STUDIES ON RARE EARTH DOPED LUMINESCENT MATERIALS FOR APPLICATIONS IN GENERAL ILLUMINATION AND BIO-PHOTONICS

Abstract

Research in the field of luminescent materials doped with rare earth (RE) and transition metal (TM) ions has taken giant strides due to the rapid development of technologies such as solid-state lighting and other display technologies. RE ions doped glassy materials also find applications in lasers, optoelectronic devices, and civil-military applications such as infrared detectors, infrared fairings, nuclear imaging, and detection. Photo luminescent glass applies these distinctive properties to photonics, lighting, and photovoltaics by applying Downconversion light from UV to visible or near-infrared (NIR) light, and it is suitable for display devices, smart windows, lasers, and optical fibers & w-LEDs, among many other applications. RE doped glasses useful for optoelectronics devices have been fabricated and characterized by many researchers because of their high transparency, low production cost, ease of shaping, and relatively high thermal stability. Rare earth activated glasses, and nanophosphor are important groups of engineering materials that can be used in a wide range of multifunctional and industrial applications. The advantages of glasses and glass ceramics have attracted a lot of attention. Inorganic glasses, which belong to the unique family of amorphous solid-state materials, are usually thermally stable and form over a wide range of glass-former concentrations. Since Snitzer first demonstrated the laser action of Nd3+ ions in barium crown glass in 1961, a great deal of work has been done on rare-Earth-doped glasses for solid-state lighting (SSL) applications, broadband optical amplifiers, up-conversion luminescence temperature sensors, and near-IR STUDIES ON RARE EARTH DOPED LUMINESCENT MATERIALS FOR APPLICATIONS IN GENERAL ILLUMINATION AND BIO-PHOTONICS vii lasers. Additionally, optical fibers that emitted infrared light were made using a variety of precursor glasses. In the current work, we have created a good glassy system (using the melt quench method) called alkali zinc alumino borosilicate (AZABS) glass doped with varying concentrations of europium ions. We have characterized them spectroscopically to gain insight into their suitability for general illumination such as w-LEDs and other related SSL device applications, all against the backdrop of the various scientific patronages of chemical species like H3BO3, SiO2, Al2O3, ZnO, and Na2CO3. Also, the spectral properties of RE doped chlorides, oxides, fluorides and phosphate have been studied extensively to understand their suitability as potential luminescent applications. However, most of the systems are sensitive to moisture and thus are not quite suitable for bio labeling except the fluoride compounds with a formula such as AREF4 (A = alkali, RE = rare earth F = Fluoride) [38,39]. Among AREF4 host lattices, KREF4 (K stands for potassium) especially have attracted much more attention because of the high reflective index and low phonon energy that make them excellent host matrix for both DS as well as UC processes For the current thesis project, the rare earth-doped luminous material's industrial and multipurpose uses are chosen based on the previously described benefits. Enhancing the luminous qualities of rare earth ion-doped glasses and nanophosphors for use in biomedical and general illumination applications is the main goal of the research. Several chapters are meeting the research's objectives. Each chapter is meant to be read independently. Chapter 1: A clearer introduction, the reason for the problem, the motivation for the research, and a review of recent literature are all included in the first chapter. This chapter 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 glasses and nanophosphors, the components involved in their formation, and their specific viii properties. Furthermore, the importance of borosilicate glasses and nanophosphor are discussed in detail. Based on the characteristics required for glass host, 35B2O3.20SiO2.15Al2O3.15ZnO. -15Na2CO3 glass composition selected to synthesize, transparent, thermally mechanically stable glass with exceptional photonic properties, which can directly be applicable in general illumination. This approach has involved a thorough exploration of the properties of the many chemical components present in the host glass. The usefulness of RE ions doped in glasses for use in photonic devices has been studied further. Also, discussed the Eu doped KYF4 (Potassium Yttrium Fluoride) nanophosphors as promising host material for biomedical application. Chapter 2: The experimental process used for producing RE-doped glasses and nanophosphor and the methods for evaluating their luminous properties are the main topics of the second chapter. There is also a detailed discussion of the melt-quench process, which is used to create as-prepared glasses. This chapter describes the use of many sophisticated experimental techniques, such as X-ray diffraction (XRD), UV-VIS spectrophotometer, Temperature Dependent PL (TDPL) Spectroscopy, PL Decay Spectroscopy, SEM, and Up-conversion processes, to study various properties, including thermal, structural, photoluminescent, and colorimetric properties. Chapter 3: In this chapter, Dy3+ doped Alkali Zinc Alumino Borosilicate (AZABS) glasses have been prepared via melt quenching technique. A series of AZABS glasses of varying concentrations of dysprosium (Dy3+) (0.1 mol% -2.5 mol%) was prepared. It was found that under UV excitation, 0.5 mol% Dy3+ doped glass exhibited maximum luminescence intensity. Subsequent photoluminescence studies like emission/excitation spectra, temperature dependent photoluminescence and decay kinetics were also performed. Dexter theory was applied to study the energy transfer mechanism between the dopant ions in the glass matrix. Positive and ix encouraging results from all the photoluminescence studies for Dy3+ doped AZABS glasses confirm that these as-prepared glasses can be used as prospective materials in general illumination. [Part of this work has been published in Journal of Materials Science: Materials in Electronic, 33 (2022) 4782–4793 (Impact Factor = 2.8)] Chapter 4: Samarium (Sm3+) ions doped AZABS glasses were synthesized via quick melt quench technique. Various spectroscopic studies like optical absorption, photoluminescence (PL) emission, PL excitation, temperature-dependent PL and PL decay kinetics were performed on the as prepared glass system. Under 402 nm excitation, three sharp bands at wavelengths 563, 599 and 645 nm corresponding to transitions 4G5/2 → 6H5/2, 6H7/2 and 6H9/2 respectively can be seen in the PL emission spectra. The 0.25 mol% Sm3+ glass has the highest intensity for these emissions. The lanthanide interaction in the glass matrix is dipole-dipole in nature as was proven from Dexter’s analysis. The direct bandgap of 0.25 mol% Sm3+ doped AZABS glass was calculated to be 2.88 eV. The lifetimes of the as prepared glasses range from 1.93 ms for the lowest concentration of Sm3+ to 0.75 ms for the highest. From temperature dependent PL studies, the activation energy for 0.25 mol% Sm3+ doped AZABS glass was found to be 0.19 eV which shows high thermal stability of this glass. We propose to utilize these Sm3+ doped AZABS glasses for general illumination such as w-LEDs and solid-state lighting (SSL) applications. [Part of this work has been published in Luminescence 28 (2023), 428-436, (Impact Factor = 3.2)] Chapter 5: In this chapter we discuss the synthesis of thermally stable borosilicate glasses doped with europium ions having chemical composition 35B2O3.20SiO2.(15-x)Al2O3.15ZnO. 15Na2CO3.xEu2O3 (x = 0.5 to 2.5 mol%) using melt quench process. A broad hump without any sharp peaks observed in the XRD spectrum recorded for an undoped glass confirm its glassy x nature. The DSC & TGA has been conducted on an undoped glass to understand thermal stability and aggregate weight loss. The absorption spectral features recorded for the as prepared glasses are used to estimate optical band gap. In the process of understanding the effective usage of the as prepared glasses in visible red photonic device applications, the spectral features such as photoluminescence (PL) excitation, PL emission and PL decay were recorded and analyzed. Under 393 nm sharp excitation, all the glass samples are showing red emission corresponds to 5D0 → 7F2 transition (612 nm) and whose intensity continuously increasing with Eu3+ ion concentration up to 2.5 mol%. The red to orange color ratio (R/O) estimated from the recorded PL spectral features varies from 3.62 to 3.92 within the variation limits of Eu3+ ions from 0.5 to 2.5 mol% indicates relatively low symmetry around Eu3+ ions in the as prepared glasses. Relatively higher R/O ratio also reveals that the nature of bonding between Eu3+ ions and the surrounding ligands as covalent. The Judd-Ofelt theory has been applied to the emission spectral features to understand the nature of bonding between the doped RE ion and its surrounding ligands along with the radiative properties of the doped RE ion. Activation energy (0.175 eV) and percentage loss (82%) in PL intensity estimated for 2.5 mol% of Eu3+ ions through temperature dependent PL (TDPL) studies reveal the superiority in thermal stability of the as prepared glasses. The PL, TDPL, PL decay studies conducted along with CIE coordinates estimated allows us to contemplate that, the as prepared glasses are quite useful in fabricating thermally stable visible red photonic devices. [Part of this work has been communicated in Journal of Non-Crystalline Solids 575 (2022) 121184 (Impact Factor = 3.2)] Chapter 6: This chapter deals with the synthesis of cubic phase KYF4:Eu3+ nanophosphors via wet chemical route. Morphological studies such as XRD, SEM and EDAX mapping were done to ascertain shape, size and composition of the as prepared nanophosphors. Debye Scherrer formula applied to the XRD spectral features of the as prepared nanophosphors reveals the xi average size in the range 3 - 4 nm. The JCPDS data analysis for KYF4:Eu3+ nanophosphors confirm cubic structure with lattice constant a = b = c = 5.448Å and α = β = γ = 90°. The SEM image mapping clearly demonstrates the uniform distribution of all the constituent elements such as potassium, yttrium, fluorine and europium. Up-conversion (UC) studies carried out using 800 nm spitfire femtosecond laser produces peaks at 576, 590, 612, 650, 700 nm pertaining to the transitions 5D0 → 7Fj (J = 0, 1, 2, 3, 4) respectively. In addition to this, three higher order peaks are also observed at 523 531, 552 nm pertaining to 5D1 → 7Fj (J = 0, 1, 2) transitions respectively. Down-shifting (DS) studies under 393 nm and 405 nm excitation were also recorded to understand the utility of the as prepared phosphors for lighting applications. These nanophosphors are capable of emitting visible emissions under UV/NIR excitations. The powder dependence studies conducted on UC and DS reveal the excitation process as two photons and single photon respectively. DS temperature dependent PL spectral revels good thermal stability for the as prepared phosphor. The interesting results obtained allow us to contemplate that the as prepared KYF4:Eu3+ nanophosphors are useful for bio-imaging (through UC) as well as lighting applications (through DS). [Part of this work has been published in Journal of Alloys and Compounds, 885 (2021), 160893, (Impact Factor = 5.8)] Chapter 7: An overview of the overall study effort and the particular conclusions drawn from the data are presented in sixth chapter of this dissertation. This chapter also looks at future directions and social impact for this study and how it could be used to inform future research directions.