra of the sample A and sample B after replacing Er3 þ with Eu3 þ . It can be seen that the presence of EDTA in the synthesization process has no obvious influence on the site symmetry of the crystal fields since the asymmetric ratios of 5 D0-7F1 to 5D0-7F2 are similar for both cases. Thus, the intensity difference for UC emission is very possible due to the existence of EDTA on the surface of microparticles. It is reported that the surface-related high-frequency vibrational modes play an important role in the UC process [23]. Fourier-transform infrared (FTIR) spectroscopy can help us to identify the existence of EDTA at the

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surface of microparticles in the presence of EDTA. The observed FTIR spectra from particles of the sample A and B is shown in Fig. 6B. The existence of EDTA in the sample A is proved by the appearance of –CH2– (2850 cm  1) stretching, –CH2– (1400 cm  1) bending, C–O (1091 cm  1) stretching and –COO– (1640 cm  1) stretching vibrations [24]. The broadband at 3400 cm  1 is assigned to the hydroxyl group (–OH) stretching vibration of water and EDTA. Therefore, one should say that the fluorescence emission intensity becomes weaker with the presence of EDTA. As can be seen from Fig. 3C, the intensity ratio of red to green is higher from the sample B than that from the sample A. Similar phenomena are also observed in NaYF4:Yb3 þ /Er3 þ nanocrystals and submicroplates [25]. This phenomenon could be understood by considering the phonon energy of LiYF4 matrix and the energy of vibrational modes of EDTA. According to the energy level diagram and UC mechanism, multiphonon nonradiative relaxations of 4I11/2-4I13/2 and 4S3/2-4F9/2 should be very weak. The reason is that their energy gaps are about 3000 cm  1 and 3600 cm  1, which are about five times higher than the phonon energy of LiYF4 that is about 570 cm  1 [17]. On the other hand, hydroxyl groups (2700–3600 cm  1) possess high-energy vibration modes that would strongly quench the excited states of Er3 þ ions and accelerate the multiphonon relaxations of 4I11/2-4I13/2 and 4S3/2-4F9/2, increasing the population of the intermediate states and finally resulting in the relative intensity change of red and green emissions [26]. Therefore, we suggest that intensity ratio change of red and green emission in the sample A is due to the presence of the hydroxyl groups on the surface of the sample A.

4. Conclusion

Fig. 5. Energy level diagram with proposed UC mechanism for Er3 þ and Yb3 þ codoped LiYF4 microparticles.

The tetragonal phase LiYF4:Yb3 þ /Er3 þ microparticles are synthesized by applying the facile hydrothermal method with EDTA and without EDTA added in the preparation process. The UC luminescence emission from single microparticle is investigated experimently with a confocal setup. The study suggests that the presence of EDTA in the synthesization process can control the morphology of micropaticle, but does not influence the site symmetry of matrix crystal. It is also found that the organic ligands of EDTA on the surface of microparticles affect the property of luminescence emission by changing the nonradiative relaxation rate.

Fig. 6. (A) Emission spectra of the sample A (a) and the sample B (b) after Er3 þ is replaced by Eu3 þ . (B) FTIR spectra of the sample A (a) and the sample B (b).

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Acknowledgement The work is supported by the National Science Foundation of China (Grant 11174190), the Fundamental Research Funds for the Central Universities (Grants GK201101006 and GK201304002). References [1] E. Downing, L. Hesselink, J. Ralston, R. Macfarlane, Science 273 (1996) 1185. [2] J. Zhou, Z. Liu, F.Y. Li, Chem. Soc. Rev. 41 (2012) 1323. [3] B. Dong, D.P. Liu, X.J. Wang, T. Yang, S.M. Miao, C.R. Li, Appl. Phys. Lett. 90 (2007) 181117. [4] G. Yi, H. Lu, S. Zhao, Y. Ge, W. Yang, D. Chen, L. Guo, Nano Lett. 4 (2004) 2191. [5] Y.S. Chen, W. He, Y.C. Jiao, H.H. Wang, X.L. Hao, J.X. Lu, S.E Yang, J. Lumin. 132 (2012) 2247. [6] V. Mahalingam, F. Vetrone, R. Naccache, A. Speghini, J.A. Capobianco, Adv. Mater. 21 (2009) 4025. [7] K. Ogasawara, S. Watanabe, H. Toyoshima, T. Ishii, M.G. Brik, H. Ikeno, I. Tanaka, J. Solid State Chem. 178 (2005) 412. [8] A. Yin, Y. Zhang, L. Sun, C. Yan, Nanoscale 2 (2010) 953. [9] F. Wang, J. Wang, X.G. Liu, Angew. Chem. 121 (2010) 7618. [10] X.J. Pei, Y.B. Hou, S.L. Zhao, Z. Xu, F. Teng, Mater. Chem. Phys. 90 (2005) 270. [11] G.Y. Chen, T.Y. Ohulchanskyy, A. Kachynski, H. Ågren, P.N. Prasad, ACS Nano 5 (2011) 4981.

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