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Comparative study of the luminescence of Y3Al5O12 nanoceramics and single crystals under excitation by synchrotron radiation
Optical Materials 35 (2013) 2049–2052
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Comparative study of the luminescence of Y3Al5O12 nanoceramics and single crystals under excitation by synchrotron radiation Yu. Zorenko a,b,⇑, T. Voznyak b, V.V. Gorbenko b, A. Doroshenko c, A. Tolmachev c, R. Yavetskiy c, I. Petrusha d, V. Turkevich d a
Institute of Physics, Kazimierz, Wielki University of Bydgoszcz, 11 Weyssenhoffa Sq., 85-072 Bydgoszcz, Poland Electronics Department, Ivan Franko National University of Lviv, 107 Gen. Tarnavsky Str., 79017 Lviv, Ukraine Institute for Single Crystals, NAS of Ukraine, 60 Lenin Ave., 61001 Kharkiv, Ukraine d Institute for Superhard Materials, 2 Avtozavodskaya Str., 04074 Kyiv, Ukraine b c
a r t i c l e
i n f o
Article history: Received 18 September 2011 Received in revised form 25 July 2012 Accepted 26 July 2012 Available online 6 September 2012 Keywords: Luminescence Y3Al5O12 garnet Nanoceramics Single crystals Antisite defects Oxygen vacancies
a b s t r a c t Comparative investigation of the luminescent properties of Y3Al5O12 (YAG) nanoceramics with the properties of single crystals counterpart is performed under excitation by synchrotron radiation in the exciton range of YAG host. Analysis of the luminescent properties of such different crystalline forms of YAG allows us to conclude that the behavior of YAG nanoceramics is close to the properties of single crystal analogue with large content of YAl antisite defects (ADs). We also have found that the relative intensity of F+–AD coupled centers is significantly higher in YAG nanoceramics than that for single crystal counterpart. This presupposes the strong coupling of the antisite defects and oxygen vacancy-related centers in YAG nanoceramics, mainly at the boundaries of grains. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Transparent micro- and nanoceramics on the basis of Y3Al5O12 (YAG) is now being considered as an alternative to single crystals of this garnet for laser media and scintillator applications [1]. Recently in works [2,3] we have compared the luminescent properties YAG:Ce in the forms of single crystals, single crystalline films, nano-powders and transparent micro-ceramic under excitation by synchrotron radiation (SR) in the VUV range. We have showed [2,3] that the differences in the spectral-kinetic characteristics of the Ce3+ luminescence in such crystalline YAG:Ce phosphors are caused by the significant differences in the methods of their preparation; namely the concentration of antisite defects (ADs) and oxygen vacancies as main types of defects in garnet compounds. These types of defects can play the role as emission and trapping centers and strongly contribute to the energy transfer processes from YAG host to Ce3+ emission centers [2,3]. In this work, we continue our research of the luminescent properties of YAG in the different crystalline forms. We shall compare the intrinsic luminescence of YAG nanoceramics [4,5] and bulk single crystal under excitation by SR in the exciton range of YAG ⇑ Corresponding author at: Institute of Physics, Kazimierz, Wielki University of Bydgoszcz, 11 Weyssenhoffa Sq., 85-072 Bydgoszcz, Poland. E-mail address: [email protected] (Yu. Zorenko). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.07.009
host. We also shall compare the obtained results on YAG crystals and nanoceramics with the same properties of undoped YAG single crystalline films presented in detail in our previous works [6,7].
2. Methods of samples preparation and experimental technique The samples of transparent YAG nanoceramics (Fig. 1) were produced by low-temperature (350–450 °C) high-pressure (7.7 GPa) sintering method. The details of YAG nanoceramics preparation are presented in works [4,5]. Fig. 2 shows the results of Rietveld refinement for a grinded pellet of YAG nanoceramics obtained under 7.7 GPa and 350 °C. According to the XRD data, the ceramics has single phase with an average crystallite size of 30.3 nm, the lattice parameters a = 12.0384 Å and the volume of elemental cell V = 1744.632 Å3. The YAG single crystals were crystallized by the Czochralski method from the melt at 1970 °C in Ar + 1.5% O2 atmosphere. Such a significant difference in the conditions of YAG crystal and nanoceramics preparation (temperature of preparation and content of atmosphere), in principle, can be strongly reflected on the concentration of YAl AD (Y cations in the positions of Al cations) and oxygen vacancies and related with them emission and trapping centers [2,3,6–8]. The comparative investigation of the luminescence of YAG single crystal and nanoceramics samples was performed at 10 K
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Yu. Zorenko et al. / Optical Materials 35 (2013) 2049–2052
Fig. 1. YAG nanoceramics produced by low-temperature sintering for 30 s under a pressure of 7.7 GPa and a temperature of 350 °C (the sample’s thickness is 1 mm) (a) and secondary electron image of the polished surface of YAG nanoceramics (b).
Fig. 2. Results of Rietveld refinement for YAG nanoceramics sintered under a pressure of 7.7 GPa and a temperature of 350 °C for 30 s. Vertical bars show the position of Bragg reflections. Upper solid line represents the calculated pattern. The dots indicate the observed intensities. The bottom solid line shows the difference between the observed and calculated intensities.
at the Superlumi station at HASYLAB at DESY (Hamburg, Germany) under excitation by synchrotron radiation (SR) with an energy of 3.7–25 eV. The emission and excitation spectra were measured both in the integral regime and in time gates of 1.2–12.2 ns and 150–200 ns (called as the fast and slow components, respectively) in the limits of SR pulse with a repetition time of 200 ns and a duration of 0.127 ns. The excitation spectra were corrected for the spectral dependence of transmittance of Al-grating and intensity of SR beam, but emission spectra were not corrected. 3. Results and discussion The luminescence spectra of YAG nanoceramics (a) and single crystal (b) at 8 K under excitation by SR with different energies in the YAG exciton range are shown in Fig. 3. The normalized emission spectra of YAG nanoceramics (Fig. 3a, curves 1–3) consist of the set of broad bands in the UV and blue ranges peaked at 260, 298, 334, 395 and 490 nm. By analogy with the well-known structure of the intrinsic luminescence of YAG single crystals (Fig. 3b) [6–8], the mentioned emission bands of YAG nanoceramics are caused by the radiation decay of self-trapped excitons (STEs) and excitons, localized around YAl AD (LE(AD) centers); the recombination emission of YAl AD; the luminescence of F+ centers (oxygen vacancy with one trapped electron), localized around YAl AD, and the emission of F centers (oxygen vacancy with two trapped electron), respectively.
The intensity of the luminescence of STE is at least by one order of magnitude lower in YAG nanoceramics (Fig. 3a) than that in the single crystal counterpart (Fig. 3a). Specifically, at 8 K the STE luminescence in the 254 nm band is dominating in the spectra of YAG single crystal (Fig. 3a) whereas for YAG nanoceramics the intensity of STE emission is comparable with the intensity of ADrelated bands and F+ centers (Fig. 3b). This result shows that YAl AD and charged oxygen vacancies strongly contribute to the total output of the intrinsic luminescence of YAG nanoceramics. It is important to note here that the concentration of these intrinsic defects is very high at the boundaries of YAG nanoceramics grains and strongly decreases in the main volume of grains [2,3]. The excitation spectra of different intrinsic emission centers in YAG nanoceramics (a) and single crystals (b) at 10 K are shown in Fig. 4. Apart from the generally the same structure of excitation bands, we find some important differences in the excitation spectrum of YAG nanoceramics with respect to the excitation spectra of the similar centers in YAG crystals. Namely, the positions of the excitation bands of STE emission in YAG nanoceramics at 7.06 and 7.5 eV differ from the positions of the same bands at 6.9 and 7.8 eV in the excitation spectrum of the single crystal counterpart. Such a significant low-energy shift (0.16 eV) of the main excitation band of the STE emission in YAG single crystal with respect to the excitation spectrum of YAG nanoceramics reflects the dominant contribution of YAl AD to their luminescent properties. As oppose to YAG single crystal, the position of the main excitation bands of the STE luminescence in YAG nanoceramics at 7.06 eV is rather close to the position of the STE bands in YAG single crystalline films at 7.17 eV [7,8], where the AD are completely absent. Therefore, the high energy shift of the excitation band of the STE emission in YAG nanoceramics with respect to the spectrum of YAG single crystal can be explained by the lower than in single crystal concentration of AD in the main volume of YAG nanoceramics (see also [2]). The main band in the excitation spectrum of the YAl AD luminescence at 334 nm in YAG nanoceramics (Fig. 4a, curve 3) is significantly broader and strongly low-energy shifted to 6.6 eV with respect to the position of the narrow peak at 6.75 eV for the AD emission in YAG crystal (Fig. 4b, curve 2). This result reflects strong involvement of other defect centers, namely, the oxygen vacancies, in the formation of energy structure of YAl centers in YAG nanoceramics. The last conclusion is also supported by very close structure of the excitation spectra of the emission of YAl AD and F+(AD) center in YAG nanoceramics in the exciton range (Fig. 4a, curves 3 and 4, respectively). The excitation spectrum of the luminescence of F+(AD) center in YAG nanoceramics (Fig. 4a, curve 4) is also notably broader in the exciton range and is shifted to 6.5 eV in comparison with the excitation spectrum of the F-center luminescence in YAG crystal (Fig. 4b, curve 3) with main maximum at
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Yu. Zorenko et al. / Optical Materials 35 (2013) 2049–2052
YAl AD F+(AD)
STE 260 nm
Intensity (arb. units)
1,0
254 nm
1,0
YAG NC T = 10 K
334 nm 395 nm
Eex = 6.62 eV (3)
LE(AD)
3 - Eex = 6.735 eV
0,6
YAl
F
0,5
STE
0,4
490 nm
300
400
500
3
200
F centers
+
F (AD)
2
0,0
600
323 nm
1
0,2 0,0 200
2 - Eex = 6.905 eV
293 nm
Eex = 7.07 eV (2)
298 nm
1 - Eex = 7.38 eV
LE(AD)
0,8
------ Eex = 7.50 eV (1)
YAG SC T = 10 K
300
490 nm
400
500
Wavelength (nm)
Wavelength (nm)
(a)
(b)
600
Fig. 3. Luminescence spectra of YAG nanoceramics (a) and single crystal (b) at 10 K under excitation by SR with energies indicated in legend of the figures, in the exciton range of YAG host.
6.5 eV
Intensity (arb. units)
1,0
6.98 eV 7.06 eV
YAG NC, T = 10 K 1 - λem= 240 nm (STE)
1,0
2 - λem= 290 nm (LE(AD))
7.5
0,8
6.75 eV 6.9 eV 6.65 eV
3 - λem= 330 nm (YAl AD)
0,6
3 - λem = 490 nm (F)
2 8.08
0,6
2
1
0,4 5.70 eV
6.11 eV
1
Eg
0,4
3
0,2
7.8 eV 2 - λ = 330 nm (Y AD) em Al
0,8
+
4 - λem= 400 nm (F (AD)
YAG SC 1 - λem = 250 nm (STE)
3
5.27 eV
0,2
4 0,0
0,0
5
6
7
8
9
10
5
11
7
6
8
10
9
11
Energy (eV)
Energy (eV)
(b)
(a)
Fig. 4. Excitation spectra of the different emission centers in YAG nanoceramics (a) and single crystal (b) at 10 K.
YAl AD
2
2
1000
Intensity (arb. units)
3
1
1000
4
STE 1 - Eex = 6.9 eV; λem= 250 nm
1 YAG NC
2 - Eex= 6.74 eV; λem= 330 nm 3 - Eex = 6.74 eV; λem = 480 nm
T = 10 K 1 - Eex = 7.075 eV; λem = 240 nm (STE)
3
100
2 - Eex = 7.075 eV; λem= 295 nm (LE(AD)
YAG SC
------- 3 - Eex = 6.96 eV; λtm = 330 nm (YAl AD)
T=10 K
+
------- 4 - Eex = 6.96 eV; λem= 400 nm (F (AD))
100
0
50
100
150
Time (ns)
(a)
F
10
0
50
100
150
Time (ns)
(b)
Fig. 5. Decay kinetics of different emission centers in YAG nanoceramics (a) and single crystal (b) at 10 K, under excitation by SR with energies, indicated in figures. P The three-exponent fits of decay curves I(t) = Aiexp( t/si), i = 3 are also presented (see Table 1 for details).
6.65 eV and bumps at 6.11 and 5.27 eV. Therefore, the strong coupling of the AD and oxygen vacancy-related centers is expected in YAG nanoceramics, mainly at the boundaries of grains [2,3].
The decay kinetics of the luminescence of STE and different defects centers in YAG nanoceramics and single crystal is shown in Fig. 5. All of the decay curves present the typical superposition of
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Yu. Zorenko et al. / Optical Materials 35 (2013) 2049–2052
Table 1 Parameters of approximation of the luminescence decay kinetic of different emission centers in YAG nanoceramics and single crystal, presented in Fig. 1. Sample
Emission band
A1
s1 (ns)
A2
s2 (ns)
A3
s3 (ns)
Nanoceramics Nanoceramics Nanoceramics Nanoceramics Crystal Crystal Crystal
STE LE(AD) YAl AD F+ AD STE YAl AD F+ AD
361.9 177.7 300.3 238.0 1395.7 3513.9 99.2
0.8 0.3 0.6 0.7 1.6 2.3 0.8
86.3 258.4 118.7 101.0 44.6 374.4 32.5
12.2 28.5 17.4 16.6 12.7 51.4 6.4
520.6 749.3 759.4 599.9 1358.4 1745.7 58.8
2242 999 1462 1560 15,486 1985 11,208
the fast (in the range of few ns) and slow (in the range of hundred ns–ls) emission components, presumably corresponding to the singlet–singlet and triplet–singlet radiation transitions [6,7]. Such a type of the decay kinetics is characteristic for the radiation decay of STE or excitons localized around all intrinsic defects as well as for the luminescence of excitons bound with the mentioned defects of YAG host [2,3,6,7]. From comparison of the approximations of the corresponding decay curves in Fig. 4a and b, presented in Table 1, we find that the decay kinetics of the same centers is faster in YAG nanoceramics than that in YAG crystal. Most probably that such differences in the shape of the decay curves for the same centers reflect the difference in the relative concentrations of YAl AD, F+, F centers and their aggregates in the samples under study. 4. Conclusions The comparative analysis of the luminescence of YAG nanoceramics and single crystal analogue allows us to conclude that the luminescence properties of nanoceramics and single crystal is rather close. Namely, we have found that similarly to YAG SC the complex intrinsic luminescence of YAG nanoceramics in 200–500 nm range is caused by the radiation decay of self-trapped excitons and exciton, localized around YAl antisite defects (ADs); the recombination emission of YAl AD themselves; the luminescence of F+ centers, localized around YAl AD as well as the intrinsic emission of F centers. This result shows that YAl AD and charged oxygen vacancies significantly contribute to the total output of the intrinsic luminescence of YAG nanoceramics. We have also found, that the relative intensity of F+(AD) centers is significantly higher in YAG nanoceramics than that for single crystal counterpart. This presupposes the strong coupling of the AD and oxygen vacancy-related centers in YAG nanoceramics, mainly at the boundaries of grains. At the same time, we have also found that the position of the main excitation band of the STE luminescence in YAG nanoceram-
ics at 7.07 eV is significantly (0.16 eV) high-energy shifted with respect to the same band in the YAG crystal counterpart and is closer to position of the main excitation band of the STE emission in YAG single crystalline films at 7.17 eV [5,6], where the AD are completely absent. These results are coherent with the strong decrease in the concentration of YAl AD in the main volume of YAG nanoceramics with respect to the single crystal analogues. Acknowledgments This research was supported by Ministry of Science and Education of Ukraine (Project SF-126 F), and NATO CBP.NUKR.CLG984305 project. Authors from ISC are grateful for the support from Target Complex Program of Fundamental Research of NAS of Ukraine «Fundamental problems of Nanostructured Systems, Nanomaterials and Nanotechnologies», Project No. 78/10-H. Authors are also grateful for Dr. V. N. Baumer for XRD analysis of YAG nanoceramics. The investigation with synchrotron radiation at the Superlumi station was performed in the framework of I-20110085 EC project. References [1] E. Zych, C. Brecher, A.J. Wojtowicz, H. Lingertat, Journal of Luminescence 75 (1997) 193. [2] Yu Zorenko, T. Voznyak, V. Gorbenko, E. Zych, S. Nizankovski, A. Danko, V. Puzikov, Journal of Luminescence 131 (2011) 17. [3] Y. Zorenko, E. Zych, A. Voloshinovskii, Optical Materials 31 (2009) 1845. [4] E.A. Vovk, T.G. Deineka, A.G. Doroshenko, et al., Journal of Superhard Materials 31 (2009) 252. [5] R.P. Yavetskiy, E.A. Vovk, A.G. Doroshenko, M.I. Danylenko, A.V. Lopin, I.A. Petrusha, V.F. Tkachenko, A.V. Tolmachev, V.Z. Turkevich, Ceramics International 37 (2011) 2477. [6] Yu Zorenko, A. Voloshinovskii, V. Savchyn, T. Vozniak, M. Nikl, K. Nejezchleb, V. Mikhailin, V. Kolobanov, D. Spassky, Physica Status Solidi (b) 244 (2007) 2180. [7] Y. Zorenko, V. Gorbenko, E. Mihokova, M. Nikl, K. Nejezchleb, A. Vedda, V. Kolobanov, D. Spassky, Radiation Measurements 42 (2007) 521. [8] A. Pujats, M. Springis, Radiation Effects and Defects in Solids 155 (2001) 65.
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Comparative study of the luminescence of Y3Al5O12 nanoceramics and single crystals under excitation by synchrotron radiation Yu. Zorenko a,b,⇑, T. Voznyak b, V.V. Gorbenko b, A. Doroshenko c, A. Tolmachev c, R. Yavetskiy c, I. Petrusha d, V. Turkevich d a
Institute of Physics, Kazimierz, Wielki University of Bydgoszcz, 11 Weyssenhoffa Sq., 85-072 Bydgoszcz, Poland Electronics Department, Ivan Franko National University of Lviv, 107 Gen. Tarnavsky Str., 79017 Lviv, Ukraine Institute for Single Crystals, NAS of Ukraine, 60 Lenin Ave., 61001 Kharkiv, Ukraine d Institute for Superhard Materials, 2 Avtozavodskaya Str., 04074 Kyiv, Ukraine b c
a r t i c l e
i n f o
Article history: Received 18 September 2011 Received in revised form 25 July 2012 Accepted 26 July 2012 Available online 6 September 2012 Keywords: Luminescence Y3Al5O12 garnet Nanoceramics Single crystals Antisite defects Oxygen vacancies
a b s t r a c t Comparative investigation of the luminescent properties of Y3Al5O12 (YAG) nanoceramics with the properties of single crystals counterpart is performed under excitation by synchrotron radiation in the exciton range of YAG host. Analysis of the luminescent properties of such different crystalline forms of YAG allows us to conclude that the behavior of YAG nanoceramics is close to the properties of single crystal analogue with large content of YAl antisite defects (ADs). We also have found that the relative intensity of F+–AD coupled centers is significantly higher in YAG nanoceramics than that for single crystal counterpart. This presupposes the strong coupling of the antisite defects and oxygen vacancy-related centers in YAG nanoceramics, mainly at the boundaries of grains. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Transparent micro- and nanoceramics on the basis of Y3Al5O12 (YAG) is now being considered as an alternative to single crystals of this garnet for laser media and scintillator applications [1]. Recently in works [2,3] we have compared the luminescent properties YAG:Ce in the forms of single crystals, single crystalline films, nano-powders and transparent micro-ceramic under excitation by synchrotron radiation (SR) in the VUV range. We have showed [2,3] that the differences in the spectral-kinetic characteristics of the Ce3+ luminescence in such crystalline YAG:Ce phosphors are caused by the significant differences in the methods of their preparation; namely the concentration of antisite defects (ADs) and oxygen vacancies as main types of defects in garnet compounds. These types of defects can play the role as emission and trapping centers and strongly contribute to the energy transfer processes from YAG host to Ce3+ emission centers [2,3]. In this work, we continue our research of the luminescent properties of YAG in the different crystalline forms. We shall compare the intrinsic luminescence of YAG nanoceramics [4,5] and bulk single crystal under excitation by SR in the exciton range of YAG ⇑ Corresponding author at: Institute of Physics, Kazimierz, Wielki University of Bydgoszcz, 11 Weyssenhoffa Sq., 85-072 Bydgoszcz, Poland. E-mail address: [email protected] (Yu. Zorenko). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.07.009
host. We also shall compare the obtained results on YAG crystals and nanoceramics with the same properties of undoped YAG single crystalline films presented in detail in our previous works [6,7].
2. Methods of samples preparation and experimental technique The samples of transparent YAG nanoceramics (Fig. 1) were produced by low-temperature (350–450 °C) high-pressure (7.7 GPa) sintering method. The details of YAG nanoceramics preparation are presented in works [4,5]. Fig. 2 shows the results of Rietveld refinement for a grinded pellet of YAG nanoceramics obtained under 7.7 GPa and 350 °C. According to the XRD data, the ceramics has single phase with an average crystallite size of 30.3 nm, the lattice parameters a = 12.0384 Å and the volume of elemental cell V = 1744.632 Å3. The YAG single crystals were crystallized by the Czochralski method from the melt at 1970 °C in Ar + 1.5% O2 atmosphere. Such a significant difference in the conditions of YAG crystal and nanoceramics preparation (temperature of preparation and content of atmosphere), in principle, can be strongly reflected on the concentration of YAl AD (Y cations in the positions of Al cations) and oxygen vacancies and related with them emission and trapping centers [2,3,6–8]. The comparative investigation of the luminescence of YAG single crystal and nanoceramics samples was performed at 10 K
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Yu. Zorenko et al. / Optical Materials 35 (2013) 2049–2052
Fig. 1. YAG nanoceramics produced by low-temperature sintering for 30 s under a pressure of 7.7 GPa and a temperature of 350 °C (the sample’s thickness is 1 mm) (a) and secondary electron image of the polished surface of YAG nanoceramics (b).
Fig. 2. Results of Rietveld refinement for YAG nanoceramics sintered under a pressure of 7.7 GPa and a temperature of 350 °C for 30 s. Vertical bars show the position of Bragg reflections. Upper solid line represents the calculated pattern. The dots indicate the observed intensities. The bottom solid line shows the difference between the observed and calculated intensities.
at the Superlumi station at HASYLAB at DESY (Hamburg, Germany) under excitation by synchrotron radiation (SR) with an energy of 3.7–25 eV. The emission and excitation spectra were measured both in the integral regime and in time gates of 1.2–12.2 ns and 150–200 ns (called as the fast and slow components, respectively) in the limits of SR pulse with a repetition time of 200 ns and a duration of 0.127 ns. The excitation spectra were corrected for the spectral dependence of transmittance of Al-grating and intensity of SR beam, but emission spectra were not corrected. 3. Results and discussion The luminescence spectra of YAG nanoceramics (a) and single crystal (b) at 8 K under excitation by SR with different energies in the YAG exciton range are shown in Fig. 3. The normalized emission spectra of YAG nanoceramics (Fig. 3a, curves 1–3) consist of the set of broad bands in the UV and blue ranges peaked at 260, 298, 334, 395 and 490 nm. By analogy with the well-known structure of the intrinsic luminescence of YAG single crystals (Fig. 3b) [6–8], the mentioned emission bands of YAG nanoceramics are caused by the radiation decay of self-trapped excitons (STEs) and excitons, localized around YAl AD (LE(AD) centers); the recombination emission of YAl AD; the luminescence of F+ centers (oxygen vacancy with one trapped electron), localized around YAl AD, and the emission of F centers (oxygen vacancy with two trapped electron), respectively.
The intensity of the luminescence of STE is at least by one order of magnitude lower in YAG nanoceramics (Fig. 3a) than that in the single crystal counterpart (Fig. 3a). Specifically, at 8 K the STE luminescence in the 254 nm band is dominating in the spectra of YAG single crystal (Fig. 3a) whereas for YAG nanoceramics the intensity of STE emission is comparable with the intensity of ADrelated bands and F+ centers (Fig. 3b). This result shows that YAl AD and charged oxygen vacancies strongly contribute to the total output of the intrinsic luminescence of YAG nanoceramics. It is important to note here that the concentration of these intrinsic defects is very high at the boundaries of YAG nanoceramics grains and strongly decreases in the main volume of grains [2,3]. The excitation spectra of different intrinsic emission centers in YAG nanoceramics (a) and single crystals (b) at 10 K are shown in Fig. 4. Apart from the generally the same structure of excitation bands, we find some important differences in the excitation spectrum of YAG nanoceramics with respect to the excitation spectra of the similar centers in YAG crystals. Namely, the positions of the excitation bands of STE emission in YAG nanoceramics at 7.06 and 7.5 eV differ from the positions of the same bands at 6.9 and 7.8 eV in the excitation spectrum of the single crystal counterpart. Such a significant low-energy shift (0.16 eV) of the main excitation band of the STE emission in YAG single crystal with respect to the excitation spectrum of YAG nanoceramics reflects the dominant contribution of YAl AD to their luminescent properties. As oppose to YAG single crystal, the position of the main excitation bands of the STE luminescence in YAG nanoceramics at 7.06 eV is rather close to the position of the STE bands in YAG single crystalline films at 7.17 eV [7,8], where the AD are completely absent. Therefore, the high energy shift of the excitation band of the STE emission in YAG nanoceramics with respect to the spectrum of YAG single crystal can be explained by the lower than in single crystal concentration of AD in the main volume of YAG nanoceramics (see also [2]). The main band in the excitation spectrum of the YAl AD luminescence at 334 nm in YAG nanoceramics (Fig. 4a, curve 3) is significantly broader and strongly low-energy shifted to 6.6 eV with respect to the position of the narrow peak at 6.75 eV for the AD emission in YAG crystal (Fig. 4b, curve 2). This result reflects strong involvement of other defect centers, namely, the oxygen vacancies, in the formation of energy structure of YAl centers in YAG nanoceramics. The last conclusion is also supported by very close structure of the excitation spectra of the emission of YAl AD and F+(AD) center in YAG nanoceramics in the exciton range (Fig. 4a, curves 3 and 4, respectively). The excitation spectrum of the luminescence of F+(AD) center in YAG nanoceramics (Fig. 4a, curve 4) is also notably broader in the exciton range and is shifted to 6.5 eV in comparison with the excitation spectrum of the F-center luminescence in YAG crystal (Fig. 4b, curve 3) with main maximum at
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Yu. Zorenko et al. / Optical Materials 35 (2013) 2049–2052
YAl AD F+(AD)
STE 260 nm
Intensity (arb. units)
1,0
254 nm
1,0
YAG NC T = 10 K
334 nm 395 nm
Eex = 6.62 eV (3)
LE(AD)
3 - Eex = 6.735 eV
0,6
YAl
F
0,5
STE
0,4
490 nm
300
400
500
3
200
F centers
+
F (AD)
2
0,0
600
323 nm
1
0,2 0,0 200
2 - Eex = 6.905 eV
293 nm
Eex = 7.07 eV (2)
298 nm
1 - Eex = 7.38 eV
LE(AD)
0,8
------ Eex = 7.50 eV (1)
YAG SC T = 10 K
300
490 nm
400
500
Wavelength (nm)
Wavelength (nm)
(a)
(b)
600
Fig. 3. Luminescence spectra of YAG nanoceramics (a) and single crystal (b) at 10 K under excitation by SR with energies indicated in legend of the figures, in the exciton range of YAG host.
6.5 eV
Intensity (arb. units)
1,0
6.98 eV 7.06 eV
YAG NC, T = 10 K 1 - λem= 240 nm (STE)
1,0
2 - λem= 290 nm (LE(AD))
7.5
0,8
6.75 eV 6.9 eV 6.65 eV
3 - λem= 330 nm (YAl AD)
0,6
3 - λem = 490 nm (F)
2 8.08
0,6
2
1
0,4 5.70 eV
6.11 eV
1
Eg
0,4
3
0,2
7.8 eV 2 - λ = 330 nm (Y AD) em Al
0,8
+
4 - λem= 400 nm (F (AD)
YAG SC 1 - λem = 250 nm (STE)
3
5.27 eV
0,2
4 0,0
0,0
5
6
7
8
9
10
5
11
7
6
8
10
9
11
Energy (eV)
Energy (eV)
(b)
(a)
Fig. 4. Excitation spectra of the different emission centers in YAG nanoceramics (a) and single crystal (b) at 10 K.
YAl AD
2
2
1000
Intensity (arb. units)
3
1
1000
4
STE 1 - Eex = 6.9 eV; λem= 250 nm
1 YAG NC
2 - Eex= 6.74 eV; λem= 330 nm 3 - Eex = 6.74 eV; λem = 480 nm
T = 10 K 1 - Eex = 7.075 eV; λem = 240 nm (STE)
3
100
2 - Eex = 7.075 eV; λem= 295 nm (LE(AD)
YAG SC
------- 3 - Eex = 6.96 eV; λtm = 330 nm (YAl AD)
T=10 K
+
------- 4 - Eex = 6.96 eV; λem= 400 nm (F (AD))
100
0
50
100
150
Time (ns)
(a)
F
10
0
50
100
150
Time (ns)
(b)
Fig. 5. Decay kinetics of different emission centers in YAG nanoceramics (a) and single crystal (b) at 10 K, under excitation by SR with energies, indicated in figures. P The three-exponent fits of decay curves I(t) = Aiexp( t/si), i = 3 are also presented (see Table 1 for details).
6.65 eV and bumps at 6.11 and 5.27 eV. Therefore, the strong coupling of the AD and oxygen vacancy-related centers is expected in YAG nanoceramics, mainly at the boundaries of grains [2,3].
The decay kinetics of the luminescence of STE and different defects centers in YAG nanoceramics and single crystal is shown in Fig. 5. All of the decay curves present the typical superposition of
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Yu. Zorenko et al. / Optical Materials 35 (2013) 2049–2052
Table 1 Parameters of approximation of the luminescence decay kinetic of different emission centers in YAG nanoceramics and single crystal, presented in Fig. 1. Sample
Emission band
A1
s1 (ns)
A2
s2 (ns)
A3
s3 (ns)
Nanoceramics Nanoceramics Nanoceramics Nanoceramics Crystal Crystal Crystal
STE LE(AD) YAl AD F+ AD STE YAl AD F+ AD
361.9 177.7 300.3 238.0 1395.7 3513.9 99.2
0.8 0.3 0.6 0.7 1.6 2.3 0.8
86.3 258.4 118.7 101.0 44.6 374.4 32.5
12.2 28.5 17.4 16.6 12.7 51.4 6.4
520.6 749.3 759.4 599.9 1358.4 1745.7 58.8
2242 999 1462 1560 15,486 1985 11,208
the fast (in the range of few ns) and slow (in the range of hundred ns–ls) emission components, presumably corresponding to the singlet–singlet and triplet–singlet radiation transitions [6,7]. Such a type of the decay kinetics is characteristic for the radiation decay of STE or excitons localized around all intrinsic defects as well as for the luminescence of excitons bound with the mentioned defects of YAG host [2,3,6,7]. From comparison of the approximations of the corresponding decay curves in Fig. 4a and b, presented in Table 1, we find that the decay kinetics of the same centers is faster in YAG nanoceramics than that in YAG crystal. Most probably that such differences in the shape of the decay curves for the same centers reflect the difference in the relative concentrations of YAl AD, F+, F centers and their aggregates in the samples under study. 4. Conclusions The comparative analysis of the luminescence of YAG nanoceramics and single crystal analogue allows us to conclude that the luminescence properties of nanoceramics and single crystal is rather close. Namely, we have found that similarly to YAG SC the complex intrinsic luminescence of YAG nanoceramics in 200–500 nm range is caused by the radiation decay of self-trapped excitons and exciton, localized around YAl antisite defects (ADs); the recombination emission of YAl AD themselves; the luminescence of F+ centers, localized around YAl AD as well as the intrinsic emission of F centers. This result shows that YAl AD and charged oxygen vacancies significantly contribute to the total output of the intrinsic luminescence of YAG nanoceramics. We have also found, that the relative intensity of F+(AD) centers is significantly higher in YAG nanoceramics than that for single crystal counterpart. This presupposes the strong coupling of the AD and oxygen vacancy-related centers in YAG nanoceramics, mainly at the boundaries of grains. At the same time, we have also found that the position of the main excitation band of the STE luminescence in YAG nanoceram-
ics at 7.07 eV is significantly (0.16 eV) high-energy shifted with respect to the same band in the YAG crystal counterpart and is closer to position of the main excitation band of the STE emission in YAG single crystalline films at 7.17 eV [5,6], where the AD are completely absent. These results are coherent with the strong decrease in the concentration of YAl AD in the main volume of YAG nanoceramics with respect to the single crystal analogues. Acknowledgments This research was supported by Ministry of Science and Education of Ukraine (Project SF-126 F), and NATO CBP.NUKR.CLG984305 project. Authors from ISC are grateful for the support from Target Complex Program of Fundamental Research of NAS of Ukraine «Fundamental problems of Nanostructured Systems, Nanomaterials and Nanotechnologies», Project No. 78/10-H. Authors are also grateful for Dr. V. N. Baumer for XRD analysis of YAG nanoceramics. The investigation with synchrotron radiation at the Superlumi station was performed in the framework of I-20110085 EC project. References [1] E. Zych, C. Brecher, A.J. Wojtowicz, H. Lingertat, Journal of Luminescence 75 (1997) 193. [2] Yu Zorenko, T. Voznyak, V. Gorbenko, E. Zych, S. Nizankovski, A. Danko, V. Puzikov, Journal of Luminescence 131 (2011) 17. [3] Y. Zorenko, E. Zych, A. Voloshinovskii, Optical Materials 31 (2009) 1845. [4] E.A. Vovk, T.G. Deineka, A.G. Doroshenko, et al., Journal of Superhard Materials 31 (2009) 252. [5] R.P. Yavetskiy, E.A. Vovk, A.G. Doroshenko, M.I. Danylenko, A.V. Lopin, I.A. Petrusha, V.F. Tkachenko, A.V. Tolmachev, V.Z. Turkevich, Ceramics International 37 (2011) 2477. [6] Yu Zorenko, A. Voloshinovskii, V. Savchyn, T. Vozniak, M. Nikl, K. Nejezchleb, V. Mikhailin, V. Kolobanov, D. Spassky, Physica Status Solidi (b) 244 (2007) 2180. [7] Y. Zorenko, V. Gorbenko, E. Mihokova, M. Nikl, K. Nejezchleb, A. Vedda, V. Kolobanov, D. Spassky, Radiation Measurements 42 (2007) 521. [8] A. Pujats, M. Springis, Radiation Effects and Defects in Solids 155 (2001) 65.