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The role of local chemical environments in the electron energy loss spectra of complex multiferroic oxides was studied using computational and experimental techniques. The evolution of the O K-edge across an interface between bismuth ferrite (BFO) and lanthanum strontium manganate (LSMO) was considered through spectral averaging over crystallographically equivalent positions to capture the periodicity of the local O environments. Computational techniques were used to investigate the contribution of individual atomic environments to the overall spectrum, and the role of doping and strain was considered. Chemical variation, even at the low level, was found to have a major impact on the spectral features, whereas strain only induced a small chemical shift to the edge onset energy. Through a combination of these methods, it was possible to explain experimentally observed effects such as spectral flattening near the interface as the combination of spectral responses from multiple local atomic environments., Peer Reviewed, Postprint (published version)

Resonant tunneling is a quantum-mechanical effect in which electron transport is controlled by the discrete energy levels within a quantum-well (QW) structure. A ferroelectric resonant tunneling diode (RTD) exploits the switchable electric polarization state of the QW barrier to tune the device resistance. Here, the discovery of robust room-temperature ferroelectric-modulated resonant tunneling and negative differential resistance (NDR) behaviors in all-perovskite-oxide BaTiO3/SrRuO3/BaTiO3 QW structures is reported. The resonant current amplitude and voltage are tunable by the switchable polarization of the BaTiO3 ferroelectric with the NDR ratio modulated by ≈3 orders of magnitude and an OFF/ON resistance ratio exceeding a factor of 2 × 104. The observed NDR effect is explained an energy bandgap between Ru-t2g and Ru-eg orbitals driven by electron–electron correlations, as follows from density functional theory calculations. This study paves the way for ferroelectric-based quantum-tunneling devices in future oxide electronics.

This thesis explores topological defects and topological defect transitions in epitaxial ultrathin ferroelectric heterostructures. Geometrical confinements in ultrathin films has enabled the realisation of several nontrivial topological polarisation arrangements in ferroelectrics, categorised as a range of topological defects such as bubble, meron, vortex, flux-closure domain, etc. These ferroelectric topological defects can be engineered by tuning depolarisation field, mechanical and electrical boundary conditions. Our model system is an ultrathin (001) oriented PbZrxTi1-xO3/ SrTiO3/ PbZrxTi1-xO3 heterostructure fabricated on La0.67Sr0.33MnO3 buffered SrTiO3 substrates. Several topological defects are realised under specific mechanical and electrical boundary conditions. Topological defect transitions are also achieved using different routes such as electric field, thickness variation, mechanical pressure and thin film milling.These topological defects have also gained immense technological interest on account of their emergent properties. This thesis further studies the functional properties in topological defects such as electrical conductivity in bubble domains. The motion of these bubble domains is also investigated.The results herein offer new insights on how to engineer topological defects and topological defect transitions in order to design multifunctional ferroelectric/multiferroic devices with enhanced operational speed, sensitivity and energy-efficiencies.

This thesis investigates the crystallography, domain morphology and electromechanical behaviour of epitaxial PbZrxTi(1-x)O3 (PZT) bilayer heterostructures deposited on silicon.As a first step, PZT bilayers are deposited on SrTiO3 crystalline substrates by pulsed laser deposition, and their high temperature crystallography is investigated by X-ray diffraction (XRD). The profile of rhombohedral and tetragonal PZT structures with temperature shows excellent bulk-like behaviour with clear crystallographic phase transformations and coefficients of thermal expansion that are comparable with literature. Comparisons to theoretical relaxed films reveal how strain within the system is enough to modify the thermal properties of the films including increasing the Curie temperature for extended operation temperatures in practical applications.Next, thin films of tetragonal PbZr0.3Ti0.7O3 (PZT-T) of varying thickness are deposited above a 40 nm rhombohedral PbZr0.54Ti0.46O3 (PZT-R) film, all grown on silicon (100) substrates. XRD is performed on the samples and compared to the crystallographic dataset for bilayers on SrTiO3 substrates. XRD and transmission electron microscopy provide evidence that the PZT-T layer has a modified, pseudo-tetragonal structure due to substrate induced strain but maintains its in-plane a-axis orientation. The structure is observed to pass through a ferroelectric-paraelectric phase transformation with an increased Curie temperature. The domain morphology, characterised by piezoresponse force microscopy, reveals that the unequal lattice parameters of the pseudo-tetragonal structure are insufficiently different to reorient the polarisation. Instead, the film is arranged as ferroelastic a1/a2 tetragonal nanodomains within a larger array of mosaic superdomains. This remains unchanged with reduced film thickness, except for a smaller periodicity of the a1/a2 twins. Interestingly, the domain pattern gives rise to a series of topological defects including vor

Scanning probe microscopy (SPM) is a powerful technique to investigate the surface properties at the nanoscale such as topography, ferroelectricity, piezoelectricity, and conductivity. In this thesis, these properties are studied in bulk WTe2, La-doped ZnO (ZnO:La) thin films, Highly Oriented Pyrolytic Graphite (HOPG) and Nb-doped SrTiO3.HOPG is firstly studied as the standard calibration material for scanning tunnelling microscopy (STM) and observed Moire patterns of twisted layers with a rotation angle of 5.4 degrees. To utilizing cross-sectional STM (XSTM), the fractured Nb-doped SrTiO3 (001) reveals different termination terraces of TiO2 and SrO on cleaved surfaces.Surface oxidation of WTe2 surfaces in different environments reveals non-uniform surface oxide dynamics, which upon saturation after several hours of atmospheric exposure yields a self-limiting approximately 2.5 nm-thick amorphous surface layer. Also, in a controlled environment involving a continuous flow of nitrogen, the surface oxide layer forms dendritic surface texture and is considerably impeded.WTe2 is a semimetal in which ferroelectricity was observed for the first time. SPM allows resolving spontaneously formed domains as well as so far un-detected stripe and ripple structures in bulk WTe2. The stripes are pushed or dragged by the electric field generated by the STM tip, and they can be bent or pinned by a non-uniform external field and defects on the surface. The ripples present merging and splitting of lines and, to some degree, are crystallographically oriented and have a concentration of point defects at the valleys of the ripples. Both defects and intrinsic strains of the material likely drive the formation of the ripple structure.Another material system investigated in this thesis is ZnO, which is highly promising for electronic and photonic devices. SPM study of spray pyrolysis grown ZnO:La thin films reveal that as-grown in-plane piezoelectric domains independent of topographic proper

Fast ionic conductors (superionic conductors) are materials that exhibit ionic conductivity several orders of magnitude larger than those of conventional materials at given temperature condition. Their prominent conductivity makes fast-ion conductors to be regarded as promising electrolytes in developing new-generation of energy conversion devices (for example, all-solid-state battery and solid oxide fuel cell). Fast-ion conductors distinguished by the different ion diffusing, as listed in the following: Na+ based: Na3.1Zr1.95Ca0.05Si2PO12 and Na3SbSe, Li+ based: Li9AlP4 and Li7La3Zr2O12 as well as O2- (oxide) based: Gd2TiO7 and Ba2In2O5.As regards oxygen-based superionic conductors, the role of oxygen vacancies concentration and diffusion is of great importance when it comes to practical applications. One of the fundamental aspects of oxygen-based superionic materials that is still not well understood is how electrical and mechanical fields affect their oxygen diffusion, a problem that is also relevant to practical energy conversion applications. These two types of external fields commonly are present in electrochemical devices, for instance: (1) electric fields inevitably present in a working electrochemical device (e.g., solid-state battery or solid oxide fuel cell) (2) the ubiquitous lattice mismatch, which leads to internal mechanical stresses, in electrode-electrolyte interfaces as well as between electrodes and their coating material. Previous studies have shown that mechanical fields may induce significant enhancement and reduction of oxygen diffusion.SrTiO3 is a typical perovskite oxide-based superionic conductor that has been employed as a substrate on which to grow epitaxial perovskite thin films, and as a key component in oxide heterostructures that exhibit fundamentally intriguing physical behaviour, for example, LaTiO3/SrTiO3 bilayers.The impact of the mechanical field on oxygen diffusion in SrTiO3 still remains controversial, as the available experime

Scanning probe microscopy (SPM) has emerged as an indispensable technique to image, characterize, manipulate, and create new functional properties of materials at the nanoscale. Concurrently, the tantalizing nanoscale properties of ferroelectric materials have aroused immense interest both scientifically and technologically. Here SPM techniques are used to probe and exploit the nanoscale ferroelectric functionalities in various material systems, and each chapter in this dissertation is allocated to one specific material.Polarization retention failure compromises the performance and life expectancy of non-volatile nanoelectronic devices and thus needs to be addressed. In a designer-defect-engineered epitaxial BiFeO3 system, an ultralong polarization retention with no degradation over one year has been discovered. SPM based dynamic switching measurements reveal a significantly increased activation field that provides sufficient energy for efficient domain wall pinning. Atomic resolution scanning transmission electron microscopy indicates that nanoscale defect pockets pervade the entire film thickness acting as domain wall pinning centers. The findings provide guidelines for improving polarization retention in ferroelectrics using defect engineering.Domain walls, as one type of topological defect, have given rise to the recent development of prototype domain wall nanoelectronics elements. Here, the nanoscale electrical conduction behaviour of single crystal 0.7Pb(Mg1/3Nb2/3)-0.3PbTiO3 (PMN-PT) with a (001) orientation is investigated. Electrical conductivity at domain walls in these crystals is observed using conductive AFM (c-AFM). The AC-conductivity response at such walls is mapped and local preferential charge relaxation along domain walls is investigated by Kelvin probe force microscopy revealing slow dynamics in the minute's range. These findings provide new insight into domain wall properties at a.c. frequencies for domain wall based nanoelectronics.Van der Waal