Structural, Spectroscopic and Photophysical Properties of Rare-earth Activated CaXO4 (X = W, Mo) Phosphors

Abstract

Photonics is predominantly elucidated as the physical science and technology for realizing, controlling and manipulating the interaction between light and matter which furnishes technological base for solar-cell devices, lighting displays, medical science, solid state lighting (WLEDs), bio-imaging, biotechnology, etc. The energy efficiency needs to be an important criterion for all such applications. In this framework, lanthanide ions hold a special place in photonics owing to their unique photo-physical properties favouring the generation and amplification of light. To accomplish the above requirements this thesis work tries to study various photo-physical processes involved in a self-activated Scheelite family like down-conversion and up-conversion. Different approaches are made including incorporation of activators, sensitizers, charge compensators and surface treatment in order to enhance the luminescence properties. The detailed synthesis procedure and the characterization of the phosphors along with the results based on the structural, morphological, vibrational and photo-physical properties of synthesized phosphors are investigated and are systematically documented. Some possible field of applications are also explored and discussed in this work. The Scheelite family with chemical formula CaXO4(X = W and Mo) have emerged as the technologically interesting materials owing to their excellent thermal and chemical stability, interesting luminescence behaviour, self-activated nature, wide emission spectra in the visible region, low afterglow to luminescence, and attractive structural properties. In this context, a series of CaWO4:0.03Eu3+:xBi3+ (x = 0.02,0.05,0.07 and 0.10) nanophosphors are studied in detail. The phosphors are synthesized through the efficient low temperature ethylene glycol route. X-ray diffraction analysis and XPS survey proves that the Bi3+ and Eu3+ ions are perfectly incorporated into Ca2+ without disturbing the lattice. The nanophosphors show orange-red luminescence which is further tuned to red via incorporation of sensitizers Bi3+ ions. This color tuning of the nanoparticles is described in the frame work of energy transfer processes from WO4 2- group and sensitizer Bi3+ ions to the activator Eu3+ ions. The energy transfer efficiency and the quenching phenomenon are discussed in detail. The CIE diagram supports this color tenability which evident the application of the phosphor in solid state lighting applications. A series of un-doped and Li+ co-doped CaMoO4:Dy3+ nanoparticles are synthesized via. modified reflux method. The host shows broad dual band emission centered in blue and green regions endowed by charge transfer and defects. Upon Dy3+ doping, the host to dopant energy transfer (hdet) cause the CaMoO4:Dy3+ nanoparticles to exhibit characteristic emission lines from Dy3+. The CIE 1931 chromaticity coordinates for the prepared phosphors are found to lie in the nearly white region of color space. The light output as well as excited state lifetimes are successfully improved by lithium co-doping by virtue of reduced charge compensating defects and sensitization via. oxygen related defects. The positron annihilation studies show that all the lithium concentration co-doped for charge compensation is merely helping in removal of cations. An attempt is made to synthesize core nanoparticles (NPs) and core-shell particles using the reflux method. The structural and photo-physical properties of the developed phosphors have been investigated through various analytical techniques. The transmission electron microscopy (TEM) analysis evidences the formation of the shell on the core NPs. An 8-fold enhancement of emission intensity is observed in the core-shell particles as compared to that of core, due to the reduced surface quenchers after silica coating. The tunable red emission on formation of core-shell structures is confirmed from the CIE diagram. These results are ascribed to the formation of the chemical bonds between CaWO4@ CaWO4:0.03Eu:0.05Bi (core) and amorphous SiO2 shell via W – O – Si bridges. Better hydrophilicity developing from active functional groups in solutions and intense luminescence behavior with a quantum efficiency of 91% allow the developed phosphors for various potential applications such as solid state lighting, bio-labelling agent for the visualization of latent fingerprints (LFPs) and anti-counterfeiting, etc. Upconversion emission in CaWO4:Er3+/Yb3+/Mn2+ phosphor synthesized via ethylene glycol route has been analyzed on excitation under 980 nm diode laser. The structural information and morphology have been widely studied through different experimental techniques. The Yb-Mn dimer is established to explicate the energy transfer mechanism which brings about two-fold enhancement in the green emission. The optical thermometric performance of the developed phosphor based on the thermally coupled green levels of Er3+ (2H11/2 and 4S3/2) ions in the range of 303-623 K has been explored using the fluorescence intensity ratio (FIR) technique. The maximum sensitivity of 1.04 % K-1 at 303 K is obtained for the optimized phosphor with a thermal resolution of 0.4 K. Additionally, the internal heating characteristics of the said phosphor developed by the variation in excitation laser power is also studied which is found to raise from 265 to 563 K. Under 980 nm excitation the prepared phosphors exhibit strong green emission with a color purity of 98%. The energy transfer mechanism, population redistribution ability and thermal stability have been discussed in detail and the possible field of applications have been explored.