INVESTIGATION OF CHARGE CARRIER DYNAMICS AND DIELECTRIC RELAXATION BEHAVIOUR IN METAL OXIDES

Abstract

Transition metal and rare earth metal oxides have gained significant attention in recent decades due to their prodigious characteristics, encompassing electronic, optical, and magnetic properties. These unique features of metal oxides have found wide-range of applications in fields of catalysis, water splitting, sensors, energy conversion and storage devices, as well as microelectronics. The designing of metal oxides based devices such as fuel cells, solar cells, batteries etc. by optimizing charge transfer properties and dielectric behavior is crucial in solving energy crisis for a sustainable future. Moreover, the investigation of charge carrier dynamics and dielectric properties carries immense potential in advancing the performance of the micro- and opto-electronic devices. The fundamental understanding of charge carrier dynamics, such as diffusion, hopping, and tunneling, facilitates researchers to find new strategies to enhance the performance of state of the art devices. Additionally, study of dielectric behavior enable researchers to develop high performance super capacitors, insulators, ceramics, gate-dielectrics, and more. This thesis aims to provide a comprehensive description of synthesis, structural properties, charge carrier dynamics, and dielectric behavior of transition metal oxides (TiO2, Fe2O3) and rare earth metal oxides (CeO2). The selected transition and rare earth metal oxides were chemically doped to improve their charge carrier dynamics and dielectric properties. Titanium dioxide (TiO2) is a chemically stable, photosensitive, and non-toxic metal oxide. The physical and chemical properties of TiO2 electrodes have been explored extensively for the last few decades. However, a detailed and comparative study of the dielectric relaxation and carrier dynamics with the dopants is still lacking. Electrochemical impedance and modulus spectroscopy were used to investigate the charge carrier conduction and dielectric relaxation behavior. The thesis confirms the decrement in crystallinity of TiO2 electrodes with dopants. The Cu doping in TiO2 reduces its bandgap to 3.06 eV from 3.28 eV. An improved charge carrier conduction at high frequencies (ω= 103 to 106 radians-s−1) with doping was achieved. The charge transfer resistance for the electrons is minimum for Cu-TiO2 compared to pristine and other TiO2 electrodes. The correlation analysis of impedance and modulus spectroscopy data provides a prodigious picture of the i charge carrier relaxation and conduction mechanism. Cerium Oxide (CeO2) is a promising material for applications like- energy storage, energy conversion, and FETs etc. due to its unique dielectric properties. The high electrical resistance ( ∼ 1 to 10 GΩ) and a wide bandgap (2.8 eV to 3.6 eV) of pristine CeO2 limits its applicability in photovoltaics and photo-electrocatalysis. To overcome this challenge, CeO2 is co-doped with transition metals (Fe2+/Fe3+ and Cu2+), which reduces its optical bandgap energy and electrical resistance. The temperature dependent dielectric relaxation and carrier dynamics in Fe, Cu co-doped CeO2 were investigated using Temperature Dependent-Electrochemical Impedance Spectroscopy (TD-EIS) and modulus spectroscopy. The temperature dependent impedance response of the sample shows Negative Temperature Coefficient of Resistance (NTCR) behaviour. The dielectric constant (ε′) of the material increases with temperature due to orientational dipole polarization. In contrast, it decreases rapidly with frequency and saturates in the high-frequency domain. The correlated study of the imaginary part of impedance (-Z") along with the imaginary part of modulus (M") shows the transition of carrier relaxation from ideal Debye type to non-Debye type with temperature. The small polaron hopping between the lattice sites gives rise to the conductivity in the sample, following the Nearest Neighbor Hopping (NNH) model. Our study explains carrier dynamics, dielectric behavior, and relaxation mechanisms in Fe, Cu-CeO2. The exceptional features of Hematite (α-Fe2O3), such as high dielectric constant, chemical stability, cost-effectiveness, and high natural abundance, make it favorable for electrode material among transition metal oxides. α-Fe2O3 displays immense potential in diverse applications, encompassing batteries/supercapacitors, gas sensors, photocatalysis, field-effect transistors, and magnetic resonance imaging. This study explores the structure and correlated barrier hopping of small polaron in α-Fe2O3 through TD-EIS. The structural study reveals the formation of the monoclinic phase of α-Fe2O3. The charge carrier dynamics and dielectric relaxation behavior mechanisms of α-Fe2O3 were investigated by impedance spectroscopy at various temperatures in the broad frequency range. Temperature-dependent conductivity spectrum suggests small polaron transport through a correlated barrier-hopping mechanism. The materials can also be modified via ion implantation; an indigenously developed low-energy tabletop Penning Ion Source shall soon be operational in our department. The thesis also includes development of one of the components of the ion source, namely the Wien Velocity Filter (WVF). The WVF separates out the undesired charged particles from the ion beam trajectory.