Advances in Materials Physics and Chemistry
Vol.04 No.09(2014), Article ID:49926,6 pages

Photoluminescence and Energy Transfer Process in Bi3+/Sm3+ Co-Doped Phosphate Zinc Lithium Glasses

Chandragiri Parthasaradhi Reddy, Varnakavi Naresh, Ballipalli Chandra Babu, Srinivasa Buddhudu

Department of Physics, Sri Venkateswara University, Tirupati, India


Copyright © 2014 by authors and Scientific Research Publishing Inc.

This work is licensed under the Creative Commons Attribution International License (CC BY).

Received 5 July 2014; revised 22 August 2014; accepted 7 September 2014


Present paper reports on luminescence characteristics of individually doped Bi3+: PZL, Sm3+: PZL and co-doped (Bi3+/Sm3+): PZL (50P2O5-30ZnO-20LiF) glasses prepared by a melt quenching method. The results revealed that Bi3+: PZL glass exhibited a broad emission peak at 440 nm (3P1®1S0) under excitation wavelength 300 nm (1S0®3P1). Sm3+: PZL doped glass has shown a prominent orange emission at 601 nm (4G5/2®6H7/2) with an excitation wavelength 403 nm (6H5/2®4F7/2). Later on Bi3+ is added to Sm3+: PZL glass by increasing its concentrations from 0.1 - 1.5 mol%. By co-doping Bi3+ to Sm3+: PZL glass, Sm3+ emission intensity has been considerably enhanced till 1.0 mol% due to energy transfer from Bi3+ to Sm3+ and when its concentration exceeds this critical value (1.0 mol%) there has been a drastic decrease in Sm3+ emission which is explained accordingly from photoluminescence spectra, energy level diagram and lifetime measurements.


Melt Quenching, Optical Glasses, Rare Earth Ions, Photoluminescence, Luminescence Quenching and Energy Transfer

1. Introduction

From the optical spectroscopy point of view, a remarkable progress has been observed in the development of rare earth ions as luminescent centres due to their narrow emission bands (f-f interactions) and high internal quantum efficiencies with suitable promising application in the field of photonics as solid state lasers and optoelectronic devices [1] - [5] . In the present study, lithium zinc phosphate glass has been taken as a host because of its combined advantages of fluoride (low phonon energy) and oxide glasses (high mechanical and thermal stability and chemical durability) [6] . Such a stable base glass 50P2O5-30ZnO-20LiF (PZL) is initially incorporated with 0.5 mol% Sm3+ and 0.5 mol% Bi3+ separately and their luminescence properties were analyzed. Later on Bi3+ ion is co-doped to 1.0 mol% Sm3+ to study the enhancement in the luminescence through energy transfer from Bi3+ to Sm3+ ion. Here, we choose Sm3+ because it has efficient luminescence (strong orange emission) in the visible region and has wide range of applications such as hole burning, high-density optical storage, colour displays and alongside samarium doped glasses can also be used as a cladding for Nd-glass laser rods [7] . Bi3+ co-doped rare earth ions are considered as prospective materials for scintillators. Bi3+ is chosen because of its closed-shell 6S2 configuration which influences the luminescence and it is considered as suitable material both as an activator and sensitizer for scintillators due to an intense and fast Bi3+-related luminescence [8] . The energy transfer occurs between a sensitizer and an activator, if the energy difference between the excited state of sensitizer (S) and ground state activator (A) is equal. The sensitizer transfers all its energy non-radiatively to activator by quenching its luminescence and enhancing emission of activator. This resonance condition is called resonance transfer of energy. This resonance condition can be tested by the spectral overlap of emission band of sensitizer (S) and absorption band of activator (A) [9] [10] . Energy transfer between doped luminescent ions in the optical materials enhances the emission due to acceptor (Sm3+) and quenches donor emission by transferring its excitation energy. So in the present work we have undertaken to co-dope Bi3+ & Sm3+ ions into the PZL glass matrix to investigate the possibility of energy transfer between these luminescent ions.

2. Experimental Studies

Lithium zinc phosphate glasses with base composition 50P2O5-30ZnO-20LiF containing singly doped Bi3+, Sm3+ ions and together doped (Sm3+/Bi3+) ions in different sets were prepared by a melt quenching method. The chemical compositions are listed below:

1) 50P2O5-30ZnO-20LiF (PZL) (host glass).

2) (50 − x)P2O5-30ZnO-20LiF-xBi2O3 (x = 0.5 mol%).

3) (50 − y)P2O5-30ZnO-20LiF-ySm2O3 (where y = 0.5 mol%).

4) (50 − x)P2O5-30ZnO-20LiF-xBi2O3-ySm2O3 (where x = 0.1, 0.5, 1.0, 1.5 mol% and y = 0.5 mol%).

Reagent grade chemicals NH4H2PO4, ZnCO3, LiF, Sm2O3, and Bi2O3 were used for the preparation of glasses. All those chemicals were weighed separately in a 10 g batch, thoroughly mixed and finely powdered using an agate mortar and pestle. Each batch of chemical mix was transferred into porcelain crucibles and each of those was sintered in an electric furnace for an hour at 950˚C separately. These melts were quenched in between two smooth surfaced brass plates to obtain circular glass discs of 2 - 3 cm in diameter with 0.3 cm in thickness. The reference PZL glass was transparent and colourless. Sm3+: PZL glass did exhibit an orange emission, Sm3+/Bi3+: PZL co-doped glasses have displayed an enhanced bright reddish-orange emission under an UV source.

3. Measurements

The optical absorption spectra of PZL glasses doped with Sm3+ and Bi3+ were recorded at room temperature in the spectral range of 250 nm - 2500 nm on a Varian-Cary-Win Spectrometer (JASCO V-570). The excitation and emission spectra of singly doped Sm3+, Bi3+ and co-doped (Sm3+/Bi3+) glasses were recorded at room temperature on a SPEX Flurolog-3 (Model-II) spectrophotometer, attached with an Xe-arc lamp (450 W) as the excitation source. This system is employed with a Datamax software package for acquiring the spectral data and decay-curve (lifetime measurement) data using a phosphorimeter and a Xe-flash lamp.

4. Results and Discussion

4.1. Photoluminescence Spectrum of Bi3+: PZL Glass

Figure 1(a) and Figure 1(b) represent excitation and emission spectra of 0.5 mol% Bi3+: PZL glass. The luminescence properties of Bi3+ (6S2) were attributed to radiative decay of triplet relaxed state of Bi3+ centres, 3P1®1S0 and 3P0®1S0 originating from the S-P inter-configurational transition [11] [12] . Usually the excitation occurs from the ground state 1S0 to the excited state 3P0, 3P1, 3P2 and 1P1 in the sequence of increasing energy. The transitions from 1S0®3PJ where J = 0 and 1 are spin forbidden. However, the 1S0®3P1 is partially allowed by mixing with the singlet and triplet states of Bi3+ ions. In the present glass matrix 1S0®3P1 transition is

(a) (b)

Figure 1. (a) Excitation and (b) emission spectra of Bi3+: PZL glass.

noticed. In the excitation spectrum a broad band in the range of 280 nm to 330 nm centred at 300 nm which corresponds to 1S0®1P1 electronic transition of Bi3+, which is partially allowed due to mixing of triplet 3P1 state with singlet 1P1 state by the spin-orbit interaction. The emission spectrum is measured by monitoring at 300 nm as excitation wavelength, a broad band with a maxima centred at 440 nm assigned to 3P1®1S0 is obtained [13] . The emission of Bi3+ ion is assigned to its electro-covalence, which arose from the S-P interactions [12] [14] .

4.2. Emission Analysis of Sm3+: PZL Glass

In Figure 2(a) and Figure 2(b) the excitation and emission spectra of Sm3+: PZL glasses for 0.5 mol% Sm3+ are shown. The spectra is measured in the range of 325 nm to 575 nm exhibiting bands at 6H5/2®4H9/2 (345 nm), 4D5/2, 6P5/2 (363 nm), 4D1/2 (376 nm), 4F7/2 (403 nm), 4M19/2 (418 nm), 4G9/2 (439 nm), 4I13/2 (463 nm), 4I11/2 (471 nm), 4G7/2 (501 nm), 4F3/2 (528 nm), 4G5/2 (563 nm) attributed to 4f-4f transition of Sm3+ [15] . Among all the transitions, the prominent excitation transition 6H5/2®4F7/2 at 403 nm has been chosen for the recording emission spectra of Sm3+ doped glasses. Upon exciting, Sm3+ ions are pumped to upper energy level 4H9/2 and from where they relax non-radiatively to 4G5/2 metastable state through 4F7/2, 4G7/2, and 4F3/2 levels. As the energy levels 4F7/2 and 4G5/2 are very close fast non-radiative relaxations takes place. The photoluminescence spectra consists of four emission bands assigned to their electronic transitions 4G5/2®6H(2j+1)/2 where j = 2, 3, 4, and 5 at (565 nm: yellow), (602 nm: orange), (647 nm: orange reddish), and (709 nm: red). Of all these transitions, 4G5/2®6H7/2 (602 nm) is the most dominant transition with intense orange emission which is partially MD- and partially ED- allowed with selection rule ∆J = ±1, therefore it can be considered to be suitable for laser emission. 4G5/2®6H5/2 (565 nm) transition is a forbidden magnetic dipole transition (∆J = 0 i.e., J ≠ 0 ↔ 0 values), 4G5/2®6H9/2 (647 nm) is purely electric dipole transition with ∆J = ±2 having moderate intensity and 4G5/2®6H11/2 (709 nm) is forbidden transition with ∆J = ±3 having feeble intensity [16] - [18] . The intensity ratio (R) between ED and MD transitions elucidates the asymmetry nature Sm3+ ion in the glass matrix. Higher is the intensity of the ED transition greater is the asymmetric nature. In the present work, 4G5/2®6H9/2 (ED) transition is less intense compared to 4G5/2®6H5/2 (MD) transition suggesting symmetric nature of Sm3+ in the host glass. The assignment of luminescent bands are made on the basis of energy level diagram given by Dieke [19] and from Carnall and his co-workers [15] .

4.3. Energy Transfer between Luminescent Bi3+ and Sm3+ Ions

Energy transfer is very important mechanism in the study of luminescent properties in co-activated glassy systems. According to Dexter’s theory the probability of energy transfer is proportional to spectral overlap of sensitizer emission and activator absorption/excitation [9] [10] . The spectral overlap of Bi3+ (sensitizer) emission and Sm3+ (acceptor) excitation is shown in the Figure 3. The primary condition for energy transfer is fulfilled from the spectral overlap of emission band of Bi3+ (440 nm; 3P1®1S0) appreciably with the excitation bands of Sm3+ (6H5/2®4F7/2 (403 nm); 4G9/2 (439 nm); 4I13/2 (463 nm), 4I11/2 (471 nm). Spectral overlap is not only the main criteria for energy transfer, even if the relative fluorescence levels are matched or if sensitizer excitation state is above the emission level of the activator ion energy transfer occurs [20] .

In Figure 4(a), the emission spectra for co-doped (Bi3+/Sm3+): PZL glasses are shown. The glass samples under study are prepared by taking Bi3+ at various concentrations (0. 1 - 1.5 mol%) and Sm3+ at a definite concentration of 0.5 mol%. The effect of co-doping Bi3+ ion on photoluminescence emission of Sm3+ is studied at excitation wavelength 403 nm. The emission spectra displayed four peaks (4G5/2®6HJ where J = 5/2, 7/2, 9/2, 11/2) attributed to Sm3+ with an additional band centred at (440 nm; 3P1®1S0) which arises from Bi3+. It is also

(a) (b)

Figure 2. (a) Excitation spectrum of Sm3+: PZL glass; (b) Emission spectra and energy level diagram of (0.5 mol%) Sm3+: PZL glass.

Figure 3. Spectral overlap of Bi3+ emission spectrum and Sm3+ excitation spectrum.

noticed from the spectra, with increasing the concentration of Bi3+ ion in the co-doped glasses, the emission intensity of Sm3+ has been enhanced greatly suggesting the energy transfer due to sensitization effect of Bi3+. In addition to this broad emission band of Bi3+ peeking at 440 nm has also been increased [21] - [23] . Beyond 1.0 mol% of Bi3+ (i.e. for 1.5 mol%) there is quenching in the emission intensity of Bi3++Sm3+ co-doped glass system. At lower concentrations of Bi2O3, the average distance between the Bi3+-Bi3+ ions is high so the interactions between them is negligible as a result more amount of energy would be migrated from Bi3+ to Sm3+. Whereas at higher concentration of Bi2O3, the distance between Bi3+-Bi3+ ions is low resulting in high interactions between the ions. Due to these high interactions, lower amount of energy would be transferred from Bi3+ to Sm3+. Therefore at higher concentrations of Bi3+, luminescence quenching is observed in case of 1.5 Bi3+ + 0.5 Sm3+ co-doped glass.

Figure 4(b) shows the excitation spectrum of the co-doped glass which exhibits the same trend as shown by the emission spectrum. The mechanism for energy transfer from Bi3+ to Sm3+ is explained from energy level diagram for the (Bi3+/Sm3+) co-doped glass is shown in Figure 5. Under near UV irradiation Bi3+ ions are pumped to 1P1 level from ground state and these ions cascade rapidly to 3P2, 3P1 levels and finally reach to ground state with emission of radiations. This emitted radiation energy of Bi3+ is partially reabsorbed by the Sm3+ ions in the ground state and jumps to the higher energy levels and there by relaxes non-radiatively to the ground state with enhanced red-orange emission. The energy transfer mechanism is further understood from excitation spectra and lifetime measurements for the co-doped PZL glass matrices.

The transfer of energy taking place from Bi3+ to Sm3+ is further explained from the lifetime measurements. Luminescence decay analysis is very useful for understanding the energy transfer mechanism and luminescence quenching. Generally, luminescent materials posses an exponential decays so that it is convenient to express the time constant as 1/e (time to decay 37% of the initial intensity) and the decay times are noticed in the range of milliseconds to several hundred nanoseconds. The emission decay lifetime for the co-doped (Bi3+/Sm3+) is shown in the Figure 6. The life time value of decay curves are calculated from the first order exponential decay method by using the equation: I = I0 exp (−t/τ) [6] . Measured emission decay lifetimes are found to be 1.23 ms, 1.27 ms, 1.30 ms, 1.32 ms, 1.37 ms. With increasing concentration of Bi3+ all the profiles of decay curves exhibited non-exponential nature due to energy migration depending on the types of interaction and on the average distance between the donor and acceptor ions. The reason for this non-exponential decay trend could be explained as follow: when the donor ion are excited in the presence of activator ions in a co-doped glass matrix, donors near to the acceptors decay first due to fast migration of energy so the decay is fast at initial stages whereas donors at far apart transfers their excitation energy for a long time to acceptors so the decay is slow and finally donors decay with their own lifetimes showing a non-exponential nature in the decay curve.

(a) (b)

Figure 4. (a) Energy transfer based emission spectra of co-doped (0.5 mol%) Sm3+ + (0.1 - 1.5 mol%) Bi3+: PZL glasses. (b) Excitation spectrum of co-doped (0.5 mol%) Sm3+ + (0.1 - 1.5 mol%) Bi3+: PZL glasses.

Figure 5. Energy level diagram for co-doped Sm3+ + Bi3+ ions in PZL glass.

Figure 6. Emission lifetime decay curve of co-doped (0.5 mol%) Sm3+ + (0.1 - 1.5 mol%) Bi3+: PZL glasses at 402 nm excitation.

5. Conclusion

In conclusion it is summarized that, stable and transparent glasses in the chemical composition 50P2O5- 30ZnO-20LiF containing Bi3+, Sm3+ ions in single combination and (Bi3+/Sm3+) dual combinations are prepared separately by employing a melt quenching method. Bi3+ glass demonstrated a broad emission peak at 440 nm (3P1®1S0) under 300 nm (1S0®3P1) excitation while Sm3+ glass had shown an intense orange emission at 601 nm (4G5/2®6H7/2) at an excitation 403 nm (6H5/2®4F7/2). Energy transfer taking place from Bi3+ to Sm3+ has been realized from spectral overlap of Bi3+ emission spectrum and Sm3+ excitation spectrum. With the addition of Bi3+ to Sm3+: PZL glass, emission due to Sm3+ has been enhanced till 1.0 mol% of Bi3+ and beyond concentration quenching in luminescence is observed. The sensitization effect of Bi3+ has been explained in terms of emission spectrum of (Bi3+/Sm3+) co-doped glasses and also from energy level diagram and emission decay curves. Such glasses could be potentially useful as orange light emitting devices in the fields of photonics and optoelectronic devices.


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