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

Metal configurations on 2D materials investigated via atomic resolution HAADF stem

The behaviour of palladium & nickel deposited on mechanically exfoliated samples of 2D transition metal dichalcogenides (MoS2, WS2, and WSe2) via e-beam evaporation was investigated.Sputtering of metals on the flakes allowed for interaction of the metal and TMD to be investigated on the Å scale in an aberration-corrected transmission electron microscope. By low energy sputtering metals can be deposited on 2D materials without causing damage to the thin flakes. The materials interaction is investigated on the atomic scale via high resolution scanning transmission electron microscopy in high angle annular dark field imaging. Initially, the effect of thermal annealing on the stability of the Pd-2D interaction was investigated, revealing the remarkable difference in particle stability between the 2D materials. Nickel deposition however only resulted in oxidised amorphous particles. The oxide particles’ cross-sectional area and circularity were independent of the TMD substrate thickness or type, and deposition rate

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

Non‑classical crystallisation pathway directly observed for a pharmaceutical crystal via liquid phase electron microscopy

Non‑classical crystallisation (NCC) pathways are widely accepted, however there is conflicting evidence regarding the intermediate stages of crystallisation, how they manifest and further develop into crystals. Evidence from direct observations is especially lacking for small organic molecules, as distinguishing these low‑electron dense entities from their similar liquid‑phase surroundings presents signal‑to‑noise ratio and contrast challenges. Here, Liquid Phase Electron Microscopy (LPEM) captures the intermediate pre‑crystalline stages of a small organic molecule, flufenamic acid (FFA), a common pharmaceutical. High temporospatial imaging of FFA in its native environment, an organic solvent, suggests that in this system a Pre‑Nucleation Cluster (PNC) pathway is followed by features exhibiting two‑step nucleation. This work adds to the growing body of evidence that suggests nucleation pathways are likely an amalgamation of multiple existing non‑classical theories and highlights the need for the direct evidence presented by in situ techniques such as LPE

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

Collective electronic excitations in the ultra violet regime in 2-D and 1-D carbon nanostructures achieved by the addition of foreign atoms

lasmons in the visible/UV energy regime have attracted great attention, especially in nano-materials, with regards to applications in opto-electronics and light harvesting; tailored enhancement of such plasmons is of particular interest for prospects in nano-plasmonics. This work demonstrates that it is possible, by adequate doping, to create excitations in the visible/UV regime in nano-carbon materials, i.e., carbon nanotubes and graphene, with choice of suitable ad-atoms and dopants, which are introduced directly into the lattice by low energy ion implantation or added via deposition by evaporation. Investigations as to whether these excitations are of collective nature, i.e., have plasmonic character, are carried out via DFT calculations and experiment-based extraction of the dielectric function. They give evidence of collective excitation behaviour for a number of the introduced impurity species, including K, Ag, B, N, and Pd. It is furthermore demonstrated that such excitations can be concentrated at nano-features, e.g., along nano-holes in graphene through metal atoms adhering to the edges of these holes

Visualising early-stage liquid phase organic crystal growth via liquid cell electron microscopy†

Here, we show that the development of nuclei and subsequent growth of a molecular organic crystal system can be induced by electron beam irradiation by exploiting the radiation chemistry of the carrier solvent. The technique of Liquid Cell Electron Microscopy was used to probe the crystal growth of flufenamic acid; a current commercialised active pharmaceutical ingredient. This work demonstrates liquid phase electron microscopy analysis as an essential tool for assessing pharmaceutical crystal growth in their native environment while giving insight into polymorph identification of nano-crystals at their very inception. Possible mechanisms of crystal nucleation due to the electron beam with a focus on radiolysis are discussed along with the innovations this technique offers to the study of pharmaceutical crystals and other low contrast materials

Plasmons in MoS2 studied via experimental and theoretical correlation of energy loss spectra

This paper takes a fundamental view of the electron energy loss spectra of monolayer and few layer MoS2. The dielectric function of monolayer MoS2 is compared to the experimental spectra to give clear criteria for the nature of different signals. Kramers-Krönig analysis allows a direct extraction of the dielectric function from the experimental data. However this analysis is sensitive to slight changes in the normalisation step of the data pre-treatment. Density functional theory provides simulations of the dielectric function for comparison and validation of experimental findings. Simulated and experimental spectra are compared to isolate the and + surface plasmon modes in monolayer MoS2. Singleparticle excitations obscure the plasmons in the monolayer spectrum and momentum resolved measurements give indication of indirect interband transitions that are excited due to the large convergence and collection angles used in the experiment

Imaging two dimensional materials and their heterostructures

Stacking different two-dimensional (2D) atomic layers is a feasible approach to create unique multilayered van der Waals heterostructures with desired properties. 2D materials, graphene, hexagonal boron nitride (h-BN), molybdenum disulphate (MoS2) and graphene based van der Waals heterostructures, such as h-BN/graphene and MoS2/graphene have been investigated by means of Scanning Transmission Electron Microscopy (STEM)

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