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Optical properties of (ZnO)0.5(P2O5)0.5 glasses doped with Gd2O3:Eu nanoparticles and Eu2O3
ARTICLE IN PRESS Journal of Luminescence 130 (2010) 1379–1383
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Optical properties of (ZnO)0.5(P2O5)0.5 glasses doped with Gd2O3:Eu nanoparticles and Eu2O3 V. Sudarsan, Dheeraj Jain, R.K. Vatsa, C.G.S. Pillai n Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
a r t i c l e in fo
abstract
Article history: Received 21 July 2009 Received in revised form 17 February 2010 Accepted 19 February 2010 Available online 6 March 2010
Binary (ZnO)0.5(P2O5)0.5 glasses doped with Eu2O3 and nanoparticles of Gd2O3:Eu were prepared by conventional melt-quench method and their luminescence properties were compared. Undoped (ZnO)0.5(P2O5)0.5 glass is characterized by a luminescent defect centre (similar to L-centre present in Na2O–SiO2 glasses) with emission around 324 nm and having an excited state lifetime of 18 ns. Such defect centres can transfer the energy to Eu3 + ions leading to improved Eu3 + luminescence from such glasses. Based on the decay curves corresponding to the 5D0 level of Eu3 + ions in both Gd2O3:Eu nanoparticles incorporated as well as Eu2O3 incorporated glasses, a significant clustering of Eu3 + ions taking place with the latter sample is confirmed. From the lifetime studies of the excited state of L-centre emission from (ZnO)0.5(P2O5)0.5 glass doped with Gd2O3:Eu nanoparticles, it is established that there exists weak energy transfer from L-centres to Eu3 + ions. Poor energy transfer from the defect centres to Eu3 + ions in Gd2O3:Eu nanoparticles doped (ZnO)0.5(P2O5)0.5 glass has been attributed to effective shielding of Eu3 + ions from the luminescence centre by Gd–O–P type of linkages, leading to an increased distance between luminescent centre and Eu3 + ions. & 2010 Elsevier B.V. All rights reserved.
Keywords: Optical properties Phosphate glasses Eu3 + ions Gd2O3:Eu nanoparticles
1. Introduction Zinc phosphate based glasses are being studied for their potential to be used as solid-state laser host materials, novel glass–polymer composites, low-melting glass to metal seals, optical glasses for precision molding, etc. [1–4]. Presence of zinc as a network former/modifier not only makes these glasses chemically durable over other phosphate based glasses but also imparts a lower glass transition temperature. A wider glass forming composition range further makes it possible to tune the glass properties over a broad spectrum. These glasses have higher transparency in the ultra-violet region as compared to conventional silicate glasses and are potential materials for optical and luminescent displays. Generally, lanthanide ions are incorporated in glasses to make various optical devices for telecommunication applications. Among the several lanthanides doped phosphate glasses currently being investigated for their emission behavior [5–7], glasses incorporated with Gd3 + ions are of significant interest due to their interesting scintillating properties [8]. Being cheap and efficient scintillating materials, these glasses have the
n Corresponding author. Tel.: + 91 22 25595355: fax: + 91 22 25505151, + 91 22 25519613. E-mail address: [email protected] (C.G.S. Pillai).
0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.02.050
potential of being used as detector materials for high energy particles and radiations. In a previous study [9], we have compared the luminescence properties of (ZnO)0.5(P2O5)0.5 glass doped with Gd2O3:Eu nanoparticles and Eu2O3. Based on these studies it is established that shielding of Eu3 + ions by Gd–O linkages is the main reason for the lack of energy transfer between the lanthanide ions and the luminescent centres present in such glasses. However, the information regarding the nature of the excited state of Eu3 + ions in both type of glasses (glasses directly incorporated with bulk Eu2O3 and glasses incorporated with Gd2O3:Eu nanoparticles) is not probed in that study. This will be useful not only to understand the interaction of Eu3 + ions in the glass host but also to improve the luminescence properties of these lanthanide ions doped glasses since the luminescence properties of Eu3 + ions with respect to its environment is well understood [10–13]. Further, the information obtained will be helpful for predicting the luminescence properties of other lanthanide ions (for example Tb3 + and Dy3 + ) doped glasses. Keeping this in mind, we have continued our studies on undoped zinc phosphate glass; zinc phosphate glass doped with Gd2O3:Eu nanoparticles and Eu2O3 and analyzed their luminescence properties. Nanoparticles of Gd2O3:Eu are chosen to ensure uniform dispersion as well as prevent the aggregation of Gd3 + and Eu3 + ions, if added individually and thereby deteriorating the luminescence properties. Luminescence properties of nanoparticles doped glasses were
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compared to glasses doped with Eu2O3. The results detailing our observations are presented in this paper. To the best of our knowledge, the presented results are the first ones on Gd2O3:Eu nanoparticles incorporated (ZnO)0.5(P2O5)0.5 glasses.
2. Experimental 2.1. Synthesis of Gd2O3:Eu3 + nanoparticles Gd2O3 nanoparticles doped with 5 at.% Eu3 + ions were prepared based on the co-precipitation of Gd3 + and Eu3 + ions in ethylene glycol medium over the temperature range 170–180 1C. Detailed procedure is available in our previous paper [9]. However, briefly it is described below. Around 0.5 g of Gd2O3 and stoichiometric amount of Eu2O3 were dissolved in 5 ml of warm concentrated hydrochloric acid in a 100 ml two necked round bottom flask. The clear solution was repeatedly evaporated to remove the residual acid until a white colored solid was obtained. This was then dissolved in 50 ml of freshly distilled ethylene glycol. The clear solution was first heated to 100 1C followed by slow heating up to 170–180 1C, and at this stage a white turbidity appeared. The temperature was maintained at 180 1C for an hour for the precipitation process to be completed. It was then cooled to room temperature and the precipitate was separated using a centrifuge and washed thoroughly with ethanol and dried under ambient conditions. The precipitate thus obtained was heated to 823 K in air to get crystalline Gd2O3:Eu nanoparticles. The nanoparticles were found to be stable under ambient conditions and used as such for preparing nanoparticles doped glass. 2.2. Synthesis of the (ZnO)0.5(P2O5)0.5 glass Both, undoped and lanthanide ions doped glasses were prepared by conventional melt-quench technique. AR grade zinc oxide (ZnO, BDH Chemicals, 99.9%) and ammonium dihydrogen phosphate (NH4H2PO4, Merck, 99.9%) were mixed in 1:2 molar ratio and ground well. The resulting powder was heated at 873 K for 10 h in a quartz crucible to remove the volatiles. This precursor was then heated to 1423 K in a recrystallized alumina crucible and soaked at this temperature for 2 h. The free-flowing melt was then quenched in between two stainless steel plates at room temperature. The glass buttons were annealed at 600 K for 1 h before their further use. Similarly, Gd2O3:Eu nanoparticles (2 wt.%) and Eu2O3 (2 wt.%) doped glasses were prepared by adding required amount of oxide nanoparticles and pure Eu2O3 to the glass precursor and subjected to extensive grinding for thorough mixing and then melting at 1423 K followed by room temperature quenching. 2.3. Measurements of luminescence properties Room temperature steady state emission and excitation spectrum were recorded using a Hittachi F-4500 machine attached with 150 W Xe lamp as the excitation source. All emission spectra were corrected for the detector response and excitation spectra for the lamp profile. For all steady state measurements, excitation and emission slit widths were kept at 3 nm. Lifetime measurements were carried out using an Edinburgh Instruments FLSP 920 system having excitation sources like 450 W Xe lamp, a microsecond flash lamp with an average power greater than 50 W at 100 Hz and a nanosecond flash lamp having hydrogen as the discharge gas. Lifetimes of the excited state level of Eu3 + in the samples were recorded using
microsecond flash lamp as the excitation source. Decay curves were obtained by multi-channel scaling of time correlated data obtained during a timed sweep of the data channels. For measuring the lifetime of the excited state of the defect centre of the host glass samples, time correlated single photon counting (TCSPC) technique was used with nanosecond flash lamp as the excitation source. Excitation wavelengths were selected using a monochromator. Slit openings on the emission and excitation side were kept at 6 nm when microsecond flash lamp was used as the excitation source and 10 nm when the nanosecond flash lamp was used for exciting the samples. Small glass pieces (both undoped as well as lanthanide ions doped) were mounted on front face sample holder for measuring the emission, excitation spectra and decay curves. For measuring luminescence from Gd2O3:Eu nanoparticles, the sample was mixed with methanol, spread onto a glass slide prior to its mounting inside the sample chamber.
3. Results and discussion XRD studies carried out for these samples confirmed that the Gd2O3:Eu3 + (5 at%) nanoparticles crystallizes in cubic fluorite structure. XRD patterns of the glass samples showed a broad hump around 241 confirming the amorphous nature of the samples [9]. Peaks due to Gd2O3:Eu nanoparticles or Eu2O3 were not seen in the XRD pattern of glass samples suggesting their homogeneous distribution in the glassy phase. Emission spectrum from the Gd2O3:Eu nanoparticles obtained after 398 nm excitation is shown in Fig. 1(a). The spectrum mainly consists of two peaks around 590 and 612 nm, characteristic of magnetic dipole allowed 5D0-7F1 transition and electric dipole allowed 5D0-7F2 transitions, respectively. Excitation spectrum corresponding to 612 nm emission of Eu3 + is shown in Fig. 1(b). In addition to the broad asymmetric peak centered 265 nm with a shoulder around 236 nm characteristic of the overlapping of O2 –Eu3 + charge transfer process and Gd–O host absorption, sharp peaks at 398, 385 and 367 nm characteristic of intra 4ftransitions (shown as an inset in Fig. 1(b)) of Eu3 + ions can be clearly seen. Observation of peak due to host absorption (236 nm) in the excitation spectrum obtained by monitoring Eu3 + emission confirmed the existence of energy transfer between the host and lanthanide ions. These results are in agreement with the previous reports on Gd2O3:Eu nanoparticles/bulk material [14]. Fig. 2 shows the (a) emission spectrum (excitation wavelength 290 nm) and (b) corresponding excitation spectrum from undoped zinc metaphosphate glass. Both, the emission and excitation spectrum consists of a broad peak centered on 324 and 290 nm, respectively. Based on the previous studies [9,16,17], these peaks can be attributed to transition between luminescence centres known as L-centres present in the glass. L-centres are nothing but color centres and were first observed in alkali silicate glasses mainly arising due to the presence of Si–O –Na + type of linkages in the glass. In binary Na2O–SiO2 glasses, the ground state of the L-centre corresponds to the ionic bonding between the nonbridging O ion and Na + ions. Excited state is formed by the transfer of an electron from O to Na + , resulting in O0 Na covalent bond. The excited state can be either a singlet or a triplet and a triplet to singlet transition in the excited state is associated with decrease in the lifetime of the emitting state (triplet state). By taking analogy of L-centres in alkali silicate glasses, the L-centres in phosphate based glasses arises due to P–O –Zn2 + – O –P type of linkages present in the glass. Similar to the luminescent levels present in Na2O–SiO2 glasses, [15,16], ground state of these color centres in (ZnO)0.5(P2O5)0.5 glasses is P–O – Zn2 + –O –P type of ionic linkages and the excited state is P–O– Zn–O–P type of covalent linkages arising from transfer of one
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Fig. 1. Emission spectrum of Gd2O3:Eu nanoparticles (a) obtained after excitation at 398 nm. The excitation spectrum monitored at 612 nm emission is shown in (b). The inset of (b) shows the enlarged view of the excitation spectrum over the region of 290–450 nm.
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electron each from P–O linkages to Zn2 + . Transition from this excited state to the ground state gives rise to the defect emission around 324 nm. Similar emission spectrum has also been observed in PbO–SiO2–Na2O glasses, due to transition between excited state Si–O–Na and ground state SiO –Na + linkages present in the glass [16]. Decay curve corresponding to this L-centre emission is shown in Fig. 2(c). The excited state corresponding to L-centre decays single exponentially with a lifetime of around 18 ns. From the emission and excitation spectrum shown in Fig. 2, it can be seen that there exists a considerable overlap between the emission and excitation spectrum. In such systems, it is possible to have significant radiation trapping as reported by many authors [18,19]. Radiation trapping results in the overestimation of the quantum yield of emission as well as the excited state lifetime [18]. Further, the excited state lifetime for such samples is a function of the thickness (optical path length) of the samples [18,19]. However in the present study, from the lifetime measurements carried out for glass samples with different thicknesses, it is observed that the lifetime of the excited state of the L-centre is almost same, indicating that the radiation trapping effect is quite weak in these samples. At this stage it is also worthwhile to mention that the L-centre emission and excitation line widths are relatively sharper compared to the ones from organic molecules. In our previous study on (ZnO)0.5(P2O5)0.5 glasses doped with Gd2O3:Eu nanoparticles and Eu2O3 [9], it is demonstrated that strong Eu3 + emission is observed from Eu2O3 doped (ZnO)0.5
(P2O5)0.5 glass whereas, very weak emission from Eu3 + energy levels is seen from glass doped with Gd2O3:Eu nanoparticles after excitation at 290 nm. This is explained mainly due to the combined effect of low concentration of Eu3 + ions and lack of energy transfer from the defect luminescent centres to Eu3 + in Gd2O3:Eu nanoparticles doped glasses compared to Eu2O3 doped glasses. Lack of energy transfer is understandable as it depends on the extent of overlap between donor (D) emission peak (L-centre emission peak in the present case) and acceptor (A) absorption peak (Eu3 + excitation peak in the present case) and expressed by [20] Z 4p 2 gD ðEÞUgA ðEÞ 9 o D,A 9HDA 9D ,A 4 9 ð1Þ PDA ¼ h where PDA is the rate of energy transfer from donor to acceptor, Dn and An represent the excited state of the donor and acceptor, HDA is the interaction Hamiltonian, gD(E) and gA(E) are normalized optical line shape functions of donor and acceptor, respectively. Value of the integral depends on the extent of overlap between the donor emission and acceptor absorption. PDA also strongly depends on the critical distance between donor and acceptor and excited state lifetimes of donor and acceptor. The critical distance is nothing but the minimum distance between donor and acceptor above which no energy transfer between them is possible. Higher PDA values indicate a stronger energy transfer whereas the lower values indicate weak or poor energy transfer. As multi-pole interactions, which are proportional to the 1/R6 where R is the
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distance between donor and acceptor, are responsible for the through space energy transfer processes, shorter distances favors higher extent of energy transfer. In the case of (ZnO)0.5(P2O5)0.5 glass containing Gd2O3:Eu nanoparticles, a very few number of Eu3 + ions (the ones present on the surface of these nanoparticles) can only interact with the defect centres present in the glass. This is because in Gd2O3:Eu nanoparticles incorporated glass samples, Eu3 + ions are covered by Gd–O–P type linkages and hence is associated with an increased distance between the Eu3 + and defect luminescent centres in the host. In addition to this, the low concentration of Eu3 + ions in Gd2O3:Eu nanoparticles incorporated (ZnO)0.5(P2O5)0.5 glass compared to Eu2O3 incorporated glass, results in the further reduction in the extent of energy transfer process. Even though the energy transfer is poor, the clustering of the Eu3 + ions in the glass can be reduced when Gd2O3:Eu nanoparticles are incorporated in the glass compared to Eu2O3 incorporated glass, as can be seen from the decay curve corresponding to the 5D0 level of Eu3 + in these samples, which are described in the following section. Fig. 3 shows the decay curves corresponding to 612 nm emission from (a) Gd2O3:Eu nanoparticles doped glass and (b) Eu2O3 incorporated glass samples. For nanoparticle incorporated glass sample, the 5D0 level decays biexponentially with lifetime values around 0.5 ms (5%) and 2.2 ms (95%). This is understandable as Eu3 + ions in the nanoparticle incorporated glass will have at least two different environments, namely the ones having near-neighbour Gd3 + ion and the other the surface Eu3 + ions with Eu–O–P type of linkages. On the other hand, for Eu2O3 doped glasses, a quick glance suggests a single exponential decay of 5D0 level with a lifetime of around 2.5 ms. However, a careful look at the decay curve reveals that there exists a growth
ð2Þ
where td and tr are decay time and rise time, respectively. Imax is the maximum intensity at 612 nm emission, I(t), emission intensity at any time t and Ib is the background intensity, if any. The rise time and decay time for Eu2O3 doped glasses is found to be 0.2 and 2.5 ms, respectively. Growth occurs due to the feeding of 5D0 level from 5D1 level of Eu3 + ions. This is because, due to the clustering of the Eu3 + ions, non-radiative decay of 5D1 level becomes faster (due to non-radiative processes like crossrelaxation and self-quenching brought about by the decrease in inter-ionic distances) resulting in the build-up of the population in the 5D0 levels. For nanoparticles incorporated glass sample, no such clustering occurs as can be clearly seen from the decay curve shown in Fig. 3(a). Hence, based on the decay curves corresponding to 5D0 level of Eu3 + ions in both Gd2O3:Eu nanoparticles and Eu2O3 doped glasses, it is confirmed that significant clustering takes place in the latter samples as compared to the former. With a view to find out whether the clustering is also reflected in the intensity of emission peak from the 5D1 level of Eu3 + in nanoparticles incorporated glasses as well as Eu2O3 incorporated glasses, emission spectrum recorded after 394 nm excitation from both the samples and are shown in Fig. 4. Emission intensity from the 5D1 level is found to be more in the case of nanoparticle incorporated glass compared to Eu2O3 incorporated glass. Lifetime of the 5D1 level of Eu3 + could not be recorded in both the sample due to their very weak nature. These results further substantiate our previous inferences that the clustering of Eu3 + ions in Eu2O3 incorporated glass sample leads to the non-radiative decay from the 5D1 level to 5D0 level of Eu3 + ion thereby reducing the intensity of emissions from 5D1 levels of Eu3 + ions in the sample. It is known that the clustering is a concentration dependent phenomena and the effect decreases with decrease in Eu3 + concentration in the glass. Fig. 5 shows the decay curve corresponding to 5D0 level of Eu3 + ions present in 0.25 wt.% Eu2O3 doped glasses. In fact it is observed that the ZnO–P2O5 glasses with 0.25 wt.% Eu2O3 showed only a single exponential decay (with lifetime of around 2.5 ms) without any rise time at the starting of the decay as can be seen clearly from inset of Fig. 5. Fig. 6 shows the decay curve corresponding to the L-centre emission from Gd2O3:Eu nanoparticles doped glass sample. Unlike in the pure (ZnO)0.5(P2O5)0.5 glass, the excited state here decays biexponentially comprising of an initial fast decay component
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doped glasses, it is confirmed that, the energy transfer taking place between the L-centre and the lanthanide ions is incomplete and this arises due to the effective shielding of Eu3 + ions from the L-centres by the Gd–O linkages of Gd2O3 host. Eu3 + ions either present on the nanoparticles surface or close to the nanoparticles surface can therefore only get energy transferred to them from these L-centres.
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Based on the 5D0 lifetime measurements of Eu3 + ions in (ZnO)0.5(P2O5)0.5 glasses containing Gd2O3:Eu nanoparticles and Eu2O3, it has been concluded that, a significant clustering of Eu3 + ions takes place in the Eu2O3 doped glass as compared to Gd2O3:Eu nanoparticles doped glass. The clustering is associated with increase in the extent of non-radiative decay of the 5D1 level to populate 5D0 level of Eu3 + , thereby decreasing the luminescence intensity of 5D1 emission in Eu2O3 incorporated glasses compared to nanoparticles incorporated glasses. From the weak biexponentially nature of the decay corresponding to the L-centre emission from nanoparticles doped glasses, it is concluded that there exists weak energy transfer between defect centre and Eu3 + ions in them.
Acknowledgement The authors are grateful to Dr. D. Das, Head, Chemistry Division for his keen interest and encouragement during the course of this work.
10 References
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Time (ns) Fig. 6. Decay curve corresponding to the L-centre emission (324 nm) from Gd2O3:Eu doped (ZnO)0.5(P2O5)0.5 glass sample. Sample was excited at 290 nm and emission monitored at 324 nm. (Black curve denotes the experimental decay of the sample, red curve represents instrument response and the blue curve is the residuals difference between the experimental and fitted profiles.)
with a lifetime of 573 ps (10%) followed by a slow decay component with a lifetime of 18 ns (90%). The fast decay component has been attributed to the L-centres, which are in close proximity with the Eu3 + ions whereas the slow decay component is due to L-centres that are away form the Eu3 + ions. As the energy transfer occurs through dipole–dipole interactions, lanthanide ions closer to these defect centres will have efficient energy transfer compared to the ones that are away from these L-centres. Hence, based on the decay curve for the L-centre emission from both undoped as well as Gd2O3:Eu nanoparticles
[1] N. Peyghambarian, T. Qiu, P. Polynkin, A. Schulzgen, L. Li, V. Temyanko, M. Mansuripur, J.V. Moloney, Opt. Photonics News (2004) 41. [2] J.W. Dadge, S.K. Aptea, S.D. Naika, B.B. Kalea, R.C. Aiyer, J. Optoelctro. Adv. Mater. 8 (2006) 368. [3] H.S. Liu, T.S. Chin, Phys. Chem. Glasses 38 (1997) 123. [4] R.K. Brow, J. Non-Cryst. Solids 263 (264) (2000) 1. [5] J.E. Marion, M.J. Weber, Eur. J. Solid State Inorg. Chem. 28 (1991) 271. [6] R. Reisfeld, L. Boehm, J. Solid State Chem. 4 (1972) 417. [7] B.C. Joshi, M.C. Joshi, J. Non-Cryst. Solids 142 (1992) 171. [8] J. Fu, M. Kobayashi, J.M. Parker, J. Lumin. 94 (95) (2001) 321. [9] D. Jain, V. Sudarsan, R.K. Vatsa, C.G.S. Pillai, J. Lumin. 129 (2009) 439. [10] V. Sudarsan, Frank C.J.M. Van Veggel, Rodeny A. Herring, M. Raudsepp, J. Mater. Chem. 15 (2005) 1332. [11] V. Sudarsan, S. Sivakumar, F.C.J.M. Van Veggel, M. Raudsepp, Chem. Mater. 17 (2005) 4736. [12] R.S. Ningthoujam, V. Sudarsan, S.K. Kulshreshtha, J. Lumin. 127 (2007) 747. [13] C.E. Secu, D. Predoi, M. Secu, M. Cernea, G. Aldica, Opt. Mater. 31 (2009) 1745. [14] C.C. Lin, K.M. Lin, Y.Y. Li, J. Lumin. 126 (2007) 795 the references therein. [15] A.N. Trukhin, J. Non-Cryst. Solids 123 (1990) 250. [16] A.N. Trukhin, J. Non-Cryst. Solids 189 (1995) 1. [17] V.K. Shrikhande, V. Sudarsan, G.P. Kothiyal, S.K. Kulshreshtha, J. Non-Cryst. Solids 353 (2007) 1341. [18] M. Liao, L. Hu, Y. Fang, S. Xu, L. Zhang, J. Opt. Soc. Am. B 24 (2007) 1498. [19] Y. Zhou, S. Dai, J. Wang, T. Xu, Q. Nie, S. Huang, J. Alloys Compd. 472 (2009) 203. [20] G. Blasse, B.C. Grabmaier, in: Luminescent Materials, Springer-Verlag, Berlin, 1994. [21] V.L. Ermolaev, E.B. Sveshnikova, Russ. Chem. Rev. 63 (1994) 905 references cited therein..
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Optical properties of (ZnO)0.5(P2O5)0.5 glasses doped with Gd2O3:Eu nanoparticles and Eu2O3 V. Sudarsan, Dheeraj Jain, R.K. Vatsa, C.G.S. Pillai n Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
a r t i c l e in fo
abstract
Article history: Received 21 July 2009 Received in revised form 17 February 2010 Accepted 19 February 2010 Available online 6 March 2010
Binary (ZnO)0.5(P2O5)0.5 glasses doped with Eu2O3 and nanoparticles of Gd2O3:Eu were prepared by conventional melt-quench method and their luminescence properties were compared. Undoped (ZnO)0.5(P2O5)0.5 glass is characterized by a luminescent defect centre (similar to L-centre present in Na2O–SiO2 glasses) with emission around 324 nm and having an excited state lifetime of 18 ns. Such defect centres can transfer the energy to Eu3 + ions leading to improved Eu3 + luminescence from such glasses. Based on the decay curves corresponding to the 5D0 level of Eu3 + ions in both Gd2O3:Eu nanoparticles incorporated as well as Eu2O3 incorporated glasses, a significant clustering of Eu3 + ions taking place with the latter sample is confirmed. From the lifetime studies of the excited state of L-centre emission from (ZnO)0.5(P2O5)0.5 glass doped with Gd2O3:Eu nanoparticles, it is established that there exists weak energy transfer from L-centres to Eu3 + ions. Poor energy transfer from the defect centres to Eu3 + ions in Gd2O3:Eu nanoparticles doped (ZnO)0.5(P2O5)0.5 glass has been attributed to effective shielding of Eu3 + ions from the luminescence centre by Gd–O–P type of linkages, leading to an increased distance between luminescent centre and Eu3 + ions. & 2010 Elsevier B.V. All rights reserved.
Keywords: Optical properties Phosphate glasses Eu3 + ions Gd2O3:Eu nanoparticles
1. Introduction Zinc phosphate based glasses are being studied for their potential to be used as solid-state laser host materials, novel glass–polymer composites, low-melting glass to metal seals, optical glasses for precision molding, etc. [1–4]. Presence of zinc as a network former/modifier not only makes these glasses chemically durable over other phosphate based glasses but also imparts a lower glass transition temperature. A wider glass forming composition range further makes it possible to tune the glass properties over a broad spectrum. These glasses have higher transparency in the ultra-violet region as compared to conventional silicate glasses and are potential materials for optical and luminescent displays. Generally, lanthanide ions are incorporated in glasses to make various optical devices for telecommunication applications. Among the several lanthanides doped phosphate glasses currently being investigated for their emission behavior [5–7], glasses incorporated with Gd3 + ions are of significant interest due to their interesting scintillating properties [8]. Being cheap and efficient scintillating materials, these glasses have the
n Corresponding author. Tel.: + 91 22 25595355: fax: + 91 22 25505151, + 91 22 25519613. E-mail address: [email protected] (C.G.S. Pillai).
0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.02.050
potential of being used as detector materials for high energy particles and radiations. In a previous study [9], we have compared the luminescence properties of (ZnO)0.5(P2O5)0.5 glass doped with Gd2O3:Eu nanoparticles and Eu2O3. Based on these studies it is established that shielding of Eu3 + ions by Gd–O linkages is the main reason for the lack of energy transfer between the lanthanide ions and the luminescent centres present in such glasses. However, the information regarding the nature of the excited state of Eu3 + ions in both type of glasses (glasses directly incorporated with bulk Eu2O3 and glasses incorporated with Gd2O3:Eu nanoparticles) is not probed in that study. This will be useful not only to understand the interaction of Eu3 + ions in the glass host but also to improve the luminescence properties of these lanthanide ions doped glasses since the luminescence properties of Eu3 + ions with respect to its environment is well understood [10–13]. Further, the information obtained will be helpful for predicting the luminescence properties of other lanthanide ions (for example Tb3 + and Dy3 + ) doped glasses. Keeping this in mind, we have continued our studies on undoped zinc phosphate glass; zinc phosphate glass doped with Gd2O3:Eu nanoparticles and Eu2O3 and analyzed their luminescence properties. Nanoparticles of Gd2O3:Eu are chosen to ensure uniform dispersion as well as prevent the aggregation of Gd3 + and Eu3 + ions, if added individually and thereby deteriorating the luminescence properties. Luminescence properties of nanoparticles doped glasses were
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compared to glasses doped with Eu2O3. The results detailing our observations are presented in this paper. To the best of our knowledge, the presented results are the first ones on Gd2O3:Eu nanoparticles incorporated (ZnO)0.5(P2O5)0.5 glasses.
2. Experimental 2.1. Synthesis of Gd2O3:Eu3 + nanoparticles Gd2O3 nanoparticles doped with 5 at.% Eu3 + ions were prepared based on the co-precipitation of Gd3 + and Eu3 + ions in ethylene glycol medium over the temperature range 170–180 1C. Detailed procedure is available in our previous paper [9]. However, briefly it is described below. Around 0.5 g of Gd2O3 and stoichiometric amount of Eu2O3 were dissolved in 5 ml of warm concentrated hydrochloric acid in a 100 ml two necked round bottom flask. The clear solution was repeatedly evaporated to remove the residual acid until a white colored solid was obtained. This was then dissolved in 50 ml of freshly distilled ethylene glycol. The clear solution was first heated to 100 1C followed by slow heating up to 170–180 1C, and at this stage a white turbidity appeared. The temperature was maintained at 180 1C for an hour for the precipitation process to be completed. It was then cooled to room temperature and the precipitate was separated using a centrifuge and washed thoroughly with ethanol and dried under ambient conditions. The precipitate thus obtained was heated to 823 K in air to get crystalline Gd2O3:Eu nanoparticles. The nanoparticles were found to be stable under ambient conditions and used as such for preparing nanoparticles doped glass. 2.2. Synthesis of the (ZnO)0.5(P2O5)0.5 glass Both, undoped and lanthanide ions doped glasses were prepared by conventional melt-quench technique. AR grade zinc oxide (ZnO, BDH Chemicals, 99.9%) and ammonium dihydrogen phosphate (NH4H2PO4, Merck, 99.9%) were mixed in 1:2 molar ratio and ground well. The resulting powder was heated at 873 K for 10 h in a quartz crucible to remove the volatiles. This precursor was then heated to 1423 K in a recrystallized alumina crucible and soaked at this temperature for 2 h. The free-flowing melt was then quenched in between two stainless steel plates at room temperature. The glass buttons were annealed at 600 K for 1 h before their further use. Similarly, Gd2O3:Eu nanoparticles (2 wt.%) and Eu2O3 (2 wt.%) doped glasses were prepared by adding required amount of oxide nanoparticles and pure Eu2O3 to the glass precursor and subjected to extensive grinding for thorough mixing and then melting at 1423 K followed by room temperature quenching. 2.3. Measurements of luminescence properties Room temperature steady state emission and excitation spectrum were recorded using a Hittachi F-4500 machine attached with 150 W Xe lamp as the excitation source. All emission spectra were corrected for the detector response and excitation spectra for the lamp profile. For all steady state measurements, excitation and emission slit widths were kept at 3 nm. Lifetime measurements were carried out using an Edinburgh Instruments FLSP 920 system having excitation sources like 450 W Xe lamp, a microsecond flash lamp with an average power greater than 50 W at 100 Hz and a nanosecond flash lamp having hydrogen as the discharge gas. Lifetimes of the excited state level of Eu3 + in the samples were recorded using
microsecond flash lamp as the excitation source. Decay curves were obtained by multi-channel scaling of time correlated data obtained during a timed sweep of the data channels. For measuring the lifetime of the excited state of the defect centre of the host glass samples, time correlated single photon counting (TCSPC) technique was used with nanosecond flash lamp as the excitation source. Excitation wavelengths were selected using a monochromator. Slit openings on the emission and excitation side were kept at 6 nm when microsecond flash lamp was used as the excitation source and 10 nm when the nanosecond flash lamp was used for exciting the samples. Small glass pieces (both undoped as well as lanthanide ions doped) were mounted on front face sample holder for measuring the emission, excitation spectra and decay curves. For measuring luminescence from Gd2O3:Eu nanoparticles, the sample was mixed with methanol, spread onto a glass slide prior to its mounting inside the sample chamber.
3. Results and discussion XRD studies carried out for these samples confirmed that the Gd2O3:Eu3 + (5 at%) nanoparticles crystallizes in cubic fluorite structure. XRD patterns of the glass samples showed a broad hump around 241 confirming the amorphous nature of the samples [9]. Peaks due to Gd2O3:Eu nanoparticles or Eu2O3 were not seen in the XRD pattern of glass samples suggesting their homogeneous distribution in the glassy phase. Emission spectrum from the Gd2O3:Eu nanoparticles obtained after 398 nm excitation is shown in Fig. 1(a). The spectrum mainly consists of two peaks around 590 and 612 nm, characteristic of magnetic dipole allowed 5D0-7F1 transition and electric dipole allowed 5D0-7F2 transitions, respectively. Excitation spectrum corresponding to 612 nm emission of Eu3 + is shown in Fig. 1(b). In addition to the broad asymmetric peak centered 265 nm with a shoulder around 236 nm characteristic of the overlapping of O2 –Eu3 + charge transfer process and Gd–O host absorption, sharp peaks at 398, 385 and 367 nm characteristic of intra 4ftransitions (shown as an inset in Fig. 1(b)) of Eu3 + ions can be clearly seen. Observation of peak due to host absorption (236 nm) in the excitation spectrum obtained by monitoring Eu3 + emission confirmed the existence of energy transfer between the host and lanthanide ions. These results are in agreement with the previous reports on Gd2O3:Eu nanoparticles/bulk material [14]. Fig. 2 shows the (a) emission spectrum (excitation wavelength 290 nm) and (b) corresponding excitation spectrum from undoped zinc metaphosphate glass. Both, the emission and excitation spectrum consists of a broad peak centered on 324 and 290 nm, respectively. Based on the previous studies [9,16,17], these peaks can be attributed to transition between luminescence centres known as L-centres present in the glass. L-centres are nothing but color centres and were first observed in alkali silicate glasses mainly arising due to the presence of Si–O –Na + type of linkages in the glass. In binary Na2O–SiO2 glasses, the ground state of the L-centre corresponds to the ionic bonding between the nonbridging O ion and Na + ions. Excited state is formed by the transfer of an electron from O to Na + , resulting in O0 Na covalent bond. The excited state can be either a singlet or a triplet and a triplet to singlet transition in the excited state is associated with decrease in the lifetime of the emitting state (triplet state). By taking analogy of L-centres in alkali silicate glasses, the L-centres in phosphate based glasses arises due to P–O –Zn2 + – O –P type of linkages present in the glass. Similar to the luminescent levels present in Na2O–SiO2 glasses, [15,16], ground state of these color centres in (ZnO)0.5(P2O5)0.5 glasses is P–O – Zn2 + –O –P type of ionic linkages and the excited state is P–O– Zn–O–P type of covalent linkages arising from transfer of one
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Fig. 1. Emission spectrum of Gd2O3:Eu nanoparticles (a) obtained after excitation at 398 nm. The excitation spectrum monitored at 612 nm emission is shown in (b). The inset of (b) shows the enlarged view of the excitation spectrum over the region of 290–450 nm.
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Fig. 2. Emission spectrum (a) and excitation spectrum (b) of pure (ZnO)0.5(P2O5)0.5 glass sample obtained by 290 nm excitation (inset shows the excitation spectrum of the same sample). Decay curve corresponding to the L-centre emission from (ZnO)0.5(P2O5)05 glass is shown in (c). Sample was excited at 290 nm and emission monitored at 324 nm. (Black curve denotes the experimental decay of the sample, red curve represents instrument response and the blue curve is the residuals difference between the experimental and fitted profiles).
electron each from P–O linkages to Zn2 + . Transition from this excited state to the ground state gives rise to the defect emission around 324 nm. Similar emission spectrum has also been observed in PbO–SiO2–Na2O glasses, due to transition between excited state Si–O–Na and ground state SiO –Na + linkages present in the glass [16]. Decay curve corresponding to this L-centre emission is shown in Fig. 2(c). The excited state corresponding to L-centre decays single exponentially with a lifetime of around 18 ns. From the emission and excitation spectrum shown in Fig. 2, it can be seen that there exists a considerable overlap between the emission and excitation spectrum. In such systems, it is possible to have significant radiation trapping as reported by many authors [18,19]. Radiation trapping results in the overestimation of the quantum yield of emission as well as the excited state lifetime [18]. Further, the excited state lifetime for such samples is a function of the thickness (optical path length) of the samples [18,19]. However in the present study, from the lifetime measurements carried out for glass samples with different thicknesses, it is observed that the lifetime of the excited state of the L-centre is almost same, indicating that the radiation trapping effect is quite weak in these samples. At this stage it is also worthwhile to mention that the L-centre emission and excitation line widths are relatively sharper compared to the ones from organic molecules. In our previous study on (ZnO)0.5(P2O5)0.5 glasses doped with Gd2O3:Eu nanoparticles and Eu2O3 [9], it is demonstrated that strong Eu3 + emission is observed from Eu2O3 doped (ZnO)0.5
(P2O5)0.5 glass whereas, very weak emission from Eu3 + energy levels is seen from glass doped with Gd2O3:Eu nanoparticles after excitation at 290 nm. This is explained mainly due to the combined effect of low concentration of Eu3 + ions and lack of energy transfer from the defect luminescent centres to Eu3 + in Gd2O3:Eu nanoparticles doped glasses compared to Eu2O3 doped glasses. Lack of energy transfer is understandable as it depends on the extent of overlap between donor (D) emission peak (L-centre emission peak in the present case) and acceptor (A) absorption peak (Eu3 + excitation peak in the present case) and expressed by [20] Z 4p 2 gD ðEÞUgA ðEÞ 9 o D,A 9HDA 9D ,A 4 9 ð1Þ PDA ¼ h where PDA is the rate of energy transfer from donor to acceptor, Dn and An represent the excited state of the donor and acceptor, HDA is the interaction Hamiltonian, gD(E) and gA(E) are normalized optical line shape functions of donor and acceptor, respectively. Value of the integral depends on the extent of overlap between the donor emission and acceptor absorption. PDA also strongly depends on the critical distance between donor and acceptor and excited state lifetimes of donor and acceptor. The critical distance is nothing but the minimum distance between donor and acceptor above which no energy transfer between them is possible. Higher PDA values indicate a stronger energy transfer whereas the lower values indicate weak or poor energy transfer. As multi-pole interactions, which are proportional to the 1/R6 where R is the
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before the decay as can be seen from the inset of Fig. 3(b). Considering both, the growth and the decay, the observed curve has been fitted using the following relation:[21] IðtÞ ¼ Imax expðt=t d Þ½1expðt=t r Þ þ Ib 100 0
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distance between donor and acceptor, are responsible for the through space energy transfer processes, shorter distances favors higher extent of energy transfer. In the case of (ZnO)0.5(P2O5)0.5 glass containing Gd2O3:Eu nanoparticles, a very few number of Eu3 + ions (the ones present on the surface of these nanoparticles) can only interact with the defect centres present in the glass. This is because in Gd2O3:Eu nanoparticles incorporated glass samples, Eu3 + ions are covered by Gd–O–P type linkages and hence is associated with an increased distance between the Eu3 + and defect luminescent centres in the host. In addition to this, the low concentration of Eu3 + ions in Gd2O3:Eu nanoparticles incorporated (ZnO)0.5(P2O5)0.5 glass compared to Eu2O3 incorporated glass, results in the further reduction in the extent of energy transfer process. Even though the energy transfer is poor, the clustering of the Eu3 + ions in the glass can be reduced when Gd2O3:Eu nanoparticles are incorporated in the glass compared to Eu2O3 incorporated glass, as can be seen from the decay curve corresponding to the 5D0 level of Eu3 + in these samples, which are described in the following section. Fig. 3 shows the decay curves corresponding to 612 nm emission from (a) Gd2O3:Eu nanoparticles doped glass and (b) Eu2O3 incorporated glass samples. For nanoparticle incorporated glass sample, the 5D0 level decays biexponentially with lifetime values around 0.5 ms (5%) and 2.2 ms (95%). This is understandable as Eu3 + ions in the nanoparticle incorporated glass will have at least two different environments, namely the ones having near-neighbour Gd3 + ion and the other the surface Eu3 + ions with Eu–O–P type of linkages. On the other hand, for Eu2O3 doped glasses, a quick glance suggests a single exponential decay of 5D0 level with a lifetime of around 2.5 ms. However, a careful look at the decay curve reveals that there exists a growth
ð2Þ
where td and tr are decay time and rise time, respectively. Imax is the maximum intensity at 612 nm emission, I(t), emission intensity at any time t and Ib is the background intensity, if any. The rise time and decay time for Eu2O3 doped glasses is found to be 0.2 and 2.5 ms, respectively. Growth occurs due to the feeding of 5D0 level from 5D1 level of Eu3 + ions. This is because, due to the clustering of the Eu3 + ions, non-radiative decay of 5D1 level becomes faster (due to non-radiative processes like crossrelaxation and self-quenching brought about by the decrease in inter-ionic distances) resulting in the build-up of the population in the 5D0 levels. For nanoparticles incorporated glass sample, no such clustering occurs as can be clearly seen from the decay curve shown in Fig. 3(a). Hence, based on the decay curves corresponding to 5D0 level of Eu3 + ions in both Gd2O3:Eu nanoparticles and Eu2O3 doped glasses, it is confirmed that significant clustering takes place in the latter samples as compared to the former. With a view to find out whether the clustering is also reflected in the intensity of emission peak from the 5D1 level of Eu3 + in nanoparticles incorporated glasses as well as Eu2O3 incorporated glasses, emission spectrum recorded after 394 nm excitation from both the samples and are shown in Fig. 4. Emission intensity from the 5D1 level is found to be more in the case of nanoparticle incorporated glass compared to Eu2O3 incorporated glass. Lifetime of the 5D1 level of Eu3 + could not be recorded in both the sample due to their very weak nature. These results further substantiate our previous inferences that the clustering of Eu3 + ions in Eu2O3 incorporated glass sample leads to the non-radiative decay from the 5D1 level to 5D0 level of Eu3 + ion thereby reducing the intensity of emissions from 5D1 levels of Eu3 + ions in the sample. It is known that the clustering is a concentration dependent phenomena and the effect decreases with decrease in Eu3 + concentration in the glass. Fig. 5 shows the decay curve corresponding to 5D0 level of Eu3 + ions present in 0.25 wt.% Eu2O3 doped glasses. In fact it is observed that the ZnO–P2O5 glasses with 0.25 wt.% Eu2O3 showed only a single exponential decay (with lifetime of around 2.5 ms) without any rise time at the starting of the decay as can be seen clearly from inset of Fig. 5. Fig. 6 shows the decay curve corresponding to the L-centre emission from Gd2O3:Eu nanoparticles doped glass sample. Unlike in the pure (ZnO)0.5(P2O5)0.5 glass, the excited state here decays biexponentially comprising of an initial fast decay component
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doped glasses, it is confirmed that, the energy transfer taking place between the L-centre and the lanthanide ions is incomplete and this arises due to the effective shielding of Eu3 + ions from the L-centres by the Gd–O linkages of Gd2O3 host. Eu3 + ions either present on the nanoparticles surface or close to the nanoparticles surface can therefore only get energy transferred to them from these L-centres.
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Based on the 5D0 lifetime measurements of Eu3 + ions in (ZnO)0.5(P2O5)0.5 glasses containing Gd2O3:Eu nanoparticles and Eu2O3, it has been concluded that, a significant clustering of Eu3 + ions takes place in the Eu2O3 doped glass as compared to Gd2O3:Eu nanoparticles doped glass. The clustering is associated with increase in the extent of non-radiative decay of the 5D1 level to populate 5D0 level of Eu3 + , thereby decreasing the luminescence intensity of 5D1 emission in Eu2O3 incorporated glasses compared to nanoparticles incorporated glasses. From the weak biexponentially nature of the decay corresponding to the L-centre emission from nanoparticles doped glasses, it is concluded that there exists weak energy transfer between defect centre and Eu3 + ions in them.
Acknowledgement The authors are grateful to Dr. D. Das, Head, Chemistry Division for his keen interest and encouragement during the course of this work.
10 References
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Time (ns) Fig. 6. Decay curve corresponding to the L-centre emission (324 nm) from Gd2O3:Eu doped (ZnO)0.5(P2O5)0.5 glass sample. Sample was excited at 290 nm and emission monitored at 324 nm. (Black curve denotes the experimental decay of the sample, red curve represents instrument response and the blue curve is the residuals difference between the experimental and fitted profiles.)
with a lifetime of 573 ps (10%) followed by a slow decay component with a lifetime of 18 ns (90%). The fast decay component has been attributed to the L-centres, which are in close proximity with the Eu3 + ions whereas the slow decay component is due to L-centres that are away form the Eu3 + ions. As the energy transfer occurs through dipole–dipole interactions, lanthanide ions closer to these defect centres will have efficient energy transfer compared to the ones that are away from these L-centres. Hence, based on the decay curve for the L-centre emission from both undoped as well as Gd2O3:Eu nanoparticles
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