Optical characterization of Eu3+ and Tb3+ ions doped cadmium lithium alumino fluoro boro tellurite glasses

Optical characterization of Eu3+ and Tb3+ ions doped cadmium lithium alumino fluoro boro tellurite glasses

Spectrochimica Acta Part A 79 (2011) 87–91 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectros...

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Spectrochimica Acta Part A 79 (2011) 87–91

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Optical characterization of Eu3+ and Tb3+ ions doped cadmium lithium alumino fluoro boro tellurite glasses K. Vemasevana Raju, S. Sailaja, C. Nageswara Raju, B. Sudhakar Reddy ∗ Department of Physics, S. V. Degree College, Kadapa 516003, India

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 20 September 2010 Received in revised form 25 January 2011 Accepted 7 February 2011

This article reports on the development and spectral results of Eu3+ and Tb3+ ions doped cadmium lithium alumino fluoro boro tellurite (CLiAFBT) glasses in the following composition.

Keywords: CLiAFBT glasses Emission Excitation Optical properties Thermal properties

(40 − x)TeO2 –30B2 O3 –10CdO–10Li2 O–10AlF3 –xEu2 O3

40TeO2 –30B2 O3 –10CdO–10Li2 O–10AlF3

(Hostglass)

(40 − x)TeO2 –30B2 O3 –10CdO–10Li2 O–10AlF3 –xTb4 O7 where x = 0.25, 0.50, 0.75, 1.0, 1.25 mol%. Glass amorphous nature and thermal properties have been studied using the XRD and DSC profiles. From the emission spectra of Eu3+ :glasses, five emission transitions have been observed at 578 nm, 592 nm, 612 nm, 653 nm, 701 nm and are assigned to the transitions 5 D0 → 7 F0 , 7 F1, 7 F2 , 7 F3 and 7 F4 , respectively, with exci = 392 nm (7 F0 → 5 L6 ). In case of Tb3+ :glasses, four emission transitions (5 D4 → 7 F6, 7 F5 , 7 F4 and 7 F3 ) are observed at 488 nm, 543 nm, 584 nm and 614 nm, respectively, with exci = 376 nm. Decay curves and energy level diagrams have been plotted to evaluate the life times and to analyze the emission mechanism. © 2011 Elsevier B.V. All rights reserved.

1. Introduction We have earlier reported on the preparation and the optical characterization of different glasses based on tellurites, phosphates, borates [1–3]. Among these, tellurite glasses have extensively been investigated in the past few years due to their good chemical durability, good thermal stability, high refractive index, good transparency in the mid-infrared region (0.35–6 ␮m), low phonon energy values (700–800 cm−1 ) and also high solubility for rare earth ions. In addition, tellurite glasses have also been suggested for use in the development of optical fibers and planar waveguides, potential optical CD memory devices. These special optical properties encourage them as important materials for different potential applications. It is well known that a pure TeO2 chemical does not form a glass but it does so, when it is mixed with certain other oxides such as B2 O3 , Li2 O, AlF3 and CdO, etc., Further, the glasses upon addition of Li2 O and AlF3 as network modifiers (NWM), could strengthen (or) enhance certain electrical, thermal and optical properties [4]. From the literature, it is quite

∗ Corresponding author. Tel.: +91 8562244367; fax: +91 856225976. E-mail address: [email protected] (B. Sudhakar Reddy). 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.02.009

clear that these optical glasses, when doped with certain rare-earth ions, could display encouraging important optical results in optical communication fibers, solid-state lasers, light converters, sensors, etc. [5–10]. Among the various RE ions, it is well known that the Eu3+ ion has an excited level 5 D0 to exhibit intense and sharp emission transitions with 7 F0 as the ground state and also this ion shows bright red colour emission having the transition 5 D0 → 7 F2. The Eu3+ -doped glasses are attracting a great deal of interest because persistent spectral hole burning can be performed in the 7 F0 → 5 D0 transition of Eu3+ at room temperature and also these have potential use in high-density optical storage. Another important one is Tb3+ ion, which can shows more intense transitions with 7 F6 as the ground state and this ion emits more intense green colour emission (5 D4 → 7 F5 ) and hence these have been used in the development of efficient green emitting phosphors and scintillating materials. Thus the rare earth ions such as Eu3+ and Tb3+ ions give strong luminescence in a variety of host lattices and hence it is interesting to study the photoluminescence properties of these ions doped cadmium lithium alumino fluoro boro tellurite glasses. In the present work, we have investigated the optical properties of tellurite based glasses with rare earth ions such as Eu3+ and Tb3+ ions as dopants.

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2. Experimental procedure 2.1. Glasses preparation The chemicals used were reagent grade of H3 BO3 , TeO2 , CdO, AlF3 , Li2 CO3 , Eu2 O3 and Tb4 O7 . The chemical compositions of Eu3+ and Tb3+ ions doped glasses are as follows: 40TeO2 –30B2 O3 –10CdO–10Li2 O–10AlF3

(Hostglass)

(40 − x)TeO2 –30B2 O3 –10CdO–10Li2 O–10AlF3 –xEu2 O3 (40 − x)TeO2 –30B2 O3 –10CdO–10Li2 O–10AlF3 –xTb4 O7 where x = 0.25, 0.50, 0.75, 1.0, 1.25 mol% All the weighed chemicals were finely powdered and then mixed thoroughly before each of batches (10 g) was melt using alumina crucibles in an electric furnace at 950 ◦ C, for an hour. These melts were quenched in between two brass plates and thus obtained 2–3 cm diameter optical glasses with a uniform thickness 0.3 cm and these glasses were annealed at 200 ◦ C for an hour in order to remove thermal strains if any in them soon after the glasses production.

Fig. 1. XRD profile of CLiAFBT glass.

2.2. Measurements Powder X-ray diffraction (XRD) spectrum was obtained on a Shi˚ madzu XD3A diffractometer with a Ni-filter and CuK␣ (=1.542 A) radiation with an applied voltage as 30 kV and 20 mA anode current, calibrated with Si at the rate of 2◦ /min. Differential scanning calorimeter (DSC) was carried out on a Netzsch STA 409C in the temperature range of 30–1200 ◦ C, at the rate of 10 ◦ C/min, under N2 -gas atmosphere. The optical absorption spectra (350–1100 nm) for all undoped glasses were measured on a Varian-Cary Win Spectrometer. Both the excitation and emission spectra of Eu3+ and Tb3+ doped glasses were measured under the steady state mode on a SPEX Fluorolog-2 Fluorimeter (model-F-II) with a datamax software to acquire the data with a Xe-arc lamp (150 W) as the excitation source. Fig. 2. UV absorption spectra CLiAFBT glass.

3. Results and discussion

where Eopt is the energy of the optical band gap. The above relation can be written as

3.1. CLiAFBT host glass The XRD pattern of the 40TeO2 –30B2 O3 –10CdO–10Li2 O–10AlF3 glass is shown in Fig. 1, which confirms amorphous nature of the glass. Fig. 2 shows the UV optical absorption spectrum of glass in the chemical composition of 40TeO2 –30B2 O3 –10CdO–10Li2 O–10AlF3 glass, in order to examine the UV transmission ability. From the absorption spectrum of 40TeO2 –30B2 O3 –10CdO–10Li2 O–10AlF3 , the optical band gap (Eopt ) values were determined the following way. The absorption coefficient “˛”, near the edge of each curve was determined at wavelength intervals of 5 nm for linear region and 2 nm for the non-linear region, using the relation [11].

 ˛ = ln

I/I0 t

 (1)

where“t” is the thickness of each sample and ln(I/I0 ) corresponds to the absorbance. The relation between ˛ and photon energy of incident radiation, hv is given by the following relation. ˛ = Const

[h − Eopt ] h

2

(2)

(˛h)

1/2

= Const(h − Eopt )

(3)

Using the relation (3), the Eopt values were determined by extrapolation of the linear region of the plots of (˛h)1/2 against h to (˛h)1/2 = 0 as shown in Fig. 3. The relation between ‘˛’ and Urbach energy (E) is given by the well known Urbach law is given by the relation ˛ = Const. exp

 h  E

(4)

E is usually interpreted as the width of the tail of the localized states in the band gap. The relation (4) can be rewritten as ln ˛ =

 h  E

− Const

(5)

Urbach plots are the plots where the natural logarithm of absorption coefficients, ln ˛ is plotted against photon energy h [12]. In the present study, an Urbach plot for a typical glass 40TeO2 –30B2 O3 –10CdO–10Li2 O–10AlF3 is as shown in Fig. 4. The

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Fig. 3. (˛h)1/2 as a function of photon energy h for CLiAFBT glass.

Fig. 4. ln ˛ plotted against photon energy h for CLiAFBT glass.

values of Urbach energy (E) were calculated by determining the slope of the linear regions of the curve and taking its reciprocal. The calculated values of Urbach energy (E) and optical band gap (Eopt ) are 0.299 eV and 2.852 eV, respectively. DSC thermo grams for host glass is shown in Fig. 5, and from this, the glass transition temperature (Tg = 383 ◦ C), crystallization temperature (Tc = 574 ◦ C) and glass melting temperature (Tm = 841 ◦ C) are evaluated and from these values the glass stability factor (S = 191 ◦ C), Hruby’s parameter (Kgl = 0.7153) have been calculated using the necessary formulae that are already reported in the literature [13], which are compared with the published articles [14,15]. From the DSC profile it is observed that the endothermic peak 1 is conventional band due to hydroxyl group and there is no controversial about the exothermic peak due to Tg and also endothermic peak is due to remelting temp (∼1150 ◦ C) of the glass. The major broad exothermic peak with minor upward kinks represents crystallization temp. The minor kinks indicate slight non-homogeneity in the glass material. The stability factor (S) addresses the glass stability and Hruby’s parameter presents the stability of the glass against the devitrification. 3.2. Eu3+ :CLiAFBT glasses Inset of Fig. 6, presents the excitation spectrum of Eu3+ :CLiAFBT glass, with four excitation bands which are assigned to the elec-

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Fig. 5. DSC profile of CLiAFBT glass.

Fig. 6. Emission spectra and inset show excitation spectrum of Eu3+ :CLiAFBT glasses.

tronic transitions of 7 F0 → 5 D4 at 360 nm, 7 F0 → 5 L7 at 381 nm, 7 F → 5 L at 392 nm and 7 F → 5 D at 412 nm. Fig. 6, shows the 0 6 0 3 emission spectra of Eu3+ :CLiAFBT glasses. From the emission spectra, different spectral lines are observed which are mainly located in the wavelength range from 550 to 750 nm. These emission lines correspond to transitions from the excited state 5 D0 of the 4f6 configuration of Eu3+ and the strongest emission is the electric dipole transition 5 D0 → 7 F2 which peaks at 612 nm when excited at 392 nm. The red emission was observed from the Eu3+ doped CLiAFBT glasses under an UV source also. The remaining four emission bands are observed at 578 nm, 592 nm, 653 nm and 701 nm and are assigned to the electronic transitions 5 D0 → 7 F0 , 5 D0 → 7 F1 , 5 D → 7 F and 5 D → 7 F respectively. The 5 D → 7 F transition 0 3 0 4 0 1 is purely magnetic dipole allowed and is usually taken as a reference transition. The forced electric dipole 5 D0 → 7 F2 transition is strongly hypersensitive to the environment of Eu3+ ions [16]. From the emission spectra it is also observed that as the Eu3+ ions concentration (0.25–1.25 mol%) increases the normalized emission intensity also increases. The normalized emission intensity is high for 1.25 mol% of Eu3+ :CLiAFBT glass when compared with other mol% containing glasses. Fig. 7, shows the energy level diagram of Eu3+ :CLiAFBT glasses. Fig. 8, presents the decay curves of emission bands of Eu3+ :CLiAFBT glass along with their life times in the same figure. The measured lifetime ( m ), the emission peak wavelength (p ), full width at half maximum (FWHM, p ) for all the

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K. Vemasevana Raju et al. / Spectrochimica Acta Part A 79 (2011) 87–91 Table 1 The measured emission peak wavelengths (p nm), full-widths at half-maximum (FWHM p nm) and life times ( m ms) of different emission transitions of Eu3+ and Tb3+ :CLiAFBT glasses. Emission transitions Eu3+ :CLiAFBT glass 5 D0 → 7 F0 5 D0 → 7 F1 5 D0 → 7 F2 5 D0 → 7 F3 5 D0 → 7 F4 Tb3+ :CLiAFBT glass 5 D4 → 7 F6 5 D4 → 7 F5 5 D4 → 7 F4 5 D4 → 7 F3

p (nm)

p (nm)

 m (ms)

Reference

578 592 612 653 701

4 13 15 4 11

1.788 1.897 1.911 1.742 1.850

[14] [15] [17]

488 543 584 614

16 10 13 4

2.067 2.075 2.045 2.028

[14] [19] [20] [21]

Fig. 7. Energy level diagram of Eu3+ :CLiAFBT glasses.

Fig. 9. Emission spectra and inset shows excitation spectrum of Tb3+ :CLiAFBT glasses.

Fig. 8. Decay curves of emission transitions of Eu3+ :CLiAFBT glasses.

five emission transitions have been calculated and compared with the previously published articles [14,15,17] and are presented in Table 1. 3.3. Tb3+ :CLiAFBT glasses Inset of Fig. 9, presents an excitation spectrum of Tb3+ glass with four excitation peaks, 7 F6 → 5 H7 (315 nm), 7 F6 → 5 D1 (330 nm), 7 F → 5 D (350 nm) and 7 F → 5 G (376 nm) [18]. Fig. 9, shows the 6 2 6 6 emission spectra of Tb3+ :CLiAFBT glasses with ␭exci = 376 nm. From the emission spectra, four emission transitions are observed at 488 nm, 543 nm, 584 nm and 614 nm and are assigned to 5 D4 → 7 F6 , 5 D → 7 F , 5 D → 7 F and 5 D → 7 F , respectively. Among these 5 4 4 4 4 3 the transition 5 D4 → 7 F5 at 543 nm has shown a strong green emission. The transition (5 D4 → 7 F5 ) at 543 nm is the magnetic dipole transition and obeys the selection rule of J = ±1. In the case of Tb3+ :CLiAFBT glasses also as the Tb3+ ions concentration (0.25–1.25 mol%) increases the normalized emission intensity also

Fig. 10. Energy level diagram of Tb3+ :CLiAFBT glasses.

increases. The normalized emission intensity is high for 1.25 mol% of Tb3+ :CLiAFBT glass when compared with other mol% containing glasses. Fig. 10, presents the energy level diagram of Tb3+ :CLiAFBT

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intensity increases. The normalized emission intensity is high for 1.25 mol% of Eu3+ and Tb3+ :CLiAFBT glasses when compared with other mol% containing glasses. Based on the spectral results, it is suggested that, primary colours (red and green) emitting 1.25 mol% of Eu3+ and Tb3+ :CLiAFBT glasses are suitable for the development of novel optical materials. Acknowledgement This work was supported by the UGC, New Delhi in the form of Major Research Project (F.No. 35-1/2008 S.R.) sanctioned to the author (B.S.R.) and also provided the financial assistance to one of the author (K.V.R.) in the form of Project Fellow, who would like to thank, Joint Secretary, UGC, New Delhi, India. References

Fig. 11. Decay curves of emission transitions of Tb3+ :CLiAFBT glasses.

glass. Fig. 11, presents the decay curves of emission bands of Tb3+ :CLiAFBT glass along with their life times in the same figure. The measured lifetime ( m ), the emission peak wavelength (p ), full width at half maximum (FWHM, p ) for all the four emission transitions have been calculated and compared with the previously published articles [14,19–21] and are presented in Table 1. 4. Conclusions In summary, it is concluded that we have prepared more stable and transparent Eu3+ and Tb3+ ions doped cadmium lithium alumino fluoro boro tellurite glasses. From the XRD and DSC profiles, the glass amorphous nature and thermal properties have been studied. The Eu3+ :CLiAFBT glasses have shown a strong red emission at 612 nm (5 D0 → 7 F2 ) and in case of Tb3+ :CLiAFBT glasses have shown a bright green emission at 543 nm (5 D4 → 7 F5 ). From both the emission spectra, it is observed that as the Eu3+ and Tb3+ ions concentration (0.25–1.25 mol%) increases the normalized emission

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