IJCRR

IJCRR - 9(13), July, 2017

Pages: 19-24

Date of Publication: 03-Jul-2017

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Investigations on Eu3+ Doped Zinc Phosphate Glasses for Photonic Applications

Author: B. Venkata Rao, G. Venkata Chalapathi, R. Jeevan Kumar

Category: General Sciences

Abstract:Zinc phosphate (ZP) glasses doped with different concentrations of europium (Eu3+) doped were prepared, and their luminescence spectra was investigated. Thermal properties were studied by differential thermal analysis (DTA) measurements. The phase composition of the product was verified by X-ray diffraction analysis. Structural properties were studied by FTIR and Raman spectrum. Optical transition properties of Eu3+ in the studied zinc phosphate glasses were evaluated in the framework of Judd - Ofelt theory. The radiative transition rates (AR), fluorescence branching ratios (\?), stimulated emission cross-sections (σe) and lifetimes (τexp) for certain transitions or levels were evaluated. Red emission of Eu3+ exhibits mainly by the 5D07F2 transition located at 612 nm. Concentration quenching was observed from fluorescence spectra. It is found to be that the lifetimes of 5D0 level increases with increase in concentration and then decreases. Intense orange-red light emission was obtained for the 2.0 mol% of Eu3+ ions in zinc phosphate glasses. This approach shows significant promise for use in reddish-orange lighting applications which is useful in photonics field.

Keywords: Zinc Phosphate glass, Eu3+, Judd-Ofelt theory, Concentration quenching, FTIR

Full Text:

Introduction
In recent years, researcher’s motivated on glasses and their optical components especially doped with lanthanide ions. The main interest is to search and develop novel optical devices for use in lasers, sensors, displays and optical amplifiers and fibers [1]. Glasses with rare earth ions are one of the functional optical materials. Glasses are in particularly suitable for rare earth ions due to higher solubility in host glass matrix [2]. Generally, rare earth elements have been widely used to modify the host properties of functional materials due to their unique physiochemical activity.

One class of host glass materials for visible and near infrared regions are phosphate glasses. Historically, phosphate glasses have been used as a host for optical applications, because of its excellent character with a wide range of rare earths solubility and high transparency. An important issue of this phosphate glass is low chemical stability and durability [3, 4].

Among rare-earth ions, Eu³⁺ is a highly active luminescence center because of simple energy level structure, its high luminescence performance as well as being a good for site selective spectroscopy. In amorphous and crystalline materials, luminescence properties of Eu³⁺ are well investigated in order to study the materials structure and local symmetry through spectroscopic probe. Optical materials based on Eu³⁺ gives strong red in phosphors and hence phosphor materials are useful for displays, lamps, illumination devices and optoelectronic devices. To get the luminescence improvement of Eu³⁺ ion, one way is co-doped method. Proper selection of co-doped material will play an important key role in energy transfer process.  From energy level analysis, it is observed that the chances for energy transfer and cooperative activation between Eu³⁺ and Sm³⁺ ions are more. By co-activating with appropriate concentration of Sm³⁺ with Eu³⁺ ion it was reported emission intense red light under UV excitation [5, 6].

Recently, Rajagukguk et al. [7] studied structural and optical characteristics of Eu³⁺ ions in sodium-lead zinc-lithium-borate glass system and concluded that decay time increases with increasing Eu³⁺ ion concentration. Ouchouicha et al. [8] prepared Eu³⁺/Eu²⁺ co-doped calcium aluminosilicate glass–ceramics and observed that during heating of the original glass, part of Eu³⁺ ions were reduced to Eu²⁺ ions. Steudel et al. [9] investigated quantum efficiency and energy transfer (ET) processes in rare-earth doped borate glass for solid-state lighting and concluded that for constant Sm³⁺ concentration and increasing Eu³⁺ doping, the efficiency increases slightly.

The present paper focused on the systematic characterization and analysis of concentration effect of Eu³⁺ on luminescence and. Thermal, structural and optical aspects of zinc phosphate glasses were done by DTA, XRD, FTIR, Raman, optical absorption, emission and lifetime measurements. To study concentration effect, proper analysis of excitation, emission and energy level analysis are needed and these were recorded for Eu³⁺ doped ZP glasses.

Methodology
Different zinc phosphate (ZP) glasses were prepared by using raw materials (99.9%), ammonium phosphate (NH₄H₂PO₄), lithium fluoride (LiF), strontium oxide (SrO), zinc oxide (ZnO) and europium oxide (Eu₂O₃). The glass compositions details as follows (mol%):

1   Eu01	59.9NH4H2PO4-10LiF-10SrO-20ZnO-0.1Eu2O3
2   Eu05	59.5NH4H2PO4-10LiF-10SrO-20ZnO-0.5Eu2O3
3   Eu10	59.0NH4H2PO4-10LiF-10SrO-20ZnO-1.0Eu2O3
4   Eu15	58.5NH4H2PO4-10LiF-10SrO-20ZnO-1.5Eu2O3
5   Eu20	58.0NH4H2PO4-10LiF-10SrO-20ZnO-2.0Eu2O3
6   Eu30	57.0NH4H2PO4-10LiF-10SrO-20ZnO-3.0Eu2O3

 

 

 

 

 

 

 

 

 

 

 

Precise amounts of the starting materials in mol% were weighed out and grinded in an agate mortar. The mixtures were placed in porcelain crucibles and were then heated in an electric furnace. After heating, the obtained liquid was poured on brass plate and then pressed by another brass plate. The obtained glasses are used for measurements.

DTA measurement was performed using a NetZsch STA 409 model with N₂ atmosphere. Amorphous nature of prepared glasses was verified by SEIFERT X-RAY diffractrometer. FTIR spectrum was recorded using a series of Perkin Elmer spectrum One FT-IR spectrophotometer. Raman spectrum was recorded JOBIN YVUON spectrometer. The optical absorption spectral recording was done on ELICO SL 218 double beam spectrophotometer. The excitation, photoluminescence spectra and decay curves of Eu³⁺ doped glass samples were recorded using FLS-980 fluorescence spectrometer with xenon lamp as excitation source.

Results
Thermal analysis- Differential thermal analysis (DTA)
In order to understand and interpret the thermal, crystallization and melting behaviour of glass system, differential thermal analysis (DTA) is important one. Through DTA analysis, phase changes are also observed during thermal heat treatment. The separated various phases like glass transition temperature (T_g) and crystallization temperature (T_c) which are depend on the different constituting oxides such as glass former and modifiers in the glass systems. DTA profile is recorded for host ZP glass matrix and is presented in Fig.1.

XRD analysis
XRD profile of 2.0 mol% of europium doped zinc phosphate glass is pictured in Fig.2.

FTIR spectrum
Fig.3 shows FTIR spectrum of 2.0 mol% doped zinc phosphate glass matrix.The bands are observed at 740 cm^-1, 910 cm^-1, 1075 cm^-1 and 1230 cm^-1.

Raman spectrum
 Fig.4 shows Raman spectrum of 2.0 mol% doped zinc phosphate glass matrix. The  bands are observed between 300-480 cm^-1 region and at 773 cm^-1.

Optical absorption of Eu³⁺ doped ZP glasses
The optical changes of the Eu³⁺doped ZP glasses were observed through the absorption spectra and these measurements for all glasses were recorded and were shown in Fig. 5 for 2.0 mol% of Eu³⁺ doped glass matrix. The absorption spectra display seven transitions from the ground levels ⁷F₀ and ⁷F₁. The absorption spectral bands originate from ⁷F₀ and ⁷F₁ levels since these levels (⁷F₀ and ⁷F₁) are being closely located to each other.

Excitation and emission spectra of Eu³⁺ doped ZP glasses
The observed excitation and emission spectra of Eu³⁺-doped ZP glasses were shown in Figs. 6 and 7 respectively. The observed excitation spectral transitions were found to be matched with the absorption transitions. In the excitation spectra, major emission intensity was observed at 393 nm due to ⁷F_O→⁵L₇ transition for all Eu³⁺-doped ZP glasses and using this as excitation source, emission spectra were recorded. Fig. 7 shows Eu³⁺ doped emission spectra for Eu01, Eu05, Eu10, Eu15, Eu20 and Eu30 glasses under ultraviolet excitation source (at 393 nm).

Decay analysis of Eu³⁺ doped ZP glasses
The emission decay time of the ⁵D₀ level (at λ_exc = 393 nm and at λ_emi=612 nm) of the different ZP glasses are shown in Fig. 8. The decay curves were fitted to the first order exponential function for all Eu³⁺ doped ZP glasses.

Discussion
 From Fig.1, an endothermic peak located nearly at 136 ^OC is observed which is due glass transition temperature (T_g). Two exothermic peaks follow after the glass transition temperature indicating the stage of crystallization (T_C) process. T_C1 and T_C2 are two crystallization events are formed during melting process. These two events are occurred nearly at 196 and 267 ^OC temperatures, respectively. From Fig. 1, where the higher T_C2 and lower Tg are used to measure the thermal stability of the amorphous zinc phosphate glass system. The thermal stability or glass forming ability can be measured by difference of glass transition and crystallization temperatures (ΔT = T_C2-T_g) [10].  In the present work, it is observed that the thermal stability is found to be 131 ^OC which is high for the zinc phosphate glass system.

 From this XRD figure 2, it is apparent that no characteristic diffraction peaks, indicating that the prepared glasses are of amorphous in nature. Fig.3 consists of the band at 740 cm^-1 is ascribed to the symmetric stretching vibration of P–O–P rings. The absorption band at 910 cm^-1 is related to asymmetric stretching vibration of P–O–P groups linked with linear metaphosphate chains. The band at 1075 cm^-1 is assigned to asymmetric stretching of PO^-2 group units. The band at 1230 cm^-1 is attributed to the stretching of the doubly bonded oxygen vibration, ν_as(P = O) modes [11].

Fig.4 shows Raman spectrum consists of the broad band at 773 cm^-1 is due to symmetric stretching of (P–O–P) bridging oxygen bonds in (P₂O₇)⁴ units. [12, 13, 14 ]. This peak is a convolution of two peaks i.e. one may due to vibrations of the two joined Q² units and the second one to those of Q¹ units. Presence of last two bands indicates that due to addition of different modifiers into glass matrix, long range order of network become cut down and formed chain like units. This type of structural units increases glass strength.

The absorption spectra (Fig.5) display seven transitions from the ground levels ⁷F₀ and ⁷F₁. The absorption spectral bands originate from ⁷F₀ and ⁷F₁ levels since these levels (⁷F₀ and ⁷F₁) are being closely located to each other. However, transitions from the ⁷F₂ level were too weak to be investigated because of the minimum fractional thermal populations at room temperature [14]. The optical bands observed due to the ⁷F₀→⁵D₄, ⁷F₀→⁵G₄, ⁷F₁→⁵G₃, ⁷F₀→⁵L₆, ⁷F₁→⁵D₃, ⁷F₀→⁵D₂ and ⁷F₁→⁵D₁ transitions, centered at 362, 376, 382, 393, 415, 465 and 535 m respectively. At room temperature, energy difference between ⁷F₁ and ⁷F₀ level was only ~350 cm^-1 and thermally populated about 35% and 65% respectively. Hence, absorption transitions were appeared from both levels [16]. So that, Judd-Ofelt analysis from the absorption spectra is very difficult. Out of these seven bands, the absorption band, ⁷F₀→⁵L₆ has more intensity than rest of the absorption bands. This transition was forbidden by the ΔS and ΔL selection rules, but it is allowed by the ΔJ selection rule. Furthermore, the intensity of ⁷F₀→⁵D₂ (electric dipole) transition has higher than that of ⁷F₀→⁵D₁ transition (magnetic dipole). Hence, the ⁷F₀→⁵D₂ transition is referred as hypersensitive transition [17,18].

The excitation spectra (at λ_emi= 611 nm) were composed of ⁷F₁→⁵I₅, ⁷F₀→⁵H₆, ⁷F₀→⁵H_3,7, ⁷F₀→⁵D_4,  ⁷F_O→⁵L₁₀, ⁷F₀→⁵G_2,3,4,5, ⁷F_O→⁵L₇, ⁷F₀→⁵L₆, ⁷F_O→⁵D₃, ⁷F_O→⁵D₂, ⁷F_O→⁵D₁ and ⁷F₁→⁵D₁ transitions with the wavelengths located at 302, 319, 326, 360, 364, 376, 381, 393, 413, 463, 522 and 531 nm respectively (Fig.6). The ⁷F₀→⁵L₆ (393 nm) and ⁷F₀→⁵D₂ (464 nm) transitions indicate relatively higher intensities than the other transitions [19]. It is observed that the peak at 440 nm is associated to the pure electronic transition (PET) and the peak at 464 nm is associated to the phonon side band (PSB) ⁷F₀→⁵D₂ occur at higher energy side. Due to excitation by the incident energy corresponding to the PET with an excess energy of vibrational modes of Eu-O bonds, PSBs were occurring. The phonon energy (?ω) can be defined as energy difference between PSB (at 22727 cm⁻¹) and PET (at 21555 cm⁻¹), it was 1172 cm⁻¹ in Eu³⁺ doped ZP glasses. Thus, PSBs result from inter mixing of vibrational modes to the IR bands in the range 1080-1270 cm^-1  related to P-O-P and P=O bands. The electron–phonon coupling strength parameter (g) was calculated from the integrated intensity ratios of PSB to PET using the formula given in Ref. [20]. The g parameter gives intensity of interaction between Eu³⁺ cation and O^2- ligand anion. This g parameter was found to be 0.1, 0.4, 0.5, 0.7 and 0.5 for Eu01, Eu05, Eu10, Eu15, Eu20 and Eu30 glass matrices respectively.

In the excitation spectra, major emission intensity was observed at 393 nm due to ⁷F_O→⁵L₇ transition for all Eu³⁺-doped ZP glasses and using this as excitation source, emission spectra  (Fig.7)were recorded. By using 393 nm excitation, transitions from ground level to ⁵L₇ level take place. The population of the ⁵L₇ level relaxes non-radiatively (NR) to the ⁵D₃ level, and then sequentially to the lower ⁵D₀ level via ⁵D₂ and ⁵D₁ levels. Emissions from the ⁵D₀ level to lower levels takes place radiatively due to higher energy gap between the excited ⁵D₀ and ⁵F₆ level and negligible multiphonon relaxation (MPR) effect. Emission occurs only from ⁵D₀ level and emission from ⁵D₁, ⁵D₂ and ⁵D₃ levels were not observed (From Fig.7) due to higher phonon energy of ZP host glass matrix. In the present work, five emission bands (in the visible region) were noticed i.e. from the excited state ⁵D₀ to lower levels i.e. ⁵D₀→⁷F₀ (578 nm, yellow), ⁵D₀→⁷F₁ (590 nm, orange), ⁵D₀→⁷F₂ (611, red), ⁵D₀→⁷F₃ (653 nm, deep red), ⁵D₀→⁷F₄ (701 nm, deep red).

The emission line associated with ⁵D₀→⁷F₂ transition has the highest emission intensity and the emission line associated with ⁵D₀→⁷F₀ transition has the lowest in emission intensity. Out of five bands, the ⁵D₀→⁷F₂ band was a forced electric dipole (ED) allowed transition and it possesses hypersensitive character due to the admixture of odd-parity character into the 4f⁶ wave functions. Meanwhile, the ⁵D₀→⁷F₁ was magnetic dipole (MD) spin-forbidden transition but it is allowed when Eu³⁺ ion is in higher symmetric position due to spin–orbit interaction. The rest of emission lines ⁵D₀→⁷F_J (J = 0 and 3) were strictly forbidden. The intensity ratio (R: ED/MD) was widely used to manifest the bonding medium and asymmetry around Eu³⁺ ions. This ratio gives information concern covalent/ionic bonding between the Eu³⁺ ions and the surrounding oxygen ligands. With increase in europium concentration, R value also increases due to increase in asymmetry and higher degree of covalency between Eu–O. Smaller values of R is attributed to higher local symmetry for Eu³⁺ ions.

Increase of Eu³⁺ content from 0.1 mol% to 2.0 mol% increases the emission intensities, and then decreases drastically when Eu³⁺ content is 3.0 mol%. As the concentration increased, the Eu³⁺ ions are crowded closer and closer together. This is owing to strong interactions among the europium ions as well as cross relaxation (CR) process at higher concentrations. These CR was from ⁵D₂ to ⁵D₁ levels and from ⁵D₃ to ⁵D₂ levels and channels like : ⁵D₂+⁷F₁→⁵D₁+⁷F₄ and ⁵D₃+⁷F₀→⁵D₂+⁷F₄. Among all ZP glasses, 2.0 mol% of Eu³⁺ doped ZP glass shows the strongest red emission intensity.

The limited level of absorption transitions and energy level structure of Eu³⁺ ion, Judd-Ofelt parameters were unable to determine from absorption spectra. Hence, the J-O intensity parameters (Ω₂ and Ω₄) [21, 22] were calculated for Eu20 ZP glass system from the emission spectra. The ⁵D₀→⁷F₂ transition has not been observed in emission spectra and hence Ω₆ parameter was not evaluated. It is observed that the Ω₂ parameter is high (7.11 × 10⁻²⁰ cm²) in 2.0 mol% Eu³⁺-doped ZP glass matrix. This indicates that high asymmetry and covalency connected around the europium ions in the Eu20 glass matrix. The J-O parameters followed with trend like Ω₂>Ω₄ in Eu20 ZP glass. This affirmation is also in consistence with the obtained R/O intensity ratios observed in Eu20 glass matrix. J-O intensity parameters were used to assess the radiative properties and are presented in Table 1 for Eu20 ZP glass matrix.

It is observed that among all emission transitions, ⁵D₀→⁷F₂ transition shows higher values of radiative transition probabilities (A_R) in all the concentrations, and this value is high in 2.0 mol%.  The experimental branching  ratios (β_exp) and stimulated emission cross-sections (σ_e) for all the emission transitions of Eu³⁺-doped ZP glass matrices are also calculated. The β_exp are obtained from the relative areas under specific transitions. From Table 1, among all emission transitions, ⁵D₀→⁷F₂ transition has higher β_exp (68%) for 2.0 mol% of Eu³⁺-doped Eu20 glass matrix. It  indicates that the dominant red emission located at 612 nm of Eu³⁺ might be useful for optical materials. The luminescence performance of the material can be decided from σ_P values. From table 1, the σ_P values are higher for ⁵D₀→⁷F₂ transition compared to other emission transitions.

The obtained lifetimes (Fig.8) were 2.3, 2.3, 2.4, 2.5, 2.6, and 2.7 ms for Eu01, Eu05, Eu10, Eu15, Eu20 and Eu30 glasses respectively. Decay time is observed to be increased with increasing Eu³⁺ ion concentrations from 0.1 mol% to 2.0 mol % and then decreases. The glass sample with 0.1 mol% shows the shortest luminescence lifetimes and sample with 2.0 mol% shows the longest luminescence lifetimes.

Conclusion
The authors prepared and characterized Eu³⁺ doped zinc phosphate glasses for luminescence applications. The thermal stability measured by using DTA technique. In the present work, it is observed that the thermal stability is found to be 131 ^OC which is high for the zinc phosphate host glass system. The phase composition of the product was verified by X-ray diffraction analysis and found to be amorphous in nature. The structural properties are studied through FTIR and Raman measurements. Optical properties of Eu³⁺ in the studied zinc phosphate glasses were evaluated in the framework of Judd–Ofelt theory. Among different ZP glasses, Eu20 has high covalency bond. Red emission of Eu³⁺ is generated mainly by the ⁵D₀→⁷F₂ transition located at 612 nm. Concentration quenching and energy transfer were observed from fluorescence spectra and decay curves, respectively. The 0.1 mol% and 3.0 mol% Eu³⁺ doped ZP samples have the longest and shortest decay times. With the increase in dopant concentration, decay time values gradually decreased. It was found that the main reason for concentration quenching was non-radiative energy transfer through cross-relaxation mechanism due to dipole–dipole interaction. This approach shows significant promise for use in bright orange-red lighting applications. The optimized properties of the Eu³⁺ doped zinc phosphate glasses are found to be promising luminescent materials.

ACKNOWLEDGEMENTS
 Authors acknowledge the immense help received from the scholars whose articles are cited and included in references of this manuscript. The authors are also grateful to authors / editors / publishers of all those articles, journals and books from where the literature for this article has been reviewed and discussed.

Source of funding: None

Conflicts of interest: None

 

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