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Structural influence of Bi3 + ions on physical properties of Na2CuSiO4 glasses photoluminescence and thermoluminescence studies
Journal of Non-Crystalline Solids 449 (2016) 50–54
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Structural influence of Bi3 + ions on physical properties of Na2CuSiO4 glasses photoluminescence and thermoluminescence studies J. Ashok a, J. Suresh Kumar b, M.P.F. Graça b, M.J. Soares b, M. Srinivasa Reddy c,e, B. Sanyal d, M. Piasecki f,⁎, N. Veeraiah a a
Department of Physics, Acharya Nagarjuna University, Nagarjuna Nagar 522 510, A.P., India Department of Physics, I3N, University of Aveiro, 3810-193 Aveiro, Portugal Department of Physics, University College of Engineering and Technology, Acharya Nagarjuna University, Nagarjuna Nagar 522 510, A.P., India d Food Technology Division, Bhabha Atomic Research Center, Trombay, Mumbai 400 085, India e Department of Physics, Kakani Venkata Ratnam College, Nandigama 521 185, A.P., India f Institute of Physics, J. Dlugosz University, Al. ArmiiKrajowej 13/15, 42-201 Czestochowa, Poland b c
a r t i c l e
i n f o
Article history: Received 1 June 2016 Received in revised form 27 June 2016 Accepted 5 July 2016 Available online xxxx Keywords: Na2CuSiO4 glasses Photoluminescence Thermoluminescence Bi3+ ions
a b s t r a c t CuO doped sodium silicate glasses mixed with different concentrations of Bi2O3 are synthesized. Photoluminescence, thermoluminescence characteristics have been investigated. Photoluminescence emission spectra exhibited a broad band in the bluish green region under excitation at 325 nm. This band is attributed to 3D1 → 1S0transition of (Cu+)2 pairs. The intensity of this emission band is observed to increase with the concentration of Bi2O3 up to 20 mol%. The increase is attributed to the increase in the concentration of Cu+ ions. Thermoluminescence (TL) emission exhibited a dosimetric peak at about 250 °C. The TL output under this glow peak is observed to decrease with increase of γ–ray dose and also with the increase of Bi2O3 content. The mechanisms responsible for TL emission and the variation in TL output with the concentration of Bi2O3 are quantitatively discussed in terms of electron and hole centers developed due to interaction of γ–rays with the glass network. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Monovalent copper ions in the amorphous materials exhibit rich emission in the broad wavelength range starting from blue to yellow regions. Hence the copper ion containing glasses are particularly useful for solar spectral conversion in photovoltaic cells and are also being used as warm colored light sources. Further in view of the broad emission range of these ions, they can also be used as sensitizers for various rare earth ions for up–conversion of emission in the visible region [1–4]. Alkali silicate glasses mixed with heavy metal oxide like Bi2O3possess high density and high refractive index. Hence such glasses are expected to have large emission cross section for the dopant luminescence ions like copper in the visible region. When an irradiated insulator like the titled glass is heated light is emitted and this emission is known as thermoluminescence emission. The measure of TL intensity is a function of absorbed dose gives the information about the suitability of the material for radiation dosimetry methods. Additionally, such studies also give the information regarding inherent structural aspects of the material and also throw some light on
⁎ Corresponding author. E-mail addresses: [email protected] (M.S. Reddy), [email protected] (M. Piasecki).
http://dx.doi.org/10.1016/j.jnoncrysol.2016.07.010 0022-3093/© 2016 Elsevier B.V. All rights reserved.
the defects created in the material due to gamma ray irradiation. Hitherto, such types of studies are mostly confined to single crystals. Of late, similar studies are also being extended to amorphous materials [5–8] with a view to examine the probable use of these materials for dosimetric applications and also to ascertain some structural information. The studies on physical properties connected with other spectroscopic characteristics of CuO doped Na2O–SiO2–Bi2O3 glasses, we have reported the influence of bismuth ions on redox behaviour of copper ions on physical properties connected with electrical and spectroscopic characteristics. These studies have indicated that with increase of Bi2O3 concentration, there is a gradual reduction of Cu2+ ions to monovalent copper ions and such ions were predicted to participate in the glass network with tetrahedral structural units and increase the degree of polymerization of the glass network. To reinforce such findings, in this part of the study we have investigated the influence of such redox behaviour of copper ions on photoluminescence (PL) and thermoluminescence (TL) characteristics of the titled glasses. The clear objectives of this investigation are (i) to study the photoluminescence as a function of Bi2O3 concentration in CuO doped Na2O–SiO2–Bi2O3 glasses d (ii) to study TL–radiation response in a broad dose range 0.5to 8 kGy important for commercial radiation processing as a function of Bi2O3 concentration in CuO doped Na2O–SiO2–Bi2O3 glass matrix and to elucidate the role of valence states of copper ions in PL and TL light outputs.
J. Ashok et al. / Journal of Non-Crystalline Solids 449 (2016) 50–54
2. Experimental The chemical composition of the glasses chosen for the present study is (49–x) Na2O–x Bi2O3–50 SiO2:1.0 CuO (with x = 4, 8,12,16, and 20 mol% and the samples are labelled as B4, B8, B12, B16 and B20, respectively). The glasses were prepared by usual melt quenching technique. The final dimensions of the samples used for the recording PL spectrum were about 1.0 cm × 1.0 cm × 0.2 cm. The PL spectra were recorded on SPEX 1702/04 spectrometer using 325 nm (He–Cd laser) as excitation source. The decay profiles were recorded, using Jobin–Yvon Fluorolog-3 spectrofluorometer augmented with a 450 W xenon lamp for excitation. All the decay curves were recorded with an excitation wavelength of 280 nm and monitored the emission at 460 nm. For TL measurements the glass samples of generic names B4, B8, B12, B16 and, B20 were distributed in six parts. One part was kept as control (no irradiated). Remaining five parts were exposed to gamma radiation from a cobalt 60 source at 0.5, 1, 2, 4, and 8 kGy doses using a gamma chamber GC 5000 (RRIT, Mumbai, India) with dose rate 38 Gy/min. Thermoluminescence of the samples was measured by increasing the temperature from ambient to 300 °C with a heating rate of 5 °C/s using a TL reader (Intech Dosimeters PvtLtd, New Delhi, India).
3. Results and discussion The optical absorption (OA) spectra of Na2O–SiO2: CuO glasses mixed with different concentrations of Bi2O3 recorded at room temperature in the wavelength region 300–1500 nm exhibited a broad absorption band in the region 550–850 nm due to 2B1g → 2B2g octahedral transition of Cu2+ ions and a distinct kink at about 340 nm. The later one is identified as being due to charge transfer of O2− → Cu2+, indicating the reduction of Cu2 + → Cu+ ions. With the gradual increase of Bi2O3 content the intensity of the band due to 2B1g → 2B2g octahedral transition of Cu2 + ions is observed to decrease, whereas the charge transfer band is observed to increase gradually. This observation clearly suggests that there is an increasing proportion of Cu+ ions with increase of Bi2O3 content in the glasses network. In Fig. 1 optical absorption spectrum of one the glasses, namely, B4, before and after gamma irradiation and the relative variations in the intensity of absorption bands due to Cu2 + and Cu+ ions with the concentration of Bi2O3 are presented as the inset. Divalent copper ions in general occupy octahedral positions and act as modifiers, whereas Cu+ ions occupy tetrahedral positions and interlink with silicate structural units with the bonds of the type Si\\O\\Cu. Such linkages are expected to be more stable relative to Si\\O\\Si bonds [9] and make the glass network more rigid. However, there are some
51
reports suggesting that Cu+ ions also participate in the glasses network in the octahedral positions [10]. The IR spectral studies have indicated that with increase of Bi2O3 content there is a gradual increase in the intensity of bands due to symmetrical vibrations of SiO4 structural units, whereas that of asymmetrical vibrations of SiO4 structural units is observed to decrease. From these studies it is concluded that there is an increase in the degree of polymerization of the glass network with increase of Bi2O3 content. This we have ascribed to the increasing participation of Cu+ ions in the network forming positions. The summary of the results of IR spectra is presented in the Fig. 2. Fig. 3 shows the photoluminescence spectra of Na2O–SiO2– Bi2O3:CuO glasses recorded at room temperature at an excitation wavelength of 325 nm. The spectrum of each glass exhibited a broad emission band in the region 350–550 nm; this band is identified as being due to 3 D1 → 1S0transition of (Cu+)2 pairs. In fact, this band is predicted to be composed of two emission bands [11]. The first one connected with the transition of (Cu+)2 pairs (expected to be in the lower wavelength side) and the other one is due to the emission from isolated Cu+ ion. Thus the results PL emission clearly demonstrate that the Cu+ cations have successfully diffused into the glasses [12] and have acted as the emission centers. With the increasing content of Bi2O3ions in the glass matrix, the half width of the band is observed to increase with a shift of meta–centre towards lower wavelength side. The luminescence output associated with this band is observed to be the maximal for the glass B20; such observation clearly indicates that there is a larger proportion of Cu2+ ions that have been reduced into Cu+ ions in the glass B20. Further, the blue shift of the emission band indicates an increasing clusters of (Cu+)2 ion pairs. Using the conventional formulae [13], the transition probability, A (between the states J′ → J) is given by 8ν2 106 A ψ J0 ; ψ J ¼ e2
ð1Þ
(where, ν is the wavenumber of the emission peak and e is the charge of electron). The emission cross-section, σEp, σEρ ¼
λ4 A ψ J0 ; ψ J 8πcnd 2 Δλ
ð2Þ
of the observed emission peak have been evaluated and presented in Table 1. In Eq. (2), l is the wavelength of the emission peak, A is the transition probability, nd is the refractive index of the sample, Δλ is the half width of the emission peak and c is the velocity of light. The value of σ Ep is found to increase gradually from the glass B4 → B20; this observation
Fig. 1. (a) Optical absorption spectra of the glass B4 before and (b) after irradiation with a dose of 0.5 kGy. The inset represents the variation of intensity of peak due to Cu2+ (2B1g → 2B2g) ion and charge transfer band with the concentration of Bi2O3 after irradiating the glasses to a dose of 0.5 kGy.
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J. Ashok et al. / Journal of Non-Crystalline Solids 449 (2016) 50–54
Eδ ¼ 0:976 kB Tm 2 =δ
Fig. 2. IR spectrum of B4 glass. Inset represents the intensity variation of BiO6 and symmetric stretching vibrations of SiO4 units with the concentration of Bi2O3.
indicates increasing luminescence efficiency with increase in the concentration of Bi2O3. To have some understanding over the lifetimes of excited states of Cu+ ions, we have recorded the fluorescence profiles of the titled glasses at room temperature and presented in Fig. 4. The curves exhibited a small fraction (about 10%) of short decay component followed by larger part of slow decay component. Short decay component in the decay profile generally arises due to the emission from clustered luminescent ions which normally give rise to non-radiative de-excitation of ions [14]. From these curves, we have evaluated radiative life time τ and presented in Table 1. The value of τ is found to increase gradually from the glass B4 to B20. Fig. 5 represents the TL glow curves (recorded within the temperature region 30 to 300 °C) of glass B4 irradiated with different gamma ray doses in the range 0.5to 8 kGy. The glow curve exhibited a TL peak (dosimetric peak) at about 250 °C. With increase in gamma ray dose a gradual decrease in TL output of the samples is observed. Further, with increase of Bi2O3 content a substantial quenching of TL is observed. For a glass mixed with 20 mol% of Bi2O3 virtually no TL could be detected in the range of dose studied. In the inset of Fig. 5 the TL glow curves of all the samples irradiated to a dose of 0.5 kGy are presented. Activation energies associated with glow peak are computed using Chen's formulae [15] (Eqs. (3) and (4)): ð3Þ Eτ ¼ 1:52 kB Tm 2 =τ −1:58ð2kB Tm Þ
ð4Þ
where, kB is the Boltzmann constant, τ = Tm − T1, δ = T2 − Tm, Tm is the glow peak temperature, T1 (rising end) and T2 (falling end) are the temperatures at the half widths of the glow peaks. It may be noted here that Chen's method does not make use of any iterative procedures and does not require knowledge of the kinetic order for evaluating trap depth parameters. The trap depth parameters Eτ and Eδ associated with dosimetric peak (for all the samples exposed to a dose of 0.5 kGy) are evaluated and presented in the Table 2. To be more specific, the parameters Eτ and Eδ, respectively, stand for activation energies corresponding to low temperature and high temperature sides of the glow curve [15]. The values of these parameters are found to increase gradually with increase of Bi2O3 content. Na+ ion and also octahedral modifiers (Bi3+ ions)normally cleave Si\\O\\Si bonds in sequence (Fig. 4). γ–ray radiation produces secondary electrons and non–bridging oxygens (O− ions) due to its interaction with the glass network. During irradiation Cu2+ ions get reduced to Cu+ state due to the trapping of holes (Cu2+ + h+ → Cu+) and act as electron and hole trapped centers [16]. The observed increase of intensity of charge transfer band in the OA spectra (Inset of Fig. 1) of irradiated samples suggests an increase in the concentration of Cu+ ions. The monovalent copper ions (Cu+ ions) participate in the substitutional positions and may form the linkages with Bi and Si ions and make the glass network more polymerized. As a result the Cu+ ions formed during irradiation may be interlocked with such linkages and act as TL killers. To be more specific the localized bonding state of substitutional neighboring Cu+ ions in tetrahedral sites, play an intermediate role in the recombination reaction either through bypassing or by resonance energy conditions, so that the radiative transition is inhibited. In other words, the site symmetry of the Cu+ ions and the covalence degree of copper–oxygen bond mainly determine the magnitude of TL output. Additionally, the electrons produced during the irradiation may be trapped at the intrinsic structural defects like [SiOi/2O]j− (where j = 1, 2, 3 and i = 3, 2, 1, respectively). Thermoluminescence occurs due to radiative recombination between the electrons (released by heating from electron center) and an anti-bonding molecular orbital of the nearest of the oxygen hole center. Further, during the thermal excitation, oxygen ions may scatter back and excite[SiOi/2O]j− groups. Such excitation is then transferred to the nearby copper ions and contributes to TL emission: j− j− j− hν þ hν; Oi − → SiOi=2 O → SiOi=2 O → SiOi=2 O þ hν→Cu2þ þ e−
Fig. 3. Photoluminescence spectra of Na2O–Bi2O3–SiO2:CuO glasses. Inset represents the intensity variation of PL peak with the concentration of Bi2O3.
Cuþ
J. Ashok et al. / Journal of Non-Crystalline Solids 449 (2016) 50–54
53
Table 1 Summary of the data on PL emission spectra of Na2O–Bi2O3–SiO2:CuO glasses. Glass
B20 B16 B12 B8 B4
Peak position (nm) (±0.1)
Emission cross–section σ E ρ ð1022 ; cm−2 Þð0:1Þ
Decay time τs (μs) (±0.1)
446 451 462 473 494
11.09 8.72 8.36 7.58 7.19
109.0 97.0 85.0 70.1 63.3
Holes generated during irradiation may be trapped at divalent copper ions. During the heating, the holes recombine with electrons released from the electron trapped centers gives rise to TL emission. For the charge compensation the local O− i captures an electron from silicate groups and form Cu+ ions. During the heating, monovalent copper ions release electrons that later recombine with silicate complexes and contributes to TL emission. The mechanism is described below: 2− − þ e− → SiO3=2 O ; Cu2þ þ h→Cuþ ; SiO2=2 O 2− − heating − ; Cuþ → Cu2þþ e− ; SiO2=2 O þ e → SiO3=2 O − 2− 2− SiO3=2 O þ SiO2=2 O þ O− →e− þ SiO2=2 O We have already mentioned that with increase of Bi2O3 content, large proportions of divalent copper ions reduced to Cu+ ions and such ions do participate in the polymerization of the glasses network with tetrahedral units by forming Si\\O\\Cu linkages. Further the fraction of octahedrally positioned Bi3+ ions concentration is also observed to decrease with increase of Bi2O3 content. Such type of local sites plays an intermediate role in the recombination reaction either through by passing or by resonance energy conditions and inhibits radiative emission. In other words, the site symmetry of Bi3+ ions and the covalence degree of Cu\\O bond mainly determine the shape of the TL glow curve. The decrease of BiO6 fragments (which normally induce structural defects in the glasses network) as has been evidenced from IR spectra, facilitates for increase of degree of polymerization of the glass and leads to increase of non–radiative losses or phonon losses and decrease radiative transition. The observed quenching of TL emission can also be interpreted in terms of exciton formation. The e–h recombine in the exciton and the released energy due to such recombination will be transferred to the excited states of d band of copper ion. Such excited ions relax to the ground state with phonon emission. This is possible because the free electron (or the hole) does not have sufficient electron–phonon coupling necessary for the self-trapping to give rise to radiative recombination; on the other hand, the exciton has a strong coupling with the
Fig. 5. TL glow curves of Na2O–SiO2–Bi2O3:CuO glasses irradiated with different γ ray – doses. Inset represents the TL glow curves all the samples irradiated with 0.5 kGy.
lattice so that predominant nonradiative recombination occurs with the phonon emission [17]. Regarding the wavelength of TL glow curve, it can be measured using optical filters. Unfortunately our TL experimental set-up is not augmented with such additionally facility. However, other earlier works indicated that the TL peak observed at about this temperature for the amorphous materials doped with Cu+ ions is associated with the wavelength 350–500 nm [18]. It is nearly the same as that of PL region observed in the present study. 4. Conclusions Copper doped sodium silicate glasses mixed with different concentrations of Bi2O3 were prepared. Photoluminescence, luminescence decay and thermoluminescence studies were carried out. (i) The photoluminescence spectra of these glasses excited at 325 nm exhibited broad emission band in the region 350– 550 nm; this band is identified as being due to 3 D1 → 1S0transition of (Cu+)2 pairs. A significant increase in the PL output of this emission is observed with increase in the concentration of Bi2O3upto 20 mol%; the increase is attributed to the increase in the concentration of Cu+ ions. (ii) The glasses samples mixed with 4.0 mol% of Bi2O3 exhibited TL emission exhibited a TL peak at about 250 °C. With increase of Bi2O3 concentration a gradual quenching of TL emission is observed. The mechanism responsible for TL emission and its variation with Bi2O3 content are discussed in terms of structural defects induced due to the interaction of γ-rays with the glass network. Acknowledgments One of the authors J. Ashok wishes to thank UGC, New Delhi, for sanctioning RGNF fellowship to carry out this work. M.S. Reddy is grateful to CSIR, New Delhi for the financial support in the form of Major Research Project to carry out this work (Grant No: 03 (1234)/12/EMR-II).
Table 2 Summary of the data on trap depth parameters Na2O–Bi2O3–SiO2:CuO glasses.
Fig. 4. Photoluminescence decay curves of Cu+ ion emission. All the decay curves were recorded with an excitation wavelength of 280 nm and monitored the emission at 460 nm.
Glass
Tm (K)(± 0.5 K)
Eτ (eV)(± 0.01)
Eδ (eV)(± 0.01)
B4 B8 B12 B16 B20
539.0 537.0 547.0 542.0 543.0
0.525 0.703 0.990 1.662 1.759
0.618 0.997 2.257 2.216 2.892
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J. Ashok et al. / Journal of Non-Crystalline Solids 449 (2016) 50–54
References [1] S. Gómez, I. Urra, R. Valiente, F. Rodríguez, Sol. Energy Mater. Sol. Cells 95 (2011) 2018. [2] A. Yasumori, F. Tada, S. Yanagida, T. Kishi, J. Electrochem. Soc. 159 (2012) J143. [3] H. Guo, R.F. Wei, X.Y. Liu, Opt. Lett. 37 (2012) 1670. [4] R.F. Wei, C.G. Ma, Y.L. Wei, J.Y. Gao, H. Guo, Opt. Express 20 (2012) 29743. [5] R. Laopaiboon, C. Bootjomchai, Mater. Des. 80 (2015) 20. [6] T.Y. Lim, H. Wagiran, R. Hussin, S. Hashim, Appl. Radiat. Isot. 102 (2015) 10. [7] M. Sundara Rao, B. Sanyal, K. Bhargavi, R. Vijay, I.V. Kityk, N. Veeraiah, J. Mol. Struct. 1073 (2014) 174. [8] I. Magdalyna, F. Mauro, K. András, Radiat. Phys. Chem. 81 (2012) 1528. [9] L. Srinivasa Rao, M. Srinivasa Reddy, D. Krishna Rao, N. Veeraiah, Solid State Sci. 11 (2009) 578.
[10] J. Gao, R. Ma, J. Zhao, Q. Xu, X. Qiao, J. Du, X. Fan, J. Alloys Compd. 664 (2015) 331. [11] Q. Zhang, G. Chen, G. Dong, G. Zhang, X. Liu, J. Qiu, Q. Zhou, Q. Chen, D. Chen, Chem. Phys. Lett. 482 (2009) 228. [12] E. Borsella, A.D. Vecchio, M.A. Garcia, C. Sada, F. Gonella, R. Polloni, A. Quaranta, L.J.G.W. van Wilderen, J. Appl. Phys. 91 (2002) 90. [13] G. Fuxi, Shanghai Scientific Technical Pub Shanghai, Springer-Verlag, 1991. [14] K. Bhargavi, M. Sundara Rao, V. Sudarsan, C.S. Rao, M. Piasecki, I.V. Kityk, M.S. Reddy, N. Veeraiah, Opt. Mater. 36 (2014) 1189. [15] R. Chen, J. Appl. Phys. 40 (1969) 570. [16] B.J.R.S. Swamy, R. BhaskarSanyal, P. Vijay, R. Babu, D.K. Rao, N. Veeraiah, Ceram. Int. 40 (2014) 3707. [17] S.G. Bishop, P.C. Taylor, Philos. Mag. B 40 (1979) 483. [18] M. Santiago, M. Lester, E. Caselli, A. Lavat, A. Ges, F. Spano, C. Kessler, J. Mater. Sci. Lett. 17 (1998) 1293.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Structural influence of Bi3 + ions on physical properties of Na2CuSiO4 glasses photoluminescence and thermoluminescence studies J. Ashok a, J. Suresh Kumar b, M.P.F. Graça b, M.J. Soares b, M. Srinivasa Reddy c,e, B. Sanyal d, M. Piasecki f,⁎, N. Veeraiah a a
Department of Physics, Acharya Nagarjuna University, Nagarjuna Nagar 522 510, A.P., India Department of Physics, I3N, University of Aveiro, 3810-193 Aveiro, Portugal Department of Physics, University College of Engineering and Technology, Acharya Nagarjuna University, Nagarjuna Nagar 522 510, A.P., India d Food Technology Division, Bhabha Atomic Research Center, Trombay, Mumbai 400 085, India e Department of Physics, Kakani Venkata Ratnam College, Nandigama 521 185, A.P., India f Institute of Physics, J. Dlugosz University, Al. ArmiiKrajowej 13/15, 42-201 Czestochowa, Poland b c
a r t i c l e
i n f o
Article history: Received 1 June 2016 Received in revised form 27 June 2016 Accepted 5 July 2016 Available online xxxx Keywords: Na2CuSiO4 glasses Photoluminescence Thermoluminescence Bi3+ ions
a b s t r a c t CuO doped sodium silicate glasses mixed with different concentrations of Bi2O3 are synthesized. Photoluminescence, thermoluminescence characteristics have been investigated. Photoluminescence emission spectra exhibited a broad band in the bluish green region under excitation at 325 nm. This band is attributed to 3D1 → 1S0transition of (Cu+)2 pairs. The intensity of this emission band is observed to increase with the concentration of Bi2O3 up to 20 mol%. The increase is attributed to the increase in the concentration of Cu+ ions. Thermoluminescence (TL) emission exhibited a dosimetric peak at about 250 °C. The TL output under this glow peak is observed to decrease with increase of γ–ray dose and also with the increase of Bi2O3 content. The mechanisms responsible for TL emission and the variation in TL output with the concentration of Bi2O3 are quantitatively discussed in terms of electron and hole centers developed due to interaction of γ–rays with the glass network. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Monovalent copper ions in the amorphous materials exhibit rich emission in the broad wavelength range starting from blue to yellow regions. Hence the copper ion containing glasses are particularly useful for solar spectral conversion in photovoltaic cells and are also being used as warm colored light sources. Further in view of the broad emission range of these ions, they can also be used as sensitizers for various rare earth ions for up–conversion of emission in the visible region [1–4]. Alkali silicate glasses mixed with heavy metal oxide like Bi2O3possess high density and high refractive index. Hence such glasses are expected to have large emission cross section for the dopant luminescence ions like copper in the visible region. When an irradiated insulator like the titled glass is heated light is emitted and this emission is known as thermoluminescence emission. The measure of TL intensity is a function of absorbed dose gives the information about the suitability of the material for radiation dosimetry methods. Additionally, such studies also give the information regarding inherent structural aspects of the material and also throw some light on
⁎ Corresponding author. E-mail addresses: [email protected] (M.S. Reddy), [email protected] (M. Piasecki).
http://dx.doi.org/10.1016/j.jnoncrysol.2016.07.010 0022-3093/© 2016 Elsevier B.V. All rights reserved.
the defects created in the material due to gamma ray irradiation. Hitherto, such types of studies are mostly confined to single crystals. Of late, similar studies are also being extended to amorphous materials [5–8] with a view to examine the probable use of these materials for dosimetric applications and also to ascertain some structural information. The studies on physical properties connected with other spectroscopic characteristics of CuO doped Na2O–SiO2–Bi2O3 glasses, we have reported the influence of bismuth ions on redox behaviour of copper ions on physical properties connected with electrical and spectroscopic characteristics. These studies have indicated that with increase of Bi2O3 concentration, there is a gradual reduction of Cu2+ ions to monovalent copper ions and such ions were predicted to participate in the glass network with tetrahedral structural units and increase the degree of polymerization of the glass network. To reinforce such findings, in this part of the study we have investigated the influence of such redox behaviour of copper ions on photoluminescence (PL) and thermoluminescence (TL) characteristics of the titled glasses. The clear objectives of this investigation are (i) to study the photoluminescence as a function of Bi2O3 concentration in CuO doped Na2O–SiO2–Bi2O3 glasses d (ii) to study TL–radiation response in a broad dose range 0.5to 8 kGy important for commercial radiation processing as a function of Bi2O3 concentration in CuO doped Na2O–SiO2–Bi2O3 glass matrix and to elucidate the role of valence states of copper ions in PL and TL light outputs.
J. Ashok et al. / Journal of Non-Crystalline Solids 449 (2016) 50–54
2. Experimental The chemical composition of the glasses chosen for the present study is (49–x) Na2O–x Bi2O3–50 SiO2:1.0 CuO (with x = 4, 8,12,16, and 20 mol% and the samples are labelled as B4, B8, B12, B16 and B20, respectively). The glasses were prepared by usual melt quenching technique. The final dimensions of the samples used for the recording PL spectrum were about 1.0 cm × 1.0 cm × 0.2 cm. The PL spectra were recorded on SPEX 1702/04 spectrometer using 325 nm (He–Cd laser) as excitation source. The decay profiles were recorded, using Jobin–Yvon Fluorolog-3 spectrofluorometer augmented with a 450 W xenon lamp for excitation. All the decay curves were recorded with an excitation wavelength of 280 nm and monitored the emission at 460 nm. For TL measurements the glass samples of generic names B4, B8, B12, B16 and, B20 were distributed in six parts. One part was kept as control (no irradiated). Remaining five parts were exposed to gamma radiation from a cobalt 60 source at 0.5, 1, 2, 4, and 8 kGy doses using a gamma chamber GC 5000 (RRIT, Mumbai, India) with dose rate 38 Gy/min. Thermoluminescence of the samples was measured by increasing the temperature from ambient to 300 °C with a heating rate of 5 °C/s using a TL reader (Intech Dosimeters PvtLtd, New Delhi, India).
3. Results and discussion The optical absorption (OA) spectra of Na2O–SiO2: CuO glasses mixed with different concentrations of Bi2O3 recorded at room temperature in the wavelength region 300–1500 nm exhibited a broad absorption band in the region 550–850 nm due to 2B1g → 2B2g octahedral transition of Cu2+ ions and a distinct kink at about 340 nm. The later one is identified as being due to charge transfer of O2− → Cu2+, indicating the reduction of Cu2 + → Cu+ ions. With the gradual increase of Bi2O3 content the intensity of the band due to 2B1g → 2B2g octahedral transition of Cu2 + ions is observed to decrease, whereas the charge transfer band is observed to increase gradually. This observation clearly suggests that there is an increasing proportion of Cu+ ions with increase of Bi2O3 content in the glasses network. In Fig. 1 optical absorption spectrum of one the glasses, namely, B4, before and after gamma irradiation and the relative variations in the intensity of absorption bands due to Cu2 + and Cu+ ions with the concentration of Bi2O3 are presented as the inset. Divalent copper ions in general occupy octahedral positions and act as modifiers, whereas Cu+ ions occupy tetrahedral positions and interlink with silicate structural units with the bonds of the type Si\\O\\Cu. Such linkages are expected to be more stable relative to Si\\O\\Si bonds [9] and make the glass network more rigid. However, there are some
51
reports suggesting that Cu+ ions also participate in the glasses network in the octahedral positions [10]. The IR spectral studies have indicated that with increase of Bi2O3 content there is a gradual increase in the intensity of bands due to symmetrical vibrations of SiO4 structural units, whereas that of asymmetrical vibrations of SiO4 structural units is observed to decrease. From these studies it is concluded that there is an increase in the degree of polymerization of the glass network with increase of Bi2O3 content. This we have ascribed to the increasing participation of Cu+ ions in the network forming positions. The summary of the results of IR spectra is presented in the Fig. 2. Fig. 3 shows the photoluminescence spectra of Na2O–SiO2– Bi2O3:CuO glasses recorded at room temperature at an excitation wavelength of 325 nm. The spectrum of each glass exhibited a broad emission band in the region 350–550 nm; this band is identified as being due to 3 D1 → 1S0transition of (Cu+)2 pairs. In fact, this band is predicted to be composed of two emission bands [11]. The first one connected with the transition of (Cu+)2 pairs (expected to be in the lower wavelength side) and the other one is due to the emission from isolated Cu+ ion. Thus the results PL emission clearly demonstrate that the Cu+ cations have successfully diffused into the glasses [12] and have acted as the emission centers. With the increasing content of Bi2O3ions in the glass matrix, the half width of the band is observed to increase with a shift of meta–centre towards lower wavelength side. The luminescence output associated with this band is observed to be the maximal for the glass B20; such observation clearly indicates that there is a larger proportion of Cu2+ ions that have been reduced into Cu+ ions in the glass B20. Further, the blue shift of the emission band indicates an increasing clusters of (Cu+)2 ion pairs. Using the conventional formulae [13], the transition probability, A (between the states J′ → J) is given by 8ν2 106 A ψ J0 ; ψ J ¼ e2
ð1Þ
(where, ν is the wavenumber of the emission peak and e is the charge of electron). The emission cross-section, σEp, σEρ ¼
λ4 A ψ J0 ; ψ J 8πcnd 2 Δλ
ð2Þ
of the observed emission peak have been evaluated and presented in Table 1. In Eq. (2), l is the wavelength of the emission peak, A is the transition probability, nd is the refractive index of the sample, Δλ is the half width of the emission peak and c is the velocity of light. The value of σ Ep is found to increase gradually from the glass B4 → B20; this observation
Fig. 1. (a) Optical absorption spectra of the glass B4 before and (b) after irradiation with a dose of 0.5 kGy. The inset represents the variation of intensity of peak due to Cu2+ (2B1g → 2B2g) ion and charge transfer band with the concentration of Bi2O3 after irradiating the glasses to a dose of 0.5 kGy.
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J. Ashok et al. / Journal of Non-Crystalline Solids 449 (2016) 50–54
Eδ ¼ 0:976 kB Tm 2 =δ
Fig. 2. IR spectrum of B4 glass. Inset represents the intensity variation of BiO6 and symmetric stretching vibrations of SiO4 units with the concentration of Bi2O3.
indicates increasing luminescence efficiency with increase in the concentration of Bi2O3. To have some understanding over the lifetimes of excited states of Cu+ ions, we have recorded the fluorescence profiles of the titled glasses at room temperature and presented in Fig. 4. The curves exhibited a small fraction (about 10%) of short decay component followed by larger part of slow decay component. Short decay component in the decay profile generally arises due to the emission from clustered luminescent ions which normally give rise to non-radiative de-excitation of ions [14]. From these curves, we have evaluated radiative life time τ and presented in Table 1. The value of τ is found to increase gradually from the glass B4 to B20. Fig. 5 represents the TL glow curves (recorded within the temperature region 30 to 300 °C) of glass B4 irradiated with different gamma ray doses in the range 0.5to 8 kGy. The glow curve exhibited a TL peak (dosimetric peak) at about 250 °C. With increase in gamma ray dose a gradual decrease in TL output of the samples is observed. Further, with increase of Bi2O3 content a substantial quenching of TL is observed. For a glass mixed with 20 mol% of Bi2O3 virtually no TL could be detected in the range of dose studied. In the inset of Fig. 5 the TL glow curves of all the samples irradiated to a dose of 0.5 kGy are presented. Activation energies associated with glow peak are computed using Chen's formulae [15] (Eqs. (3) and (4)): ð3Þ Eτ ¼ 1:52 kB Tm 2 =τ −1:58ð2kB Tm Þ
ð4Þ
where, kB is the Boltzmann constant, τ = Tm − T1, δ = T2 − Tm, Tm is the glow peak temperature, T1 (rising end) and T2 (falling end) are the temperatures at the half widths of the glow peaks. It may be noted here that Chen's method does not make use of any iterative procedures and does not require knowledge of the kinetic order for evaluating trap depth parameters. The trap depth parameters Eτ and Eδ associated with dosimetric peak (for all the samples exposed to a dose of 0.5 kGy) are evaluated and presented in the Table 2. To be more specific, the parameters Eτ and Eδ, respectively, stand for activation energies corresponding to low temperature and high temperature sides of the glow curve [15]. The values of these parameters are found to increase gradually with increase of Bi2O3 content. Na+ ion and also octahedral modifiers (Bi3+ ions)normally cleave Si\\O\\Si bonds in sequence (Fig. 4). γ–ray radiation produces secondary electrons and non–bridging oxygens (O− ions) due to its interaction with the glass network. During irradiation Cu2+ ions get reduced to Cu+ state due to the trapping of holes (Cu2+ + h+ → Cu+) and act as electron and hole trapped centers [16]. The observed increase of intensity of charge transfer band in the OA spectra (Inset of Fig. 1) of irradiated samples suggests an increase in the concentration of Cu+ ions. The monovalent copper ions (Cu+ ions) participate in the substitutional positions and may form the linkages with Bi and Si ions and make the glass network more polymerized. As a result the Cu+ ions formed during irradiation may be interlocked with such linkages and act as TL killers. To be more specific the localized bonding state of substitutional neighboring Cu+ ions in tetrahedral sites, play an intermediate role in the recombination reaction either through bypassing or by resonance energy conditions, so that the radiative transition is inhibited. In other words, the site symmetry of the Cu+ ions and the covalence degree of copper–oxygen bond mainly determine the magnitude of TL output. Additionally, the electrons produced during the irradiation may be trapped at the intrinsic structural defects like [SiOi/2O]j− (where j = 1, 2, 3 and i = 3, 2, 1, respectively). Thermoluminescence occurs due to radiative recombination between the electrons (released by heating from electron center) and an anti-bonding molecular orbital of the nearest of the oxygen hole center. Further, during the thermal excitation, oxygen ions may scatter back and excite[SiOi/2O]j− groups. Such excitation is then transferred to the nearby copper ions and contributes to TL emission: j− j− j− hν þ hν; Oi − → SiOi=2 O → SiOi=2 O → SiOi=2 O þ hν→Cu2þ þ e−
Fig. 3. Photoluminescence spectra of Na2O–Bi2O3–SiO2:CuO glasses. Inset represents the intensity variation of PL peak with the concentration of Bi2O3.
Cuþ
J. Ashok et al. / Journal of Non-Crystalline Solids 449 (2016) 50–54
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Table 1 Summary of the data on PL emission spectra of Na2O–Bi2O3–SiO2:CuO glasses. Glass
B20 B16 B12 B8 B4
Peak position (nm) (±0.1)
Emission cross–section σ E ρ ð1022 ; cm−2 Þð0:1Þ
Decay time τs (μs) (±0.1)
446 451 462 473 494
11.09 8.72 8.36 7.58 7.19
109.0 97.0 85.0 70.1 63.3
Holes generated during irradiation may be trapped at divalent copper ions. During the heating, the holes recombine with electrons released from the electron trapped centers gives rise to TL emission. For the charge compensation the local O− i captures an electron from silicate groups and form Cu+ ions. During the heating, monovalent copper ions release electrons that later recombine with silicate complexes and contributes to TL emission. The mechanism is described below: 2− − þ e− → SiO3=2 O ; Cu2þ þ h→Cuþ ; SiO2=2 O 2− − heating − ; Cuþ → Cu2þþ e− ; SiO2=2 O þ e → SiO3=2 O − 2− 2− SiO3=2 O þ SiO2=2 O þ O− →e− þ SiO2=2 O We have already mentioned that with increase of Bi2O3 content, large proportions of divalent copper ions reduced to Cu+ ions and such ions do participate in the polymerization of the glasses network with tetrahedral units by forming Si\\O\\Cu linkages. Further the fraction of octahedrally positioned Bi3+ ions concentration is also observed to decrease with increase of Bi2O3 content. Such type of local sites plays an intermediate role in the recombination reaction either through by passing or by resonance energy conditions and inhibits radiative emission. In other words, the site symmetry of Bi3+ ions and the covalence degree of Cu\\O bond mainly determine the shape of the TL glow curve. The decrease of BiO6 fragments (which normally induce structural defects in the glasses network) as has been evidenced from IR spectra, facilitates for increase of degree of polymerization of the glass and leads to increase of non–radiative losses or phonon losses and decrease radiative transition. The observed quenching of TL emission can also be interpreted in terms of exciton formation. The e–h recombine in the exciton and the released energy due to such recombination will be transferred to the excited states of d band of copper ion. Such excited ions relax to the ground state with phonon emission. This is possible because the free electron (or the hole) does not have sufficient electron–phonon coupling necessary for the self-trapping to give rise to radiative recombination; on the other hand, the exciton has a strong coupling with the
Fig. 5. TL glow curves of Na2O–SiO2–Bi2O3:CuO glasses irradiated with different γ ray – doses. Inset represents the TL glow curves all the samples irradiated with 0.5 kGy.
lattice so that predominant nonradiative recombination occurs with the phonon emission [17]. Regarding the wavelength of TL glow curve, it can be measured using optical filters. Unfortunately our TL experimental set-up is not augmented with such additionally facility. However, other earlier works indicated that the TL peak observed at about this temperature for the amorphous materials doped with Cu+ ions is associated with the wavelength 350–500 nm [18]. It is nearly the same as that of PL region observed in the present study. 4. Conclusions Copper doped sodium silicate glasses mixed with different concentrations of Bi2O3 were prepared. Photoluminescence, luminescence decay and thermoluminescence studies were carried out. (i) The photoluminescence spectra of these glasses excited at 325 nm exhibited broad emission band in the region 350– 550 nm; this band is identified as being due to 3 D1 → 1S0transition of (Cu+)2 pairs. A significant increase in the PL output of this emission is observed with increase in the concentration of Bi2O3upto 20 mol%; the increase is attributed to the increase in the concentration of Cu+ ions. (ii) The glasses samples mixed with 4.0 mol% of Bi2O3 exhibited TL emission exhibited a TL peak at about 250 °C. With increase of Bi2O3 concentration a gradual quenching of TL emission is observed. The mechanism responsible for TL emission and its variation with Bi2O3 content are discussed in terms of structural defects induced due to the interaction of γ-rays with the glass network. Acknowledgments One of the authors J. Ashok wishes to thank UGC, New Delhi, for sanctioning RGNF fellowship to carry out this work. M.S. Reddy is grateful to CSIR, New Delhi for the financial support in the form of Major Research Project to carry out this work (Grant No: 03 (1234)/12/EMR-II).
Table 2 Summary of the data on trap depth parameters Na2O–Bi2O3–SiO2:CuO glasses.
Fig. 4. Photoluminescence decay curves of Cu+ ion emission. All the decay curves were recorded with an excitation wavelength of 280 nm and monitored the emission at 460 nm.
Glass
Tm (K)(± 0.5 K)
Eτ (eV)(± 0.01)
Eδ (eV)(± 0.01)
B4 B8 B12 B16 B20
539.0 537.0 547.0 542.0 543.0
0.525 0.703 0.990 1.662 1.759
0.618 0.997 2.257 2.216 2.892
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