Color-tunable properties of Eu3+- and Dy3+-codoped Y2O3 phosphor particles
© Atabaev et al.; licensee Springer. 2012
Received: 9 July 2012
Accepted: 21 September 2012
Published: 8 October 2012
Rare-earth phosphors are commonly used in display panels, security printing, and fluorescent lamps, and have potential applications in lasers and bioimaging. In the present study, Eu3+- and Dy3+-codoped uniform-shaped Y2O3 submicron particles were prepared using the urea homogeneous precipitation method. The structure and morphology of the resulting particles were characterized by X-ray diffraction, field emission scanning electron microscope, and field emission transmission electron microscope, whereas their optical properties were monitored by photoluminescence spectroscopy. The room-temperature luminescence color emission of the synthesized particles can be tuned from red to yellow by switching the excitation wavelength from 254 to 350 nm. The luminescence intensities of red and yellow emissions could be altered by varying the dopant concentration. Strong quenching was observed at high Eu3+ and Dy3+ concentrations in the Y2O3 host lattice.
KeywordsY2O3 particles Luminescence Urea homogeneous precipitation Eu3+ and Dy3+ codoped
The development of novel luminescent phosphor materials with a controllable size and morphology has been a major focus in the field of photonics and optoelectronics . Phosphor nanocrystals are exceptionally promising materials in many fields of technology including photonics, luminescent displays, fluorescent lamps, lasers, cathodoluminescence, and biotechnology . Moreover, the emission wavelength of rare-earth-doped nanoparticles is independent of the particle size and depends only on the dopant type, leading to lower synthesis cost. They also offer excellent chemical stability as well as high quantum yield. Different methods have been used to fabricate nanocrystalline phosphor particles, such as flame spray pyrolysis , co-precipitation method , sol–gel method , and solvothermal method . The urea homogeneous precipitation method was recognized to be a green route for the high-yield mass production of spherical ceramic submicron particles with controllable sizes. Spherical-shaped particles can improve the optical performance due to the high packing density and reduction of light scattering .
Yttrium oxide (Y2O3) has been investigated widely as a host material for rare-earth (RE) ion doping in optical applications [3, 4, 6, 7] on account of its excellent chemical stability, broad transparency range (0.2 to 8 μm) with a band gap of 5.6 eV, high refractive index, and low phonon energy . Furthermore, the similarities in the chemical properties and ionic radius of RE ions and Y2O3 make it an attractive choice as a host material [6, 8].
The color tunability of yttria-based phosphors can be achieved by codoping the host material with some specific rare-earth elements. For example, in our previous report we investigated the color-tunability effect of Eu3+- and Tb3+-codoped Y2O3 submicron particles . We showed that the color emission of synthesized particles could be tuned precisely from red to green by a simple variation of the Tb/Eu ratio and excitation wavelength. Strong energy transfer (ET) from Tb to Eu ions was observed in Tb/Eu-codoped Y2O3 submicron particles, but back ET from Eu3+ to Tb3+ was not significant. Ishiwada et al. investigated the Tb/Tm-codoped Y2O3 phosphor for high-temperature thermometry application . The synthesized Tb/Tm-codoped Y2O3 phosphor showed a distinct change of visible emission colors from green to blue with increasing temperature. Therefore, research into Y2O3 codoped with other different RE activators is important because the color-tunable properties can be used in a wide range of applications. Although many studies have examined the optical properties of RE ion-doped Y2O3 phosphors, only a few have investigated the codoping of two or more different ions in the same yttria host material [6, 8, 9].
In recent years, the Eu/Dy codoping in a single host material has attracted a great deal of attention and has been extensively investigated. For example, the SrAl2O4 host material codoped with Eu3+ and Dy3+ is known as a new generation long-lasting luminescent phosphor material . It is well known that Eu3+ doping into Y2O3 host material results in a red emission, whereas doping with Dy3+ results in blue-yellow emission . To the best of the authors' knowledge, there are no reports of an Y2O3 phosphor codoped with Eu3+ and Dy3+. In addition, there is an optimum concentration of dopant ions for all RE phosphors, but the optimum concentration also depends on several parameters, such as the size of the phosphors and synthetic route.
In the present study, urea homogeneous precipitation synthesis method was used for the preparation of Eu3+- and Dy3+-codoped Y2O3 submicron particles. The morphology and particles characteristics were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), field emission transmission electron microscopy (FETEM), and energy dispersive X-ray (EDX) spectroscopy. The optical properties of the synthesized particles were explored by photoluminescence spectroscopy (PL). Y2O3 submicron particles with different concentrations of codoped Eu3+ and Dy3+ were investigated, and the luminescence intensity of these particles were found to be strongly dependent on the activators concentration. The emission color of the Eu3+- and Dy3+-codoped Y2O3 particles could be switched from red to yellow by variation of the excitation wavelength.
Analytical grade Y2O3 (99.9%), europium oxide (Eu2O3; 99.9%), dysprosium oxide (Dy2O3; 99.9%), nitric acid (HNO3; 70%), and urea (99% to 100.5%) were purchased from Sigma-Aldrich Corporation (MO, USA) and were used without further purification.
Uniform-shaped sub-micron Eu3+- and Dy3+-codoped Y2O3 particles were synthesized according to the reported protocols [6, 8]. Phosphor precipitates were prepared by heating the corresponding RE nitrates (0.001 mol each sample) in aqueous solution of urea (40 ml H2O and 0.5 g urea). The concentration of Eu3+ varied between 1 to 3 mol%, whereas the Dy3+ concentration varied from 1 to 2 mol%.
The structure of the prepared powders was examined by XRD using a Bruker D8 Discover diffractometer (Bruker Optics Inc., MA, USA) with Cu Kα radiation (λ = 0.15405 nm) and a 2θ scan range of 20 to 60°. The structural properties were also analyzed using Fourier transform infrared spectroscopy (Jasco FT/IR6300, JASCO Corp., Easton, MD, USA). The morphologies of the particles were characterized by FESEM (Hitachi S-4700, Hitachi, Ltd., Tokyo, Japan) and FETEM (JEOL JEM-2100F, JEOL Ltd., Tokyo, Japan). Elemental analysis was carried out by EDX (Horiba 6853-H, HORIBA Jobin Yvon Inc., Edison, NJ, USA). The PL measurements were performed with a Hitachi F-7000 spectrophotometer equipped with a 150-W xenon lamp as an excitation source. All the measurements were performed at room temperature.
Results and discussion
Morphology and structure
Calculated mean crystallite sizes of Eu 3+ - and Dy 3+ -codoped Y 2 O 3 particles
36.42 ± 0.11
36.72 ± 0.09
36.96 ± 0.10
36.67 ± 0.14
37.01 ± 0.16
37.28 ± 0.07
The mean distance between the dopant ions at high concentrations (R = 0.62/(N)1/3, where N is the concentration of ions) was much shorter. Therefore, ions can interact through an electric multipolar process leading to energy migration. The dipole-dipole quenching process is inversely proportional to the sixth power of ion-ion separation and, thus, to the square of the dopant concentration [14, 15]. In Figures 4a and 5a, except for a decrease of the luminescence intensity, the emission spectrum of Y2O3 particles codoped with different Eu3+ and Dy3+ concentrations was similar due to the same f-f transitions within the specific RE ion. Therefore, the concentration of codoped RE ions plays an important role and should be strongly considered during the phosphor fabrication process.
Such behavior suggests that there is some energy transfer (ET) occurring between the two codoped ions. On the other hand, the ET observed in Eu3+- and Dy3+-codoped Y2O3 was not as strong as that observed in Tb3+-Eu3+-codoped Y2O3 phosphor . Therefore, the emission wavelength and color output of the same Y2O3:1% Eu3+-1% Dy3+ particles can be adjusted by switching the radiation from 254 to 350 nm. The digital photographs of eye-visible luminescence emissions from Y2O3:1% Eu3+-1% Dy3+ particles upon 254 and 350 nm excitations are shown in the Figure 8. It is obvious that the same host material emit red or yellow color, depending on the excitation wavelength.
In conclusion, Eu3+- and Dy3+-codoped Y2O3 submicron spherical particles were synthesized using the urea homogeneous precipitation method. The crystal structure and morphology of synthesized particles were characterized by XRD, FESEM, EDX, and FETEM. The PL spectroscopy was used to examine luminescent properties of Eu3+- and Dy3+-codoped Y2O3 particles. PL measurements revealed strong concentration quenching at high-codoped Eu3+ and Dy3+ concentrations. The luminescence color emission could be controlled by the excitation wavelength and the incorporation of Dy3+ and Eu3+ at the appropriate concentrations into Y2O3 structure. Weak energy transfer between the codoped ions was observed. These color-tunable Y2O3:1%Eu3+-1%Dy3+ phosphor particles can be used for security printing, solid state illumination, or for optical displays.
This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (grant no.: 2010–0010575) and by the Korean government (grant no. 2012R1A1B3001357). The author would like to thank Prof. J. B. Lee for allowing the use of his equipment for the preparation and characterization of the samples.
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