Spectral properties of Eu3+: ZnO–B2O3–SiO2 glasses

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Abstract

Studies on the vibronic spectral analysis of Eu3+ ions doped in zinc borosilicate glass have been performed in order to correlate them with the local environment of the dopant ion in this host matrix on the basis of the Fourier Transform infrared (FT-IR) spectroscopy. It is noticed that the Eu3+ ions are bonded to the bridging and non-bridging oxygens of BO3, BO4 and SiO4 structural units. The Stark splittings exhibited especially by the emission transitions: D0F17, D05F27 and their intensity ratio (EMIR) could indicate that the Eu3+ ions are taking the sites, which lack inversion symmetry. Judd–Ofelt intensity (Ωλ, λ=2, 4, 6) parameters have been obtained from the measured absorption spectra of Eu3+: Zinc borosilicate glass and these intensity defining factors are used in the computation of the emission transition probabilities, radiative rates, braching ratios and radiative lifetimes of the fluorescence transitions. Decay curves of the emission transitions (D05F0,1,2,3,47) were measured and their lifetimes have been estimated. Besides this, Optical band gap energies (EOpt) have also been evaluated from the base glass absorption spectrum and the values are found to be 4.82 and 4.60 eV for direct and indirect band gaps, respectively.

Introduction

In recent years, ZnO and the materials based on it are drawing more attention due to their interesting optical, electrical and magnetic properties in combination with its non toxicity, non-hygroscopic nature and low cost. Its direct wide band gap, intrinsic emitting property and a large exciton binding energy makes it a potential candidate for development of optoelectronic devices [1], [2], UV emitting lasers [3], solar energy converters [4], gas sensors [5]. ZnO based materials in the form of single crystals, polycrystalline powders, thin films or nano particles activated with dopant ions are widely used as luminescent phosphors in TV, CRT screens, lamps, X-ray intensifiers, flat panel displays, plasma display panels [6]. It has been found that, Zinc borosilicate glass in the chemical composition of 60ZnO–20B2O3–20SiO2 with 0.2 mol% Tb2O3 could exhibit green phosphorescence upon excitation with an UV source at 254 nm [7]. This phosphorescence arises from f–f transitions of Tb3+ ions and has readily been noticed by our eyes, in the dark for about an hour even after the stoppage of UV-excitation, which could be due to the recombination process of oxygen ion vacancies associated with the network modifying Zn2+ created during UV irradiation with the multivalent luminescent ions. A strongly enhanced luminescence efficiency of zinc-borate glasses and glass fibers doped with trivalent europium or terbium has been expected because of large amount of direct gap ZnO nanocrystallites which function as sensitizers for the rare-earth ions. Subsequently, a long lasting phosphorescence (LLP) and photo-stimulated long lasting phosphorescence (PSLLP) has become possible from Mn2+ doped zinc borosilcate glass [8], [9]. Earlier, we have studied the fluorescence as well as upconversion emission properties of several rare earth doped zinc borosilicate glasses [10], [11], [12], [13], [14], [15]. It is well known that, optical spectra of rare earth ions often exhibit vibronic features. According to the general theory, the transitions following ΔJ=0, 2 selection rule accompany these vibronic spectral features and for this matter, Eu3+ ions have widely been considered as probing ions due their simple energy level structures and the occurance of vibronic features accompanied with the transitions (F07D05, D25 or D05F07, F27) in absorption, excitation or emission spectra. In the present paper, we report on the vibrionic spectral features associated with the excitation transition F07D25 of Eu3+ in zinc borosilicate glass in correlation with its Fourier Transform Infrared (FT-IR) vibrational spectra in order to understand the local site environments. Besides this, the paper also presents the optical band gap energies (Eopt) determined for the base glass absorption along with the estimated values of Judd–Ofelt intensity parameters (Ωλ cm2, where λ=2, 4, 6), radiative emission properties such as spontaneous emission probability (A s−1), total radiative rate (AT s−1), branching ratios (βR) and ratiadive lifetimes of the emission transitions of the Eu3+ doped zinc borosilicate glass. Also lifetimes of all the recorded fluorescence transitions (D05F0,1,2,3,47) for the Eu3+ (1 mol%) doped zinc borosilicate glass have been measured and reported.

Section snippets

Experimental

Zinc borosilicate glasses of the following chemical composition 60ZnO–20B2O3–(20-x) SiO2-x Eu2O3 (x=0 and 1) in molar percentage were prepared by employing the quenching technique. The raw chemicals used were of spectral pure ZnO, SiO2, H3BO3, and Eu2O3. The glass preparation procedures were similar to those as were explained earlier [10], [11], [12], [13], [14], [15]. The annealed glass samples were processed into desired dimensions for different property measurements and 2 mm thick glass

Results and discussion

Fig. 1a shows the absorption spectrum of zinc borosilicate reference glass (un-doped glass sample) in the wavelength range of 200–1100 nm. In the NIR and visible regions, the glass exhibited a good transparency but below 300 nm its absorbance started to increase rapidly. The non-sharp UV absorption edge in the profile confirms the amorphous nature of the glass under study. There are two types of allowed optical transitions that are involved in the occurrence of UV edge when electromagnetic

Conclusions

In summary, it is concluded that we have successfully analyzed the vibronic features accompanying the pure electronic excitation transition F07D25 of Eu3+ ions in zinc borosilicate glass and interpreted the local environment around the dopant ions based on IR vibrational spectra of the glass obtained from FT-IR measurement. It could be observed that, Eu3+ ions incorporated in the glass network are replacing the Zn2+ modifier cations, which are predominantly bonding with the bridging oxygens of

Acknowledgements

We would like to thank Dr H.S. Maiti, Director, CGCRI for his kind encouragement in the present work. One of us (MD) is grateful to the CGCRI, CSIR for the award of a Research Internship to her.

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