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Effect of tin ions on enhancing the intensity of narrow luminescence line at 311 nm of Gd3+ ions in Li2OPbOP2O5 glass system
Optical Materials 57 (2016) 39e44
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Effect of tin ions on enhancing the intensity of narrow luminescence line at 311 nm of Gd3þ ions in Li2OePbOeP2O5 glass system Y. Gandhi a, b, P. Rajanikanth a, b, M. Sundara Rao c, V. Ravi Kumar a, N. Veeraiah a, M. Piasecki d, * a
Department of Physics, Acharya Nagarjuna University, Nagarjuna Nagar, Guntur 522 510, AP, India Department of Physics, Kakani Venkata Ratnam College, Nandigama 521 185, AP, India Department of Physics, P.B. Siddhartha College of Arts and Science, Vijayawada 520 010, AP, India d Institute of Physics, J. Dlugosz University, Ul. Armii Krajowej 13/15, 42-201 Czestochowa, Poland b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 February 2016 Received in revised form 31 March 2016 Accepted 8 April 2016 Available online 16 April 2016
This study is mainly focused on enriching the UVB 311 narrow emission band of Gd3þ ions in Li2OePbOeP2O5 glasses doped with 1.0 mol% of Gd2O3 and mixed with different concentrations of SnO2 (0 e7.0 mol%). The emission spectra SnO2 free glasses exhibited intense narrow UVB band at 311 nm due to 6 P7/2 / 8S7/2 transition of Gd3þ ions when excited at 273 nm. The intensity of this band is found to be enhanced nearly four times when the glasses are mixed with 3.0 mol% of SnO2. The reasons for this enhancement have been explored in the light of energy transfer from Sn4þ to Gd3þ ions with the help of rate equations. The declustering of Gd3þ ions (that reduce cross relaxation losses) by tin ions is also found to the other reason for such enrichment. The 311 nm radiation is an efficient in the treatment of various skin diseases and currently it is one of the most desirable and commonly utilised UVB in the construction of phototherapy devices. © 2016 Elsevier B.V. All rights reserved.
Keywords: UVB311 emission Gd3þ and Sn4þ ions Borophosphate glasses Phototherapy Vitiligo vulgaris
1. Introduction Emission and absorption characteristics of Gd3þ ions in amorphous materials are relatively less investigated when compared with those of other rare earth ions in spite of the fact that these ions give intense UVB emission [1,2]. Probably, one of the reasons is that the absorption and emission bands of Gd3þ ions lie below the cut off wavelength of several glass hosts. Further, Judd-Ofelt (JO) theory could hardly be applied for Gd3þ ion for the simple reason that the reduced matrix elements of several transitions of this ion are zeros. Because of this reason, U4 is indeterminate, whereas U2 is found to be erroneous in several cases for this ion [3,4]. PbO added alkaline/alkaline phosphate glass systems are advantageous for hosting UV luminous ions like Gd3þ. PbO makes the glasses more stable against devitrification, increases the moisture resistant capability, density and refractive index of the glass [5e7]. As a result, the cut-off wavelength of the glass will be lowered significantly and pave the way for observing absorption and
* Corresponding author. E-mail address: [email protected] (M. Piasecki). http://dx.doi.org/10.1016/j.optmat.2016.04.015 0925-3467/© 2016 Elsevier B.V. All rights reserved.
emission bands of rare earth ions like Gd3þ in the high energy region. Among all the rare earth ions, it is the Gd3þ ion that gives intense ESR signal at g~2.0 at room temperature and this signal is attributed to the clusters of gadolinium ions in the glass matrix [8,9]. By de-clustering (which can be verified by ESR spectra) these ions, the quenching losses due to cross relaxation can be minimized and the intensity of blue emission of Gd3þ ions can be significantly increased. Gd3þions possess strong excitation band at about 273 nm due to 8S7/2 / 6IJ transitions [10,11] and gives the emission at 311 nm. This particular beam has got important applications especially in the treatment of vitiligo vulgaris. Several dermatologists have attempted to use this beam in the treatment of such skin diseases [12,13]. Apart from the aforementioned application to vitiligo the narrowband UVB radiation is most frequently used for the treatment of psoriasis and a wide range of skin diseases, atopic dermatitis, early stages of mycosis fungoides and pruritic disorders. Interestingly, the semiconducting tin ion (Sn4þ ion) also exhibits emission band at about the same region due to S0 / S2singlet transition [14,15]. Hence the co-doping of Gd3þ with Sn4þ ions facilitates for the energy transfer between the two ions. In view of this co-doping of Gd3þ with Sn4þis an added advantage for enriching UV emission. Further, it was reported that the tin ions
40
Y. Gandhi et al. / Optical Materials 57 (2016) 39e44
exist in divalent as well as tetravalent states in the glass matrices. The divalent ions were found to be acting as modifiers of the glass network, whereas Sn4þ ions were reported to be participating in network forming [16]. Hence by identifying optimal concentration of tin ions, one can achieve the de-clustering of Gd3þ ions. Thus the admixing of tin ions along with Gd3þ ions to the PbO mixed lithium phosphate glasses, enhances UV emission not only by energy transfer but also by acting as de-clustering agent of Gd3þ ions. In our earlier studies, we have reported the influence of tin ions on electrical properties of lithium lead phosphate glass system. We have also investigated the enrichment of orange emission of Er3þ ions due to co-doping with Sn4þ ions in the same glass system [17,18]. Even though some studies are available on emission characteristics of rare earth ions co-doped with Sn4þ ions in alkaline earth glasses like SrOeP2O5 [14], Rigorous studies especially dequenching effects and the application of JO theory to characterize the spectra of Gd3þ ions, are very rare. Motivated by these observations and in view of the important applications of UVB 311 emission in medical therapy, the presented study is devoted to throw some light on the influence of tin ions on the enrichment of UV emission of Gd3þ ions in Li2OePbOeP2O5 glass system.
The following composition contents (all in mol%) of the glasses are chosen for the present study: S0G0: 20Li2Oe20PbOe60P2O5 S0G: 20Li2Oe20PbOe59P2O5e1.0 Gd2O3 S1G: 20Li2Oe20PbOe58P2O5e1.0 Gd2O3: 1.0 SnO2 S3G: 20Li2Oe20PbOe56P2O5e1.0 Gd2O3: 3.0 SnO2 S5G: 20Li2Oe20PbOe54P2O5e1.0 Gd2O3: 5.0 SnO2 S7G: 20Li2Oe20PbOe52P2O5e1.0 Gd2O3: 7.0 SnO2 The details of preparation of the samples were reported in our earlier papers [17,18]. Optical absorption spectra of the glasses were recorded in the wavelength region 200e500 nm using JASCO Model V-670 UVeviseNIR spectrophotometer. The excitation and photoluminescence spectra (with the specified excitation wavelengths) were recorded using PerkineElmer LS-55 luminescence spectrophotometer with Xe lamp as the source of light. The ESR spectra of the fine powders of the samples were recorded at room temperature on JEOL JES-TES100 X-band ESR spectrometer. 3. Results and discussion Gd3þ ion concentration Ni and mean Gd3þion separation ri were evaluated using the measured values of density d and calculated average molecular weight M in Li2OePbOeP2O5eGd2O3eSnO2 glasses and presented in Table 1. Optical absorption spectra (Fig. 1) of glasses co-doped with Gd3þ and Sn4þ ions recorded at ambient temperature in the UV region exhibited several following absorption bands of Gd3þ ions: S7/2 / 6D9/2,
7/2, 5/2,
Glass
Density (g/cm3)
Ni (1022 ions/cm3)
Ri (A )
Refractive Index (n)
S0 S1G S3G S5G S7G
3.742 3.833 3.914 3.996 4.077
e 1.67 1.71 1.74 1.77
e 3.91 3.88 3.86 3.84
1.599 1.601 1.606 1.607 1.610
were found to be relatively more intense, whereas the intensity of the bands due to 8S7/2 / 6DJ, PJ transitions were found to be weak but distinct. Further, most of these transitions are due to induced electric dipole transitions. However, some contribution to induced magnetic dipoles was reported due to 8S7/2 / 6PJ transitions [19]. The total oscillator strength, including both magnetic dipole and induced electric dipole contributions, is given by Ref. [20,21].
fcal
2 D E2 8p2 mcy 4 X ¼ ced Ul f N ½g; S; LJ U l f N ½g0 ; S0 ; L0 J 0 3hð2J þ 1Þ 2;4;6 3 2 þ cmd hJkL þ 2SkJ 0 i 5
2. Experimental
8
Table 1 Physical parameters of Li2OePbOeP2O5eSnO2eGd2O3 glasses.
6
I17/2, 15/2, 13/2, 11/2,9/2,7/2 6P7/2,
5/2.
In addition, the spectra also exhibited a significant band at about 480 nm due to S0 / S2 (triplet state) transition of Sn4þ ions. Among various bands of Gd3þ ions, the bands due to 8S7/2 / 6IJ transitions
ðn2 þ2Þ2
Where ced ¼ d9nd and cmd ¼ nd.nd is the refractive index and the 0 0 0 0 bra- and ket-vectors hf N ½g; S; LJj, f N ½g ; S ; L J i stand for the initial and final states, respectively, with all necessary sets of quantum numbers in square brackets. U l are the reduced matrix elements of the unit tensor operators calculated between the states involved intoa considered transition were taken from the literature [22]. All 2 the U 4 elements are zero or have a negligible value. Hence it is not possible to determine the U4 parameter. It can be determined only if transitions due to 6GJ are observed in the absorption spectra. The values of U2 and U6 obtained for the studied samples are furnished in Table 2. Among these two parameters, U2 is mainly determined from weak 6DJ transitions, whereas U4 is determined from 6IJ transitions. The values of U2 and U6 evaluated for the studied glasses were found to be comparable with those reported for various several other hosts [23,24]. The comparison of the values of U2 parameter obtained for the glasses mixed with different concentrations of SnO2 shows the minimal value for the glass S3G. The value of this parameter as per JO theory is normally connected to the structural change in the neighbourhood of rare earth ions. Earlier, Ehrt [25] based on Mossbauer studies reported that tin ions do exist in both Sn2þ and Sn4þ states in certain SnO2 mixed phosphate composites and amorphous materials. Our earlier studies on electrical properties of SnO2 mixed Li2OePbOeP2O5 glasses (of the same composition) have also confirmed the presence of Sn2þions in addition to the Sn4þions [17]. The analysis of these results further suggested that the concentration of such Sn2þ ions is the maximum in the glass mixed with 3.0 mol% of SnO2. These Sn2þ ions, similar to Liþ ions act as modifiers, depolymerize phosphate network and increase the average distance between PeOeP chains in the glass network. Such increase also leads to elongation of GdeO bonds and may weaken the field strength in the region of Gd3þions and cause to lower the value of U2. In Fig. 2 the ESR spectra of Li2OePbOeP2O5eGd2O3eSnO2 glass samples recorded at room temperature are presented. The spectra exhibited several resonance signals at g ¼ 5.99, 4.65, 3.55 and 1.98. Among these, the resonance signals at g ¼ 1.98 exhibited the minimum intensity for the glass S3G. For further increase of SnO2 content a gradual increase in the intensity of the signal is observed. Among these signals the low field signals are attributed to strong
Y. Gandhi et al. / Optical Materials 57 (2016) 39e44
41
Fig. 1. Optical absorption spectra of Li2OePbOeP2O5eSnO2eGd2O3 glasses. All the transitions are from the ground state 8S7/2 of Gd3þ ions. S0 / S2 represents the band due to Sn4þ ions.
Table 2 The summary of J-O parameters Ul (1020 cm2). Glass
U2
U4
U6
S1 G S3 G S5 G S7 G
3.84 3.45 3.67 3.78
e e e e
2.93 2.25 2.65 2.73
and well defined rhombic crystal field, whereas the signal at g~2.0 is ascribed to formation of clusters Gd3þ ions and it is due to weak crystal fields, for which Zeeman term dominates [26e29]. The
lowest intensity of this signal observed in the spectrum of glass S3G suggests the lower degree of clustering of Gd3þ ions in this glass matrix. The excitation spectra of these glasses recorded at room temperature (monitored at lem ¼ 311 nm) exhibited two significant bands at 255 nm and 273 nm, respectively, due to 8S7/2 / 6D9/2 and 8 S7/2 / 6IJ transitions of Gd3þ ions (Fig. 3(a)) [10,11]. Among these two, the band due to 8S7/2 / 6IJ found to be more intense and sharp. The same was used as excitation for recording photoluminescence (PL) spectra. The PL spectra recorded in the UV region exhibited a sharp intense band at about 311 nm. This band is attributed to 6P7/2 / 8S7/2 emission transition of Gd3þ ions
Fig. 2. The ESR spectra of Li2OePbOeP2O5Gd2O3 glasses mixed with different concentrations of SnO2 recorded at room temperature.
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Y. Gandhi et al. / Optical Materials 57 (2016) 39e44
Fig. 3. (a) Excitation (lem ¼ 311 nm) spectra of Li2OePbOeP2O5eSnO2eGd2O3 glasses. (b) Emission (lexc ¼ 273 nm) spectra of Li2OePbOeP2O5MACROBUTTON InsGlyph eGd2O3 glasses mixed with varying concentrations of SnO2. S0G represents the emission spectrum of SnO2 free Gd2O3 mixed glass.
(Fig. 3(b)) [10,11]. In addition this region also consists of emission due to S2 / S0 transition of Sn4þ ions [29]. With increase in the concentration of SnO2 upto 3.0 mol%, a significant hike in the intensity of the 6P7/2 / 8S7/2 emission is observed. To assess the influence of SnO2 on the luminescence emission of Gd3þ ions in the titled glass, we have also recorded PL spectrum for SnO2 free glass (S0G in Fig. 3(b)). The spectra undoubtedly suggest that Sn4þ ions enhanced the luminescence emission (nearly 4 times) of 6P7/ 8 3þ ions due to energy transfer. 2 / S7/2 (UVB311) transition of Gd Weak fluorescence intensity observed for S1G glass indicates more concentration of Gd3þ clusters that are responsible for luminescence quenching in this glass matrix [19]. The enhancement in the intensity of the band due to 6P7/2 / 8S7/2 transition in the PL spectra with increase in the SnO2 content from 1.0 to 3.0 mol % indicates that tin ions are responsible for the increase of PL output. As we have mentioned earlier the simultaneous presence of tin ions in two valence states viz., Sn2þ and Sn4þ is possible in this glass matrix. The iso-exchange replacement of Sn2þ with Gd3þ ion is also possible because difference of ionic radius of Gd3þ (~0.094 nm) and Sn2þ (~0.093 nm) ions is very small. Hence, Gd3þ ions participate with Sn2þ forming SneOeGd bonds. As a result the Gd3þions in the glass matrix decluster and the spacing between Gd3þ ions gets enlarged. Such dispersion of Gd3þ ions is also evidenced from the ESR spectral results. The decrease in the intensity of ESR signal at g~2.0 in fact supports this view point. Such departing of Gd3þ ions reduces the emission losses due to cross relaxation and thereby enhances the emission intensity. The Sn4þ ions also exhibited emission due to S2 / S0 transition of at about 480 nm in addition to contributing to the emission at 311 nm. The intensity of this band is decreased with increase of SnO2 concentration beyond 3.0 mol%. As said above when the concentration of SnO2 is raised beyond 3.0 mol%, the tetravalent tin ions that take part network forming positions prevail over the Sn2þ ion concentration. Such Sn4þ ions mostly interlocked with phosphate structural groups and contribute for non-radiative emission transitions of Sn4þ ions. The decrease of intensity of both the emission bands at 311 nm and 480 nm with the increase of SnO2 content beyond 3.0 mol% of SnO2 may be attributed to this reason. The fluorescence decay curve of the 6P7/2 excited level of Gd3þ doped glasses is shown in Fig. 4; the curves viewed to be single exponential and the lifetimes evaluated from these curves are presented in Table 3. The lifetime is found to be the highest for the glass S3G and this observation indicates that tin ions gradually disperse Gd3þ ions more uniformly in this glass matrix and reduced
Fig. 4. Fluorescence decay curve of Li2OePbOeP2O5eSnO2eGd2O3 glasses recorded at room temperature corresponding to the emission line 6P7/2 / 8S7/2 at 311 nm.
fluorescence quenching due to cross-relaxation when compared with that of other glasses. For having more understanding energy transfer from Sn4þ to 3þ Gd for reinforcing UVB311 emission as predicted in the energy level diagram (Fig. 5) we have developed the rate equations of emission probability from the 6P7/2 level as per the following procedure. The rate equations for 6DJ, 6IJ and 6PJenergy levels Gd3þ ions are:
dna ¼ Gca nc þ Gba nb Gag na þ f2b0 a nb0 þ f2ga ng Qa dt þ ta 1 na
(1)
dnb ¼ Gcb nc Gba nb þ f2gb ng Qb þ tb 1 nb dt
(2)
dnc ¼ f2gc ng Gca nc Gcb nc þ Qc þ tc 1 nc dt
(3)
Y. Gandhi et al. / Optical Materials 57 (2016) 39e44
43
Table 3 Radiative lifetimes and energy transfer efficiencies of Li2OePbOeP2O5eSnO2eGd2O3 glasses. SnO2 conc. (mol%)
Radiative lifetime tSn
1.0 3.0 5.0 7.0
3.87 4.78 4.00 2.95
þ Gd
(ms)
Energy transfer efficiency h % 39.86 43.79 34.48 33.52
f3 2ga 2gb 2gc Gca Gba ng nb nc na ¼ Qa þ ta 1 Qb þ tb 1 Qc þ tc 1
(6)
The intensity of the UV emission transition 6P7/2 / 8S7/2 is:
Zuag f3 2ga 2gb 2gc Gca Gba ng nb nc Ia ¼ Qa þ ta 1 Qb þ tb 1 Qc þ tc 1
(7)
Similarly the intensity of probable UV emission of Sn4þ ion is:
2 3 Zub0 g0 f2g0 c0 Gc0 b0 ng0 Gb0 a 4 1 5 Ib0 ¼ Qb0 þ tb0 1 Qc0 þ tc0 1 Qb0 þ tb0 1
(8)
The resultant UV emission
IUV ¼ Ia þ Ib0 Zuag f3 2ga 2gb 2gc Gca Gba ng nb nc Qa þ ta 1 Qb þ tb 1 Qc þ tc 1 2 3 Zub0 g0 f2g0 c0 Gc0 b0 ng0 Gb0 a 4 1 5 þ Qb0 þ tb0 1 Qc0 þ tc0 1 Qb0 þ tb0 1
¼
Fig. 5. Energy level diagram with various transitions of Gd3þ and Sn4þ ions of glass S3G.
Similarly for singlet and triplet states of S2 level of Sn4þ ions can be written as:
dna0 ¼ Gc0 a0 nc0 Ga0 g0 na0 Qa0 þ ta0 1 na0 dt
(9)
From the Eqs. (7)e(9), it is clear that the intensity of possible UV emission is proportional to the radiative life times of the upper levels. The life time t for 6P7/2 level is found to be the largest for the glass S3G (Table 2). Thus the observed highest intensity of the UV emission in the emission spectra for the glass S3G glass is well in consistent with the theoretical predictions. Further, the intensity of this emission is strongly dependent on energy transfer efficiency (h ¼ 1tSn þ Gd/tSn) from Sn4þ / Gd3þ ions. Using tSn þ Gd and tSn values we have evaluated the parameter h and presented in the Table 3. The value of h is found to be the highest for the glass S3G. This observation clearly indicates the maximum energy transfer from Sn4þ / Gd3þ ions in the glass mixed with 3.0 mol% of SnO2.
(4) 4. Conclusions
dnb0 ¼ Gc0 b0 nc0 Gb0 a0 nb0 Gb0 g0 nb0 f2b0 a nb0 Qb0 þ tb0 1 nb0 dt (5) In the Eqs. (1)e(5) ni represents the population of ith excited level, Gi,j indicates the transition probability from i / j, ti and Qi, respectively, represent life time and quenching rates of ith level, whereas f represents pumping and the absorption cross section is denoted by zij. To evaluate the intensity of UV emission of Gd3þ ions from 6P7/ level, we have solved rate equations under the stable situation by 2 ignoring the accumulations of middle levels. We thus obtain
Li2OePbOeP2O5 glasses doped with 1.0 mol% of Gd2O3 and mixed with different concentrations of SnO2 have been synthesized. The luminescence spectra of the glasses exhibited intense UVB band at about 311 nm due to 6P7/2 / 8S7/2 transition of Gd3þ ion. A significant hike (nearly 4 times) in the intensity of this UVB band is observed when the glasses are mixed with 3.0 mol% of SnO2. The reasons for this enhancement have been have explored in the light of energy transfer from Sn4þ to Gd3þ ions and due to the de-clustering of Gd3þ ions by SnO2. The energy transfer mechanism is further discussed quantitatively with the rate equations. Finally we emphasize that this particular glass (S3G) is useful as efficient radiation source for phototherapy in vitiligo.
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Y. Gandhi et al. / Optical Materials 57 (2016) 39e44
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Y. Tong, Z. Yan, H. Zeng, G. Chen, J. Lumin. 145 (2014) 438. S.Y. Lee, Y.H. Shin, Y. Kim, S. Kim, S. Ju, J. Lumin. 131 (2011) 2565. S. Das, V. Jayaraman, Prog. Mater. Sci. 66 (2014) 112. P. Rajanikanth, M.A. Valente, Y. Gandhi, M. Piasecki, N. Veeraiah, Ionics (2015) 1e13. P. Rajanikanth, Y. Gandhi, N. Veeraiah, Opt. Mater. 48 (2015) 51. €rller-Walrand, J.L. Adam, Chem. Phys. Lett. 280 (1997) 333. K. Binnemans, C. Go B.R. Judd, Optical absorption intensities of rare-earth ions, Phys. Rev. 127 (1962) 750. G.S. Ofelt, Intensities of crystal spectra of rare-earth ions, J. Chem. Phys. 37 (1962) 511. W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4412. J. McDougall, D.B. Hollis, M.J.B. Payne, Phys. Chem. Glas. 35 (1994) 258. P.J. Alonso, V.M. Orera, R. Cases, R. Alcala, V.D. Rodrıguez, J. Lumin. 39 (1988) 275. D. Ehrt, J. Non Cryst. Solids 354 (2008) 546. L. Cugunov, J. Kliava, J. Phys. C. Solid State Phys. 15 (1982) L933eL936. G. Chiodelli, A. Magistris, M. Villa, Solid State Ionics 18 (1986) 356. A. Magistris, G. Chiodelli, M. Duclot, Solid State Ionics 10 (1983) 611. C. Bouzidi, H. Elhouichet, A. Moadhen, M. Oueslati, J. Lumin. 129 (2009) 30.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Effect of tin ions on enhancing the intensity of narrow luminescence line at 311 nm of Gd3þ ions in Li2OePbOeP2O5 glass system Y. Gandhi a, b, P. Rajanikanth a, b, M. Sundara Rao c, V. Ravi Kumar a, N. Veeraiah a, M. Piasecki d, * a
Department of Physics, Acharya Nagarjuna University, Nagarjuna Nagar, Guntur 522 510, AP, India Department of Physics, Kakani Venkata Ratnam College, Nandigama 521 185, AP, India Department of Physics, P.B. Siddhartha College of Arts and Science, Vijayawada 520 010, AP, India d Institute of Physics, J. Dlugosz University, Ul. Armii Krajowej 13/15, 42-201 Czestochowa, Poland b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 February 2016 Received in revised form 31 March 2016 Accepted 8 April 2016 Available online 16 April 2016
This study is mainly focused on enriching the UVB 311 narrow emission band of Gd3þ ions in Li2OePbOeP2O5 glasses doped with 1.0 mol% of Gd2O3 and mixed with different concentrations of SnO2 (0 e7.0 mol%). The emission spectra SnO2 free glasses exhibited intense narrow UVB band at 311 nm due to 6 P7/2 / 8S7/2 transition of Gd3þ ions when excited at 273 nm. The intensity of this band is found to be enhanced nearly four times when the glasses are mixed with 3.0 mol% of SnO2. The reasons for this enhancement have been explored in the light of energy transfer from Sn4þ to Gd3þ ions with the help of rate equations. The declustering of Gd3þ ions (that reduce cross relaxation losses) by tin ions is also found to the other reason for such enrichment. The 311 nm radiation is an efficient in the treatment of various skin diseases and currently it is one of the most desirable and commonly utilised UVB in the construction of phototherapy devices. © 2016 Elsevier B.V. All rights reserved.
Keywords: UVB311 emission Gd3þ and Sn4þ ions Borophosphate glasses Phototherapy Vitiligo vulgaris
1. Introduction Emission and absorption characteristics of Gd3þ ions in amorphous materials are relatively less investigated when compared with those of other rare earth ions in spite of the fact that these ions give intense UVB emission [1,2]. Probably, one of the reasons is that the absorption and emission bands of Gd3þ ions lie below the cut off wavelength of several glass hosts. Further, Judd-Ofelt (JO) theory could hardly be applied for Gd3þ ion for the simple reason that the reduced matrix elements of several transitions of this ion are zeros. Because of this reason, U4 is indeterminate, whereas U2 is found to be erroneous in several cases for this ion [3,4]. PbO added alkaline/alkaline phosphate glass systems are advantageous for hosting UV luminous ions like Gd3þ. PbO makes the glasses more stable against devitrification, increases the moisture resistant capability, density and refractive index of the glass [5e7]. As a result, the cut-off wavelength of the glass will be lowered significantly and pave the way for observing absorption and
* Corresponding author. E-mail address: [email protected] (M. Piasecki). http://dx.doi.org/10.1016/j.optmat.2016.04.015 0925-3467/© 2016 Elsevier B.V. All rights reserved.
emission bands of rare earth ions like Gd3þ in the high energy region. Among all the rare earth ions, it is the Gd3þ ion that gives intense ESR signal at g~2.0 at room temperature and this signal is attributed to the clusters of gadolinium ions in the glass matrix [8,9]. By de-clustering (which can be verified by ESR spectra) these ions, the quenching losses due to cross relaxation can be minimized and the intensity of blue emission of Gd3þ ions can be significantly increased. Gd3þions possess strong excitation band at about 273 nm due to 8S7/2 / 6IJ transitions [10,11] and gives the emission at 311 nm. This particular beam has got important applications especially in the treatment of vitiligo vulgaris. Several dermatologists have attempted to use this beam in the treatment of such skin diseases [12,13]. Apart from the aforementioned application to vitiligo the narrowband UVB radiation is most frequently used for the treatment of psoriasis and a wide range of skin diseases, atopic dermatitis, early stages of mycosis fungoides and pruritic disorders. Interestingly, the semiconducting tin ion (Sn4þ ion) also exhibits emission band at about the same region due to S0 / S2singlet transition [14,15]. Hence the co-doping of Gd3þ with Sn4þ ions facilitates for the energy transfer between the two ions. In view of this co-doping of Gd3þ with Sn4þis an added advantage for enriching UV emission. Further, it was reported that the tin ions
40
Y. Gandhi et al. / Optical Materials 57 (2016) 39e44
exist in divalent as well as tetravalent states in the glass matrices. The divalent ions were found to be acting as modifiers of the glass network, whereas Sn4þ ions were reported to be participating in network forming [16]. Hence by identifying optimal concentration of tin ions, one can achieve the de-clustering of Gd3þ ions. Thus the admixing of tin ions along with Gd3þ ions to the PbO mixed lithium phosphate glasses, enhances UV emission not only by energy transfer but also by acting as de-clustering agent of Gd3þ ions. In our earlier studies, we have reported the influence of tin ions on electrical properties of lithium lead phosphate glass system. We have also investigated the enrichment of orange emission of Er3þ ions due to co-doping with Sn4þ ions in the same glass system [17,18]. Even though some studies are available on emission characteristics of rare earth ions co-doped with Sn4þ ions in alkaline earth glasses like SrOeP2O5 [14], Rigorous studies especially dequenching effects and the application of JO theory to characterize the spectra of Gd3þ ions, are very rare. Motivated by these observations and in view of the important applications of UVB 311 emission in medical therapy, the presented study is devoted to throw some light on the influence of tin ions on the enrichment of UV emission of Gd3þ ions in Li2OePbOeP2O5 glass system.
The following composition contents (all in mol%) of the glasses are chosen for the present study: S0G0: 20Li2Oe20PbOe60P2O5 S0G: 20Li2Oe20PbOe59P2O5e1.0 Gd2O3 S1G: 20Li2Oe20PbOe58P2O5e1.0 Gd2O3: 1.0 SnO2 S3G: 20Li2Oe20PbOe56P2O5e1.0 Gd2O3: 3.0 SnO2 S5G: 20Li2Oe20PbOe54P2O5e1.0 Gd2O3: 5.0 SnO2 S7G: 20Li2Oe20PbOe52P2O5e1.0 Gd2O3: 7.0 SnO2 The details of preparation of the samples were reported in our earlier papers [17,18]. Optical absorption spectra of the glasses were recorded in the wavelength region 200e500 nm using JASCO Model V-670 UVeviseNIR spectrophotometer. The excitation and photoluminescence spectra (with the specified excitation wavelengths) were recorded using PerkineElmer LS-55 luminescence spectrophotometer with Xe lamp as the source of light. The ESR spectra of the fine powders of the samples were recorded at room temperature on JEOL JES-TES100 X-band ESR spectrometer. 3. Results and discussion Gd3þ ion concentration Ni and mean Gd3þion separation ri were evaluated using the measured values of density d and calculated average molecular weight M in Li2OePbOeP2O5eGd2O3eSnO2 glasses and presented in Table 1. Optical absorption spectra (Fig. 1) of glasses co-doped with Gd3þ and Sn4þ ions recorded at ambient temperature in the UV region exhibited several following absorption bands of Gd3þ ions: S7/2 / 6D9/2,
7/2, 5/2,
Glass
Density (g/cm3)
Ni (1022 ions/cm3)
Ri (A )
Refractive Index (n)
S0 S1G S3G S5G S7G
3.742 3.833 3.914 3.996 4.077
e 1.67 1.71 1.74 1.77
e 3.91 3.88 3.86 3.84
1.599 1.601 1.606 1.607 1.610
were found to be relatively more intense, whereas the intensity of the bands due to 8S7/2 / 6DJ, PJ transitions were found to be weak but distinct. Further, most of these transitions are due to induced electric dipole transitions. However, some contribution to induced magnetic dipoles was reported due to 8S7/2 / 6PJ transitions [19]. The total oscillator strength, including both magnetic dipole and induced electric dipole contributions, is given by Ref. [20,21].
fcal
2 D E2 8p2 mcy 4 X ¼ ced Ul f N ½g; S; LJ U l f N ½g0 ; S0 ; L0 J 0 3hð2J þ 1Þ 2;4;6 3 2 þ cmd hJkL þ 2SkJ 0 i 5
2. Experimental
8
Table 1 Physical parameters of Li2OePbOeP2O5eSnO2eGd2O3 glasses.
6
I17/2, 15/2, 13/2, 11/2,9/2,7/2 6P7/2,
5/2.
In addition, the spectra also exhibited a significant band at about 480 nm due to S0 / S2 (triplet state) transition of Sn4þ ions. Among various bands of Gd3þ ions, the bands due to 8S7/2 / 6IJ transitions
ðn2 þ2Þ2
Where ced ¼ d9nd and cmd ¼ nd.nd is the refractive index and the 0 0 0 0 bra- and ket-vectors hf N ½g; S; LJj, f N ½g ; S ; L J i stand for the initial and final states, respectively, with all necessary sets of quantum numbers in square brackets. U l are the reduced matrix elements of the unit tensor operators calculated between the states involved intoa considered transition were taken from the literature [22]. All 2 the U 4 elements are zero or have a negligible value. Hence it is not possible to determine the U4 parameter. It can be determined only if transitions due to 6GJ are observed in the absorption spectra. The values of U2 and U6 obtained for the studied samples are furnished in Table 2. Among these two parameters, U2 is mainly determined from weak 6DJ transitions, whereas U4 is determined from 6IJ transitions. The values of U2 and U6 evaluated for the studied glasses were found to be comparable with those reported for various several other hosts [23,24]. The comparison of the values of U2 parameter obtained for the glasses mixed with different concentrations of SnO2 shows the minimal value for the glass S3G. The value of this parameter as per JO theory is normally connected to the structural change in the neighbourhood of rare earth ions. Earlier, Ehrt [25] based on Mossbauer studies reported that tin ions do exist in both Sn2þ and Sn4þ states in certain SnO2 mixed phosphate composites and amorphous materials. Our earlier studies on electrical properties of SnO2 mixed Li2OePbOeP2O5 glasses (of the same composition) have also confirmed the presence of Sn2þions in addition to the Sn4þions [17]. The analysis of these results further suggested that the concentration of such Sn2þ ions is the maximum in the glass mixed with 3.0 mol% of SnO2. These Sn2þ ions, similar to Liþ ions act as modifiers, depolymerize phosphate network and increase the average distance between PeOeP chains in the glass network. Such increase also leads to elongation of GdeO bonds and may weaken the field strength in the region of Gd3þions and cause to lower the value of U2. In Fig. 2 the ESR spectra of Li2OePbOeP2O5eGd2O3eSnO2 glass samples recorded at room temperature are presented. The spectra exhibited several resonance signals at g ¼ 5.99, 4.65, 3.55 and 1.98. Among these, the resonance signals at g ¼ 1.98 exhibited the minimum intensity for the glass S3G. For further increase of SnO2 content a gradual increase in the intensity of the signal is observed. Among these signals the low field signals are attributed to strong
Y. Gandhi et al. / Optical Materials 57 (2016) 39e44
41
Fig. 1. Optical absorption spectra of Li2OePbOeP2O5eSnO2eGd2O3 glasses. All the transitions are from the ground state 8S7/2 of Gd3þ ions. S0 / S2 represents the band due to Sn4þ ions.
Table 2 The summary of J-O parameters Ul (1020 cm2). Glass
U2
U4
U6
S1 G S3 G S5 G S7 G
3.84 3.45 3.67 3.78
e e e e
2.93 2.25 2.65 2.73
and well defined rhombic crystal field, whereas the signal at g~2.0 is ascribed to formation of clusters Gd3þ ions and it is due to weak crystal fields, for which Zeeman term dominates [26e29]. The
lowest intensity of this signal observed in the spectrum of glass S3G suggests the lower degree of clustering of Gd3þ ions in this glass matrix. The excitation spectra of these glasses recorded at room temperature (monitored at lem ¼ 311 nm) exhibited two significant bands at 255 nm and 273 nm, respectively, due to 8S7/2 / 6D9/2 and 8 S7/2 / 6IJ transitions of Gd3þ ions (Fig. 3(a)) [10,11]. Among these two, the band due to 8S7/2 / 6IJ found to be more intense and sharp. The same was used as excitation for recording photoluminescence (PL) spectra. The PL spectra recorded in the UV region exhibited a sharp intense band at about 311 nm. This band is attributed to 6P7/2 / 8S7/2 emission transition of Gd3þ ions
Fig. 2. The ESR spectra of Li2OePbOeP2O5Gd2O3 glasses mixed with different concentrations of SnO2 recorded at room temperature.
42
Y. Gandhi et al. / Optical Materials 57 (2016) 39e44
Fig. 3. (a) Excitation (lem ¼ 311 nm) spectra of Li2OePbOeP2O5eSnO2eGd2O3 glasses. (b) Emission (lexc ¼ 273 nm) spectra of Li2OePbOeP2O5MACROBUTTON InsGlyph eGd2O3 glasses mixed with varying concentrations of SnO2. S0G represents the emission spectrum of SnO2 free Gd2O3 mixed glass.
(Fig. 3(b)) [10,11]. In addition this region also consists of emission due to S2 / S0 transition of Sn4þ ions [29]. With increase in the concentration of SnO2 upto 3.0 mol%, a significant hike in the intensity of the 6P7/2 / 8S7/2 emission is observed. To assess the influence of SnO2 on the luminescence emission of Gd3þ ions in the titled glass, we have also recorded PL spectrum for SnO2 free glass (S0G in Fig. 3(b)). The spectra undoubtedly suggest that Sn4þ ions enhanced the luminescence emission (nearly 4 times) of 6P7/ 8 3þ ions due to energy transfer. 2 / S7/2 (UVB311) transition of Gd Weak fluorescence intensity observed for S1G glass indicates more concentration of Gd3þ clusters that are responsible for luminescence quenching in this glass matrix [19]. The enhancement in the intensity of the band due to 6P7/2 / 8S7/2 transition in the PL spectra with increase in the SnO2 content from 1.0 to 3.0 mol % indicates that tin ions are responsible for the increase of PL output. As we have mentioned earlier the simultaneous presence of tin ions in two valence states viz., Sn2þ and Sn4þ is possible in this glass matrix. The iso-exchange replacement of Sn2þ with Gd3þ ion is also possible because difference of ionic radius of Gd3þ (~0.094 nm) and Sn2þ (~0.093 nm) ions is very small. Hence, Gd3þ ions participate with Sn2þ forming SneOeGd bonds. As a result the Gd3þions in the glass matrix decluster and the spacing between Gd3þ ions gets enlarged. Such dispersion of Gd3þ ions is also evidenced from the ESR spectral results. The decrease in the intensity of ESR signal at g~2.0 in fact supports this view point. Such departing of Gd3þ ions reduces the emission losses due to cross relaxation and thereby enhances the emission intensity. The Sn4þ ions also exhibited emission due to S2 / S0 transition of at about 480 nm in addition to contributing to the emission at 311 nm. The intensity of this band is decreased with increase of SnO2 concentration beyond 3.0 mol%. As said above when the concentration of SnO2 is raised beyond 3.0 mol%, the tetravalent tin ions that take part network forming positions prevail over the Sn2þ ion concentration. Such Sn4þ ions mostly interlocked with phosphate structural groups and contribute for non-radiative emission transitions of Sn4þ ions. The decrease of intensity of both the emission bands at 311 nm and 480 nm with the increase of SnO2 content beyond 3.0 mol% of SnO2 may be attributed to this reason. The fluorescence decay curve of the 6P7/2 excited level of Gd3þ doped glasses is shown in Fig. 4; the curves viewed to be single exponential and the lifetimes evaluated from these curves are presented in Table 3. The lifetime is found to be the highest for the glass S3G and this observation indicates that tin ions gradually disperse Gd3þ ions more uniformly in this glass matrix and reduced
Fig. 4. Fluorescence decay curve of Li2OePbOeP2O5eSnO2eGd2O3 glasses recorded at room temperature corresponding to the emission line 6P7/2 / 8S7/2 at 311 nm.
fluorescence quenching due to cross-relaxation when compared with that of other glasses. For having more understanding energy transfer from Sn4þ to 3þ Gd for reinforcing UVB311 emission as predicted in the energy level diagram (Fig. 5) we have developed the rate equations of emission probability from the 6P7/2 level as per the following procedure. The rate equations for 6DJ, 6IJ and 6PJenergy levels Gd3þ ions are:
dna ¼ Gca nc þ Gba nb Gag na þ f2b0 a nb0 þ f2ga ng Qa dt þ ta 1 na
(1)
dnb ¼ Gcb nc Gba nb þ f2gb ng Qb þ tb 1 nb dt
(2)
dnc ¼ f2gc ng Gca nc Gcb nc þ Qc þ tc 1 nc dt
(3)
Y. Gandhi et al. / Optical Materials 57 (2016) 39e44
43
Table 3 Radiative lifetimes and energy transfer efficiencies of Li2OePbOeP2O5eSnO2eGd2O3 glasses. SnO2 conc. (mol%)
Radiative lifetime tSn
1.0 3.0 5.0 7.0
3.87 4.78 4.00 2.95
þ Gd
(ms)
Energy transfer efficiency h % 39.86 43.79 34.48 33.52
f3 2ga 2gb 2gc Gca Gba ng nb nc na ¼ Qa þ ta 1 Qb þ tb 1 Qc þ tc 1
(6)
The intensity of the UV emission transition 6P7/2 / 8S7/2 is:
Zuag f3 2ga 2gb 2gc Gca Gba ng nb nc Ia ¼ Qa þ ta 1 Qb þ tb 1 Qc þ tc 1
(7)
Similarly the intensity of probable UV emission of Sn4þ ion is:
2 3 Zub0 g0 f2g0 c0 Gc0 b0 ng0 Gb0 a 4 1 5 Ib0 ¼ Qb0 þ tb0 1 Qc0 þ tc0 1 Qb0 þ tb0 1
(8)
The resultant UV emission
IUV ¼ Ia þ Ib0 Zuag f3 2ga 2gb 2gc Gca Gba ng nb nc Qa þ ta 1 Qb þ tb 1 Qc þ tc 1 2 3 Zub0 g0 f2g0 c0 Gc0 b0 ng0 Gb0 a 4 1 5 þ Qb0 þ tb0 1 Qc0 þ tc0 1 Qb0 þ tb0 1
¼
Fig. 5. Energy level diagram with various transitions of Gd3þ and Sn4þ ions of glass S3G.
Similarly for singlet and triplet states of S2 level of Sn4þ ions can be written as:
dna0 ¼ Gc0 a0 nc0 Ga0 g0 na0 Qa0 þ ta0 1 na0 dt
(9)
From the Eqs. (7)e(9), it is clear that the intensity of possible UV emission is proportional to the radiative life times of the upper levels. The life time t for 6P7/2 level is found to be the largest for the glass S3G (Table 2). Thus the observed highest intensity of the UV emission in the emission spectra for the glass S3G glass is well in consistent with the theoretical predictions. Further, the intensity of this emission is strongly dependent on energy transfer efficiency (h ¼ 1tSn þ Gd/tSn) from Sn4þ / Gd3þ ions. Using tSn þ Gd and tSn values we have evaluated the parameter h and presented in the Table 3. The value of h is found to be the highest for the glass S3G. This observation clearly indicates the maximum energy transfer from Sn4þ / Gd3þ ions in the glass mixed with 3.0 mol% of SnO2.
(4) 4. Conclusions
dnb0 ¼ Gc0 b0 nc0 Gb0 a0 nb0 Gb0 g0 nb0 f2b0 a nb0 Qb0 þ tb0 1 nb0 dt (5) In the Eqs. (1)e(5) ni represents the population of ith excited level, Gi,j indicates the transition probability from i / j, ti and Qi, respectively, represent life time and quenching rates of ith level, whereas f represents pumping and the absorption cross section is denoted by zij. To evaluate the intensity of UV emission of Gd3þ ions from 6P7/ level, we have solved rate equations under the stable situation by 2 ignoring the accumulations of middle levels. We thus obtain
Li2OePbOeP2O5 glasses doped with 1.0 mol% of Gd2O3 and mixed with different concentrations of SnO2 have been synthesized. The luminescence spectra of the glasses exhibited intense UVB band at about 311 nm due to 6P7/2 / 8S7/2 transition of Gd3þ ion. A significant hike (nearly 4 times) in the intensity of this UVB band is observed when the glasses are mixed with 3.0 mol% of SnO2. The reasons for this enhancement have been have explored in the light of energy transfer from Sn4þ to Gd3þ ions and due to the de-clustering of Gd3þ ions by SnO2. The energy transfer mechanism is further discussed quantitatively with the rate equations. Finally we emphasize that this particular glass (S3G) is useful as efficient radiation source for phototherapy in vitiligo.
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Y. Gandhi et al. / Optical Materials 57 (2016) 39e44
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