PREPARATION AND SPECTROSCOPIC INVESTIGATIONS OF RARE EARTH IONS DOPED TUNGSTATE-TELLURITE GLASSES FOR PHOTONIC APPLICATIONS

Abstract

All over the universe, artificial illumination has significant adverse effects on human civilization, biotic communities, and eco-friendly environments. Since the emergence of human civilization, scientists and researchers have worked to develop efficient illumination technologies that are beneficial for illumination applications in a daily life. The development of the light emitting diode was a crucial innovation in the 20th century. In the current years, bright illumination emitting sources, i.e., white light emitting diodes (w-LEDs) are considered to be the 4th generation solid-state lighting (SSL) devices because of their favorable characteristics, including higher reliability, low energy consumption, longer lifespan, eco-friendly and high luminous efficiency. At this time, w-LEDs available in the market consist of an inorganic phosphor and a n-UV/blue LED chip along with organic epoxy resin, which has negative features such as high correlated color temperature (CCT), low color rendering index (CRI) and low efficiency. Since the organic epoxy resin used to combine the inorganic phosphor has weak thermal stability thus suffering the lifespan and color quality of the w-LEDs. Hence, inorganic glasses and phosphor in glass (PiG) have the potential to alleviate these shortcomings of inorganic phosphor-based w-LEDs because they exhibit exceptional high thermal stability and can perform both as encapsulating materials and luminescence converters at the same time. They are considered to be superior to their crystalline equivalents and include exceptional applications such as optical fibre, optical data storage systems, optical thermometer sensors, lasers, w-LEDs, display and lighting devices, etc. Moreover, PiG has gained much attention because it has several advantages over single-crystal and transparent ceramics. The advantages are (i) luminescence can be controlled via mixing diverse phosphors and glass host compositions, (ii) simple method of preparation, and (iii)

xiv reproducible consistently. Thus, PiG has been deemed as an appropriate luminescence converter for high-power w-LEDs as a result of its robustness, excellent thermal stability, minimal thermal expansion coefficient, and high temperature as well as humidity resistance. Tellurite-based glasses have been considered the most admirable glass formers among the various glass-forming oxides owing to their numerous unique characteristics, including low melting temperature (≈ 850 ºC), wide transmission ranges from 0.4 to 5.0 µm, minimum phonon energy (≈ 800 cm-1 ), maximum refractive index (≈ 1.9-2.5) and better solubility of REIs. As tellurite glasses possess low phonon energy, the light output shall be enhanced by reducing the non-radiative transition (NRT) between the higher and lower energy states of REIs. The addition of WO3 in tellurite glasses can act as an excellent modifier helping to increase chemical stability, quantum efficiency and also to reduce non-radiative losses. Further, the introduction of alkali oxides such as Li2O, Na2CO3 and K2CO3 in the tellurite-based glasses helps to decrease the non radiative losses and enhances the strength of the host glass matrix. Moreover, the inclusion of heavy metal oxide such as bismuth trioxide (Bi2O3) serves as an unconventional network former in the glass host matrix. These characteristics of tungstate-tellurite glasses make them an appropriate choice for their applications in photonic devices. This thesis focuses on the optimization of rare earth doped glass system and a specific type of orthophosphate phosphor (Eu3+ -activated Ca3Bi(PO4)3 phosphor) in undoped tungstate-tellurite glasses, also known as development of PiG development. Chapter 1 starts with a brief introduction, the origin of the problem, the motivation of the research work, and an overview of the current literature. This chapter provides an introduction to different types of glass, the components involved in the glass formation, and their specific properties. Furthermore, the importance of the tungstate-tellurite glasses and phosphor in glass (PiG) have been discussed in detail. Thereafter, the chapter focuses on the principle of

xv photoluminescence, the energy transfer processes and the utility of rare earth-doped glass for illumination and display devices. The shortfalls and limitations of the existing white light technology and how to overcome those problems have been discussed. The motivation to carry out the present research work and the objectives of the thesis work are stated at the end of the chapter. Chapter 2 describes the experimental technique to synthesize and characterize the tungstate-tellurite glasses and phosphor in the optimized tungstate-tellurite glass (i.e., development of PiG). The chapter describes the melt-quenching technique in detail to obtain the desired glass. Then the chapter follows the different analytical techniques that have been used to analyze the as-prepared glasses for their suitable applications. The thermal, structural, vibrational, morphological, and photoluminescent properties of the as-prepared samples have been investigated by thermogravimetric analysis (TGA)-differential scanning calorimetry (DSC), X ray diffraction (XRD), Raman spectroscopy, Fourier Transform Infrared Spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM), Spectrofluorophotometer. A brief introduction to the above-mentioned characterization techniques has been discussed in this chapter. Chapter 3 explains the synthesis of transparent TeO2 – Li2O – WO3 – ZnO – Bi2O3 (tungstate-tellurite) glasses doped with Dy3+ ions via employing a conventional melt quenching procedure. A broad hump in the X-ray diffraction profile confirmed the non-crystalline or amorphous nature of the prepared glasses. The absorption spectrum exhibits several bands between the 400-1800 nm range, which confirms that the transitions initiate from the lowest energy state (6H15/2) to the numerous excited states. The photoluminescence (PL) spectral profiles reveal three significant peaks centred at 481 (blue), 575 (yellow), and 664 nm (red) related to the Dy3+ ions under n-UV (388 nm) excitation. Furthermore, the chromaticity coordinates of the

xvi prepared tungstate-tellurite glasses were situated in the white light region and nearest to the standard white light (0.33, 0.33). The decay profiles show the bi-exponential behaviour of the Dy3+ doped tungstate-tellurite (x = 0.1, 1.0, and 2.0 mol%) glasses. The energy transfer mechanism between the Dy3+ - Dy3+ ions has been determined to be a dipole-dipole in nature using the Inokuti Hirayama (I-H) model to the decay profiles of the prepared tungstate-tellurite glasses. Moreover, temperature dependent PL spectra demonstrate the appreciable thermal constancy of the prepared glasses having a high value of activation energy. The above results indicate that the Dy3+ doped tungstate-tellurite glasses are potential luminescent materials to utilize in solid state lighting applications, especially for white LEDs. [Part of this work has been published in the International Journal of Applied Glass Science 13 (2022) 645-654] (I.F.: 2.10) Chapter 4 presents the optimization and preparation of the alkali tungstate-tellurite glasses in molar composition: 49.0 TeO2 – 20.0 R2O – 15.0 WO3 – 10.0 ZnO – 5.0 Bi2O3 – 1.0 Eu2O3 (R = Li, Na and K) using the melt quenching process. The luminescent characteristics of the prepared glasses have been examined in detail to reveal the optimization of the alkali ions in the prepared glasses. It is evident from the luminescent studies that the potassium tungstate-tellurite glasses exhibit comparatively stronger luminescence than the emission for the other two alkali-based tungstate-tellurite glasses, indicating the better quality of the potassium tungstate-tellurite glasses to proceed for further studies. Based on the above analysis, potassium tungstate-tellurite glass matrices with molar composition (50.0 – x) TeO2 – 20.0 K2O – 15.0 WO3 – 10.0 ZnO – 5.0 Bi2O3 – x Eu2O3 (where x = 1.0, 3.0, 5.0 and 7.0 mol%) were synthesized to determine the optimal concentration of dopant (Eu2O3). Further, to estimate the aggregate weight loss, glass transition temperature (𝑇𝑔) and thermal stability factor (∆T) of the prepared host glass matrix, thermogravimetric analysis - differential scanning calorimetry (TGA – DSC) were utilized. Various functional groups were revealed via employing Fourier transform infrared (FT–IR)

xvii spectroscopy. The optical bandgap (𝐸𝑂𝑝𝑡) values for all the prepared Eu3+ doped potassium tungstate-tellurite glasses have been evaluated by employing the absorption spectra. Under n-UV and blue excitations, Eu3+ doped potassium tungstate-tellurite glasses are demonstrating reddish emission at 614 nm ascribed to the 5D0 → 7F2 transition, in which the emission intensity is increasing continuously with Eu3+ ions content up to 5.0 mol%. The experimental lifetime profiles demonstrate the single exponential nature of the prepared glasses under n-UV excitation. Furthermore, temperature dependent photoluminescence (TDPL) spectra indicate excellent thermal stability of the prepared glasses with a high value of activation energy (∆E). The prototype organic epoxy resin/binder-free device has been developed using the optimized glass matrix (i.e., 5.0 mol% Eu3+ doped potassium tungstate-tellurite glass) and n-UV LED chip. All the aforementioned findings validate that the optimized Eu3+ doped potassium tungstate-tellurite glass is an auspicious candidate for the red component to fabricate organic epoxy-free white LEDs. [Part of this work has been published in the Current Applied Physics 58 (2024) 11-20 (I.F.: 2.40)] Chapter 5 explains the energy transfer and luminescent properties of Sm3+ doped, (Dy3+/Sm3+) and (Dy3+/Eu3+) co-doped potassium tungstate-tellurite transparent glasses, prepared via melt quenching technique. The optical characteristics of the prepared glass matrices were examined with the aid of the absorption spectra. Sm3+ doped potassium tungstate-tellurite glass matrices reveal a strong intense emission peak at ~600 nm associated with the orange-red region band (4G5/2 → 6H7/2) transition. In Dy3+/Sm3+ co-doped glasses, the emission intensity progressively upsurges with an increment in Sm3+ ion content (up to 1.5 mol%) confirming the energy transfer between Dy3+ and Sm3+ ions. Likewise, Dy3+/Eu3+ dual-doped glasses demonstrate intense emission peaks in the blue, yellow, and red regions of the electromagnetic spectrum. Application of Dexter’s energy transfer formulation with Reisfeld’s approximation revealed that the energy transfer mechanism involves a non-radiative dipole-dipole (d-d) interaction between (Dy3+ and Sm3+) and (Dy3+ and Eu3+) ions. The average lifetime values for the 4F9/2 level of the

xviii Dy3+ ion in the above-mentioned glasses were measured and found that these values decreased with increasing activator (Sm3+ and Eu3+) ion contents. The color coordinates (x, y) and the correlated color temperature (CCT) values of the prepared glasses were estimated. Based on the chromaticity coordinates and CCT values, it has been found that the desired colour of the prepared glass samples can be modulated from warm white light to orange-red light and warm white to red region by precisely adjusting the concentration of Sm3+ and Eu3+ ions and at specified n-UV/blue excitation wavelengths, respectively. Furthermore, the emission intensity observed at 373 K was 83.83% for TWKZBi: Dy3+/Sm3+ glasses and 85.92% at 373 K for TWKZBi: Dy3+/Eu3+ glasses compared to the emission intensity at the initial temperature, indicating the excellent thermal stability of the prepared glasses. Hence, the above results confirm that the prepared (Dy3+/Sm3+) and (Dy3+/Eu3+) co-doped potassium tungstate-tellurite transparent glasses can be a promising candidate for white light and other photonic devices. [Part of this work has been published in the Journal of Luminescence 266 (2024) 120276 (I.F.: 3.30) and part of this work has been communicated to the Journal of Molecular Structure (I.F.: 4.00).] Chapter 6 presents the successful incorporation of Tb3+/Eu3+ into the transparent tungstate tellurite glass matrix prepared using the melt quenching approach. The existence of functional units corresponding to the different vibrations has been examined via Raman spectroscopy. Several emission peaks have been observed in the Tb3+ doped TWKZBi glasses under n-UV and blue excitations and the maximum luminescent intensity has been detected for 2.0 mol% of the Tb3+ doped TWKZBi glass sample. The color coordinates were obtained to be (0.303, 0.560) and (0.343, 0.646) under n-UV and blue excitations, which lies in the pure green region and suggests its utility as a green component for white LEDs and display devices. While Eu3+ doped TWKZBi glass revealed the strongest emission at 614 nm associated with the 5D0 → 7F2 transition. The CIE coordinates (0.650, 0.348) lie in the red region indicating its potential application for fabrication in red component LEDs and display devices. The emission spectra of the co-doped Tb3+ and Eu3+

xix ions in the TWKZBi glasses have been studied and the maximum energy transfer efficiency is found to be 32.82% under n-UV excitation. The energy transfer from sensitizer (Tb3+) to activator (Eu3+) ion happens through dipole-dipole interaction, as confirmed by Dexter’s and Resisfeld’s approximation. The emission colour of the prepared Tb3+/Eu3+ co-activated potassium tungstate tellurite glasses has been easily tunable from green to the reddish region in the CIE diagram by varying the Eu3+ ion concentration. Furthermore, the decay profiles for the 5D4 level of the Tb3+ ions diminish with varying the concentration of Eu3+ ions, confirming the energy transfer from Tb3+ to Eu3+ ions. Moreover, temperature dependent photoluminescence studies indicate that the Tb3+/Eu3+ co-activated potassium tungstate-tellurite glasses have good thermal stability. All the aforementioned results reveal the suitability of the Tb3+/Eu3+ co-activated potassium tungstate tellurite glasses for photonic applications. [Part of this work has been published in the Journal of Physics D: Applied Physics 57 (2024) 195301] (I.F.: 3.10) Chapter 7 focused on the development of red-emitting phosphor in glass (PiG) with different concentrations with the help of the melt quenching method via the incorporation of the Eu3+: CBP phosphor into the tungstate-tellurite glass. X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) analysis validate the successful inclusion of the Eu3+: CBP phosphor within the tungstate-tellurite glass. Rietveld refinement analysis of the PiG systems also confirms the crystal peaks indeed belong to the Eu3+: CBP phosphor. When excited with the n-UV and blue lights, the emission profiles of the Eu3+: CBP phosphor and PiG systems demonstrate the hypersensitive transition around 612 nm (5D0 → 7F2), which has been stronger than the other observed transitions. The emission intensity of the TWKZBi: CBPEu PiG system exhibits strong intensity in comparison to the Eu3+ doped CBP phosphor. CIE colour coordinates of the fabricated PiG systems under n-UV and blue excitations were situated in the red emission region and demonstrated excellent colour purity. Moreover, the thermal quenching behaviour was improved from 84.72 to 91.52% at 100 ⁰C in the temperature range from room temperature to

xx 200 ⁰C and activation energy increased from 0.1938 to 0.2698 eV due to the homogenous behavior and better network stability. Thus, the combination of the aforementioned findings ensures that the red-emitting PiG systems have the potential to be utilized in photonic applications. [Part of this work has been communicated to the Optical Materials] (I.F.: 3.80) Chapter 8 summarizes the relevant conclusions based on the results obtained in the previous chapters and outlines the future scope of the work.