Günter Möbus

New high-voltage atomic resolution microscope approaching 1 Å point resolution installed in Stuttgart

Abstract The basic characteristics of the new high-voltage atomic resolution microscope (JEOL JEM-ARM1250) recently installed in Stuttgart are described. Owing to its high electrical stability and proper installation conditions this instrument reaches its theoretical point resolution of 0.105 nm at an accelerating voltage of 1250 kV. The defocus spread due to chromatic aberrations was determined quantitatively by extensive diffractogram tests. An information transfer limit between 0.08 and 0.09 nm can be deduced from the results of these tests. First application are presented, which demonstrate the performance of the instrument in structural studies of various materials.

Nanoscale tomography in materials science

In materials science, various techniques for three-dimensional reconstruction of microstructures have been applied successfully for decades, such as X-ray tomography and mechanical sectioning. However, in the last decade the family tree of methods has grown significantly. This is partly through advances in instrumentation. The introduction of the focused ion beam microscope and the transformation of transmission electron microscopy into a multipurpose analytical and structural tool have made major impacts. The main driving force for progress is perhaps the advent of nanotechnology with the need to achieve nanometer-scale resolution and the desire to get a real three-dimensional view of the nanoscale world.

Spectroscopic electron tomography.

The elemental mapping techniques in analytical transmission electron microscopy (TEM), energy filtered imaging (EFTEM) and EDX-mapping, are shown to provide new routes for tomographic reconstructions of 3D chemical maps on the nanoscale. The inelastic scattering does not only provide chemical sensitivity but also improves the linear projection relationship between mass density and image intensity, which often fails in bright field TEM of crystalline materials due to diffraction contrast. Instrumental requirements and artefact sources within the contrast formation mechanisms and within the numerical reconstruction are assessed.

3D determination of grain shape in a FeAl-based nanocomposite by 3D FIB tomography

Abstract The 3D shapes of individual grains in an extruded FeAl nanocomposite have been determined by a new method of 3D focused ion beam (FIB) tomographic analysis. Sequential 2D sectioning and imaging of grains using a FIB, and computer reconstruction can locate the grain boundaries with better than 100 nm precision.

Three-dimensional reconstruction of buried nanoparticles by element-sensitive tomography based on inelastically scattered electrons

Energy-filtered transmission electron microscopy is used to image the nanocomposite FeAl+Y2O3, an oxide-dispersion-strengthened intermetallic alloy, over a tilt range of ±60° using inelastically scattered electrons only. The properties of electron spectroscopic imaging are exploited to recover a projection relationship between the three-dimensional chemical concentration distribution and the micrographs. This allows recovery of the full information on volume shape, distribution, and homogeneity of the buried nanoparticles by backprojection. Restrictions to low atomic number, common in bio-objects, are here overcome at the expense of higher electron exposures.

Structure determination of metal-ceramic interfaces by numerical contrast evaluation of HRTEM micrographs

Abstract Iterative digital image matching (IDIM) between high-resolution electron micrographs and simulated images is presented as a suitable technique of structure refinement of crystal defects. Demonstrated on an MBE-grown interface between niobium and sapphire, results on the translation vector between the half-crystals of niobium and sapphire as well as on the monolayer relaxation at the boundary are obtained. The key features of the procedure are: (i) high-precision image simulation which is performed by refining the imaging conditions by IDIM using bulk-structural units near the boundary, (ii) multiple calculation of image agreement factors in parallel, and (iii) refinement of parameterized model structures by scanning the parameter space.

Subsurface damage analysis by TEM and 3D FIB crack mapping in alumina and alumina/5vol.%SiC nanocomposites

Abstract TEM and 3D crack analysis by focused ion beam (FIB) cross sectioning have been used to quantify the subsurface damage beneath scratches made by a 120° cone indenter loaded to 1 N in monolithic polycrystalline alumina and alumina/5vol.%SiC nanocomposites. In the nanocomposite, an extensive plastic deformation zone was found under the scratch grooves, extending beyond the first layer of grains to a maximum plastic deformation depth of ~7 μm below the surface of the track. In the alumina, however, the plastically deformed region only extends to a maximum depth of ~4 μm and is contained within the first layer of grains adjacent to the groove surface. The 3D morphologies of the cracks under the scratches have been determined by FIB sectioning, showing that a high density of microcracks exists under the scratches in both ceramics. Differences between the plastic deformation and subsurface facture modes of the alumina and the nanocomposite are discussed.

Iterative structure retrieval techniques in HREM: a comparative study and a modular program package

A retrieval technique for crystal structures using high‐resolution electron microscopy is presented. The inversion of the complex structure‐to‐image relation is performed by numerical optimization of the configuration space of object models. Unlike structure refinement in X‐ray crystallography, the method operates on unknown defect structures on a nanometre scale. The diversity of crystal defects examined and the differences in microscope types and alignment conditions makes it necessary to adapt the basic algorithm to a broad variety of needs resulting in a modular program package for general use. New developments, such as a real space slice function calculation, problem adapted optimization strategies, and finally an examination of iterative matching of image series and exit wavefunctions are presented.

Synthesis of analytical and high-resolution transmission electron microscopy to determine the interface structure of Cu/Al2O3

Abstract The structure of Cu/Al2O3 interfaces has been studied by high-resolution transmission electron microscopy (HRTEM) and analytical TEM. The use of electron energy-loss spectroscopy (EELS) in the determination of the local coordinations and bonding mechanisms at the interface is shown to be an essential extension for atomic structure evaluation by quantitative high-resolution TEM. A refined atomic model of the Cu/Al2O3 interface is then retrieved by iterative digital image matching. Multiple scattering calculations based on this model are vital to explain the interface specific components in the EELS O-K edge. The atomistic configuration at the interface is described by a structure model with Cu O bonds across the interface.

Retrieval of crystal defect structures from HREM images by simulated evolution I. Basic technique

Abstract Image interpretation of crystal defect structures by means of HREM and image simulations is performed using an advanced method of iterative digital image matching. HREM image analysis is treated as a high-dimensional numerical optimization problem. The configuration space is found to be highly multi-modal with local optima in both the sub-spaces for the structures and for the imaging parameters. Steepest descent optimization is therefore not always sufficient. The global optimization algorithm of simulated evolution by Schwefel [Numerical Optimization of Computer Models (Wiley, New York, 1981)] is tested in this first part by simulations on metal-sapphire interfaces. The capability of arriving at the globally optimal crystal structure within reasonable error bars is studied as a function of four basic quantities: (i) deviation of the initial structure model from the true solution, (ii) signal-to-noise ratio, (iii) microscope resolution, and (iv) smallest projected bond length. For each of these items a well defined limit exists which, upon crossing, leads to a failure of the technique.