alexa Sensitization Effect of Yb3+ in Upconversion Luminescence of Eu3+Codoped Y2O3 Phosphor | Open Access Journals
ISSN: 2161-0398
Journal of Physical Chemistry & Biophysics
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Sensitization Effect of Yb3+ in Upconversion Luminescence of Eu3+Codoped Y2O3 Phosphor

Anurag Pandey, Riya Dey and Vineet Kumar Rai*
Laser and Spectroscopy Laboratory, Department of Applied Physics, Indian School of Mines, Dhanbad-826004, Jharkhand, India
Corresponding Author : Vineet Kumar Rai
Laser and Spectroscopy Laboratory
Department of Applied Physics
Indian School of Mines
Dhanbad-826004, India
Tel: +91-326-223-5404/5282
E-mail: [email protected]; [email protected]
Received August 22, 2013; Accepted September 27, 2013; Published September 30, 2013
Citation: Pandey A, Dey R, Rai VK (2013) Sensitization Effect of Yb3+ in Upconversion Luminescence of Eu3+ Codoped Y2O3 Phosphor. J Phys Chem Biophys 3:129. doi:10.4172/2161-0398.1000129
Copyright: © 2013 Pandey A, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

For structural information, the X-ray diffraction analysis of Y2O3:Eu3+ phosphors codoped with Yb3+ synthesized by combustion synthesis process has been performed. The upconversion emission study of the Y2O3:Eu3+ phosphor codoped with Yb3+ ions on excitation with 980 nm diode laser in the visible region has been done. The upconversion emissions corresponding to the Eu3+ ions is due to sensitization from Yb3+ ions in developed phosphor, and has been explained on the basis of cooperative energy transfer process. The orange colour emitted from the codoped samples is visualized by CIE diagram. The results show the applicability of the present phosphor as suitable NIR (near infrared) to visible upconverter, and in other photonic devices.

Keywords
Combustion synthesis; Cooperative emission; Photoluminescence; Rare earths
Introduction
The rare earths (RE) doped luminescent materials have been the subject of significant interest in recent years due to their potential applications in different colour fields [1-4]. In these materials, mainly the rare earth elements are responsible for the generation of radiation of light of different by changing the dopants, which are useful for various applications [5,6]. Several oxide host materials are available for the preparation of rare earth doped luminescent materials, but the Y2O3 is chosen due to their high optical band gap, low phonon frequency and ionic radii comparable with most of the rare earths. Several studies have been reported on synthesis and optical characterisation of nanocrystalline Eu3+ doped phosphors [7,8]. But, the upconversion (UC) emission with NIR excitation in singly Eu3+ doped materials is not possible. So, in order to get visible UC emission from Eu3+ ion, another rare earth ion can be used as sensitizer to excite Eu3+ ions. In our previous study, we have excited Eu3+ ion by a 980 nm laser using Er3+ as sensitizer and studied its UC behavior [9]. In most of the cases, Yb3+ ion is taken as the sensitizer because of its higher absorption crosssection corresponding to 980 nm excitation.
In the present work, we have synthesized Y2O3: Eu3+-Yb3+ phosphors through solution combustion synthesis process. The XRD analysis and upconversion emission studies of the synthesized material have been performed, and the process responsible for the UC emissions is discussed in detail.
Experimental Procedure
The Eu3+, Yb3+ codoped Y2O3 Phosphor powders have been prepared by low temperature solution combustion method. The compositions of the compounds used were as follows:
(100-x-y)Y2O3+x Eu2O3+y Yb2O3
where x=1.0 mol%, y=1.0, 3.0, 5.0 mol%.
Firstly, the Y2O3, Eu2O3 and Yb2O3 were dissolved in HNO3 to convert in the form of nitrates. The nitrate forms of host and dopants were mixed with urea solution and stirred about 2 hours at 65°C, till transparent gel was obtained. The formed gel was taken in an alumina crucible and placed inside an electrical furnace preheated at 600°C where combustion took place. The obtained samples were grinded to get fine and homogeneous powder, and then heat treated at higher temperature about 800°C. These heat treated samples have been used for further measurements.
Results and Discussion
X-ray diffraction analysis
The X-ray diffraction pattern of heat treated Eu3+, Yb3+ codoped Y2O3 phosphor has been shown in Figure 1. The observed peaks matches very well with the peaks of JCPDS card no. 25-1200. The results indicated cubic structure of synthesized material with lattice parameters a=b=c=10.60 Å and α=β=γ=90°. The average crystallite size of developed phosphor has been found around 20 nm using well known Scherrer’s formula [10].
Upconversion emission study
The UC emission spectra of Eu3+, Yb3+ codoped Y2O3 phosphor upon excitation at 980 nm with fixed Eu3+ (1.0 mol%) and varying Yb3+ (1.0, 3.0, 5.0 mol%) concentrations have been recorded, and for 1.0 mol% Eu3++3.0 mol% Yb3+ combination the maximum UC emission intensity has been observed. In Figure 2, we have shown the UC emission spectra corresponding to the optimized concentration of Eu3+ and Yb3+ ions. The Eu3+ ions cannot be excited directly by using a 980 nm diode laser excitation due to unavailability of energy levels in Eu3+ ions. But, in the Eu3+, Yb3+ codoped system upconversion emission spectrum is observed (Figure 2). This indicates that the Eu3+ ions are excited due to presence of the Yb3+ ions. The UC emission bands peaking about 526 nm, 550 nm, 586 nm, 611 nm and 658 nm are assigned as 5D17F0, 5D17F2, 5D07F0,1 and 5D07F2, 5D07F3 transitions of Eu3+ ion, respectively [11]. The orange colour light emitted from synthesized sample can be seen by naked eyes.
No UC emission bands have been observed in Eu3+ doped phosphor, but are found in the Eu3+, Yb3+ codoped phosphor. This is basically due to sensitization of Eu3+ ions by Yb3+ ions via cooperative energy transfer process. The contribution of two Yb3+ ions have been observed in UC emissions from Eu3+, Yb3+ codoped materials [11]. A simplified energy level scheme of Eu3+ and Yb3+ ion is shown in Figure 3.
The Yb3+ ions from its 2F7/2 ground state are firstly excited to the 2F5/2 state, and then transfer their energy cooperatively in such a way that one of them (acceptor) after gaining energy from the another one (donor) occupies the virtual state (V) (i.e. 2F5/2+2F5/2→22F5/2). After that the excited Yb3+ ions from its virtual state transfer their excitation energy directly to the ground state Eu3+ ions and promoting them to the 5D1 and 5D0 level of Eu3+ ions [12]. Then, the relaxations from 5D1 and 5D0 levels gives radiative emissions at ~526 nm, ~550 nm, ~586 nm, ~611 nm and ~658 nm, corresponding to the 5D17F0, 5D17F2, 5D07F0,1, 5D07F2 and 5D07F3 transitions, respectively.
The CIE color coordinates corresponding to 1.0 mol% Eu3+-y mol% Yb3+ (y=1.0, 3.0, 5.0 mol%) codoped Y2O3 phosphors have been calculated as shown in Figure 4. The calculated values of color coordinates are found to be (0.56, 0.41), (0.57, 0.40), (0.58, 0.40), respectively. This does not show any significant variation in color emitted from the phosphor with increasing content of the Yb3+ ions, which makes such phosphor materials suitable to use in display devices.
Conclusion
The cubic structured Eu3+, Yb3+ codoped Y2O3 phosphors have been synthesized via low temperature combustion technique successfully. The average crystallite size ~20 nm has been confirmed with the help of XRD analysis. The study of upconversion emission observed from the synthesized phosphor by 980 nm excitation support the possibility of cooperative energy transfer from Yb3+ to Eu3+ ions. The color emitted from the developed phosphors is confirmed from CIE diagram. The present study shows utility of the Eu3+, Yb3+ codoped Y2O3 phosphors as NIR to visible upconverter, and its applicability for the development of other photonic devices.
Acknowledgements
Authors are grateful to the University Grants Commission, New Delhi, India for providing the financial assistance {F. No. 39-534/2010(SR)}. One of the authors, Riya Dey is grateful to Indian School of Mines, Dhanbad for providing the fellowship.
References

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