STRUCTURAL AND LUMINESCENT PROPERTIES OF RARE EARTH ACTIVATED CALCIUM ALUMINOZINCATE PHOSPHOR FOR SOLID STATE LIGHTING APPLICATIONS

Abstract

Recently, the increasing requirement for energetically efficient, high resolution displays and lighting devices necessitate the development of such devices with enhanced brightness and good color purity. Solid state lighting (SSL) technology involves white Light Emitting Diodes (wLEDs) which are globally used for wide-spread applications in consumer electronics and general illumination. It offers many scientific and technological patronages such as environmental friendliness, lower operating temperature and effective energy utilization by saving 80% of energy consumption over conventional lighting devices. In the light of the aforementioned advantages, wLEDs are considered to be the next generation lighting technology. Generally, there are three different approaches which can be used for generating white light based on solid state lighting LEDs: (1) by mixing red, green, blue (RGB) LEDs, (2) by using an ultraviolet (UV) LED to stimulate RGB phosphors, and (3) by using a blue-emitting GaN LED chip that excites a yellow-emitting phosphor (YAG: Ce) embedded in an organic material; the combination of blue and yellow light makes a white-emitting LED (pc-wLEDs). However, pc-wLEDs made by means of blue-LED + YAG: Ce yellow phosphors suffer some weaknesses, such as poor color rendering index (CRI) and low stability of color temperature. Since the white light is generated by the combination of blue light emitted by an LED chip and yellow light emitted by YAG: Ce phosphors, deterioration of the chip or YAG: Ce phosphors would cause some significant color changes. The instability of color temperature also exists for RGB LEDs as the degradation of different color LEDs or variations of driving current which also complicate its fabrication. 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 ix with tunable emission containing white emission or new red phosphor such that luminous efficiency, CRI and correlated color temperature (CCT) can be improved. Phosphors have been considered as key and technologically important components of the functionality and success of many lighting and display systems over the past several years. Versatile physical properties of the phosphors facilitate their usage in many areas, both scientific research and practical applications. Inorganic compound incorporated with the rare earth (RE) elements is the most prevalent protocol to synthesize brilliant luminescence phosphor in visible region. Among different inorganic oxides, zincates have been long studied because of its high chemical stability, high brightness, high melting point, and long persistence time without radioactive radiations. Moreover, zincate-based compounds in suitable composition could be potential candidates for white light emitting phosphors. When excited with LED emission in UV or n-UV region, these novel materials have the capability to convert the ultraviolet emission of a UV-LED into visible emission. Therefore, the work done for the thesis is based on novel ternary calcium aluminozincate (Ca3Al4ZnO10,) as a host material for SSL application. By incorporating suitable RE ions, the present host can be tuned for different emissions in visible region of the electromagnetic spectrum for SSL applications. The present thesis embraces seven chapters to accomplish the research objectives. The brief summary of each chapter is as follows: Chapter1 emphasize on the brief history and technological advancements of white light generation, their advantages, related issues. Spectroscopy of RE ions and theoretical models adopted for the analysis of observed spectral data, ionic interaction and energy transfer between RE ions. The Judd-Ofelt (J-O) theory that elucidates radiative transition probability (AR), branching ratio (β) and radiative lifetime (τR). The emission spectral data enables to evaluate the CIE coordinates, color purity and correlated color temperature (CCT). x Chapter 2 describes the methods to prepare calcium aluminozincate (Ca3Al4ZnO10: CAZ) phosphors activated with different RE ions (Eu3+, Sm3+, and Dy3+) with varying concentrations. It also explains the experimental tools/equipment’s used to analyse structural, morphological and optical properties of the CAZ phosphors. The thermal, structural and vibrational properties determined from thermal gravimetric analysis (TGA), X-ray Diffractometer, Fourier transform infrared (FT-IR) spectroscopy, Diffuse reflectance absorbance (DRA) have also been discussed. The morphology of the as-prepared phosphors investigated from scanning electron microscopy and the photoluminescence properties of the CAZ phosphors doped with different RE ions were also discussed in this chapter. Chapter 3 explains single phase CAZ phosphor doped with Sm3+ ions with varying concentrations prepared at 1300°C by conventional solid-state reaction (SSR) method. The crystal structure and phase analysis of the as-prepared phosphor has been carried out by XRD studies. Morphology and functional groups present in the phosphor have been investigated thoroughly by using SEM and FT-IR spectral measurements respectively. Under 401nm excitation, the as-prepared phosphor exhibits intense visible orange emission at 601nm. The Sm3+ ions concentration is optimized to 1.0 mol% to achieve intense visible orange emission. The PL analysis reveals that the dipole-dipole interaction is primarily responsible for the concentration quenching observed beyond 1.0 mol% of Sm3+ ions. The PL decay study reveals bi-exponential behaviour of decay curves with an average lifetime of the order of microseconds. The CIE coordinates (x= 0.574 and y= 0.424) measured for the optimized phosphor are very close to the intense orange emission coordinates specified by Nichia Corporation developed Amber LED NSPAR 70BS (0.57, 0.42). The Spectroscopic, PL and PL decay studies suggest the potential use of Sm3+ doped CAZ phosphors for display and white light emitting devices. xi Chapter 4 describes about the Eu3+ ions doped CAZ phosphors without flux and blended with various fluxes (NaF, NaCl and Na2B4O7) synthesized by using conventional SSR method and characterized by employing XRD, SEM, DRA and Spectrofluorophotometer to study the structural, morphological and PL properties. The Judd-Ofelt (J-O) analysis has been carried out to get insights of optical properties. All the XRD peaks are matching well with the standard ICDD card confirms that all the prepared phosphors consist of single phase with orthorhombic structure. The band gap has been calculated from DRA spectra. The PL spectra recorded under near-UV/blue excitations demonstrate a very distinct and intense red emission from all the phosphors. The intensity of emission in these phosphors increases up to 2.0 mol% of Eu3+ ions concentration and beyond concentration quenching is observed. The PL studies exhibit significant enhancement in the red emission for Eu3+ doped CAZ phosphor with the incorporation of Na2B4O7. The emission intensity has been found to increase approximately 1.5 times using NaCl and 2.5 times for Na2B4O7 in Eu3+ doped CAZ phosphor as compared to the phosphor without any flux. The energy transfer mechanism has also been studied in detail. The CIE chromaticity coordinates measured from the PL spectra of the prepared samples lies in the red region of the visible spectrum. From the measured PL and CIE chromaticity co ordinates, it was found that Eu3+ doped CAZ phosphors with optimized concentration of Eu3+ ions as 2.0 mol% and optimized flux (Na2B4O7) could be used as red phosphor in the development of white LEDs. Chapter 5 described about the Eu3+ doped CAZ phosphors synthesized via solid-state reaction (SSR), combustion (CS) and Pechini sol-gel (SG) methods for better luminescent properties and optimization of the synthesis method. The XRD, SEM, PL and PL decay curve measurements have been recorded for the detailed investigation of the luminescence properties of the as-synthesized phosphor. The XRD peaks indicate the complete matching of the diffraction peaks of the as-synthesized sample with the standard data for Ca3Al4ZnO10. The xii morphology of the sample synthesized via SG method shows homogeneous distribution of agglomerated particles with smaller particle size than those obtained from CS and SSR methods. The detail investigation shows significant red emission enhancement in Eu3+ doped CAZ phosphors synthesized via SG method. The obtained results suggest that the Eu3+ doped CAZ phosphor synthesized via SG method could be a great choice as red emitter for SSL applications. Chapter 6 describes the preparation and characterization of a series of Dy3+ doped and Dy3+/Eu3+ co-doped CAZ phosphors using Pechini sol-gel method. The crystal structure was analysed by recording XRD patterns and further it was confirmed from Rietveld refinement using Fullprof suite software. The morphology of the as-prepared phosphors was studied by recording SEM images. The PL properties were analysed from excitation and emission spectra. The colorimetric properties were studied by evaluating CIE coordinates and CCT. The decay curves were recorded, and energy transfer mechanism was analysed using I-H model, Dexter theory and Reisfeld’s approximation. The obtained results indicate that the white light from warm to neutral to cool emission can be achieved and color tunability can also be achieved in Dy3+/Eu3+ co-activated CAZ phosphor by varying the excitation or activator concentration in the host lattice. Chapter 7 summarizes the results obtained from the content presented in chapters 3 to 6. The futuristic scope of the thesis work has also been elaborated with respect to the expected applications of these phosphors in versatile fields.