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Article
Code: M035200902029
First author: QIAO Yanmin
First author's unit: zhejiang normal university
Other author:

ZHANG Xinbo: Department of Physics,Zhejiang Normal University,Jinhua 321004,China

YE Xiao: Department of Physics,Zhejiang Normal University,Jinhua 321004,China

CHEN Yan: Department of Physics,Zhejiang Normal University,Jinhua 321004,China

GUO Hai: Department of Physics,Zhejiang Normal University,Jinhua 321004,China;Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces,Institute of Physical Chemistry,Zhejiang Normal University,Jinhua 321004,China

Published year: 2009
Roll: 27
Issue: 2
Page: 323-326
About the Journal
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Subject: Materials Science, Multidisciplinary
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Long Abstract
Abstract

Objectives Europium ions doped phosphors have been used in a wide variety field. The red light emission of Eu3+ is centered near 612 nm, which owing to intra-4f transition. While the emission from Eu2+ originating from the dipole allowed 5d-4f transition, varies in a wide range from red to ultraviolet which depending on the crystal structure of host materials. Recently, Sr2SiO4 has attracted great interest due to its special structure features and excellent physical and chemical stability. The aim of this paper is to investigate the luminescent behaviors of Sr2SiO4:Eu3+ and Sr2SiO4:Eu2+ phosphors. Results show that dual behavior of Sr2SiO4:Eu3+ and Sr2SiO4:Eu2+ phosphors may play a positive role in practical applications. Methods: Sr2SiO4:Eu3+ phosphors were prepared by a conventional solid-state reaction method and Sr2SiO4:Eu2+ phosphors were prepared by annealing Sr2SiO4:Eu3+ phosphors at 1100 ?C in a reduction atmosphere. The crystalline structures of the prepared phosphors were investigated by X-ray diffraction (XRD). The luminescent properties of phosphors were investigated by excitation, emission spectra and Commission International del?ˉEclairange (CIE) chromaticity coordination. Results: 1. From XRD patterns, Sr2SiO4:Eu3+ and Sr2SiO4:Eu2+ phosphors are the mixtures of the orthorhombic |aˉ-Sr2SiO4 with the space group of Pnma and monoclinic |-Sr2SiO4 with the space group of P21/n. |aˉ-Sr2SiO4 is the major phase of Sr2SiO4:Eu3+ phosphors, while for Sr2SiO4:Eu2+ phosphors, the proportion of the two phases are almost equal. Further works are in process to improve the purity of the obtained samples. 2. In the structure of |aˉ-Sr2SiO4 and |-Sr2SiO4, Sr2+ ions locate at two kinds of unequivalent lattice sites, and their coordination numbers are 9 and 10, respectively. When the Eu3+ and Eu2+ ions are introduced into the Sr2SiO4 matrix, they take the place of the Sr2+. 3. The excitation spectrum of Sr2SiO4:Eu3+ consists of an intense broad band with maximum at 239 nm and some sharp weak lines in the range from 318 nm to 464 nm. The broad band is related to the O2--Eu3+ charge transfer band (CTB). Such result shows that the phosphor can be excited efficiently by low-pressure mercury lamp. 4. The emission spectrum of Sr2SiO4:Eu3+ shows the typical emissions of Eu3+ ions, and the emission peaks can be assigned to the 5D0, 1-7FJ (J=0,1, 2, 3, 4) transitions. The main peak located at 613 nm corresponds to 5D0??2 transition. 5. The excitation spectrum of Sr2SiO4:Eu2+ exhibits a broad band ranges from approximately 300 to 400 nm with a peak maximum at 370 nm. The strong absorption at 370 nm is attributed to the transitions from the 4f7(8S7/2) ground state to the 4f65d1 excited state of the Eu2+ ions. The broadness of the excitation spectrum means that the phosphors can be well excited by near-ultraviolet light from 360-380 nm, matches well with the emission bands of the near-UV LED chips. 6. The emission spectrum of Sr2SiO4:Eu2+ phosphors shows two intense broad emission bands peaked at 470 and 547 nm. The broad emission band is ascribed to a strong coupling interaction between the Eu2+ ions and the Sr2SiO4 host matrix. 7. The CIE chromaticity coordination of Sr2SiO4:Eu2+ phosphors under an excitation of 370 nm is (x=0.30, y=0.40). The color temperature calculated from emission spectrum is Tc=6500 K. These two features make Sr2SiO4:Eu2+ phosphors have great potential application as candidate phosphors for white LEDs pumped by a near-UV chip and our method is a good method to elaborate Sr2SiO4:Eu2+ phosphors. Conclusion: An effective and simple solid-state reaction technique has been used to prepare Sr2SiO4:Eu3+ and Sr2SiO4:Eu2+ phosphors. The XRD pattern show that the phosphors are composed of orthorhombic |aˉ-Sr2SiO4 and monoclinic |-Sr2SiO4 phase with different concentration. Sr2SiO4:Eu3+ phosphors show intense emission in the red region when it was excited by 256 nm light. Sr2SiO4:Eu2+ phosphors give intense white emissions (x=0.30, y=0.40, Tc=6500 K) ranging from 425 to 650 nm when it is excited by near-UV light. The excitation efficiency is very high in the range from 300 nm to 400 nm and centered at 370 nm. The excellent luminescent properties make Sr2SiO4:Eu2+ suitable for white LEDs phosphors pumped by near-UV chips.

Objectives
Objectives Europium ions doped phosphors have been used in a wide variety field. The red light emission of Eu3+ is centered near 612 nm, which owing to intra-4f transition. While the emission from Eu2+ originating from the dipole allowed 5d-4f transition, varies in a wide range from red to ultraviolet which depending on the crystal structure of host materials. Recently, Sr2SiO4 has attracted great interest due to its special structure features and excellent physical and chemical stability. The aim of this paper is to investigate the luminescent behaviors of Sr2SiO4:Eu3+ and Sr2SiO4:Eu2+ phosphors. Results show that dual behavior of Sr2SiO4:Eu3+ and Sr2SiO4:Eu2+ phosphors may play a positive role in practical applications.
Methods
Methods: Sr2SiO4:Eu3+ phosphors were prepared by a conventional solid-state reaction method and Sr2SiO4:Eu2+ phosphors were prepared by annealing Sr2SiO4:Eu3+ phosphors at 1100 ?C in a reduction atmosphere. The crystalline structures of the prepared phosphors were investigated by X-ray diffraction (XRD). The luminescent properties of phosphors were investigated by excitation, emission spectra and Commission International del?ˉEclairange (CIE) chromaticity coordination.
Results
Results: 1. From XRD patterns, Sr2SiO4:Eu3+ and Sr2SiO4:Eu2+ phosphors are the mixtures of the orthorhombic |aˉ-Sr2SiO4 with the space group of Pnma and monoclinic |-Sr2SiO4 with the space group of P21/n. |aˉ-Sr2SiO4 is the major phase of Sr2SiO4:Eu3+ phosphors, while for Sr2SiO4:Eu2+ phosphors, the proportion of the two phases are almost equal. Further works are in process to improve the purity of the obtained samples. 2. In the structure of |aˉ-Sr2SiO4 and |-Sr2SiO4, Sr2+ ions locate at two kinds of unequivalent lattice sites, and their coordination numbers are 9 and 10, respectively. When the Eu3+ and Eu2+ ions are introduced into the Sr2SiO4 matrix, they take the place of the Sr2+. 3. The excitation spectrum of Sr2SiO4:Eu3+ consists of an intense broad band with maximum at 239 nm and some sharp weak lines in the range from 318 nm to 464 nm. The broad band is related to the O2--Eu3+ charge transfer band (CTB). Such result shows that the phosphor can be excited efficiently by low-pressure mercury lamp. 4. The emission spectrum of Sr2SiO4:Eu3+ shows the typical emissions of Eu3+ ions, and the emission peaks can be assigned to the 5D0, 1-7FJ (J=0,1, 2, 3, 4) transitions. The main peak located at 613 nm corresponds to 5D0??2 transition. 5. The excitation spectrum of Sr2SiO4:Eu2+ exhibits a broad band ranges from approximately 300 to 400 nm with a peak maximum at 370 nm. The strong absorption at 370 nm is attributed to the transitions from the 4f7(8S7/2) ground state to the 4f65d1 excited state of the Eu2+ ions. The broadness of the excitation spectrum means that the phosphors can be well excited by near-ultraviolet light from 360-380 nm, matches well with the emission bands of the near-UV LED chips. 6. The emission spectrum of Sr2SiO4:Eu2+ phosphors shows two intense broad emission bands peaked at 470 and 547 nm. The broad emission band is ascribed to a strong coupling interaction between the Eu2+ ions and the Sr2SiO4 host matrix. 7. The CIE chromaticity coordination of Sr2SiO4:Eu2+ phosphors under an excitation of 370 nm is (x=0.30, y=0.40). The color temperature calculated from emission spectrum is Tc=6500 K. These two features make Sr2SiO4:Eu2+ phosphors have great potential application as candidate phosphors for white LEDs pumped by a near-UV chip and our method is a good method to elaborate Sr2SiO4:Eu2+ phosphors.
Conclusion
Conclusion: An effective and simple solid-state reaction technique has been used to prepare Sr2SiO4:Eu3+ and Sr2SiO4:Eu2+ phosphors. The XRD pattern show that the phosphors are composed of orthorhombic |aˉ-Sr2SiO4 and monoclinic |-Sr2SiO4 phase with different concentration. Sr2SiO4:Eu3+ phosphors show intense emission in the red region when it was excited by 256 nm light. Sr2SiO4:Eu2+ phosphors give intense white emissions (x=0.30, y=0.40, Tc=6500 K) ranging from 425 to 650 nm when it is excited by near-UV light. The excitation efficiency is very high in the range from 300 nm to 400 nm and centered at 370 nm. The excellent luminescent properties make Sr2SiO4:Eu2+ suitable for white LEDs phosphors pumped by near-UV chips.
Keywords
sr2sio4:eu2+; white light-emitting diodes (led); rare earths
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