Optical and radiative properties of Sm3+ions activated alkali-bismuth-germanate glasses

doi:10.1016/j.jlumin.2019.116566

Journal of Luminescence

Volume 214, October 2019, 116566

https://doi.org/10.1016/j.jlumin.2019.116566 Get rights and content

Highlights

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Samarium (Sm³⁺)-doped bismuth-germanate (GeBiNaGdBaSm) glasses have been synthesized by conventional melt-quenching technique.
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Judd-Ofelt (JO) intensity parameters (Ω₂, Ω₄, Ω₆) have been evaluated from the absorption spectrum.
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Radiative properties have been obtained by utilizing the JO parameters and refractive index.
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CIE chromaticity coordinates have revealed a tunable emission from orange to red region.

Abstract

Trivalent samarium (Sm³⁺)-doped alkali-bismuth-germanate glasses of a composition (40-x) GeO₂ + 20 Bi₂O₃ + 20 Na₂O + 10 BaO + 10 Gd₂O₃ + x Sm₂O₃ (x = 0.05, 0.1, 0.5, 1.0, 1.5 and 2.5 mol%) (GeBiNaBaGdSm) were made from a typical melt-quenching procedure. An optical absorption, photoluminescence excitation, emission and decay curves of GeBiNaBaGdSm glasses were studied. For the ⁴G_5/2 → ⁶H_7/2 transition, an intense orange emission at 601 nm was observed when the Sm³⁺ ions pumped by 405 nm. A high value of 15.59 $\times$ 10⁻²² cm² stimulated emission cross-section and 20.59 $\times$ 10⁻²⁸ cm³ of optical gain bandwidth for the 1.5 mo% Sm₂O₃-doped GeBiNaBaGdSm15 glass were obtained. The experimental lifetime (τ_exp) was increased up to GeBiNaBaGdSm05 glass, thereafter diminished with further increase of Sm³⁺ ion concentration. Decay profiles of Sm³⁺ ions were fitted using I-H model. Energy transfer rate (W_ET) for GeBiNaBaGdSm10 glass is evaluated to be 554 s⁻¹. The average red to orange (R/O) intensity ratio of these glasses is found to be 0.123. Furthermore, CIE chromaticity coordinates demonstrate that the emission was observed in the orange region and then shifted to red region with the increase of Sm³⁺ ion concentration. All the studies revealed that these glasses could be useful for tunable color display applications.

Introduction

Germanate glasses are of great attention for design and development of luminescent materials owing to their high refractive index, low phonon energies, and sensitivity to UV photon irradiation. Usually, germanate glasses have found applications in nonlinear optical devices, optical waveguides, Bragg gratings and optical fiber telecommunications [[1], [2], [3]]. With a significant addition of alkali metals ions such as Li⁺ and Na⁺ to these glasses which acts as mixed glass formers (MGF). The MGF contained glass types of electrolytes are exceptional candidates for next generation solid-state electrolytes. Besides, a better thermal stability and stronger mechanical strength of these glasses because of their sturdy inter-ionic force between Ge⁴⁺ and O²⁻ ions in comparison with infrared (IR) transmitting glasses that include fluoride, tellurite and chalcogenide glasses [4].

Additionally, neutron scattering studies revealed that the GeO₂−P₂O₅ and K₂O−GeO₂−P₂O₅ glasses have been shown the existence of germanium atoms with a coordination number more than four. As a result, the structure of the GeO₂ has changed from tetrahedral to hexagonal. This higher coordinated germanium would cause a significant modifications in the structure of the binary alkali glasses, especially the alkali concentration less than 20 mol%, where nonlinear trends were noticed in the macroscopic properties that is said to be germinate anomaly [[5], [6], [7]]. The germanate glasses are superior in the optical fiber technology compared to silicate glasses due to their transparency in the mid infrared (mid-IR) region. But the development of these glasses was hindered by Rayleigh scattering losses [8]. These losses can be minimized with the addition of alkali metal ion modifiers in the germanate glass.

Among lanthanide (Ln³⁺) ions, Gd₂O₃ is a widespread material because of proficient energy transfer occurs to the Ln³⁺ ions from Gd³⁺ ions consequently high thermal neutron capture cross-section that increases the light yield of emission. Addition of alkali metal ions (Li⁺, Na⁺, K⁺) modifies, the Ge network leads to the formation of non-bridging (NBO) oxygens. The high Na⁺ ion concentration may decrease ion pair function due to the result of bridging oxygen (BO) to non-bridging where Na–NBO distances are smaller in Na–O–Na bonds compared to Na–O–Bi/Ge that attributed to strong coulomb attractive Ge–O interaction. The addition of Na⁺ also decreases defects in solids due to its smaller cation size [[9], [10], [11]].

Samarium (Sm³⁺) ion is a good choice among the Ln³⁺ ions as its lowest emitting energy state ⁴G_5/2 possesses higher quantum efficiency with different quenching channels and exhibits a very small amount of probability for non-radiative decay, which are worthy characteristics of laser applications. Sm³⁺ions activated glasses have realized in the promising applications as high density optical storage, color displays, solid state lasers (in visible region), photodynamic therapy (PDT) light sources and telecommunication (undersea) [12]. Sm³⁺ -doped glasses show a strong orange-red emission in the visible region. Sm³⁺ ions exhibit broad emission intensity bands in the NIR region due to ⁴G_5/2 → ⁶H_J transitions (where J = 5/2, 7/2, 9/2 and 11/2). A large energy gap (~7250 cm⁻¹) between meta stable state ⁴G_5/2 and lower ⁶F_11/2 state in Sm³⁺ ions would cause a reddish orange emission around 600 nm which is not stimulated dreadfully by the phonon energy of the glass matrix [[13], [14], [15]]. For the purpose of energy transfer studies, Sm³⁺ ion can be used as co-doped ion with other Ln³⁺ ions [[16], [17], [18], [19], [20], [21], [22]].

This study is aimed to synthesize the Sm³⁺-doped alkali bismuth-germanate (GeO₂ + Bi₂O₃ + Na₂O + BaO + Gd₂O₃, GeBiNaBaGd) glasses and to investigate their optical, photoluminescence and radiative properties with the variation of Sm³⁺ ion concentration. The radiative transition probabilities (A), branching ratio (β) and stimulated emission cross-sections (σ_SE) of GeBiNaBaGdSm15 glass were evaluated with the help of Judd-Ofelt (JO) theory. Inokuti and Hirayama (IH) model is employed to fit the non-exponential decay profiles of Sm³⁺ ions in these glasses, with an aim to know the multipolar interactions among Sm³⁺ ions and explore the energy transfer processes.

Section snippets

Glass fabrication procedure

A batch of 10 gm from high purity chemical composition of (40-x) GeO₂ + 20 Bi₂O₃ + 20 Na₂O + 10 BaO+ 10 Gd₂O₃+ x Sm₂O₃ (x = 0.05, 0.1, 0.5, 1.0, 1.5 and 2.5) (GeBiNaBaGdSm) glasses were prepared using conventional melt-quenching technique. These glasses were coded as GeBiNaBaGdSm005, GeBiNaBaGdSm01, GeBiNaBaGdSm05, GeBiNaBaGdSm10, GeBiNaBaGdSm15 and GeBiNaBaGdSm25. The stoichiometrical amounts of chemicals were mixed homogeneously by grinding all the high purity chemicals using agate mortar and

Optical (UV- visible- NIR) absorption spectrum

The ultraviolet (UV) – visible – near infrared (NIR) optical absorption spectrum of GeBiNaBaGdSm15 glass was recorded in the 350–2500 nm spectral region at room temperature and is shown in Fig. 1. Twelve optical absorption bands from visible and NIR were observed in the absorption spectrum owing to 4f⁵-4f⁵ transitions. Six absorption bands observed in the range of 400–500 nm are shown in Fig. 1(a), and the rest of the bands noticed in the NIR region in the range of 900–2200 nm for

Photoluminescence (PL) excitation spectra

The PL excitation spectra of the Sm³⁺-doped GeBiNaBaGdSm glasses were recorded by monitoring the emission at 601 nm in the range of 350–540 nm at room temperature and are shown in Fig. 2. The spectra consists of twelve excitation bands positioned at 361 nm, 374 nm, 389 nm, 401 nm, 414 nm, 420 nm, 437 nm, 461 nm, 471 nm, 489 nm, 500 nm and 525 nm are ascribed to the transitions from the ground state, ⁶H_5/2 to ⁴D_3/2, ⁶P_7/2, ⁴L_5/2, ⁶P_3/2, ⁶P_5/2, ⁴M_19/2, ⁴G_9/2, ⁴I_13/2, ⁴I_11/2 + ⁴I_9/2, ⁴M_15/2, ⁴G_7/2

PL spectra

PL spectra of GeBiNaBaGdSm glasses with variation of Sm³⁺ ion concentration were recorded upon excitation of 405 nm in the spectral range of 530–740 nm, as shown in Fig. 3. The spectra revealed four emission bands of yellow, orange, orange-redand red were positioned at 561 nm (17825 cm⁻¹), 597 nm (16750 cm⁻¹), 644 nm (15527 cm⁻¹) and 707 nm (14144 cm⁻¹), which are ascribed to ⁴G_5/2 → ⁶H_5/2, ⁶H_7/2, ⁶H_9/2 and ⁶H_11/2 transitions, respectively. Similar emissions were noticed in the reported

Photoluminescent decay curves of ⁴G_5/2 level of Sm³⁺ -doped GeBiNaBaGdSm glasses were recorded upon the excitation of 405 nm. The decay curve of GeBiNaBaGdSm005 glass reveal the exponential behaviour, as shown in Fig. 5 and is analysed by using a single exponential equation. The non-exponential behaviour was observed for the glasses from GeBiNaBaGdSm01 to GeBiNaBaGdSm25, which analysed by using a double exponential equation and Inokuti and Hirayama (I-H) model. The I-H model is used to fit the

CIE chromaticity diagram

CIE chromaticity diagram was used for evaluating the color representation of the visible emission of Sm³⁺ - doped GeBiNaBaGdSm glasses. The chromaticity coordinates are found to be (0.53, 0.39), (0.54, 0.38), (0.56, 0.36), (0.56, 0.35), (0.55, 0.36), and (0.62, 0.35) for the Sm³⁺ concentration of 0.05, 0.1, 0.5, 1.0, 1.5 and 2.5 mol%, respectively, by following the procedure has been described elsewhere [42]. The locations of all the coordinates were shown in Fig. 7. The emission from the CIE

Conclusion

Alkali bismuth germanate glasses doped with different Sm³⁺ ions concentration have been fabricated using melt quenching technique and studied their optical and spectroscopic properties. Nephelauxetic ratio and bonding parameters of the GeBiNaBaGdSm15 glass were determined and revealed the ionic bond nature of the Sm³⁺ ion – ligand. Judd-Ofelt (JO) parameters have been carried out to find the radiative properties for the excited states of Sm³⁺ ions in the GeBiNaBaGdSm15 glass. Upon 405 nm

Acknowledgements

The authors acknowledge MoU-DAE-BRNS Project (No. 2009/34/36/BRNS/3174), Department of Physics, Sri Venkateswara University, Tirupati, India for extending the PL experimental facility. We extend thanks to the Central Electro Chemical Research Institute (CECRI-CSIR), Karaikudi and Indian Institute of Technology Madras (IIT-M) for providing UV-VIS-NIR absorption and Ellipsometry studies, respectively.