In several projects the StEM group is investigating the structure and the chemistry, including the valence state, of defects and interfaces at up to atomic resolution. Often it is the defects that control the useful properties of novel materials, particularly of thin films. Interfaces can be considered a special type of defect, and again the properties of interfaces often have a large influence on the behaviour of the material system that encompasses them.
The study of plasmons offers opportunities to take advantage of cutting edge TEM technology and to combine and compare the resulting experimental data with highly developed theoretical calculations. The practical appeal of plasmons arises from their potential use as carriers of information, at a size scale useful to bridge the gap between high speed optical data transfer, with components that are relatively large, and the most highly developed electronic processing components, which are very small but speed limited. The research of the StEM group is generally directed towards investigating the fundamentals of surface plasmon behaviour in different materials and geometries. Some current projects in this area are described.
Bio-materials possess a wide variety of interesting structures that inspire the design of new-man-made materials targeted to specific applications. In some cases a biological organism itself can be recruited to assist in the creation of a new composite material. The micro-and nanostructure of biological materials, the details of their chemical composition and their performance in inducing or generating new material structures is a vibrant and growing field of study. StEM investigates the structure and composition of naturally occurring bio-minerals and bio-composites, and also the development and properties of new materials generated with biological assistance.
Inelastic interactions of electrons with the optical density of states results in some resonances in the low loss regime of electron energy-loss spectra (EELS). How are the electrons coupled to several modes of the nanostructures? How large are the contributions of the different mechanisms of radiation, such as Larmor radiation, transition radiation, and Cherenkov radiation, to the EELS or cathodoluminescence spectra? How much recoil does the electron receive, due to the interaction with its self-field? How can the self-inertia or the radiation resistance of the electrons be overcome in the design of efficient electron-driven photon-sources? These are all questions that we would like to answer with our theoretical methods, within the research area of electron and electromagnetic field interactions.
Hydrogen-based electricity generation is an important component in ‘Energiewende’, and ceramic membranes applied in fuel cells and hydrogen production can play a key role in improving the efficiency of the process. The goal of this collaborative project is to optimize the transport properties of selected ceramic permeation membranes while developing the ability to manufacture them at device-scales.
Graphene is a two-dimensional, one atom thick (0.34 nm), sp2 hybridized carbon material that has a honeycomb-like arrangement of atoms in its crystal lattice. It has found itself in an elite club of advanced materials since its discovery in 2004, and promises to enlighten the future with its unique electrical (high mobility of electrons) and mechanical properties, which have been explored and tested in recent years. This project will characterize organic and inorganic molecules deposited on single layer graphene, using advanced TEM techniques.
Topological insulators are called a new state of matter. The bulk material shows insulating behavior whereas metallic conductivity is exhibited at the surface. We are investigating topological insulators, mainly Bi2Se3 and Bi2Te3, using various transmission electron microscopy techniques. In conventional transmission electron microscopy, the image contains information only of the electron wave’s amplitude and phase information is lost after image formation. Inline electron holography is used to determine the electrical properties of the topological insulator surfaces, knowledge of which allows the reconstruction of the exit wave function.
Many electromechanical devices use ferroelectric perovskites, often PbTiO3 or Pb(Zr,Ti)O3. Efforts to reduce the use of lead in these devices require a replacement material and KNbO3 (KNO) is a suitable candidate. KNO is a ferroelectric perovskite with appropriate electronic and optical properties, and good chemical stability. Its structure varies with temperature more than some perovskites; it is cubic and not ferroelectric above 435°C, tetragonal between 435°C and 225°C and orthorhombic at room temperature. The aim of this study is to investigate the structure and behaviour of dislocations and domain walls in KNO after plastic deformation.
Atomic-level modeling, simulation and electron microscopy investigation of dislocations and related structural defects in perovskite oxides
Strontium titanate (SrTiO3) exhibits a ductile to brittle to ductile transition (DBDT). The reason for this behavior should be found by analyzing the dislocation microstructure. TEM observations reveal that below room temperature dislocations are predominantly screw-type and most of them are very straight (fig), suggesting restricted mobility. As the temperature increases a microstructure composed of bent dislocations appears, indicating that the dislocations can glide easily. It is assumed that the reason for this transition and for why screw dislocations show such a low mobility must be due to changes in the dislocation core structure. This is studied by HRTEM.
Atomic-level study of transition metal oxide superlattices
Superlattices of transition metal oxides, for example LaNiO3/LaAlO3, have unique properties that significantly differ from the bulk properties. This is mainly caused by the interfaces between the layers. We study the microstructure as well as the electronic structure of such systems on an atomic level. This is mainly done with scanning transmission electron microscopy (STEM) in combination with electron energy-loss spectroscopy (EELS). The electronic structure can be determined by studying the fine structure of absorption edges (ELNES). Furthermore, strain mapping is performed on the superlattices.
Proton conducting oxides are promising electrolyte materials for intermediate temperature solid oxide fuel cells (SOFCs). Among the oxide proton conductors, Y-doped BaZrO3 offers a combination of high conductivity and good chemical stability. However, low conductivity of grain boundaries compared to the grains (bulk) is the main obstacle for using this material as an electrolyte. Consequently, there is a great demand for increasing total conductivity of this material by improving the electrical properties of grain boundaries. In this respect, we studied the effect of high-temperature annealing and the segregation behavior of two different trivalent dopant cations (Sc3+ and Zr4+) on the grain boundary properties. This project is in collaboration with the Department Maier (Max Planck Institute for Solid State Research).
Electron tomography of nanomaterials
Electron tomography has been widely used not only in biological sciences but also in materials science for obtaining three-dimensional (3D) information about specimens. Due to diffraction contrast from crystalline materials, which does not follow the projection principle required by tomography, the use of tomography in materials science is still limited. By using EFTEM (energy-filtered TEM) tomography, not only can the effect of diffraction contrast be minimized, but also the 3D elemental distributions of various materials in particular nanosystems can be obtained.
In-situ studies with particular emphasis on atomic resolution are carried out using the ARM1250 high-voltage microscope. Recent projects involve the study of surface phenomena such as faceting, surface reconstruction and island formation in strontium titanate at temperatures between 900 and 1000 ºC, studies on the formation of an incommensurably modulated phase in the Brownmillerite-type oxygen deficient perovskite Ca2Fe2O5 and the formation of antiphase boundaries accompanying this process, or the metal-induced crystallization of amorphous silicon at Al grain boundaries. The development of special specimen holders for in-situ TEM is pursued as well.
Simulation of STEM and TEM images
We are developing and maintaining software for simulating HRTEM and (HAADF-) STEM images as well as CBED patterns of arbitrarily large structures.
We have developed new methods to efficiently map strain in, for example, semiconductor devices. These methods are based on either high resolution TEM or dark-field inline holography.
Structure analysis of ionic solids using electron diffraction
Within the framework of a DFG priority programme, we studied the structural development of alkalihalogenides during the transition from the amorphous to the crystalline state. One of the goals was the identification of metastable phases. The structural studies were performed using electron diffraction. We determined reduced density functions and compared these with results from molecular dynamics. Pair distribution functions extracted from elaborated electron diffraction experiments yielded information on the structural transformations from amorphous to crystalline phases. From the comparison of theoretical modelling, such as molecular dynamics simulations, and experimental results details of the short‐range order in the amorphous as well as the distorted crystalline phase could be retrieved. [A. Bach et al. Inorg. Chem. 50 (2011) 1563]