Investigating Structure, Morphology and Photophysical Properties of Rare-earth Activated Fluorides

Abstract

The scientific community has shown a growing interest in relating to the rare-earth based luminescent materials owing to their versatility and potentiality to improve the lives of mankind. Photo-physics of luminescent materials plays an important role for their diversified practical applications viz. solid state lighting (pcLED), spectral converters in solar cells, optical thermometry, fluorescent based lamps, liquid crystal display, field emission display etc. In this endeavor, this thesis work focuses on the fluoride based down- and up-conversion phosphors as they possess low phonon energy, wide band gap and high chemical stability as compared to other host matrices. The primary goal is to design the fluoride based phosphors by the various sensitizer-activator approach as well as the incorporation of impurity ions and to establish their structural, morphological, spectroscopic and photo-physical properties which can evince their practical utility in variety of applications. The material science description of the synthesis techniques used to prepare different series of inorganic fluoride based phosphors and characterization of the phosphors along with the results based on the structural, morphological, vibrational and spectroscopic techniques are discussed in a systematic manner. The luminescence properties of synthesized phosphors and the underlying mechanisms are extensively studied in this work and are systematically documented. An effort has been made to prepare a series of Eu3+ (x) activated YF3 nanophosphors with a minimal concentration (0.05) of Gd3+ as sensitizer. The energy transfer mechanism from Gd3+ to Eu3+ in YF3 host matrix is explored by using the photoluminescence spectra at the indirect excitation of 272 nm. The intensity of the dominant emission peak at 591 nm corresponds to 5D0→7F1 transition increases with rising concentration of Eu3+ ion and quenching occurs above x = 0.05. Further, 94.78% of efficient energy transfer process between Gd3+ and Eu3+ is governed by dominant dipole-dipole transitions. Quantum efficiency, colorimetric parameters are also calculated and the results indicate the suitability of this red nanophosphor for solid state lighting application. In an attempt to obtain an intense green emission, a series of YF3:xTb3+ nanophosphors are sensitized with minimal Gd3+ doping. The structures, morphologies, and optical properties of the synthesized nanophosphors are analyzed in detail. The characteristic emissions of both the Gd3+ ( 6P7/2 → 8S7/2) and Tb3+ ( 5D4 → 7FJ) ions can be observed in the photoluminescence spectra at the 272 nm excitation of Gd3+ ions and the Gd3+ → Tb3+ energy transfer leading to 15-fold enhancement in the green emission of the trivalent terbium ion. The possible energy transfer mechanism from Gd3+ to Tb3+ is presented schematically and 88.92% energy transfer ix efficiency is achieved, which is dominated by electric dipole–dipole interactions. The calculated branching ratio (for 5D4 → 7F5), quantum efficiency of 89% and obtained colorimetric parameters suggest the applicability of the synthesized nanophosphors in ultraviolet excitable phosphors for white light-emitting diodes and solid-state green lasers. In a similar approach, a series of -Na(Y0.95-x,Gd0.05)F4:xEu3+ phosphors with minimal concentration of Gd3+ is developed using hydrothermal techniques. Characteristics photoluminescence emissions of Gd3+ ( 6P7/2 8S7/2) and Eu3+ ( 5D0  7Fj) are observed in the doped and co-doped phosphors by an indirect excitation at 272 nm (of Gd3+). Local environment of Eu3+ ions is analyzed using Judd-Ofelt model and the observed 2 > 4 values suggests a local asymmetricity around Eu3+ ions in NaYF4 host matrix. The emission peak intensity at 615 nm varies with Eu3+ concentrations and quenching occurs at higher doping level (x > 0.10). Moreover, the emission spectra and luminescence lifetime based on Inokuti-Hirayama model reveals an efficient energy transfer from Gd3+ to Eu3+ which is mainly mediated through dipole-dipole interaction. A detail schematic representation of the energy transfer process between activator ions is also documented. Comparatively a better emission color tunability and color purity (90.08%) of red emission is achieved in Gd3+ -Eu3+ doped phosphors for indirect excitation of Eu3+ ions at 272 nm. The effective tunability of the photo-physical properties of these UV excitable phosphors suggest its applicability in fabricating w-LED using UVchips and spectral converters for solar cell. Double impurities doping approach has been adopted to enhance the up-conversion luminescence in β-NaYF4:0.2Yb3+/0.02Ho3+ phosphors via simultaneous co-doping of xMg2+/ySc3+ ions. The correlation among structure, morphology and up-conversion properties of prepared phosphors has been well established. The effect of Mg2+/Sc3+ co-doping on the UC green emission ( 5F4, 5S2→5 I8) is studied and about 31-fold enhancement is observed for the 0.08Mg2+/0.12Sc3+ co-doped phosphor. The down-conversion properties of the phosphors is also discussed to showcase the dual mode luminescence. The temperature sensing performance is investigated using luminescence intensity ratio (LIR) technique for non thermally coupled levels (5F5/ 5F4, 5S2) of Ho3+ ions and maximum relative sensitivity of 0.258% K-1 at 374 K is obtained for the optimized phosphor. The phosphor is found to be sustainable at high temperature as the UC emission intensity retains about 65% and 36% at 423 K and 574 K. From chromaticity diagram, the maximum color purity of 91.37% is obtained for 0.08Mg2+/0.12Sc3+ doping concentration. The significant enhancement in UC emission, estimated thermal parameters and color purity of Mg2+/Sc3+ co-doped β NaYF4:0.2Yb3+/0.02Ho3+ UC phosphors suggest their applicability in fabricating temperature sensors and LEDs.