Real time computer simulation of transmission electron microscope images with tilted illumination: grain boundary applications.

Computer programs have been developed to simulate electron microscope images from digitized graphically represented model structures. Via a television rate image processing system, these programs allow real time, interactive modification of the microscope objective lens parameters, incident beam inclination, and incident beam energy. In addition to explaining the computational methods, the need for using tilted beam illumination is explored to extend microscope resolution. For this study, the subject of grain boundary imaging is analyzed for a copper sigma = 5, 36.9 degrees, (310) tilt boundary with a [001] common rotation axis. The Cu [200] lattice spacings of approximately 1.8A on both sides of the interface cannot be reliably resolved under axial illumination conditions in a 200 kV microscope. Therefore, either tilted beam modes or higher incident beam energies were explored and the types of image features correlated with atomic position data through the digital frame store system.

A fast image simulation technique for high resolution electron microscopy with multicomponent atomic species.

A new method has been developed for simulating high resolution electron microscope images of weak phase objects via a digital television frame store system and fast Fourier transforms of a graphical representation of structures containing several atomic species. Here masks are constructed which consist of circular disk regions whose areas are proportional to the scattering power of the different atom types. These masks represent the object transmission function. The method extends the previous work on image computations of monatomic species objects using small point-like model representations of atom positions. Several examples will be given to verify the relative scattering power from masks corresponding to different atomic species such as Y, Ba, Cu, and O, as well as the limitations of this method for representing objects. Image simulations, which are in agreement with experiments, will also be presented for superconducting oxide materials of the form YBa2Cu3O7 using the circular disk method.

Extending the limit of atomic level grain boundary structure imaging using high-resolution electron microscopy.

It has been possible to image sigma = 21/[111]21.8 degrees tilt boundaries in thin Au films and to deduce their atomic arrangements. These results represent an electron microscopic resolution level of 1.43 A, attainable with a small amount of image processing, which produces interpretable structure images. This substantial improvement over other recent grain boundary studies, which required about 1.9-2.0 A resolution, clearly demonstrates that many more tilt grain boundary orientations are now accessible instead of a limited subset.

Evaluation of high-resolution electron microscopy as a method for studying Y-Ba-Cu-O superconductors.

High-resolution transmission electron microscopes operating at 300 and 400 kV were used to investigate the crystallography and microstructure of the perovskitelike YBa2Cu3O7-x. In this paper, we evaluate the performance attainable with these microscopes both empirically and by computer modelling. Based upon the assumption that oxygen may be a key to superconductivity properties, we have also investigated the visibility of the oxygen sites as well as the heavier yttrium and barium ion positions and the lighter Cu atom positions. We propose a scheme for observing different twin orientations in these structures and hence the oxygen atom positions seen in projection for the [100] and [010]. Our observations of both thick and thin regions of Y-Ba-Cu-O materials are reported as well as the problems of adjusting microscope parameters and specimen alignment to obtain interpretable images. We also give a preliminary report on the effects of heat treatment as seen in high-resolution micrographs to assess disorder of the heavy atoms and oxygen vacancies.

Applications of electronically controlled illumination in the conventional transmission electron microscope.

A device used to produce electronic cone illumination in an analog fashion in the conventional transmission electron microscope has been applied to a number of materials problems which require special diffraction conditions not readily achieved in the microscope's normal operating mode. The device manipulates the primary beam tilt to produce a variety of virtual condenser aperture conditions, and hence electron diffraction patterns can be recorded which reflect the manner in which the direct beam is tilted during the exposure of a micrograph. For single crystalline material, the device provides an improvement over convergent-beam electron diffraction for systematic row reflections and allows direct observation of dynamical beam interactions. It has also been applied to imaging defects in thin crystalline films which would often be obscured under normal microscope conditions. The device allows the imaging of polycrystalline material and the selection of given diffraction orders to determine the orientation of crystallites in a large field of view. It can also modify amorphous patterns to extend the information contained in dark-field images beyond normal tilted-beam dark-field imaging. Control of the incident beam can be accomplished digitally for more varied beam manipulation requirements. A few cases of manipulation of diffraction patterns will be considered.

A method for producing hollow cone illumination electronically in the conventional transmission microscope.

An electronic device manipulates the primary beam in the conventional transmission microscope to produce a hollow cone of illumination with its apex located at the specimen. The device uses the existing tilt coils of the microscope, and modulates the D.C. signals to both x and y tilt directions simultaneously with various waveforms to produce Lissajous figures in the back-focal plane of the objective lens. Electron diffraction patterns can be recorded which reflect the manner in which the direct beam is tilted during exposure of a micrograph. In the bright-field imaging mode the device provides a microscope transfer function without zeros in all spatial directions and has been used to obtain high resolution images which are also free from the effect of chromatic aberration. A standard second condenser aperture is employed and the width of the cone annulus is readily controlled by defocusing the second condenser lens. The cone azimuthal angle is also controlled electronically; hence the device can also be used in the dark-field imaging mode. This device has been applied to imaging both amorphous and crystalline materials including biomolecular specimens.

Computer experiments for tilted beam dark-field imaging.

Simulated high resolution tilted beam dark-field electron micrographs for the conventional transmission microscope were obtained by performing wave optical calculations with a high speed computer. Various organometallic molecules and point defects in crystals were studied to assess whether the image structures resembled the orginal object in terms of atom positions and atom correlations for a variety of microscope conditions. For the organometallic molecules close agreement was found between actual experimental micrographs and calculated images for specific combinations of microscope parameters. The images of point defects indicate that it should be possible to identify these structures based upon image size and intensity which in turn are highly dependent on the strain field surrounding the defect.