Synthesis And Studies Of Perovskite Nanostructures

Abstract

The group of materials with ABO3 type perovskite structure are very important due to their attractive electrical and magnetic properties for technological applications and have been studied in the form of single crystals, bulk polycrystalline materials and thin films. Recently, efforts have been made to synthesize and understand the growth of ABO3 type perovskite nanostructures because of their distinctive physical properties and potential applications in the nanodevices. The primary aim of the present thesis is to synthesize the perovskites at nano-scale, with zero-dimension (0D), and one-dimension (1D) configurations. Basic work was carried in terms of synthesis – structure – composition correlation. Due to the small nature of the synthesized materials, few attempts were done to examine the physical properties, but to a limited extant. Efforts were also done to emphasize the structural behavior of nano perovskite in comparison with their bulk counterparts. Chapter 1 provides a brief introduction to perovskite materials and nanostructures, their technological applications and the fundamental physics involved. A brief review of the perovskite nanostructures both from fundamental science and technological point of view is provided. Finally the specific objectives of the current research are outlined. Chapter 2 deals with the experimental studies carried out in this thesis. It describes the methods used for the synthesis, experimental set up and the basic operation principles of various structural and physical characterizations such as X-ray diffraction (XRD), thermal analysis, Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), compositional analysis (EDX), focused ion beam (FIB), electrical and magnetic studies of the materials prepared. Chapter 3 describes the fabrication of porous anodic aluminum oxide (AAO) templates with different pore size, basic steps for synthesis of nanotubes and the possible growth mechanism of nanotubes in the AAO template. In chapter 4, we report the synthesis of ferroelectric Ba1-xSrxTiO3 (x = 0.0, 0.3) nanoparticles (diameter range: 20-40nm) and Ba1-xSrxTiO3 (x = 0.0, 0.4) nanotubes with diameter about 200nm by the sol-gel method. The Ba1-xSrxTiO3 nanostructures so obtained were characterized by number of techniques, including FE-SEM, XRD, DTA/TGA, FTIR spectroscopy, TEM, HRTEM as well as EDX and SAED. Formation of Y-junctions and multi-branches in Ba1-xSrxTiO3 nanotubes were also observed. The wall of the nanotubes were found to be made of randomly oriented nanoparticles which were confirmed from the HRTEM image. The average thickness of the wall of the nanotubes was found around 15(±5) nm and nanoparticles consisting the wall were found to be in the range of 5-10nm. Diffused phase transition (cubic to tetragonal), shifted to lower temperature side and leaky ferroelectric P–E loops were observed in Ba1-xSrxTiO3 (x = 0.0) ceramic prepared from nanoparticles. Curie temperature was observed at 120oC in the BT nanotube array as confirmed by the dielectric study. The P–E loops of as-prepared Ba1-xSrxTiO3 (x = 0.0) nanotube array were also measured and the hysteresis clearly demonstrates the room temperature ferroelectricity in the as prepared nanotubes, indicating these nanotube array is potential media as ferroelectric information storage. In chapter 5, we report the synthesis of single crystalline nanoparticles and polycrystalline nanotubes of Pb0.76Ca0.24TiO3 (PCT24) by sol-gel processing and characterized by various techniques. The crystallinity and phase purity of the PCT24 nanoparticles and nanotubes were confirmed by the XRD and SAED pattern. Compositional homogeneity and their crystalline structure confirms the formation of the tetragonal perovskite phase. The wall of the nanotubes was found to be made of nanoparticles which were confirmed from the HRTEM analysis. The average thickness of the wall of the nanotubes was found around 20nm and nanoparticles consisting the wall were found to be in the range of 5-8nm. Formation of some single crystalline PCT24 nanorods was also observed as confirmed by SAED and HRTEM analysis. Formations of Y-junctions and multi-branches in this complex functional oxide were observed. Dielectric measurements shows the diffuse phase transition and frequency dependence of Tm (temperature at which real part of dielectric constant shows maxima) suggesting the relaxor type behavior in the PCT24 ceramic prepared from nanoparticles. Polarization study was carried out on PCT24 nanotube array, which shows the ferroelectric nature at room temperature. Chapter 6 reports the synthesis and studies of PbZrO3 (PZ) nanoparticles and PbZr1-xTixO3 for x = 0.0, 0.48 and 1.0 nanotubes. PZ nanoparticles were prepared by a novel sol-gel method based on diol-based solution. Initially, PZ was crystallized with some intermediate m-Z and t-Z phases at 400-550oC and start transforming to orthorhombic at around 600oC and then finally transformed into pure orthorhombic PZ phase at about 700oC. XRD and TEM confirmed the nanocrystalline nature of PZ particles. Curie temperature in the PZ ceramic prepared from PZ nanoparticles was observed around at 205oC, which is lower as compared to the bulk (233oC). P–E hysteresis loops of PZ ceramic prepared from nanoparticles were measured at different applied voltages and single ferroelectric loops of leaky nature were observed rather than antiferroelectrics. The lead zirconate nanoparticles produced may have potential applications as materials used in microelectronics and microelectromechanical systems. PbZr1-xTixO3 for x = 0.0 (PZ), 0.48 (PZT48) and 1 (PT) nanotubes were fabricated by sol-gel method within the closely packed porous alumina templates and characterized by various techniques. The crystallinity of the PZ, PZT48 and PT nanotubes were confirmed via XRD and SAED studies. EDX analysis demonstrated that stoichiometry was formed. Formation of Y-junctions in this complex functional oxide was also observed. The wall of the nanotubes was found to be made up of randomly oriented nanoparticles, which were confirmed by the HRTEM studies and also by a typical SEM image. The average thickness of the wall of the nanotubes was found to be around 10-20nm and nanoparticles consisting the wall was found to be in the range of 3 – 8nm. The Curie temperature was observed at 220oC in the PZ nanotube array. For the first time, PLD has been employed for the synthesis of lead zirconate nanotubes using AAO template. Well-registered arrays of these nanotubes could function as three dimensional (3D) device elements in miniaturized ferroelectric random access memory (FRAM). In chapter 7, we report the synthesis of single crystalline 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 (PMN-PT) nanoparticles. PMN-PT nanoparticles were developed by a novel sol-gel method based on diol route. After partial calcination at 450oC/1h, PMN-PT powder morphology started transforming from pyrochlore to perovskite phase. It is interesting to note that this partially crystallized PMN-PT powder was unstable under electron beam and generated freestanding lead nanoparticles after absorbing energy from a focused electron beam. PMN-PT powder annealed at 700°C was fully transformed to perovskite phase and was stable under electron beam. XRD calculations and TEM imaging confirmed the nanocrystalline nature of PMN-PT particles. Magnetic measurements on PMN-PT nanoparticles prepared at 650 and 750oC show room temperature ferromagnetic hysteresis, whereas the bulk or the agglomerated particles show diamagnetic behavior. With an increase of annealing temperature or the particle size the magnetic moment decreases. PMN-PT nanotubes with diameter about 200nm were fabricated successfully by the sol-gel method based on diol route within the closely packed porous nanochannel alumina templates. Phase purity and crystalline perovskite phase formation of PMN-PT nanotubes were confirmed by the XRD and SAED pattern. EDX analysis demonstrated that stoichiometry was formed within accepted limit. The wall of the nanotubes was found to be made of nanoparticles which were confirmed from the HRTEM analysis. The average thickness of the wall of the nanotube was found around 20 nm and nanoparticles consisting the wall were found to be in the range of 10-20 nm. Since electroceramic materials are following a similar trend to miniaturization as conventional semiconductors, the synthesis of nanosized oxidic building blocks is moving into the focus of scientific and technological interest. Ferroelectrics are promising class of materials for the fabrication of electronic devices, as they are already an integral part of modern nanotechnological operations. Chapter 8 deals with the synthesis and properties of BiFeO3 (BFO) nanoparticles and nanotubes. Single crystalline BFO nanoparticles of different size and polycrystalline BFO nanotubes were prepared by sol-gel method. As prepared nanostructures were characterized by various techniques such as XRD, TGA-DTA, FTIR, scanning electron microscope (SEM), transmission electron microscope (TEM), selected-area electron diffraction (SAED), high resolution TEM and energy-dispersive X-ray spectroscopy (EDX). The crystallinity and phase purity of the BFO nanoparticles and nanotubes were confirmed by the XRD, SAED pattern and HRTEM analysis. Compositional homogeneity and their crystalline structure confirms the formation of the rhombohedrally distorted perovskite phase. EDX analysis demonstrated that stoichiometric BiFeO3 was formed within accepted limit. The HRTEM analysis confirmed that wall of the BFO nanotubes was made of nanoparticles, which were randomly oriented in the wall. The average thickness of the wall of the nanotubes was found to be around 15 nm and nanoparticles consisting the wall were found to be in the range of 3-6nm. Formation of Y-junctions in this complex functional oxide was observed. Magnetic measurements show clearly the enhancement of ferromagnetism in BFO nanotubes and ferroelectric loops were also observed in these nanotubes, that indicates the multiferroic nature of these nanotubes. BFO nanostructures at a large scale might be important for many applications such as memory elements in nanoscale devices in future. Chapter 9 reports the synthesis of a series of crystalline La1-xCaxMnO3 (x = 0, 0.3, 0.5, 0.7) nanoparticles with average diameter about 20 nm by an improved sol-gel method. The crystallinity and phase formation of as prepared nanoparticles was confirmed via XRD, SAED and HRTEM studies. EDX analysis demonstrated that desired stoichiometric was formed. Magnetic characterization reveals that the PM-FM transitions (Tc) occurs around at 205, 235, 235 and 230 K for x = 0, 0.3, 0.5, 0.7, respectively. The strong irreversibility between zero field cooling (ZFC) and field cooling (FC) magnetization curves, a cusplike peak in ZFC curve and unusual shape of M versus H loop at T = 5 K gives strong support for surface spin glass behavior. The highly stable charge ordering state in bulk manganites is suppressed, while the ferromagnetism is enhanced in these nanoparticles (x = 0.5 and 0.7). La0.7Ca0.3MnO3 were fabricated by sol-gel method within the closely packed porous alumina templates. The wall of the nanotubes was found to be made up of randomly oriented nanoparticles (8-12nm) as confirmed by HRTEM studies. The strong irreversibility between ZFC and FC magnetization curves as well as a cusplike peak in ZFC curve gives strong support for surface spin glass behavior. Magnetization value as obtained from M-H loop was about 28.5% of expected value, suggesting the existence of a magnetic dead layer, which avoids the propagation of exchange interaction between magnetic grains. The PM-FM transition was observed at 235 K. Chapter 10 gives the summary and conclusions of the present study and also discusses the possible future work that could after more insights into the understanding of the perovskite nanostructures. Highlight of the present work (i) Successful growth of nanostructures in both particles and tube forms, and study of their structure – composition correlations. (ii) Present work could optimize the necessary chemistry to successfully grow nanoparticles and nanotubes of various perovskite compositions. (iii) Successful studies of physical properties of nanoparticles and nanotubes, ofcourse, to a limited extent. However the properties observed in the present nanostructures have a strong indication of nonlinear phenomena similar to their bulk counterparts. (iv) It was reported in the literature, the observation of ferromagnetic behavior in several nonmagnetic compositions at nano-scale. Surprisingly, similar ferroelectric behavior was noticed even in our perovskite complex oxides such as relaxors (PMN-PT). A clear interaction of magnetic spin and an electric dipole was evident in these oxides such as relaxors and also multiferroics at nano-scale (~10-20 nm). (v) In ferromagnetic compositions such as LCMO, a very interesting spin-glass type behavior was observed.