Role of modifier ion radius in luminescence enhancement from ⁵D₄ level of Tb³⁺ ion doped alkali-alumino-telluroborate glasses

Highlights

• A phonon sideband was obtained from the excitation spectra of Tb³⁺ ion.

• Phonon energy and multiphonon relaxation rate were estimated by PSB analysis.

• Luminescence from ⁵D₄ (Tb³⁺) level is enhanced with the modifier ion having large radius.

• The optical properties of Tb³⁺ were analyzed by using the Judd-Ofelt theory.

Abstract

Alkali-alumino-telluroborate glasses with the different network modifier elements were prepared by melt–quenching method. Absorption, excitation, luminescence spectra as well as decay curves of ⁵D₄ level were measured at room temperature. A phonon side band of ⁷F₆ → ⁵D₄ transition has been obtained from the excitation spectra of Tb³⁺ ion. The role of modifier ion radius in luminescence enhancement from ⁵D₄ level of Tb³⁺ ion was discussed in detail. In prepared glasses, multiphonon relaxation rate from ⁵D₃ level to ⁵D₄ is enhanced in the host glasses containing modifier ions with large radius. This is main reason leading to the enhancement of luminescence from ⁵D₄ level of Tb³⁺ ion in alkali-alumino-telluroborate glasses under excitation at 350 nm. Additionally, the Judd-Ofelt theory was used to estimate the features of ligand field and radiative properties of Tb³⁺ ion. The calculated results have shown that the ⁵D₄→⁷F₅ transition has potential applications for laser action and optical amplifier.

Introduction

Recently, there has been a great attraction in studying the fluorescence of trivalent rare earth (RE³⁺) doped crystals and glasses because of their applications in photonic such as laser, lighting, optical amplifier, etc [[1], [2], [3], [4], [5]]. In comparison with crystals, the glass materials show more advantages because they could be prepared in different shapes and sizes, which accept rare-earth ions without inducing any crystallization [3,5,6]. The glass materials also display good optical transparency [6,7]. Moreover, the low cost and rare earth solubility are also other advantages of glasses. Among the inorganic glasses, borate glass has been studied extensively due to its special physical properties such as excellent heat stability and lower melting temperature in comparison with other glasses [[5], [6], [7]]. The most disadvantages of this glass are low mechanical reliability and high phonon energy which leads to the large loss of excitation energy through multiphonon relaxation [3,7]. To improve these disadvantages, the TeO₂ components are often added to borate glass to create B₂O₃–TeO mixture glasses. The addition of TeO₂ oxide in borate glass can cause the reduction of the phonon energy and the enhancement of the durability of the material [3,6].

Trivalent terbium (Tb³⁺) is abundant rare earth element which is a promising activator for applications in fluorescent tubes, plasma display panels, lighting, laser, solar cells and light frequency conversion [6,[8], [9], [10], [11]]. The strong emission of green band at wavelength around 543 nm corresponding to ⁵D₄→⁷F₅ transition in Tb³⁺ ions that can relate to nonradiative process from ⁵D₃ level to ⁵D₄ [12,13]. This process includes two mechanisms: multiphonon relaxation (MR) and energy transfer through cross relaxation (CR) channels of (⁵D₃→⁵D₄)→(⁷F₆→⁷F_J). In the first mechanism, the MR rate increases with an increase in the phonon energy of materials. In the second one, the CR rate increases with the raise of Tb³⁺ concentration. The luminescence enhancement from ⁵D₄ level may result from the increase of the MR and CR rates from ⁵D₃ level to ⁵D₄ [[12], [13], [14], [15]]. So far there have been many investigations on optical properties of Tb³⁺ in materials such as single crystal, glass and polycrystalline [[12], [13], [14], [15], [16], [17]]. The influence of the Tb³⁺ concentration on its emission characteristic was presented in the literature [12,15,16]. The studies showed that the rate of (⁵D₃→⁵D₄)→(⁷F₆→⁷F_J) CR channels increases with the increase of Tb³⁺ concentration and this leads to the enhancement of luminescence bands originating from ⁵D₄ level. The dependence of Tb³⁺ fluorescence on glass compositions was also reported in literature [14,16,17]. In these studies, authors have shown that the luminescence features such as colour tone and intensity can be adjusted by changing the ratio of host compositions.

Unlike the mentioned studies, in this work, we would like to focus on the role of the modifier ion radius in luminescence enhancement from ⁵D₄ level of Tb³⁺ ion doped alkali-alumino-telluroborate glasses. This enhancement is the result of the increase of nonradiative rate from ⁵D₃ level to ⁵D₄. It should be noted that Wada (2007) carried out the similar research but for Eu³⁺ ion doped borate, silicate and phosphate glasses [18]. However, to the best of our knowledge, there have been no published reports on this topic for Tb³⁺ ions.

In prepared glasses, the observed emission spectra (see Fig. 4) indicates that the probability of nonradiative process is much higher than that of radiative emission process, since the integrated intensity of the emission bands from ⁵D₄ level is significantly greater than that of the emission bands from ⁵D₄ level. Through the evaluation of the CR and MP rates, the dominant role of the ⁵D₃→⁵D₄ multiphonon relaxation process in luminescence enhancement of ⁵D₄ level has been analyzed. In addition, the influence of the modifier ion radius on the optical properties of Tb³⁺ ions as well as the ligand field features have been also discussed in detail by using the Judd-Ofelt theory [19,20].