STRUCTURAL AND PHOTOLUMINESCENCE PROPERTIES OF RARE EARTH IONS DOPED STRONTIUM TUNGSTEN YTTRIUM OXIDE PHOSPHOR FOR SOLID STATE LIGHTING APPLICATIONS

Abstract

Recent developments in solid-state lighting (SSL) technologies have revolutionized the lighting industry. SSL devices are compact, long-lasting, energy-efficient, and environmentally friendly. White light-emitting diodes (w-LEDs) built on SSL technology offer significant advantages over traditional light sources like incandescent bulbs, electric lamps, and fluorescent tubes. These benefits include longer operational lifespans, reduced energy consumption, enhanced color rendering, smaller sizes, and eco-friendliness. Phosphor- converted w-LEDs (pc-wLEDs) stand out due to their high luminous efficiency and lower power usage. Typical pc-wLEDs use blue chips combined with green and red phosphors or blue chips with yellow phosphors. However, pc-wLEDs that use blue LEDs and YAG:Ce3+ yellow phosphors face challenges such as low color rendering and color temperature instability. Tricolor (RGB) pc-wLEDs, which are excited by n-UV/UV LEDs, have lower efficiency in red phosphors compared to green and blue ones, and their degradation rates vary. This highlights the need for single-phase phosphors with tunable emissions to improve luminous efficiency, color rendering index (CRI), and correlated color temperature (CCT). Phosphors are utilized in both up-conversion and down-conversion processes to optimize light emissions. Doping lanthanide ions into host matrices has long been an effective method for achieving desired luminous properties. Lanthanides, or rare earth elements, are typically excited by UV or near-UV light and emit energy as visible light. For the luminescence to occur, the host material needs an activator ion. In some cases, the activator alone does not efficiently absorb and convert energy into visible light, so a sensitizer ion is used to absorb energy and transfer it to the activator, enhancing light emission. Host materials like vanadates and tungstates can self-excite under UV light and emit radiation in the near-UV or visible range, acting as host luminophores. For pc-LEDs, phosphors must absorb UV or near-UV light and emit visible light. Tungstates-based phosphors are particularly attractive due to their broad SHEETAL KUMARI vi bandgap and high refractive index, which improve emission intensity. Adjusting synthesis parameters and doping with 3d or 4f ions allows for control over the chromaticity of phosphors. This thesis focuses on the investigation of Strontium Tungsten Yttrium oxide (Sr9Y2W4O24) phosphor, which have not been widely studied in terms of their structural and luminescent properties when doped with rare-earth ions. The thesis is organized into eight chapters, each contributing to the research objectives while allowing for individual reading. Chapter 1 introduces the study by discussing the research background, the motivation for the work, and a summary of recent literature. It highlights advancements in white light generation technology, emphasizing its benefits and challenges. The chapter delves into theoretical frameworks for analyzing spectral data, ionic interactions, and energy transfer processes among rare-earth (RE) ions, using spectroscopy principles. Dexter theory is employed to understand the interaction mechanisms between donor and acceptor ions. Emission spectral data are analyzed to determine CIE coordinates, color purity, and correlated color temperature. Furthermore, the chapter examines photoluminescence (PL) decay curves to assess the lifetimes of activator and sensitizer ions. Chapter 2 outlines on the synthesis of Sr9Y2W4O24 phosphors doped with various rare-earth ions, including Eu3+, Sm3+, Ho3+, Yb3+, Er3+, and Dy3+, at different concentrations using solid state reaction synthesis approaches. It provides a detailed overview of the experimental methods used to investigate the structural, morphological, optical, and photoluminescence (PL) properties of the materials. The analysis includes structural, vibrational, and photo-luminescent features evaluated using techniques such as X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, Raman spectroscopy, diffuse reflectance spectroscopy (DRS), and PL spectroscopy. Chapter 3 focuses structural and photoluminescence properties of Sm3+ ions doped Strontium Yttrium Tungstate Phosphors for Reddish-orange Photonic device applications. In the present research work, Sm3+ ions doped Strontium Yttrium Tungstate (Sr9Y2W4O24) phosphors entitled to emit reddish-orange light have been synthesized using the conventional solid-state reaction method and analysed their structure and morphological behaviour using X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM). Different vibrational bands present in the titled phosphor have been examined using Fourier Transform Infrared (FT-IR) spectroscopy. The optical band gap values have been measured using diffuse reflectance SHEETAL KUMARI vii spectra (DRS). The photoluminescence (PL) spectral features recorded for the Sm3+ ions activated phosphor under 405 nm excitation reveal strong visible reddish-orange emission at 604 nm pertaining to the 4G5/2 → 6H7/2 transition of Sm3+ ions. From the PL spectra, the CIE chromaticity coordinates estimated for Sr9Y2W4O24:2.0mol% of Sm3+ phosphor are falling in deep visible reddish orange region. The PL decay spectral profiles recorded at 604 nm emission under 405 nm excitation are showing double exponential behavior and the experimental lifetimes are found to be decreasing with increase in Sm3+ ions concentration in the as synthesized phosphors. The thermal stability of the phosphors was revealed by temperature- dependent PL (TD-PL) analysis. All the investigations carried out finally allows us to contemplate the suitability of Sm3+ ions doped Sr9Y2W4O24 phosphor for reddish-orange photonic device applications. Chapter 4 contains investigations on photoluminescence and energy transfer studies of Sm3+ and Eu3+ ions doped Sr9Y2W4O24 red emitting phosphors with high color purity for w-LEDs. The Sr9Y2W4O24 phosphor is tetragonal in structure and belongs to the I41/a space group. The emission intensity pertaining to Eu3+ ions increases at the expense of decrease in intensity of Sm3+ ions with increase in the concentration of Eu3+ ion in the co-doped phosphors. The Reisfeld’s approximation applied to the emission spectra reveal quadrupole-quadrupole as the mechanism responsible for energy transfer (ET) from Sm3+ to Eu3+ ions. The optimised sample exhibited a remarkably larger QY of 51.2%. As the Eu3+ ions concentration increases, the τexp values of Sm3+ ions decline, indicating that energy is transferred from Sm3+ to Eu3+. With increase in Eu3+ ion concentration the CIE parameters computed are gradually shifting towards the deep red region of the visible spectrum. Furthermore, the as-synthesized phosphor has an activation energy of 0.306 eV, indicating reasonably substantial thermal stability. The results obtained in the present investigation allows us to explore the utility of Sm3+/Eu3+ co-doped Sr9Y2W4O24 phosphors as an effective deep red emitter needed to fabricate white LEDs. Chapter 5 describes green emission of erbium doped Sr9Y2W4O24 (SYW) phosphors for optical thermometry and solid-state lighting. Erbium-doped SYW phosphors were synthesised through a solid-state reaction process and assessed for use in optical thermometry and solid- state lighting. The structural analysis revealed a single-phase tetragonal configuration with an I41/a space group. Morphological examination showed irregular polyhedral structures, and UV- Vis diffuse reflectance measurements identified an optical bandgap of 4.25 eV. Upon excitation SHEETAL KUMARI viii at 380 nm, the phosphors emitted green light, with a dominant peak at 563 nm corresponding to the 4S3/2 →4I15/2, transition. TD-PL experiments demonstrated excellent thermal stability, with a calculated activation energy of 0.247 eV. The fluorescence intensity ratio (FIR) technique was employed to evaluate optical sensing capabilities, achieving a peak relative sensitivity of 1.33% K−1and an absolute sensitivity of 0.307% K−1. These findings highlights the potential of SYW:Er3+ phosphors for non-contact thermal sensing and advanced photonic technologies. Chapter 6 delves up-conversion luminescence in Sr9Y2W4O24:Ho3+,Yb3+ phosphors for applications in temperature sensing and w-LEDs. Optical bandgap values were determined using diffuse reflectance spectra, while photoluminescence properties were assessed through excitation and emission studies, including decay lifetime analysis. When excited at 980 nm, the phosphors displayed up-conversion luminescence with emission peaks at 488 nm, 551 nm, and 668 nm, corresponding to Ho3+ transitions. The emission mechanism was attributed to a two-photon absorption process, validated by analyzing the relationship between pump power and luminescence intensity. Temperature-dependent photoluminescence (TD-PL) and fluorescence intensity ratio (FIR) measurements confirmed the phosphors' thermal stability, highlighting their suitability for applications in optical temperature sensing and solid-state lighting. Chapter 7 deals with Structural characterization and luminescence characteristics of Dy3+ doped Sr9Y2W4O24 phosphor for application in White-LEDs. A series of Sr9Y2W4O24 (SYWO) phosphors doped with Dy3+ ions were developed and explored for their suitability in white light-emitting diodes (w-LEDs) and display technologies. XRD and FT-IR spectroscopy confirmed the formation of single-phase particles without impurities, with characteristic vibrational bands present in the host lattice. The optical band gaps were estimated using diffuse reflectance spectroscopy (DRS). Emission measurements for SYWO: xDy3+ (1.0–5.0 mol%) under excitations at 354, 367, and 391 nm showed prominent emissions in the blue and yellow regions. Chromaticity coordinates plotted on the CIE 1931 diagram confirmed white-light emission. The decay analysis revealed a reduction in Dy3+ ion lifetimes with increasing dopant concentrations. Furthermore, temperature-dependent photoluminescence (TD-PL) studies indicated strong thermal stability, with an activation energy of 0.231 eV. These results SHEETAL KUMARI ix underscore the potential of Dy3+ -doped SYWO phosphors for use in w-LEDs and advanced photonic devices. Chapter 8 indicates that the thesis conclusion with a summary, a brief recapitulation of the research presented in previous chapters and possible future approaches for extending work addressed. References also form part along with bibliography at the end of thesis.