SYNTHESIS OF PURE AND DOPED CuO NANOSTRUCTURES AND THEIR MULTIFUNCTIONAL PROPERTIES

Abstract

In recent years nanotechnology has been among one of the most exciting areas, which has revolutionized the scientific, medical and technological research having vast potential for usage in energy, environment, health and agriculture. Nanotechnology is not limited to the understanding of physical phenomena occurring at this scale but leads to creating improved materials, devices and systems exploiting their unique properties for the betterment of human life. At this scale the physicochemical properties of nanomaterials become size and shape dependent and have been largely attributed to the phenomena of quantum confinement and surface effect. It has been observed that with the reduction of the size of nanostructured materials to less than or equal to the de Broglie wavelength of the charge carriers, quantum mechanical phenomenon become pronounced altering the different properties. In 0-D nanostructures like quantum dots, the carriers are confined in all three dimensions while in 1-D and 2-D nanostructures like nanowires and thin film respectively, carriers can freely move in one and two dimensions, respectively. Moreover, it was observed that the longer dimension of 1-D nanostructures (nanowires, nanoneedles and nanorods) makes them suitable to connect with the macroscopic world for electrical, electronic, magnetic and many other physical measurements. Therefore, 1-D nanostructure is more appropriate not only for the fabrication of nanoelectronic devices like gas sensors, electron-field emitters and logic devices etc but also leads to exhibit excellent optical, electrical, magnetic and mechanical properties. Furthermore it was observed that the self assembly of these nanostructures into porous nanostructures influenced optical, electrical and magnetic properties due to high specific surface area of these nanostructures. In order to have a control over the different material properties and developing functional devices, it is very essential to synthesize nanostructures with high degree of regularity in size, shape, dimensionality and alignment. The synthesis of the nanoscale materials with precise control in size, shape and dimensionality still remains a challenge to the scientific community. However, a large number of approaches have been developed to synthesize the nanomaterials of different size and shape. These methods can be broadly classified into two categories: Top down approach (physical methods) and Bottom up approach (chemical methods). Among all nanoscale materials, metal oxides nanostructures have fascinated the scientific community extensively because of their tremendous applications and rich physics behind iv the different properties at this scale. The magnetic and optical phenomena have contributed to the wide ranging applications. As a result, the 3d transition metal oxides have been studied extensively for theoretical and experimental investigations in the recent years in these areas of research. Among these, CuO has aroused the great interest in oxide semiconductor physics because of its attractive physicochemical properties such as magnetic, optical, electrical and ferroelectric properties. Narrow direct band gap (1.2.eV in bulk) p-type semiconductor CuO have monoclinic structure with four CuO molecules in unit cell in which Cu atom is attached to four nearly coplanar O atoms and O atoms linked to Cu atoms at distorted tetrahedron. CuO is antiferromagnetic with Neel temperature from 213 to 230 K. It has been proposed in literature that in a highly symmetric divalent copper monoxide structure, the Jahn –Teller distortion introduces a strong electron phonon interaction which causes the high Tc superconductivity in layered cuprates. Due to its low symmetry, CuO has been found to exhibit ferroelectric properties. Various optical studies show CuO as a charge transfer insulator, but the presence of micro traces of Cu3+ introduces excess holes in CuO which makes it a semiconductor. CuO nanostructures possess superior and remarkably different physicochemical properties from their bulk counterparts due to large surface area, size and shape effects. These nanostructures have been greatly investigated in recent years due to their extensive applications like gas sensing, super capacitor/Li-ion batteries and waste water purification. The origin of ferromagnetism in pure and doped CuO nanostructures is interesting due to 3d electron induced strong coupling and its applications in spintronics and magneto-optics devices. Furthermore the coupling of magnetic order parameter with other parameter like ferroelectricity is also of fundamental importance and added one more feather in the applications of CuO nanostructures. Synthesis, properties and applications of a variety of CuO nanostructures has been published in several reviews. From the above, it is clear that it requires a systematic approach to synthesize CuO nanostructures with controllable morphology and to explore the effect of morphology/doping on the observed optical and magnetic properties. Hence, an attempt has been made in this thesis to synthesize low dimensional pure and doped CuO nanostructures with different morphologies without using any surfactant and investigate their optical, magnetic, ferroelectric and dielectric properties thoroughly. The effect of morphology and doping on these properties have been analyzed and discussed.