Synthesis and Luminescent Properties of LaAlO3:RE3+ (RE = Tm, Tb

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Synthesis and Luminescent Properties of LaAlO3:RE3+ (RE ) Tm, Tb) Nanocrystalline Phosphors via a SolsGel Process Xiaoming Liu,†,‡ Liushui Yan,† and Jun Lin*,‡ School of EnVironment and Chemical Engineering, Nanchang Hangkong UniVersity, Nanchang 330063, China, and State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ReceiVed: February 15, 2009; ReVised Manuscript ReceiVed: March 29, 2009

LaAlO3:Tm3+ and LaAlO3:Tb3+ phosphors were prepared through a Pechini-type sol-gel process. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), photoluminescence, and cathodoluminescence (CL) spectra were utilized to characterize the synthesized phosphors. The XRD results reveal that the fully crystalline pure LaAlO3 phase can be obtained at 800 °C. The FE-SEM image indicates that the phosphor samples are composed of aggregated spherical particles with sizes ranging from 40 to 80 nm. Under the excitation of ultraviolet light (230 nm) and low-voltage electron beams (1-3 kV), the LaAlO3: Tm3+ and LaAlO3:Tb3+ phosphors show the characteristic emissions of Tm3+ (1D2f3H6,4,3F4 transitions) and Tb3+ (5D3,4f7F6,5,4,3 transitions), respectively. The CL of the LaAlO3:Tm3+ phosphors have high color purity and comparable intensity to the Y2SiO5:Ce3+ commercial product, and the CL colors of Tb3+-doped LaAlO3 phosphors can be tuned from blue to green by changing the doping concentration of Tb3+ to some extent. These phosphors have potential applications in area of field emission displays. Introduction Field emission display (FED) is a new technology for flatpanel displays.1,2 It combines the high image quality of the cathode ray tube (CRT) with the thinness and light weight of the flat panel display and has the potential to provide displays with thin panel thickness, self-emission, wide viewing, quick response, high brightness, high contrast ratio, light weight, and low-power consumption.3-6 In the development of FED, it is important to develop phosphors that show high efficiency and good stability at low voltage electron excitation and high current density.7-17 While many efficient sulfide-based compounds have been explored as possible low-voltage phosphors, the volatility of sulfur has prohibited their use in the FEDs. Sulfide-base phosphors often degrade under the high-energy electron bombardment due to dissociation of the cation-sulfur bonds. The process generates corrosive sulfur-bearing gas species that contaminate emission tips and shorten the device lifetime.18-25 Oxide-based phosphors are more stable and environmentally friendly in comparison to sulfides. Therefore, rare-earth-doped oxide-based phosphors for FED have been of great interest due to their excellent light output, color-rendering properties, and superior stability under electron bombardment.26-32 Lanthanum aluminate (LaAlO3) possesses a crystal structure with the hexagonal space group R3jc (No. 167), and the La3+ ion has D3 site symmetry. The structure is stable at room temperature; at temperatures above 530 °C the cell is cubic with space group Pm3jm (No. 221).33-36 Aside from its luminescence host lattice potential, recent interest in LaAlO3 lanthanum orthoaluminate bulk crystals is driven by possible applications of this material in the areas of high frequency capacitors; magneto-hydrodynamic generators; substrate for super conducting, ferroelectric thin films; and colossal magnetoresistance.37-40 * Corresponding author. E-mail: [email protected]. † Nanchang Hangkong University. ‡ Chinese Academy of Sciences.

Rare earth ion doped crystallite has attracted considerable research interest owing to its excellent luminescent properties. Since the 4f electrons of rare earth ions are shielded by the outer 5s and 5p electrons, the intra-4f emission spectra of rare earth ions are characterized by narrow lines with high color purity.41,42 Thulium-doped phosphors have attracted substantial attention in recent years because Tm3+ ions provide blue luminescence with appropriate lifetimes, color-rendering properties, and potential application in CRT screens, FEDs, plasma display panels, and electroluminescence devices, such as Y2O3: Tm3+, Y3Al5O12:Tm3+, SrHfO3:Tm3+, Y3GaO6:Tm3+, LaPO4: Tm3+, LaGaO3:Tm3+, and LaAlGe2O7:Tm3+.43-48 The emissions of Tb3+ are mainly due to transitions 5D4f7FJ (J ) 6, 5, 4, 3) in the green region. Often there is a considerable contribution to the emission from the higher-level emission 5D3f7FJ, mainly in the blue region. As far as we know, less information is available concerning the rare earth ion (RE3+) doped LaAlO3 phosphors.49-51 In those reports for Tm3+- or Tb3+-doped LaAlO3, there is not a clear description about their photoluminescence (PL), and a detailed investigation on the cathodoluminescence (CL) of LaAlO3:Tm3+ and LaAlO3:Tb3+ have not been performed. Accordingly, in this paper, we report the synthesis of LaAlO3:Tm3+ and LaAlO3:Tb3+ samples using a Pechini-type sol-gel process and investigate the PL and CL properties of the samples in more detail. It is of great importance and interest to note that the CL of the LaAlO3:Tm3+ phosphors had high color purity and comparable intensity to the Y2SiO5: Ce3+ commercial product (Product No.1047, Nichia Kagaku Kogyo Kabushiki), and the CL color of the LaAlO3:Tb3+ phosphors can be turned from blue to green by changing the doping concentration of Tb3+ ion in LaAlO3 host to some extent. Due to the excellent CL and cheap materials of the LaAlO3: RE3+ (RE ) Tm, Tb) phosphors, the obtained LaAlO3:RE3+ (RE ) Tm, Tb) nanocrystalline phosphors have great potential applications in CL devices areas, such as CRT, FED, and vacuum fluorescent display (VFD) devices.

10.1021/jp9013724 CCC: $40.75  2009 American Chemical Society Published on Web 04/20/2009

Synthesis and Luminescent Properties of LaAlO3:RE3+

J. Phys. Chem. C, Vol. 113, No. 19, 2009 8479

Experimental Section The LaAlO3:Tm3+ and LaAlO3:Tb3+ samples were all prepared by a Pechini-type sol-gel process.52-54 The doping concentrations of Tm3+ and Tb3+ are 0.05-6 atom % of La3+ in LaAlO3. The stoichiometric amounts of La2O3, Tm2O3, and Tb4O7 (99.99%, Shanghai Yuelong Non-Ferrous Metals Ltd.) were dissolved in dilute HNO3 [analytical reagent (AR), Beijing Fine Chemical Co.,] under stirring and heating, resulting in the formation of colorless solutions of La(NO3)3, Tm(NO3)3, and Tb(NO3)3 The solutions of La(NO3)3 and Tm(NO3)3 [or Tb(NO3)3] were mixed together followed by the addition of the stoichiometric amounts of Al(NO3)3 · 9H2O (AR, Beijing Fine Chemical Co.) under stirring. Then citric acid and polyethylene glycol (PEG, molecular weight ) 10 000) were dissolved in the above solution (CPEG ) 0.01 M, citric acid/metal ion ) 2:1). The resultant mixtures were stirred for 1 h and condensed at 75 °C in a water bath until dry 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 at 800 °C for 3 h in air (in the mixture of hydrogen and nitrogen for LaAlO3:Tb3+) to produce the final samples. X-ray diffraction (XRD) measurements were carried out on a Rigaku-Dmax 2500 diffractometer using Cu KR radiation (λ ) 0.154 05 nm). The morphologies of the samples were inspected using a field emission scanning electron microscope (FE-SEM, XL30, Philips). The PL measurements were performed on a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The CL measurements were carried out in an ultrahigh vacuum chamber ( 0.01 in La(1-x)AlO3:xTb3+ phosphors), due to the cross-relaxation between 5D3f5D4 and 7F0f7F6 of two Tb3+ ions, the blue emission of 5D3f7FJ transition is not as strong as the 5D4f7FJ transition at this doping concentration and the emission is dominated by the 5D4f7FJ transitions (shown in Figure 8d).42,61,62 The obtained phosphors give a strong green emission (for example, the CIE chromaticity coordinates are x

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) 0.2455 and y ) 0.5448 for LaAlO3:0.05Tb3+). When doping concentration is moderate (0.003 e x e 0.01) in La(1-x)AlO3: xTb3+ phosphors, under the excitation of low-voltage electron beam, the obtained phosphors give the strong blue-green luminescence (for example, the CIE chromaticity coordinates are x ) 0.1923 and y ) 0.2128 for La2O3:0.003Tb3+, shown in Figure 8b). The CL color of La(1-x)AlO3:xTb3+ can be tuned from blue to blue-green to green by changing the doping concentration of Tb3+ ions. Figure 7 shows the corresponding CIE chromaticity diagram of La(1-x)AlO3:xTb3+ phosphors with different doping concentration of Tb3+ ions. The cross dots indicate the CIE chromaticity coordinates positions. The CIE chromaticity coordinates change from x ) 0.1995, y ) 0.1584 (blue) to x ) 0.2538, y ) 0.5894 (green) by changing the doping concentration of Tb3+ from x ) 0.0005 to 0.06 in La(1-x)AlO3:xTb3+ phosphors. The corresponding luminescence color can change from blue to blue-green to green. The CL emission intensities of the LaAlO3:Tm3+ and LaAlO3: xTb3+ phosphors have been investigated as a function of the accelerating voltage and the filament current. Figure 9 shows the typical CL emission intensities as a function of the accelerating voltage and the filament current of LaAlO3: 0.01Tm3+ and LaAlO3:0.05Tb3+phosphors, respectively. When the filament current is fixed at 15 mA, the CL intensity increases with raising the accelerating voltage from 1.0 to 3.0 kV (Figure 9a). Similarly, under 1.5 kV electron beam excitation, the CL intensity also increases with increasing the filament current from 14 to 18 mA (Figure 9b). The increase in CL brightness with an increase in electron energy and filament current is attributed to the deeper penetration of the electrons into the phosphors body and the larger electron beam current density. The electron penetration depth can be estimated using the empirical formula: L [Å] ) 250(A/F)(E/Z1/2)n, where n ) 1.2/(1-0.29 log10 Z), and A is the atomic or molecular weight of the material, F is the bulk density, Z is the atomic number or the number of electrons per molecule in the case compounds, and E is the accelerating voltage (kV).65 For LaAlO3:Tm3+ (Tb3+), Z ) 138.9, A ) 213.9, F ) 5.38 g/cm3, and the estimated electron penetration depth at 2 kV is about 6.7 nm. For CL, the Tm3+ and Tb3+ ions are excited by the plasma produced by the incident electrons. The deeper the electron penetration depth, the more plasma will be produced, which results in more Tm3+ and Tb3+ ions being excited; thus, the CL intensity increases.54 Due to its strong lowvoltage CL intensity, good CIE chromaticity, high stability, and environmentally friendly properties, the LaAlO3:Tm3+ and LaAlO3:Tb3+ phosphors are promising for application in CRT, FED, and VFD devices. Conclusions The LaAlO3:Tm3+ and LaAlO3:Tb3+ nanocrystalline phosphors were prepared by a Pechini-type sol-gel process. Under the excitation of low-voltage electron beams, the LaAlO3:Tm3+ phosphors show blue luminescence with high color purity and comparable intensity to the Y2SiO5:Ce3+ commercial product. The CL color of the LaAlO3:xTb3+ phosphors can be tuned from blue to blue-green to green by changing the doping concentration of Tb3+ ion in LaAlO3 host. Due to its excellent CL and cheap materials, the obtained LaAlO3:Tm3+ and LaAlO3:Tb3+ phosphors have potential applications in CRT, FED, and VFD devices. Acknowledgment. This project is financially supported by the National Basic Research Program of China (2007CB935502), and the National Natural Science Foundation of China (NSFC

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