Structure and photoluminescent properties of (200)-oriented Eu-doped CeO2 thin films fabricated on fused silica substrates by chemical solution deposition
Introduction
Over the last decade, cerium oxide with fluorite structure (Fm-3 m) has raised great interests for its wide applications in catalysis, solid oxide fuel cells, oxygen gas sensors, ultraviolet (UV) blocking and shielding materials [1], [2], [3], [4], [5], [6]. With a high dielectric constant (~ 26), excellent lattice match with Si, easy control of the thickness of insulating CeO2 layer, and lowering defect density, CeO2 is a potential gate dielectric in the application of metal-oxide-semiconductor field-effect transistors [7], [8]. In addition, due to its high thermal stability and appropriate lattice constants, CeO2 is also supposed to be an ideal buffer layer or substrate for fabricating epitaxially-grown high-temperature superconducting thin films and perovskite oxide thin films [9], [10].
Furthermore, its fascinating optical properties, such as relative large band gap energy (indirect Eg ≈ 3.2 eV), high refractory index (~ 2.6), and considerable high transmittance over visible light range, interest many researchers to study CeO2 as a host material for lanthanide ions, for potential applications in light emitting diodes, flat panel display, and biomedicine [11], [12], [13]. It is known that Ce4 + ions do not have 4f electrons, thus, the energy transfer to doped rare-earth ions via charge transfer transition from O2 − to Ce4 + should be an available and efficient path to activate 4f–4f transitions of doped rare-earth ions. Recent reports on luminescence properties of rare-earth ions doped ceria nanocrystallites have confirmed CeO2 to be a good host material for some lanthanide ions [14], [15], [16].
Trivalent europium ion (Eu3 +), as one of the most important luminescent centers, has been widely used in light emitting diodes, lasers, lamp phosphors due to its two 4f–4f transitions: 5D0 → 7F1 (~ 590 nm) and 5D0 → 7F2 (~ 612 nm) [17], [18]. Owing to the shielding effects of outer electrons (5 s and 5p), weak interactions between 4f electrons and external environment have made the two emissions high color rendering indexes. In addition, Eu3 +-doped materials can be used in various sensitive applications, such as bar codes, anticounterfeiting ink, and biomedical imaging [19].
Although some investigations on Eu3 +, Sm3 +, Tb3 +, and Er3 + doped CeO2 nanopowders have been done recently, preparation of preferentially oriented Eu-doped CeO2 (Ce1 − xEuxO2, CEO) thin films and their photoluminescent properties have never been reported. Herein, we report on synthesis and photoluminescent properties of (200)-oriented CEO thin films fabricated on fused silica substrates by a chemical solution deposition method. The effects of Eu-doping concentration on photoluminescence, structural asymmetry, and decay curves were studied in detail. Our study indicated that the CEO thin films exhibited strong photoluminescent properties.
Section snippets
Experimental procedure
Ce1 − xEuxO2 (x = 0, 0.01, 0.02, 0.04, 0.06, 0.08, and 0.1) thin films were prepared by a chemical solution deposition method, using Eu(NO3)3·6H2O and Ce(NO)3·6H2O, as raw materials, diethanolamine as stabilizing reagent and a mixture of acetic acid and 2-methoxyethanol with a volume ratio of 2:1 as co-solvents. The final transparent and colorless precursor solutions were adjusted to 0.2 M. The thin films were prepared by spin-coating the precursor solutions at a spinning rate of 3000 rpm for 30 s on
Results and discussion
Fig. 1(a) shows the XRD patterns of Ce0.94Eu0.06O2 thin films deposited on fused silica substrates annealed at 500 °C, 600 °C, and 700 °C in air atmosphere for 3 h. It shows evidently that the thin films are a-axis oriented, and with increasing annealing temperature, the diffraction peaks become sharper, suggesting enhanced crystallinity of Ce0.94Eu0.06O2 thin films. Thus, in order to get Eu-doped thin films with good crystallinity, the CEO thin films with different Eu-doping contents on fused
Conclusions
(200)-oriented Ce1 − xEuxO2 (x = 0, 0.01, 0.02, 0.04, 0.06, 0.08, and 0.1) thin films have been fabricated on fused silica substrates by a chemical solution deposition method. Under UV excitation, the thin films showed bright orange-red emission originated from the 5D0 → 7F1 and 5D0 → 7F2 transitions of Eu3 + ions. Structure distortions induced by Eu-doping contributed to the light emission of electric dipole transition 5D0 → 7F2. Our study indicates that Eu3 +-doped CeO2 thin films will have potential
Acknowledgments
The authors gratefully acknowledge the financial support from the Natural Science Foundation of China (No. 51172289), National Basic Research Program (973 Program) of China (No. 2012CB619302), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20110171130004).
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