Cone Dark Field Research Articles

Measurement of interdiffusion by high angle hollow cone illumination

High angle hollow cone darkfield or Z-contrast imaging is used to supress dynamical contributions like bend or extinction contours to electron imaging. Using a multiple scattering model it is possible to interpret image intensities quantitatively and to analyze specimen composition on a nanometer scale without specialized detectors like energy-energy-loss spectroscopy or energy dispersive spectroscopy. This method is practically tested on the interdiffusion of Cu-Au couples. Measured thin film interdiffusion coefficents, are comparable to literature data of the bulk diffusion coefficent in this system.

Transmission electron microscopy detection of microtexture variations and their effects on thin film stability

We have studied grain growth and the development of preferred orientation in thin gold films using transmission electron microscopy conical dark field imaging. The technique is capable of detecting texture variations characterized by mean orientation differences smaller than 5°. We have observed microscopic regions with distinct 〈111〉 fiber textures that differ, primarily, in the mean azimuthal deviation from the fiber axis. For one texture, the 〈111〉 axes of individual grains are found to have an average deviation of 1.5° from the foil normal, a second is characterized by a mean deviation of 5°, and a third is more nearly random with only a slight 〈111〉 orientation preference. The shapes of the differently oriented regions depend on the preparation of the sodium chloride substrate before deposition. We have studied the dependence of preferential orientation development on substrate surface topography and the evolution of microstructure in differently textured regions. Hillock growth, observed only in nearly randomly oriented regions of the films, cannot be explained by current theories of grain growth or hillock formation.

Localized Texture Formation and its Detection in Polycrystalline Thin Films of Gold

ABSTRACTWe have developed a technique for detecting microscopic variations in texture in thin film TEM specimens using conical dark field imaging. This technique has been used to identify texture variations in 25nm thick gold films which were deposited on rock salt and annealed at 150°C. After annealing, the films were Mostly (111) textured with one micrometer wide bands of contrast in which the grain remained randomly oriented. When the contrast due to the variation in texture was first observed, the films were columnar-structured and the grain sizes in both the textured and un-textured regions were uniform, with a mean equivalent diameter of 54nm; no abnormal grains were observed. After additional annealing, abnormal (111) oriented grains appeared in some untextured regions of the film. The evolution of texture during grain growth and the influence of substrate morphology on texture formation will be discussed.

Calculation of the trapping force in a strongly focused laser beam

We present what is, to the best of our knowledge, the first detailed calculation of the reverse trapping force acting on a dielectric sphere when it is illuminated by a strongly focused laser beam. The calculation is carried out within the geometrical optics approximation. The phenomenon of laser trapping was discovered experimentally by Ashkin et al. [ Opt. Lett.11, 288 ( 1986)] and is of great practical interest in view of the possibility it offers for freely manipulating biological particles, such as viruses and bacteria, in a nondestructive manner. We support our calculations by a qualitative experiment that clearly shows the accessibility of the trapping effect in practice. We use, as an experimental improvement, an objective with a central field stop producing a conical dark field. This enhances the relative contribution from high-N.A. illumination and makes it easier to achieve optical trapping.

Hollow cone illumination—I. Dark-field electron microscopy of biological specimens

Fixed beam, hollow cone dark-field electron microscopy was set out to investigate the applicability to various biological specimens. The image quality of plain carbon films showed a significant improvement compared to those illuminated with tilted beam. Nucleic acid specimens, unstained or stained, were recorded with far better contrast than their stained counterparts. Apart from the fact that the dark-field micrographs were free from defocus-dependent granulation, microbiological specimens did not show features other than those already known from bright-field imaging. Specimens positively stained with heavy atom compounds, can provide complementary information to the negative staining technique employed in bright-field. Aldehyde-fixed material was successfully imaged in hollow cone dark-field. If sections were cut thin enough (≤ 40nm), post-fixation and staining was not required. In these specimens, dark-field micrographs revealed membrane-like structures not observed in bright-field.

Bright field and hollow cone dark field electron microscopy of palladium catalysts

SUMMARYThe usefulness of hollow cone dark field imaging for the study of Pd catalyst particles on γ‐alumina substrates is investigated. By varying the depth of the specimen in the objective lens field, different Bragg reflections can be conveniently selected. Typically, about 70% of the particles appear in either the 111 or 200 Bragg reflections. Rather more particles can be visible in bright field because of other Bragg reflections as well as absorption effects. Size distributions in bright field and dark field are compared.