J. Phys. Chem. C 2007, 111, 16601-16607
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Tunable Luminescence Properties of CaIn2O4:Eu3+ Phosphors Xiaoming Liu,†,‡ Chunxia Li,†,‡ Zewei Quan,†,‡ Ziyong Cheng,† and Jun Lin*,† State Key Laboratory of Application of Rare Earth Resources, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ReceiVed: June 22, 2007; In Final Form: August 16, 2007
CaIn2O4:Eu3+ phosphors were prepared by a Pechini sol-gel process. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), photoluminescence (PL), cathodoluminescence (CL) spectra as well as lifetimes were utilized to characterize the samples. The XRD results reveal that the samples begin to crystallize at 800 °C, and the crystallinity increases upon raising the annealing temperature. The FE-SEM images indicate that the CaIn2O4:Eu3+ samples consist of fine and spherical grains with size around 200-400 nm. Under the excitation of ultraviolet light and low-voltage electron beams, the CaIn2O4:Eu3+ phosphors show the characteristic emissions of Eu3+ (5DJ-7FJ′ J, J′ ) 0, 1, 2, 3 transitions). The luminescence color can be tuned from white to orange to red by adjusting the doping concentration of Eu3+. The corresponding luminescence mechanisms have been proposed.
Introduction Nowadays the use of phosphors represents a fast growing industry due to the wide range of applications: light-emitting diodes (LEDs), cathode ray tubes (CRTs), field emission displays (FEDs), vacuum fluorescent displays (VFDs), plasma display panels (PDPs), and X-ray imaging scintillators.1-7 Oxide phosphors have gained interest due to their better thermal and chemical stability and environmental friendliness compared with sulfides, which are currently used in the screen of flat-panel displays (FPDs), VFDs, and FEDs.8-18 Therefore, more attention has been paid to the improvement of original oxide phosphors and (or) developing new oxide phosphors materials with respect to the wide possible applications.19-26 Ultraviolet light-emitting diodes (UVLEDs) based on wide band gap III-nitride compound semiconductors have attracted the attention of researchers because of their potential applications for solid-state white lighting.27 The quest for new UVLED converted phosphors has triggered active research efforts in the investigation of whiteemitting materials using longer UVLED (300-410 nm) instead of the blue irradiation (460 nm) from GaInN chips as the excitation source.28 This has generated a great interest in developing new white-emitting phosphors via chemical approaches.29-32 Rare earth ions have been playing an important role in modern lighting and display fields due to the abundant emission colors based on their 4f-4f or 5d-4f transitions.33 The trivalent europium ion (Eu3+) is well-known as a red-emitting activator due to its 5D0-7FJ transitions (J ) 0, 1, 2, 3, 4, usually ranging from 578 nm for J ) 0 to 700 nm for J ) 4, with 5D0-7F2 around 610-625 nm as the most prominent group). These emission lines have found an important application in the lighting (such as Y2O3:Eu3+ and YVO4:Eu3+ used in lamps) and display (such as Y2O2S:Eu3+ used in color television) fields. In addition to the above emission lines, those from higher 5D * Author to whom all correspondence should be addressed. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Graduate University of the Chinese Academy of Sciences.
levels, such as 5D1 (green), 5D2 (green, blue), and 5D3 (blue), are often observed depending upon the host lattice (phonon frequency as well as the crystal structure) and the doping concentration of Eu3+.34 In this case, both the phonon frequencies of the host lattices and the doping concentration of Eu3+ should be low enough to avoid the multiphonon relaxation and cross-relaxation occurring among the energy levels of Eu3+, respectively. CaIn2O4 is a semiconductor with a reported band gap (Eg) of 3.9 eV, belonging to the ordered CaFe2O4 structures with the Pca21 or Pbcm space group and the lattice parameters a ) 9.70 Å, b ) 11.30 Å, and c ) 3.21 Å for Z ) 4,35 which has the potential to serve as a new host material in phosphor applications.36 CaIn2O4 shows only weak self-activated blue luminescence when excited under ultraviolet light. Recently, we have realized the white emission from Eu3+ in the CaIn2O4 host.36b As a continuation and extension of this work, here we report the synthesis of CaIn2O4:Eu3+ phosphors, using a Pechini-type sol-gel process, and their luminescence properties in more detail. It is of great importance and interest to note that the luminescence color of CaIn2O4:Eu3+ phosphors can be tuned from white to orange to red by adjusting the doping concentration of Eu3+. The corresponding luminescence mechanisms have been proposed. The prepared phosphors are potentially applied for UVLED and FEDs devices. Experimental Section The CaIn2O4:Eu3+ samples were all prepared by a Pechinitype sol-gel process.37-39 The doping concentrations of Eu3+ are 0.5-10 atom % of Ca2+ in CaIn2O4. The Eu2O3 (99.99%, Shanghai Yuelong Non-Ferrous Metals Ltd.) was dissolved in dilute HNO3 (analytical reagent, A. R. Beijing Fine Chemical Co.) under stirring and heating, resulting in the formation of a colorless solution of Eu(NO3)3. Stoichiometric amounts of CaCO3 (A. R. Beijing Fine Chemical Co.) were also dissolved in dilute HNO3 to get Ca(NO3)2 solution and then stoichiometric amounts of In(NO3)3‚4.5H2O (A. R. Beijing Fine Chemical Co.), Ca(NO3)2, and Eu(NO3)3 solution were mixed in deionized water under stirring. The citric acid and polyethylene glycol (PEG,
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Liu et al.
Figure 2. Crystal structure of CaIn2O4.
Figure 1. XRD patterns of CaIn2O4:0.02Eu3+ annealed at different temperatures, with the standard data for CaIn2O4 (JCPDS No. 17-0643) being used as a reference (*, In2O3; #, CaO).
molecular weight ) 10 000) were dissolved in the above solution [CPEG ) 0.01 M, citric acid/metal ion ) 2:1 (mol)]. The resultant mixtures were stirred for 1 h and heated at 75 °C in a water bath until homogeneous gels formed. After being dried in an oven at 110 °C for 10 h, the gels were ground and prefired at 450 °C for 4 h in air. Then the samples were fully ground and fired to the desired temperatures (700-1000 °C) for 3 h. The X-ray diffraction (XRD) measurements were carried out on a Rigaku-Dmax 2500 diffractometer using Cu KR radiation (λ ) 0.154 05 nm). The Fourier transform infrared spectroscopy (FT-IR) spectra were measured with a Perkin-Elmer 580B infrared spectrophotometer and the KBr pellet technique. The apparatus provided a resolution of 4 cm-1. The morphologies of the samples were inspected using a field emission scanning electron microscope (FE-SEM, XL30, Philips). The photoluminescence (PL) measurements were performed on a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The cathodoluminescent (CL) measurements were carried out in an ultrahigh-vacuum chamber (
