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Synthesis and photoluminescence of blue LED excitable La4Ti9O24:Eu3+ phosphor for red-light emission
Materials Research Bulletin 51 (2014) 185–188
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Synthesis and photoluminescence of blue LED excitable La4Ti9O24:Eu3+ phosphor for red-light emission Bin Gao a, Jing Yin b, Zhi-Yong Mao a,c, Da-Jian Wang a, Le-Xi Zhang a, Li-Jian Bie a,* a
School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China School of Environmental Science & Safety Engineering, Tianjin University of Technology, Tianjin 300384, China c Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 March 2013 Received in revised form 19 August 2013 Accepted 13 December 2013 Available online 19 December 2013
Eu3+ activated La4Ti9O24 phosphors were prepared by firing precursors from sol–gel method. Under the excitation of 465 nm light, the phosphor with optimized concentration at 3% shows strong red light emission peaked at 613 nm with high color purity owing to 5D0 ! 7F2 emission of Eu3+. When adequate amount of either Si4+ or Bi3+ is incorporated in La4Ti9O24 host, the photoluminescence intensity of asprepared La4Ti9O24:Eu3+ phosphor can be enhanced by 12% and 19.4%, respectively. As the (La0.97Eu0.03)4Ti9O24 is pumped with blue-light, high purity red emission with chromaticity coordinates (0.6380, 0.3616) is achieved at the optimized condition. This phosphor might be applied in the solidstate white light emission devices based on blue light-emitting diodes. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds A. Optical materials B. Chemical synthesis C. Electron microscopy D. Luminescence
1. Introduction White light-emitting diodes (W-LEDs) are considered as the next generation solid-state lighting devices owing to the advantages of energy saving, high efficiency, environmental friendly, and long lifetime [1]. At present, most commercial W-LEDs products are fabricated by combining GaN-based blue LED (440–465 nm) and YAG:Ce (YAG) yellow-emitting phosphor. However, this kind of white light has poor color rendering due to the color deficiency in the red region [2]. One way to improve the color rendering index for this GaN chip/YAG combination can be achieved by adding redlight component. The other patterns to generate white light with high color rendering can be achieved through tri-color mix with red, green and blue phosphors as pumped by InGaN-based near-UV LED (370–410 nm), or with red, green phosphors pumped by blue LED [3]. Up to now, most commercial red phosphors are based on Eu2+-doped binary alkaline earth sulfides [4] or Eu3+-doped Y2O2S [5], the drawbacks of which are their chemical instability, short lifetime and low efficiency [6]. Therefore, developing of red phosphor with high efficiency, excellent chemical stability, and efficient absorption in blue light (around 460 nm) or near-UV light (around 400 nm) has attracted wide interests.
* Corresponding author. Tel.: +86 22 60216800; fax: +86 22 60216800. E-mail addresses: [email protected], [email protected] (L.-J. Bie). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.12.024
Recently, lanthanide doped La–Ti–O system, such as La2Ti2O7:Pr3+ [7], (La, Pr)2Ti2O7 [8] and (La0.95Eu0.05)2Ti2O7 [9], have been investigated for their up-conversion and red-emitting properties. As a member of La–Ti–O system, La4Ti9O24 consists of a complex network of distorted, octahedral-coordinated titanium sharing corners or edges, linked by two six-coordinated and one eight-coordinated lanthanum ions [10], the La3+ sites in this compound have low symmetry, which might be a good host for lanthanide ions. As Eu3+ ions usually show a typical 5D0–7F2 lineshaped emission around 612 nm when occupying lattice sites without centro-symmetry [11], indicating that La4Ti9O24:Eu3+ is suitable to be a red-light-emitting phosphor to compensate red component for white-light. In this paper, Eu3+ activated La4Ti9O24 red-light emitting phosphor samples, which could be excited by blue light, were prepared by firing precursors from sol–gel method. The photoluminescence of the phosphor and the influence of Si4+ or Bi3+ incorporation on the emission intensities of the phosphors were investigated. 2. Experimental Sol–gel method has many advantages over solid-state method, such as good homogeneity, lower sintering temperature, and narrow particle size distribution, which are all beneficial to increase the efficiency of phosphor [12]. La4Ti9O24:Eu3+ phosphors were prepared by sol–gel method: La(NO3)36H2O (A.R.) and
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Eu(NO3)36H2O (A.R.) were dissolved in acetic acid (A.R.) to form solution A; Ti(C4H9O)4 (A.R.) was dissolved in anhydrous ethanol (A.R.) to obtain solution B; then stoichiometric amount of solution A was added to solution B under vigorous stirring to produce a clear solution, after that, the as-prepared solution was kept at 80 8C to obtain a transparent gel. The obtained gel was heat-treated at 400 8C for 2 h, and annealed for 2 h subsequently at a temperature range from 800 8C to 1000 8C to obtain phosphor samples. Stoichiometric amount of Si(OC2H5)4 (A.R.) was added into solution B for the co-doping of Si4+, and the co-doping of Bi3+ was achieved by adding stoichiometric amount of Bi(NO3)35H2O (A.R.) to solution A. X-ray diffractions (XRD) were examined on Rigaku D/MAX2500 using Cu-Ka radiation. The morphology of the samples was characterized using a JEOL JEM-2100 transmission electron microscope (TEM) operated at 200 kV. The photoluminescence measurements were carried out using a Shimadzu RF-5301 PC fluorescence spectrophotometer equipped with a 150 W xenon lamp as the excitation source (parameters of excitation and emission slit widths were set to be 1.5 nm). The colorimetry parameters were measured on a PMS-50 Plus UV-Vis-near IR spectro-photocolorimeter (Everfine, China). The lifetimes were measured on a Horiba Jobin Yvon FL3-2-iHR320 fluorescence spectrophotometer. All measurements were carried out at room temperature.
3. Results and discussion The X-ray diffraction (XRD) patterns of La4Ti9O24:Eu3+ powders obtained after heating the dried gels at different temperature for 2 h are shown in Fig. 1. The diffraction peaks of samples heated at 800 8C and 900 8C match well with the patterns in JCPDS No. 830946 of La4Ti9O24. As the calcination temperature increases to 1000 8C, the intensity of the diffraction peaks increases, and the half width tends to be narrowed, revealing that a high purity, well crystallized La4Ti9O24:Eu3+ sample is obtained. Therefore, the sample obtained at the firing temperature of 1000 8C is chosen to discuss the luminescence property in the experiment. As can be seen from the transmission electron microscopy (TEM) image of La4Ti9O24:Eu3+ sample in Fig. 2, the particles of the sample are uniform and in a spherical morphology with a mean size of 100 nm, which might be a good candidate to form phosphor
Fig. 1. XRD patterns of as-prepared La4Ti9O24 powders with JCPDS No. 83-0946.
Fig. 2. TEM image of the as-prepared La4Ti9O24:Eu3+ particles.
layer with higher packing density and lower surface scattering for manufacturing W-LEDs. Fig. 3 shows the excitation spectra of La4Ti9O24:Eu3+ phosphor in the range of 250–590 nm monitored at 613 nm. The featured excitation lines mainly exist between 380 nm and 600 nm, attributed to transitions from the 7F0 ground state to the excited 5 DJ (J = 0, 1, 2, 3, 4) and 5L6 levels of the 4f7 configuration of Eu3+, and the intensities are much higher than that of CT band positioned at 320 nm originated from the charge transfer transition of O2Eu3+ and O2-Ti4+. The peaks at 395 nm, 404 nm, and 417 nm are owing to the transitions of 7F0 ! 5L6 and 7F0 ! 5D3. Commonly, the strongest excitation peak for Eu3+ is the 7F0–5L6. However, the dominant excitation peaks in the excitation spectra of La4Ti9O24:Eu3+ phosphor are 7F0 ! 5D2 electric-dipole transition, 7 F0 ! 5D1 magnetic-dipole transition and forbidden 7F0 ! 5D0
Fig. 3. Excitation spectra of La4Ti9O24:Eu3+ at room temperature as monitored at 613 nm.
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transition. It was well known that the electric-dipole transition (7F0 ! 5D2) in 4f6 configurations for Eu3+ with an approximate wavenumber 21.5 103 cm1 (465 nm) is a forbidden transition in view of the Laporte selection rule. Indeed, the forbidden transition becomes allowed due to the spin–orbit coupling at the excited state as well as the noncentrosymmetric crystal field, this phenomenon is classically defined as hypersensitive transition for Eu3+. As that in our work, the strong 7F0 ! 5D1 magnetic-dipole transition and forbidden 7F0 ! 5D0 transition detected on excitation spectra are also deduced to have communication with the Eu3+ in a noncentrosymmetric crystal field in La4Ti9O24. While the transition of 7F0 ! 5L6 is magnetic-dipole forbidden in term of J–J coupling and suppressed. Taking the coupling method into consideration, we speculate that J–J coupling approximation would be more suitable for Eu3+ in La4Ti9O24, whereas l–s coupling approximation is selected for Eu3+ in other host commonly. The excitation properties of La4Ti9O24:Eu3+ is different from that of Eu3+ doped La2Ti2O7 phosphors, in which the CT band positioned at 280 nm was the dominant component of the excitation spectrum. Interestingly, the strongest excitation peaks related to the 7 F0 ! 5D2 hypersensitive transition of Eu3+ in La4Ti9O24 host ensures La4Ti9O24:Eu3+ phosphors match well with the output wavelength of commercial GaN-based blue LED for the potential application in W-LEDs. Fig. 4 shows the emission spectra from 530 nm to 650 nm under the 465 nm excitation. The main emission line is 5D0 ! 7F2 transition of Eu3+ at 613 nm, other transitions from 5DJ (J = 0, 1) to 7FJ (J = 1, 2) ground states are relatively weaker owing to the influence of energy absorption, energy transfer, interaction with phonon and relaxation effect, which is beneficial for a phosphor with high color purity. As for the transition mechanism of Eu3+ energy level in the host, 5 D0 is the unsplitted singlet band, simplifying in a significant way the applications of the group theory and electronic transition selection rules. The electric-dipole allowed transition might be dominant as Eu3+ occupies the lattice site of noncentrosymmetric environment in the La4Ti9O24 phase [13]. The emission lines at 590 nm and 613 nm are classified as the magnetic-dipole transition and electric-dipole transition, respectively. The intensity of 5D0 ! 7F2 (electric-dipole transition) is found to be much higher than that of 5D0 ! 7F1 (magnetic-dipole transition), implying that the Eu3+ ion occupies the non-centro symmetry site.
The emission intensities (5D0 ! 7F2 transition of Eu3+) of La4Ti9O24:Eu3+ phosphors with different Eu3+ content (x) are illustrated in the inset of Fig. 4. The emission intensity of (La1xEux)4Ti9O24 increases with the increase of Eu3+ concentration until a maximum intensity is reached at the concentration of around 3% Eu3+, and then decreases with the increasing Eu3+ concentration. An increase in the Eu3+ concentration results in shorting the distance between Eu3+. The interaction of the ions and the transfer of energy tend to be intensified. On the other hand, a decrease in Eu3+ concentration reduces the photon energy stored by the quenching centers. Consequently, there is an optimum in the Eu3+ concentration to take into account of a trade-off of the above two factors. Fig. 5 portrays the colorimetry parameters of the (La0.97Eu0.03)4Ti9O24 phosphor under 465 nm excitation in the Commission Internationale de l’Eclairage (CIE) chromaticity diagram. The obtained CIE (x, y) chromaticity coordinate of the phosphor is (0.6380, 0.3616), which is close to the value from standard of National Television Systems Committee for red (x = 0.67, y = 0.33), indicating that this red phosphor could be an excellent red-lightemitting phosphor in illuminating and display devices excited by blue LED. The photograph of the (La0.97Eu0.03)4Ti9O24 phosphor excited emission is shown in the inset of Fig. 5, a high intensity luminescence can be observed with naked eye as excited with blue light. Furthermore, doping of Si4+ or Bi3+ to (La0.97Eu0.03)4Ti9O24 phosphor can increase the intensity of red-light emission under the excitation of 465 nm, without changing positions of the emission peaks. The emission intensity of samples is derived form the emission spectra, which was generally characterized under the same measurement conditions, including the amount of samples, scan speed, slit width and measure temperature. The effective ionic radii of Si4+ is 0.41 A˚, much smaller than that of La3+ (1.06 A˚) but closer to that of Ti4+ (0.68 A˚), indicating a more likely occupation of Si4+ on Ti4+ sites in the lattice of (La0.97Eu0.03)4Ti9O24 phosphor [14]. Fig. 6 shows the emission intensity with different Si4+ contents (y) in (La0.97Eu0.03)4(Ti1ySiy)9O24. The emission intensity increases 12% at the Si4+ doping concentration of 5%. The photoluminescence enhancement can be attributed to the distortion of the host lattice around the luminescent center resulted from the substitution of Ti4+ by smaller Si4+ ion, which might optimize the lattice environment for the energy transfer and luminescence
Fig. 4. Emission spectra of La4Ti9O24:Eu3+ under 465 nm excitation at room temperature (the insert shows the intensity observed at 613 nm vs. Eu3+ concentration).
Fig. 5. Colorimetry parameters of the (La0.97Eu0.03)4Ti9O24 phosphor in the Commission Internationale de l’Eclairage (CIE) diagram; the insert is a photograph of the (La0.97Eu0.03)4Ti9O24 phosphor under 465 nm excitation.
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Fig. 6. Emission spectral intensities of (La0.97Eu0.03)4(Ti1ySiy)9O24 phosphor vs. Si4+ concentration under 465 nm excitation.
Fig. 7. Emission spectral intensities of (La0.97zBizEu0.03)4Ti9O24 phosphor vs. Bi3+ concentration under 465 nm excitation.
of Eu3+ to certain degree. The lifetimes for 5D0 ! 7F2 emission (613 nm) of (La0.97Eu0.03)4Ti9O24 and (La0.97Eu0.03)4(Ti0.94Si0.06)9O24 phosphors under 465 nm excitation were measured to be 0.732 ms and 0.701 ms, respectively. The variation of lifetime indicates the change of local crystal environment resulted from the replacement of Ti4+ by Si4+ in host lattice. The effective ionic radii of Bi3+ (1.02 A˚) is similar to that of La3+ (1.06 A˚), therefore Bi3+ might take up the site of the La3+ site in (La0.97Eu0.03)4Ti9O24 phosphor. The emission intensity vs. Bi3+ contents (z) in the system of (La0.97zBizEu0.03)4Ti9O24 are shown in Fig. 7. The intensity of emission spectra increases 19.4% as the Bi3+ doping concentration is 6%. The reason might be the same as that for Si4+ doping [15]. The lifetimes for 5D0 ! 7F2 emission (613 nm) of (La0.97Eu0.03)4Ti9O24 and (La0.92Bi0.05Eu0.03)4Ti9O24 phosphors under the excitation of 465 nm were measured to be 0.732 ms and 0.748 ms relatively, which also implying the change of local crystal environment resulted from the replacement of La3+ by Bi3+.
respectively. Since the excitation spectra of the phosphor at 465 nm match exactly with the output wavelength of commercial blue LED, this Eu3+ doped titanate red-light phosphor could be a new candidate for color mixing in white light-emitting diode.
4. Conclusions A novel red-light-emitting La4Ti9O24:Eu3+ phosphors with average particle size of 100 nm are obtained by firing precursors from sol–gel method at 1000 8C for 2 h. Under 465 nm excitation, the La4Ti9O24:Eu3+ phosphors show strong red 5D0 ! 7F2 emission peaked at 613 nm with color purity of 100%. Doping with adequate amount of either Si4+ or Bi3+ can enhance the emission intensity of the synthesized La4Ti9O24:Eu3+ phosphor by 12% and 19.4%,
Acknowledgements This work is financially supported by National Natural Science Foundation of China (NSFC, Grant No. 21271139) and Tianjin Natural Science Foundation (Grant No. 08JCZDJC18700).
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Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Synthesis and photoluminescence of blue LED excitable La4Ti9O24:Eu3+ phosphor for red-light emission Bin Gao a, Jing Yin b, Zhi-Yong Mao a,c, Da-Jian Wang a, Le-Xi Zhang a, Li-Jian Bie a,* a
School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China School of Environmental Science & Safety Engineering, Tianjin University of Technology, Tianjin 300384, China c Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 March 2013 Received in revised form 19 August 2013 Accepted 13 December 2013 Available online 19 December 2013
Eu3+ activated La4Ti9O24 phosphors were prepared by firing precursors from sol–gel method. Under the excitation of 465 nm light, the phosphor with optimized concentration at 3% shows strong red light emission peaked at 613 nm with high color purity owing to 5D0 ! 7F2 emission of Eu3+. When adequate amount of either Si4+ or Bi3+ is incorporated in La4Ti9O24 host, the photoluminescence intensity of asprepared La4Ti9O24:Eu3+ phosphor can be enhanced by 12% and 19.4%, respectively. As the (La0.97Eu0.03)4Ti9O24 is pumped with blue-light, high purity red emission with chromaticity coordinates (0.6380, 0.3616) is achieved at the optimized condition. This phosphor might be applied in the solidstate white light emission devices based on blue light-emitting diodes. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds A. Optical materials B. Chemical synthesis C. Electron microscopy D. Luminescence
1. Introduction White light-emitting diodes (W-LEDs) are considered as the next generation solid-state lighting devices owing to the advantages of energy saving, high efficiency, environmental friendly, and long lifetime [1]. At present, most commercial W-LEDs products are fabricated by combining GaN-based blue LED (440–465 nm) and YAG:Ce (YAG) yellow-emitting phosphor. However, this kind of white light has poor color rendering due to the color deficiency in the red region [2]. One way to improve the color rendering index for this GaN chip/YAG combination can be achieved by adding redlight component. The other patterns to generate white light with high color rendering can be achieved through tri-color mix with red, green and blue phosphors as pumped by InGaN-based near-UV LED (370–410 nm), or with red, green phosphors pumped by blue LED [3]. Up to now, most commercial red phosphors are based on Eu2+-doped binary alkaline earth sulfides [4] or Eu3+-doped Y2O2S [5], the drawbacks of which are their chemical instability, short lifetime and low efficiency [6]. Therefore, developing of red phosphor with high efficiency, excellent chemical stability, and efficient absorption in blue light (around 460 nm) or near-UV light (around 400 nm) has attracted wide interests.
* Corresponding author. Tel.: +86 22 60216800; fax: +86 22 60216800. E-mail addresses: [email protected], [email protected] (L.-J. Bie). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.12.024
Recently, lanthanide doped La–Ti–O system, such as La2Ti2O7:Pr3+ [7], (La, Pr)2Ti2O7 [8] and (La0.95Eu0.05)2Ti2O7 [9], have been investigated for their up-conversion and red-emitting properties. As a member of La–Ti–O system, La4Ti9O24 consists of a complex network of distorted, octahedral-coordinated titanium sharing corners or edges, linked by two six-coordinated and one eight-coordinated lanthanum ions [10], the La3+ sites in this compound have low symmetry, which might be a good host for lanthanide ions. As Eu3+ ions usually show a typical 5D0–7F2 lineshaped emission around 612 nm when occupying lattice sites without centro-symmetry [11], indicating that La4Ti9O24:Eu3+ is suitable to be a red-light-emitting phosphor to compensate red component for white-light. In this paper, Eu3+ activated La4Ti9O24 red-light emitting phosphor samples, which could be excited by blue light, were prepared by firing precursors from sol–gel method. The photoluminescence of the phosphor and the influence of Si4+ or Bi3+ incorporation on the emission intensities of the phosphors were investigated. 2. Experimental Sol–gel method has many advantages over solid-state method, such as good homogeneity, lower sintering temperature, and narrow particle size distribution, which are all beneficial to increase the efficiency of phosphor [12]. La4Ti9O24:Eu3+ phosphors were prepared by sol–gel method: La(NO3)36H2O (A.R.) and
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Eu(NO3)36H2O (A.R.) were dissolved in acetic acid (A.R.) to form solution A; Ti(C4H9O)4 (A.R.) was dissolved in anhydrous ethanol (A.R.) to obtain solution B; then stoichiometric amount of solution A was added to solution B under vigorous stirring to produce a clear solution, after that, the as-prepared solution was kept at 80 8C to obtain a transparent gel. The obtained gel was heat-treated at 400 8C for 2 h, and annealed for 2 h subsequently at a temperature range from 800 8C to 1000 8C to obtain phosphor samples. Stoichiometric amount of Si(OC2H5)4 (A.R.) was added into solution B for the co-doping of Si4+, and the co-doping of Bi3+ was achieved by adding stoichiometric amount of Bi(NO3)35H2O (A.R.) to solution A. X-ray diffractions (XRD) were examined on Rigaku D/MAX2500 using Cu-Ka radiation. The morphology of the samples was characterized using a JEOL JEM-2100 transmission electron microscope (TEM) operated at 200 kV. The photoluminescence measurements were carried out using a Shimadzu RF-5301 PC fluorescence spectrophotometer equipped with a 150 W xenon lamp as the excitation source (parameters of excitation and emission slit widths were set to be 1.5 nm). The colorimetry parameters were measured on a PMS-50 Plus UV-Vis-near IR spectro-photocolorimeter (Everfine, China). The lifetimes were measured on a Horiba Jobin Yvon FL3-2-iHR320 fluorescence spectrophotometer. All measurements were carried out at room temperature.
3. Results and discussion The X-ray diffraction (XRD) patterns of La4Ti9O24:Eu3+ powders obtained after heating the dried gels at different temperature for 2 h are shown in Fig. 1. The diffraction peaks of samples heated at 800 8C and 900 8C match well with the patterns in JCPDS No. 830946 of La4Ti9O24. As the calcination temperature increases to 1000 8C, the intensity of the diffraction peaks increases, and the half width tends to be narrowed, revealing that a high purity, well crystallized La4Ti9O24:Eu3+ sample is obtained. Therefore, the sample obtained at the firing temperature of 1000 8C is chosen to discuss the luminescence property in the experiment. As can be seen from the transmission electron microscopy (TEM) image of La4Ti9O24:Eu3+ sample in Fig. 2, the particles of the sample are uniform and in a spherical morphology with a mean size of 100 nm, which might be a good candidate to form phosphor
Fig. 1. XRD patterns of as-prepared La4Ti9O24 powders with JCPDS No. 83-0946.
Fig. 2. TEM image of the as-prepared La4Ti9O24:Eu3+ particles.
layer with higher packing density and lower surface scattering for manufacturing W-LEDs. Fig. 3 shows the excitation spectra of La4Ti9O24:Eu3+ phosphor in the range of 250–590 nm monitored at 613 nm. The featured excitation lines mainly exist between 380 nm and 600 nm, attributed to transitions from the 7F0 ground state to the excited 5 DJ (J = 0, 1, 2, 3, 4) and 5L6 levels of the 4f7 configuration of Eu3+, and the intensities are much higher than that of CT band positioned at 320 nm originated from the charge transfer transition of O2Eu3+ and O2-Ti4+. The peaks at 395 nm, 404 nm, and 417 nm are owing to the transitions of 7F0 ! 5L6 and 7F0 ! 5D3. Commonly, the strongest excitation peak for Eu3+ is the 7F0–5L6. However, the dominant excitation peaks in the excitation spectra of La4Ti9O24:Eu3+ phosphor are 7F0 ! 5D2 electric-dipole transition, 7 F0 ! 5D1 magnetic-dipole transition and forbidden 7F0 ! 5D0
Fig. 3. Excitation spectra of La4Ti9O24:Eu3+ at room temperature as monitored at 613 nm.
B. Gao et al. / Materials Research Bulletin 51 (2014) 185–188
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transition. It was well known that the electric-dipole transition (7F0 ! 5D2) in 4f6 configurations for Eu3+ with an approximate wavenumber 21.5 103 cm1 (465 nm) is a forbidden transition in view of the Laporte selection rule. Indeed, the forbidden transition becomes allowed due to the spin–orbit coupling at the excited state as well as the noncentrosymmetric crystal field, this phenomenon is classically defined as hypersensitive transition for Eu3+. As that in our work, the strong 7F0 ! 5D1 magnetic-dipole transition and forbidden 7F0 ! 5D0 transition detected on excitation spectra are also deduced to have communication with the Eu3+ in a noncentrosymmetric crystal field in La4Ti9O24. While the transition of 7F0 ! 5L6 is magnetic-dipole forbidden in term of J–J coupling and suppressed. Taking the coupling method into consideration, we speculate that J–J coupling approximation would be more suitable for Eu3+ in La4Ti9O24, whereas l–s coupling approximation is selected for Eu3+ in other host commonly. The excitation properties of La4Ti9O24:Eu3+ is different from that of Eu3+ doped La2Ti2O7 phosphors, in which the CT band positioned at 280 nm was the dominant component of the excitation spectrum. Interestingly, the strongest excitation peaks related to the 7 F0 ! 5D2 hypersensitive transition of Eu3+ in La4Ti9O24 host ensures La4Ti9O24:Eu3+ phosphors match well with the output wavelength of commercial GaN-based blue LED for the potential application in W-LEDs. Fig. 4 shows the emission spectra from 530 nm to 650 nm under the 465 nm excitation. The main emission line is 5D0 ! 7F2 transition of Eu3+ at 613 nm, other transitions from 5DJ (J = 0, 1) to 7FJ (J = 1, 2) ground states are relatively weaker owing to the influence of energy absorption, energy transfer, interaction with phonon and relaxation effect, which is beneficial for a phosphor with high color purity. As for the transition mechanism of Eu3+ energy level in the host, 5 D0 is the unsplitted singlet band, simplifying in a significant way the applications of the group theory and electronic transition selection rules. The electric-dipole allowed transition might be dominant as Eu3+ occupies the lattice site of noncentrosymmetric environment in the La4Ti9O24 phase [13]. The emission lines at 590 nm and 613 nm are classified as the magnetic-dipole transition and electric-dipole transition, respectively. The intensity of 5D0 ! 7F2 (electric-dipole transition) is found to be much higher than that of 5D0 ! 7F1 (magnetic-dipole transition), implying that the Eu3+ ion occupies the non-centro symmetry site.
The emission intensities (5D0 ! 7F2 transition of Eu3+) of La4Ti9O24:Eu3+ phosphors with different Eu3+ content (x) are illustrated in the inset of Fig. 4. The emission intensity of (La1xEux)4Ti9O24 increases with the increase of Eu3+ concentration until a maximum intensity is reached at the concentration of around 3% Eu3+, and then decreases with the increasing Eu3+ concentration. An increase in the Eu3+ concentration results in shorting the distance between Eu3+. The interaction of the ions and the transfer of energy tend to be intensified. On the other hand, a decrease in Eu3+ concentration reduces the photon energy stored by the quenching centers. Consequently, there is an optimum in the Eu3+ concentration to take into account of a trade-off of the above two factors. Fig. 5 portrays the colorimetry parameters of the (La0.97Eu0.03)4Ti9O24 phosphor under 465 nm excitation in the Commission Internationale de l’Eclairage (CIE) chromaticity diagram. The obtained CIE (x, y) chromaticity coordinate of the phosphor is (0.6380, 0.3616), which is close to the value from standard of National Television Systems Committee for red (x = 0.67, y = 0.33), indicating that this red phosphor could be an excellent red-lightemitting phosphor in illuminating and display devices excited by blue LED. The photograph of the (La0.97Eu0.03)4Ti9O24 phosphor excited emission is shown in the inset of Fig. 5, a high intensity luminescence can be observed with naked eye as excited with blue light. Furthermore, doping of Si4+ or Bi3+ to (La0.97Eu0.03)4Ti9O24 phosphor can increase the intensity of red-light emission under the excitation of 465 nm, without changing positions of the emission peaks. The emission intensity of samples is derived form the emission spectra, which was generally characterized under the same measurement conditions, including the amount of samples, scan speed, slit width and measure temperature. The effective ionic radii of Si4+ is 0.41 A˚, much smaller than that of La3+ (1.06 A˚) but closer to that of Ti4+ (0.68 A˚), indicating a more likely occupation of Si4+ on Ti4+ sites in the lattice of (La0.97Eu0.03)4Ti9O24 phosphor [14]. Fig. 6 shows the emission intensity with different Si4+ contents (y) in (La0.97Eu0.03)4(Ti1ySiy)9O24. The emission intensity increases 12% at the Si4+ doping concentration of 5%. The photoluminescence enhancement can be attributed to the distortion of the host lattice around the luminescent center resulted from the substitution of Ti4+ by smaller Si4+ ion, which might optimize the lattice environment for the energy transfer and luminescence
Fig. 4. Emission spectra of La4Ti9O24:Eu3+ under 465 nm excitation at room temperature (the insert shows the intensity observed at 613 nm vs. Eu3+ concentration).
Fig. 5. Colorimetry parameters of the (La0.97Eu0.03)4Ti9O24 phosphor in the Commission Internationale de l’Eclairage (CIE) diagram; the insert is a photograph of the (La0.97Eu0.03)4Ti9O24 phosphor under 465 nm excitation.
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Fig. 6. Emission spectral intensities of (La0.97Eu0.03)4(Ti1ySiy)9O24 phosphor vs. Si4+ concentration under 465 nm excitation.
Fig. 7. Emission spectral intensities of (La0.97zBizEu0.03)4Ti9O24 phosphor vs. Bi3+ concentration under 465 nm excitation.
of Eu3+ to certain degree. The lifetimes for 5D0 ! 7F2 emission (613 nm) of (La0.97Eu0.03)4Ti9O24 and (La0.97Eu0.03)4(Ti0.94Si0.06)9O24 phosphors under 465 nm excitation were measured to be 0.732 ms and 0.701 ms, respectively. The variation of lifetime indicates the change of local crystal environment resulted from the replacement of Ti4+ by Si4+ in host lattice. The effective ionic radii of Bi3+ (1.02 A˚) is similar to that of La3+ (1.06 A˚), therefore Bi3+ might take up the site of the La3+ site in (La0.97Eu0.03)4Ti9O24 phosphor. The emission intensity vs. Bi3+ contents (z) in the system of (La0.97zBizEu0.03)4Ti9O24 are shown in Fig. 7. The intensity of emission spectra increases 19.4% as the Bi3+ doping concentration is 6%. The reason might be the same as that for Si4+ doping [15]. The lifetimes for 5D0 ! 7F2 emission (613 nm) of (La0.97Eu0.03)4Ti9O24 and (La0.92Bi0.05Eu0.03)4Ti9O24 phosphors under the excitation of 465 nm were measured to be 0.732 ms and 0.748 ms relatively, which also implying the change of local crystal environment resulted from the replacement of La3+ by Bi3+.
respectively. Since the excitation spectra of the phosphor at 465 nm match exactly with the output wavelength of commercial blue LED, this Eu3+ doped titanate red-light phosphor could be a new candidate for color mixing in white light-emitting diode.
4. Conclusions A novel red-light-emitting La4Ti9O24:Eu3+ phosphors with average particle size of 100 nm are obtained by firing precursors from sol–gel method at 1000 8C for 2 h. Under 465 nm excitation, the La4Ti9O24:Eu3+ phosphors show strong red 5D0 ! 7F2 emission peaked at 613 nm with color purity of 100%. Doping with adequate amount of either Si4+ or Bi3+ can enhance the emission intensity of the synthesized La4Ti9O24:Eu3+ phosphor by 12% and 19.4%,
Acknowledgements This work is financially supported by National Natural Science Foundation of China (NSFC, Grant No. 21271139) and Tianjin Natural Science Foundation (Grant No. 08JCZDJC18700).
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