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Synthesis and luminescence properties of an (Er2Te4O11) nanocrystals dispersed highly efficient upconverting lead free tellurite glass
Chemical Physics Letters 474 (2009) 331–335
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Synthesis and luminescence properties of an (Er2Te4O11) nanocrystals dispersed highly efficient upconverting lead free tellurite glass R. Debnath *, A. Ghosh, S. Balaji Central Glass and Ceramic Research Institute, Kolkata 700 032, India
a r t i c l e
i n f o
Article history: Received 3 December 2008 In final form 4 May 2009 Available online 7 May 2009
a b s t r a c t Synthesis, and luminescence properties of Er3+ in an Er2Te4O11 nanocrystals dispersed tellurite glass under direct and upconversion excitation are reported. The nanocrystals which could be grown by ageing or subjecting the glass to stress, were grown, in this case, by ageing and were characterized by X-ray, FESEM and TEM studies. The growth of nanocrystals in the matrix significantly enhances both the normal and upconversion luminescence efficiency of the glass. The reason of such enhancement of efficiency is explained and the photo-physics involved in the upconversion luminescence process has been proposed. High upconversion luminescence efficiency of the glass shows the prospect of its use as solar IR concentrator and NIR sensor. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Rare earth ion activated glasses and crystals [1–28], constitute a large fraction of components of the present day optics and optoelectronics. They are extensively used in solid state lasers [1– 3,18–21], optical communications [1–8], optical signal amplification [1–20], energy upconversion [4–25], biological labeling [22– 25], anti-Stokes luminescence cooling [26], IR quantum informatics [27] etc. In the recent years upconversion luminescence properties of such materials i.e. emitting light of higher energy (preferably in the visible range) on the absorption of light of lower energy (normally in the NIR range) has received special attention because of their prospective use in solar NIR concentration for photovoltaic exploitation [28], IR sensing and biological labeling. To designate an affected tissue inside a biological system, fluorescent markers are frequently used. Excitation bands of such markers in many cases, lie in the UV region of light which some times, may be harmful to the tissues. In that respect, NIR excited markers are safe. Upconverting nanocrystals like hexagonal NaYF4: Er3+, Yb3+ [22,23] have already been shown to be very efficient and prospective material for this purpose. In case of other applications like solar IR concentration or IR sensing big panels either wholly made up of upconverting materials or panels dispersed with upconverting crystals are preferred. In that context nanocrystals dispersed upconverting glasses are expected to be the ideal candidate both in terms of efficiency and larger area coverage. Amongst the various types of glasses, oxides glasses are superior to others in respect of their stability. Considering again the fact that the phonon structure of a host plays an important role in determining the lumines* Corresponding author. Fax: +91 033 2473 0957. E-mail address: [email protected] (R. Debnath). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.05.001
cence efficiency and the excited state dynamics of an incorporated activator ion, an oxide glass with low phonon energy should be suitable for use as a host to reduce the nonradiative energy loss of the excited activator ions through multi phonon relaxation (MPR). Of the various known oxide glasses, tellurite glasses have considerably low phonon energy, high refractive index and greater power to dissolve high concentration [29] of rare earth ions. So rare earth activated tellurite glasses are thought to be the ideal material for development of large area solar concentrator/IR sensor. Choice of appropriate rare earth ion to achieve efficient upconversion luminescence, on the other hand, requires [2] consideration of availability of suitable energy levels disposition in the energy level spectrum of the ion so that the processes like, excited state co-operative energy transfer (ESET) and excited state absorption (ESA) can occur. In that respect Er3+ ion is found to be an excellent candidate because of existence of a number of suitable near infrared (NIR) excited states (e.g. 4I11/2 and 4I13/2) with long life time, in its energy level spectrum. Relatively longer life-time helps growth of population of ions in these excited states which in turn facilitates both the (ESET) and (ESA) processes. In the present Letter, we report synthesis and studies of normal and upconversion luminescence properties of an (Er2Te4O11) nanocrystals dispersed fluoro-tellurite glass. 2. Experimental The Er3+ doped tellurite glass of composition (mol%) 80TeO2– 15(BaF2 + BaO)–4La2O3–1Er2O3, and its base glass i.e. devoid of Er3+, were prepared by melting at 700–750 °C in an electrical furnace using platinum crucible. The size of batch each time was approximately 50 g. All the chemicals used were of AR grade
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(Sigma-Aldrich). Glasses were annealed for 12 h at 230–250 °C. The (Er2Te4O11) nanocrystals in the glass were grown by ageing the as cast glass under ambient atmosphere. The rate of growth of nanocrystals was found to be relatively fast and ageing of the as cast glass for one week was found to be sufficient to obtain maximum extent of growth of the nanocrystals. The density of the glass was measured by usual method (D = 5.729 gms/cc). Concentration of the Er3+ in the glass was calculated to be 4.12 1020 ions cm3. The X-ray powder diffraction spectrum of the powdered glass was recorded in an X-ray diffractometer (model: PW1710 BASED, make: Philips) using a copper anode. A Field Emission Scanning Electron Microscope (FESEM, Supra-35 VP, make: Carlzeiss SMT, Germany) was used to record the micrographs of the samples. The transmission electron micrographs (TEM) of the nanocrystals in the glass were recorded in a 200 kV HR TEM, model: JEM2011, make: JEOL, Japan. To record the absorption spectrum of Er3+ in the glass, plateshaped polished sample were prepared using the Er3+ doped aged sample. The spectrum was recorded at 303 K in an UV–Vis-NIR absorption spectrophotometer (Shimadzu, Japan, model: 10001). A polished plate-shaped base glass sample of same thickness was used as the reference. The emission and excitation spectra were recorded at room temperature in a Perkin Elmer LS55 Spectrofluorimeter. To see the effect of growth of (Er2Te4O11) nanocrystals on the luminescence properties of the glass, a quantitative study of the emission of the glass before and after the growth of nanocrystals was made by using front surface illumination technique and keeping the sample position, emission and excitation slits as well as the lamp voltage unaltered. The quantitative upconversion luminescence of the glass was recorded by using a fibre pig-tailed 976 nm diode laser with variable output power up to a maximum of 200 mwatts, (Thor Lab, USA) as an excitation source and fixing the sample in a sample holder at a definite distance from the tip of the laser output fibre. Other parameters were maintained similar to those described before. The excited state dynamics of the 4S3/2 green luminescent state of Er3+ was studied in a luminescence lifetime measurement setup (model: Fluorocube, IBH, UK) using multichannel analyzer technique. A Spectra-LED of 372 nm with pulse duration of 100 ls and repetition rate of 100 Hz, was used as the excitation source. To avoid contamination of the excitation profile with the sample’s decay, only the photon-channels of time region beyond the pulsewidth of the excitation pulse were considered to study of the nature of the decay.
Fig. 1. Absorption spectrum of the base glass (. . .); as recorded absorption spectrum of Er3+ doped glass (- - -); and the base glass corrected absorption spectrum of Er3+ in the glass (___). [Sample thickness in both the cases d 2.15 mm].
The base glass corrected absorption spectrum exhibits a number of distinct strong absorption bands of Er3+ in the near UV and Vis-NIR region namely at 376, 405, 486, 519, 542, 650, 798, 974 and 1532 nm. Comparing the energy and intensity of the bands with those of the reported similar bands of Er3+ ion [1–28] the corresponding associated transitions of the bands can be identified as transitions from the 4I15/2 ground state to 4G11/2, 2H9/2, 4F7/2, 2H11/2, 4 S3/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/2 excited states of Er3+ ions of the glass. Since the tellurite glasses are prone to devitrefication, to check the amorphous characteristics of the glass, microscopic studies were carried out using samples of different stages of ageing. Fig. 2 shows the FESEM picture of a glass sample aged for one week. It shows existence of nanoparticles of size 50 nm in the matrix. To reveal crystalline phase as well as the chemical composition of the nanoparticles, X-ray powder diffraction spectrum of the glass was recorded. This is shown in Fig. 3. As evident from the spectrum, that although the major part of the glass is in the amorphous state, there is definitely a fraction which is crystalline. The crystalline phase, exhibits one broad diffraction band peaking in between 2h = 26 and 28.5° along with a broad hallow in the higher 2h region. Careful scrutiny of the more expanded version of the spectrum shows that the broad peak is actually a composite band of three relatively strong lines with respective ‘d’ values 3.25, 3.16 and 3.09 A°. Two weak peak positions are also located at ‘d’ = 3.72 and 2.96 A°. Consideration of the network structure and
3. Results and discussion 3.1. Absorption spectrum and microscopic study of the glass Fig. 1 shows the absorption spectrum of Er3+ activated tellurite glass and that of its base-glass along with the base glass corrected absorption spectrum of Er3+ in the glass. Consideration of the absorption characteristics of the basic units of the glass composition namely BaF2, BaO, La2O3 and different oxides of tellurium, the onset of UV absorption edge of the base glass from around 400 nm region may safely be attributed to the result of collective absorption of all forms of tellurium oxide groups, namely, [Te4O11]6, [Te3O9]6 and [TeO6]6 present in the glass [30–32]. In case of Er3+ doped glass some of the Er3+ bands (namely 4 I15/2 ? 2G9/2, 4I15/2 ? 4G11/2 transitions) lie in this near UV region, and hence absorption intensity in this region in the case of Er3+ doped glass gets enhanced due to additive absorption of Er3+ and the oxides of tellurium of the glass. This is clearly evident from the as recorded absorption spectrum of the Er3+ doped glass.
Fig. 2. FESEM picture of the glass after the growth of the nanocrystals.
R. Debnath et al. / Chemical Physics Letters 474 (2009) 331–335
Fig. 3. Powder X-ray diffraction spectrum of the Er3+-doped glass after the growth of the nanocrystals.
chemical composition of the glass, suggests that crystalline phases like Er2[Te4O11] [30], Er2[TeO6] [31] and/or La2[Te3O9] [32] may grow in the glass during the period of ageing. A comparison of the ‘d’ values and the relative intensities of the observed diffraction lines with those of the possible different phases mentioned above shows, that the observed lines are the result of overlapping of the main XRD lines of La2Te3O9 [Cubic, d = 3.75 (I 20), d = 3.22 (I 80), d = 3.14 (I 100), d = 3.09 (I 80)] [32] and of Er2Te4O11 [monoclinic, d = 3.81 (I 37, h, k, l = 112, 004), d = 3.25 (I 100, h, k, l = 212), d = 2.95 (I 46, h, k, l = 311)] [31]. Transmission electron micrograph (TEM) of the nanoparticles is shown in Fig. 4. It shows that each nanoparticle of the glass consists of several nanocrystallites of size 10–12 nm. It is however difficult to estimate from the XRD-spectrum, the proportion of the two phases in the glass but it can be argued that since the concentration of lanthanum oxide in the glass is four times higher than that of erbium oxide, possibility of formation of La2Te3O9 phase should be higher than that of Er2Te4O11 phase. However, as far as the role of the nanocrystals in the phenomenon of enhancement of luminescence efficiency of Er3+ is concerned, we are interested only in the Er2Te4O11 phase. 3.2. Growth of (Er2Te4O11) nanocrystals and the luminescence efficiency of the glass The Er3+ activated glass under 376 nm light excitation (4I15/2 ? 2G11/2 absorption of Er3+) exhibits a bright green lumines-
Fig. 4. TEM picture of the nanocrystals in the glass.
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cence due to the [(2H11/2, 4S3/2) ? 4I15/2] transition of Er3+. To see the effect of growth of (Er2Te4O11) nanocrystals in the glass on the luminescence properties of Er3+, a quantitative study on the efficiency of the green emission under 376 nm – excitation was made before and after the growth of nanocrystals. This is shown in Fig. 5. Surprisingly, it is observed that the growth of nanocrystals, significantly enhances the luminescence efficiency of the glass. An estimate of the ratio of integrated emissions (IE) of the sample measured quantitatively before and after the growth of the nanocrystals, shows that the efficiency is enhanced almost three times. In addition to the enhancement of luminescence a significant change in the spectral resolution of the Stark split components as well as a reversal in their peak intensities, are also noted. The results show that the Er3+ ions in the glass experience a stronger and regular crystal field after the growth of the nanocrystals. Before the Er2Te4O11 nanocrystals are grown, the erbium ions remain randomly dispersed in the in the interstitial of the network structure of the glass. After the formation of the nanocrystal, Er3+ ions of the nanocrystals have a well defined surrounding and hence it is expected to experience a well defined crystal field resulting well resolved stark split of the emission bands. The result also indicates that the Er3+ ions are directly associated with the nanocrystals. It is clear from the absorption spectrum of the base glass that the UV absorption edge of the glass onsets right from 400 nm. As stated before that the onset of UV absorption edge of the base glass from 400 nm region results from collective absorption of different forms of tellurium oxide groups present in the glass [30–32] and as a result the Er3+ doped glass, exhibits greater absorption intensity in the spectral region towards UV beyond 400 nm, due to additive absorption of the glass and the Er3+ ions. After the growth of (Er2Te4O11) nanocrystals if the band gap of the nanocrystal falls in the similar wave length region, excitation of the glass under 376 nm light may lead to excitation of Er3+ ions remaining within the nanocrystals (through the band gap) as well as those remaining in the glass phase (through direct absorption) causing a significant enhancement in luminescence efficiency. The decay profile of the 4S3/2 state measured by monitoring the 548 nm green emission is shown in Fig. 6. An analysis of the decay shows that it has bi-exponential character with a faster component of lifetimes of 16 ls and a slower component of 39 ls. The life time values obtained are found to be consistent with those reported earlier for the 4S3/2 state by others [13,20]. The bi-exponential nature of the decay is in fact, related to two different groups of Er3+ ions in the glass one staying within the crystalline frame work of the nanocrystal and the other randomly distributed in the interstial space of the network structure of the tellurite glass.
Fig. 5. Green luminescence spectra due to 4S3/2 ? 4I15/2 transition of Er3+ in the glass before (___) and after (- - -) the growth of nanocrystals.
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Fig. 6. Excitation and decay profile of the 4S3/2 excited state of Er3+ in the glass. Inset: Log plot of the decay profile of 4S3/2 excited state; kMoni = 546 nm.
3.3. Upconversion luminescence and the effect of variation excitation power The glass also exhibits the same green emission when it is excited through upconversion route (4I15/2 ? 4I11/2 transition of Er3+) using 976 nm diode laser. Fig. 7, presents upconversion emission spectra of the glass as a function of laser power. A bright upconversion green luminescence from Er3+ peaking at 532 and 550 nm due its (2H11/2, 4S3/2) ? 4I15/2 transitions is observed along with a weak red emission peaking at 690 nm due to (F9/2 ? 4I15/2) transition. The intensity of the red emission here is found to be enhanced compared to that obtained under direct excitation. The upconversion green luminescence of the glass is very bright and can be seen with naked eye even under excitation of 07 mwatts power of laser and it looks like a bright green light source when under excitation of 200 mwatts power of the laser (shown in the
right top of Fig. 7). The upconversion luminescence efficiency of the glass is thus also found to be significantly enhanced due to growth of the (Er2Te4O11) nanocrystals in the matrix. Since the base glass is optically transparent beyond 420 nm, one can expect better penetration of 976 nm light in the glass and hence better a excitation of Er3+ ions of the nanocrystals which have higher value of surface to volume ratio. More over, since the nanocrystals consist of erbium related compound e.g. (Er2Te4O11) concentration of Er3+ ions in the crystals should be more than that in the glassy part. Thus an overall enhancement of efficiency of upconversion emission is highly likely due to the increased excitation efficiency of Er3+ ions. The glass thus seems to be potential material for use as large area NIR solar concentrator and NIR sensor. In case of an upconversion luminescence, upconversion luminescence intensity Iupc is related to the intensity of the excitation source Iexci by the following relation. Iupc / Im exci , ‘m’ is the number of IR photons absorbed per visible photon emitted. A plot of log Iupc vs. log Iexci, should therefore, give a straight line with a slope = m. The inset (left) of the Fig. 7, shows such plots for different upconversion emissions of Er3+ in the glass. The values of ‘m’ obtained for 532, 550 and 690 nm emissions are 2.08, 2.11, and 2.00. The results indicate that all the upconversion emission processes are biphotonic in nature. After population of the Er3+ ions in the 4I11/2 state upon excitation with the 976 nm laser, a fraction of the Er3+ ions may relax to the immediate low lying 4I13/2 state but major fraction of the ions because of the relatively long life time [16] of the 4I11/2 state gets the opportunity of absorbing second photon through excited state absorption (ESA) and reaches the 2H11/2 and subsequently relaxes to the ground state emitting the green luminescence through (2H11/2, 4S3/2) ? 4I15/2 transitions. As the laser power is increased the process of excited state absorption (ESA) also increases and consequently intensity of the
Fig. 7. Upconversion luminescence of Er3+ under 978 nm laser excitation (4I15/2 ? 4I11/2) as a function of laser power. Inset (left): log – plots of different luminescence peak intensities vs. laser power. Inset (right): a photo-graph of the upconversion luminescence of the glass under the 980 nm laser excitation.
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The upconversion luminescence property of the glass was also studied using a 976 nm diode laser. An unusually intense green upconversion luminescence of the glass demonstrated that the glass is a prospective material for use as large area solar concentrator as well as NIR sensor. Acknowledgement Present work was carried out under the sponsorship of CSIR, India, through the OLP Project No. OLP-0210 of CGCRI. References
Fig. 8. Energy level diagram and possible scheme of green and red upconversion luminescence of Er3+ under 976 nm excitation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
green luminescence increases. Since the concentration of Er3+ in the present glass is 1 mol% which is sufficiently high, an additional phenomenon of co-operative energy transfer (ET) should also occur through cross relaxation, enhancing further the intensity of the green luminescence. The life time of 4I13/2 state is also high few milliseconds [3,5,6] so the fraction of ions that comes to this level from the upper 4I11/2 level through nonradiative relaxation can also undergo excited state absorption (ESA) of second 976 nm photons. Energy acquired by an Er3+ ion of 4I13/2 state through this process of excited state absorption (ESA), however, lies in between 4S3/2 and 4F9/2 levels, the ions therefore, possibly reach to some higher phononic level above the 4F9/2 state and subsequently cascade to the red 4F9/2 emitting state and relax to the ground state through 4F9/2 ? 4I15/2 transition creating red luminescence. Photo-physical path ways proposed, respectively, for the green and red upconversion luminescence are shown in Fig. 8. 4. Conclusion Studies on the microscopic and luminescence properties of an Er3+ ion (1 mol%) doped lead free fluoro–barium tellurite glass showed the efficiency of luminescence of the glass get enhanced by almost three times after nanocrystalline phase of (Er2Te4O11) is grown in the matrix.
[1] L.F. Johnson, H.J. Guggenheim, Appl. Phys. Lett. 19 (1971) 44. [2] AlexanderA Kaminskii, Laser Crystals (Their Physics and Properties), second edn., Springer-Verlag, Berlin, Heidelberg, New York, 1990. [3] A. Mori, Y. Ohishi, S. Sudo, Electron. Lett. 33 (1997) 863. [4] Y. Ding et al., Opt. Mater. 15 (2000) 123. [5] R. Balda, O. Sanz, J. Goazalo, M. Zimenez de Castro, A. Perea, J. M. Fernandez– Navarro, C.N. Afanso, J. Fernandez, in: Proceedings of SPIE, No. 4990, 2003, p. 30. [6] H. Lin, G. Meredith, S. Jiang, X. Peng, Tao Luo N. Peghambarian, E. Y-Bun Pun, J. Appl. Phys. 93 (2003) 186. [7] H. Chen, Y.H. Liu, Y.F. Zhou, Q.Y. Zhang, Z.H. Jiang, J. Non-Cryst. Solids 351 (2005) 3060. [8] I.R. Martin, P. Velez, V.D. Rodriguez, U.R. Rodriguez-Mendoza, V. Lavin, Spectrochim. Acta, Part-A 55 (1999) 935. [9] L.A. Diaz-Torres, E. Dela Rossa –Cruz, P. Salas, C. Angeles Chavez, J. Phys. D, Appl. Phys. 37 (2004) 2489. [10] A. Aebischer, S. Heer, D. Biner, K. Kramer, M. Haase, H.U. Gudel, Chem. Phys. Lett. 407 (2005) 124. [11] J.F. Suyver, J. Grimm, K.W. Kramer, H.U. Gudel, J. Lumin. 114 (2005) 53. [12] H. Sun et al., J. Lumin. 17 (2006) 179. [13] J.F. Suyver, J. Grimm, M.K. Van Veen, D. Biner, K.W. Kramer, H.U. Gudel, J. Lumin. 117 (2006) 1. [14] S. Dai, C. Yu, G. Zhou, J. Zhang, G. Wang, L. Hu, J. Lumin. 117 (2006) 39. [15] H. Lin et al., J. Lumin. 124 (2007) 167. [16] F. Vetrone, J.C. Boyer, J.A. Capobianco, A. Speghini, M. Bettinelli, Appl. Phys. Lett. 80 (2002) 1702. [17] A.B. de Souza, M.T. de Araujo, M.V.D. Vermelho, F.C. Cassanjes, S.J.L. Ribeiro, Y. Massaddeq, J. Non-Cryst. Solids 353 (2007) 94. [18] A. Nayak, P. Kundu, R. Debnath, J. Non-Cryst. Solids 353 (2007) 1414. [19] D.K. Sardar, D.M. Dee, K.L. Nash, R.M. Yow, J.B. Gruber, R. Debnath, J. Appl. Phys. 102 (2007) 83105. [20] A. Ghosh, R. Debnath, Opt. Mater. 31 (2009) 604. [21] A. Ghosh, R. Debnath, Solid Stat. Commun. (2009), doi:10.1016/j.s.s.c.2009. 04.002. [22] J.C. Boyer, L.A. Cuccia, J.A. Capabianco, Nano Lett. 7 (2007) 847. [23] J.-H.Z. Ji Su, Z.-H. Li, R.-X. Yan, Y.-D. Li, Adv. Mater. 17 (2005) 2119. [24] K.W. Kramer, D. Biner, G. Frei, H.U. Gudel, M.P. Hehlen, S.R. Luthi, Chem. Matter 16 (2004) 1244. [25] O. Ehlert, R. Thomann, M. Darbandi, T. Nann, ACSnano 2 (2008) 120. [26] C.W. Hoyt, M. Sheik Bahae, R.I. Epstein, B.C. Edwards, J.E. Anderson, Phys. Rev. Lett. 85 (2000) 3600. [27] J. Wasylak, I.V. Kityk, J. Eur. Cer. Soc. 27 (2007) 1703. [28] A. Shalav, B.S. Richards, M.A. Green, Sol. Energy Mater. Sol. Cells 91 (2007) 29. [29] R.A.H. El-Mallawany, Tellurite Glass Hand Book, CRC Press, Boca Ratan F1, 2002. [30] Y.-Ling Shen, J.-Gao Mao, J. Alloys Compd. 385 (2004) 86 (and JCPDS file no. 47–17). [31] S. Natansohn, J. Inorg. Nucl. Chem. 30 (1968) 741 (and JCPDS file no. 21–325). [32] M.J. Redman, W.P. Binnie, J.R. Carter, J. Less Common Mets 16 (1968) 407.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Synthesis and luminescence properties of an (Er2Te4O11) nanocrystals dispersed highly efficient upconverting lead free tellurite glass R. Debnath *, A. Ghosh, S. Balaji Central Glass and Ceramic Research Institute, Kolkata 700 032, India
a r t i c l e
i n f o
Article history: Received 3 December 2008 In final form 4 May 2009 Available online 7 May 2009
a b s t r a c t Synthesis, and luminescence properties of Er3+ in an Er2Te4O11 nanocrystals dispersed tellurite glass under direct and upconversion excitation are reported. The nanocrystals which could be grown by ageing or subjecting the glass to stress, were grown, in this case, by ageing and were characterized by X-ray, FESEM and TEM studies. The growth of nanocrystals in the matrix significantly enhances both the normal and upconversion luminescence efficiency of the glass. The reason of such enhancement of efficiency is explained and the photo-physics involved in the upconversion luminescence process has been proposed. High upconversion luminescence efficiency of the glass shows the prospect of its use as solar IR concentrator and NIR sensor. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Rare earth ion activated glasses and crystals [1–28], constitute a large fraction of components of the present day optics and optoelectronics. They are extensively used in solid state lasers [1– 3,18–21], optical communications [1–8], optical signal amplification [1–20], energy upconversion [4–25], biological labeling [22– 25], anti-Stokes luminescence cooling [26], IR quantum informatics [27] etc. In the recent years upconversion luminescence properties of such materials i.e. emitting light of higher energy (preferably in the visible range) on the absorption of light of lower energy (normally in the NIR range) has received special attention because of their prospective use in solar NIR concentration for photovoltaic exploitation [28], IR sensing and biological labeling. To designate an affected tissue inside a biological system, fluorescent markers are frequently used. Excitation bands of such markers in many cases, lie in the UV region of light which some times, may be harmful to the tissues. In that respect, NIR excited markers are safe. Upconverting nanocrystals like hexagonal NaYF4: Er3+, Yb3+ [22,23] have already been shown to be very efficient and prospective material for this purpose. In case of other applications like solar IR concentration or IR sensing big panels either wholly made up of upconverting materials or panels dispersed with upconverting crystals are preferred. In that context nanocrystals dispersed upconverting glasses are expected to be the ideal candidate both in terms of efficiency and larger area coverage. Amongst the various types of glasses, oxides glasses are superior to others in respect of their stability. Considering again the fact that the phonon structure of a host plays an important role in determining the lumines* Corresponding author. Fax: +91 033 2473 0957. E-mail address: [email protected] (R. Debnath). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.05.001
cence efficiency and the excited state dynamics of an incorporated activator ion, an oxide glass with low phonon energy should be suitable for use as a host to reduce the nonradiative energy loss of the excited activator ions through multi phonon relaxation (MPR). Of the various known oxide glasses, tellurite glasses have considerably low phonon energy, high refractive index and greater power to dissolve high concentration [29] of rare earth ions. So rare earth activated tellurite glasses are thought to be the ideal material for development of large area solar concentrator/IR sensor. Choice of appropriate rare earth ion to achieve efficient upconversion luminescence, on the other hand, requires [2] consideration of availability of suitable energy levels disposition in the energy level spectrum of the ion so that the processes like, excited state co-operative energy transfer (ESET) and excited state absorption (ESA) can occur. In that respect Er3+ ion is found to be an excellent candidate because of existence of a number of suitable near infrared (NIR) excited states (e.g. 4I11/2 and 4I13/2) with long life time, in its energy level spectrum. Relatively longer life-time helps growth of population of ions in these excited states which in turn facilitates both the (ESET) and (ESA) processes. In the present Letter, we report synthesis and studies of normal and upconversion luminescence properties of an (Er2Te4O11) nanocrystals dispersed fluoro-tellurite glass. 2. Experimental The Er3+ doped tellurite glass of composition (mol%) 80TeO2– 15(BaF2 + BaO)–4La2O3–1Er2O3, and its base glass i.e. devoid of Er3+, were prepared by melting at 700–750 °C in an electrical furnace using platinum crucible. The size of batch each time was approximately 50 g. All the chemicals used were of AR grade
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(Sigma-Aldrich). Glasses were annealed for 12 h at 230–250 °C. The (Er2Te4O11) nanocrystals in the glass were grown by ageing the as cast glass under ambient atmosphere. The rate of growth of nanocrystals was found to be relatively fast and ageing of the as cast glass for one week was found to be sufficient to obtain maximum extent of growth of the nanocrystals. The density of the glass was measured by usual method (D = 5.729 gms/cc). Concentration of the Er3+ in the glass was calculated to be 4.12 1020 ions cm3. The X-ray powder diffraction spectrum of the powdered glass was recorded in an X-ray diffractometer (model: PW1710 BASED, make: Philips) using a copper anode. A Field Emission Scanning Electron Microscope (FESEM, Supra-35 VP, make: Carlzeiss SMT, Germany) was used to record the micrographs of the samples. The transmission electron micrographs (TEM) of the nanocrystals in the glass were recorded in a 200 kV HR TEM, model: JEM2011, make: JEOL, Japan. To record the absorption spectrum of Er3+ in the glass, plateshaped polished sample were prepared using the Er3+ doped aged sample. The spectrum was recorded at 303 K in an UV–Vis-NIR absorption spectrophotometer (Shimadzu, Japan, model: 10001). A polished plate-shaped base glass sample of same thickness was used as the reference. The emission and excitation spectra were recorded at room temperature in a Perkin Elmer LS55 Spectrofluorimeter. To see the effect of growth of (Er2Te4O11) nanocrystals on the luminescence properties of the glass, a quantitative study of the emission of the glass before and after the growth of nanocrystals was made by using front surface illumination technique and keeping the sample position, emission and excitation slits as well as the lamp voltage unaltered. The quantitative upconversion luminescence of the glass was recorded by using a fibre pig-tailed 976 nm diode laser with variable output power up to a maximum of 200 mwatts, (Thor Lab, USA) as an excitation source and fixing the sample in a sample holder at a definite distance from the tip of the laser output fibre. Other parameters were maintained similar to those described before. The excited state dynamics of the 4S3/2 green luminescent state of Er3+ was studied in a luminescence lifetime measurement setup (model: Fluorocube, IBH, UK) using multichannel analyzer technique. A Spectra-LED of 372 nm with pulse duration of 100 ls and repetition rate of 100 Hz, was used as the excitation source. To avoid contamination of the excitation profile with the sample’s decay, only the photon-channels of time region beyond the pulsewidth of the excitation pulse were considered to study of the nature of the decay.
Fig. 1. Absorption spectrum of the base glass (. . .); as recorded absorption spectrum of Er3+ doped glass (- - -); and the base glass corrected absorption spectrum of Er3+ in the glass (___). [Sample thickness in both the cases d 2.15 mm].
The base glass corrected absorption spectrum exhibits a number of distinct strong absorption bands of Er3+ in the near UV and Vis-NIR region namely at 376, 405, 486, 519, 542, 650, 798, 974 and 1532 nm. Comparing the energy and intensity of the bands with those of the reported similar bands of Er3+ ion [1–28] the corresponding associated transitions of the bands can be identified as transitions from the 4I15/2 ground state to 4G11/2, 2H9/2, 4F7/2, 2H11/2, 4 S3/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/2 excited states of Er3+ ions of the glass. Since the tellurite glasses are prone to devitrefication, to check the amorphous characteristics of the glass, microscopic studies were carried out using samples of different stages of ageing. Fig. 2 shows the FESEM picture of a glass sample aged for one week. It shows existence of nanoparticles of size 50 nm in the matrix. To reveal crystalline phase as well as the chemical composition of the nanoparticles, X-ray powder diffraction spectrum of the glass was recorded. This is shown in Fig. 3. As evident from the spectrum, that although the major part of the glass is in the amorphous state, there is definitely a fraction which is crystalline. The crystalline phase, exhibits one broad diffraction band peaking in between 2h = 26 and 28.5° along with a broad hallow in the higher 2h region. Careful scrutiny of the more expanded version of the spectrum shows that the broad peak is actually a composite band of three relatively strong lines with respective ‘d’ values 3.25, 3.16 and 3.09 A°. Two weak peak positions are also located at ‘d’ = 3.72 and 2.96 A°. Consideration of the network structure and
3. Results and discussion 3.1. Absorption spectrum and microscopic study of the glass Fig. 1 shows the absorption spectrum of Er3+ activated tellurite glass and that of its base-glass along with the base glass corrected absorption spectrum of Er3+ in the glass. Consideration of the absorption characteristics of the basic units of the glass composition namely BaF2, BaO, La2O3 and different oxides of tellurium, the onset of UV absorption edge of the base glass from around 400 nm region may safely be attributed to the result of collective absorption of all forms of tellurium oxide groups, namely, [Te4O11]6, [Te3O9]6 and [TeO6]6 present in the glass [30–32]. In case of Er3+ doped glass some of the Er3+ bands (namely 4 I15/2 ? 2G9/2, 4I15/2 ? 4G11/2 transitions) lie in this near UV region, and hence absorption intensity in this region in the case of Er3+ doped glass gets enhanced due to additive absorption of Er3+ and the oxides of tellurium of the glass. This is clearly evident from the as recorded absorption spectrum of the Er3+ doped glass.
Fig. 2. FESEM picture of the glass after the growth of the nanocrystals.
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Fig. 3. Powder X-ray diffraction spectrum of the Er3+-doped glass after the growth of the nanocrystals.
chemical composition of the glass, suggests that crystalline phases like Er2[Te4O11] [30], Er2[TeO6] [31] and/or La2[Te3O9] [32] may grow in the glass during the period of ageing. A comparison of the ‘d’ values and the relative intensities of the observed diffraction lines with those of the possible different phases mentioned above shows, that the observed lines are the result of overlapping of the main XRD lines of La2Te3O9 [Cubic, d = 3.75 (I 20), d = 3.22 (I 80), d = 3.14 (I 100), d = 3.09 (I 80)] [32] and of Er2Te4O11 [monoclinic, d = 3.81 (I 37, h, k, l = 112, 004), d = 3.25 (I 100, h, k, l = 212), d = 2.95 (I 46, h, k, l = 311)] [31]. Transmission electron micrograph (TEM) of the nanoparticles is shown in Fig. 4. It shows that each nanoparticle of the glass consists of several nanocrystallites of size 10–12 nm. It is however difficult to estimate from the XRD-spectrum, the proportion of the two phases in the glass but it can be argued that since the concentration of lanthanum oxide in the glass is four times higher than that of erbium oxide, possibility of formation of La2Te3O9 phase should be higher than that of Er2Te4O11 phase. However, as far as the role of the nanocrystals in the phenomenon of enhancement of luminescence efficiency of Er3+ is concerned, we are interested only in the Er2Te4O11 phase. 3.2. Growth of (Er2Te4O11) nanocrystals and the luminescence efficiency of the glass The Er3+ activated glass under 376 nm light excitation (4I15/2 ? 2G11/2 absorption of Er3+) exhibits a bright green lumines-
Fig. 4. TEM picture of the nanocrystals in the glass.
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cence due to the [(2H11/2, 4S3/2) ? 4I15/2] transition of Er3+. To see the effect of growth of (Er2Te4O11) nanocrystals in the glass on the luminescence properties of Er3+, a quantitative study on the efficiency of the green emission under 376 nm – excitation was made before and after the growth of nanocrystals. This is shown in Fig. 5. Surprisingly, it is observed that the growth of nanocrystals, significantly enhances the luminescence efficiency of the glass. An estimate of the ratio of integrated emissions (IE) of the sample measured quantitatively before and after the growth of the nanocrystals, shows that the efficiency is enhanced almost three times. In addition to the enhancement of luminescence a significant change in the spectral resolution of the Stark split components as well as a reversal in their peak intensities, are also noted. The results show that the Er3+ ions in the glass experience a stronger and regular crystal field after the growth of the nanocrystals. Before the Er2Te4O11 nanocrystals are grown, the erbium ions remain randomly dispersed in the in the interstitial of the network structure of the glass. After the formation of the nanocrystal, Er3+ ions of the nanocrystals have a well defined surrounding and hence it is expected to experience a well defined crystal field resulting well resolved stark split of the emission bands. The result also indicates that the Er3+ ions are directly associated with the nanocrystals. It is clear from the absorption spectrum of the base glass that the UV absorption edge of the glass onsets right from 400 nm. As stated before that the onset of UV absorption edge of the base glass from 400 nm region results from collective absorption of different forms of tellurium oxide groups present in the glass [30–32] and as a result the Er3+ doped glass, exhibits greater absorption intensity in the spectral region towards UV beyond 400 nm, due to additive absorption of the glass and the Er3+ ions. After the growth of (Er2Te4O11) nanocrystals if the band gap of the nanocrystal falls in the similar wave length region, excitation of the glass under 376 nm light may lead to excitation of Er3+ ions remaining within the nanocrystals (through the band gap) as well as those remaining in the glass phase (through direct absorption) causing a significant enhancement in luminescence efficiency. The decay profile of the 4S3/2 state measured by monitoring the 548 nm green emission is shown in Fig. 6. An analysis of the decay shows that it has bi-exponential character with a faster component of lifetimes of 16 ls and a slower component of 39 ls. The life time values obtained are found to be consistent with those reported earlier for the 4S3/2 state by others [13,20]. The bi-exponential nature of the decay is in fact, related to two different groups of Er3+ ions in the glass one staying within the crystalline frame work of the nanocrystal and the other randomly distributed in the interstial space of the network structure of the tellurite glass.
Fig. 5. Green luminescence spectra due to 4S3/2 ? 4I15/2 transition of Er3+ in the glass before (___) and after (- - -) the growth of nanocrystals.
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Fig. 6. Excitation and decay profile of the 4S3/2 excited state of Er3+ in the glass. Inset: Log plot of the decay profile of 4S3/2 excited state; kMoni = 546 nm.
3.3. Upconversion luminescence and the effect of variation excitation power The glass also exhibits the same green emission when it is excited through upconversion route (4I15/2 ? 4I11/2 transition of Er3+) using 976 nm diode laser. Fig. 7, presents upconversion emission spectra of the glass as a function of laser power. A bright upconversion green luminescence from Er3+ peaking at 532 and 550 nm due its (2H11/2, 4S3/2) ? 4I15/2 transitions is observed along with a weak red emission peaking at 690 nm due to (F9/2 ? 4I15/2) transition. The intensity of the red emission here is found to be enhanced compared to that obtained under direct excitation. The upconversion green luminescence of the glass is very bright and can be seen with naked eye even under excitation of 07 mwatts power of laser and it looks like a bright green light source when under excitation of 200 mwatts power of the laser (shown in the
right top of Fig. 7). The upconversion luminescence efficiency of the glass is thus also found to be significantly enhanced due to growth of the (Er2Te4O11) nanocrystals in the matrix. Since the base glass is optically transparent beyond 420 nm, one can expect better penetration of 976 nm light in the glass and hence better a excitation of Er3+ ions of the nanocrystals which have higher value of surface to volume ratio. More over, since the nanocrystals consist of erbium related compound e.g. (Er2Te4O11) concentration of Er3+ ions in the crystals should be more than that in the glassy part. Thus an overall enhancement of efficiency of upconversion emission is highly likely due to the increased excitation efficiency of Er3+ ions. The glass thus seems to be potential material for use as large area NIR solar concentrator and NIR sensor. In case of an upconversion luminescence, upconversion luminescence intensity Iupc is related to the intensity of the excitation source Iexci by the following relation. Iupc / Im exci , ‘m’ is the number of IR photons absorbed per visible photon emitted. A plot of log Iupc vs. log Iexci, should therefore, give a straight line with a slope = m. The inset (left) of the Fig. 7, shows such plots for different upconversion emissions of Er3+ in the glass. The values of ‘m’ obtained for 532, 550 and 690 nm emissions are 2.08, 2.11, and 2.00. The results indicate that all the upconversion emission processes are biphotonic in nature. After population of the Er3+ ions in the 4I11/2 state upon excitation with the 976 nm laser, a fraction of the Er3+ ions may relax to the immediate low lying 4I13/2 state but major fraction of the ions because of the relatively long life time [16] of the 4I11/2 state gets the opportunity of absorbing second photon through excited state absorption (ESA) and reaches the 2H11/2 and subsequently relaxes to the ground state emitting the green luminescence through (2H11/2, 4S3/2) ? 4I15/2 transitions. As the laser power is increased the process of excited state absorption (ESA) also increases and consequently intensity of the
Fig. 7. Upconversion luminescence of Er3+ under 978 nm laser excitation (4I15/2 ? 4I11/2) as a function of laser power. Inset (left): log – plots of different luminescence peak intensities vs. laser power. Inset (right): a photo-graph of the upconversion luminescence of the glass under the 980 nm laser excitation.
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The upconversion luminescence property of the glass was also studied using a 976 nm diode laser. An unusually intense green upconversion luminescence of the glass demonstrated that the glass is a prospective material for use as large area solar concentrator as well as NIR sensor. Acknowledgement Present work was carried out under the sponsorship of CSIR, India, through the OLP Project No. OLP-0210 of CGCRI. References
Fig. 8. Energy level diagram and possible scheme of green and red upconversion luminescence of Er3+ under 976 nm excitation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
green luminescence increases. Since the concentration of Er3+ in the present glass is 1 mol% which is sufficiently high, an additional phenomenon of co-operative energy transfer (ET) should also occur through cross relaxation, enhancing further the intensity of the green luminescence. The life time of 4I13/2 state is also high few milliseconds [3,5,6] so the fraction of ions that comes to this level from the upper 4I11/2 level through nonradiative relaxation can also undergo excited state absorption (ESA) of second 976 nm photons. Energy acquired by an Er3+ ion of 4I13/2 state through this process of excited state absorption (ESA), however, lies in between 4S3/2 and 4F9/2 levels, the ions therefore, possibly reach to some higher phononic level above the 4F9/2 state and subsequently cascade to the red 4F9/2 emitting state and relax to the ground state through 4F9/2 ? 4I15/2 transition creating red luminescence. Photo-physical path ways proposed, respectively, for the green and red upconversion luminescence are shown in Fig. 8. 4. Conclusion Studies on the microscopic and luminescence properties of an Er3+ ion (1 mol%) doped lead free fluoro–barium tellurite glass showed the efficiency of luminescence of the glass get enhanced by almost three times after nanocrystalline phase of (Er2Te4O11) is grown in the matrix.
[1] L.F. Johnson, H.J. Guggenheim, Appl. Phys. Lett. 19 (1971) 44. [2] AlexanderA Kaminskii, Laser Crystals (Their Physics and Properties), second edn., Springer-Verlag, Berlin, Heidelberg, New York, 1990. [3] A. Mori, Y. Ohishi, S. Sudo, Electron. Lett. 33 (1997) 863. [4] Y. Ding et al., Opt. Mater. 15 (2000) 123. [5] R. Balda, O. Sanz, J. Goazalo, M. Zimenez de Castro, A. Perea, J. M. Fernandez– Navarro, C.N. Afanso, J. Fernandez, in: Proceedings of SPIE, No. 4990, 2003, p. 30. [6] H. Lin, G. Meredith, S. Jiang, X. Peng, Tao Luo N. Peghambarian, E. Y-Bun Pun, J. Appl. Phys. 93 (2003) 186. [7] H. Chen, Y.H. Liu, Y.F. Zhou, Q.Y. Zhang, Z.H. Jiang, J. Non-Cryst. Solids 351 (2005) 3060. [8] I.R. Martin, P. Velez, V.D. Rodriguez, U.R. Rodriguez-Mendoza, V. Lavin, Spectrochim. Acta, Part-A 55 (1999) 935. [9] L.A. Diaz-Torres, E. Dela Rossa –Cruz, P. Salas, C. Angeles Chavez, J. Phys. D, Appl. Phys. 37 (2004) 2489. [10] A. Aebischer, S. Heer, D. Biner, K. Kramer, M. Haase, H.U. Gudel, Chem. Phys. Lett. 407 (2005) 124. [11] J.F. Suyver, J. Grimm, K.W. Kramer, H.U. Gudel, J. Lumin. 114 (2005) 53. [12] H. Sun et al., J. Lumin. 17 (2006) 179. [13] J.F. Suyver, J. Grimm, M.K. Van Veen, D. Biner, K.W. Kramer, H.U. Gudel, J. Lumin. 117 (2006) 1. [14] S. Dai, C. Yu, G. Zhou, J. Zhang, G. Wang, L. Hu, J. Lumin. 117 (2006) 39. [15] H. Lin et al., J. Lumin. 124 (2007) 167. [16] F. Vetrone, J.C. Boyer, J.A. Capobianco, A. Speghini, M. Bettinelli, Appl. Phys. Lett. 80 (2002) 1702. [17] A.B. de Souza, M.T. de Araujo, M.V.D. Vermelho, F.C. Cassanjes, S.J.L. Ribeiro, Y. Massaddeq, J. Non-Cryst. Solids 353 (2007) 94. [18] A. Nayak, P. Kundu, R. Debnath, J. Non-Cryst. Solids 353 (2007) 1414. [19] D.K. Sardar, D.M. Dee, K.L. Nash, R.M. Yow, J.B. Gruber, R. Debnath, J. Appl. Phys. 102 (2007) 83105. [20] A. Ghosh, R. Debnath, Opt. Mater. 31 (2009) 604. [21] A. Ghosh, R. Debnath, Solid Stat. Commun. (2009), doi:10.1016/j.s.s.c.2009. 04.002. [22] J.C. Boyer, L.A. Cuccia, J.A. Capabianco, Nano Lett. 7 (2007) 847. [23] J.-H.Z. Ji Su, Z.-H. Li, R.-X. Yan, Y.-D. Li, Adv. Mater. 17 (2005) 2119. [24] K.W. Kramer, D. Biner, G. Frei, H.U. Gudel, M.P. Hehlen, S.R. Luthi, Chem. Matter 16 (2004) 1244. [25] O. Ehlert, R. Thomann, M. Darbandi, T. Nann, ACSnano 2 (2008) 120. [26] C.W. Hoyt, M. Sheik Bahae, R.I. Epstein, B.C. Edwards, J.E. Anderson, Phys. Rev. Lett. 85 (2000) 3600. [27] J. Wasylak, I.V. Kityk, J. Eur. Cer. Soc. 27 (2007) 1703. [28] A. Shalav, B.S. Richards, M.A. Green, Sol. Energy Mater. Sol. Cells 91 (2007) 29. [29] R.A.H. El-Mallawany, Tellurite Glass Hand Book, CRC Press, Boca Ratan F1, 2002. [30] Y.-Ling Shen, J.-Gao Mao, J. Alloys Compd. 385 (2004) 86 (and JCPDS file no. 47–17). [31] S. Natansohn, J. Inorg. Nucl. Chem. 30 (1968) 741 (and JCPDS file no. 21–325). [32] M.J. Redman, W.P. Binnie, J.R. Carter, J. Less Common Mets 16 (1968) 407.