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Dependence of photoluminescence (PL) emission intensity on Eu3+ and ZnO concentrations in Y2O3:Eu3+ and ZnO·Y2O3:Eu3+ nanophosphors
Optical Materials 33 (2011) 1495–1499
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Dependence of photoluminescence (PL) emission intensity on Eu3+ and ZnO concentrations in Y2O3:Eu3+ and ZnOY2O3:Eu3+ nanophosphors G.H. Mhlongo a,b, M.S. Dhlamini a,c, H.C. Swart b, O.M. Ntwaeaborwa b, K.T. Hillie a,b,⇑ a
National Centre for Nano-structured Materials, Council for Scientific and Industrial Research, 1-Meiring Naude Road, Brummeria, PO Box 395, Pretoria 0001, South Africa Department of Physics, University of Free State, Bloemfontein ZA9300, South Africa c Department of Physics, University of South Africa, Pretoria, South Africa b
a r t i c l e
i n f o
Article history: Received 13 October 2010 Received in revised form 22 February 2011 Accepted 3 March 2011 Available online 7 April 2011 Keywords: Energy transfer Phosphors Concentration quenching effect
a b s t r a c t Y2O3:Eu3+ and ZnOY2O3:Eu3+ nanophosphor powders with different concentrations of Eu3+ ions were synthesized by a sol–gel method and their luminescence properties were investigated. The red photoluminescence (PL) from Eu3+ ions with the main emission peak at 612 nm was observed to increase with Eu3+ concentration from 0.25 to 0.75 mol% and decreased notably when the concentration was increased to 1 mol%. The decrease in the PL intensity at higher Eu3+ concentrations can be associated with concentration quenching effects. The red emission at 612 nm was shown to increase considerable when ZnO nanoparticles were incorporated in Y2O3:Eu3+ while green emission from ZnO was suppressed. The increase is attributed to energy transfer from ZnO to Eu3+. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
The study of the synthesis and characterization of rare earth oxide based nanophosphors has increased rapidly after the realization that semiconductor nanoparticles can transfer energy via electronic interactions to luminescent centres in a host matrix, resulting in enhanced luminescence. These nanophosphors have potential application in light emitting devices such as fluorescent lamps, solid state lasers, optical wave guides, and flat panel displays [1,2]. Numerous synthetic approaches used to prepare these phosphors have been reported. However, the sol–gel method is preferable since it is relatively inexpensive, provides good control of particle size, uniform morphology, and high homogeneity. Although the red emitting Y2O3:Eu3+ with different concentration of Eu3+ has been researched extensively with particles in micron scale and recently nanoparticles with each size domain pursued for specific technologies, there are still inconsistent reports about the exact amount of Eu3+ which cause concentration quenching in these phosphors [3–5]. On the other hand, the improved luminescence induced by energy transfer from ZnO nanocrystals to Eu3+ encapsulated in SiO2 matrix has been reported [6], and similar phenomenon was investigated in Y2O3 host. In the present work, we investigated the effect of Eu3+ and ZnO concentration on nanocrystalline Y2O3:Eu3+ phosphor powders synthesized by the sol–gel method. Energy transfer mechanism and concentration quenching effect are discussed.
Y2O3: Eu3+ was prepared following the method described by Zhang et al. [7] but with a few modifications. Y2O3 (99.99%), Eu2O3 (99.99%), nitric acid (65% A.U.) were used as the starting materials and citric acid (99.5%, A.C.S. reagent) was used as a chelating agent. Y2O3 and Eu2O3 were dissolved in a dilute nitric acid followed by a dropwise addition of citric acid resulting in a clear sol. The amount of Y2O3 was fixed in all the samples while the Eu3+ concentration was varied from 0.25 to 1 mol%. The molar ratio of metal ions to citric acid was 2:1. A homogeneous gel was obtained by heating the clear sol at 115 °C for 2.5 h, which was then calcined in air at 900 °C for 3 h, and ground into a fine powder. ZnO.Y2O3:Eu3+ was prepared by mixing Y2O3:Eu3+ sol with ZnO nanoparticles suspended in ethanol. A detailed preparation of ZnO nanoparticles with an average particle size of 4 nm in diameter was reported elsewhere [8]. The particle morphology and surface topography was analyzed using JEOL JSM-7500F, Field Emission Scanning Electron Microscopy (SEM) and Multimode Atomic Force Microscope (Nano Scope Version (R) IV) respectively. The crystal structure of the samples was analysed by Phillips Xpert X-ray diffractometer using the Cu Ka source. Photoluminescence (PL) data were collected using an Ar laser and a monochromatized Xenon lamp at an excitation wavelength of 475 and 325 nm, respectively while cathodoluminescence data was collected using an Ocean Optics S2000 spectrometer when the samples were irradiated with 2 keV and 8.5 lA beam of electrons in a high vacuum chamber at a base pressure of 1.2 10 8 Torr.
⇑ Corresponding author. Fax: +27 12 841 2229. E-mail address: [email protected] (K.T. Hillie). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.03.009
G.H. Mhlongo et al. / Optical Materials 33 (2011) 1495–1499 222
3+
622
400
440
0.05 mol% ZnO.Y 2O3 :0.75 mol% Eu
433 600 611 026 145
125
332
422 134
420
411
211
Intensity (a.u)
10000
Y2O3 3+ Y2O3 :0.75 mol% Eu
136 444 543 046 721 642
1496
1000
20
30
40
50
60
70
2 theta (degree) Fig. 1. The XRD patterns of the ZnO.Y2O3:Eu3+, Y2O3:Eu3+ nanophosphors, and Y2O3 nanoparticles.
(b)
(a)
200 nm
Fig. 2. (a) 3D rendition AFM image and (b) SEM image of the Y2O3:Eu3+ nanophosphors with the Eu3+ concentration of 0.75 mol% after calcination at 900 °C for 2h.
5
3+
Y2O3: 0.75 mol% Eu
7
D0-7F3
5
7
5
7
D0- F4
5
D0- F1 (C2)
5
7
D0- F5
5
5 D -7F (C ) D0-7F1 (S6) 0 0 2
PL intensity (a.u)
D0- F2 (C2)
570
580
590
600
610
620
630
640
650
660
670
680
690
700
Wavelength (nm) Fig. 3. PL emission spectra of the Y2O3:Eu3+ (0.75 mol%) nanophosphor after excitation at a wavelength of 475 nm using Ar laser source.
1497
PL intensity (a.u)
G.H. Mhlongo et al. / Optical Materials 33 (2011) 1495–1499
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Eu concentration (mol%) Fig. 4. PL intensity of the 5D0 ? 7F2 transition of Y2O3:Eu3+ in 0.25, 0.5, 0.75, and 1 mol%, under 475 nm excitation using an Ar laser source.
3. Results and discussions Fig. 1 depicts the X-ray Diffraction patterns of Y2O3, Y2O3:Eu3+ and ZnOY2O3:Eu3+ nanophosphor powders. The patterns are consistent with a single cubic phase of Y2O3 referenced in JCPDS file No.89-5591. ZnO nanoparticles and Eu3+ were not detected in the XRD patterns probably because of their relatively low concentrations in Y2O3 matrix. In addition, the absence of ZnO nanoparticles diffraction patterns suggest that the particles were well dispersed in the matrix and they remained small even after calcining in air at 900 °C for 3 h. The 3D AFM and 2D SEM images in Fig. 2a and b, respectively, depict the agglomeration of spherical particles of Y2O3:Eu3+ (0.75 mol%) nanophosphor with an average particle size of 30– 50 nm in diameter as estimated from both images. Similar images (not shown) were observed from the rest of the samples.
PL Intensity (a.u)
0.015
The PL emission spectra from Y2O3:Eu3+ powders with different concentrations of Eu3+ were characterized by the main emission at 612 nm due to 5D0 ? 7F2 and minors peaks attributed to 5D0 ? 7FJ (J = 0, 1, 2, 3, 4, . . .) of Eu3+. Note that the radiative transitions were note measured in this study and they were assigned according to the literature cited [4,5,9]. The most intense emission was observed from the sample doped with 0.75 mol% of Eu3+. Figs. 3 and 4 show respectively the PL emission spectrum of Y2O3:(0.75 mol%)Eu3+ and the maximum PL intensity as a function of Eu3+ concentrations. Shown in Fig. 4 is that the PL intensity from 5 D0 ? 7F2 transition was shown to increase with Eu3+ concentration from 0.25 mol%, was maximized at 0.75 mol% and decreased when the concentration was increased to 1 mol%+. The decrease of the PL intensity at higher activator/dopant concentration can be attributed to concentration quenching effect [4,9]. The quenching starts to occur at a certain concentration, for which there is a sufficient reduction in the average distance between emission centres to favour energy transfer [10]. Two mechanisms have been proposed to explain the luminescence concentration quenching. Firstly, due to efficient energy transfer, the excitation energy can migrate nonradiatively about a large number of centres before being emitted. This can result in the energy being transferred to/ intercepted by incidental defects/impurities, which will subsequently relax to their ground state by multiphonon or infrared emission [10,11]. Therefore, these defects act as energy sink within the transfer chain and they are called killers or quenching traps. Secondly, luminescence concentration quenching can occur when the excitation energy is lost from the emitting state via cross relaxation mechanism. This kind of relaxation occurs by resonant energy transfer between two identical adjacent centres, due to the particular energy-level structure of these centres. That is, for two neighbouring identical centres a resonant energy transfer can occur via cross relaxation in which one of the centres (the donor) transfers part of its excitation energy to the other centre (the acceptor). The luminescence concentration quenching in this study can be attributed to one or both of the mechanisms. In the case of efficient energy transfer, it is possible that migration of excitation energy was transferred nonradiatively from one Eu3+ ion to its neighbouring Eu3+ ion via exchange interaction, through a number of transfer steps and finally to a killer/quenching site. This lumi-
3+
0.05 mol% ZnO.Y2O3:Eu
0.010
3+
0 mol% ZnO.Y2O3:Eu
0.005
3+
0.15 mol% ZnO.Y2O3:Eu
3+
0.25 mol% ZnO.Y2O3:Eu
0.000 580
600
620
640
660
680
700
Wavelength (nm) Fig. 5. PL emission spectra of Y2O3:Eu3+, and ZnOY2O3:Eu3+ (0.75 mol%) after excitation at a wavelength of 475 nm using an Ar laser source.
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PL Intensity (a.u)
120
Y2O3:Eu3+. The enhancement was largest when the concentration of ZnO was 0.05 mol%. The PL intensity from ZnOY2O3:Eu3+ was shown to decrease with increasing concentration of ZnO nanoparticles, but the emission colour was still red. As shown in Fig. 6 the PL emission spectrum from ZnO nanoparticles showed the green emission at around 517 nm which can be associated with recombination of delocalized electrons at singly occupied oxygen vacancies with deep trapped holes [8]. Direct bandgap emission due to recombination of excitonic centres in ZnO was also observed at 365 nm. The green emission from ZnO nanoparticles was suppressed when ZnO nanoparticles were incorporated into Y2O3:Eu3+ suggesting that energy was transferred from ZnO nanoparticles to Eu3+. Energy transfer from ZnO to Eu3+ in silica glass is explained in details in Refs. [6,8]. The luminescence enhancement with 0.05 mol% ZnO incorporated into Y2O3:Eu3+ nanophosphor was also observed from CL emission spectra presented in Fig. 7. However, an appearance of the new emission peaks from the 5D1 level to 7F1 and 7F2 ground states were observed between 480 and 560 nm range. The decrease of the PL intensity with increasing ZnO concentration observed from both spectra (Figs. 5 and 7) can be ascribed to pairing and aggregation of ZnO nanoparticles in the Y2O3 matrix. The difference in the PL and CL spectra can be attributed to different mechanisms involved in the PL and CL excitations. In the PL process, the photons used for excitation have energy less than the bandgap of the host matrix so that no electro-hole pairs are produced. The luminescent centre is excited using a wavelength lying in the absorption band which will then after a short while nonradiatively relax to the 5 D0 level then radiatively relax to the ground state by emitting photons corresponding to specific transitions localized within the Eu3+ itself. In the CL process, the accelerated electrons create a multiplicity charge carriers (holes and electrons) and the excitation energy is transferred by excitons to luminescent centres.
517 nm
ZnO nanoparticles λ exc = 325 nm
80
40
365 nm
350
400
450
500
550
Wavelength (nm) Fig. 6. PL emission spectra of ZnO nanoparticles after an excitation of 325 nm using a Xenon lamp.
nescence concentration quenching dominated when the Eu3+ was increased above 0.75 mol%. When the concentration of Eu3+ is less than 0.75 mol% i.e. (0.25 and 0.5) mol%, the Eu3+ nearest ions can be thought of as isolated and only few Eu3+ ions having traps (defects) nearby will give their energy to the traps. Therefore, the luminescence quenching is not significant. As the activator concentration increases to 0.75 mol%, the neighbouring Eu3+ ions may be close enough excitation energy resonantly between each other. When the luminescent centre concentration exceeds 0.75 mol%, the distance between the Eu3+ neighbouring ions is short, therefore the excitation energy is lost nonradiatively between the Eu3+ ions favouring quenching effect and decreasing the luminescent intensity. Fig. 5 shows the PL emission spectra from Y2O3:Eu3+ without and with different concentrations of ZnO nanoparticles. Incorporation of ZnO nanoparticles into Y2O3:Eu3+ did not change the radiative relaxation process of Eu3+ ions, instead the PL emission from ZnOY2O3:Eu3+ was significantly enhanced compared to pure
1500
4. Conclusion ZnO nanoparticles incorporated in Y2O3:Eu3+ nanophosphor powders were successfully prepared using the sol–gel method. The incorporation of ZnO nanoparticles into Eu3+ doped Y2O3 resulted in quenching of green emission from ZnO nanoparticles
0.25 mol% ZnO.Y 2O3:0.75 mol% Eu
3+
0.15 mol% ZnO.Y 2O3:0.75 mol% Eu
3+
0.05 mol% ZnO.Y 2O3:0.75 mol% Eu
3+
0 mol% ZnO.Y2O3:0.75 mol% Eu
5
7
D 0- F 2
3+
CL Intensity (a.u)
1250
1000
750
500 5
250
5
7
D 1- F 2
5
7
D 0- F 1
5
7
D 0- F 3
7
D 1- F 1
5
0
5
7
D 0- F 4
5
7
D 0- F 5
7
D 0- F 6
480 500 520 540 560 580 600 620 640 660 680 700 720 740
Wavelength (nm) Fig. 7. CL emission spectra of ZnOY2O3:Eu3+ (0.75 mol%) after bombardment with beam of electrons (2 keV and 8.5 lA) in a high vacuum pressure of 1.2 10
8
Torr.
G.H. Mhlongo et al. / Optical Materials 33 (2011) 1495–1499
and an energy transfer to Eu3+ ions, hence the PL enhancement. The influence of Eu3+ and ZnO amounts on the PL intensity from Y2O3:Eu3+ and ZnOY2O3:Eu3+ nanophosphors were also investigated. The decrease of PL intensity with increasing Eu3+ and ZnO concentration in Y2O3 matrix was attributed to concentration quenching effect and the origin of this effect was explained.
Acknowledgements This project is financially supported by the Department of Science and Technology of South Africa and the Council for Scientific and Industrial Research of South Africa. Authors would also like to thank Mart-Mari Duvenhage for assisting with the CL measurements.
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References [1] C.-H. Kim, I.-E. Kwon, C.-H. Park, Y.-J. Hwang, H.-S. Bae, B.-Y Yu, C.-H. Pyun, G.Y. Hong, J. Alloys Compd. 311 (2000) 33. [2] C.R. Ronda, T. Justel, H. Nikol, J. Alloys Compd. 275–277 (1998) 669. [3] W.-W. Zhang, W.-P. Zhang, P.-B. Xie, M. Yin, H.-T. Chen, L. Jing, Y.-S. Zhang, L.-R. Lou, S.-D. Xia, J. Colloid Interface Sci. 262 (2003) 588. [4] M.-G. Kwak, J.-H. Park, S. Shon, Solid State Commun. 130 (2004) 199. [5] Y. Satoh, H. Najafov, S. Ohshio, H. Saitoh, Adv. Technol Mater Mater Process. 17 (1) (2005) 43–46. [6] J. Bang, H. Yang, P.H. Holloway, J. Chem. Phys. 123 (2005) 084709. [7] J. Zhang, Z. Zhang, Z. Tang, Y. Lin, Y.L. Zheng, J. Mater. Process. Technol. 121 (2002) 265. [8] G.H. Mhlongo, O.M. Ntwaeaborwa, M.S. Dhlamini, H.C. Swart, K.T. Hillie, J. Mater. Sci. 45 (2010) 5228–5236. [9] J. Heber, K.H. Hellwege, U. Kobler, H. Murmann, Z. Physik 237 (1970) 189. [10] J.G. Sole, L.E. Bausa, D. Jaque, Chichester (2005) An Introduction To The Optical Spectroscopy Of Solids. John Wiley & Sons [11] G. Blasse, B.C. Grabmaier, Luminescent Material, Springer, Berlin, 1994.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Dependence of photoluminescence (PL) emission intensity on Eu3+ and ZnO concentrations in Y2O3:Eu3+ and ZnOY2O3:Eu3+ nanophosphors G.H. Mhlongo a,b, M.S. Dhlamini a,c, H.C. Swart b, O.M. Ntwaeaborwa b, K.T. Hillie a,b,⇑ a
National Centre for Nano-structured Materials, Council for Scientific and Industrial Research, 1-Meiring Naude Road, Brummeria, PO Box 395, Pretoria 0001, South Africa Department of Physics, University of Free State, Bloemfontein ZA9300, South Africa c Department of Physics, University of South Africa, Pretoria, South Africa b
a r t i c l e
i n f o
Article history: Received 13 October 2010 Received in revised form 22 February 2011 Accepted 3 March 2011 Available online 7 April 2011 Keywords: Energy transfer Phosphors Concentration quenching effect
a b s t r a c t Y2O3:Eu3+ and ZnOY2O3:Eu3+ nanophosphor powders with different concentrations of Eu3+ ions were synthesized by a sol–gel method and their luminescence properties were investigated. The red photoluminescence (PL) from Eu3+ ions with the main emission peak at 612 nm was observed to increase with Eu3+ concentration from 0.25 to 0.75 mol% and decreased notably when the concentration was increased to 1 mol%. The decrease in the PL intensity at higher Eu3+ concentrations can be associated with concentration quenching effects. The red emission at 612 nm was shown to increase considerable when ZnO nanoparticles were incorporated in Y2O3:Eu3+ while green emission from ZnO was suppressed. The increase is attributed to energy transfer from ZnO to Eu3+. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
The study of the synthesis and characterization of rare earth oxide based nanophosphors has increased rapidly after the realization that semiconductor nanoparticles can transfer energy via electronic interactions to luminescent centres in a host matrix, resulting in enhanced luminescence. These nanophosphors have potential application in light emitting devices such as fluorescent lamps, solid state lasers, optical wave guides, and flat panel displays [1,2]. Numerous synthetic approaches used to prepare these phosphors have been reported. However, the sol–gel method is preferable since it is relatively inexpensive, provides good control of particle size, uniform morphology, and high homogeneity. Although the red emitting Y2O3:Eu3+ with different concentration of Eu3+ has been researched extensively with particles in micron scale and recently nanoparticles with each size domain pursued for specific technologies, there are still inconsistent reports about the exact amount of Eu3+ which cause concentration quenching in these phosphors [3–5]. On the other hand, the improved luminescence induced by energy transfer from ZnO nanocrystals to Eu3+ encapsulated in SiO2 matrix has been reported [6], and similar phenomenon was investigated in Y2O3 host. In the present work, we investigated the effect of Eu3+ and ZnO concentration on nanocrystalline Y2O3:Eu3+ phosphor powders synthesized by the sol–gel method. Energy transfer mechanism and concentration quenching effect are discussed.
Y2O3: Eu3+ was prepared following the method described by Zhang et al. [7] but with a few modifications. Y2O3 (99.99%), Eu2O3 (99.99%), nitric acid (65% A.U.) were used as the starting materials and citric acid (99.5%, A.C.S. reagent) was used as a chelating agent. Y2O3 and Eu2O3 were dissolved in a dilute nitric acid followed by a dropwise addition of citric acid resulting in a clear sol. The amount of Y2O3 was fixed in all the samples while the Eu3+ concentration was varied from 0.25 to 1 mol%. The molar ratio of metal ions to citric acid was 2:1. A homogeneous gel was obtained by heating the clear sol at 115 °C for 2.5 h, which was then calcined in air at 900 °C for 3 h, and ground into a fine powder. ZnO.Y2O3:Eu3+ was prepared by mixing Y2O3:Eu3+ sol with ZnO nanoparticles suspended in ethanol. A detailed preparation of ZnO nanoparticles with an average particle size of 4 nm in diameter was reported elsewhere [8]. The particle morphology and surface topography was analyzed using JEOL JSM-7500F, Field Emission Scanning Electron Microscopy (SEM) and Multimode Atomic Force Microscope (Nano Scope Version (R) IV) respectively. The crystal structure of the samples was analysed by Phillips Xpert X-ray diffractometer using the Cu Ka source. Photoluminescence (PL) data were collected using an Ar laser and a monochromatized Xenon lamp at an excitation wavelength of 475 and 325 nm, respectively while cathodoluminescence data was collected using an Ocean Optics S2000 spectrometer when the samples were irradiated with 2 keV and 8.5 lA beam of electrons in a high vacuum chamber at a base pressure of 1.2 10 8 Torr.
⇑ Corresponding author. Fax: +27 12 841 2229. E-mail address: [email protected] (K.T. Hillie). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.03.009
G.H. Mhlongo et al. / Optical Materials 33 (2011) 1495–1499 222
3+
622
400
440
0.05 mol% ZnO.Y 2O3 :0.75 mol% Eu
433 600 611 026 145
125
332
422 134
420
411
211
Intensity (a.u)
10000
Y2O3 3+ Y2O3 :0.75 mol% Eu
136 444 543 046 721 642
1496
1000
20
30
40
50
60
70
2 theta (degree) Fig. 1. The XRD patterns of the ZnO.Y2O3:Eu3+, Y2O3:Eu3+ nanophosphors, and Y2O3 nanoparticles.
(b)
(a)
200 nm
Fig. 2. (a) 3D rendition AFM image and (b) SEM image of the Y2O3:Eu3+ nanophosphors with the Eu3+ concentration of 0.75 mol% after calcination at 900 °C for 2h.
5
3+
Y2O3: 0.75 mol% Eu
7
D0-7F3
5
7
5
7
D0- F4
5
D0- F1 (C2)
5
7
D0- F5
5
5 D -7F (C ) D0-7F1 (S6) 0 0 2
PL intensity (a.u)
D0- F2 (C2)
570
580
590
600
610
620
630
640
650
660
670
680
690
700
Wavelength (nm) Fig. 3. PL emission spectra of the Y2O3:Eu3+ (0.75 mol%) nanophosphor after excitation at a wavelength of 475 nm using Ar laser source.
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PL intensity (a.u)
G.H. Mhlongo et al. / Optical Materials 33 (2011) 1495–1499
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Eu concentration (mol%) Fig. 4. PL intensity of the 5D0 ? 7F2 transition of Y2O3:Eu3+ in 0.25, 0.5, 0.75, and 1 mol%, under 475 nm excitation using an Ar laser source.
3. Results and discussions Fig. 1 depicts the X-ray Diffraction patterns of Y2O3, Y2O3:Eu3+ and ZnOY2O3:Eu3+ nanophosphor powders. The patterns are consistent with a single cubic phase of Y2O3 referenced in JCPDS file No.89-5591. ZnO nanoparticles and Eu3+ were not detected in the XRD patterns probably because of their relatively low concentrations in Y2O3 matrix. In addition, the absence of ZnO nanoparticles diffraction patterns suggest that the particles were well dispersed in the matrix and they remained small even after calcining in air at 900 °C for 3 h. The 3D AFM and 2D SEM images in Fig. 2a and b, respectively, depict the agglomeration of spherical particles of Y2O3:Eu3+ (0.75 mol%) nanophosphor with an average particle size of 30– 50 nm in diameter as estimated from both images. Similar images (not shown) were observed from the rest of the samples.
PL Intensity (a.u)
0.015
The PL emission spectra from Y2O3:Eu3+ powders with different concentrations of Eu3+ were characterized by the main emission at 612 nm due to 5D0 ? 7F2 and minors peaks attributed to 5D0 ? 7FJ (J = 0, 1, 2, 3, 4, . . .) of Eu3+. Note that the radiative transitions were note measured in this study and they were assigned according to the literature cited [4,5,9]. The most intense emission was observed from the sample doped with 0.75 mol% of Eu3+. Figs. 3 and 4 show respectively the PL emission spectrum of Y2O3:(0.75 mol%)Eu3+ and the maximum PL intensity as a function of Eu3+ concentrations. Shown in Fig. 4 is that the PL intensity from 5 D0 ? 7F2 transition was shown to increase with Eu3+ concentration from 0.25 mol%, was maximized at 0.75 mol% and decreased when the concentration was increased to 1 mol%+. The decrease of the PL intensity at higher activator/dopant concentration can be attributed to concentration quenching effect [4,9]. The quenching starts to occur at a certain concentration, for which there is a sufficient reduction in the average distance between emission centres to favour energy transfer [10]. Two mechanisms have been proposed to explain the luminescence concentration quenching. Firstly, due to efficient energy transfer, the excitation energy can migrate nonradiatively about a large number of centres before being emitted. This can result in the energy being transferred to/ intercepted by incidental defects/impurities, which will subsequently relax to their ground state by multiphonon or infrared emission [10,11]. Therefore, these defects act as energy sink within the transfer chain and they are called killers or quenching traps. Secondly, luminescence concentration quenching can occur when the excitation energy is lost from the emitting state via cross relaxation mechanism. This kind of relaxation occurs by resonant energy transfer between two identical adjacent centres, due to the particular energy-level structure of these centres. That is, for two neighbouring identical centres a resonant energy transfer can occur via cross relaxation in which one of the centres (the donor) transfers part of its excitation energy to the other centre (the acceptor). The luminescence concentration quenching in this study can be attributed to one or both of the mechanisms. In the case of efficient energy transfer, it is possible that migration of excitation energy was transferred nonradiatively from one Eu3+ ion to its neighbouring Eu3+ ion via exchange interaction, through a number of transfer steps and finally to a killer/quenching site. This lumi-
3+
0.05 mol% ZnO.Y2O3:Eu
0.010
3+
0 mol% ZnO.Y2O3:Eu
0.005
3+
0.15 mol% ZnO.Y2O3:Eu
3+
0.25 mol% ZnO.Y2O3:Eu
0.000 580
600
620
640
660
680
700
Wavelength (nm) Fig. 5. PL emission spectra of Y2O3:Eu3+, and ZnOY2O3:Eu3+ (0.75 mol%) after excitation at a wavelength of 475 nm using an Ar laser source.
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PL Intensity (a.u)
120
Y2O3:Eu3+. The enhancement was largest when the concentration of ZnO was 0.05 mol%. The PL intensity from ZnOY2O3:Eu3+ was shown to decrease with increasing concentration of ZnO nanoparticles, but the emission colour was still red. As shown in Fig. 6 the PL emission spectrum from ZnO nanoparticles showed the green emission at around 517 nm which can be associated with recombination of delocalized electrons at singly occupied oxygen vacancies with deep trapped holes [8]. Direct bandgap emission due to recombination of excitonic centres in ZnO was also observed at 365 nm. The green emission from ZnO nanoparticles was suppressed when ZnO nanoparticles were incorporated into Y2O3:Eu3+ suggesting that energy was transferred from ZnO nanoparticles to Eu3+. Energy transfer from ZnO to Eu3+ in silica glass is explained in details in Refs. [6,8]. The luminescence enhancement with 0.05 mol% ZnO incorporated into Y2O3:Eu3+ nanophosphor was also observed from CL emission spectra presented in Fig. 7. However, an appearance of the new emission peaks from the 5D1 level to 7F1 and 7F2 ground states were observed between 480 and 560 nm range. The decrease of the PL intensity with increasing ZnO concentration observed from both spectra (Figs. 5 and 7) can be ascribed to pairing and aggregation of ZnO nanoparticles in the Y2O3 matrix. The difference in the PL and CL spectra can be attributed to different mechanisms involved in the PL and CL excitations. In the PL process, the photons used for excitation have energy less than the bandgap of the host matrix so that no electro-hole pairs are produced. The luminescent centre is excited using a wavelength lying in the absorption band which will then after a short while nonradiatively relax to the 5 D0 level then radiatively relax to the ground state by emitting photons corresponding to specific transitions localized within the Eu3+ itself. In the CL process, the accelerated electrons create a multiplicity charge carriers (holes and electrons) and the excitation energy is transferred by excitons to luminescent centres.
517 nm
ZnO nanoparticles λ exc = 325 nm
80
40
365 nm
350
400
450
500
550
Wavelength (nm) Fig. 6. PL emission spectra of ZnO nanoparticles after an excitation of 325 nm using a Xenon lamp.
nescence concentration quenching dominated when the Eu3+ was increased above 0.75 mol%. When the concentration of Eu3+ is less than 0.75 mol% i.e. (0.25 and 0.5) mol%, the Eu3+ nearest ions can be thought of as isolated and only few Eu3+ ions having traps (defects) nearby will give their energy to the traps. Therefore, the luminescence quenching is not significant. As the activator concentration increases to 0.75 mol%, the neighbouring Eu3+ ions may be close enough excitation energy resonantly between each other. When the luminescent centre concentration exceeds 0.75 mol%, the distance between the Eu3+ neighbouring ions is short, therefore the excitation energy is lost nonradiatively between the Eu3+ ions favouring quenching effect and decreasing the luminescent intensity. Fig. 5 shows the PL emission spectra from Y2O3:Eu3+ without and with different concentrations of ZnO nanoparticles. Incorporation of ZnO nanoparticles into Y2O3:Eu3+ did not change the radiative relaxation process of Eu3+ ions, instead the PL emission from ZnOY2O3:Eu3+ was significantly enhanced compared to pure
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4. Conclusion ZnO nanoparticles incorporated in Y2O3:Eu3+ nanophosphor powders were successfully prepared using the sol–gel method. The incorporation of ZnO nanoparticles into Eu3+ doped Y2O3 resulted in quenching of green emission from ZnO nanoparticles
0.25 mol% ZnO.Y 2O3:0.75 mol% Eu
3+
0.15 mol% ZnO.Y 2O3:0.75 mol% Eu
3+
0.05 mol% ZnO.Y 2O3:0.75 mol% Eu
3+
0 mol% ZnO.Y2O3:0.75 mol% Eu
5
7
D 0- F 2
3+
CL Intensity (a.u)
1250
1000
750
500 5
250
5
7
D 1- F 2
5
7
D 0- F 1
5
7
D 0- F 3
7
D 1- F 1
5
0
5
7
D 0- F 4
5
7
D 0- F 5
7
D 0- F 6
480 500 520 540 560 580 600 620 640 660 680 700 720 740
Wavelength (nm) Fig. 7. CL emission spectra of ZnOY2O3:Eu3+ (0.75 mol%) after bombardment with beam of electrons (2 keV and 8.5 lA) in a high vacuum pressure of 1.2 10
8
Torr.
G.H. Mhlongo et al. / Optical Materials 33 (2011) 1495–1499
and an energy transfer to Eu3+ ions, hence the PL enhancement. The influence of Eu3+ and ZnO amounts on the PL intensity from Y2O3:Eu3+ and ZnOY2O3:Eu3+ nanophosphors were also investigated. The decrease of PL intensity with increasing Eu3+ and ZnO concentration in Y2O3 matrix was attributed to concentration quenching effect and the origin of this effect was explained.
Acknowledgements This project is financially supported by the Department of Science and Technology of South Africa and the Council for Scientific and Industrial Research of South Africa. Authors would also like to thank Mart-Mari Duvenhage for assisting with the CL measurements.
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References [1] C.-H. Kim, I.-E. Kwon, C.-H. Park, Y.-J. Hwang, H.-S. Bae, B.-Y Yu, C.-H. Pyun, G.Y. Hong, J. Alloys Compd. 311 (2000) 33. [2] C.R. Ronda, T. Justel, H. Nikol, J. Alloys Compd. 275–277 (1998) 669. [3] W.-W. Zhang, W.-P. Zhang, P.-B. Xie, M. Yin, H.-T. Chen, L. Jing, Y.-S. Zhang, L.-R. Lou, S.-D. Xia, J. Colloid Interface Sci. 262 (2003) 588. [4] M.-G. Kwak, J.-H. Park, S. Shon, Solid State Commun. 130 (2004) 199. [5] Y. Satoh, H. Najafov, S. Ohshio, H. Saitoh, Adv. Technol Mater Mater Process. 17 (1) (2005) 43–46. [6] J. Bang, H. Yang, P.H. Holloway, J. Chem. Phys. 123 (2005) 084709. [7] J. Zhang, Z. Zhang, Z. Tang, Y. Lin, Y.L. Zheng, J. Mater. Process. Technol. 121 (2002) 265. [8] G.H. Mhlongo, O.M. Ntwaeaborwa, M.S. Dhlamini, H.C. Swart, K.T. Hillie, J. Mater. Sci. 45 (2010) 5228–5236. [9] J. Heber, K.H. Hellwege, U. Kobler, H. Murmann, Z. Physik 237 (1970) 189. [10] J.G. Sole, L.E. Bausa, D. Jaque, Chichester (2005) An Introduction To The Optical Spectroscopy Of Solids. John Wiley & Sons [11] G. Blasse, B.C. Grabmaier, Luminescent Material, Springer, Berlin, 1994.