STRUCTURAL, MORPHOLOGICAL AND PHOTOLUMINESCENCE STUDIES OF CERTAIN DOPED TITANATE PHOSPHORS FOR PHOTONIC APPLICATIONS

Abstract

Recent advances in solid-state lighting (SSL) devices have made a substantial and practical contribution to the lighting industries. SSL devices are small, robust, and energy-efficient and also environmentally friendly. The SSL based white light emitting diodes (w-LEDs) are more superior when compared to traditional conventional light sources like incandescent lamps, electric bulbs, and fluorescent tubes because they last longer, require less energy, have excellent colour rendering index, compact, and are ecologically friendly. The phosphor converted w-LEDs (pc-wLEDs) are appealing because of their low power consumption, excellent luminous efficacy, and environmentally beneficial features. The most widely used and readily accessible pc-wLEDs are built using a mix of blue-chip, green, and red phosphors or blue-chip and yellow phosphor. However, pc-wLEDs made by means of blue-LED and YAG: Ce3+ yellow phosphors suffer with some weaknesses, such as poor color rendering index (CRI) and low stability of color temperature. In tricolor (RGB) phosphors, the excitation is achieved by n-UV/UV LEDs. Moreover, in tricolor pc-wLED the efficiency of red phosphors is low in comparison with green and blue phosphors and the rate of degradation of tricolor phosphors is different. Therefore, it is necessary to develop single phase phosphor with tunable emission containing white emission such that luminous efficiency, CRI and correlated color temperature (CCT) can be improved. Both the up-conversion and down-conversion are the fields in which phosphors are used. The most well-known method of obtaining the good luminous characteristics has traditionally been the doping of lanthanide ions/transition metal ions in an appropriate host matrix. Lanthanides are part of the f-block series of elements and are more commonly known as rare earths (RE). The typical behavior of lanthanide ions is to become excited by absorbing UV or near-UV light and then release the energy in the form of visible light. To trigger the luminescence phenomena in such xiii hosts, they need a luminescent ion (or activator). Even in some cases, the activator may occasionally fall short of effectively absorbing the energy and transforming it into visible light emission. In such cases, a sensitizer ion plays a supporting role by effectively absorbing energy and transferring part or all of it to the activator, significantly improving the light emission capacity of activator. Some hosts such as vanadates and tungstates have the ability to excite themselves by UV light and emit radiation in the near-UV or visible range. Such hosts are called as host luminophores. The primary need for a phosphor to be used in a pc-LED is that they must have a substantial absorption in the UV or near-UV area and must have an effective emission in the visible light zone. Titanate-based phosphors have gathered the most attention among all of these substances because of the broad bandgap and high refractive index of the titanate group, which improve emission intensity. By changing some synthesis parameters and doping them with a 3d or 4f ions, titanate-based systems' chromaticity can be managed. This thesis comprises the work done on Barium Molybdenum Titanate (Ba3MoTiO8) (now onwards abbreviated as BMT) phosphor lattice as the structural and luminescent properties of this ternary compound doped with RE ions has not been extensively studied till now. Seven chapters make up this thesis' structure in order to meet the objectives of my research. Each chapter is structured such that it may be read alone. Chapter 1 includes an explanation of the introduction, the origin of the problem, the motivation for the study, and an overview of recent literature. This chapter focuses on the white light generation's brief technological history, benefits, and related difficulties. Theoretical models for analyzing observed spectrum data, ionic interaction, and energy transfer between RE ions are used in conjunction with the spectroscopy of RE ions. Dexter theory was used to know the type of interaction between the acceptor/donor ions. Evaluation of the CIE coordinates, color purity, and corresponding color temperature are made possible by the emission spectral data. In addition to xiv this, analysis of PL decay curves was also discussed to know the lifetime of these activator/ sensitizer ions. Chapter 2 describes the synthesis of BMT phosphors doped with certain RE ions/post transition metal ions (Eu3+, Sm3+, Ho3+, and Yb3+, Bi3+) at variable concentrations using different techniques. Additionally, it describes the experimental apparatus used to examine the structural, morphological, optical and photoluminescence (PL) characteristics of BMT phosphors. Discussion has also been extended to the structural, vibrational and PL characteristics discovered by X-ray diffractometer (XRD), Fourier transform infrared (FT-IR) spectroscopy, Raman spectroscopy, Diffuse reflectance spectroscopy (DRS) and PL Spectroscopy. Chapter 3 explains synthesis optimization and efficiency enhancement in Eu3+ doped BMT phosphors for w-LED applications. Ba3MoTiO8 phosphors activated with Eu3+ ions have been synthesized successfully by three different routes such as solid-state reaction method (SSM), combustion method (CMM), and sol-gel combustion method (SGM) to optimize relatively best synthesis technique and temperature of synthesis. XRD patterns have been used to confirm the phase and crystallinity of the as-prepared phosphor. SEM and Raman spectra have been used to study the morphological behavior and different vibrational bands present in the structure of the titled phosphor respectively. DRS have been recorded to find out the optical band gap of phosphors. PL spectral features recorded under 393 & 465 nm excitation wavelengths reveal an intense red emission at 612 nm corresponding to 5D0→7F2 transition of the doped Eu3+ ions. CMM synthesized sample has shown maximum PL intensity out of the three synthesis methods and further studies have been carried out by preparing a series of europium ions doped BMT phosphors using the CMM technique. The PL decay curves recorded for 612 nm red emission under 393 nm excitation show non-linear double exponential nature with a decrease in an experimental lifetime as Eu3+ ions xv concentration increases in the as-prepared phosphors. All the morphological and PL studies carried out on the Eu3+ ions doped BMT phosphor emphasize the suitability of the as-prepared phosphor for a red component in w-LEDs fabrication. Chapter 4 contains linear and non-linear PL studies of Ho3+/Yb3+ co-doped titanate phosphors for photonic applications. This work presents the morphological and PL studies of the single Ho3+ ion doped and Ho3+/Yb3+ co-doped BMT: xHo3+/yYb3+ phosphors synthesized by conventional SSM reaction method. Phase confirmation of all the as-prepared samples was done by XRD patterns. The morphological behavior and vibrational frequency bands were studied by SEM and FT-IR techniques respectively. DRS has been recorded for the singly doped and co-doped samples to find out the optical bandgap. The linear and non-linear emission spectral studies under 448 & 980 nm excitation wavelengths respectively showed three bands in the green, red, and blue regions. A relatively more intense band observed in the green region under 980 nm excitation shows promising usage of the titled phosphor for non-linear applications. A plot drawn between non linear PL emission intensity vs pump power reveals the information pertaining to the number of photons involved in the process. In addition, CIE coordinates calculated from the emission spectral features lie in the green region. All the studies show the broad application of the as-prepared phosphors in linear and non-linear luminescence process-based green emitting diodes, display devices, security inks, phototherapy, and SSL applications. Chapter 5 describes energy transfer induced color tunable PL performance of thermally stable Sm3+/Eu3+ co-doped BMT phosphors for w-LED applications. The combustion process has been successfully used to synthesize BMT: Sm3+ and BMT: Sm3+/Eu3+ co-doped phosphors for w-LED applications. For the as-prepared phosphors, XRD, SEM, FT-IR, DRS, and PL spectroscopy techniques have been used to investigate phase confirmation, morphological analysis, and PL xvi properties. The PL emission spectra of singly doped Sm3+ and Sm3+/Eu3+ co-doped phosphors recorded under 409 nm wavelength show intense emission in reddish-orange and red regions, respectively. For Sm3+ -Eu3+ co-doped phosphors, the intensity of Sm3+ peaks show a decrease with an increase in Eu3+ ion concentration, demonstrating the energy transfer from Sm3+ to Eu3+ ions. Quadrupole-quadrupole interaction is the mechanism responsible for energy transfer from Sm3+ ions to Eu3+ ions according to Dexter’s hypothesis as applied to the PL spectra. This result is in consonance with the results of PL decay curves recorded under 409 nm excitation. The Commission Internationale de I’ Eclairage coordinates calculated for singly Sm3+ doped and Sm3+/Eu3+ co-doped phosphors show colour tunability from reddish-orange to pure red regions. All the results obtained finally revealed the utility of the titled phosphors as tunable red emitters needed to fabricate w-LEDs. Chapter 6 provides structural and luminescence studies on thermally stable Bi3+ activated BMT phosphors for near UV pumped w-LED applications. This research presents the outcomes of diverse morphological and PL studies performed on Bi3+ ions doped BMT phosphors prepared via conventional SSM method. Phase assessment was carried out via the XRD studies. SEM and EDAX analysis have been used to study the surface morphology and elemental composition. FT-IR was used to study the character and vibrational frequencies of bonds in the titled host lattice. Room-temperature PL was performed under 275 & 386 nm excitations displaying a broad band in the blue region corresponding to 3P1 → 1S0 transition of Bi3+ ions. The CIE coordinates and correlated colour temperature values show that the as-prepared phosphors emit in the blue vicinity. The PL decay curves under both excitation wavelengths show a single exponential behavior. Temperature-dependent PL studies confirmed the high thermal stability of the phosphor. All the xvii results obtained endorses the utility of the titled phosphors of this chapter as a blue-emitting component in w-LED and SSL applications. Chapter 7 summarizes the overall research results presented in this thesis work as well as the specific conclusions drawn from the findings. At the end of this chapter the future scope of the present investigations has been explained.