Optimizing compositional and atomic-level information of oxides in atom probe tomography

Atom probe tomography (APT) is a 3D analysis technique that offers unique chemical accuracy and sensitivity with sub-nanometer spatial resolution. Recently, there is an increasing interest in the application of APT to complex oxides materials, giving new insight into the relation between local variations in chemical composition and emergent physical properties. However, in contrast to the field of metallurgy, where APT is routinely applied to study materials at the atomic level, complex oxides and their specific field evaporation mechanisms are much less explored. Here, we perform APT measurements on the hexagonal manganite ErMnO3 and systematically study the effect of different experimental parameters on the measured composition and atomic structure. We demonstrate that both the mass resolving power (MRP) and compositional accuracy can be improved by increasing the charge-state ratio (CSR) working at low laser energy (< 5 pJ). Furthermore, we observe a substantial preferential retention of Er atoms, which is suppressed at higher CSRs. We explain our findings based on fundamental field evaporation concepts, expanding the knowledge about the impact of key experimental parameters and the field evaporation process in complex oxides in general

Observation of Antiferroelectric Domain Walls in a Uniaxial Hyperferroelectric

Ferroelectric domain walls are a rich source of emergent electronic properties and unusual polar order. Recent studies showed that the configuration of ferroelectric walls can go well beyond the conventional Ising-type structure. N\'eel-, Bloch-, and vortex-like polar patterns have been observed, displaying strong similarities with the spin textures at magnetic domain walls. Here, we report the discovery of antiferroelectric domain walls in the uniaxial ferroelectric Pb$_{5}$Ge$_{3}$O$_{11}$. We resolve highly mobile domain walls with an alternating displacement of Pb atoms, resulting in a cyclic 180$^{\circ}$ flip of dipole direction within the wall. Density functional theory calculations reveal that Pb$_{5}$Ge$_{3}$O$_{11}$ is hyperferroelectric, allowing the system to overcome the depolarization fields that usually suppress antiparallel ordering of dipoles along the longitudinal direction. Interestingly, the antiferroelectric walls observed under the electron beam are energetically more costly than basic head-to-head or tail-to-tail walls. The results suggest a new type of excited domain-wall state, expanding previous studies on ferroelectric domain walls into the realm of antiferroic phenomena

Visualizing Ferroelectric Domain Structures in ErMnO3 and Pb5Ge3O11 by Electron Microscopy

Ferroelektriske domenevegger har nylig tiltrukket økende oppmerksomhet som nye kandidater for funksjonelle 2D-systemer. Disse atomskarpe grensesnittene som kan flyttes, opprettes og fjernes med elektriske felt, kan brukes til å lage elektroniske enheter med en fleksibilitet som ikke finnes i andre materialer. For mange multiferroiske materialer er imidlertid mye av den underliggende fysikken til ladede domenevegger fortsatt ukjent som begrenser potensialet for å utvikle enheter. Fullstendig undersøkelser av domenevegger krever en romlig oppløsning i en rekkevidde fra over mesoskopisk til mindre enn nanoskopisk som transmisjonselektronmikroskopet (TEM) dekker. Domenestrukturene gir imidlertid ikke en dominerende bildekontrast, så spesialiserte teknikker er nødvendig for å visualisere dem. Selv med disse teknikkene, er kontrasten lett å misforstå og samsvare høyoppløsningsresultater fra TEM med andre teknikker som sveipelektronmikroskopet (SEM) kan være svært fordelaktig. I denne oppgaven har både SEM og TEM blitt anvendt for å undersøke teknikker og rutiner for å studere de ferroelektriske domenestrukturer av ErMnO3 og Pb5Ge3O11. Prøver fra begge materialer prepareres for første gang ved mekanisk stativpolering, og danner en stor (> 1 mm) elektron-gjennomsiktig kant uten et amorft lag eller potensiell skade fra ioner som vanligvis brukes i tynningsprosessen. En SEM brukes til å gi en oversikt over domenevegger langs den store kanten. Domener er observert å bare være synlige over en kritisk tykkelse på 427 nm i ErMnO3. Et to-trinns preparering ble utviklet, hvor først en ble brukt til å ta oversiktsbilder på en prøve over kritisk tykkelse. Forsiktig Ar-ioneetsing ble deretter gjort for TEM-analyse av områdene kartlagt av SEM. For ErMnO3 er 002 refleksjonen funnet gjennom simuleringer til å være det beste valget for mørkefelt bilder. Stativpolering innførte imidlertid for mange defekter for å finne domenevegger ved bruk av mørkefelt bilder. Prøvene var fortsatt anvendbare for høy kvalitets atom avbildning med høyoppløselig TEM og sveip transmisjonselektronmikroskopi (STEM). Polariseringen ble funnet i en perfekt pulverprøve ved bruk av en ringformet detektor i mørkefelt STEM ved å ta en serie bilder med rekonstruksjon i et ikke-korrigert STEM. Pb5Ge3O11 stativpolerte prøver var derimot fri for induserte defekter, men overflaten ble skadet i stedet. Elektronmikroskopstudier av stativpolerte prøver lider sterkt av oppladningseffekter og stråleskader. De amorfiserte raskt med en kritisk dose på 0.64 C/cm^2 (3.99 ∗ 10^2 e/Å^2) per nanometer i STEM-modus. I TEM modus dekomponerer materialet til Pb-partikler med en kritisk dose på 5.78 ×10^3 C/cm^2 (3.61 ∗ 10^6 e/Å^2). Angående de ferroelektriske domenene til Pb5Ge3O11, ble de funnet å bli lett omskrevet under avbildning i SEM, selv med en lav elektronstrøm (≤0.1 nA) og spenning (≤5 keV). Både tykkelse og geometri spiller en rolle, hvor tynnere områder og skarpe kanter er de mest sårbare for å bli omskrevet av elektronstrålen. På grunn av oppladningseffekter var kantene spesielt utfordrende for avbildning, så rutinen med å samsvare TEM og SEM på samme lokasjon i prøven kunne ikke brukes for å studere domenestrukturen. I stedet ble SEM brukt til tykkere deler av de stativpolerte prøvene og TEM på den tynne kanten. For Pb5Ge3O11 ble 003-refleksjonen funnet gjennom simuleringer til å være det ideelle valget for mørkefelt bilder, og flere grensesnitt som ligner på domenemurer ble studert. Konvergerende elektron diffraksjon ble brukt til å finne polarisering lokalt i en pulverprøve, men kunne ikke brukes på stativpolerte prøver på grunn av oppladningseffekter. Resultatene av dette studiet identifiserer grensene for avbildning av atomstrukturen i domenevegger i Pb5Ge3O11, som aldri har blitt oppnådd, men er essensielt for videre undersøkelser av de grunnleggende egenskapene av de fascinerende domeneveggene

Ferroelectric Domain Engineering Using Structural Defect Ordering

ISSN:0897-475

Characterization of ferroelectric domain walls by scanning electron microscopy

Ferroelectric domain walls are a completely new type of functional interface, which have the potential to revolutionize nanotechnology. In addition to the emergent phenomena at domain walls, they are spatially mobile and can be injected, positioned, and deleted on demand, giving a new degree of flexibility that is not available at conventional interfaces. Progress in the field is closely linked to the development of modern microscopy methods, which are essential for studying their physical properties at the nanoscale. In this article, we discuss scanning electron microscopy (SEM) as a powerful and highly flexible imaging technique for scale-bridging studies on domain walls, continuously covering nano- to mesoscopic length scales. We review seminal SEM experiments on ferroelectric domains and domain walls, provide practical information on how to visualize them in modern SEMs, and provide a comprehensive overview of the models that have been proposed to explain the contrast formation in SEM. Going beyond basic imaging experiments, recent examples for nano-structuring and correlated microscopy work on ferroelectric domain walls are presented. Other techniques, such as 3D atom probe tomography, are particularly promising and may be combined with SEM in the future to investigate individual domain walls, providing new opportunities for tackling the complex nanoscale physics and defect chemistry at ferroelectric domain walls

Introducing a Dynamic Reconstruction Methodology for Multilayered Structures in Atom Probe Tomography

International audienceAbstract Atom probe tomography (APT) is a powerful three-dimensional nanoanalyzing microscopy technique considered key in modern materials science. However, progress in the spatial reconstruction of APT data has been rather limited since the first implementation of the protocol proposed by Bas et al. in 1995. This paper proposes a simple semianalytical approach to reconstruct multilayered structures, i.e., two or more different compounds stacked perpendicular to the analysis direction. Using a field evaporation model, the general dynamic evolution of parameters involved in the reconstruction of this type of structure is estimated. Some experimental reconstructions of different structures through the implementation of this method that dynamically accommodates variations in the tomographic reconstruction parameters are presented. It is shown both experimentally and theoretically that the depth accuracy of reconstructed APT images is improved using this method. The method requires few parameters in order to be easily usable and substantially improves atom probe tomographic reconstructions of multilayered structures

Atomic-scale 3D imaging of individual dopant atoms in an oxide semiconductor

The physical properties of semiconductors are controlled by chemical doping. In oxide semiconductors, small variations in the density of dopant atoms can completely change the local electric and magnetic responses caused by their strongly correlated electrons. In lightly doped systems, however, such variations are difficult to determine as quantitative 3D imaging of individual dopant atoms is a major challenge. We apply atom probe tomography to resolve the atomic sites that donors occupy in the small band gap semiconductor Er(Mn,Ti)O3 with a nominal Ti concentration of 0.04 at. %, map their 3D lattice positions, and quantify spatial variations. Our work enables atomic-level 3D studies of structure-property relations in lightly doped complex oxides, which is crucial to understand and control emergent dopant-driven quantum phenomena.ISSN:2041-172

Imaging and structure analysis of ferroelectric domains, domain walls, and vortices by scanning electron diffraction

Direct electron detectors in scanning transmission electron microscopy give unprecedented possibilities for structure analysis at the nanoscale. In electronic and quantum materials, this new capability gives access to, for example, emergent chiral structures and symmetry-breaking distortions that underpin functional properties. Quantifying nanoscale structural features with statistical significance, however, is complicated by the subtleties of dynamic diffraction and coexisting contrast mechanisms, which often results in low signal-to-noise and the superposition of multiple signals that are challenging to deconvolute. Here we apply scanning electron diffraction to explore local polar distortions in the uniaxial ferroelectric Er(Mn,Ti)O$_3$. Using a custom-designed convolutional autoencoder with bespoke regularization, we demonstrate that subtle variations in the scattering signatures of ferroelectric domains, domain walls, and vortex textures can readily be disentangled with statistical significance and separated from extrinsic contributions due to, e.g., variations in specimen thickness or bending. The work demonstrates a pathway to quantitatively measure symmetry-breaking distortions across large areas, mapping structural changes at interfaces and topological structures with nanoscale spatial resolution

Observation of Unconventional Dynamics of Domain Walls in Uniaxial Ferroelectric Lead Germanate

Application of scanning probe microscopy techniques such as piezoresponse force microscopy (PFM) opens the possibility to re‐visit the ferroelectrics previously studied by the macroscopic electrical testing methods and establish a link between their local nanoscale characteristics and integral response. The nanoscale PFM studies and phase field modeling of the static and dynamic behavior of the domain structure in the well‐known ferroelectric material lead germanate, Pb5Ge3O11, are reported. Several unusual phenomena are revealed: 1) domain formation during the paraelectric‐to‐ferroelectric phase transition, which exhibits an atypical cooling rate dependence; 2) unexpected electrically induced formation of the oblate domains due to the preferential domain walls motion in the directions perpendicular to the polar axis, contrary to the typical domain growth behavior observed so far; 3) absence of the bound charges at the 180° head‐to‐head (H–H) and tail‐totail (T–T) domain walls, which typically exhibit a significant charge density in other ferroelectrics due to the polarization discontinuity. This strikingly different behavior is rationalized by the phase field modeling of the dynamics of uncharged H–H and T–T domain walls. The results provide a new insight into the emergent physics of the ferroelectric domain boundaries, revealing unusual properties not exhibited by conventional Ising‐type walls