Helmut Kohl

Transmission electron microscopy : physics of image formation.

Particle Optics of Electrons.- Wave Optics of Electrons.- Elements of a Transmission Electron Microscope.- Electron-Specimen Interactions..- Scattering and Phase Contrast.- Theory of Electron Diffraction.- Electron-Diffraction Modesand Applications ..- Imaging of Crystalline Specimens and Their Defects..- Elemental Analysis by X-ray and Electron Energy-Loss Spectroscopy..- Specimen Damage by Electron Irradiation.

Experimental evidence of self-limited growth of nanocrystals in glass.

Growth of nanocrystals precipitated in glasses with specific compositions can be effectively limited by diffusion barriers forming around crystallites. For the first time, we do experimentally prove this concept of self-limited growth on the nanoscale for a SiO(2)/Al(2)O(3)/Na(2)O/K(2)O/BaF(2) glass in which BaF(2) nanocrystals are formed. As shown by advanced analytical transmission electron microscopy techniques, the growth of these BaF(2) crystals, having great potential for photonic applications, is inherently limited by the formation of a ca. 1 nm wide SiO(2) shell.

Detection limits in elemental distribution images produced by energy filtering TEM : case study of grain boundaries in Si3N4

Abstract Using an energy filtering transmission electron microscope (EFTEM) we have investigated the detection limits of elemental distribution images obtained by electron spectroscopic imaging (ESI). As a model system we have chosen the amorphous grain boundary films in sintered Si3N4 ceramics. The analyzed films consist of SiO2 with a thickness between 0.7 and 1.5 nm corresponding roughly to 2–5 monolayers of oxygen. The experimental results are compared with theoretical predictions. Imaging conditions yielding optimum detectability are determined. The detection limit is discussed quamtitatively.

THEORETICAL AND EXPERIMENTAL INVESTIGATIONS OF RESOLUTION AND DETECTION LIMITS IN ENERGY-FILTERING ELECTRON MICROSCOPY

Abstract The energy-filtering transmission electron microscope (EFTEM) offers the possibility of obtaining two-dimensional elemental maps using characteristic inner-shell loss electrons. The characteristic signal lies on a background stemming from outer-shell ionizations. To subtract this unspecific background one acquires several images at energy losses below the characteristic edge to extrapolate the background. We used small uranium clusters to compare different background models and to investigate the detection and resolution limits. We found that the jump-ratio method yields the best results for the background correction, followed by the least-squares fit estimation for quantitative analyses. Particles with sizes down to 2.3 nm were detectable with a signal-to-noise ratio greater than five. To obtain the resolution limit, the technique of Youngs fringes and the cross-correlation function were used. For our experiments this resulted in a resolution limit down to 2.4 nm. Theoretical calculations of the detection limit confirm these results.

Computation and interpretation of contrast in crystal lattice images formed by inelastically scattered electrons in a transmission electron microscope

Abstract With modern transmission electron microscopes elemental mapping with atomic resolution seems feasible. For crystalline specimens the interpretation of contrast in such energy-loss micrographs is not a simple straightforward matter, as the so-called preservation of elastic contrast can play an important role. Hence the computation of theoretical micrographs for comparison is necessary. The calculation method presented in this article is based on well-known theoretical approaches. To enable numerical evaluation we use an exciton-like representation for the excited electronic state of the crystal. The practicability of this method is demonstrated by computing lattice fringes and demonstrating the influence of the preservation of elastic contrast.

The resolution limit for elemental mapping in energy-filtering transmission electron microscopy

Abstract In recent years electron microscopes with energy filtering devices have found widespread acceptance. Combined with an effective detection system, such as a slow-scan CCD camera, these microscopes allow the rapid acquisition of elemental maps of large areas. Due to the low cross-sections for the relevant inner-shell excitations, the resolution attainable with a given object is limited by the signal-to-noise ratio. We shall propose a resolution criterion which includes the statistical properties of the images and discuss the dependence of this “object resolution limit” on instrumental parameters.

High-resolution STEM imaging with a quadrant detector--conditions for differential phase contrast microscopy in the weak phase object approximation.

Differential phase contrast is a contrast mechanism that can be utilized in the scanning transmission electron microscope (STEM) to determine the distribution of magnetic or electric fields. In practice, several different detector geometries can be used to obtain differential phase contrast. As recent high resolution differential phase contrast experiments with the STEM are focused on ring quadrant detectors, we evaluate the contrast transfer characteristics of different quadrant detector geometries, namely two ring quadrant detectors with different inner detector angles and a conventional quadrant detector, by calculating the corresponding phase gradient transfer functions. For an ideal microscope and a weak phase object, this can be done analytically. The calculated phase gradient transfer functions indicate that the barely illuminated ring quadrant detector setup used for imaging magnetic fields in the specimen reduces the resolution limit to about 2.5Å for an aberration corrected STEM. Our results show that the resolution can be drastically improved by using a conventional quadrant detector instead.

Quantitative comparison of energy-filtering transmission electron microscopy and atom probe tomography

Whereas transmission electron microscopy (TEM) is a well established method for the analysis of thin film structures down to the sub-nanometer scale, atom probe tomography (APT) is less known in the microscopy community. In the present work, local chemical analysis of sputtered Fe/Cr multilayer structures was performed with energy-filtering transmission electron microscopy (EFTEM) and APT. The single-layer thickness was varied from 1 to 6nm in order to quantify spatial resolution and chemical sensitivity. While both the methods are able to resolve the layer structure, even at 2nm thickness, it is demonstrated that the spatial resolution of the APT is about a factor of two, higher in comparison with the unprocessed EFTEM data. By calculating the influence of the instrumental parameters on EFTEM images of model structures, remaining interface roughness is indicated to be the most important factor that limits the practical resolution of analytical TEM.

Methods for ELNES-quantification: characterization of the degree of inversion of Mg–Al-spinels

The energy loss near-edge structures in electron energy-loss spectra contain information about bonding characteristics, the electronic structure and coordinations of the excited atoms. We have calculated sets of reference spectra for the normal and for the inverse Mg-Al-spinel using a full multiple scattering approach. By a quantitative comparison of these reference spectra with experimental data ELNES-quantification becomes possible. We characterized the degree of inversion lambda by the analysis of the relative peak-intensities and the relative peak-positions within 35 eV beyond the edge onset. The results demonstrate that by using the provided methods ELNES-quantification will become possible when uncertainties in the experiment are reduced and a better fit of the simulations to the experiment is achieved.

Optimization of EFTEM image acquisition by using elastically filtered images for drift correction

Because of its high spatial resolution, energy-filtering transmission electron microscopy (EFTEM) has become widely used for the analysis of the chemical composition of nanostructures. To obtain the best spatial resolution, the precise correction of instrumental influences and the optimization of the data acquisition procedure are very important. In this publication, we discuss a modified image acquisition procedure that optimizes the acquisition process of the EFTEM images, especially for long exposure times and measurements that are affected by large spatial drift. To alleviate the blurring of the image caused by the spatial drift, we propose to take several EFTEM images with a shorter exposure time (sub-images) and merge these sub-images afterwards. To correct for the drift between these sub-images, elastically filtered images are acquired between two subsequent sub-images. These elastically filtered images are highly suitable for spatial drift correction based on the cross-correlation method. The use of the drift information between two elastically filtered images permits to merge the drift-corrected sub-images automatically and with high accuracy, resulting in sharper edges and an improved signal intensity in the final EFTEM image. Artefacts that are caused by prominent noise-peaks in the dark reference image have been suppressed by calculating the dark reference image from three images. Furthermore, using the information given by the elastically filtered images, it is possible to drift-correct a set of EFTEM images already during the acquisition. This simplifies the post-processing for elemental mapping and offers the possibility for active drift correction using the image shift function of the microscope, leading to an increased field of view.