| dc.description.abstract | Luminescent materials, commonly referred to as 'phosphors,' have gathered significant scientific interest due to their diverse applications in areas such as lighting, solar cells, sensors, and biomedical imaging. Phosphors typically consist of a crystalline or amorphous host lattice, which incorporates dopant atoms to introduce additional energy levels do not present in the host lattice. This doping process enables the achievement of desired emission for particular purpose. Lanthanide (Ln3+) ions are commonly used as dopants in phosphors, due to their characteristic emission spectra, long luminescence decay times, efficient energy transfer, photo-stability, and environmentally friendly. Lanthanide ions, also known as rare-earth ions, are capable of both downconversion and upconversion emission processes through well-shielded f-f transitions. In certain cases, these ions can exhibit broad optical emission through d-f transitions, influenced by the host lattice's crystal field, thus offering a wide range of emission wavelengths. The incorporation of non-lanthanide can further enhance the luminescence intensity of the emission spectrum, by producing distortion in the crystal lattice, which breaks the local site symmetry and enhances the emission intensity. Various oxide lattice hosts, such as phosphates, borates, vanadates, tungstates, garnets, and molybdates, have been employed for phosphors. Among these, double molybdate ALn(MoO4)2 (where Ln is a trivalent rare earth (RE) ion and A is Li, Na, K, Rb and Cs) is quite interesting due to their outstanding properties like low phonon energy, easy and environment-friendly synthesis, good thermal and chemical stability, near-ultraviolet (nUV) absorption, and broadband blue emission.
This thesis consists of eight chapters, which is focuses on exploring multimodal light emission such as DC, UC and QC in rare earth ion activated NaLa(MoO4)2 phosphors and co-doped with monovalent (Li+), divalent (Ca2+) and trivalent (Bi3+) ions for applications in LED devices and plant growth, solar cells and optical thermometry. The thesis is organized as follows: Chapter 1. Discusses the fundamental principles of excitation and emission processes in rare-earth ions, the criteria for suitable host materials for effective luminescence, and provides an overview of the objectives, current research trends, and potential applications of these phosphors. Chapter 2. Details of the synthesis methods used for sample preparation and the instrumentation employed for characterizing the prepared samples, including basic concepts and schematic diagrams of the instruments used. Chapter 3. In this chapter we study a series of Dy3+-doped NaLa(MoO4)2 phosphors using the conventional solid-state method at 750 °C for 4 hours. The n-UV excitations were used to get luminescence through downconversion emission processes. The n-UV excitation has resulted into two characteristic emissions of Dy3+ ions: blue (4F9/2 → 6H15/2) at 487 nm and yellow (4F9/2 → 6H13/2) at 574 nm. The optimal concentration of Dy3+ ions was determined to be 3 mol%, beyond which quenching occurred due to multipolar interactions. Further enhancement of emission intensity was achieved by co-doping with monovalent (Li+), divalent (Ca2+), and trivalent (Bi3+) ions. Additionally, incorporating Eu3+ ions into the NaLa(MoO4)2:Dy3+ system enabled tuning of the emission color from white to red, attributed to an energy transfer from Dy3+ to Eu3+ ions. The intensity parameters (Ω2, Ω4) and radiative properties, including transition probabilities (AT), radiative lifetime (τrad), and branching ratios, were calculated using the Judd-Ofelt theory. The CIE color coordinates, and correlated color temperature (CCT) values indicated that these phosphors exhibit excellent white emission. The determined radiative properties, CIE coordinates, and CCT results revealed that Dy3+-activated NaLa(MoO4)2 phosphors are promising materials for developing white LEDs and optoelectronic devices. Chapter 4, This study examines the effects of co-doping with monovalent (Li+), divalent (Ca2+), and trivalent (Bi3+) ions on the luminescence and temperature-sensing properties of NaLa(MoO4)2:Eu3+ phosphors. All the phosphors were synthesized using a conventional solid-state method at 750°C for 4 hours, resulting in a tetragonal crystal structure confirmed by XRD and the Rietveld refinement method. The photoluminescence (PL) emission intensity increased with the Eu3+ content up to 11 mol%. Further, on co-doping with Li+, Ca2+, and Bi3+ enhanced the emission intensity by factors of 15.5, 1.4, and 1.6, respectively, in the NaLa(MoO4)2:Eu3+ (7 mol%) sample. The Judd-Ofelt theory was used to calculate intensity parameters and radiative properties. A high activation energy of 0.33 eV was achieved for the Li+ co-doped phosphor, indicating strong thermal stability, with 57% of luminescence intensity retained at 423 K. The phosphors were also assessed for temperature-sensing applications using the fluorescence intensity ratio method, achieving maximum relative sensitivities of 0.29% K-1 and 0.65% K-1 for specific peak ratios at 300 K. The phosphors exhibited red CIE color coordinates and correlated color temperatures ranging from 1621 K to 2314 K. These properties suggest the synthesized phosphors are promising multifunctional materials for red components in solid-state lighting, laser applications, and non-contact optical temperature sensors.
Chapter 5. In this study, we explored the non-thermal coupling energy levels of Tm3+, Ho3+, and Yb3+ ions in tri-doped NaLa(MoO4)2 (NLMO) phosphors. A comparative analysis of temperature-dependent upconversion (UC) properties was conducted for Tm3+/Yb3+, Ho3+/Yb3+, and Tm3+/Ho3+/Yb3+ doped NLMO phosphors. The Tm3+/Ho3+/Yb3+ tri-doped phosphor achieved a maximum relative sensitivity of approximately 1.63% K-1 at 308 K, with a temperature uncertainty range of 0.08 to 0.26 K. Repeatability tests over five cycles confirmed the phosphor's excellent thermal stability. The Tm3+/Ho3+ co-doped phosphors demonstrated efficient energy transfer, with around 99% transfer from Tm³⁺ to Ho³⁺ ions, facilitating color tunability from blue to green. This effective energy transfer was corroborated by decay analysis. Additionally, the excitation at 358 nm aligns well with commercially available near-UV chips. These findings suggest that Tm3+/Ho3+/Yb3+ doped NLMO phosphors are promising candidates for up-converting temperature sensors and down-converting color-tunable phosphors. Chapter 6. This study focuses on the development of dual-mode light-emitting phosphors for use as optical thermometers, which are capable of functioning in both cryogenic and high-temperature environments. Specifically, Er3+ doped and Yb3+/Er3+ co-doped NaLa(MoO4)2 phosphors were synthesized via a solid-state reaction method. The Judd-Ofelt theory was applied to determine intensity parameters, providing insights into the nature of the bonding and emission characteristics. Under 377 nm excitation, the phosphors exhibited intense green and weak red emissions due to Er3+ electronic transitions from 2H11/2/4S3/2→4I15/2 and 4F9/2 → 4I15/2, respectively, through stokes emission process. Additionally, anti-stokes emission was observed in Er3+ and Yb3+/Er3+ doped samples under 980 nm excitation. Furthermore, the dynamic studies of Er3+ and Yb3+/Er3+ doped samples were conducted to explore different emission modes. For practical applications as contactless optical thermometers, the temperature-dependent emission spectra were analyzed using the luminescence intensity ratio (LIR) technique. This involved evaluating both thermally and non-thermally coupled levels for temperature sensing in the range of 100 K to 543 K. The study demonstrates the potential of these phosphors in a wide temperature range for advanced technological applications. Chapter 7. This study investigates the multimodal light emission-including downshifting (DS), quantum cutting (QC), and upconversion (UC) of Pr3+/Yb3+ activated NaLa(MoO4)2 phosphors for multifunctional applications. The co-doped phosphors emit visible light under blue (449 nm) and near-infrared (NIR, 980 nm) excitations through DS and UC processes, respectively, which are attributed to various f-f transitions of Pr3+ ions. Additionally, under blue light excitation, the phosphors emit a NIR band ranging from 900 to 1050 nm due to Yb3+ f-f transitions, facilitated by energy transfer from a single Pr3+ ion to a pair of Yb3+ ions through the QC process. The study also explores concentration and thermal quenching mechanisms. The phosphors demonstrate potential for optical thermometry applications using the luminescence intensity ratio (LIR) technique, achieving a maximum relative sensitivity of 0.41% K-1 at 448 K. Moreover, a phosphor-coated LED (pc-LED) was constructed by coupling NaLa0.97Pr0.03(MoO4)2 with a blue LED chip (InGaN). The study further discusses the application of the phosphor's optical properties in enhancing photovoltaic performance for solar cells and indoor plant growth. Chapter 8. This section presents a summary and conclusions of the current study. Additionally, the future prospects of the work are briefly discussed. | en_US |
