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Effect of Zn2+ and Li+ codoping ions on nanosized Gd2O3:Eu3+ phosphor
Journal of Alloys and Compounds 440 (2007) 341–345
Effect of Zn2+ and Li+ codoping ions on nanosized Gd2O3:Eu3+ phosphor Bingjie Liu, Mu Gu ∗ , Xiaolin Liu, Chen Ni, Di Wang, Lihong Xiao, Riu Zhang Laboratory of Waves & Microstructure Materials, Pohl Institute of Solid State Physics, Tongji University, Shanghai 200092, People’s Republic of China Received 21 July 2006; accepted 10 September 2006 Available online 13 October 2006
Abstract Nanosized Gd1.92−x−y Znx Liy Eu0.08 O3−δ phosphor was fabricated by combustion synthesis. The effect of Zn2+ and Li+ ions on the crystallization behavior, morphology, and luminescence property of Gd2 O3 :Eu3+ was investigated. The results indicated that incorporation of Zn2+ and Li+ ions into Gd2 O3 :Eu3+ nanoparticles (NPs) could lead to a remarkable increase of photoluminescence or X-ray excited luminescence, and the intensity at 612 nm was increased by a factor of 7.1 or 21.5 in comparison with that of undoped sample. The enhanced luminescence was regarded as the results of the creation of oxygen vacancies due to the Gd3+ sites occupied by Li+ ions, the alteration of the crystal field surrounding the activator Eu3+ ions owing to the incorporation Zn2+ ions into interstitial sites, and the flux effect of Zn2+ and Li+ ions. The Zn- and Li-codoped Gd2 O3 :Eu3+ phosphor with highly enhanced luminescence is very encouraging for applications in high-resolution display devices. © 2006 Elsevier B.V. All rights reserved. Keywords: Phosphors; Nanofabrications; Optical properties; Luminescence
1. Introduction The resolution of display devices is related closely to the particle size of phosphor. In the last decade, significant interest in investigation of nanosized rare-earth oxide phosphor had been emerged due to the possibilities for advanced applications, especially for high-resolution displays. A number of methods, including colloidal and coprecipitation [1], sol–gel process [2], and combustion synthesis [3–5], etc. were employed to fabricate the oxide nanoparticles (NPs). Among these methods, combustion synthesis is even more attractive owing to the advantages of the requirement of simple apparatus, the direct crystallization of NPs, low process temperature, reduced time, and easily controlled particle size. The Gd2 O3 :Eu3+ phosphor has attracted much attention because of its high density (7.64 g cm−3 ), good luminescent performances, stable chemical property [6]. The synthesis and characterization of Gd2 O3 :Eu3+ NPs have been reported by many researchers
∗
Corresponding author. Tel.: +86 21 65980219; fax: +86 21 65980219. E-mail address: [email protected] (M. Gu).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.09.036
[3,7]. It is generally regarded that the luminescent efficiency of phosphor is reduced with the decrease of the grain size due to a large contribution of the surface states to the nonradiative transition [2,5]. On the other hand, a change in the composition of phosphor, such as the incorporation of Zn2+ or Li+ ion into host material, is an effective way to enhance its luminescent performance [4,8]. Several mechanisms were proposed for Zn2+ ion doping, such as flux effect [8], creation of oxygen vacancy [9], and diffusivity of Zn2+ ions in the host lattice [10]. And the role of the Li+ ion is mainly attributed to the flux effect and the creation of oxygen vacancy [4,11]. Therefore, it is expected that the luminescent efficiency will be improved further by codoping with several dopants if their effects are different. To the best of our knowledge, the influence of Zn2+ and Li+ codoping ions on the photoluminescence (PL) property of Gd2 O3 :Eu3+ NPs, especially on X-ray excited luminescence (XEL), has not been reported. In this work, Zn-doped, Li-doped, and Zn-, Li-codoped Gd2 O3 :Eu3+ NPs were prepared by combustion synthesis. The results indicate that both of PL and XEL are enhanced further by codoping with Zn2+ and Li+ ions. And the roles of dopants are discussed.
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2. Experimental Gd1.92−x−y Znx Liy Eu0.08 O3−δ (0 ≤ x ≤ 1.1, 0 ≤ y ≤ 0.1) NPs were synthesized directly by combustion synthesis. Stoichiometric amounts of Gd2 O3 (99.95%) and Eu2 O3 (99.99%) were dissolved in a solution of dilute HNO3 (A.R.), and a certain amounts of Zn(NO3 )2 ·6H2 O (99.00%), Li2 CO3 (99.99%), and glycine (Gly, H2 NCH2 COOH) were added. The molar ratio of glycine to nitrate (Gly/NO3 − ) was remained at 0.3 for all mixtures. The mixture was stirred until a uniform and transparent solution was obtained, and then gradually heated up in a crucible until it became sticky with the vaporizing of water. The spontaneous ignition was occurred, and the combustion flame temperature was controlled by the ratio of Gly/NO3 − . The combustion was finished after a few seconds, and foamy white powder was obtained. All as-prepared samples were annealed at 500 ◦ C for 1 h. No residual glycine and NO3 − would be detected by IR spectroscopy after this post heat-treatment [12]. The structural characteristic was examined by a Bruker D8 X-ray powder ˚ operated at 40 kV and 50 mA. diffractmeter with Cu K␣ radiation (λ = 1.5405 A) The morphology was measured by a JEOL JEM-1230 transmission electronic microscopy (TEM). PL spectrum and fluorescent lifetime were recorded on a Perkin-Elmer LS-55 luminescence spectrometer with a xenon flash lamp. The XEL spectrum was measured by X-ray excited spectrometer, where an F-30 X-ray tube (W anticathode target) operated under 80 kV and 4 mA was used as X-ray source, a SBP-300 monochromator and a Hamamatsu PMTH-S1-CR131 photomutiplier were used to record the luminescence spectrum. All measurements were carried out at room temperature.
3. Results and discussion For determination of the doping concentration of Zn2+ or Li+ ions, the PL intensity at 612 nm, which is the strongest emission peak for Eu3+ activator, was recorded as illustrated in Fig. 1. The PL intensity was associated with the Zn2+ or Li+ contents, and the optimal doping concentration of Zn2+ or Li+ ions was x = 0.55, y = 0 or x = 0, y = 0.08, respectively. In this work, x = 0.55, y = 0.08 was chosen as the Zn2+ and Li+ codoping concentration. To understand the effect of Zn2+ and Li+ ions on the structure of Gd1.92−x−y Znx Liy Eu0.08 O3−δ NP, XRD patterns were taken as given in Fig. 2. All diffraction peaks were assigned to cubic and monoclinic Gd2 O3 according to Powder Diffraction File PDF 76-0155 or PDF 43-1015. No Eu2 O3 , Li2 O and ZnO diffraction peaks were detected, which means the Eu3+ , Zn2+ , Li+ ions incorporated into Gd2 O3 host lattice homogeneously. It also presented that the cubic (C-type) and monoclinic phases coexisted in the undoped (x = 0, y = 0) and Li-doped (x = 0, y = 0.08) samples, and only cubic structure was exhibited in Zn-doped (x = 0.55, y = 0) or Li-, Zn-copoded (x = 0.55, y = 0.08) sample. It indicates that both of Zn2+ and Li+ ions can bring on a structure transform. The incorporation of Li+ or Zn2+ ions into Gd2 O3 matrix will lead to the lattice distortion, which causes a strong tendency for the matrix to form a more stable structure [3], i.e. the cubic phase. But the structural variation resulted from Zn2+ doping is much more evident than that from Li+ doping. On the other hand, the relative intensity of diffraction peak was improved due to the Li+ or/and Zn2+ doping, and the full width at half maximum (FWHM) became narrower. Using the Scherrer formula D = Kλ/β cos θ, where λ is the wavelength of ˚ β is the FWHM of the diffraction peaks, θ the X-ray (1.5405 A), is the Bragg diffraction angle, and parameter K equals to 0.89, the
Fig. 1. The dependence of PL intensity of Gd2 O3 :Eu3+ NPs on the Zn2+ and Li+ doping concentrations: (a) x = 0–1.1, y = 0 and (b) x = 0, y = 0–0.1.
mean grain size D was calculated, which are about 12, 24, 29, and 52 nm for undoped, Li-doped, Zn-doped, and Li-, Zn-codoped Gd2 O3 :Eu3+ NPs, respectively. Better crystallization and larger grain size can be regarded as the result of the flux effect of Zn2+ and Li+ ions during the preparation process, which plays the role in effectively promoting an incorporation of Eu2 O3 and Gd2 O3 , as well as Li+ and Zn2+ ions themselves into the host lattice [13].
Fig. 2. The XRD patterns of Gd2 O3 :Eu3+ NPs: (a) undoped (x = 0, y = 0), (b) Li-doped (x = 0, y = 0.08), (c) Zn-doped (x = 0.55, y = 0), and (d) Li-, Zn-codoped (x = 0.55, y = 0.08). The asterisks indicate the peaks originated from the monoclinic Gd2 O3 .
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Fig. 3. The TEM images of Gd2 O3 :Eu3+ NPs: (a) undoped (x = 0, y = 0), (b) Li-doped (x = 0, y = 0.08), (c) Zn-doped (x = 0.55, y = 0), and (d) Li-, Zn-codoped (x = 0.55, y = 0.08).
Morphology of phosphor material is an important factor on luminescent property. The morphology of Gd1.92−x−y Znx Liy Eu0.08 O3−δ NP was measured by using TEM as shown in Fig. 3. It illustrated that the grain size was affected apparently by the introduction of dopants. The agglomeration was obvious in undoped sample, and the mean grain size was less than 20 nm. However, the particles were spherical-like and less agglomeration, and the grain size was larger than 20 nm for Zn- or Li-doped sample. Furthermore, the mean grain size was larger than 50 nm for Zn-, Li-codoped Gd2 O3 :Eu3+ NP. That is also consistent with the result of XRD. As discussion above, all of the data indicate that the flux effect of two dopants is much stronger than that of single one. And it is also expected that the luminescent intensity will be improved further by codoping with Zn2+ and Li+ ions. The PL spectra of Gd1.92−x−y Znx Liy Eu0.08 O3−δ NPs are illustrated in Fig. 4. The peak near 250 nm in the excitation spectrum is known as the charge transfer (CT) process which
Fig. 4. PL spectra of undoped, Li-doped, Zn-doped, and Li-, Zn-codoped Gd2 O3 :Eu3+ NPs, the dash and solid lines indicate the excitation and emission spectra, respectively, λex = 230 nm, λem = 612 nm.
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attributes to the transition from O2− 2p state to Eu3+ 4f state. The peak near 230 nm originates from the excitation of Gd2 O3 host lattice (HL). The HL and CT peaks of the doped samples were improved distinctly in comparison with that of undoped sample. Whereas, it is noticeable that the CT peak was almost unchanged in Zn-doped sample. In emission spectrum, the strongest peak situated at 612 nm is assigned to the 5 D0 → 7 F2 transition of Eu3+ ions, and the peak around 588 nm is related to 5 D0 → 7 F1 transition. The peaks from 5 D1 → 7 FJ (J = 1, 2) transitions were also detected in the range of 525–570 nm. It is obvious that the PL intensity of Gd2 O3 :Eu3+ NPs can be enhanced dramatically by codoping with Zn2+ and Li+ ions. Comparing with the undoped sample, the PL intensities are improved up to about 2.4, 2.7 and 7.1 times for Li-doped, Zn-doped, and Li-, Zn-codoped samples, respectively. Fig. 5 shows the XEL spectra of Gd1.92−x−y Znx Liy Eu0.08 O3−δ NPs. The XEL spectra are similar to PL spectra, and the luminescent intensities of Li-doped, Zn-doped, and Li-, Zn-codoped Gd2 O3 :Eu3+ NPs achieve 3.1, 6.3, and 21.5 times of that of undoped sample, respectively. Apparently, enhanced luminescence by X-ray excitation is much higher than that by UV light excitation. That implies the Li-, Zn-codoped Gd2 O3 :Eu3+ NP is possessed of a superior XEL property, and would have a promising application in high-resolution X-ray imaging. The influence of the Zn2+ and Li+ ions on the luminescent performance may be attributed to several aspects. Firstly and the most obviously one is the flux effect of the Zn2+ and Li+ . It gives a better crystallization and larger grain size as shown in Figs. 2 and 3, resulting in higher oscillating strengths for the optical transitions [13], and also reducing the luminescence quenching due to the surface states [5], thus bringing on the increase of the luminescent intensity. Additional, both of Zn2+ and Li+ ions promote the structure transformed from monoclinic to cubic Gd2 O3 . This structural change is favourable for lumi˚ ion is smaller nescence. The effective ionic radius of Li+ (0.76 A) 3+ ˚ than that of Gd ion (0.94 A). That is to say the Li+ ions are suitable for occupation of the Gd3+ sites, which will give rise to a number of oxygen vacancies for the charge neutrality. Lopez et al. reported that the oxygen vacancy might act as a sensitizer for
Fig. 5. XEL spectra of undoped, Li-doped, Zn-doped, and Li-, Zn-codoped Gd2 O3 :Eu3+ NPs.
the energy transfer to the rare earth ion owing to the strong mixing of CT states [11]. Hence, the second aspect can be ascribed to the creation of oxygen vacancies due to Gd3+ sites occupied by smaller Li+ ions, resulting in the improvement of luminescent intensity. As discussed in Figs. 2 and 4, the CT peak was unvaried in Zn-doped sample, and the Zn2+ ions were incorporated into Gd2 O3 host lattice homogeneously. It should be unsuitable for interpreting the enhanced luminescence with the oxygen vacancies in Zn-doped Gd2 O3 :Eu3+ sample. Additional, the difference of Pauling’s electronegativity between Zn (1.65) and Gd (1.2) is larger than that between Li (0.98) and Gd element. Therefore, the other is assumed that Zn2+ ions might occupy easily into interstitial sites, altering the crystal field surrounding the activator Eu3+ . The sites offered for Eu3+ ions will have a more reduced symmetry, which is able to lift the parity selection rule and increase transition probability of electron, then result in the increase of luminescent intensity. In a word, enhanced luminescence by Zn2+ and Li+ codoping is mainly regarded as the results of the flux effect of Li+ and Zn2+ ions, the creation of oxygen vacancy by Li+ ion doping, and the alteration of crystal field due to Zn2+ ion doping. Furthermore, the modification of dopants on the fluorescent lifetime was also examined. The lifetimes of 5 D0 → 7 F2 transition for different samples were measured under the excitation of the 230 nm UV light, and obtained by single exponential fitting. As illustrated in Fig. 6, the lifetimes are 1.33, 1.47, 1.46, and 1.39 ms, respectively, for undoped, Li-doped, Zn-doped, and Li, Zn-codoped samples. It can be seen that the lifetime of undoped Gd2 O3 :Eu3+ NP is different from that of the doped samples. It is reported that the lifetime of Eu3+ ions at the particle surface would be longer than that of those inside the particle without additional nonradiative transition paths, but the lifetime would be shorter for those closer to or at the surface with quenching centers [14]. It may be the surface states, which act as quenching centers, give rise to a shorter lifetime and lower luminescent intensity for the undoped sample. On the other hand, the lifetime decreases with the increase of luminescent intensity for doped samples. It can be attributed to the improvement
Fig. 6. The fluorescent lifetimes of 5 D0 → 7 F2 (λem = 612 nm) of undoped, Li-doped, Zn-doped, and Li-, Zn-codoped Gd2 O3 :Eu3+ NPs excited by the ultraviolet radiation (λex = 230 nm).
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of radiative transition rate by doping with Zn2+ and Li+ ions [5].
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Research Field Project, Shuguang Research Project (Grant No. 02SG19), and the Shanghai Natural Science Foundation (Grant No. 05ZR14123).
4. Conclusion The influence of Zn2+ and Li+ ions on luminescence performance of nanosized Gd2 O3 :Eu3+ phosphor is discussed. The enhanced luminescence by codoping with Zn2+ and Li+ ions is much more effective than that by doping with single dopant, and Zn2+ and Li+ ions might exhibit different functions. The oxygen vacancy is result from the occupation of Gd3+ site by Li+ ion, acting a sensitizer for the effective energy transfer. By incorporation of Zn2+ ions into interstitial sites, the crystal field is altered, which lifts the parity selection rule and promotes transition probability of electron. And the flux effect of Zn2+ and Li+ ions results in enlargement of the grain size, improvement of the crystalline behavior, as well as promotion of the matrix structure to perfect cubic phase. The Zn-, Li-codoped Gd2 O3 :Eu3+ phosphor with highly enhanced luminescence is very encouraging for applications in high-resolution display devices. Acknowledgments This work is supported by Science and Technology Commission of Shanghai Municipality (Grant No. 05JC14062), the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of MOE, PR China, the Foundation of Shanghai Educational Commission for Key
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Effect of Zn2+ and Li+ codoping ions on nanosized Gd2O3:Eu3+ phosphor Bingjie Liu, Mu Gu ∗ , Xiaolin Liu, Chen Ni, Di Wang, Lihong Xiao, Riu Zhang Laboratory of Waves & Microstructure Materials, Pohl Institute of Solid State Physics, Tongji University, Shanghai 200092, People’s Republic of China Received 21 July 2006; accepted 10 September 2006 Available online 13 October 2006
Abstract Nanosized Gd1.92−x−y Znx Liy Eu0.08 O3−δ phosphor was fabricated by combustion synthesis. The effect of Zn2+ and Li+ ions on the crystallization behavior, morphology, and luminescence property of Gd2 O3 :Eu3+ was investigated. The results indicated that incorporation of Zn2+ and Li+ ions into Gd2 O3 :Eu3+ nanoparticles (NPs) could lead to a remarkable increase of photoluminescence or X-ray excited luminescence, and the intensity at 612 nm was increased by a factor of 7.1 or 21.5 in comparison with that of undoped sample. The enhanced luminescence was regarded as the results of the creation of oxygen vacancies due to the Gd3+ sites occupied by Li+ ions, the alteration of the crystal field surrounding the activator Eu3+ ions owing to the incorporation Zn2+ ions into interstitial sites, and the flux effect of Zn2+ and Li+ ions. The Zn- and Li-codoped Gd2 O3 :Eu3+ phosphor with highly enhanced luminescence is very encouraging for applications in high-resolution display devices. © 2006 Elsevier B.V. All rights reserved. Keywords: Phosphors; Nanofabrications; Optical properties; Luminescence
1. Introduction The resolution of display devices is related closely to the particle size of phosphor. In the last decade, significant interest in investigation of nanosized rare-earth oxide phosphor had been emerged due to the possibilities for advanced applications, especially for high-resolution displays. A number of methods, including colloidal and coprecipitation [1], sol–gel process [2], and combustion synthesis [3–5], etc. were employed to fabricate the oxide nanoparticles (NPs). Among these methods, combustion synthesis is even more attractive owing to the advantages of the requirement of simple apparatus, the direct crystallization of NPs, low process temperature, reduced time, and easily controlled particle size. The Gd2 O3 :Eu3+ phosphor has attracted much attention because of its high density (7.64 g cm−3 ), good luminescent performances, stable chemical property [6]. The synthesis and characterization of Gd2 O3 :Eu3+ NPs have been reported by many researchers
∗
Corresponding author. Tel.: +86 21 65980219; fax: +86 21 65980219. E-mail address: [email protected] (M. Gu).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.09.036
[3,7]. It is generally regarded that the luminescent efficiency of phosphor is reduced with the decrease of the grain size due to a large contribution of the surface states to the nonradiative transition [2,5]. On the other hand, a change in the composition of phosphor, such as the incorporation of Zn2+ or Li+ ion into host material, is an effective way to enhance its luminescent performance [4,8]. Several mechanisms were proposed for Zn2+ ion doping, such as flux effect [8], creation of oxygen vacancy [9], and diffusivity of Zn2+ ions in the host lattice [10]. And the role of the Li+ ion is mainly attributed to the flux effect and the creation of oxygen vacancy [4,11]. Therefore, it is expected that the luminescent efficiency will be improved further by codoping with several dopants if their effects are different. To the best of our knowledge, the influence of Zn2+ and Li+ codoping ions on the photoluminescence (PL) property of Gd2 O3 :Eu3+ NPs, especially on X-ray excited luminescence (XEL), has not been reported. In this work, Zn-doped, Li-doped, and Zn-, Li-codoped Gd2 O3 :Eu3+ NPs were prepared by combustion synthesis. The results indicate that both of PL and XEL are enhanced further by codoping with Zn2+ and Li+ ions. And the roles of dopants are discussed.
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2. Experimental Gd1.92−x−y Znx Liy Eu0.08 O3−δ (0 ≤ x ≤ 1.1, 0 ≤ y ≤ 0.1) NPs were synthesized directly by combustion synthesis. Stoichiometric amounts of Gd2 O3 (99.95%) and Eu2 O3 (99.99%) were dissolved in a solution of dilute HNO3 (A.R.), and a certain amounts of Zn(NO3 )2 ·6H2 O (99.00%), Li2 CO3 (99.99%), and glycine (Gly, H2 NCH2 COOH) were added. The molar ratio of glycine to nitrate (Gly/NO3 − ) was remained at 0.3 for all mixtures. The mixture was stirred until a uniform and transparent solution was obtained, and then gradually heated up in a crucible until it became sticky with the vaporizing of water. The spontaneous ignition was occurred, and the combustion flame temperature was controlled by the ratio of Gly/NO3 − . The combustion was finished after a few seconds, and foamy white powder was obtained. All as-prepared samples were annealed at 500 ◦ C for 1 h. No residual glycine and NO3 − would be detected by IR spectroscopy after this post heat-treatment [12]. The structural characteristic was examined by a Bruker D8 X-ray powder ˚ operated at 40 kV and 50 mA. diffractmeter with Cu K␣ radiation (λ = 1.5405 A) The morphology was measured by a JEOL JEM-1230 transmission electronic microscopy (TEM). PL spectrum and fluorescent lifetime were recorded on a Perkin-Elmer LS-55 luminescence spectrometer with a xenon flash lamp. The XEL spectrum was measured by X-ray excited spectrometer, where an F-30 X-ray tube (W anticathode target) operated under 80 kV and 4 mA was used as X-ray source, a SBP-300 monochromator and a Hamamatsu PMTH-S1-CR131 photomutiplier were used to record the luminescence spectrum. All measurements were carried out at room temperature.
3. Results and discussion For determination of the doping concentration of Zn2+ or Li+ ions, the PL intensity at 612 nm, which is the strongest emission peak for Eu3+ activator, was recorded as illustrated in Fig. 1. The PL intensity was associated with the Zn2+ or Li+ contents, and the optimal doping concentration of Zn2+ or Li+ ions was x = 0.55, y = 0 or x = 0, y = 0.08, respectively. In this work, x = 0.55, y = 0.08 was chosen as the Zn2+ and Li+ codoping concentration. To understand the effect of Zn2+ and Li+ ions on the structure of Gd1.92−x−y Znx Liy Eu0.08 O3−δ NP, XRD patterns were taken as given in Fig. 2. All diffraction peaks were assigned to cubic and monoclinic Gd2 O3 according to Powder Diffraction File PDF 76-0155 or PDF 43-1015. No Eu2 O3 , Li2 O and ZnO diffraction peaks were detected, which means the Eu3+ , Zn2+ , Li+ ions incorporated into Gd2 O3 host lattice homogeneously. It also presented that the cubic (C-type) and monoclinic phases coexisted in the undoped (x = 0, y = 0) and Li-doped (x = 0, y = 0.08) samples, and only cubic structure was exhibited in Zn-doped (x = 0.55, y = 0) or Li-, Zn-copoded (x = 0.55, y = 0.08) sample. It indicates that both of Zn2+ and Li+ ions can bring on a structure transform. The incorporation of Li+ or Zn2+ ions into Gd2 O3 matrix will lead to the lattice distortion, which causes a strong tendency for the matrix to form a more stable structure [3], i.e. the cubic phase. But the structural variation resulted from Zn2+ doping is much more evident than that from Li+ doping. On the other hand, the relative intensity of diffraction peak was improved due to the Li+ or/and Zn2+ doping, and the full width at half maximum (FWHM) became narrower. Using the Scherrer formula D = Kλ/β cos θ, where λ is the wavelength of ˚ β is the FWHM of the diffraction peaks, θ the X-ray (1.5405 A), is the Bragg diffraction angle, and parameter K equals to 0.89, the
Fig. 1. The dependence of PL intensity of Gd2 O3 :Eu3+ NPs on the Zn2+ and Li+ doping concentrations: (a) x = 0–1.1, y = 0 and (b) x = 0, y = 0–0.1.
mean grain size D was calculated, which are about 12, 24, 29, and 52 nm for undoped, Li-doped, Zn-doped, and Li-, Zn-codoped Gd2 O3 :Eu3+ NPs, respectively. Better crystallization and larger grain size can be regarded as the result of the flux effect of Zn2+ and Li+ ions during the preparation process, which plays the role in effectively promoting an incorporation of Eu2 O3 and Gd2 O3 , as well as Li+ and Zn2+ ions themselves into the host lattice [13].
Fig. 2. The XRD patterns of Gd2 O3 :Eu3+ NPs: (a) undoped (x = 0, y = 0), (b) Li-doped (x = 0, y = 0.08), (c) Zn-doped (x = 0.55, y = 0), and (d) Li-, Zn-codoped (x = 0.55, y = 0.08). The asterisks indicate the peaks originated from the monoclinic Gd2 O3 .
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Fig. 3. The TEM images of Gd2 O3 :Eu3+ NPs: (a) undoped (x = 0, y = 0), (b) Li-doped (x = 0, y = 0.08), (c) Zn-doped (x = 0.55, y = 0), and (d) Li-, Zn-codoped (x = 0.55, y = 0.08).
Morphology of phosphor material is an important factor on luminescent property. The morphology of Gd1.92−x−y Znx Liy Eu0.08 O3−δ NP was measured by using TEM as shown in Fig. 3. It illustrated that the grain size was affected apparently by the introduction of dopants. The agglomeration was obvious in undoped sample, and the mean grain size was less than 20 nm. However, the particles were spherical-like and less agglomeration, and the grain size was larger than 20 nm for Zn- or Li-doped sample. Furthermore, the mean grain size was larger than 50 nm for Zn-, Li-codoped Gd2 O3 :Eu3+ NP. That is also consistent with the result of XRD. As discussion above, all of the data indicate that the flux effect of two dopants is much stronger than that of single one. And it is also expected that the luminescent intensity will be improved further by codoping with Zn2+ and Li+ ions. The PL spectra of Gd1.92−x−y Znx Liy Eu0.08 O3−δ NPs are illustrated in Fig. 4. The peak near 250 nm in the excitation spectrum is known as the charge transfer (CT) process which
Fig. 4. PL spectra of undoped, Li-doped, Zn-doped, and Li-, Zn-codoped Gd2 O3 :Eu3+ NPs, the dash and solid lines indicate the excitation and emission spectra, respectively, λex = 230 nm, λem = 612 nm.
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attributes to the transition from O2− 2p state to Eu3+ 4f state. The peak near 230 nm originates from the excitation of Gd2 O3 host lattice (HL). The HL and CT peaks of the doped samples were improved distinctly in comparison with that of undoped sample. Whereas, it is noticeable that the CT peak was almost unchanged in Zn-doped sample. In emission spectrum, the strongest peak situated at 612 nm is assigned to the 5 D0 → 7 F2 transition of Eu3+ ions, and the peak around 588 nm is related to 5 D0 → 7 F1 transition. The peaks from 5 D1 → 7 FJ (J = 1, 2) transitions were also detected in the range of 525–570 nm. It is obvious that the PL intensity of Gd2 O3 :Eu3+ NPs can be enhanced dramatically by codoping with Zn2+ and Li+ ions. Comparing with the undoped sample, the PL intensities are improved up to about 2.4, 2.7 and 7.1 times for Li-doped, Zn-doped, and Li-, Zn-codoped samples, respectively. Fig. 5 shows the XEL spectra of Gd1.92−x−y Znx Liy Eu0.08 O3−δ NPs. The XEL spectra are similar to PL spectra, and the luminescent intensities of Li-doped, Zn-doped, and Li-, Zn-codoped Gd2 O3 :Eu3+ NPs achieve 3.1, 6.3, and 21.5 times of that of undoped sample, respectively. Apparently, enhanced luminescence by X-ray excitation is much higher than that by UV light excitation. That implies the Li-, Zn-codoped Gd2 O3 :Eu3+ NP is possessed of a superior XEL property, and would have a promising application in high-resolution X-ray imaging. The influence of the Zn2+ and Li+ ions on the luminescent performance may be attributed to several aspects. Firstly and the most obviously one is the flux effect of the Zn2+ and Li+ . It gives a better crystallization and larger grain size as shown in Figs. 2 and 3, resulting in higher oscillating strengths for the optical transitions [13], and also reducing the luminescence quenching due to the surface states [5], thus bringing on the increase of the luminescent intensity. Additional, both of Zn2+ and Li+ ions promote the structure transformed from monoclinic to cubic Gd2 O3 . This structural change is favourable for lumi˚ ion is smaller nescence. The effective ionic radius of Li+ (0.76 A) 3+ ˚ than that of Gd ion (0.94 A). That is to say the Li+ ions are suitable for occupation of the Gd3+ sites, which will give rise to a number of oxygen vacancies for the charge neutrality. Lopez et al. reported that the oxygen vacancy might act as a sensitizer for
Fig. 5. XEL spectra of undoped, Li-doped, Zn-doped, and Li-, Zn-codoped Gd2 O3 :Eu3+ NPs.
the energy transfer to the rare earth ion owing to the strong mixing of CT states [11]. Hence, the second aspect can be ascribed to the creation of oxygen vacancies due to Gd3+ sites occupied by smaller Li+ ions, resulting in the improvement of luminescent intensity. As discussed in Figs. 2 and 4, the CT peak was unvaried in Zn-doped sample, and the Zn2+ ions were incorporated into Gd2 O3 host lattice homogeneously. It should be unsuitable for interpreting the enhanced luminescence with the oxygen vacancies in Zn-doped Gd2 O3 :Eu3+ sample. Additional, the difference of Pauling’s electronegativity between Zn (1.65) and Gd (1.2) is larger than that between Li (0.98) and Gd element. Therefore, the other is assumed that Zn2+ ions might occupy easily into interstitial sites, altering the crystal field surrounding the activator Eu3+ . The sites offered for Eu3+ ions will have a more reduced symmetry, which is able to lift the parity selection rule and increase transition probability of electron, then result in the increase of luminescent intensity. In a word, enhanced luminescence by Zn2+ and Li+ codoping is mainly regarded as the results of the flux effect of Li+ and Zn2+ ions, the creation of oxygen vacancy by Li+ ion doping, and the alteration of crystal field due to Zn2+ ion doping. Furthermore, the modification of dopants on the fluorescent lifetime was also examined. The lifetimes of 5 D0 → 7 F2 transition for different samples were measured under the excitation of the 230 nm UV light, and obtained by single exponential fitting. As illustrated in Fig. 6, the lifetimes are 1.33, 1.47, 1.46, and 1.39 ms, respectively, for undoped, Li-doped, Zn-doped, and Li, Zn-codoped samples. It can be seen that the lifetime of undoped Gd2 O3 :Eu3+ NP is different from that of the doped samples. It is reported that the lifetime of Eu3+ ions at the particle surface would be longer than that of those inside the particle without additional nonradiative transition paths, but the lifetime would be shorter for those closer to or at the surface with quenching centers [14]. It may be the surface states, which act as quenching centers, give rise to a shorter lifetime and lower luminescent intensity for the undoped sample. On the other hand, the lifetime decreases with the increase of luminescent intensity for doped samples. It can be attributed to the improvement
Fig. 6. The fluorescent lifetimes of 5 D0 → 7 F2 (λem = 612 nm) of undoped, Li-doped, Zn-doped, and Li-, Zn-codoped Gd2 O3 :Eu3+ NPs excited by the ultraviolet radiation (λex = 230 nm).
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of radiative transition rate by doping with Zn2+ and Li+ ions [5].
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Research Field Project, Shuguang Research Project (Grant No. 02SG19), and the Shanghai Natural Science Foundation (Grant No. 05ZR14123).
4. Conclusion The influence of Zn2+ and Li+ ions on luminescence performance of nanosized Gd2 O3 :Eu3+ phosphor is discussed. The enhanced luminescence by codoping with Zn2+ and Li+ ions is much more effective than that by doping with single dopant, and Zn2+ and Li+ ions might exhibit different functions. The oxygen vacancy is result from the occupation of Gd3+ site by Li+ ion, acting a sensitizer for the effective energy transfer. By incorporation of Zn2+ ions into interstitial sites, the crystal field is altered, which lifts the parity selection rule and promotes transition probability of electron. And the flux effect of Zn2+ and Li+ ions results in enlargement of the grain size, improvement of the crystalline behavior, as well as promotion of the matrix structure to perfect cubic phase. The Zn-, Li-codoped Gd2 O3 :Eu3+ phosphor with highly enhanced luminescence is very encouraging for applications in high-resolution display devices. Acknowledgments This work is supported by Science and Technology Commission of Shanghai Municipality (Grant No. 05JC14062), the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of MOE, PR China, the Foundation of Shanghai Educational Commission for Key
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