J. Phys. Chem. B 2007, 111, 12693-12699
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Novel Class of Aeschynite Structure LaNbTiO6-Based Orange-Red Phosphors via a Modified Combustion Approach Qian Ma, Aiyu Zhang, Mengkai Lu1 ,* Yuanyuan Zhou, Zifeng Qiu, and Guangjun Zhou State Key Laboratory of Crystal Materials, Shandong UniVersity, Shanda Nan Road, Jinan 250100, People’s Republic of China ReceiVed: May 21, 2007; In Final Form: August 31, 2007
A novel class of orange-red phosphors based on Eu3+-activated LaNbTiO6 was successfully fabricated by a wet chemical method, called a modified combustion approach. XRD, TG-DTA, SEM, and EDS results show that the heat-treatment of the powders above 1000 °C is enough to obtain highly crystallized and phase-pure LaNbTiO6 and Eu3+-doped samples, which is of prime importance in investigating the optical properties of the novel phosphors using LaNbTiO6 as the host material. UV-vis diffuse reflectance spectroscopy reveals that the direct band gap of LaNbTiO6 with large grains (above 200 nm) is calculated to be 3.27 eV, while the absorption edge of the small particles shows an obvious blue-shift. Two blue emission bands centered at 440 and 470 nm ascribed to the self-trapped exciton emission of the distorted NbO6 and TiO6 groups for the pure LaNbTiO6 can be obtained. Photoluminescence spectra of the Eu3+-doped phosphor particles illuminated the simultaneous occurrence of several intense orange-red band emissions due to the characteristic transitions of 5 D0,1 f 7FJ (J ) 0, 1, 2, 3, 4) of Eu3+ under 395 nm excitation. The mechanism of these multiplets possibly arising from the odd-parity distortions of the Eu3+ ion environment and the effect of crystallanity of the compounds on luminescence were discussed, respectively. The highly bright and color-uniform fluorescence images of the doped samples with short luminescence decay times (nanosecond magnitude) confirmed the potential applications of the phosphors in luminescence and display devices.
Introduction In recent years, there has been a widespread and growing interest in the investigation of new and novel families of highperformance luminescence materials for the development of solid-state lighting (SSL) and flat panel display (FPD) devices.1 Inorganic oxide compounds doped with rare earth (RE) ions are an available class of phosphors for many photonic applications since the oxide phosphors offer potential advantages utilizing their superior stability under electron bombardment and excellent luminescence properties.2 As a prominent red luminescence activator for many different host lattices, trivalent europium (Eu3+) is of extensive scientific significance because it has played an important role in modern lighting and emissive display technologies, such as filed emission displays (FEDs) and vacuum fluorescent displays (VFDs), due to the abundant emission colors based on 4f-4f and 5d-4f transitions. With regard to inorganic luminescence materials, it is well-known that the 5D0,1 f 7F0-6 transitions of Eu3+ primarily depend on the site symmetries of the local environment around Eu3+ in the host lattice and that Eu3+-activated phosphors prefer multiband emissions rather than a monochromatic sharp emission generally ascribed to the influence of asymmetry ratio and crystal-field splitting of the ground states.3 Very recently, a mass of different crystal structure-type materials as a host lattice for phosphors doped with Eu3+ has been investigated, such as TiO2/ Eu3+,4 LaPO4/Eu3+,5 Y2O3/Eu3+,6 Y2O2S/Eu3+,7 YVO4/Eu3+,8 and Gd2O3/Eu3+,9 etc. Although many improved works have been carried out, there are also a series of limitations existing in those practical systems for luminescence devices. At present, * Corresponding author. E-mail:
[email protected].
many investigations have been focused on enhancing the luminescence of Eu3+ by incorporating it into the appropriate crystal lattice location of host materials. Actually, the microstructure of the host material is believed to have a crucial impact on the luminescence properties of the corresponding phosphors. A more simple and direct way to improve the performance of current materials, based on the correlation between microstructure and properties, is to change the structure of the host materials.10 The development of novel phosphors is the most important aspect in designing luminescence devices. Therefore, our interest is to obtain and establish a novel high-performance promising luminescence system for the red phosphors. The ternary oxide of aeschynite-type RENbTiO6, orthorhombic with space group Pnma, has been studied as a microwave dielectric resonator for communication applications. Particularly, many ceramics and single crystals of this group of materials have been lately regarded as ideal gain media for miniature solid-state and diode pumped lasers because of their exciting optical properties.11 As a typical aeschynite-type compound, LaNbTiO6 has attracted much attention based on its potential application as a novel host material for the luminescence centers since the complicated structure of a framework formed with edge- and corner-shared NbO6 and TiO6 distorted octahedra and La3+ cations filled in the interstices has more feasibility of gaining abundant multistage transition pathways and broad distributing emission spectra.12 In comparison with conventional methods, combustion synthesis has promising advantages in its products, such as more homogeneity, less impurities, and larger surface areas. In addition, the citrate-gel method, which involves the formation of a mixed ion citrate due to the three-ligand nature of citric
10.1021/jp0739162 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/16/2007
12694 J. Phys. Chem. B, Vol. 111, No. 44, 2007 acid and results in a transparent three-dimensional network upon drying, allows the preparation of single-phase multicomponent oxides at low temperatures.13 Therefore, in this study, novel orange-red phosphors based on a LaNbTiO6 system have been synthesized by a facile and energy-saving modified citrate-gel combustion approach. To our best knowledge, the homogeneous aeschynite-type powders prepared by the soft chemical route have not been reported yet. Our research shows that the orangered phosphors with Eu3+ introduced into the LaNbTiO6 host lattice display visible light efficiently under 395 nm excitation and have excellent PL properties confirmed by the decay time measurement and fluorescence images, so that they are promising candidates as new orange-red components for optical devices. Experimental Procedures Pure and doped LaNbTiO6 samples were prepared by a simple sol-gel combustion reaction and a subsequent heat-treatment process. Common chemical reagents including lanthanum oxide [La2O3(AR)], europium oxide [Eu2O3(AR)], niobium oxide [Nb2O5(AR)], and tetra-n-butyl titanate [Ti(C4H9O)4(CR)] were used as starting materials. Lanthanum nitrate [La(NO3)3] and europium nitrate [Eu(NO3)3] solutions were first obtained by dissolving La2O3 and Eu2O3 into diluted nitric acid, respectively. As an eximious complexing agent, citric acid (C6H8O7‚H2O(AR)) was not only employed in dissolving the unsoluble compounds at aqueous solution environments such as the Ti(C4H9O)4 and Nb2O5‚nH2O precipitate produced on the original step but also in energizing the sol and gel formation and melioration process. Meanwhile, the appropriate additive ratio of citric acid to ammonium nitrate [NH4NO3(AR)] makes for complete combustion. All the reagents were used without further purification. In a typical synthesis, first, a stoichiometric amount of Nb2O5 was dissolved into HF acid (40%) after heating in a hot water bath for 5 h, and then ammonia solution was added dropwise into this aqueous solution under magnetic stirring at room temperature to obtain Nb2O5‚nH2O. The final mixture was alkalescence with a pH value around 9 to ensure the complete reaction of the reagents, and the precipitate was immediately filtered and washed with deionized water several times to avoid the intervention of F- ions. Afterward, citric acid and Nb2O5‚ nH2O in an appropriate molar ratio were dissolved in a small amount of deionized water under heating at 80 °C. Then, stoichiometric amounts of Ti(C4H9O)4, La(NO3)3, and excessive NH4NO3 were added and mixed homogeneously under continuous stirring and heating at about 100 °C. As water evaporated, the solution was turned into a transparent sol slowly and finally formed a yellow gel with a high viscosity. The gel was then introduced into a muffle furnace preheated at 550 °C for 15 min. The ignition occurred rapidly, the combustion went on vigorously for a few seconds, and then a fluffy product was obtained after the combustion reaction. After being ground, the products were annealed at 700, 800, 900, 1000, 1100, and 1200 °C for another 1 h, respectively. Eu3+-doped samples were synthesized just as was previously mentioned except that Eu(NO3)3 was introduced. The doping concentration of Eu3+ was varied from 1 to 12 mol %. The doped samples were also annealed at 1100 °C for 1, 3, 5, 7, and 10 h, respectively. Finally, the samples were ground into powder for characterization. The phase composition and structure were characterized by powder XRD patterns (Germany Bruker Axs D8-Avance X-ray diffractometer with graphite monochromatized Cu KR irradiation (λ ) 1.5418 Å)), and data were collected over the 2θ range of
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Figure 1. XRD patterns of the samples annealed at different temperatures: (a) the powder obtained by direct combustion at 550 °C; (b-f and h) heat-treated LaNbTiO6 powders at 700, 800, 900, 1000, 1100, and 1200 °C for 1 h, respectively; and (g) 12 mol % Eu3+-doped sample annealed at 1100 °C for 1 h.
10-70°, with a step width of 0.02° and a count time of 0.2 s/step. Thermal analysis of the powder that was dried at 100 °C for 1 week was carried out from 20 to 1200 °C using thermogravimetry-differential thermal analysis (TG-DTA) (PerkinElmer corporation, Diamond TG-DTA) with a heating rate of 20 °C/min. The microstructure and stoichiometry data were obtained by SEM (Hitachi, S-4800) and EDS (Horiba EMAX Energy, EX-350), respectively. UV-vis diffuse reflectance spectroscopy (UV-DRS) were measured on a UV-vis spectrophotometer (Shimadzu, UV-2550). The PL properties and lifetime measurements of the obtained phosphors were recorded on a fluorescence spectrophotometer (Edinburgh, FL920). The fluorescence microscope (Nikon Eclipse, E600) was used for the luminescence observation of the samples. All the measurements were performed at room temperature. Results and Discussion LaNbTiO6 exhibits an aeschynite structure in the orthorhombic space group Pnma, with lattice parameters a ) 1.093 nm, b ) 0.7572 nm, c ) 0.5446 nm, and Z ) 4. As early literature reported, the Nb and Ti ions coordinated by six oxygen atoms are in the center of the distorted octahedra, which are joined in pairs by edge-sharing, with each pair being connected to six other pairs by corner-sharing. The La3+ ions occupy the interstices of the three-dimensional framework with eight oxygen atom coordination, forming an irregular polyhedron.14 The typical XRD patterns are shown in Figure 1. Pattern a represents the XRD of the powder prepared by a direct combustion reaction at 550 °C and only displays the formation of amorphous products or very small crystallites, most probably reflecting the presence of a partially disordered structure. Patterns b-d suggest that single monoclinic LaNbTiO6 formed and took on a better crystallinity as the annealing temperature was higher than 700 °C. Patterns e, f, and h present the XRD of the products annealed for 1 h in air in the temperature range of 10001200 °C. All the diffraction peaks of samples annealed above 1000 °C can be assigned exactly to the standard data of LaNbTiO6 with the aeschynite structure (JCPDS, No. 73-1059), meaning that the phase transition from the monoclinic to orthorhombic unit cell occurred during 900-1000 °C. It can be observed clearly that the peaks become sharper and stronger with an increasing annealing temperature, indicating the improvement of crystallinity, and no characteristic peaks of possible impurity phases such as La2O3, TiO2, or Nb2O5 were detected. Moreover, the temperature in our preparation process is much lower than that in the conventional solid-state reaction
LaNbTiO6-Based Orange-Red Phosphors
Figure 2. TG-DTA curves of LaNbTiO6 precursor gel powder.
method (above 1400 °C), confirming the advantage of the adopted method here. There is no distinguishing detail between the XRD patterns of the 12 mol % Eu3+-doped LaNbTiO6 (Figure 1g) and the undoped samples, which proves the formation of a solid solution, including a small quantity of substitutes of Eu3+ with concentrations ranging from 1 to 12 mol % into the LaNbTiO6 host. Actually, the Eu3+ ions and La3+ ions have similar environments and locations in the framework due to the fact that the ionic radius and atomic mass do not vary significantly as for the RE ions. The TG-DTA curves of the xerogel powder without any dopant dried at 100 °C are shown in Figure 2. The TG curve displays two main stages of the weight loss process below 700 °C. The first weight loss is approximately 75% below 295 °C, accompanied by two exothermic peaks around 122 and 290 °C in the DTA curve because of the evaporation of water and the combustion of organic components such as citric acid and other organic residues. The second stage of weight loss at temperatures between 300 and 700 °C is primarily due to the further combustion of the organic groups and the citrate, as well as the dehydroxylation and oxidation of the decomposed fragment, accompanied by a broad exothermic DTA peak in the range of 520-682 °C. The exothermic peak around 728 °C, combined with the analysis of Figure 1b-d, can be identified as the beginning of monoclinic phase formation. Furthermore, a broad endothermic band centered at 925 °C emerges, and a wide extension of the exothermic trend reaches 1200 °C, without a significant weight loss observed in the TG curve. This can be ascribed to the phase transition and crystallization process, according to the results of XRD in Figure 1. Figure 3 shows the typical SEM micrographs of 12 mol % Eu3+-doped samples obtained under different annealing temperatures and time. All the particles are irregular in shape and agglomerated, and both the particle size and the extent of agglomeration increase under higher temperature and longer heat-treated time. Figure 3a displays the image of the sample prepared at 1000 °C for 1 h, and most particles are smaller than 200 nm in diameter, including some below 100 nm. Figure 3b shows the SEM observation of the powders annealed at 1100 °C for 1 h, and the particles are in the scale of 200-300 nm in size. Figure 3c,d shows the micrographs of the samples obtained at 1100 °C for 5 h and 1200 °C for 1 h, respectively. It is evident that most particles appear to be more irregular in agglomeration and elongated shapes, with the particle diameters increasing to 350-500 nm and above 800 nm, respectively. Additionally, EDS analysis was utilized to determine the chemical composition of the pure and doped samples. The EDS spectrum in Figure 4a reveals that the pure sample exactly consists of lanthanum, niobium, titanium, and oxygen elements,
J. Phys. Chem. B, Vol. 111, No. 44, 2007 12695 in agreement with the XRD analysis in Figure 1. Furthermore, the atomic ratio measurement of the given elements from the EDS spectrum presents La/Nb/Ti ≈ 1:1.03:1.06, very close to La/Nb/Ti ) 1:1:1, which demonstrates that the sample probably is composed of LaNbTiO6. To the 12 mol % Eu3+-doped LaNbTiO6, the actual Eu3+ concentration estimated by Figure 4b is 11.80 mol %, clearly indicating the presence of appropriate Eu3+ ions incorporated into the phosphor. The UV-DR spectra of the pure samples at different annealing temperatures are shown in Figure 5. The strong absorption of LaNbTiO6 in the ultraviolet region is mainly attributed to the charge transfer from the oxygen ligand to the central niobium atom and titanium atom inside the NbO6 and TiO6 groups. The absorption edge of the LaNbTiO6 annealed at 1100 °C is about 380 nm, and the corresponding band gap is calculated to be 3.27 eV from Figure 5b. The UV-DR spectrum of the sample annealed at 1200 °C is similar to that of the one annealed at 1100 °C, indicating that when the particle sizes extend above 200 nm (Figure 3d), the size effect on the band gap of LaNbTiO6 can be negligible. All the spectra consist of two stages due to the influence of defect absorption in the host, while LaNbTiO6 annealed at 1000 °C exhibits an obvious blue-shift. Since the observation of SEM in Figure 3a has revealed the presence of nanometer-sized crystallites, the origin of the blue-shift may be elucidated by the combination of the quantum size effect and surface effect in the nanoparticles.15 Figure 6 shows the excitation (λem ) 440 nm) and emission (λex ) 370 nm) spectra of the pure samples. A sharp excitation band with a maximum around 372 nm can be observed for the LaNbTiO6 sample prepared at 1100 °C, as well as a weak and smooth peak centered at 268 nm. Therefore, the emission spectra of pure LaNbTiO6 were taken under excitation at 370 nm. Two blue emission bands centered at 440 and 470 nm, respectively, due to the NbO6 and TiO6 groups as the luminescence centers, can be detected. Those blue emissions might be assigned to the transition of self-trapped exciton (STE) emission of LaNbTiO6 related to the octahedral coordinated niobium and titanium. In the aeschynite-type LaNbTiO6 structure, the Nb5+ and Ti4+ ions are incorporated into the crystal framework as MO6 octahedra, distributed on the same crystallographic site randomly.16 The oxygen coordination polyhedra of Nb (Ti) ions are slightly distorted octahedra joined in pairs by edge-sharing with each pair being connected to six other pairs by corner-sharing, while each RE3+ ion is surrounded by eight O2- ions, which form an irregular polyhedron connected closely by edges into chains along the [010] direction.14 It is known that the isolated and edge- or face-shared MO6 octahedral groups show efficient luminescence with a large Stokes shift, while corner-sharing of MO6 groups leads to exciton delocalization, smaller Stokes shift, lower energy band, energy migration, and consequent luminescence quenching.17 Taking account of the crystal structure, the blue emission bands can be ascribed to the distorted NbO6 and TiO6 groups. Moreover, with the annealing temperature increasing, the emission intensity becomes lower, illuminating that the annealing temperature has a visible effect on the emission of LaNbTiO6. It can be assumed that higher temperatures might lead to the recombination of the NbO6 and TiO6 groups, along with configuration conversion of the octahedra chains from edge- to corner-sharing. As the most common defects in host crystals, oxygen vacancies can usually be generated during the sample preparation because of the partially incomplete crystallization, particularly consisting of the powders prepared by the rapid combustion reaction with a large quantity. It has been proven that crystallization accelerates with the increase of
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Figure 3. SEM micrographs of 12 mol % Eu3+-doped samples obtained under different annealing temperatures and times: (a) 1000 °C, 1 h; (b) 1100 °C, 1 h; (c) 1100 °C, 5 h; and (d) 1200 °C, 1 h.
Figure 5. UV-DR spectra of LaNbTiO6 samples annealed for 1 h at (a) 1000 °C, (b) 1100 °C, and (c) 1200 °C, respectively.
Figure 4. EDS spectra of different samples: (a) LaNbTiO6 and (b) 12 mol % Eu3+-doped LaNbTiO6.
temperature. In our case, oxygen defects cannot be eliminated completely, although heat-treatment processes were adopted and can act effectively as radiative centers in the host luminescence.
Thus, the emission band centered at 524 nm can be attributed to electron transition mediated by the oxygen vacancies in the band gap. The defect luminescence decreases with the temperature increase because of the lack of defects. Consequently, LaNbTiO6 has not only a higher quenching temperature, but the luminescence property of the LaNbTiO6 host also depends on the intrinsic factors of the structure. Figure 7a shows the excitation spectrum of Eu3+-doped LaNbTiO6 (6 mol %) annealed at 1100 °C. The spectrum mainly consists of a series of lines ascribed to the typical Eu3+ intra4f6 transitions, including the peaks with maxima at 362 nm (7F0 f 5D4), 384 nm (7F0 f 5G2-4), 395 nm (7F0 f 5L6), 412 nm (7F0 f 5D3), 464 nm (7F0 f 5D2), 525 nm (7F0 f 5D1), and 533 nm (7F1 f 5D1), respectively. Among them, the intensity of the 395, 464, and 533 nm excitation peaks is much stronger than the others, indicating that violet, blue, and green laser diodes (LDs) and light-emitting diodes (LEDs) are efficient
LaNbTiO6-Based Orange-Red Phosphors
Figure 6. Excitation spectrum (λem ) 440 nm) of pure LaNbTiO6 prepared at 1100 °C and emission spectra (λex ) 370 nm) of pure samples prepared at (a) 1000 °C, (b) 1100 °C, and (c) 1200 °C, respectively.
Figure 7. (a) Excitation spectrum of 6 mol % Eu3+-doped LaNbTiO6 (λem ) 614 nm) and (b) emission spectra of the Eu3+-doped LaNbTiO6.
pumping sources in obtaining Eu3+ emissions.18 When the Eu3+ concentration changed from 1 to 12 mol %, the excitation spectra were similar except that the intensity of the excitation peaks based on the f-f transitions of Eu3+ reinforced continuously. Varying the excitation wavelength does not result in obvious changes to the emission lines (not shown here); thus, the peak at 395 nm with the largest absorbed intensity is employed as the irradiation source wavelength. The multiband emissions obtained under excitation at 395 nm are shown in Figure 7b. Those emission spectra consisting of lines in the orange and red spectral range exhibit exclusively the characteristic f-f transitions of Eu3+, namely, 5D1 f 7F1 (540 nm), 5D1 f 7F2 (556 nm), 5D0 f 7F0 (580 nm), 5D0 f 7F1 (588 and 596 nm), 5D f 7F (614 and 624 nm), 5D f 7F (656 nm), and 5D f 0 2 0 3 0 7F (698 and 702 nm), respectively.2 From Figure 7, it is 4 observed that, with the Eu3+ concentration increasing, all of
J. Phys. Chem. B, Vol. 111, No. 44, 2007 12697 the emission lines are enhanced significantly, reaching a maximum intensity at a concentration of 12 mol %. The luminescence intensity of the 12 mol % Eu3+-doped sample augments approximately 13 multiples as high as that of the 1 mol % sample. No distinct diversifications of the emission spectra shapes and positions occurred when the concentration of Eu3+ varied in a wide range. Yet, nearly no emission from the host could be observed because of the strong luminescence of Eu3+, indicating an efficient energy transfer from the host to Eu3+ in the phosphors. The emission spectrum not only has the most intense red peaks at 614 nm due to the electric dipole transition 5D0 f 7F2, which indicates that Eu3+ occupies a site lacking inversion symmetry,19 but also has other powerful peaks in the range of 580-710 nm, of which involve available peaks at 588 and 596 nm ascribed to the 5D0 f 7F1 magnetic dipole transition, as an internal standard to gain some idea as to the relative transition strengths of the other transitions of Eu3+.20 It can be ensured visibly that the Eu3+ ions are at least situated around two different local situations in the LaNbTiO6 host due to the presence of two distinct lines for the 5D0 f 7F1 transition. Recently, it was reported that the ratio between the integrated intensity of the 5D0 f 7F2 and 5D0 f 7F1 transitions, I0-2/I0-1, can be used as a local crystal-field probe on the nature of the local surrounding cations.21 That is to say, this intensity ratio supplies a new avenue to estimate the degree of distortion from the inversion symmetry of the Eu3+ ion local environment. The calculations of the I0-2/I0-1 ratio were 2.21 and 2.04 for the different Eu3+ doping concentrations at 1 and 12 mol %, respectively. The decreasing trend indicates a higher symmetry local environment of the cations with the increasing Eu3+ doping amount. The lattice site of Eu3+ is an important factor for the phosphor emission efficiency. In general, it is not difficult for Eu3+ to substitute La3+ due to the same valence states and similar ion sizes between La3+(1.16 Å, CN ) 8) and Eu3+(1.066 Å, CN ) 8). Thus, in our case of the aeschynite-type structure, Eu3+ ions are assumed to substitute La3+ sites easily with the same coordination number. The introduction of Eu3+ ions can result in the reduction of the (La, Eu)-O bond length and unit cell volume because the lattice volume of the host material correlates closely with the ionic radii of the constituent elements. It can be supposed that when a small quantity of Eu3+ ions (such as 1 mol %) enter into the host lattice, they can occupy the La3+ sites randomly and be located in a disordered environment. In this case, the Eu3+ ions orient in the messy sites at a situation with different quantities and kinds of ions, so as to have lower asymmetry surroundings. With a higher doping level (such as 12 mol %), a large number of Eu3+ ions can distribute homogeneously and possess similar atomic conditions in the crystal lattice, which is considered to be beneficial for lowering the distortion extent and improving the symmetry of the structure. Thus, the site symmetry of Eu3+doped samples increases as the doping concentration rises. It is well-known that Eu3+ is sensitive to the surrounding environment, and the effect of the crystal field will cause shifts and splittings of the crystal field levels.22 As expected from the crystal structure considerations, the REO813- polyhedra are distorted, with distortions being of both even and odd parity. Even parity terms in the crystal field expansion cause splitting of the electronic energy levels of the RE3+ ions, whereas odd parity terms enhance the rates of dipole transitions between the RE3+ ion multiplets.14 The complicated emission lines of Eu3+doped LaNbTiO6 phosphors involving large-scale 5D0,1 f 7FJ
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Figure 8. Eu3+ concentration dependence of the intensity of the red and orange emissions in Eu3+-doped LaNbTiO6 phosphors: (a) 614 nm, (b) 596 nm, (c) 588 nm, (d) 702 nm, (e) 624 nm, and (f) 656 nm, respectively.
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Figure 10. Luminescence decay curve of the 614 nm emission for 12 mol % Eu3+-doped LaNbTiO6.
Figure 11. Microscope fluorescence images of 6 mol % (a) and 12 mol % (b) Eu3+-doped LaNbTiO6 samples.
TABLE 1: Luminescence Decay Time of Different Eu3+ (12 mol %) Emission Peaks λem (nm) λex (nm) decay time (ns)
Figure 9. Influence of annealing temperature (a) and time (b) on the luminescence intensity to 12 mol % Eu3+-doped phosphors.
(J ) 0, 1, 2, 3, 4) transitions can be ultimately ascribed to the odd parity distortions of the environment around the Eu3+ ions. The variation of the luminescence intensity with the doping amount ranging from 1 to 12 mol % in the LaNbTiO6 host is revealed in Figure 8. Six curves (a-f) represent the relative intensity of different emission peaks at 614, 596, 588, 702, 624, and 656 nm, respectively. Obviously, the relative intensity of every wavelength peak dramatically depends on the concentration of the Eu3+ ions. As we know, excessive impurities in the host materials can lead to the formation of the deep trap center of the doped ion, which can bring about fluorescence quenching;23 however, the intensity enhances continuously until there is a higher doping amount (12 mol %), in our case without concentration quenching. Since the luminescence spectra comprised of orange and red bands with equal contributions exhibit suitable and well-proportioned distributions as well as a series of powerful emission bands (as shown in Figure 7b), the Eu3+-
588 395 1.52
596 395 1.45
614 395 1.64
702 395 1.68
doped LaNbTiO6 material can be regarded as a novel orangered phosphor for optical devices. The variation curves of the luminescence intensity referred to in the 12 mol % Eu3+-doped phosphor as a function of the annealing temperature and time are shown in Figure 9, which exhibit the close relationship between PL intensity and particle integrity in the field of crystallography. In general, a high annealing temperature and long annealing time can be considered as effective approaches to improve the crystallinity of phosphor particles, resulting in the decreased surface area, nonradiative recombination, and fluorescence quenching centers (crystal defects and adsorbed species) and then an increase in emission intensity.24 On the contrary, once the particle size overcomes a certain critical value under different heat-treated conditions, for instance, in the present case, when the annealing temperature increased to 1200 °C or the annealing time exceeded 1 h at 1100 °C, the PL intensity becomes markedly weak. As previously reported, the less intense luminescence can be attributed to the smaller cross-section for the absorption of light caused by the larger scale particles and conglomerations within the phosphor.25 With respect to the conjunct influences of the annealing temperature and the particle size, the 12 mol % Eu3+doped sample prepared at 1100 °C for 1 h is regarded as the optimal phosphor based on our experiment. Figure 10 shows the luminescence decay curve of the 614 nm emission for 12 mol % Eu3+-doped LaNbTiO6 at room temperature. The decay curve reveals good agreement with a single-exponential curve as well as the decay time calculated to be 1.64 ns. Moreover, attention is also paid to the Eu3+ decay time of different emission peaks under the excitation of λex ) 395 nm. As shown in Table 1, all the luminescence lifetimes are in the range of nanoseconds, much shorter than those
LaNbTiO6-Based Orange-Red Phosphors reported in other host materials with Eu3+ activators.26-28 Since the short decay time can produce display devices with higher resolutions, the phosphors may be suitable for practical application in FEDs. In view of practicality, it is more interesting to investigate how promising the luminance and color-uniform levels of Eu3+doped LaNbTiO6 phosphors are than other research aspects. Thus, fluorescence images of the phosphors were obtained by a fluorescence microscope. Figure 11 exhibits the images of the 6 mol % (a) and 12 mol % (b) Eu3+-doped LaNbTiO6 samples. It is clear that the images are orange-red from a full review with luminescence effects, including the high brightness and high homogeneity. Figure 11b shows a better performance than Figure 11a due to the increase of Eu3+ ion doping concentration, in accordance with the emission spectra as mentioned previously. Conclusion In summary, a novel class of orange-red phosphors based on Eu3+-activated LaNbTiO6 has been investigated for the first time. The modified combustion method has been confirmed to be valid and efficient for the fabrication of these phosphors. The amorphous phase precursor prevails in a larger amount during the combustion process, while the single-crystalline phase dominates after heat-treatment, indicating that the heat-treatment process introduced in this work is applicable for the formation of a homogeneous aeschynite-type structure. Because of the quantum size effect and surface effect of the nanoparticles, the UV-DR spectrum of the low temperature annealed sample displays a blue-shift of the absorption edge. The STE emissions of the distorted NbO6 and TiO6 octahedra in the LaNbTiO6 host crystal arise, and the luminescent intensity has a visible decrease as the annealing temperature increases. The emission spectra of the phosphors consist of several intense peaks in the orange-red spectral range that closely correspond to the characteristic transitions of Eu3+ from 5D0,1 to 7FJ (J ) 0, 1, 2, 3, 4). The short luminescence decay times of 12 mol % Eu3+-doped LaNbTiO6 under 395 nm excitation show potential applications in the display devices with a higher resolution. With regard to the results of the excellent fluorescence images, it can be concluded that the Eu3+-doped LaNbTiO6 system with a higher doping concentration under the excitation of longer UV (around 395
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