Aberration corrected STEM techniques to investigate polarization in ferroelectric domain walls and vortices

Ferroelectric domain wall (DW) based nano-electronics is an emerging new field of research. It is only recently with advancements in electron and atomic force microscopy instrumentation that the complex nature of these 2D entities can be probed. In this Research Update, the advances in aberration corrected scanning transmission electron microscopy applied to ferroelectric topological defects are summarized. We discuss sub-atomic imaging and diffraction techniques used to observe changes in polarization, chemical composition, charge density, and strain at DWs and vortices. We further highlight the current achievements in mapping the 3D nature of ferroelectric polar skyrmions and in situ biasing. This Review will focus on both the fundamental physics of DW and polar vortex formation and their dynamics. Finally, we discuss how electron spectroscopy can be used to relate the quantified structural distortions of polar topological entities to changes in their oxidation state and band structur

Rapid polarisation mapping in ferroelectrics using fourier masking

Summary Ferroelectric materials, and more specifically ferroelectric domain walls (DWs) have become an area of intense research in recent years. Novel physical phenomena have been discovered at these nanoscale topological polarization discontinuities by mapping out the polarization in each atomic unit cell around the DW in a scanning transmission electron microscope (STEM). However, identifying these features requires an understanding of the polarization in the overall domain structure of the TEM sample, which is often a time‐consuming process. Here, a fast method of polarization mapping in the TEM is presented, which can be applied to a range of ferroelectric materials. Due to the coupling of polarization to spontaneous strain, we can isolate different strain states and demonstrate the fast mapping of the domain structure in ferroelectric lead titanate (PTO). The method only requires a high‐resolution TEM or STEM image and is less sensitive to zone axis or local strain effects, which may affect other techniques. Thus, it is easily applicable to in‐situ experiments. The complimentary benefits of Fourier masking with more advanced mapping strategies and its application to other materials are discussed. These results imply that Fourier masked polarization mapping will be a useful tool for electron microscopists in streamlining their analysis of ferroelectric TEM samples. Lay Description This paper addresses a problem that often occurs when looking at a ferroelectric material in the Transmission Electron Microscope (TEM). Ferroelectric samples are interesting because they form tiny areas inside themselves with arrow of charge in each one. The thinner the sample, the smaller these regions, called “domains” become. These arrows of charge point in different directions in each domain of the sample. The boundary where these domains meet have interesting properties to study in a TEM but it's important to figure out which way the arrows point in the domains around the boundary. What causes the arrows in the different domains is tiny shifts of different atoms in unit cell away from their neutral position, usually because they're being squeezed by pressure from the domains nearby. The problem is that these tiny atoms moving are difficult to measure and see where the charged arrow is pointing, often it's hard to know how many different domains are even in the sample and where they begin. This paper discusses a method called “Fourier masking” to quickly see what's going on in the overall TEM sample, where the domains are and roughly where the arrows point. It does this by looking at the spacings of the atoms from a magnification where you can just about see the lines of atoms. In lead titanate the unit cell is a rectangle and the arrow always points in line with the long side of the rectangle. The Fourier masking lets you see which direction the long side of the rectangular unit cell is pointing in different parts of your TEM image. The big advantage is that it takes about two minutes to do and uses software that almost every TEM already has. That lets the TEM user quickly know where the domains are in their TEM samples and roughly which way the arrows of charge are pointing. Then they can choose the most interesting features focus on for higher resolution analysis

Charge carriers in dynamic ferroelectric domain walls

Ferroelectric domain walls (DWs) are the subject of intense research at present in the search for high dielectric, gigahertz responsive materials with novel functionalities[1]. Crucial to the integration of DWs into nanoelectronics is a proper understanding of the local electronic landscape around the wall and the influence this has on the behaviour of the DW under variable electric fields. A high degree of mobility under small electric fields is especially desirable for low power applications which escape from the critical current thresholds required to move magnetic domain walls[2]. Perovskite oxides are prime candidates for tuning the thermodynamic variables affecting the energy landscape of DWs and thus controlling their orientation/charge state[3]. Here we present an investigation into the behaviour of ferroelectric DWs under dynamic fields and the specific charge carriers present at DWs

In-situ TEM investigation of the amorphous to crystalline phase change during electrical breakdown of highly conductive polymers at the atomic scale

Flexible electronics has been a field of intense research focus for the diverse and new class of applications not achievable by wafer-based electronics. [1-4] Polymers that are both conductive and stretchable have been put forward as a promising candidate for these device platforms. Due to the often amorphous nature of these material platforms the failure analysis knowledge gained from more traditional devices cannot be applied. The progression and innovation of flexible nanoelectronic manufacturing is dependent on understanding the fundamental physics governing the electronic breakdown of such materials and how to avoid this. In this study we investigate the highly conductive flexible amorphous 2D PEDOT [5-7] layers formed via liquid-liquid interface growth, Figure 1 (a). Utilising aberration corrected TEM and new fast camera technology we study the phase change from amorphous to crystalline at the atomic resolution by in-situ biasing

Self-pixelation through fracture in VO2 thin films

Vanadium dioxide (VO2) is an archetypal Mott material with a metal-insulator transition (MIT) near room temperature. In thin films, this transition is affected by substrate-induced strain but, as film thickness increases, the strain is gradually relaxed and the bulk properties are recovered. Epitaxial films of VO2 on (001)-oriented rutile titanium dioxide (TiO2) relax substrate strain by forming a network of fracture lines that crisscross the film along well-defined crystallographic directions. This work shows that the electronic properties associated with these lines result in a pattern that resembles a “street map” of fully strained metallic VO2 blocks separated by insulating VO2 stripes. Each block of VO2 is thus electronically self-insulated from its neighbors and its MIT can be locally induced optically with a laser, or electronically via the tip of a scanning probe microscope, so that the films behave functionally as self-patterned pixel arrays

Germanium tin alloy nanowires as anode materials for high performance Li-Ion batteries

The combination of two active Li-ion materials (Ge and Sn) can result in improved conduction paths and higher capacity retention. Here we report for the first time, the implementation of Ge1–xSnx alloy nanowires as anode materials for Li-ion batteries. Ge1−xSnx alloy nanowires have been successfully grown via vapor–liquid–solid technique directly on stainless steel current collectors. Ge1−xSnx (x=0.048) nanowires were predominantly seeded from the Au0.80Ag0.20 catalysts with negligible amount of growth was also directly catalyzed from stainless steel substrate. The electrochemical performance of the the Ge1−xSnx nanowires as an anode material for Li-ion batteries was investigated via galvanostatic cycling and detailed analysis of differential capacity plots (DCPs). The nanowire electrodes demonstrated an exceptional capacity retention of 93.4% from the 2nd to the 100th charge at a C/5 rate, while maintaining a specific capacity value of ∼921mAhg−1 after 100 cycles. Voltage profiles and DCPs revealed that the Ge1−xSnx nanowires behave as an alloying mode anode material, as reduction/oxidation peaks for both Ge and Sn were observed, however it is clear that the reversible lithiation of Ge is responsible for the majority of the charge stored

Probing the dynamics of topologically protected charged ferroelectric domain walls with the electron beam at the atomic scale

Dynamic charged ferroelectric domain walls (CDWs) overturn the classical idea that our electronic circuits need to consist of fixed components of hardware.[1,2] With their own unique electronic properties and exotic functional behaviours all confined to their nanoscale width, DWs represent a completely new 2D material phase.[3-5] The most exciting aspect of CDWs in single crystals is that they can be easily created, destroyed and moved simply by an applied stimulus. The dynamic nature of CDWs gives them the edge over other novel systems and may lead to them being the next promising disruptive quantum technology. This is an area of research at its very early stages with endless possible applications. However, to harness their true potential there is a great deal of the fundamental physics yet to uncover. As the region of interest (CDW) is atomically thin and dynamic, it is essential for the physical characterisation to be at this scale spatially and time-resolved

Charged domain wall and polar vortex topologies in a room temperature magnetoelectric multiferroic thin

Multiferroic topologies are an emerging solution for future low-power magnetic nanoelectronics due to their combined tuneable functionality and mobility. Here, we show that in addition to being magnetoelectric multiferroic at room temperature, thin-film Aurivillius phase Bi6TixFeyMnzO18 is an ideal material platform for both domain wall and vortex topology based nanoelectronic devices. Utilizing atomic-resolution electron microscopy, we reveal the presence and structure of 180°-type charged head-to-head and tail-to-tail domain walls passing throughout the thin film. Theoretical calculations confirm the subunit cell cation site preference and charged domain wall energetics for Bi6TixFeyMnzO18. Finally, we show that polar vortex-type topologies also form at out-of-phase boundaries of stacking faults when internal strain and electrostatic energy gradients are altered. This study could pave the way for controlled polar vortex topology formation via strain engineering in other multiferroic thin films. Moreover, these results confirm that the subunit cell topological features play an important role in controlling the charge and spin state of Aurivillius phase films and other multiferroic heterostructures

Highly charged 180 degree head-to-head domain walls in lead titanate

Charged domain walls (DWs) in ferroelectric materials are an area of intense research. Microscale strain has been identified as a method of inducing arrays of twin walls to meet at right angles, forming needlepoint domains which exhibit novel material properties. Atomic scale characterisation of the features exhibiting these exciting behaviours was inaccessible with the piezoresponse force microscopy resolution of previous work. Here we use aberration corrected scanning transmission electron microscopy to observe short, stepped, highly charged DWs at the tip of the needle points in ferroelectric PbTiO3. Reverse Ti4+ shift polarisation mapping confirms the head-to-head polarisation in adjacent domains. Strain mapping reveals large deviations from the bulk and a wider DW with a high Pb2+ vacancy concentration. The extra screening charge is found to stabilise the DW perpendicular to the opposing polarisation vectors and thus constitutes the most highly charged DW possible in PbTiO3. This feature at the needle point junction is a 5 nm × 2 nm channel running through the sample and is likely to have useful conducting properties. We envisage that similar junctions can be formed in other ferro elastic materials and yield exciting phenomena for future researc

Ferroelectric domain wall memristor

A domain wall-enabled memristor is created, in thin film lithium niobate capacitors, which shows up to twelve orders of magnitude variation in resistance. Such dramatic changes are caused by the injection of strongly inclined conducting ferroelectric domain walls, which provide conduits for current flow between electrodes. Varying the magnitude of the applied electric-field pulse, used to induce switching, alters the extent to which polarization reversal occurs; this systematically changes the density of the injected conducting domain walls in the ferroelectric layer and hence the resistivity of the capacitor structure as a whole. Hundreds of distinct conductance states can be produced, with current maxima achieved around the coercive voltage, where domain wall density is greatest, and minima associated with the almost fully switched ferroelectric (few domain walls). Significantly, this “domain wall memristor” demonstrates a plasticity effect: when a succession of voltage pulses of constant magnitude is applied, the resistance changes. Resistance plasticity opens the way for the domain wall memristor to be considered for artificial synapse applications in neuromorphic circuit