STRUCTURAL, MORPHOLOGICAL AND PHOTOLUMINESCENCE CHARACTERIZATION OF SOME RARE EARTH IONS DOPED LITHIUM BARIUM TUNGSTATE PHOSPHOR FOR PHOTONIC APPLICATIONS

Abstract

The shift from traditional light sources like incandescent and fluorescent bulbs to solid-state lighting (SSL) is driven by the superior performance of phosphor-converted white light emitting diodes (pc-w-LEDs), which offer extended lifetimes, high energy efficiency, fast response, and environmental benefits. pc-w-LEDs are widely used for indoor and outdoor applications, but conventional methods-such as blending red, green, and blue phosphors or using yellow phosphors on blue LEDs-face limitations, including poor color rendering index (CRI) and high correlated color temperature (CCT). These challenges can be addressed by developing single-phase phosphors doped with rare earth (RE) ions, enabling efficient white light emission via energy transfer processes, thus improving luminous efficiency, CCT, and CRI. Furthermore, phosphor materials are promising for non-contact thermometry applications due to their high selectivity, sensitivity, and precision in harsh environments. The present research focuses on synthesizing and characterizing novel, color-tunable Lithium Barium Tungstate (Li2Ba5W3O15) phosphors doped with RE ions. These materials exhibit excellent optical properties, stability, and enhanced luminescence when doped with alkali and alkaline earth metal ions, positioning them as advanced candidates for photonic devices and optical thermometry in next-generation SSL technologies. The present thesis spans nine chapters, encompassing morphology, structural, optical, and thermal characterizations to scale up for industrial applications. Chapter 1 presents an introduction to the research problem, motivation, and a comprehensive review of recent literature. It highlights the technological history, advantages, and challenges associated with white light generation. Theoretical models such as Dexter theory, Dexter and Reisfeld’s approximation, and the Inokuti-Hirayama (I-H) model are employed to analyze the observed spectra, ionic interactions, and energy transfer mechanisms between RE ions. The Photoluminescence (PL) emission spectra provide insights into CIE coordinates, color purity ABSTRACT vi (CP), and CCT. PL decay curves are analyzed to determine the lifetimes of activator/sensitizer ions, while temperature-dependent PL (TDPL) explores how luminescence behavior varies with temperature in optimized phosphor samples. Additionally, temperature sensing parameters such as fluorescence intensity ratio (FIR), relative sensitivity (Sr), and absolute sensitivity (Sa) are explained. Chapter 2 focuses on the synthesis of Lithium Barium Tungstate (Li2Ba5W3O15: LBW) phosphors doped with various RE3+ (Pr3+, Sm3+, Eu3+, Dy3+) ions at different concentrations using high temperature solid state reaction (SSR) method. The chapter details the experimental setup for analyzing the structural, morphological, and optical properties of the LBW phosphors. Key characterization methods include X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, diffuse reflectance spectroscopy (DRS), and PL studies. The influence of temperature variation on the doped phosphors' luminescent properties, along with surface morphology observations via field effect scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX), are discussed. Chapter 3 presents the structural and luminescence properties of Dy3+ -activated LBW phosphors, specifically for near-ultraviolet (n-UV) pumped w-LED and non-contact thermometry applications. Dy3+ -doped LBW phosphors were synthesized via high-temperature SSR, and their crystal structure and morphology were confirmed using XRD, Rietveld refinements, and FE-SEM with EDAX analysis. PL and XRD results validated the substitution of Ba2+ by Dy3+, with prominent emission peaks at 493 and 583 nm corresponding to the 4F9/2 →6H15/2 and 4F9/2 →6H13/2 transitions under 368 nm excitation. The optical thermometric performance of Dy³⁺-doped LBW phosphors was evaluated using thermal quenching in the 4F9/2 →6H15/2, 13/2 transitions over a temperature range of 293-443 K. These phosphors exhibit high sensitivity, with a maximum Sr of 0.7% K-1 at 293 K, Sa of 4.68 K-1 at 443 K, and a temperature resolution of ~0.51 K at 443 K, demonstrating excellent potential for non-contact ABSTRACT vii thermometry. [The content of this chapter has been published in an international journal, Sensors & Actuators: A. Physical, 372 (2024) 115336] (I.F.= 4.1) Chapter 4 presents the synthesis and analysis of Pr3+ -doped LBW phosphors, prepared using the conventional high-temperature SSR method. Structural and phase characterization was carried out via XRD, while FT-IR spectroscopy confirmed the presence of vibrational bands. The optical band gaps of both doped and undoped LBW samples were determined using DRS. Under 319 nm excitation, the Pr³⁺-activated LBW phosphor exhibited strong blue (488 nm) and red (647 nm) emissions, corresponding to the 3P0 → 3H4 and 3F2 transitions. The CIE color coordinates indicated that the emissions lie in the white region, and TDPL measurements demonstrated good thermal stability. The optical sensing properties of LBW:Pr3+ phosphors were assessed using FIR measurements, with a maximum Sr of 1.03% K-1 , showing promise for optical thermometry. The PL decay analysis at 647 nm exhibited a double exponential trend, with decreasing lifetime as Pr3+ concentration increased. Overall, Pr3+ -doped LBW phosphors show excellent potential for solid-state lighting and optical thermometry applications due to their emission characteristics, thermal stability, and optical sensing capabilities. [The content of this chapter has been published in an international journal, Optical Materials, 145 (2023) 114476] (I.F.= 3.8) Chapter 5 focuses on the synthesis of LBW phosphors doped with Sm3+ ions using a high temperature SSR. Comprehensive analyses of their structural, morphological, and photoluminescent properties were conducted. XRD confirmed the crystallinity and purity of the phosphors, with an average crystallite size of approximately 83 nm. SEM revealed irregular polyhedral shapes and mild clustering, with particle sizes in the micrometer range. FT-IR spectroscopy indicated diverse vibrational modes and molecular bonding within the host matrix. The optical bandgap (Eg) ranged from 3.74 to 3.63 eV, suggesting suitability for photonic applications. PL analysis showed three emission bands, with the strongest at 581 nm, ABSTRACT viii corresponding to the 4G5/2→ 6H5/2 transition upon 336 nm excitation. Concentration quenching was observed at a Sm3+ concentration of 2.0 mol%, elucidated by Dexter's dipole-dipole interaction theory. The optimized sample exhibited CIE coordinates of (0.586, 0.412), indicating a deep orange-red emission. PL decay analysis revealed a double exponential fit with decay lifetimes in the millisecond range, and thermal stability was confirmed through TDPL analysis showing high activation energy. These findings underscore the promising potential of Sm3+ -doped LBW phosphors for the development of advanced photonic devices, including w LEDs. [The content of this chapter has been published in an international journal, Journal of Fluorescence, 34 (2024) 2391–2403] (I.F.= 2.7) Chapter 6 presents the successful synthesis of Eu3+ -activated orange-red phosphors Li2Ba(5-x)(WO5)3:xEu3+ (1 ≤ x ≤ 9, ∆x = 2, mol%) via high-temperature SSR method. Phase identification and structural investigations were conducted through XRD and SEM, while the optical band gap was measured using DRS. PL emission spectra recorded under 322 nm and 396 nm excitation displayed strong orange-red emission at 595 nm, attributed to the 5D0→7F1 transition of Eu3+ ions, corroborated by the deep orange-red region in CIE chromaticity coordinates. PL decay profiles at λem = 595 nm and λex = 322 nm exhibited a consistent exponential pattern, revealing decreased lifetimes with increasing Eu3+ concentration. To assess the thermal response, PL measurements were performed over a temperature range of 298-448 K, showing a maximum Sr of approximately 1.21% K-1 at 298 K, indicating enhanced sensitivity at lower temperatures. These findings suggest that the synthesized LBW:Eu3+ phosphors are promising candidates for non-contact optical thermometry and w-LED applications. [The content of this chapter has been published in an international journal, Journal of Materials Science: Materials in Electronics, 34 (2023) 2191] (I.F.= 2.8) Chapter 7 delves into the characteristics of Dy3+/Eu3+ co-doped LBW phosphors, focusing on their structural, compositional, luminescent, and temperature-dependent properties. XRD ABSTRACT ix analysis, combined with the Williamson-Hall (W-H) plot, confirmed that Dy3+ and Eu3+ ions primarily occupy Ba2+ lattice sites in the LBW matrix. The addition of these activator ions enabled tunable luminescence at specific excitation wavelengths, allowing color shifts from natural white to orange-red with varying Eu3+ concentrations, achieving warm white light emission suitable for indoor applications. Increased Eu3+ concentrations resulted in shorter decay times due to increase in non-radiative transitions, with a maximum energy transfer efficiency of 68.74% observed for D3E1.25 phosphors. Notably, the two emission peaks exhibited distinct temperature-dependent behaviors. A FIR model revealed a maximum sensitivity of 0.97% K-1 for the D3E0.75 sample at 303 K. Overall, the findings suggest that Dy3+/Eu3+ co-doped LBW phosphors are promising candidates for UV-excitable warm white LEDs with color tunability and applications in non-contact optical thermometry. [The content of this chapter has been published in an international journal, Journal of Luminescence, 269 (2024) 120444] (I.F.= 3.3) Chapter 8 focuses on a series of LBW: RE3+ (RE = Dy, Sm) phosphors, synthesized using a high-temperature SSR method. XRD and SEM with EDAX confirmed that the crystal structure matched standard LBW, featuring small irregularly shaped particles. The DRS data was used to determine the band gaps. Fluorescence studies revealed that LBW doped with Dy3+ emitted blue light at 496 nm, while Sm3+ produced distinct emissions at 582 nm (green-yellow), 612 nm (yellow), and 669 nm (red) upon excitation with n-UV light (336 nm). Notably, energy transfer between Dy3+ and Sm3+ ions was observed, influencing the luminescence of the co doped phosphors. Variations in activator doping concentrations altered the emission color from neutral white light to deep orange red. Additionally, decay curves indicated a decrease in luminescence lifetime with increasing Sm3+ concentration. The thermal degradation characteristics of the D3S5 phosphor, analyzed under 336 nm excitation, demonstrated excellent luminescence thermal stability with an activation energy of 0.16 eV. The low CCT ABSTRACT x and favorable thermal stability suggest that the synthesized phosphors hold promise for use in warm white LED applications stimulated by UV chips. [The content of this chapter has been published in an international journal, Journal of Physics D: Applied Physics, 57 (2024) 315107] (I.F.= 3.1) Chapter 9 summarized the overall research effort presented in this thesis work as well as the specific conclusions drawn from the findings. This chapter also considers how the existing research may be improved upon and used going ahead to direct new avenues of inquiry.