- Email: [email protected]
Down-conversion process in Tb3+–Yb3+ co-doped Calibo glasses
Journal of Luminescence 132 (2012) 1678–1682
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Down-conversion process in Tb3 þ –Yb3 þ co-doped Calibo glasses I.A.A. Terra a,n, L.J. Borrero-Gonza´lez a, T.R. Figueredo a, J.M.P. Almeida a, A.C. Hernandes a, L.A.O. Nunes a, O.L. Malta b a b
~ Carlos, Universidade de Sao ~ Paulo, CP 369, 13560-970 Sa~ o Carlos, SP, Brazil Instituto de Fı´sica de Sao ´ria, Recife-PE 50670-901, Brazil Departamento de Quı´mica Fundamental, Universidade Federal de Pernambuco-CCEN, Cidade Universita
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 November 2011 Received in revised form 26 January 2012 Accepted 7 February 2012 Available online 15 February 2012
The down-conversion process in Tb3 þ –Yb3 þ co-doped Calibo glasses was studied. The emission, excitation and time-resolved measurements indicated the existence of an energy conversion through the excitation of Tb3 þ ions to near-infrared emission by Yb3 þ ions. The emission intensity dependence on excitation power confirms that the one-photon process is responsible for the Yb3 þ emission. An enhanced Yb3 þ emission was observed with Yb3 þ doping and an optimal energy transfer efficiency of 32% was obtained before reaching near-infrared emission quenching. The mechanism of the nonresonant energy transfer from Tb3 þ to Yb3 þ is discussed in terms of the Tb3 þ –Yb3 þ cross-relaxation and multiphonon decay processes. & 2012 Elsevier B.V. All rights reserved.
Keywords: Down-conversion Energy transfer Calibo Tb3 þ –Yb3 þ ions
1. Introduction Recently, there has been a strong interest in sustainable energy production to minimize the negative impact of global energy consumption on the environment. One alternative is to use the energy of the solar spectrum through solar cells; however, the energy provided by these cells is limited because of a relatively high kilowatt hour cost. A reduction in price, however, may be achieved by either lowering the production cost or increasing the conversion efficiency [1]. Single junction solar cell efficiency is limited to 30% (the Schockley–Queisser theoretical limit) mainly because of losses caused by spectral mismatch, in which the largest part of the 70% energy loss is related to the thermalization electron–hole pairs and silicon transparency for the sub-band-gap incident photons [1]. Some theoretical and experimental researches are aimed at developing solar energy converters to enhance solar cell efficiency [2–5]. A great challenge is to efficiently convert the energy from the visible region of the solar spectrum, which has a maximum intensity at 550 nm, to the near-infrared region so that a single junction crystalline silicon solar cell has a major spectral response ( 1000 nm) [6]. Since the first proposal by Dexter (1957) [7], it is well known that downconversion (DC) is a suitable mechanism to convert high-energy photons into low-energy photons. Trupke et al. [5] showed that a DC material, in combination with a single junction crystalline
n
Corresponding author. Tel.: þ55 16 33739835. E-mail address: [email protected] (I.A.A. Terra).
0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2012.02.019
solar cell, could minimize energy losses due to thermalization electron-hole pairs, achieving a theoretical limit for the conversion efficiency of up to 40%. In addition, there have been investigations on DC through the cooperative energy transfer (CET) process (also known as quantum cutting, i.e. the energy conversion from one visible photon to two NIR photons) in different Tb3 þ –Yb3 þ co-doped materials [4,8–12]. The combination of one Tb3 þ (sensitizer) and two Yb3 þ (activator) ions is a promising system to achieve visible–NIR quantum cutting because the Tb3 þ :5D4-7F6 transition occurs at approximately twice the energy of the Yb3 þ :2F7/2-7F5/2 absorption, which will result in a two photon emission approximately at 1000 nm. However, the CET process in Tb3 þ –Yb3 þ systems is a second order energy transfer process (low probability) and is not the only process to achieve NIR emission from Yb3 þ ions. After Tb3 þ excitation, a Tb3 þ –Yb3 þ cross-relaxation mechanism is also possible to excite one Yb3 þ ion with a subsequent energy loss through multiphonon decay process in Tb3 þ ions (see Fig. 1). It is interesting to note that various authors have been analyzing the emission and excitation spectra in combination with lifetime measurements in Tb3 þ –Yb3 þ co-doped in different materials [8–12] and concluded that the Yb3 þ emission is due to quantum cutting through the CET process. However, those authors did not perform power dependent luminescence experiment, which is fundamental to elucidate the origin of the DC process [13,14]. To the best of our knowledge, down-conversion in Tb3 þ –Yb3 þ co-doped lithium calcium borate (Calibo) glass has not yet been reported. Calibo glass is a good host to examine the effects of chemical environments on the optical properties of rare-earth
I.A.A. Terra et al. / Journal of Luminescence 132 (2012) 1678–1682
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Excitation measurements were performed using a Xe-lamp as the excitation source, and the excitation light was dispersed by a double-grating (0.22 m, SPEX/1680) monochromator. Lifetime values were estimated from the luminescence decay curves; they were measured by exciting the samples at different wavelengths (488 and 920 nm) with an optical parametrical oscillator (OPO; Surelite/Continumm model SLII-10) pumped by the third harmonic (355 nm) of a Nd–YAG laser (Surelite I/Continumm, 10 Hz, 5 ns), using the same single monochromator (0.3 m) and the PMT or the InGaAs detector. A digital oscilloscope (TekTronix/TDS380) was used to register the decay curves. All of the spectroscopic measurements were performed at room temperature.
3. Results and analysis
Fig. 1. Energy level diagram of Yb3 þ and Tb3 þ ions. The cross-relaxation and multiphonon decay as well as excitation and emission transitions are indicated by dashed dotted, dashed and solid lines, respectively.
ions, due to the high transparency, low melting point and high thermal stability of the glass [15]. Another important characteristic of this glass is its acceptance of a large concentration of rareearth ions. For this purpose, Calibo glass has been selected as the host material in this work, and samples with different concentrations of Yb3 þ (from 0 to 25.9 1020 ions/cm3) and with fixed Tb3 þ (4.0 1020 ions/cm3) were prepared. The down-conversion in the Tb3 þ –Yb3 þ system was investigated using emission and excitation spectra, time-resolved measurements and power dependent luminescence. In our present study, the down-conversion process in Tb3 þ – Yb3 þ co-doped Calibo glasses was studied. One-photon process is responsible for the Yb3 þ emission. An enhanced Yb3 þ emission was observed with increasing the Yb3 þ concentration after a direct excitation of Tb3 þ ions. The mechanism of the nonresonant energy transfer from Tb3 þ to Yb3 þ is discussed in terms of the cross-relaxation and multiphonon decay processes. Our results suggest a potential material for NIR down-conversion.
Fig. 1 shows a simplified energy level diagram of the Tb3 þ and Yb3 þ ions, with the relevant optical transitions for analysis. The cross-relaxation and multiphonon decay as well as excitation and emission transitions are indicated by dashed dotted, dashed and solid lines, respectively. The absorption spectra of the Tb3 þ –Yb3 þ co-doped Calibo glasses were collected in a range between 250 and 1200 nm and the absorption bands corresponding to the Tb3 þ ions excited by transitions from the 7F6 ground state, and the 2F7/2-2F5/2 transition of Yb3 þ ions, were observed (results not shown here). A linear dependence was also observed for the Yb3 þ transition (at 950 nm) versus the concentration of Yb3 þ ions; this is an indication of the successful incorporation of the nominal Yb3 þ doping concentration for the prepared glass samples. The visible photoluminescence spectra of the Tb3 þ –Yb3 þ co-doped Calibo glasses for a fixed concentration of Tb3 þ (4.0 1020 ions/cm3) as a function of the Yb3 þ concentration and under 325 nm excitation are shown in Fig. 2. From these spectra, emission bands centered at 488, 544, 586 and 622 nm are clearly observed. These peaks can be assigned to the 5D4-7FJ (J¼6,5,4,3) transitions of Tb3 þ , and their intensities are drastically reduced by increasing the concentration of Yb3 þ . In addition, the UV emission intensity (not shown in Fig. 2) was weak due to the presence of OH in the samples. The inset shows the integrated intensity of emission band centered at 544 nm as a function of the
2. Experimental details Glass samples with the compositions: (99.5 x)Caliboþ0.5 Tb4O7 þxYb2O3 with x¼0; 0.1; 0.5; 1; 2.5; 5; 7 (% mol) and 93Caliboþ7Yb2O3 (% mol) were prepared by the conventional melt quenching method. A Calibo glass matrix (60B2O3 þ30CaOþ 10Li2O; % mol) was prepared using high-purity B2O3, Li2CO3 and CaCO3. Thereafter, Tb4O7 and Yb2O3 co-doped samples were obtained by melting the Calibo matrix together with the rareearth oxides at approximately 1200 1C for 1 h. The glasses were cut and polished into a plate shape with the following dimensions: 3 10 10 mm. Ground state absorption spectra were obtained by using a Perkin Elmer Lambda 900 spectrophotometer in the range from 250 to 1200 nm. The emission spectra were obtained in the visible and infrared regions using either a HeCd laser (Kimmon/ IK5652R-G) at 325 nm or an Ar þ laser (spectra physics 166/excel beta-I) at 488 nm, respectively, as excitation sources. The visible and infrared luminescence signals were dispersed by a monochromator (0.3 m, Thermo Jarrel Ash/82497), detected with either a photomultiplier tube (PMT) (Hamamatsu/R928) or an InGaAs detector (EG&G/J12D), and amplified using a lock-in amplifier.
Fig. 2. Visible photoluminescence spectra of the Tb3 þ –Yb3 þ co-doped Calibo glasses for a fixed concentration of Tb3 þ (4.0 1020 ions/cm3) as a function of the Yb3 þ concentration under 325 nm excitation. The inset shows the integrated intensity of the emission band of the Tb3 þ :5D4-7F5 transition as a function of the concentration of Yb3 þ .
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concentration of Yb3 þ , showing that the intensity decreases approximately two times as the concentration of Yb3 þ increases from 0.0 to 25.9 1020 ions/cm3. These results indicate the presence of down-conversion process. Some authors [12] propose that this process is a quantum cutting in the Tb3 þ –Yb3 þ co-doped glasses due to a cooperative energy transfer process converting one absorbed photon by Tb3 þ ion to two emitted photons by Yb3 þ ions (Tb3 þ :5D4-Yb3 þ :7F5/2 þYb3 þ :7F5/2). This down-conversion process is analyzed in further detail in the following section. The emission spectra in the NIR (approximately 980 nm) were studied by resonant pumping of the 5D4 level of Tb3 þ at 488 nm. Both NIR emission spectra were monitored and the NIR-integrated intensities were calculated as a function of the concentration of Yb3 þ (see Fig. 3) with a fixed Tb3 þ concentration (4.0 1020 ions/cm3). Fig. 3 shows the emission bands (excited with 488 nm) located at 900–1175 nm due to the 2F5/2-2F7/2 transition of Yb3 þ for concentrations of 0.4, 4.0 and 25.9 1020 ions/cm3 and are represented by solid, dotted and dashed lines, respectively. As shown, the spectra profile changes substantially with the Yb3 þ concentration and there is an apparent shift to the lower energy side due to the Yb3 þ reabsorption process [16]. The inset in Fig. 3 shows the NIR-integrated intensity as a function of the Yb3 þ concentration. The increase in the intensity is consistent with the results presented in Fig. 2, i.e., the presence of down-conversion process. Moreover, the NIR intensity increases with the concentration of Yb3 þ up to 19.0 1020 ions/cm3 and thereafter the intensity decreases for higher Yb3 þ concentration. This decrease in the intensity is probably due to relaxation (losses) mechanisms which include the energy migration between Yb3 þ ions and the OH losses. To further investigate the down-conversion process, the excitation spectrum for the 4.0 1020 ions/cm3 Tbþ 19.0 1020 ions/cm3 Yb co-doped sample excited with a Xe-lamp was collected for the Yb3 þ : 2 F5/2-2F7/2 (lm ¼980 nm monitored wavelength), and the result is shown in Fig. 4. The spectrum clearly shows the Tb3 þ absorption structure from the ground energy level 7F6 to 5D4, and 5D3, etc, and this result confirms the presence of down-conversion process. Time-resolved measurements were performed to extract the decay times and to calculate the energy transfer efficiency from Tb3 þ to Yb3 þ . Fig. 5 shows the luminescence decay times of the Yb3 þ :2F5/2-2F7/2 level (980 and 1070 nm emissions) as a
Fig. 3. NIR emission bands located at 900–1175 nm due to the 2F5/2-2F7/2 transition of Yb3 þ for concentrations of 0.4, 4.0 and 25.9 1020 ions/cm3 in Calibo glasses are represented by solid, dotted and dashed lines, respectively. The inset shows integrated intensity of the Yb3 þ emission via the down-conversion process as a function of the concentration of Yb3 þ with fixed Tb3 þ concentration (4.0 1020 ions/cm3). The excitation wavelength was lexc ¼ 488 nm resonant with the Tb3 þ :5D4 level. The solid line represents a visual guide.
Fig. 4. Excitation spectrum of the Tb3 þ –Yb3 þ (4.0 1020–25.9 1020 ions/cm3, respectively) co-doped Calibo glass, with a monitoring emission of Yb3 þ : 2 F5/2-2F7/2 at lm ¼ 980 nm.
Fig. 5. Decay times for the Yb3 þ :2F5/2-2F7/2 transition at 980 and 1070 nm in Calibo glasses obtained with excitations at 488 and 920 nm (represented by the square and circle symbols, respectively) as a function of the concentration of Yb3 þ .
function of the Yb3 þ concentration with different excitation wavelengths (488 and 920 nm) using, as an excitation source, an OPO pumped by a Nd–YAG laser (5 ns). The circle symbol represents the decay times for excitation at 920 nm in resonance with the Yb3 þ :2F5/2 level. With an excitation at 920 nm, the lifetime is equal to 0.75 ms for the lowest Yb3 þ concentration (0.4 1020 ions/cm3) and this value is approximately the same with the calculated radiative lifetime (0.70 ms) for Yb3 þ : 2F5/ 2 2- F7/2 level emission using McCumber’s theory [17,18]. In addition, the Yb3 þ decay time decreases monotonically (almost three times) with the Yb3 þ concentration up to 25.9 1020 ions/ cm3 due to energy migration between Yb3 þ ions and nearby unintentionally introduced impurities as OH radicals and the up-conversion process 2Yb3 þ -Tb3 þ [19]. However, for the excitation at 488 nm in resonance with the Tb3 þ :5D4 level, the Yb3 þ decay times (represented by the square symbol in Fig. 5) are higher than for the corresponding excitation at 920 nm. This difference is due to the additional time needed to account for the energy transfer process Tb3 þ -Yb3 þ when exciting the Tb3 þ :5D4 level. As can be observed, the Yb3 þ decay time increases with Yb3 þ concentration and reaches a maximum at 4.0 1020 ions/cm3. This lengthening in the Yb3 þ decay time is due to both the energy migration among Yb3 þ neighbor ions and
I.A.A. Terra et al. / Journal of Luminescence 132 (2012) 1678–1682
Fig. 6. Luminescence decay of the Tb3 þ :5D4-7F5 emission at 544 nm in Calibo glasses excited with a 488 nm pulsed laser (5 ns). The inset shows the dependence of the average decay time of the Tb3 þ :5D4-7F5 emission (the dot symbol) and the energy transfer efficiency (the square symbol) as a function of the Yb3 þ concentration.
the Yb3 þ reabsorption process. These processes are present in all the samples; however, the decay time decreases for concentrations higher than 4.0 1020 ions/cm3 because the probability to activate the mechanism from OH losses is privileged. Fig. 6 shows the dependence of the luminescence decay curves of Tb3 þ monitored at 544 nm (5D4-7F5) as a function of the Yb3 þ concentration under 488 nm excitation. The transient presents a single exponential behavior for the Tb3 þ single-doped sample. However, for the Tb3 þ –Yb3 þ co-doped samples, the decays are non-exponential due to the down-conversion process. The average decay time of the Tb3 þ :5D4-7F5 level was 2.38 ms for the Tb3 þ single-doped sample, and this value is the radiative lifetime for this level because the absence of Yb3 þ rules out any presence of energy transfer from Tb3þ to Yb3 þ . Radiative lifetimes between 2 and 3 ms are the typical values for Tb3þ :5D4-7F5 emission reported for phosphates [12], silicates [15,20] and borates [21] glasses. The Tb3þ :5D4-7F5 decay time decreases with Yb3þ doping until 1.32 ms for a Yb3þ concentration equals 25.9 1020 ions/cm3. This result could be attributed to a depopulation of the Tb3 þ :5D4 level, mainly via the down-conversion process due to the increasing of Yb3 þ acceptor ions (see Fig. 2). The inset in Fig. 6 presents the Tb3þ :5D4-7F5 emission average decay time as a function of the concentration of Yb3þ (the dot symbol). From these decay times, it is possible to calculate the energy transfer efficiencies (ZET) between sensitizer (Tb3 þ ) and acceptor (Yb3 þ ) ions, defined as the ratio of sensitizers that are depopulated by energy transfer to acceptors to the total number of sensitizers being excited. This can be expressed as [4]
ZET ¼ 1
tTb2xYb , tTb20Yb
ð1Þ
where tTb–xYb and tTb–0Yb ¼2.38 ms are the luminescence decay times of the Tb3 þ :5D4 level with and without Yb3 þ doping, respectively. The inset in Fig. 6 also shows the dependence of energy transfer efficiency on the Yb3 þ concentration (the square symbol). The value of ZET is found to increase linearly up to 45% when the concentration of Yb3 þ is increased from 0.4 1020 to 25.9 1020 ions/cm3. This value is comparable to the values reported in the literature in Tb3 þ –Yb3 þ co-doped in different materials: transparent glass ceramics containing CaF2 nanocrystals (55%) [4] and for Y2O3 phosphor (37%) [10]. However, an optimal energy transfer efficiency of 32% was obtained before reaching NIR
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Fig. 7. Emission intensity dependence of down-conversion at 980 nm as a function of the excitation power. The inset shows the energy transfer mechanism of the down-conversion process.
emission quenching for the (4.0Tb3 þ –19.0Yb3 þ ) 1020 ions/cm3 co-doped sample. Up to now, all the above results clearly show a down-conversion process, but those experiments do not clarify the kind of energy transfer mechanism. Excitation power measurements allow elucidation of the origin of the down-conversion process [13,22]. A Pn power law has been found for an n-photon process, in which n is the slope of the double-logarithmic plots of the intensity emission versus excitation power. This slope has been found to be 0.5 [13,22] for a quantum cutting mechanism through the CET process, where two-low energy photons are obtained upon the absorption of one high-energy photon. On the other hand, other authors have obtained n ¼1, indicating a conversion of a high energy into a low energy photon. In addition, it is interesting to note that Strek et al. [13] have observed a temperature dependence for the slope with n ¼0.5 at room temperature and n¼1 at lower temperatures 77 K, indicating the non-resonant nature of the quantum cutting energy transfer process. Therefore, excitation power dependence of the emission intensity of Yb3 þ was studied to elucidate the n-photon process involved in the energy transfer. Fig. 7 shows the dependence of the emission intensity for Yb3 þ at 980 nm as a function of the excitation power through a resonant pumping of Tb3 þ :5D4 level at 488 nm for the 4.0 1020 ions/cm3 Tbþ19.0 1020 ions/cm3 Yb co-doped sample. All the samples showed the slope equals 1.02, similar to the work reported by Ye et al. [14]. Then, the downconversion process of our system is mediated by a one-photon energy transfer and is explained in terms of a Tb3 þ –Yb3 þ crossrelaxation followed by multiphonon decay (see the inset in Fig. 7). The energy transfer rates for a first-order energy transfer are about 104–108 higher than for a second-order energy transfer (CET) [10].
4. Conclusions The down-conversion process in Tb3 þ –Yb3 þ co-doped Calibo glasses has been studied. The conversion of high-energy photon in the visible range to low-energy photon in NIR was confirmed using emission and excitation spectra in combination with time-resolved measurements. The excitation power dependence of the emission intensity of Yb3 þ elucidated that a one-photon process is
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responsible for the Yb3 þ emission under the direct excitation of Tb3 þ ions. An enhanced Yb3 þ emission was observed with increasing Yb3 þ concentration after a direct excitation of Tb3 þ ions. An optimal energy transfer efficiency of 32% was obtained before reaching NIR emission quenching for the (4.0Tb3 þ – 19.0Yb3 þ ) 1020 ions/cm3 co-doped sample. The down-conversion process mediated by a one-photon energy transfer was explained in terms of a Tb3 þ –Yb3 þ cross-relaxation following by multiphonon decay. Finally, all the results showed that Tb3 þ –Yb3 þ co-doped Calibo glass has a potential application as a down-converter material to be used in the NIR region.
Acknowledgments We would like to acknowledge the financial support of the Coordenac- a~ o de Aperfeic-oamento de Pessoal de Nı´vel Superior (CAPES), Fundac- a~ o de Amparo a Pesquisa do Estado de Sa~ o Paulo (FAPESP), Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico (CNPq) and Instituto Nacional de Nanotecnologia para Marcadores Integrados (INAMI).
References [1] B.M. van der Ende, L. Aarts, A. Meijerink, Phys. Chem. Chem. Phys. 11 (2009) 11081. [2] T. Trupke, M.A. Green, P. Wurfel, J. Appl. Phys. 92 (2002) 1668. [3] B.S. Richards, Sol. Energy Mater. Sol. Cells 90 (2006) 1189. [4] S. Ye, B. Zhu, J. Chen, J. Luo, J.R. Qiu, Appl. Phys. Lett. 92 (2008) 141112. ¨ [5] T. Trupke, M.A. Green, P. Wurfel, J. Appl. Phys. 92 (2002) 4117. ¨ ¨ [6] J.J. Eilers, D. Biner, J.T. van Wijngaarden, K. Kramer, H.U. Gudel, A. Meijerink, Appl. Phys. Lett. 96 (2010) 151106. [7] D. Dexter, Phys. Rev. 108 (1957) 630. [8] Q.Y. Zhang, C.H. Yang, Y.X. Pan, Appl. Phys. Lett. 90 (2007) 021107. [9] Q.Y. Zhang, C.H. Yang, Z.H. Jiang, X.H. Ji, Appl. Phys. Lett. 90 (2007) 061914. [10] J.L. Yuan, X.Y. Zeng, J.T. Zhao, Z.J. Zhang, H.H. Chen, X.X. Yang, J. Phys. D 41 (2008) 105406. [11] X. Liu, S. Ye, Y. Qiao, G. Dong, B. Zhu, D. Chen, G. Lakshminarayana, J. Qiu, Appl. Phys. B—Lasers Opt. 96 (2009) 51. [12] P. Vergeer, T. Vlugt, M. Kox, M. den Hertog, J. van der Eerden, A. Meijerink, Phys. Rev. B. 71 (2005) 014119. [13] W. Strek, A. Bednarkiewicz, P.J. Deren, J. Lumin. 92 (2001) 229. [14] S. Ye, Y. Katayama, S. Tanabe, J. Non-Cryst. Solids 357 (2011) 2268. [15] G. Tripathi, V.K. Rai, S.B. Rai, Appl. Phys. B—Lasers Opt. 84 (2006) 459. [16] S. Dai, J. Lumin. 104 (2003) 55. [17] D.E. McCumber, Phys. Rev. 134 (1964) A299. [18] W.J. Miniscalco, R.S. Quimby, Opt. Lett. 16 (1991) 258. [19] T. Yamashita, Y. Ohishi, J. Non-Cryst. Solids 354 (2008) 1883. [20] R. Brown, J. Lumin. 63 (1995) 1. [21] A. David Pearson, J. App.Phys. 37 (1966) 729. [22] D. Chen, Y. Wang, Y Yu, P. Huang, F. Weng, J. Appl. Phys. 104 (2008) 116105.
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Down-conversion process in Tb3 þ –Yb3 þ co-doped Calibo glasses I.A.A. Terra a,n, L.J. Borrero-Gonza´lez a, T.R. Figueredo a, J.M.P. Almeida a, A.C. Hernandes a, L.A.O. Nunes a, O.L. Malta b a b
~ Carlos, Universidade de Sao ~ Paulo, CP 369, 13560-970 Sa~ o Carlos, SP, Brazil Instituto de Fı´sica de Sao ´ria, Recife-PE 50670-901, Brazil Departamento de Quı´mica Fundamental, Universidade Federal de Pernambuco-CCEN, Cidade Universita
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 November 2011 Received in revised form 26 January 2012 Accepted 7 February 2012 Available online 15 February 2012
The down-conversion process in Tb3 þ –Yb3 þ co-doped Calibo glasses was studied. The emission, excitation and time-resolved measurements indicated the existence of an energy conversion through the excitation of Tb3 þ ions to near-infrared emission by Yb3 þ ions. The emission intensity dependence on excitation power confirms that the one-photon process is responsible for the Yb3 þ emission. An enhanced Yb3 þ emission was observed with Yb3 þ doping and an optimal energy transfer efficiency of 32% was obtained before reaching near-infrared emission quenching. The mechanism of the nonresonant energy transfer from Tb3 þ to Yb3 þ is discussed in terms of the Tb3 þ –Yb3 þ cross-relaxation and multiphonon decay processes. & 2012 Elsevier B.V. All rights reserved.
Keywords: Down-conversion Energy transfer Calibo Tb3 þ –Yb3 þ ions
1. Introduction Recently, there has been a strong interest in sustainable energy production to minimize the negative impact of global energy consumption on the environment. One alternative is to use the energy of the solar spectrum through solar cells; however, the energy provided by these cells is limited because of a relatively high kilowatt hour cost. A reduction in price, however, may be achieved by either lowering the production cost or increasing the conversion efficiency [1]. Single junction solar cell efficiency is limited to 30% (the Schockley–Queisser theoretical limit) mainly because of losses caused by spectral mismatch, in which the largest part of the 70% energy loss is related to the thermalization electron–hole pairs and silicon transparency for the sub-band-gap incident photons [1]. Some theoretical and experimental researches are aimed at developing solar energy converters to enhance solar cell efficiency [2–5]. A great challenge is to efficiently convert the energy from the visible region of the solar spectrum, which has a maximum intensity at 550 nm, to the near-infrared region so that a single junction crystalline silicon solar cell has a major spectral response ( 1000 nm) [6]. Since the first proposal by Dexter (1957) [7], it is well known that downconversion (DC) is a suitable mechanism to convert high-energy photons into low-energy photons. Trupke et al. [5] showed that a DC material, in combination with a single junction crystalline
n
Corresponding author. Tel.: þ55 16 33739835. E-mail address: [email protected] (I.A.A. Terra).
0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2012.02.019
solar cell, could minimize energy losses due to thermalization electron-hole pairs, achieving a theoretical limit for the conversion efficiency of up to 40%. In addition, there have been investigations on DC through the cooperative energy transfer (CET) process (also known as quantum cutting, i.e. the energy conversion from one visible photon to two NIR photons) in different Tb3 þ –Yb3 þ co-doped materials [4,8–12]. The combination of one Tb3 þ (sensitizer) and two Yb3 þ (activator) ions is a promising system to achieve visible–NIR quantum cutting because the Tb3 þ :5D4-7F6 transition occurs at approximately twice the energy of the Yb3 þ :2F7/2-7F5/2 absorption, which will result in a two photon emission approximately at 1000 nm. However, the CET process in Tb3 þ –Yb3 þ systems is a second order energy transfer process (low probability) and is not the only process to achieve NIR emission from Yb3 þ ions. After Tb3 þ excitation, a Tb3 þ –Yb3 þ cross-relaxation mechanism is also possible to excite one Yb3 þ ion with a subsequent energy loss through multiphonon decay process in Tb3 þ ions (see Fig. 1). It is interesting to note that various authors have been analyzing the emission and excitation spectra in combination with lifetime measurements in Tb3 þ –Yb3 þ co-doped in different materials [8–12] and concluded that the Yb3 þ emission is due to quantum cutting through the CET process. However, those authors did not perform power dependent luminescence experiment, which is fundamental to elucidate the origin of the DC process [13,14]. To the best of our knowledge, down-conversion in Tb3 þ –Yb3 þ co-doped lithium calcium borate (Calibo) glass has not yet been reported. Calibo glass is a good host to examine the effects of chemical environments on the optical properties of rare-earth
I.A.A. Terra et al. / Journal of Luminescence 132 (2012) 1678–1682
1679
Excitation measurements were performed using a Xe-lamp as the excitation source, and the excitation light was dispersed by a double-grating (0.22 m, SPEX/1680) monochromator. Lifetime values were estimated from the luminescence decay curves; they were measured by exciting the samples at different wavelengths (488 and 920 nm) with an optical parametrical oscillator (OPO; Surelite/Continumm model SLII-10) pumped by the third harmonic (355 nm) of a Nd–YAG laser (Surelite I/Continumm, 10 Hz, 5 ns), using the same single monochromator (0.3 m) and the PMT or the InGaAs detector. A digital oscilloscope (TekTronix/TDS380) was used to register the decay curves. All of the spectroscopic measurements were performed at room temperature.
3. Results and analysis
Fig. 1. Energy level diagram of Yb3 þ and Tb3 þ ions. The cross-relaxation and multiphonon decay as well as excitation and emission transitions are indicated by dashed dotted, dashed and solid lines, respectively.
ions, due to the high transparency, low melting point and high thermal stability of the glass [15]. Another important characteristic of this glass is its acceptance of a large concentration of rareearth ions. For this purpose, Calibo glass has been selected as the host material in this work, and samples with different concentrations of Yb3 þ (from 0 to 25.9 1020 ions/cm3) and with fixed Tb3 þ (4.0 1020 ions/cm3) were prepared. The down-conversion in the Tb3 þ –Yb3 þ system was investigated using emission and excitation spectra, time-resolved measurements and power dependent luminescence. In our present study, the down-conversion process in Tb3 þ – Yb3 þ co-doped Calibo glasses was studied. One-photon process is responsible for the Yb3 þ emission. An enhanced Yb3 þ emission was observed with increasing the Yb3 þ concentration after a direct excitation of Tb3 þ ions. The mechanism of the nonresonant energy transfer from Tb3 þ to Yb3 þ is discussed in terms of the cross-relaxation and multiphonon decay processes. Our results suggest a potential material for NIR down-conversion.
Fig. 1 shows a simplified energy level diagram of the Tb3 þ and Yb3 þ ions, with the relevant optical transitions for analysis. The cross-relaxation and multiphonon decay as well as excitation and emission transitions are indicated by dashed dotted, dashed and solid lines, respectively. The absorption spectra of the Tb3 þ –Yb3 þ co-doped Calibo glasses were collected in a range between 250 and 1200 nm and the absorption bands corresponding to the Tb3 þ ions excited by transitions from the 7F6 ground state, and the 2F7/2-2F5/2 transition of Yb3 þ ions, were observed (results not shown here). A linear dependence was also observed for the Yb3 þ transition (at 950 nm) versus the concentration of Yb3 þ ions; this is an indication of the successful incorporation of the nominal Yb3 þ doping concentration for the prepared glass samples. The visible photoluminescence spectra of the Tb3 þ –Yb3 þ co-doped Calibo glasses for a fixed concentration of Tb3 þ (4.0 1020 ions/cm3) as a function of the Yb3 þ concentration and under 325 nm excitation are shown in Fig. 2. From these spectra, emission bands centered at 488, 544, 586 and 622 nm are clearly observed. These peaks can be assigned to the 5D4-7FJ (J¼6,5,4,3) transitions of Tb3 þ , and their intensities are drastically reduced by increasing the concentration of Yb3 þ . In addition, the UV emission intensity (not shown in Fig. 2) was weak due to the presence of OH in the samples. The inset shows the integrated intensity of emission band centered at 544 nm as a function of the
2. Experimental details Glass samples with the compositions: (99.5 x)Caliboþ0.5 Tb4O7 þxYb2O3 with x¼0; 0.1; 0.5; 1; 2.5; 5; 7 (% mol) and 93Caliboþ7Yb2O3 (% mol) were prepared by the conventional melt quenching method. A Calibo glass matrix (60B2O3 þ30CaOþ 10Li2O; % mol) was prepared using high-purity B2O3, Li2CO3 and CaCO3. Thereafter, Tb4O7 and Yb2O3 co-doped samples were obtained by melting the Calibo matrix together with the rareearth oxides at approximately 1200 1C for 1 h. The glasses were cut and polished into a plate shape with the following dimensions: 3 10 10 mm. Ground state absorption spectra were obtained by using a Perkin Elmer Lambda 900 spectrophotometer in the range from 250 to 1200 nm. The emission spectra were obtained in the visible and infrared regions using either a HeCd laser (Kimmon/ IK5652R-G) at 325 nm or an Ar þ laser (spectra physics 166/excel beta-I) at 488 nm, respectively, as excitation sources. The visible and infrared luminescence signals were dispersed by a monochromator (0.3 m, Thermo Jarrel Ash/82497), detected with either a photomultiplier tube (PMT) (Hamamatsu/R928) or an InGaAs detector (EG&G/J12D), and amplified using a lock-in amplifier.
Fig. 2. Visible photoluminescence spectra of the Tb3 þ –Yb3 þ co-doped Calibo glasses for a fixed concentration of Tb3 þ (4.0 1020 ions/cm3) as a function of the Yb3 þ concentration under 325 nm excitation. The inset shows the integrated intensity of the emission band of the Tb3 þ :5D4-7F5 transition as a function of the concentration of Yb3 þ .
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concentration of Yb3 þ , showing that the intensity decreases approximately two times as the concentration of Yb3 þ increases from 0.0 to 25.9 1020 ions/cm3. These results indicate the presence of down-conversion process. Some authors [12] propose that this process is a quantum cutting in the Tb3 þ –Yb3 þ co-doped glasses due to a cooperative energy transfer process converting one absorbed photon by Tb3 þ ion to two emitted photons by Yb3 þ ions (Tb3 þ :5D4-Yb3 þ :7F5/2 þYb3 þ :7F5/2). This down-conversion process is analyzed in further detail in the following section. The emission spectra in the NIR (approximately 980 nm) were studied by resonant pumping of the 5D4 level of Tb3 þ at 488 nm. Both NIR emission spectra were monitored and the NIR-integrated intensities were calculated as a function of the concentration of Yb3 þ (see Fig. 3) with a fixed Tb3 þ concentration (4.0 1020 ions/cm3). Fig. 3 shows the emission bands (excited with 488 nm) located at 900–1175 nm due to the 2F5/2-2F7/2 transition of Yb3 þ for concentrations of 0.4, 4.0 and 25.9 1020 ions/cm3 and are represented by solid, dotted and dashed lines, respectively. As shown, the spectra profile changes substantially with the Yb3 þ concentration and there is an apparent shift to the lower energy side due to the Yb3 þ reabsorption process [16]. The inset in Fig. 3 shows the NIR-integrated intensity as a function of the Yb3 þ concentration. The increase in the intensity is consistent with the results presented in Fig. 2, i.e., the presence of down-conversion process. Moreover, the NIR intensity increases with the concentration of Yb3 þ up to 19.0 1020 ions/cm3 and thereafter the intensity decreases for higher Yb3 þ concentration. This decrease in the intensity is probably due to relaxation (losses) mechanisms which include the energy migration between Yb3 þ ions and the OH losses. To further investigate the down-conversion process, the excitation spectrum for the 4.0 1020 ions/cm3 Tbþ 19.0 1020 ions/cm3 Yb co-doped sample excited with a Xe-lamp was collected for the Yb3 þ : 2 F5/2-2F7/2 (lm ¼980 nm monitored wavelength), and the result is shown in Fig. 4. The spectrum clearly shows the Tb3 þ absorption structure from the ground energy level 7F6 to 5D4, and 5D3, etc, and this result confirms the presence of down-conversion process. Time-resolved measurements were performed to extract the decay times and to calculate the energy transfer efficiency from Tb3 þ to Yb3 þ . Fig. 5 shows the luminescence decay times of the Yb3 þ :2F5/2-2F7/2 level (980 and 1070 nm emissions) as a
Fig. 3. NIR emission bands located at 900–1175 nm due to the 2F5/2-2F7/2 transition of Yb3 þ for concentrations of 0.4, 4.0 and 25.9 1020 ions/cm3 in Calibo glasses are represented by solid, dotted and dashed lines, respectively. The inset shows integrated intensity of the Yb3 þ emission via the down-conversion process as a function of the concentration of Yb3 þ with fixed Tb3 þ concentration (4.0 1020 ions/cm3). The excitation wavelength was lexc ¼ 488 nm resonant with the Tb3 þ :5D4 level. The solid line represents a visual guide.
Fig. 4. Excitation spectrum of the Tb3 þ –Yb3 þ (4.0 1020–25.9 1020 ions/cm3, respectively) co-doped Calibo glass, with a monitoring emission of Yb3 þ : 2 F5/2-2F7/2 at lm ¼ 980 nm.
Fig. 5. Decay times for the Yb3 þ :2F5/2-2F7/2 transition at 980 and 1070 nm in Calibo glasses obtained with excitations at 488 and 920 nm (represented by the square and circle symbols, respectively) as a function of the concentration of Yb3 þ .
function of the Yb3 þ concentration with different excitation wavelengths (488 and 920 nm) using, as an excitation source, an OPO pumped by a Nd–YAG laser (5 ns). The circle symbol represents the decay times for excitation at 920 nm in resonance with the Yb3 þ :2F5/2 level. With an excitation at 920 nm, the lifetime is equal to 0.75 ms for the lowest Yb3 þ concentration (0.4 1020 ions/cm3) and this value is approximately the same with the calculated radiative lifetime (0.70 ms) for Yb3 þ : 2F5/ 2 2- F7/2 level emission using McCumber’s theory [17,18]. In addition, the Yb3 þ decay time decreases monotonically (almost three times) with the Yb3 þ concentration up to 25.9 1020 ions/ cm3 due to energy migration between Yb3 þ ions and nearby unintentionally introduced impurities as OH radicals and the up-conversion process 2Yb3 þ -Tb3 þ [19]. However, for the excitation at 488 nm in resonance with the Tb3 þ :5D4 level, the Yb3 þ decay times (represented by the square symbol in Fig. 5) are higher than for the corresponding excitation at 920 nm. This difference is due to the additional time needed to account for the energy transfer process Tb3 þ -Yb3 þ when exciting the Tb3 þ :5D4 level. As can be observed, the Yb3 þ decay time increases with Yb3 þ concentration and reaches a maximum at 4.0 1020 ions/cm3. This lengthening in the Yb3 þ decay time is due to both the energy migration among Yb3 þ neighbor ions and
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Fig. 6. Luminescence decay of the Tb3 þ :5D4-7F5 emission at 544 nm in Calibo glasses excited with a 488 nm pulsed laser (5 ns). The inset shows the dependence of the average decay time of the Tb3 þ :5D4-7F5 emission (the dot symbol) and the energy transfer efficiency (the square symbol) as a function of the Yb3 þ concentration.
the Yb3 þ reabsorption process. These processes are present in all the samples; however, the decay time decreases for concentrations higher than 4.0 1020 ions/cm3 because the probability to activate the mechanism from OH losses is privileged. Fig. 6 shows the dependence of the luminescence decay curves of Tb3 þ monitored at 544 nm (5D4-7F5) as a function of the Yb3 þ concentration under 488 nm excitation. The transient presents a single exponential behavior for the Tb3 þ single-doped sample. However, for the Tb3 þ –Yb3 þ co-doped samples, the decays are non-exponential due to the down-conversion process. The average decay time of the Tb3 þ :5D4-7F5 level was 2.38 ms for the Tb3 þ single-doped sample, and this value is the radiative lifetime for this level because the absence of Yb3 þ rules out any presence of energy transfer from Tb3þ to Yb3 þ . Radiative lifetimes between 2 and 3 ms are the typical values for Tb3þ :5D4-7F5 emission reported for phosphates [12], silicates [15,20] and borates [21] glasses. The Tb3þ :5D4-7F5 decay time decreases with Yb3þ doping until 1.32 ms for a Yb3þ concentration equals 25.9 1020 ions/cm3. This result could be attributed to a depopulation of the Tb3 þ :5D4 level, mainly via the down-conversion process due to the increasing of Yb3 þ acceptor ions (see Fig. 2). The inset in Fig. 6 presents the Tb3þ :5D4-7F5 emission average decay time as a function of the concentration of Yb3þ (the dot symbol). From these decay times, it is possible to calculate the energy transfer efficiencies (ZET) between sensitizer (Tb3 þ ) and acceptor (Yb3 þ ) ions, defined as the ratio of sensitizers that are depopulated by energy transfer to acceptors to the total number of sensitizers being excited. This can be expressed as [4]
ZET ¼ 1
tTb2xYb , tTb20Yb
ð1Þ
where tTb–xYb and tTb–0Yb ¼2.38 ms are the luminescence decay times of the Tb3 þ :5D4 level with and without Yb3 þ doping, respectively. The inset in Fig. 6 also shows the dependence of energy transfer efficiency on the Yb3 þ concentration (the square symbol). The value of ZET is found to increase linearly up to 45% when the concentration of Yb3 þ is increased from 0.4 1020 to 25.9 1020 ions/cm3. This value is comparable to the values reported in the literature in Tb3 þ –Yb3 þ co-doped in different materials: transparent glass ceramics containing CaF2 nanocrystals (55%) [4] and for Y2O3 phosphor (37%) [10]. However, an optimal energy transfer efficiency of 32% was obtained before reaching NIR
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Fig. 7. Emission intensity dependence of down-conversion at 980 nm as a function of the excitation power. The inset shows the energy transfer mechanism of the down-conversion process.
emission quenching for the (4.0Tb3 þ –19.0Yb3 þ ) 1020 ions/cm3 co-doped sample. Up to now, all the above results clearly show a down-conversion process, but those experiments do not clarify the kind of energy transfer mechanism. Excitation power measurements allow elucidation of the origin of the down-conversion process [13,22]. A Pn power law has been found for an n-photon process, in which n is the slope of the double-logarithmic plots of the intensity emission versus excitation power. This slope has been found to be 0.5 [13,22] for a quantum cutting mechanism through the CET process, where two-low energy photons are obtained upon the absorption of one high-energy photon. On the other hand, other authors have obtained n ¼1, indicating a conversion of a high energy into a low energy photon. In addition, it is interesting to note that Strek et al. [13] have observed a temperature dependence for the slope with n ¼0.5 at room temperature and n¼1 at lower temperatures 77 K, indicating the non-resonant nature of the quantum cutting energy transfer process. Therefore, excitation power dependence of the emission intensity of Yb3 þ was studied to elucidate the n-photon process involved in the energy transfer. Fig. 7 shows the dependence of the emission intensity for Yb3 þ at 980 nm as a function of the excitation power through a resonant pumping of Tb3 þ :5D4 level at 488 nm for the 4.0 1020 ions/cm3 Tbþ19.0 1020 ions/cm3 Yb co-doped sample. All the samples showed the slope equals 1.02, similar to the work reported by Ye et al. [14]. Then, the downconversion process of our system is mediated by a one-photon energy transfer and is explained in terms of a Tb3 þ –Yb3 þ crossrelaxation followed by multiphonon decay (see the inset in Fig. 7). The energy transfer rates for a first-order energy transfer are about 104–108 higher than for a second-order energy transfer (CET) [10].
4. Conclusions The down-conversion process in Tb3 þ –Yb3 þ co-doped Calibo glasses has been studied. The conversion of high-energy photon in the visible range to low-energy photon in NIR was confirmed using emission and excitation spectra in combination with time-resolved measurements. The excitation power dependence of the emission intensity of Yb3 þ elucidated that a one-photon process is
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responsible for the Yb3 þ emission under the direct excitation of Tb3 þ ions. An enhanced Yb3 þ emission was observed with increasing Yb3 þ concentration after a direct excitation of Tb3 þ ions. An optimal energy transfer efficiency of 32% was obtained before reaching NIR emission quenching for the (4.0Tb3 þ – 19.0Yb3 þ ) 1020 ions/cm3 co-doped sample. The down-conversion process mediated by a one-photon energy transfer was explained in terms of a Tb3 þ –Yb3 þ cross-relaxation following by multiphonon decay. Finally, all the results showed that Tb3 þ –Yb3 þ co-doped Calibo glass has a potential application as a down-converter material to be used in the NIR region.
Acknowledgments We would like to acknowledge the financial support of the Coordenac- a~ o de Aperfeic-oamento de Pessoal de Nı´vel Superior (CAPES), Fundac- a~ o de Amparo a Pesquisa do Estado de Sa~ o Paulo (FAPESP), Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico (CNPq) and Instituto Nacional de Nanotecnologia para Marcadores Integrados (INAMI).
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