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Yb2+: MO-Al2O3 (M=Ca, Sr and Ba) phosphors
ARTICLE IN PRESS Journal of Luminescence 130 (2010) 1248–1253
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Up and down conversion fluorescence studies on combustion synthesized Yb3 + /Yb2 + : MO-Al2O3 (M =Ca, Sr and Ba) phosphors R.K. Verma a, Anita Rai b, K. Kumar a, S.B. Rai a,n a b
Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi, UP-221005, India P. P. N. College, Kanpur, UP, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 24 June 2009 Received in revised form 17 February 2010 Accepted 19 February 2010 Available online 24 February 2010
The ytterbium ions doped MO-Al2O3 (M = Ca, Sr and Ba) phosphors have been synthesized through combustion technique and their up and down conversion fluorescence properties have been studied and compared. The samples were calcinated at different temperatures and their FTIR and XRD spectra have shown a close relationship. With 976 nm excitation all these phosphors show cooperative upconversion emission at 488 nm from the pairs of two Yb3 + ions along with an unexpected broad upconversion band in the blue green region and has been assigned to arise from the defect centers. Contrary to this upconversion emission, calcium aluminate phosphor exhibits bright and very broad down-conversion fluorescence (FWHM E 160 nm) upon UV (266 nm) excitation due to Yb2 + ions. The inter-conversion between the 3 + and 2 + valence states of Yb ion has been observed on calcinations of samples in open atmosphere and has been correlated to the emission properties. The Yb2 + ions containing calcium aluminate phosphor has been found suitable for producing broad band light in the visible region (white light). Lifetime of the emitting states of Yb3 + and Yb2 + ions have also been measured and discussed. & 2010 Published by Elsevier B.V.
Keywords: Combustion Phosphor Rare-earth ions Upconversion
1. Introduction Though a number of optical devices already employ Rare earth (RE) ions as emitting centers with good optical efficiency, the new generation of RE doped nano-phosphors has increased their attraction due to their prospective uses. The advanced phosphor materials prepared by newer techniques e.g. combustion with organic fuel has shown improved luminescent properties. The combustion technique has been shown to produce highly homogeneous nano-crystallized fine phosphor materials [1–3]. These RE doped phosphors show better fluorescence (Stokes emission), upconversion (anti-Stokes emission) and longer duration afterglow properties as compared to materials prepared by other methods [4–12]. Frequency upconversion (UC) of incident infrared radiation into visible/ultraviolet radiation by RE doped phosphors is another topic of interest [13–17]. Ytterbium is of special interest as it can be used as a luminescence center as well as a sensitizer, for other codoped rare earths as Yb can exist in Yb2 + as well as Yb3 + states simultaneously in different crystals and glassy systems [18,19]. The heat treatment of the as-synthesized phosphors has been reported to have very significant effect on the optical properties.
n
Corresponding author. Tel.: + 91 5422307308; fax: + 91 5422369889. E-mail address: [email protected] (S.B. Rai).
0022-2313/$ - see front matter & 2010 Published by Elsevier B.V. doi:10.1016/j.jlumin.2010.02.033
It has been conjectured that the heat treatment creates small (nano-sized) crystallites in the phosphor material which also enhance the luminescence efficiency [20]. If there are different valence states of the doped ions present in the sample or an uncontrolled dopant coexists then the luminescence spectrum becomes very complex. The present work reports a comparative study of upconversion and down-conversion fluorescence properties of Yb3 + /Yb2 + ions in MAl2O4 (M= Ca, Sr, Ba) phosphors and also the effect of calcinations on the emission and fluorescence lifetime of Yb3 + / Yb2 + ions. The correlation between the FTIR and XRD spectra of the samples has also been established.
2. Experimental The phosphor samples have been prepared through combustion method using urea as an organic fuel as described earlier [21]. The following composition of the phosphor has been selected for study: 40MO þ ð60-xÞAl2 O3 þxYb2 O3 where M=Ca, Sr and Ba and x =1.0–5.0 mol% To produce crystallization in the as-synthesized samples, the samples were calcinated at different temperatures for different durations. The existence of crystallinity is checked by recording
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the powder X-ray diffraction (XRD) using CuKa radiation (1.5406 A˚). The absorption spectra of all the samples (both as-synthesized and calcinated) were monitored in reflectance mode using Perkin Elmer, Lambda-35 spectrophotometer. The upconversion luminescence was recorded using 976 nm radiation from a diode laser while the down-conversion fluorescence was recorded using the 266 nm radiation from an Nd: YAG laser and a iHR320, Horiba Jobin Yuon, spectrometer. Perkin Elmer Spectrum RX1 was used to record the Fourier Transform Infrared (FTIR) spectra of the samples. Lifetime of the Yb3 + level involved in cooperative upconversion was recorded by chopping the continuous 976 nm radiation while for the 4f125d level of Yb2 + the pulsed excitation at 266 nm was used.
3. Results and discussion Samples with different concentration of Yb2O3 were prepared and the upconversion emission intensity was found strongest for 2.5 mol% Yb2O3. All further studies therefore were made with samples having 2.5 mol% concentration. Calcination was done at 1073, 1273 and 1473 K for 2 h. 3.1. Phase content by X-ray diffraction studies Fig. 1 shows the XRD patterns for the MO-Al2O3 (M = Ca, Sr and Ba) phosphor samples; the as-synthesized and the calcinated at three different temperatures. No feature of crystallinity has been observed for the calcium aluminate (CA) sample (Fig. 1a) below 1073 K. However, when the calcination temperature is raised beyond 1073 K characteristic peaks of crystalline phase begin to appear. This may be due to the large thermal stability of the CA sample. The XRD pattern of the sample at 1473 K is shown in Fig. 1a. All the observed peaks can be assigned to Ca12Al14O33 and only a single phase appears to be present (JCPDS Cards 090413). The Ca12Al14O33 structure is cubic with a cell parameter a =11.982 A˚ and belongs to the I43d space group. The Ca12Al14O33 phase is well known as an oxygen trap and is also an electronic conductor [22]. The barium aluminate (BA) and the strontium aluminate (SA) samples show crystalline behavior even at the assynthesized stage. However in the initial stage several phases are seen to be present which tend to a single phase when calcination temperature is raised. The SA at the as-synthesized stage shows the presence of SrCO3 phase as the major constituent in the assynthesized strontium sample (Fig. 1b). However at 1473 K, all the prominent peaks correspond to the Sr3Al2O6. The presence of CO3 group is also confirmed by the TGA/DTA graph (supporting information). All the diffraction peaks could be indexed to the cubic system (Space Group: Pa3) of Sr3Al2O6 (JCPDS Cards 24–1187). The XRD results demonstrate that the crystallization of CA and SA precursor samples entirely completed at temperature below 1473 K and single Ca12Al14O33 and Sr3Al2O6 phases are observed. These temperatures are much lower than that used to prepare these by conventional solid state reaction, as shown in the phase diagram of the CaO and SrO–Al2O3 systems (above 1700 K [23]). These results show the essential advantage of the chemical route. The precursors are homogeneously mixed at the molecular level, leading to high reactivity of starting materials and a reduction in calcination temperature. Doping of Yb3 + ions has shown no effect on the crystal phases of the samples. The crystallite size was calculated using the Scherer equation t ¼ l 0:9=b Cos y where, t is the crystallite size for (h k l) planes, l the wavelength of the incident X-ray [CuKa (0.154056)], b the full width at half
Fig. 1. X-ray powder diffraction pattern of calcinated phosphors (a) calcium aluminate and (b) shows that strontium alluminate is crystalline even in as synthesized one and consist of two phases viz strontium corbonate and strontium aluminate. It is strontium aluminate which exists even at higher temperature.
maximum (FWHM) in radians and y the diffraction angle for (h k l) plane. Three most significant peaks were selected for the calculation in different samples and the average crystallite size was found to be in the range 40–50 nm.
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3.2. FTIR studies
3.3. Effect of crystallization on the FTIR spectrum
The FTIR spectra of the different samples recorded in the 4000–400 cm 1 region and for CA and SA samples are shown in Fig. 2. The presence of a broad peak in the region 3400– 3600 cm 1 clearly indicates presence of water in the samples calcinated even at such a high temperature. Similarly, the presence of absorption band near 1600 cm 1 shows that NO2 group also retains its identity. The band at 780 cm 1 is ascribed to Al–O stretching mode. The impurity peaks due to the OH , NO3 , etc. in the calcinated samples are found to decrease as the calcination temperature is increased. A decrease in concentration of these groups which have fluorescence quenching properties might be one of the reasons for increased luminescence from calcinated samples.
Fig. 2a emphasizes the difference between the FTIR spectra of as-synthesized and the calcinated samples of CA. This difference is obviously due to the development of crystallites in the calcinated samples. The spectrum of the as-synthesized sample is similar to that of mixed lattices of CaO and Al2O3. The calcinated samples on the other hand contain crystallites of Ca12Al14O33 and new absorption bands are therefore seen. These additional peaks are seen in the FTIR spectra of the sample calcinated even at 1073 K. These peaks must appear in FTIR spectra if the crystallization has taken place even minutely. The XRD pattern however does not show any crystallinity at this temperature. The crystallites appear in XRD only at higher temperatures of calcinations. The crystallites absorption is characterized by peaks in the frequency region 400–900 cm 1 and our observation indicates that FTIR spectrum can be used to detect onset of crystallization more sensitively in such samples. A comparison of the FTIR spectrum of CA sample with those for SA (Fig. 2b) indicates that the presence of different phases can also be deflected by the changing pattern of the bands in the same frequency range. The presence of different phases is also responsible for different symmetry of the environment around the rare earth ions, so the luminescence spectra are also modified. Of course the number of bands will also differ. 3.4. Upconversion luminescence studies The excitation of CA, SA and BA phosphors with 976 nm wavelength gives two emission bands; a sharp one at 488 nm and an anomalous broad one in the blue green region. In Fig. 3a upconversion spectra of CA, SA and BA phosphors are compared. The inset of Fig. 3a shows the resolved pattern of both peaks by Gaussian peak fitting. The 488 nm emission is the result of the cooperative emission from pairs of Yb3 + ions and is stronger when the distance between a pair of Yb ions lies between 3–5 A˚ (at higher concentrations) [24]. At the molar concentration of Yb3 + used in our samples such distances can appear in clusters. Cooperative emission is described by the process
Fig. 2. Fourier transform infrared spectra of calcinated phosphors (a) calcium aluminate and (b) strontium aluminate.
Yb
3þ
ðG:S:Þ þ hnð976 nmÞ-Yb
3þ
Yb
3þ
ðExc:Þ þ Yb
3þ
3þ
ðExc:Þ-Yb
ðExcitedÞ ðG:S:Þ þYb
3þ
ðG:S:Þ þ hnð488 nmÞ
The wavelength of this sharp emission is the same for all the three hosts, CA, SA and BA but its intensity shows a decrease in the order CA 4SA4BA. The larger upconversion emission intensity in CA may be related to the higher covalence of the Ca–O band and the shortest distance of Yb–O band. Two additional emission bands are observed in CA sample in visible region at 535 nm, 548 nm and are due to Er3 + as uncontrolled impurity. These peaks do not appear in the emission spectrum of SA or BA samples and in none in absorption spectra. These peaks arise in CA samples due to the cooperative energy transfer from Yb3 + to Er3 + and is probably favored in the case of CA samples [30]. The observation of the broad band emission in the upconversion mode is dependent on the host matrix e.g. for CA as host it is at 470 nm while for SA and BA phosphors its position is near 508 nm. The intensity of the band is seen to increase as the calcination temperature is raised up to 1273 K but for higher calcination temperatures the intensity decreases in all cases (Fig. 3b). The appearance of a broad band emission may either be an emission involving Yb2 + or it may be due to defect centers. The presence of Yb2 + ions has been observed only in the CA samples through its UV–vis–IR absorption spectrum (Fig. 4) where, the
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Fig. 4. Absorption spectra of calcium, strontium and barium aluminate phosphors in reflectance mode.
Fig. 3. (a) A comparison of upconversion emission spectra of calcium, strontium and barium aluminate phosphors and (b) a comparison of emission spectra of barium aluminate phosphor calcinated at different temperatures. Inset of 3(a) shows the Gaussian fit of the peak and inset of 3(b) shows intensity variation of bands with calcination temperature.
two absorption bands appear at 325 and 380 nm wavelengths [19,26–28]. The broad emission observed in all the three cases must be attributed to a source other than the Yb2 + ions. A similar broad emission has been observed by Kaczmarek et al. [18,19] in the down conversion emission spectra of single crystal containing Yb2 + and has been explained as a consequence of defect centers. The defects in the host lattice arise when an alkaline earth cation (e.g. Ca2 + ) is replaced by Yb3 + ion. In this case the charge neutrality requires that the Yb3 + ion changes into Yb2 + and a positive hole is created. The resulting color center may be responsible for the broad emission seen in the upconversion. Such emission from color centers has been reported in many materials by several workers [19,20,25–30]. The dependence of the intensities on the calcination temperature is different for the 488 and the 508 nm bands (inset of Fig. 3b). Gain in intensity of the 508 nm peak is more rapid than for the 488 nm peak. This is not surprising since more defect centers are likely to be created at higher temperatures as more Yb3 + ions are changed into Yb2 + . This should decrease the emission due to the Yb3 + at 488 nm. The
Fig. 5. A model showing the emission mechanisms of Yb2 + and Yb3 + .
observed increase in intensity is perhaps due to reduction in the fluorescence quenching impurities e.g. NO3 and OH at higher calcination temperatures. A model of the excitation and emission mechanism for the broad peak is shown in Fig. 5. The Yb3 + ion is excited by the incident radiation and then two excited ions emit at 488 nm wavelength. This radiation is absorbed by the electrons trapped at color centers creating free electron which recombine radiatively with the holes and emit at 508 nm [31]. The lifetime of the blue emission band (488 nm) has been measured as 0.515 ms in case of the CA phosphor (Fig. 6) which is nearly half of the lifetime of the 2F5/2-2F7/2 transition of Yb3 + [32] and confirming the involvement of cooperative excitations of a pair of Yb3 + ions.
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3.5. Down conversion fluorescence studies The 266 nm excitation excites luminescence only in the CA phosphor (Fig. 7). Similar to the upconversion emission, the intensity of emission at first increases with calcination temperature but decreases subsequently. The emission comprises of two bands one at 442 nm and the other at 548 nm. The inset in Fig. 7 shows a Gaussian fit to the emission band. The FWHM of peak I at 442 nm is 73 nm and for peak II at 548 nm is 128 nm. The combination of two emission bands shows perception of white light. The intensity of the two bands varies with concentration of Yb in different manners and they probably involve two different levels of Yb2 + . Henke et al. [33] have reported three overlapping
emissions with peaks at 480, 500 and 570 nm for Yb in YAlO3 matrix and attributed these to Yb2 + ions. They also remarked that the three excited levels have different lifetimes. Similar studies on Yb2 + have been conducted by Xie et al. [34]. The 548 nm emission in our case arises from excited levels of Yb2 + ion as reported earlier [25,35]. The lifetime of levels responsible for the 442 and 548 nm bands are measured to be 230 and 270 ms at 20 K temperature indicating their distinctly different origins. The absence of any luminescence from SA and BA phosphors on excitations with 266 nm can be understood from their absorption spectrum (Fig. 4). In these phosphors no absorption attributed to Yb2 + is seen. It appears that in these phosphors no stabilization of Yb2 + state takes place even after calcinations. The reaction may take place as Yb
3þ
Normal...heating
2þ
þe !Yb
But at higher temperatures, Yb2 + ions get oxidized and again converts to Yb3 + as 2þ
Yb
Oxidation
3þ
!Yb
þ e
The temperature at which the reaction rates become comparable would depend on the ion and the host matrix. The relative magnitudes of the rates for the two reactions may be responsible for the difference noted in the behavior of three phosphors.
4. Conclusions
Fig. 6. Decay curve of Yb3 + ion on 976 nm excitation. The fitting of the curve shows single exponential decay.
Ytterbium doped MO-Al2O4 (M =Ca, Sr and Ba) nanocrystalline phosphors have been prepared through combustion technique followed by calcinations at different temperatures. The samples were structurally analyzed using XRD and FTIR techniques. Upconversion luminescence excited by 976 nm and down conversion emission excited by 266 nm have been recorded and explained in terms of Yb3 + , Yb2 + and defect centers. Both Yb3 + and Yb2 + have been detected in CA phosphors but for SA and BA, Yb2 + is not seen. The UV excitation of CA phosphor shows bright
Fig. 7. Emission spectra of calcium aluminate phosphor on UV (266 nm) excitation. The inset shows Gaussian fit of the peak.
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white light emission and indicates that this phosphor has potential for white light generating devices.
Acknowledgements Authors thankfully acknowledge Dr. H. Mishra, MMV, BHU, Varanasi for providing Ocean Optics QE65000 spectrometer for fluorescence measurements and AVH Germany for Nd: YAG laser. Authors are also thankful to DST, New Delhi for financial assistance. References [1] I.Y. Jung, Y. Cho, S.G. Lee, S.H. Sohn, D.K. Kim, Y.M. Kweon, Appl. Phys. Lett. 87 (2005) 191908. [2] K.C. Patil, M.S. Hegde, T. Rattan, in: Chemistry of Nanocrystalline Oxide Materials: Combustion Synthesis, Properties and Applications, World Scientific Pub. Co. Inc., 2008. [3] D.J. Sordelet, M. Akinc, M.L. Panchula, Y. Han, M.H. Han, J. Eur. Ceram. Soc. 14 (1994) 123. [4] B. Lei, Y. Liu, Z. Ye, C. Shi, Chin. Chem. Lett. 15 (2004) 335. [5] B. Liu, C. Shi, Z. Qi, J. Phys. Chem. Solids 67 (2006) 1674. [6] D. Jia, W.M. Yen, J. Lumin. 101 (2003) 115. [7] P.K. Bandyopadhyay, G.P. Summers, Phys. Rev. B: Condens. Matter 31 (1985) 2422. [8] A. Ibarra, F.J. Lapez, M. Jimenez de Castro, Phys. Rev. B 44 (1991) 7256. [9] T. Katsumata, R. Sakai, S. Komutro, T. Morikawa, H. Kimura, J. Cryst. Growth 869 (1999) 198. [10] P. Singh, M.S. Hegde, J. Solid State Chem. 181 (2008) 3248.
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[11] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 143 (1996) 2670. [12] Y. Mita, T. Ide, T. Katase, H. Yamamoto, J. Lumin. 72 (1997) 959. [13] H.A. Hoppe, Angew. Chem. Int. Ed. 48 (2009) 2. [14] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1687. [15] C. Joshi, K. Kumar, S.B. Rai, J. Appl. Phys. 105 (2009) 123103. [16] Y. Dwivedi, S.N. Thakur, S.B. Rai, Appl. Phys. B 89 (2007) 45. [17] V. Venkatramu, D. Falcomer, A. Speghini, M. Bettinelli, C.K. Jayasankar, J. Lumin. 128 (2008) 811. [18] S.M. Kaczmarek, A. Bensalah, G. Boulon, Opt. Mater. 28 (2006) 123. [19] S.M. Kaczmarek, T. Tsuboi, M. Ito, G. Boulon, G. Lenie, J. Phys.: Condens. Matter 17 (2005) 3771. [20] Y. Dwivedi, S.B. Rai, Opt. Mater. 31 (2008) 87. [21] S.K. Singh, K. Kumar, S.B. Rai, Appl. Phys. B 94 (2009) 165. [22] M. Lacerda, J.T.S. Irvine, F.P. Glasser, A.R. West, Nature 332 (1988) 525. [23] R.J.D. Tilley, in: Understanding Solids: The Science of Materials, Published by John Wiley and Sons, ISBN 9780470852750, 2004. [24] F. Auzel, P. Goldner, Opt. Mater. 16 (2001) 93. [25] N.A. Ivanov, E.E. Penzina, S.A. Zilov, Opt. Spectrosc. 92 (2002) 63. [26] Y. Kawamoto, Y. Kitai, N. Tokura, J. Qiu, Proceedings of SPIE, vol. 5350, SPIE, Bellingham, WA, 2004, p. 269. [27] H.P. Ho, W.W. Wong, S.Y. Wu, Opt. Eng. 42 (2003) 2349. [28] M. Mortier, F. Auzel, J. Non-Cryst. Solids 256 (1999) 361. [29] Z.A. Kazei, N.P. Kolmakova, V.I. Sokolov, Book Chapter: Information on colour centres In Book: Landolt-Bo¨rnstein—Group III Condensed Matter, vol. 27e, Springer-Verlag, Heidelberg, pp. 186–189. [30] S.M. Kaczmarek, G. Leniec, J. Typek, G. Boulon, A. Bensalah, J. Lumin. 129 (2009) 1568. [31] P.A. Chang-Kui Duan, J. Tanner, Phys. Condens. Matter 20 (2008) 215228. [32] G. Boulon, J. Alloys Compd. 451 (2008) 1. [33] M. Henke, J. Perbon, S. Kuck, J. Lumin. 87 (2000) 1049. [34] R.J. Xie, N. Hirosaki, M. Mitomo, K. Uheda, T. Suehiro, X. Xu, Y. Yamamoto, T. Sekiguchi, J. Phys. Chem. B 109 (2005) 9490. [35] I. Nicoara, M. Stef, A. Pruna, J. Cryst. Growth 310 (2008) 1470.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Up and down conversion fluorescence studies on combustion synthesized Yb3 + /Yb2 + : MO-Al2O3 (M =Ca, Sr and Ba) phosphors R.K. Verma a, Anita Rai b, K. Kumar a, S.B. Rai a,n a b
Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi, UP-221005, India P. P. N. College, Kanpur, UP, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 24 June 2009 Received in revised form 17 February 2010 Accepted 19 February 2010 Available online 24 February 2010
The ytterbium ions doped MO-Al2O3 (M = Ca, Sr and Ba) phosphors have been synthesized through combustion technique and their up and down conversion fluorescence properties have been studied and compared. The samples were calcinated at different temperatures and their FTIR and XRD spectra have shown a close relationship. With 976 nm excitation all these phosphors show cooperative upconversion emission at 488 nm from the pairs of two Yb3 + ions along with an unexpected broad upconversion band in the blue green region and has been assigned to arise from the defect centers. Contrary to this upconversion emission, calcium aluminate phosphor exhibits bright and very broad down-conversion fluorescence (FWHM E 160 nm) upon UV (266 nm) excitation due to Yb2 + ions. The inter-conversion between the 3 + and 2 + valence states of Yb ion has been observed on calcinations of samples in open atmosphere and has been correlated to the emission properties. The Yb2 + ions containing calcium aluminate phosphor has been found suitable for producing broad band light in the visible region (white light). Lifetime of the emitting states of Yb3 + and Yb2 + ions have also been measured and discussed. & 2010 Published by Elsevier B.V.
Keywords: Combustion Phosphor Rare-earth ions Upconversion
1. Introduction Though a number of optical devices already employ Rare earth (RE) ions as emitting centers with good optical efficiency, the new generation of RE doped nano-phosphors has increased their attraction due to their prospective uses. The advanced phosphor materials prepared by newer techniques e.g. combustion with organic fuel has shown improved luminescent properties. The combustion technique has been shown to produce highly homogeneous nano-crystallized fine phosphor materials [1–3]. These RE doped phosphors show better fluorescence (Stokes emission), upconversion (anti-Stokes emission) and longer duration afterglow properties as compared to materials prepared by other methods [4–12]. Frequency upconversion (UC) of incident infrared radiation into visible/ultraviolet radiation by RE doped phosphors is another topic of interest [13–17]. Ytterbium is of special interest as it can be used as a luminescence center as well as a sensitizer, for other codoped rare earths as Yb can exist in Yb2 + as well as Yb3 + states simultaneously in different crystals and glassy systems [18,19]. The heat treatment of the as-synthesized phosphors has been reported to have very significant effect on the optical properties.
n
Corresponding author. Tel.: + 91 5422307308; fax: + 91 5422369889. E-mail address: [email protected] (S.B. Rai).
0022-2313/$ - see front matter & 2010 Published by Elsevier B.V. doi:10.1016/j.jlumin.2010.02.033
It has been conjectured that the heat treatment creates small (nano-sized) crystallites in the phosphor material which also enhance the luminescence efficiency [20]. If there are different valence states of the doped ions present in the sample or an uncontrolled dopant coexists then the luminescence spectrum becomes very complex. The present work reports a comparative study of upconversion and down-conversion fluorescence properties of Yb3 + /Yb2 + ions in MAl2O4 (M= Ca, Sr, Ba) phosphors and also the effect of calcinations on the emission and fluorescence lifetime of Yb3 + / Yb2 + ions. The correlation between the FTIR and XRD spectra of the samples has also been established.
2. Experimental The phosphor samples have been prepared through combustion method using urea as an organic fuel as described earlier [21]. The following composition of the phosphor has been selected for study: 40MO þ ð60-xÞAl2 O3 þxYb2 O3 where M=Ca, Sr and Ba and x =1.0–5.0 mol% To produce crystallization in the as-synthesized samples, the samples were calcinated at different temperatures for different durations. The existence of crystallinity is checked by recording
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the powder X-ray diffraction (XRD) using CuKa radiation (1.5406 A˚). The absorption spectra of all the samples (both as-synthesized and calcinated) were monitored in reflectance mode using Perkin Elmer, Lambda-35 spectrophotometer. The upconversion luminescence was recorded using 976 nm radiation from a diode laser while the down-conversion fluorescence was recorded using the 266 nm radiation from an Nd: YAG laser and a iHR320, Horiba Jobin Yuon, spectrometer. Perkin Elmer Spectrum RX1 was used to record the Fourier Transform Infrared (FTIR) spectra of the samples. Lifetime of the Yb3 + level involved in cooperative upconversion was recorded by chopping the continuous 976 nm radiation while for the 4f125d level of Yb2 + the pulsed excitation at 266 nm was used.
3. Results and discussion Samples with different concentration of Yb2O3 were prepared and the upconversion emission intensity was found strongest for 2.5 mol% Yb2O3. All further studies therefore were made with samples having 2.5 mol% concentration. Calcination was done at 1073, 1273 and 1473 K for 2 h. 3.1. Phase content by X-ray diffraction studies Fig. 1 shows the XRD patterns for the MO-Al2O3 (M = Ca, Sr and Ba) phosphor samples; the as-synthesized and the calcinated at three different temperatures. No feature of crystallinity has been observed for the calcium aluminate (CA) sample (Fig. 1a) below 1073 K. However, when the calcination temperature is raised beyond 1073 K characteristic peaks of crystalline phase begin to appear. This may be due to the large thermal stability of the CA sample. The XRD pattern of the sample at 1473 K is shown in Fig. 1a. All the observed peaks can be assigned to Ca12Al14O33 and only a single phase appears to be present (JCPDS Cards 090413). The Ca12Al14O33 structure is cubic with a cell parameter a =11.982 A˚ and belongs to the I43d space group. The Ca12Al14O33 phase is well known as an oxygen trap and is also an electronic conductor [22]. The barium aluminate (BA) and the strontium aluminate (SA) samples show crystalline behavior even at the assynthesized stage. However in the initial stage several phases are seen to be present which tend to a single phase when calcination temperature is raised. The SA at the as-synthesized stage shows the presence of SrCO3 phase as the major constituent in the assynthesized strontium sample (Fig. 1b). However at 1473 K, all the prominent peaks correspond to the Sr3Al2O6. The presence of CO3 group is also confirmed by the TGA/DTA graph (supporting information). All the diffraction peaks could be indexed to the cubic system (Space Group: Pa3) of Sr3Al2O6 (JCPDS Cards 24–1187). The XRD results demonstrate that the crystallization of CA and SA precursor samples entirely completed at temperature below 1473 K and single Ca12Al14O33 and Sr3Al2O6 phases are observed. These temperatures are much lower than that used to prepare these by conventional solid state reaction, as shown in the phase diagram of the CaO and SrO–Al2O3 systems (above 1700 K [23]). These results show the essential advantage of the chemical route. The precursors are homogeneously mixed at the molecular level, leading to high reactivity of starting materials and a reduction in calcination temperature. Doping of Yb3 + ions has shown no effect on the crystal phases of the samples. The crystallite size was calculated using the Scherer equation t ¼ l 0:9=b Cos y where, t is the crystallite size for (h k l) planes, l the wavelength of the incident X-ray [CuKa (0.154056)], b the full width at half
Fig. 1. X-ray powder diffraction pattern of calcinated phosphors (a) calcium aluminate and (b) shows that strontium alluminate is crystalline even in as synthesized one and consist of two phases viz strontium corbonate and strontium aluminate. It is strontium aluminate which exists even at higher temperature.
maximum (FWHM) in radians and y the diffraction angle for (h k l) plane. Three most significant peaks were selected for the calculation in different samples and the average crystallite size was found to be in the range 40–50 nm.
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3.2. FTIR studies
3.3. Effect of crystallization on the FTIR spectrum
The FTIR spectra of the different samples recorded in the 4000–400 cm 1 region and for CA and SA samples are shown in Fig. 2. The presence of a broad peak in the region 3400– 3600 cm 1 clearly indicates presence of water in the samples calcinated even at such a high temperature. Similarly, the presence of absorption band near 1600 cm 1 shows that NO2 group also retains its identity. The band at 780 cm 1 is ascribed to Al–O stretching mode. The impurity peaks due to the OH , NO3 , etc. in the calcinated samples are found to decrease as the calcination temperature is increased. A decrease in concentration of these groups which have fluorescence quenching properties might be one of the reasons for increased luminescence from calcinated samples.
Fig. 2a emphasizes the difference between the FTIR spectra of as-synthesized and the calcinated samples of CA. This difference is obviously due to the development of crystallites in the calcinated samples. The spectrum of the as-synthesized sample is similar to that of mixed lattices of CaO and Al2O3. The calcinated samples on the other hand contain crystallites of Ca12Al14O33 and new absorption bands are therefore seen. These additional peaks are seen in the FTIR spectra of the sample calcinated even at 1073 K. These peaks must appear in FTIR spectra if the crystallization has taken place even minutely. The XRD pattern however does not show any crystallinity at this temperature. The crystallites appear in XRD only at higher temperatures of calcinations. The crystallites absorption is characterized by peaks in the frequency region 400–900 cm 1 and our observation indicates that FTIR spectrum can be used to detect onset of crystallization more sensitively in such samples. A comparison of the FTIR spectrum of CA sample with those for SA (Fig. 2b) indicates that the presence of different phases can also be deflected by the changing pattern of the bands in the same frequency range. The presence of different phases is also responsible for different symmetry of the environment around the rare earth ions, so the luminescence spectra are also modified. Of course the number of bands will also differ. 3.4. Upconversion luminescence studies The excitation of CA, SA and BA phosphors with 976 nm wavelength gives two emission bands; a sharp one at 488 nm and an anomalous broad one in the blue green region. In Fig. 3a upconversion spectra of CA, SA and BA phosphors are compared. The inset of Fig. 3a shows the resolved pattern of both peaks by Gaussian peak fitting. The 488 nm emission is the result of the cooperative emission from pairs of Yb3 + ions and is stronger when the distance between a pair of Yb ions lies between 3–5 A˚ (at higher concentrations) [24]. At the molar concentration of Yb3 + used in our samples such distances can appear in clusters. Cooperative emission is described by the process
Fig. 2. Fourier transform infrared spectra of calcinated phosphors (a) calcium aluminate and (b) strontium aluminate.
Yb
3þ
ðG:S:Þ þ hnð976 nmÞ-Yb
3þ
Yb
3þ
ðExc:Þ þ Yb
3þ
3þ
ðExc:Þ-Yb
ðExcitedÞ ðG:S:Þ þYb
3þ
ðG:S:Þ þ hnð488 nmÞ
The wavelength of this sharp emission is the same for all the three hosts, CA, SA and BA but its intensity shows a decrease in the order CA 4SA4BA. The larger upconversion emission intensity in CA may be related to the higher covalence of the Ca–O band and the shortest distance of Yb–O band. Two additional emission bands are observed in CA sample in visible region at 535 nm, 548 nm and are due to Er3 + as uncontrolled impurity. These peaks do not appear in the emission spectrum of SA or BA samples and in none in absorption spectra. These peaks arise in CA samples due to the cooperative energy transfer from Yb3 + to Er3 + and is probably favored in the case of CA samples [30]. The observation of the broad band emission in the upconversion mode is dependent on the host matrix e.g. for CA as host it is at 470 nm while for SA and BA phosphors its position is near 508 nm. The intensity of the band is seen to increase as the calcination temperature is raised up to 1273 K but for higher calcination temperatures the intensity decreases in all cases (Fig. 3b). The appearance of a broad band emission may either be an emission involving Yb2 + or it may be due to defect centers. The presence of Yb2 + ions has been observed only in the CA samples through its UV–vis–IR absorption spectrum (Fig. 4) where, the
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Fig. 4. Absorption spectra of calcium, strontium and barium aluminate phosphors in reflectance mode.
Fig. 3. (a) A comparison of upconversion emission spectra of calcium, strontium and barium aluminate phosphors and (b) a comparison of emission spectra of barium aluminate phosphor calcinated at different temperatures. Inset of 3(a) shows the Gaussian fit of the peak and inset of 3(b) shows intensity variation of bands with calcination temperature.
two absorption bands appear at 325 and 380 nm wavelengths [19,26–28]. The broad emission observed in all the three cases must be attributed to a source other than the Yb2 + ions. A similar broad emission has been observed by Kaczmarek et al. [18,19] in the down conversion emission spectra of single crystal containing Yb2 + and has been explained as a consequence of defect centers. The defects in the host lattice arise when an alkaline earth cation (e.g. Ca2 + ) is replaced by Yb3 + ion. In this case the charge neutrality requires that the Yb3 + ion changes into Yb2 + and a positive hole is created. The resulting color center may be responsible for the broad emission seen in the upconversion. Such emission from color centers has been reported in many materials by several workers [19,20,25–30]. The dependence of the intensities on the calcination temperature is different for the 488 and the 508 nm bands (inset of Fig. 3b). Gain in intensity of the 508 nm peak is more rapid than for the 488 nm peak. This is not surprising since more defect centers are likely to be created at higher temperatures as more Yb3 + ions are changed into Yb2 + . This should decrease the emission due to the Yb3 + at 488 nm. The
Fig. 5. A model showing the emission mechanisms of Yb2 + and Yb3 + .
observed increase in intensity is perhaps due to reduction in the fluorescence quenching impurities e.g. NO3 and OH at higher calcination temperatures. A model of the excitation and emission mechanism for the broad peak is shown in Fig. 5. The Yb3 + ion is excited by the incident radiation and then two excited ions emit at 488 nm wavelength. This radiation is absorbed by the electrons trapped at color centers creating free electron which recombine radiatively with the holes and emit at 508 nm [31]. The lifetime of the blue emission band (488 nm) has been measured as 0.515 ms in case of the CA phosphor (Fig. 6) which is nearly half of the lifetime of the 2F5/2-2F7/2 transition of Yb3 + [32] and confirming the involvement of cooperative excitations of a pair of Yb3 + ions.
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3.5. Down conversion fluorescence studies The 266 nm excitation excites luminescence only in the CA phosphor (Fig. 7). Similar to the upconversion emission, the intensity of emission at first increases with calcination temperature but decreases subsequently. The emission comprises of two bands one at 442 nm and the other at 548 nm. The inset in Fig. 7 shows a Gaussian fit to the emission band. The FWHM of peak I at 442 nm is 73 nm and for peak II at 548 nm is 128 nm. The combination of two emission bands shows perception of white light. The intensity of the two bands varies with concentration of Yb in different manners and they probably involve two different levels of Yb2 + . Henke et al. [33] have reported three overlapping
emissions with peaks at 480, 500 and 570 nm for Yb in YAlO3 matrix and attributed these to Yb2 + ions. They also remarked that the three excited levels have different lifetimes. Similar studies on Yb2 + have been conducted by Xie et al. [34]. The 548 nm emission in our case arises from excited levels of Yb2 + ion as reported earlier [25,35]. The lifetime of levels responsible for the 442 and 548 nm bands are measured to be 230 and 270 ms at 20 K temperature indicating their distinctly different origins. The absence of any luminescence from SA and BA phosphors on excitations with 266 nm can be understood from their absorption spectrum (Fig. 4). In these phosphors no absorption attributed to Yb2 + is seen. It appears that in these phosphors no stabilization of Yb2 + state takes place even after calcinations. The reaction may take place as Yb
3þ
Normal...heating
2þ
þe !Yb
But at higher temperatures, Yb2 + ions get oxidized and again converts to Yb3 + as 2þ
Yb
Oxidation
3þ
!Yb
þ e
The temperature at which the reaction rates become comparable would depend on the ion and the host matrix. The relative magnitudes of the rates for the two reactions may be responsible for the difference noted in the behavior of three phosphors.
4. Conclusions
Fig. 6. Decay curve of Yb3 + ion on 976 nm excitation. The fitting of the curve shows single exponential decay.
Ytterbium doped MO-Al2O4 (M =Ca, Sr and Ba) nanocrystalline phosphors have been prepared through combustion technique followed by calcinations at different temperatures. The samples were structurally analyzed using XRD and FTIR techniques. Upconversion luminescence excited by 976 nm and down conversion emission excited by 266 nm have been recorded and explained in terms of Yb3 + , Yb2 + and defect centers. Both Yb3 + and Yb2 + have been detected in CA phosphors but for SA and BA, Yb2 + is not seen. The UV excitation of CA phosphor shows bright
Fig. 7. Emission spectra of calcium aluminate phosphor on UV (266 nm) excitation. The inset shows Gaussian fit of the peak.
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white light emission and indicates that this phosphor has potential for white light generating devices.
Acknowledgements Authors thankfully acknowledge Dr. H. Mishra, MMV, BHU, Varanasi for providing Ocean Optics QE65000 spectrometer for fluorescence measurements and AVH Germany for Nd: YAG laser. Authors are also thankful to DST, New Delhi for financial assistance. References [1] I.Y. Jung, Y. Cho, S.G. Lee, S.H. Sohn, D.K. Kim, Y.M. Kweon, Appl. Phys. Lett. 87 (2005) 191908. [2] K.C. Patil, M.S. Hegde, T. Rattan, in: Chemistry of Nanocrystalline Oxide Materials: Combustion Synthesis, Properties and Applications, World Scientific Pub. Co. Inc., 2008. [3] D.J. Sordelet, M. Akinc, M.L. Panchula, Y. Han, M.H. Han, J. Eur. Ceram. Soc. 14 (1994) 123. [4] B. Lei, Y. Liu, Z. Ye, C. Shi, Chin. Chem. Lett. 15 (2004) 335. [5] B. Liu, C. Shi, Z. Qi, J. Phys. Chem. Solids 67 (2006) 1674. [6] D. Jia, W.M. Yen, J. Lumin. 101 (2003) 115. [7] P.K. Bandyopadhyay, G.P. Summers, Phys. Rev. B: Condens. Matter 31 (1985) 2422. [8] A. Ibarra, F.J. Lapez, M. Jimenez de Castro, Phys. Rev. B 44 (1991) 7256. [9] T. Katsumata, R. Sakai, S. Komutro, T. Morikawa, H. Kimura, J. Cryst. Growth 869 (1999) 198. [10] P. Singh, M.S. Hegde, J. Solid State Chem. 181 (2008) 3248.
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