William Krakow

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.

Origin of the fringe structure observed in high resolution bright-field electron micrographs of amorphous materials

Abstract The lattice-like fringes observed in high resolution tilted beam bright-field images of amorphous Ge and carbon films are investigated by developing the transfer theory of the electron microscope. To compare our experimental results with theory, we calculate the spatial power spectrum of the electron microscope image of a modified white noise object taking into account tilted beam illumination, partial coherence and inelastic scattering. These objects closely approximate the observed angular electron scattering distribution of an amorphous film. The experimental power spectrum is obtained from optical Fraunhofer diffractograms of bright-field micrographs of amorphous C and Ge. Our results demonstrate that the appearance of the pseudo fringe structures is an image artifact produced by spatial filtering in the electron microscope which is caused by both the transfer function of the microscope and an incoherent super-position of the image intensity distributions of elastically and inelastically scat...

Computer modeling of high resolution transmission electron microscope images of the (001) Au film surface

Abstract Using a computer-generated model of the (001) surface of Au a number of image computations were performed to match experimental electron micrographs taken at 100 kV and 1 MeV. The analysis included the effects of microscope objective lens, partial coherence, energy spread of the incident beam, energy loss of the primary beam in the specimen and beam tilting. Close agreement between the computations and experiments was attained; however, exact identification of atomic sites at 100 kV was not possible since the apparent phase contrast artifacts dominate image contrast. The power spectra of 100 kV computed images reveal the fundamental lattice periodicity. Methods for correcting or improving images at 100 kV were considered as well as the possibility of direct interpretation of images at 500 kV and 1 MeV incident beam energies.

Structural multiplicity observed at a Σ = 5/[001] 53·1° tilt boundary in gold

Abstract Using high-resolution transmission electron microscopy the observation of two different metastable structures at a Σ = 5/[001]53·1° (210) symmetrical tilt boundary are reported upon. A structural unit approach is used to describe the differences between these structures. The results are consistent with computer simulation studies where a small difference in minimum energy states was previously reported.

On the possibility of the direct imaging of point defects in crystals using transmission electron microscopy

Abstract Images with dimensions of 3–5 A were observed in the tilted beam dark field mode electron microscope using the diffuse scattering from b.c.c. Zr-Nb alloys. In an attempt to identify the origin of the image contrast, computer calculations were made of the dark field images from various crystalline defects with atomic dimensions. For the experimental conditions of this study, the calculated image profile for a vacancy had a width at half maximum of ∼ 3 A and a peak intensity of 6 × 10−4 of the incident intensity. The measured intensities from the observed images were in the range of 4 × 10−3 to 1·2 × 10−2 and a comparison of the calculated and experimental values showed that the observed images could not have resulted from the simple defects used for the contrast calculations. The present status of this work is that the observed contrast results either from another type of crystalline defect, possibly associated with a phase change in this alloy system, or from a surface film on the thin foil speci...

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.

A computer modeling study of high resolution electron microscope images of amorphous-to-crystalline transition boundaries

Abstract For the conventional transmission electron microscope, simulated high resolution bright-field micrographs of the interface between amorphous and crystalline materials were obtained by performing wave-optical calculations on a high speed computer. Various interface configurations were considered for fcc and simple cubic structures which included: a 20 A amorphous-to-crystalline transition region, a sharp transition region of one monolayer and a faceted boundary. Several histograms of position data and radial distribution functions have been obtained for these models. Images were calculated using kinematical scattering under a variety of microscope conditions which included up to 2500 atoms arranged in a thin film a few atom layers thick. In addition to the effect of the microscope objective lens, both the tilted beam and axial imaging modes and the effect of inelastic scattering on image features were evaluated. The computed images indicate the conditions under which it should be possible to identify atomic structure at these interfaces when viewed edge-on. Particularly, such tructural features as crystalline protrusions or islands at the interface and phase separation or nonstoichiometry within a quasi-amorphous region can be considered in the same context.

δ-FeO (OH) and its solutions

Decomposition products of δ-FeO(OH)-type Fe1−xMxO1−x(OH)1+x phases (M=Mg, Zn, Ca, Cd) have been studied by X-ray diffraction, electron diffraction and high-resolution transmission electron microscopy. It has been shown that the M=Mg and Cd δ-phases decompose to α-Fe2O3-based solid solutions which in turn undergo exsolution to form some MgO and CdO at a higher temperature. In the case of Fe1−xZnxO1−x(OH)1+x, the decomposition proceeds over α-Fe2O3 ss to an unstable spinel solid solution. All decomposition products are topotactically related to their precursors in the decomposition chain. In the electron microscope some δ-type phases undergo in situ decomposition under intense beam bombardment with somewhat different results than obtained for thermal decomposition products under ambient conditions. The plate-like morphology and crystal size is retained in the decomposition products; however, the products have a more pitted appearance after decomposition.

?-FeO(OH) and its solid solutions: Part 2 High resolution transmission electron microscopy of pure ?-FeO(OH)

A transmission electron microscope imaging investigation was performed on small δ-FeO(OH) crystallites less than 50 Å thick and several hundred angstroms across. We have observed faceting, and a hexagonal plate-like morphology with topological features near atomic step heights. Because of the mutual magnetic attraction on these particles, they tend to align with their thin direction (c-axis) either parallel or perpendicular to the support film surface. It is therefore possible to view dislocations or buckling of lattice planes of these plates either edge-on or perpendicular to this direction by direct lattice imaging in both the bright-field and dark-field modes. A highly distorted lattice is apparent when viewing the particles edge-on, and it is possible to show lattice projections to a resolution of 2.1 Å.

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 microscopes 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.