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Journal of Luminescence 130 (2010) 1248–1253

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Journal of Luminescence

0022-23

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jlumin

Up and down conversion fluorescence studies on combustion synthesizedYb3+/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 Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi, UP-221005, Indiab P. P. N. College, Kanpur, UP, India

a r t i c l e i n f o

Article history:

Received 24 June 2009

Received in revised form

17 February 2010

Accepted 19 February 2010Available online 24 February 2010

Keywords:

Combustion

Phosphor

Rare-earth ions

Upconversion

13/$ - see front matter & 2010 Published by

016/j.jlumin.2010.02.033

esponding author. Tel.: +91 5422307308; fax

ail address: sbrai49@yahoo.co.in (S.B. Rai).

a b s t r a c t

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 (FWHME160 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.

1. Introduction

Though a number of optical devices already employ Rare earth(RE) ions as emitting centers with good optical efficiency, thenew generation of RE doped nano-phosphors has increased theirattraction due to their prospective uses. The advanced phosphormaterials prepared by newer techniques e.g. combustion withorganic fuel has shown improved luminescent properties. Thecombustion technique has been shown to produce highlyhomogeneous nano-crystallized fine phosphor materials [1–3].

These RE doped phosphors show better fluorescence (Stokesemission), upconversion (anti-Stokes emission) and longer dura-tion afterglow properties as compared to materials prepared byother methods [4–12]. Frequency upconversion (UC) of incidentinfrared radiation into visible/ultraviolet radiation by RE dopedphosphors is another topic of interest [13–17]. Ytterbium is ofspecial interest as it can be used as a luminescence center as wellas a sensitizer, for other codoped rare earths as Yb can exist inYb2 + as well as Yb3 + states simultaneously in different crystalsand glassy systems [18,19].

The heat treatment of the as-synthesized phosphors has beenreported to have very significant effect on the optical properties.

Elsevier B.V.

: +91 5422369889.

It has been conjectured that the heat treatment creates small(nano-sized) crystallites in the phosphor material which alsoenhance the luminescence efficiency [20]. If there are differentvalence states of the doped ions present in the sample or anuncontrolled dopant coexists then the luminescence spectrumbecomes very complex.

The present work reports a comparative study of upconversionand down-conversion fluorescence properties of Yb3 +/Yb2 + ionsin MAl2O4 (M=Ca, Sr, Ba) phosphors and also the effect ofcalcinations on the emission and fluorescence lifetime of Yb3 +/Yb2 + ions. The correlation between the FTIR and XRD spectra ofthe samples has also been established.

2. Experimental

The phosphor samples have been prepared through combus-tion method using urea as an organic fuel as described earlier[21]. The following composition of the phosphor has beenselected for study:

40MOþð60-xÞAl2O3þxYb2O3

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 differentdurations. 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-synthesizedand calcinated) were monitored in reflectance mode using PerkinElmer, Lambda-35 spectrophotometer. The upconversion lumi-nescence was recorded using 976 nm radiation from a diode laserwhile the down-conversion fluorescence was recorded using the266 nm radiation from an Nd: YAG laser and a iHR320, HoribaJobin Yuon, spectrometer. Perkin Elmer Spectrum RX1 was usedto record the Fourier Transform Infrared (FTIR) spectra of thesamples. Lifetime of the Yb3 + level involved in cooperativeupconversion was recorded by chopping the continuous 976 nmradiation while for the 4f125d level of Yb2 + the pulsed excitationat 266 nm was used.

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.

3. Results and discussion

Samples with different concentration of Yb2O3 were preparedand the upconversion emission intensity was found strongest for2.5 mol% Yb2O3. All further studies therefore were made withsamples having 2.5 mol% concentration. Calcination was done at1073, 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, Srand Ba) phosphor samples; the as-synthesized and the calcinatedat three different temperatures. No feature of crystallinity hasbeen observed for the calcium aluminate (CA) sample (Fig. 1a)below 1073 K. However, when the calcination temperature israised beyond 1073 K characteristic peaks of crystalline phasebegin to appear. This may be due to the large thermal stability ofthe CA sample. The XRD pattern of the sample at 1473 K is shownin Fig. 1a. All the observed peaks can be assigned to Ca12Al14O33

and only a single phase appears to be present (JCPDS Cards 09-0413). The Ca12Al14O33 structure is cubic with a cell parametera=11.982 A and belongs to the I43d space group. The Ca12Al14O33

phase is well known as an oxygen trap and is also an electronicconductor [22]. The barium aluminate (BA) and the strontiumaluminate (SA) samples show crystalline behavior even at the as-synthesized stage. However in the initial stage several phases areseen to be present which tend to a single phase when calcinationtemperature is raised. The SA at the as-synthesized stage showsthe presence of SrCO3 phase as the major constituent in the as-synthesized strontium sample (Fig. 1b). However at 1473 K, allthe prominent peaks correspond to the Sr3Al2O6. The presence ofCO3 group is also confirmed by the TGA/DTA graph (supportinginformation). All the diffraction peaks could be indexed to thecubic system (Space Group: Pa3) of Sr3Al2O6 (JCPDS Cards24–1187). The XRD results demonstrate that the crystallizationof CA and SA precursor samples entirely completed attemperature below 1473 K and single Ca12Al14O33 and Sr3Al2O6

phases are observed. These temperatures are much lower thanthat used to prepare these by conventional solid state reaction, asshown in the phase diagram of the CaO and SrO–Al2O3 systems(above 1700 K [23]). These results show the essential advantage ofthe chemical route. The precursors are homogeneously mixed atthe molecular level, leading to high reactivity of starting materialsand a reduction in calcination temperature. Doping of Yb3 + ionshas shown no effect on the crystal phases of the samples. Thecrystallite size was calculated using the Scherer equation

t¼ l� 0:9=bCosy

where, t is the crystallite size for (h k l) planes, l the wavelengthof the incident X-ray [CuKa (0.154056)], b the full width at half

maximum (FWHM) in radians and y the diffraction angle for(h k l) plane. Three most significant peaks were selected for thecalculation in different samples and the average crystallite sizewas found to be in the range �40–50 nm.

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3.2. FTIR studies

The FTIR spectra of the different samples recorded in the4000–400 cm�1 region and for CA and SA samples are shownin Fig. 2. The presence of a broad peak in the region 3400–3600 cm�1 clearly indicates presence of water in the samplescalcinated even at such a high temperature. Similarly, thepresence of absorption band near 1600 cm�1 shows that NO2

group also retains its identity. The band at 780 cm�1 is ascribed toAl–O stretching mode. The impurity peaks due to the OH� , NO3

� ,etc. in the calcinated samples are found to decrease as thecalcination temperature is increased. A decrease in concentrationof these groups which have fluorescence quenching propertiesmight be one of the reasons for increased luminescence fromcalcinated samples.

Fig. 2. Fourier transform infrared spectra of calcinated phosphors (a) calcium

aluminate and (b) strontium aluminate.

3.3. Effect of crystallization on the FTIR spectrum

Fig. 2a emphasizes the difference between the FTIR spectra ofas-synthesized and the calcinated samples of CA. This difference isobviously due to the development of crystallites in the calcinatedsamples. The spectrum of the as-synthesized sample is similar tothat of mixed lattices of CaO and Al2O3. The calcinated samples onthe other hand contain crystallites of Ca12Al14O33 and newabsorption bands are therefore seen. These additional peaks areseen in the FTIR spectra of the sample calcinated even at 1073 K.These peaks must appear in FTIR spectra if the crystallization hastaken place even minutely. The XRD pattern however does notshow any crystallinity at this temperature. The crystallites appearin XRD only at higher temperatures of calcinations. The crystal-lites absorption is characterized by peaks in the frequency region400–900 cm�1 and our observation indicates that FTIR spectrumcan be used to detect onset of crystallization more sensitively insuch samples.

A comparison of the FTIR spectrum of CA sample with those forSA (Fig. 2b) indicates that the presence of different phases can alsobe deflected by the changing pattern of the bands in the samefrequency range. The presence of different phases is alsoresponsible for different symmetry of the environment aroundthe 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 nmwavelength gives two emission bands; a sharp one at 488 nm andan anomalous broad one in the blue green region. In Fig. 3aupconversion spectra of CA, SA and BA phosphors are compared.The inset of Fig. 3a shows the resolved pattern of both peaks byGaussian peak fitting. The 488 nm emission is the result of thecooperative emission from pairs of Yb3 + ions and is stronger whenthe distance between a pair of Yb ions lies between 3–5 A(at higher concentrations) [24]. At the molar concentration ofYb3 + used in our samples such distances can appear in clusters.Cooperative emission is described by the process

Yb3þðG:S:Þþhnð976nmÞ-Yb3þ

ðExcitedÞ

Yb3þðExc:ÞþYb3þ

ðExc:Þ-Yb3þðG:S:ÞþYb3þ

ðG:S:Þþhnð488nmÞ

The wavelength of this sharp emission is the same for all thethree hosts, CA, SA and BA but its intensity shows a decrease inthe order CA4SA4BA. The larger upconversion emissionintensity in CA may be related to the higher covalence of theCa–O band and the shortest distance of Yb–O band. Twoadditional emission bands are observed in CA sample in visibleregion at 535 nm, 548 nm and are due to Er3 + as uncontrolledimpurity. These peaks do not appear in the emission spectrum ofSA or BA samples and in none in absorption spectra. These peaksarise in CA samples due to the cooperative energy transfer fromYb3 + to Er3 + and is probably favored in the case of CA samples[30].

The observation of the broad band emission in the upconver-sion mode is dependent on the host matrix e.g. for CA as host it isat 470 nm while for SA and BA phosphors its position is near508 nm. The intensity of the band is seen to increase as thecalcination temperature is raised up to 1273 K but for highercalcination temperatures the intensity decreases in all cases(Fig. 3b). The appearance of a broad band emission may either bean emission involving Yb2 + or it may be due to defect centers. Thepresence of Yb2 + ions has been observed only in the CA samplesthrough its UV–vis–IR absorption spectrum (Fig. 4) where, the

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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.

Fig. 4. Absorption spectra of calcium, strontium and barium aluminate phosphors

in reflectance mode.

Fig. 5. A model showing the emission mechanisms of Yb2+ and Yb3+.

R.K. Verma et al. / Journal of Luminescence 130 (2010) 1248–1253 1251

two absorption bands appear at 325 and 380 nm wavelengths[19,26–28]. The broad emission observed in all the three casesmust be attributed to a source other than the Yb2 + ions. A similarbroad emission has been observed by Kaczmarek et al. [18,19] inthe down conversion emission spectra of single crystal containingYb2 + 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 chargeneutrality requires that the Yb3 + ion changes into Yb2 + and apositive hole is created. The resulting color center may beresponsible for the broad emission seen in the upconversion.Such emission from color centers has been reported in manymaterials by several workers [19,20,25–30]. The dependence ofthe intensities on the calcination temperature is different for the488 and the 508 nm bands (inset of Fig. 3b). Gain in intensityof the 508 nm peak is more rapid than for the 488 nm peak. This isnot surprising since more defect centers are likely to be created athigher temperatures as more Yb3 + ions are changed into Yb2 +.This should decrease the emission due to the Yb3 + at 488 nm. The

observed increase in intensity is perhaps due to reduction in thefluorescence quenching impurities e.g. NO�3 and OH� at highercalcination temperatures. A model of the excitation and emissionmechanism for the broad peak is shown in Fig. 5. The Yb3 + ion isexcited by the incident radiation and then two excited ions emitat 488 nm wavelength. This radiation is absorbed by the electronstrapped at color centers creating free electron which recombineradiatively with the holes and emit at 508 nm [31].

The lifetime of the blue emission band (488 nm) has beenmeasured as 0.515 ms in case of the CA phosphor (Fig. 6) which isnearly half of the lifetime of the 2F5/2-

2F7/2 transition of Yb3 +[32]and confirming the involvement of cooperative excitations of apair of Yb3 + ions.

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3.5. Down conversion fluorescence studies

The 266 nm excitation excites luminescence only in the CAphosphor (Fig. 7). Similar to the upconversion emission, theintensity of emission at first increases with calcination tempera-ture but decreases subsequently. The emission comprises of twobands one at 442 nm and the other at 548 nm. The inset in Fig. 7shows a Gaussian fit to the emission band. The FWHM of peak I at442 nm is 73 nm and for peak II at 548 nm is 128 nm. Thecombination of two emission bands shows perception of whitelight. The intensity of the two bands varies with concentration ofYb in different manners and they probably involve two differentlevels of Yb2 +. Henke et al. [33] have reported three overlapping

Fig. 6. Decay curve of Yb3 + ion on 976 nm excitation. The fitting of the curve

shows single exponential decay.

Fig. 7. Emission spectra of calcium aluminate phosphor on UV (2

emissions with peaks at 480, 500 and 570 nm for Yb in YAlO3

matrix and attributed these to Yb2 + ions. They also remarked thatthe three excited levels have different lifetimes. Similar studies onYb2 + have been conducted by Xie et al. [34]. The 548 nm emissionin our case arises from excited levels of Yb2 + ion as reportedearlier [25,35]. The lifetime of levels responsible for the 442 and548 nm bands are measured to be 230 and 270 ms at 20 Ktemperature indicating their distinctly different origins.

The absence of any luminescence from SA and BA phosphorson excitations with 266 nm can be understood from theirabsorption spectrum (Fig. 4). In these phosphors no absorptionattributed to Yb2 + is seen. It appears that in these phosphorsno stabilization of Yb2 + state takes place even after calcinations.The reaction may take place as

Yb3þþe���������!

Normal...heatingYb2þ

But at higher temperatures, Yb2 + ions get oxidized and againconverts to Yb3 + as

Yb2þ��������!

OxidationYb3þ

þe�

The temperature at which the reaction rates become compar-able would depend on the ion and the host matrix. The relativemagnitudes of the rates for the two reactions may be responsiblefor the difference noted in the behavior of three phosphors.

4. Conclusions

Ytterbium doped MO-Al2O4 (M=Ca, Sr and Ba) nanocrystallinephosphors have been prepared through combustion techniquefollowed by calcinations at different temperatures. The sampleswere structurally analyzed using XRD and FTIR techniques.Upconversion luminescence excited by 976 nm and downconversion emission excited by 266 nm have been recorded andexplained 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

66 nm) excitation. The inset shows Gaussian fit of the peak.

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white light emission and indicates that this phosphor haspotential for white light generating devices.

Acknowledgements

Authors thankfully acknowledge Dr. H. Mishra, MMV, BHU,Varanasi for providing Ocean Optics QE65000 spectrometer forfluorescence measurements and AVH Germany for Nd: YAG laser.Authors are also thankful to DST, New Delhi for financialassistance.

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 OxideMaterials: Combustion Synthesis, Properties and Applications, World Scien-tific 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.

[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-Bornstein—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.