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Luminescence properties of Nd:YAG nanoceramics prepared by low temperature high pressure sintering method
Optical Materials 29 (2007) 1244–1251 www.elsevier.com/locate/optmat
Luminescence properties of Nd:YAG nanoceramics prepared by low temperature high pressure sintering method D. Hreniak a
a,*
, R. Fedyk b, A. Bednarkiewicz a, W. Stre˛k a, W. Łojkowski
b
Institute of Low Temperature and Structure Research, Polish Academy of Science, ul. Oko´lna 2, P Box 1410, 50-422 Wrocław, Poland b Institute of High Pressure Physics, Polish Academy of Sciences, ul. Sokołowska 29/37, 01-142 Warsaw, Poland Received 9 March 2006; received in revised form 28 April 2006; accepted 4 May 2006 Available online 20 September 2006
Abstract Nd:YAG nanoceramics composed of 25 nm grains were obtained by a low temperature high pressure sintering process. Their absorption and luminescence spectra were measured. It was found that the transparency of Nd:YAG nanoceramics increases with the pressure of the fabrication process. This effect was explained in terms of reducing porosity of the ceramics with increasing the sintering pressure. It was found that the emission properties of Nd3+ ion in YAG nanoceramic were substantially different compared to its single crystalline counterpart. In particular the emission intensities and decay times strongly depends on the applied pressure. The intensity ratio of the 4 F3/2 ! 4I9/2 and 4F3/2 ! 4I11/2 emission transitions was much higher than in a single crystal and decreased with increasing the pressure. An efficient hot emission from the 4F5/2,4H9/2 terms was observed at room temperature. Ó 2006 Elsevier B.V. All rights reserved. Keywords: YAG; Nanoceramics; Neodymium; Pressure dependence; Sintering; Effective refractive index
1. Introduction There is a great interest in developing new techniques of preparation of yttrium aluminum garnet (YAG) and other cubic nanopowders as starting materials for fabrication of ceramic optical materials [1–3]. In contrast to other techniques of fabrication, based mainly on the solid state reaction of oxides [1–3], the method of fabrication of ceramics from nanopowders allows to use materials of the same chemical and phase composition as the final ceramics, for instance nanocrystalline YAG [4]. Such ceramics, prepared in a low cost way compared to processing of the single crystals, have comparable optical and thermomechanical properties and can substitute them in many applications [4–6]. YAG nanopowders can be synthesized by means of a variety of methods: sol–gel [7], Pechini [8], solvothermal [9], combustion [10] and co-precipitation [11]. It is very impor-
*
Corresponding author. Tel.: +48 71343 5021; fax: +48 71344 1029. E-mail address: [email protected] (D. Hreniak).
0925-3467/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.05.014
tant to obtain the nanocrystals at the lowest possible crystallization temperature since the crystal size is increased with temperature of synthesis [12]. Preparation methods and optical properties of transparent ceramics made by sintering Nd:YAG nanopowders as starting materials were reported by several groups [13,14]. These ceramics were characterized by microsized grains and their optical properties were almost identical to those reported for single crystalline Nd:YAG. An alternative approach for production of transparent ceramics may be based on a concept of nanoceramics with the grain less than 100 nm. Such ceramics, having low residual porosity, should be in principle highly transparent due to the restricted Rayleigh scattering [15]. Recently, Krell et al. [16,17] developed transparent alumina ceramics having grain sizes of 0.4–0.6 lm at relative densities of >99.9%. In this work we present the luminescence studies of Nd:YAG nanoceramics grain size in the range 30–50 nm. Such dense nanoceramics with were produced using the low temperature high pressure sintering (LTHP) technique. Preliminary work showing the feasibility of fabrication of
D. Hreniak et al. / Optical Materials 29 (2007) 1244–1251
Nd:YAG nanoceramics by means of LHPT technique was reported by us earlier [18] and the pressure and temperature range where this process takes place without phase transformation of YAG was also established [19]. The purpose of present work was to investigate the effect of applied pressure on optical and luminescence properties of Nd:Y3Al5O12 nanoceramics.
2. Experimental The powder used as starting material for nanoceramic was prepared by a modified sol–gel method similar to that
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used by Pechini and described by us elsewhere [20]. The microstructure and mechanical properties of nanoceramics obtained under high pressure at relatively low sintering temperature are reported in details separately [21]. Briefly, the nanoceramic samples were obtained at 450 °C under high pressure in a range 2–8 GPa. At these sintering conditions there is no decomposition of nanocrystalline Y3Al5O12 into YAlO3 and Al2O3 [19]. Additionally, we studied also the optical properties of a green body sample made by cold pressing the powder under pressure of 0.8 GPa. The transmittance spectrum was measured on a Carry 2300 UV–VIS–NIR spectrophotometer. Argon laser (514.5 nm) and the second harmonic of pulsed Nd3+:YAG
Fig. 1. The emission spectra of Nd:YAG nanoceramics measured at 300 K (a) and 77 K (b).
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D. Hreniak et al. / Optical Materials 29 (2007) 1244–1251
laser (532 nm) were used as excitation sources for the emission spectra and decays measurements, respectively. The emission spectra were measured with a Jobin–Yvon TRW 1000 monochromator and a Hamamatsu R406 photomultiplier. Emission decays were recorded with a LeCroy WaveSurfer 400 oscilloscope. The spectra were measured at room and 77 K temperatures. The luminescence spectra of Nd:YAG nanoceramics were measured as a function of pressure applied during their densification. 3. Results and discussion The detailed studies on the preparation, morphology and structural properties of Nd:YAG nanoceramics are
published separately [21]. The reported procedures permitted to obtain samples of relatively high transparency reaching 52% at 1064 nm for sample thickness 1.16 mm (extinction coefficient 13.6 cm1) and the Vickers microhardness 6.5 GPa. The microstructure studies have shown that there is a fine distribution of nano-sized pores between the grains. The results of density measurements using the Archimedes method as a function of compaction pressure showed changes from 76% to 98% of the theoretical density of pellets, for 2 GPa and 8 GPa pressure, respectively. The observed increase of density is obviously connected with a decrease of porosity of the pellets. According to this relation, the highest porosity was noticed for the green body sample (51%) fabricated at the lowest pressure.
Fig. 2. The effect of applied pressure on the half-intensity width (Dm1/2) (a) and the intensity ratio of the R2 ! Z5 to R1 ! Z5 emission transitions (b).
D. Hreniak et al. / Optical Materials 29 (2007) 1244–1251
3.1. Luminescence spectra Fig. 1 shows the pressure effect on the luminescence spectra of Nd:YAG nanoceramics. The observed luminescence bands were assigned to the 4F3/2 ! 4I9/2 and 4I11/2 and (4F5/2,4H9/2) ! 4I9/2 transitions. The assignment of Stark components of the observed luminescence lines is identical to those observed for the single Nd:YAG crystal. The most interesting difference of the measured luminescence spectra at room temperature is an appearance of relatively intense (4F5/2,2H9/2) ! 4I9/2 transition band which was not observed in a Nd:YAG single crystal counterpart. This band disappeared with decreasing temperature to 77 K. So its sensitivity to temperature decreasing allows us to ascribed its origin to hot luminescence. Such behaviour was earlier observed by us for Nd:YAG nanopowders [23]. We have shown that it was also dependent on the sizes of Nd:YAG nanograin. 3.2. The 4F3/2 ! 4I9/2, 4I11/2 luminescence The observed luminescence spectrum of Nd:YAG nanoceramics was substantially different in comparison to that observed for the Nd:YAG single crystal [22]. First of all the intensity of 4F3/2 ! 4I11/2 emission transition band in nanoceramics was much weaker than in the single crystal. This observation was earlier reported by us for Nd:YAG nanocrystalline powder and nanoceramics [23,24]. A careful analysis of luminescence properties of Nd:YAG nanoceramics shows that an increase of the applied pressure leads to broadening of emission bands (Fig. 1). The full-width at half maximum (Dm1/2) of emission lines increases almost linearly for hot pressed samples (Fig. 2a). The effect was more pronounced for the spectra measured at RT com-
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pared to those measured at 77 K. It should also be noticed that the effect was stronger for the emission lines measured for the first from the hot-pressed samples (2 GPa, 450 °C) comparing to the cold-pressed green body (0.8 GPa, RT). It points on a role of heat treatment in relaxation of pressure induced stresses. It is reported by us separately, that even the most transparent hot-pressed nanoceramic (8 GPa, 450 °C) displays microstrains close to 0.9% [21]. The respective values of half intensity widths for the 4 F3/2R2 ! 4I9/2Z5 (R2 and Z5 are the Stark components of 4F3/2 and 4I9/2 terms, respectively) transitions for a single Nd:YAG crystal are provided for comparison (Fig. 2b). The emission line widths of Nd:YAG nanoceramics for high pressured samples were more than twice broader compared to the single crystal. A careful analysis of emission intensities associated with the 4F3/2R2 ! 4I9/2Z5 and 4F3/2R1 ! 4I9/2Z5 transitions (Fig. 2b) shows that their intensity ratio, denoted briefly as IR2/IR1, is dependent on the sintering pressure. It decreased with applied pressure at room temperature whereas at low temperature 77 K it was practically constant. It means that population of the excited Stark level R2 was dependent on the applied pressure and decreased with its enhancement It is interesting to note that the intensity ratio IR2/IR1 approaches its counterpart the single crystalline Nd:YAG (IR2/IR1 = 0.73). Another interesting feature observed in the emission spectra of Nd:YAG nanoceramics is a variation of overall intensities of the 4F3/2 ! 4I9/2 and 4F3/2 ! 4I11/2 transitions with applied pressure. Their branching ratio (denoted by b = I9/2/I11/2) decreased with pressure at room temperature whereas at 77 K was only slightly enhanced. All these observations point on the role of pressure induced stresses in Nd:YAG nanoceramics which may be useful in controlling
Fig. 3. The transmittance and luminescence spectrum of transparent Nd:YAG nanoceramic measured in a range 560–820 nm at 77 K.
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emission features of Nd3+ ion. We suppose that these processes are responsible for enhancement of the resonant transition associated with the 4F3/2 ! 4I9/2 transition band which obviously is more effective at room temperature. This transition is responsible for an efficiency of excitation energy migration. 3.3. The (4F5/2,4H9/2) ! 4I9/2 luminescence One can note that the group of emission lines assigned to the (4F5/2,2H9/2) ! 4I9/2 located at the range 800–
850 nm is not observed for a single crystal of Nd:YAG. That emission is associated with thermalization of the (4F3/2,4H9/2) terms because it is not seen at low temperature 77 K measurements. So the (4F5/2,4H9/2) ! 4I9/2 transitions is in its nature the hot luminescence band. An appearance of hot luminescence was earlier reported by us for Nd, Yb:YAG nanoceramics [25,26] and discussed in terms of enhanced thermalization processes. It is interesting to see that the intensity of hot luminescence does not depend on the sintering pressure what indicates on the presence of nanoporosity even in the sample obtained at the highest
Fig. 4. The luminescence decays of Nd:YAG nanoceramics measured at 300 K (a) and 77 K (b).
D. Hreniak et al. / Optical Materials 29 (2007) 1244–1251
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pressure (8 GPa). The problem of lower thermal conductivity in nanoceramics needs further elaborated studies and is in progress. Another interesting behaviour associated with the hot luminescence band (4F5/2,4H9/2) ! 4I9/2 is that at low temperature 77 K we have observed holes (Fig. 3) instead the peaks (Fig. 1a). Such behaviour may be associated with the reabsorption of some emission bands associated with the colour centres. In fact the luminescence spectra of Nd:YAG nanoceramic measured in the range 540–820 nm demonstrated the broad emission band centred at 670 nm and some number of well resolved dips at the edge (Fig. 3). The assignment of energy features of the dips is given in the figure. An appearance of colour centre emission is most probably associated with some pressure induced defects or/ and residual gases O2, N2 adsorbed on the surfaces of Nd:YAG grains. The colour centre emission is probably quenched with increasing temperature and does not affect the luminescence from the 4F3/2 term at room temperature.
those measured at 77 K. It may be due to the enhanced role of radiationless multiphonon relaxation. The luminescence decay time for the sample pressed at 8 GPa was shorter (ca. 20%) than measured for a Nd:YAG single crystal with the same concentration of Nd3+ ions (240 ls) [27]. The observed increase of the luminescence lifetimes with decreasing density can be related to the changes of radiative transition rates via the enhancement of the effective refractive index. Following the recent studies [28–31], the effective refractive index for Y3Al5O12 nanocrystals in air is defined as
3.4. Luminescence decay times
may be recasted in the form
The measured decay profiles of all samples were exponential and well fitted with single exponential function both at 298 K and 77 K (Fig. 4). The luminescence decay times of Nd:YAG nanoceramics were measured for the samples obtained at the different applied pressures and are shown in Fig. 5. The luminescence lifetimes measured at room temperature were found to decrease with pressure since 260 ls for the green body to 190 ls for the nanoceramic obtained at 8 GPa. The same behaviour was observed at low temperature. At 77 K the decay times varied in the range 310– 220 ls. One can note that the luminescence decay times were in general lower at room temperature compared to
sR /
neff ðxÞ ¼ xnY3 Al5 O12 þ ð1 xÞnair
ð1Þ
where x is the ‘filling factor’ illustrating the interface of grains and air. As result, using neff in the place of n [29], the radiative lifetime sR of the electric dipole transitions, expressed as sR /
1 k20 1 f ðEDÞ ðn2 þ 2Þ 2 n 3
ð2Þ
1 f ðEDÞ h
1 ðxnY3 Al5 O12 3
k20
i2 þ ð1 xÞnair Þ2 þ 2Þ ðxnY3 Al5 O12 þ ð1 xÞnair Þ
ð3Þ
Here f(ED) is the oscillator strength for the electric dipole transition and k0 is the wavelength in vacuum. The radiative lifetime should be always lower for the nanocrystalline powder measured in air than for a dense nonporous material. Assuming that only x is a variable in Eq. (3) and the surface area of the grains is sublinearly dependent on the density of samples, we can find how the radiative lifetime,
Fig. 5. The dependence of the density (left axis) and the luminescence decay times (right axis) of Nd:YAG nanoceramics on the sintering pressure.
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D. Hreniak et al. / Optical Materials 29 (2007) 1244–1251
Fig. 6. The dependence of radiative lifetimes on the filling factor (bottom-left axes) and the measured luminescence lifetimes (in relation to the luminescence lifetime measured for the transparent nanoceramic) on the density of samples (top-right axes).
compared to the values calculated for a full dense material (x = 1), should change with the filling factor (Fig. 5 bottom-left axes). As it can be seen, the measured dependence of the relative change of luminescence lifetimes on the density of samples denoted as sexp(density (%))/sexp(density of transparent sample), (Fig. 5 top-right axes) is in a good agreement to the proposed relationship of relative changes of radiative lifetimes and filling factor (sR(x)/sR(x = 1)) (Fig. 6).
Acknowledgments
4. Conclusions
References
The luminescence spectra of Nd:YAG nanoceramics were substantially different to those reported for the single crystalline Nd3+:YAG. The emission line widths of 4 F3/2 ! 4I9/2 transitions were broader and increased as a function of sintering pressure. Also the intensity branching ratio of the 4F3/2 ! 4I9/2 and 4F3/2 ! 4I9/2 transition bands changed with the applied pressure. A specific feature of luminescence of Nd:YAG nanoceramics is appearance of hot emission transitions which originate most probably from enhanced thermalization of higher located terms from the 4F3/2 state. The measured luminescence decay times of the 4F3/2 state decreased with increase of sintering pressure. We discussed this behaviour in terms of porosity and density of nanoceramics affecting macroscopic optical properties such as a refractive index. The emission from the pressure induced colour centres was observed at low temperature. Further investigations including the study of the spectroscopic properties after thermal stress of nanoceramics are needed in order to obtain a detailed physical description of the observed effects.
[1] G. de With, H.J.A. Dijk van, Mater. Res. Bull. 19 (10) (1984) 1669. [2] A. Ikesue, T. Kinoshita, K. Kamata, K. Yoshida, J. Am. Ceram. Soc. 78 (1995) 1033. [3] E. Zych, C. Brecher, A.J. Wojtowicz, H. Lingertat, J. Lumin. 75 (1997) 193. [4] J. Lu, K.I. Ueda, H. Yagi, T. Yanagitani, Y. Akiyama, A.A. Kaminskii, J. Alloys Compd. 341 (2002) 220. [5] K. Takaichi, H. Yagi, A. Shirakawa, K. Ueda, S. Hosokawa, T. Yanagitani, A.A. Kaminskii, Phys. Status Solidi A 202 (1) (2005) R13. [6] G. Qin, J. Lu, J.F. Bisson, Y. Feng, K.-I. Ueda, H. Yagi, T. Yanagitani, Solid State Commun. 132 (2004) 103. [7] E.T. Goldburt, B. Kulkarni, R.N. Bharagava, J. Tylor, M. Libera, J. Lumin. 72–74 (1997) 190. [8] M.P. Pechini, US Patent 3 330 697, 1967. [9] Y. Liu, J. Zhan, M. Ren, K. Tang, W. Yu, Y. Qian, Mater. Res. Bull. 36 (2001) 1231. [10] L. Sun, J. Yao, C. Liu, C. Liao, C. Yan, J. Lumin. 87–89 (2000) 447. [11] D. Gallagher, R. Bhargava, J. Racz, US Patent 5,525,377, 1996. [12] H. Ferkel, R.J. Hellmig, Nanostruct. Mater. 11 (1999) 617. [13] J.E. Geusic, H.M. Marcos, L.G. Uitert van, Appl. Phys. Lett. 4 (1964) 182. [14] Alexander A. Kaminskii, Crystalline Lasers: Physical Processes and Operating Schemes, CRC Press, 1996. [15] M.M. Braun, L. Pilon, Thin Solid Films 496 (2006) 505.
Studies were supported by the Polish Committee for Scientific Research (KBN) under Grant No. 4 T08D 039 24, Network Namic, and SPUB COST D30 Project. The authors thank Mr. P. Mazur, Mr. M. Bereza and Mr. P. Gluchowski for technical assistance, Mrs. T. MorawskaKowal for absorption measurements, and Dr. S. Gierlotka for help in sintering experiments.
D. Hreniak et al. / Optical Materials 29 (2007) 1244–1251 [16] A. Krell, P. Blank, H.W. Ma, T. Hutzler, M.P.B. Bruggen van, R.J. Apetz, J. Am. Ceram. Soc. 86 (2003) 12. [17] A. Krell, P. Blank, H.W. Ma, T. Hutzler, M. Nebelung, J. Am. Ceram. Soc. 86 (2003) 546. [18] W. Strek et al., J. Lumin., in press, doi:10.1016/j.jlumin.2006.01.100. [19] D. Hreniak, S. Gierlotka, W. Lojkowski, W. Strek, P. Mazur, R. Fedyk, Diffus. Defect Data, Part. B 106 (2005) 17. [20] P. Mazur, D. Hreniak, J. Niittykoski, W. Stre˛k, J. Ho¨lsa¨, Mater. Sci. 23 (2005) 261. [21] R. Fedyk et al., Opt. Mater., in press, doi:10.1016/j.optmat. 2006.05. 016. [22] T. Kushida, H.M. Marcos, J.E. Geusic, Phys. Rev. 167 (1968) 289. [23] D. Hreniak, W. Strek, J. Alloys Compd. 341 (2002) 183.
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[24] D. Hreniak, M. Jasiorski, A. Hreniak, W. Dudzinski, K. Maruszewski, W. Strek, J. Sol–Gel Sci. Technol. 26 (2003) 971. [25] A. Bednarkiewicz, W. Strek, J. Phys. D: Appl. Phys. 35 (2002) 2503. [26] A. Bednarkiewicz, D. Hreniak, P. Deren, W. Strek, J. Lumin. 102– 103 (2003) 438. [27] R.C. Powell, Physics of Solid-State Laser Materials, (Chapter 9), p. 349. [28] H.S. Yang, K.S. Hong, S.P. Feofilov, B.M. Tissue, R.S. Melzer, W.M. Dennis, J. Lumin. 83-4 (1999) 139. [29] R.S. Meltzer, S.P. Feofilov, B.M. Tissue, H.B. Yuan, Phys. Rev. B 60 (1999) R14012. [30] F. Vetrone, J.-C. Boyer, J.A. Copobianco, A. Speghini, M. Bettinelli, Nanotechnology 15 (2004) 75. [31] H. Schniepp, V. Sandoghdar, Phys. Rev. Lett. 89 (2002) 257403.
Luminescence properties of Nd:YAG nanoceramics prepared by low temperature high pressure sintering method D. Hreniak a
a,*
, R. Fedyk b, A. Bednarkiewicz a, W. Stre˛k a, W. Łojkowski
b
Institute of Low Temperature and Structure Research, Polish Academy of Science, ul. Oko´lna 2, P Box 1410, 50-422 Wrocław, Poland b Institute of High Pressure Physics, Polish Academy of Sciences, ul. Sokołowska 29/37, 01-142 Warsaw, Poland Received 9 March 2006; received in revised form 28 April 2006; accepted 4 May 2006 Available online 20 September 2006
Abstract Nd:YAG nanoceramics composed of 25 nm grains were obtained by a low temperature high pressure sintering process. Their absorption and luminescence spectra were measured. It was found that the transparency of Nd:YAG nanoceramics increases with the pressure of the fabrication process. This effect was explained in terms of reducing porosity of the ceramics with increasing the sintering pressure. It was found that the emission properties of Nd3+ ion in YAG nanoceramic were substantially different compared to its single crystalline counterpart. In particular the emission intensities and decay times strongly depends on the applied pressure. The intensity ratio of the 4 F3/2 ! 4I9/2 and 4F3/2 ! 4I11/2 emission transitions was much higher than in a single crystal and decreased with increasing the pressure. An efficient hot emission from the 4F5/2,4H9/2 terms was observed at room temperature. Ó 2006 Elsevier B.V. All rights reserved. Keywords: YAG; Nanoceramics; Neodymium; Pressure dependence; Sintering; Effective refractive index
1. Introduction There is a great interest in developing new techniques of preparation of yttrium aluminum garnet (YAG) and other cubic nanopowders as starting materials for fabrication of ceramic optical materials [1–3]. In contrast to other techniques of fabrication, based mainly on the solid state reaction of oxides [1–3], the method of fabrication of ceramics from nanopowders allows to use materials of the same chemical and phase composition as the final ceramics, for instance nanocrystalline YAG [4]. Such ceramics, prepared in a low cost way compared to processing of the single crystals, have comparable optical and thermomechanical properties and can substitute them in many applications [4–6]. YAG nanopowders can be synthesized by means of a variety of methods: sol–gel [7], Pechini [8], solvothermal [9], combustion [10] and co-precipitation [11]. It is very impor-
*
Corresponding author. Tel.: +48 71343 5021; fax: +48 71344 1029. E-mail address: [email protected] (D. Hreniak).
0925-3467/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.05.014
tant to obtain the nanocrystals at the lowest possible crystallization temperature since the crystal size is increased with temperature of synthesis [12]. Preparation methods and optical properties of transparent ceramics made by sintering Nd:YAG nanopowders as starting materials were reported by several groups [13,14]. These ceramics were characterized by microsized grains and their optical properties were almost identical to those reported for single crystalline Nd:YAG. An alternative approach for production of transparent ceramics may be based on a concept of nanoceramics with the grain less than 100 nm. Such ceramics, having low residual porosity, should be in principle highly transparent due to the restricted Rayleigh scattering [15]. Recently, Krell et al. [16,17] developed transparent alumina ceramics having grain sizes of 0.4–0.6 lm at relative densities of >99.9%. In this work we present the luminescence studies of Nd:YAG nanoceramics grain size in the range 30–50 nm. Such dense nanoceramics with were produced using the low temperature high pressure sintering (LTHP) technique. Preliminary work showing the feasibility of fabrication of
D. Hreniak et al. / Optical Materials 29 (2007) 1244–1251
Nd:YAG nanoceramics by means of LHPT technique was reported by us earlier [18] and the pressure and temperature range where this process takes place without phase transformation of YAG was also established [19]. The purpose of present work was to investigate the effect of applied pressure on optical and luminescence properties of Nd:Y3Al5O12 nanoceramics.
2. Experimental The powder used as starting material for nanoceramic was prepared by a modified sol–gel method similar to that
1245
used by Pechini and described by us elsewhere [20]. The microstructure and mechanical properties of nanoceramics obtained under high pressure at relatively low sintering temperature are reported in details separately [21]. Briefly, the nanoceramic samples were obtained at 450 °C under high pressure in a range 2–8 GPa. At these sintering conditions there is no decomposition of nanocrystalline Y3Al5O12 into YAlO3 and Al2O3 [19]. Additionally, we studied also the optical properties of a green body sample made by cold pressing the powder under pressure of 0.8 GPa. The transmittance spectrum was measured on a Carry 2300 UV–VIS–NIR spectrophotometer. Argon laser (514.5 nm) and the second harmonic of pulsed Nd3+:YAG
Fig. 1. The emission spectra of Nd:YAG nanoceramics measured at 300 K (a) and 77 K (b).
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D. Hreniak et al. / Optical Materials 29 (2007) 1244–1251
laser (532 nm) were used as excitation sources for the emission spectra and decays measurements, respectively. The emission spectra were measured with a Jobin–Yvon TRW 1000 monochromator and a Hamamatsu R406 photomultiplier. Emission decays were recorded with a LeCroy WaveSurfer 400 oscilloscope. The spectra were measured at room and 77 K temperatures. The luminescence spectra of Nd:YAG nanoceramics were measured as a function of pressure applied during their densification. 3. Results and discussion The detailed studies on the preparation, morphology and structural properties of Nd:YAG nanoceramics are
published separately [21]. The reported procedures permitted to obtain samples of relatively high transparency reaching 52% at 1064 nm for sample thickness 1.16 mm (extinction coefficient 13.6 cm1) and the Vickers microhardness 6.5 GPa. The microstructure studies have shown that there is a fine distribution of nano-sized pores between the grains. The results of density measurements using the Archimedes method as a function of compaction pressure showed changes from 76% to 98% of the theoretical density of pellets, for 2 GPa and 8 GPa pressure, respectively. The observed increase of density is obviously connected with a decrease of porosity of the pellets. According to this relation, the highest porosity was noticed for the green body sample (51%) fabricated at the lowest pressure.
Fig. 2. The effect of applied pressure on the half-intensity width (Dm1/2) (a) and the intensity ratio of the R2 ! Z5 to R1 ! Z5 emission transitions (b).
D. Hreniak et al. / Optical Materials 29 (2007) 1244–1251
3.1. Luminescence spectra Fig. 1 shows the pressure effect on the luminescence spectra of Nd:YAG nanoceramics. The observed luminescence bands were assigned to the 4F3/2 ! 4I9/2 and 4I11/2 and (4F5/2,4H9/2) ! 4I9/2 transitions. The assignment of Stark components of the observed luminescence lines is identical to those observed for the single Nd:YAG crystal. The most interesting difference of the measured luminescence spectra at room temperature is an appearance of relatively intense (4F5/2,2H9/2) ! 4I9/2 transition band which was not observed in a Nd:YAG single crystal counterpart. This band disappeared with decreasing temperature to 77 K. So its sensitivity to temperature decreasing allows us to ascribed its origin to hot luminescence. Such behaviour was earlier observed by us for Nd:YAG nanopowders [23]. We have shown that it was also dependent on the sizes of Nd:YAG nanograin. 3.2. The 4F3/2 ! 4I9/2, 4I11/2 luminescence The observed luminescence spectrum of Nd:YAG nanoceramics was substantially different in comparison to that observed for the Nd:YAG single crystal [22]. First of all the intensity of 4F3/2 ! 4I11/2 emission transition band in nanoceramics was much weaker than in the single crystal. This observation was earlier reported by us for Nd:YAG nanocrystalline powder and nanoceramics [23,24]. A careful analysis of luminescence properties of Nd:YAG nanoceramics shows that an increase of the applied pressure leads to broadening of emission bands (Fig. 1). The full-width at half maximum (Dm1/2) of emission lines increases almost linearly for hot pressed samples (Fig. 2a). The effect was more pronounced for the spectra measured at RT com-
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pared to those measured at 77 K. It should also be noticed that the effect was stronger for the emission lines measured for the first from the hot-pressed samples (2 GPa, 450 °C) comparing to the cold-pressed green body (0.8 GPa, RT). It points on a role of heat treatment in relaxation of pressure induced stresses. It is reported by us separately, that even the most transparent hot-pressed nanoceramic (8 GPa, 450 °C) displays microstrains close to 0.9% [21]. The respective values of half intensity widths for the 4 F3/2R2 ! 4I9/2Z5 (R2 and Z5 are the Stark components of 4F3/2 and 4I9/2 terms, respectively) transitions for a single Nd:YAG crystal are provided for comparison (Fig. 2b). The emission line widths of Nd:YAG nanoceramics for high pressured samples were more than twice broader compared to the single crystal. A careful analysis of emission intensities associated with the 4F3/2R2 ! 4I9/2Z5 and 4F3/2R1 ! 4I9/2Z5 transitions (Fig. 2b) shows that their intensity ratio, denoted briefly as IR2/IR1, is dependent on the sintering pressure. It decreased with applied pressure at room temperature whereas at low temperature 77 K it was practically constant. It means that population of the excited Stark level R2 was dependent on the applied pressure and decreased with its enhancement It is interesting to note that the intensity ratio IR2/IR1 approaches its counterpart the single crystalline Nd:YAG (IR2/IR1 = 0.73). Another interesting feature observed in the emission spectra of Nd:YAG nanoceramics is a variation of overall intensities of the 4F3/2 ! 4I9/2 and 4F3/2 ! 4I11/2 transitions with applied pressure. Their branching ratio (denoted by b = I9/2/I11/2) decreased with pressure at room temperature whereas at 77 K was only slightly enhanced. All these observations point on the role of pressure induced stresses in Nd:YAG nanoceramics which may be useful in controlling
Fig. 3. The transmittance and luminescence spectrum of transparent Nd:YAG nanoceramic measured in a range 560–820 nm at 77 K.
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emission features of Nd3+ ion. We suppose that these processes are responsible for enhancement of the resonant transition associated with the 4F3/2 ! 4I9/2 transition band which obviously is more effective at room temperature. This transition is responsible for an efficiency of excitation energy migration. 3.3. The (4F5/2,4H9/2) ! 4I9/2 luminescence One can note that the group of emission lines assigned to the (4F5/2,2H9/2) ! 4I9/2 located at the range 800–
850 nm is not observed for a single crystal of Nd:YAG. That emission is associated with thermalization of the (4F3/2,4H9/2) terms because it is not seen at low temperature 77 K measurements. So the (4F5/2,4H9/2) ! 4I9/2 transitions is in its nature the hot luminescence band. An appearance of hot luminescence was earlier reported by us for Nd, Yb:YAG nanoceramics [25,26] and discussed in terms of enhanced thermalization processes. It is interesting to see that the intensity of hot luminescence does not depend on the sintering pressure what indicates on the presence of nanoporosity even in the sample obtained at the highest
Fig. 4. The luminescence decays of Nd:YAG nanoceramics measured at 300 K (a) and 77 K (b).
D. Hreniak et al. / Optical Materials 29 (2007) 1244–1251
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pressure (8 GPa). The problem of lower thermal conductivity in nanoceramics needs further elaborated studies and is in progress. Another interesting behaviour associated with the hot luminescence band (4F5/2,4H9/2) ! 4I9/2 is that at low temperature 77 K we have observed holes (Fig. 3) instead the peaks (Fig. 1a). Such behaviour may be associated with the reabsorption of some emission bands associated with the colour centres. In fact the luminescence spectra of Nd:YAG nanoceramic measured in the range 540–820 nm demonstrated the broad emission band centred at 670 nm and some number of well resolved dips at the edge (Fig. 3). The assignment of energy features of the dips is given in the figure. An appearance of colour centre emission is most probably associated with some pressure induced defects or/ and residual gases O2, N2 adsorbed on the surfaces of Nd:YAG grains. The colour centre emission is probably quenched with increasing temperature and does not affect the luminescence from the 4F3/2 term at room temperature.
those measured at 77 K. It may be due to the enhanced role of radiationless multiphonon relaxation. The luminescence decay time for the sample pressed at 8 GPa was shorter (ca. 20%) than measured for a Nd:YAG single crystal with the same concentration of Nd3+ ions (240 ls) [27]. The observed increase of the luminescence lifetimes with decreasing density can be related to the changes of radiative transition rates via the enhancement of the effective refractive index. Following the recent studies [28–31], the effective refractive index for Y3Al5O12 nanocrystals in air is defined as
3.4. Luminescence decay times
may be recasted in the form
The measured decay profiles of all samples were exponential and well fitted with single exponential function both at 298 K and 77 K (Fig. 4). The luminescence decay times of Nd:YAG nanoceramics were measured for the samples obtained at the different applied pressures and are shown in Fig. 5. The luminescence lifetimes measured at room temperature were found to decrease with pressure since 260 ls for the green body to 190 ls for the nanoceramic obtained at 8 GPa. The same behaviour was observed at low temperature. At 77 K the decay times varied in the range 310– 220 ls. One can note that the luminescence decay times were in general lower at room temperature compared to
sR /
neff ðxÞ ¼ xnY3 Al5 O12 þ ð1 xÞnair
ð1Þ
where x is the ‘filling factor’ illustrating the interface of grains and air. As result, using neff in the place of n [29], the radiative lifetime sR of the electric dipole transitions, expressed as sR /
1 k20 1 f ðEDÞ ðn2 þ 2Þ 2 n 3
ð2Þ
1 f ðEDÞ h
1 ðxnY3 Al5 O12 3
k20
i2 þ ð1 xÞnair Þ2 þ 2Þ ðxnY3 Al5 O12 þ ð1 xÞnair Þ
ð3Þ
Here f(ED) is the oscillator strength for the electric dipole transition and k0 is the wavelength in vacuum. The radiative lifetime should be always lower for the nanocrystalline powder measured in air than for a dense nonporous material. Assuming that only x is a variable in Eq. (3) and the surface area of the grains is sublinearly dependent on the density of samples, we can find how the radiative lifetime,
Fig. 5. The dependence of the density (left axis) and the luminescence decay times (right axis) of Nd:YAG nanoceramics on the sintering pressure.
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Fig. 6. The dependence of radiative lifetimes on the filling factor (bottom-left axes) and the measured luminescence lifetimes (in relation to the luminescence lifetime measured for the transparent nanoceramic) on the density of samples (top-right axes).
compared to the values calculated for a full dense material (x = 1), should change with the filling factor (Fig. 5 bottom-left axes). As it can be seen, the measured dependence of the relative change of luminescence lifetimes on the density of samples denoted as sexp(density (%))/sexp(density of transparent sample), (Fig. 5 top-right axes) is in a good agreement to the proposed relationship of relative changes of radiative lifetimes and filling factor (sR(x)/sR(x = 1)) (Fig. 6).
Acknowledgments
4. Conclusions
References
The luminescence spectra of Nd:YAG nanoceramics were substantially different to those reported for the single crystalline Nd3+:YAG. The emission line widths of 4 F3/2 ! 4I9/2 transitions were broader and increased as a function of sintering pressure. Also the intensity branching ratio of the 4F3/2 ! 4I9/2 and 4F3/2 ! 4I9/2 transition bands changed with the applied pressure. A specific feature of luminescence of Nd:YAG nanoceramics is appearance of hot emission transitions which originate most probably from enhanced thermalization of higher located terms from the 4F3/2 state. The measured luminescence decay times of the 4F3/2 state decreased with increase of sintering pressure. We discussed this behaviour in terms of porosity and density of nanoceramics affecting macroscopic optical properties such as a refractive index. The emission from the pressure induced colour centres was observed at low temperature. Further investigations including the study of the spectroscopic properties after thermal stress of nanoceramics are needed in order to obtain a detailed physical description of the observed effects.
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Studies were supported by the Polish Committee for Scientific Research (KBN) under Grant No. 4 T08D 039 24, Network Namic, and SPUB COST D30 Project. The authors thank Mr. P. Mazur, Mr. M. Bereza and Mr. P. Gluchowski for technical assistance, Mrs. T. MorawskaKowal for absorption measurements, and Dr. S. Gierlotka for help in sintering experiments.
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