Optical and Morphological Studies of Alkali/Alkaline Earth Metal Co-doped Gd2O3:Eu Phosphors

Abstract

Gd 2 O 3 :Eu is a promising red light-emitting phosphor and holds tremendous potential applications in various optoelectronic and biomedical devices. Gd 2 O 3 is an ideal host matrix in case of Eu 3+ dopant. This is because the ionic radii of Eu 3+ ions (0.947 Å) is close to the Gd 3+ ions (0.938 Å) and can be easily incorporated into the Gd 2 O 3 matrix. As compared to Y 2 O 3 :Eu, Gd 2 O 3 :Eu has higher strong charge transfer band and luminescent intensity. The electronegativity of the Gd 3+ ions (1.20) is smaller than the Y 3+ ions (1.22), which allows easier charge transfer from the 2p orbitals of oxygen to the 4f orbitals of the europium ions. The present research work is focused on the synthesis of Gd 2 O 3 :Eu phosphors via co-precipitation route and also to study the different parameters affecting its luminescent properties. The effect of synthesis parameters, doping concentration, role of alkali and alkaline-earth metal co-doping and use of different surfactants/templates on the structural, morphological, optical and luminescent properties of Gd 2 O 3 :Eu phosphors are studied in detail. The entire thesis work is divided into eight chapters. The state-of-art of each chapter is outlined in the following sections. Chapter 1 describes the role of rare-earth (RE) oxide phosphors as potential host materials because of their outstanding properties such as large Stoke shifts, physical and chemical stability, long lifetimes and low phonon energy. In this chapter, various terms related to luminescence phenomena and mechanism involved are discussed. Further, luminescence in the rare-earth oxides and lanthanide ions are elucidated. At the end of the chapter, brief introduction of Gd 2 O 3 , its structures and applications in various fields are elaborated. Chapter 2 summarizes the literature survey done on Eu doped Gd 2 O 3 phosphors. This chapter discusses the state-of-art synthesis of Gd 2 O 3 :Eu phosphors and various factors affecting its luminescent characteristics. The synthesis of Gd 2 O 3 :Eu phosphors via different synthesis routes and their consequences on the size, morphology, and phases are discussed. The various parameters such as doping, co-doping,morphology, and phases affecting the luminescence features of the Gd 2 O 3 phosphors, are discussed in detail. After peer-review of the existing literature on Gd 2 O 3 :Eu phosphors, the objectives of the thesis work are proposed. Chapter 3 presents the experimental details for the synthesis of phosphors and their characterization to analyze their properties. Gd 2 O 3 phosphors with varying concentrations of Eu 3+ ions are synthesized via co- precipitation route, the details of which are discussed in this chapter. The synthesis procedures employed to synthesize the alkali and alkaline earth metals co-doped and surfactant/templates assisted Gd 2 O 3 :4mol%Eu phosphors have been given. The characterization techniques adopted such as X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), UV- Visible spectroscopy, and photoluminescence (PL) techniques for structural, morphological, optical, and luminescent studies, respectively are briefly discussed in this chapter. Chapter 4 gives an insight into the variation in the synthesis parameters that influence the different features of Gd 2 O 3 :Eu phosphors with alkali metal ions co-doping and their contribution in the luminescent enhancement. The concentration of the Eu 3+ ion in the Gd 2 O 3 host lattice is varied from 1 to 5 mol%. The maximum PL intensity is obtained for the Gd 2 O 3 sample with 4 mol% Eu 3+ ion concentration, synthesized at pH 10 and calcination temperature 800 ℃, respectively. Further, different concentrations of alkali metal ions are co- doped in Gd 2 O 3 :4mol%Eu sample to investigate their influence on the structural, morphological, and optical properties. The photometric parameters are analyzed by calculating CIE coordinates and color- coordinated temperature (CCT) values. The radiative and Judd Ofelt parameters (Ω 2 , Ω 4 ) of all the samples are calculated to investigate the luminescent properties further. Chapter 5 illustrates the effect of alkaline- earth metal (AEM =Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ ) co-doping on the structural, morphological, and optical properties of Gd 2 O 3 :Eu phosphors. The concentration of the Eu 3+ ions is fixed 4 mol% to avoid the chances of the concentration quenching. A varying concentrations of alkali metal ions areco-doped in Gd 2 O 3 :4mol%Eu host lattice. The photometric parameters, such as CIE coordinates and CCT values are calculated. In order to further probe the luminescent properties and the influence of the alkaline-earth metals on the local environment of the Eu 3+ ions in the host lattice, spectral parameters are calculated employing Judd Ofelt theory from emission spectra. Chapter 6 illustrates the role of surfactants to engineer the morphology of and further its effects on altering the luminescent features of the Gd 2 O 3 :4mol%Eu phosphors. Trioctylphosphine oxide (TOPO) and cetyltrimethylammonium bromide (CTAB) are used as surfactants in different molar ratios during synthesis. The concentration of the surfactants is varied as 1:0.5, 1:1 and 1:2 i.e., metal ion to surfactant ratio during synthesis. The effect of these surfactants on the structural, morphological, and luminescent properties of Gd 2 O 3 :4mol%Eu phosphors has been studied. The photometric parameters such as CIE and CCT values are calculated for all the samples. Chapter 7 describes the effect of morphology with the use of different templates and their effect on the optical and luminescent properties of the Gd 2 O 3 :4mol%Eu phosphors. For this purpose, three water soluble soft templates such as Thioglycerol (TG), Dextran, and Polyvinylpyrrolidone (PVP) are used during the synthesis of the phosphors. The amount of all the templates is varied from 1 to 2.5 wt% during synthesis. The effect of the templates on the structural, morphological, optical and luminescent characteristics of Gd 2 O 3 :4mol%Eu are studied in detail. The photometric parameters such as CIE coordinates and CCT values are calculated to check their applicability for practical applications. Chapter 8 summarizes the results obtained in Chapter 4, 5, 6 and 7. The effects of variation in synthesis parameters, co-doping of alkali and alkaline-earth metals and use of different surfactants/templates on the structural, morphological, optical and luminescent features of Gd 2 O 3 :Eu phosphors are discussed in this chapter. Based on the conclusions drawn from different findings in the present work, some suggestions to carry out future work in this field are given at the end of this chapter.