Biomedical subjects

J M Cowley

Publications and source records attributed to J M Cowley.

11 recordsLinked to original sources

PubMed1992

Stepped surfaces of sapphire (alpha-Al2O3) with low Miller indices.

Oxygen-annealed surfaces of sapphire with low Miller indices ((0001), [1010], [1120], [1011]) have been studied in both transmission electron microscopy (TEM) and reflection electron microscopy (REM) configurations. The significance of REM diffraction conditions for the determination of the nature of the step heights is discussed. The relationship between the TEM and REM images is explained. The structural features are those that might be expected from considerations of the atom arrangement in the low Miller index planes. The structural features on the surfaces varied with respect to annealing temperature and surface condition. Thermally stable structures that might appear from consideration of the equilibrium-annealing temperature are proposed.

Aluminum Oxide↗

PubMed1992

Observation of double line contrast in surface imaging.

The double line contrast of a single-atom height step observed in surface imaging for a single crystal in reflection electron microscopy is studied under a variety of experimental conditions. It is suggested that this abnormal contrast is directly associated with the dynamical electron diffraction process. The behavior of the double line contrast is closely related to the order of the Bragg reflected beam, and can be observed mostly under one of the two commonly cited resonance conditions. This phenomenon clearly reveals the differences in the surface imaging for various resonance conditions.

Crystallography↗

PubMed1992

Preparation and characterization of MgO surfaces by reflection electron microscopy.

We have employed several different methods to prepare (100) and (111) surfaces of MgO crystals. (100) surfaces prepared by simple cleaving give good reflection high energy electron diffraction (RHEED) patterns and surfaces with a high density of coarse steps. Chemical polishing of this surface results in a roughening of the topography whilst annealing in oxygen considerably smoothens the surfaces although they appear to be contaminated. Under certain conditions we find that the MgO crystals will cleave along the (111) plane. Both cleaved and mechanically polished (111) surfaces are atomically flat and reconstructed after oxygen annealing.

Crystallography↗

PubMed1992

Characterization of the annealed (0001) surface of sapphire (alpha-Al2O3) and interaction with silver by reflection electron microscopy and scanning reflection electron microscopy.

Annealed (0001) surfaces of single-crystal sapphire (alpha-Al2O3) rod have been studied in the electron microscope using reflection electron microscopy (REM), scanning reflection electron microscopy (SREM), and reflection high energy electron diffraction (RHEED). Annealed surfaces of (0001) sapphire are vicinal and characterized by close-packed (0001)-oriented terraces separated by faceted multiple-height steps, with edges parallel to energetically preferred low-index directions (less than 1010 greater than and less than 1120 greater than). These structural features are not seen on cleaved surfaces or polished surfaces treated at temperatures less than 1,250 degrees C. Oxygen-annealing produces clean surfaces which prove useful for investigating the interaction of deposited metals with the (0001) sapphire. Both REM and SREM (with microdiffraction spots) techniques have been used to observe fine structure of flat Ag islands on the scale of 1-100 nm on the (0001)-oriented terraces as well as aggregations at the steps. A preliminary result on interaction with Cu is also included.

Aluminum Oxide↗

PubMed1989

Observation of microdiffraction patterns with a dedicated STEM instrument.

A two-dimensional detector system, designed for the observation and recording of microdiffraction patterns formed in an HB 5 scanning transmission electron microscopy (STEM) is described and discussed. Possibilities are described and demonstrated for the simultaneous or successive recording of microdiffraction patterns from regions of diameter 3 A or more, bright- or dark-field STEM images, EELS spectra, secondary electron images, and in-line holograms. Applications of the system have been made to studies of catalyst particles, reflection-mode imaging of bulk surfaces, and image reconstruction from microdiffraction patterns obtained from each point of a STEM image.

Microscopy, Electron, Scanning↗

PubMed1987

Effects of the coherence of illumination on electron microdiffraction pattern intensities.

Microdiffraction is capable of revealing the local structure within an area of the specimen consisting of only a few, or a few tens of, unit cells. However, the extent to which the diffraction pattern intensities can show the local structure depends strongly on the coherence of the illumination. If the coherence width of the illumination is smaller than the diameter of the electron probe at the specimen level, the details within the diffraction spots, which indicate deviations of the local structure from the periodicity of the crystal, will be lost. The differences in the amount of spot splitting observed in microdiffraction patterns from out-of-phase domain boundaries, observed with two instruments, are attributed to differences in the effective source sizes.

Computer Simulation↗

PubMed1979

Phase and amplitude contrast in electron microscopy of stained biological objects.

For biological objects negatively stained with heavy atom material, electron microscope images show best contrast for image detail on the scale of 10--20 A when a small objective aperture is used. In images taken under the optimum phase contrast imaging conditions of Scherzer, the required image detail is lost in unwanted noise. Both of these conditions may be described in terms of phase contrast imaging for a thin phase object. Calculations of image intensities and noise are reported for a model object consisting of heavy and light atoms randomly distributed to simulate a negatively stained protein molecule. The results are consistent with experimental observations.

Fourier Analysis↗

PubMed1976

The extension of scanning transmission electron microscopy by use of diffraction information.

The information contained in the intensity distribution of the convergent beam electron diffraction pattern, produced in the detector plane for each incident beam position in a scanning transmission electron microscope, may be used to enhance the resolution of the microscope or else to decrease the electron irradiation of the specimen involved in deducing information at any particular level of resolution. The structural information concerning the specimen may be obtained, for example by interpretation of the Patterson function using image-seeking methods. The statistical error involved due to the finite number of electrons available may be derived by considering the efficiency of use of the information contained in Patterson function peaks. The most efficient means for using the available information appears to be that based on information theory concepts, which employ the integral over the product of the observed diffraction pattern intensity and the intensity calculated for known or postulated groupings of atoms. The reduction in radiation damage possible by use of this method, relative to that involved in the separate imaging of individual atoms, may be by a factor of approximately n, where n is the number of atoms in the known or postulated group being sought.

Electrons↗

PubMed1976

Scanning transmission electron microscopy of thin specimens.

The intensity distributions in bright field and dark field images of thin specimens obtained in scanning transmission electron microscopy are evaluated in terms of the phase of object approximation for coherent and partially coherent illumination. Previous treatments, based on a weak phase object approximation are shown to contain unwarranted assumptions in some cases, resulting in predictions of limited validity. The probable errors due to the neglect of higher order terms in power series expansion and in the assumption that the dark-field signal from an annular detector is proportional to the total elastic scattering are evaluated. Questions of optimizing the signal intensity and contrast are examined by considering the detector configuration in relationship to the form of the convergent beam diffraction pattern formed in the detector plane and the implications for the use of diffraction pattern information to enhance the imaging process are considered.

Lighting↗

PubMed1975

Aperture contrast in thick amorphous specimens using scanning transmission electron microscopy.

The contrast observed in thick amorphous specimens using a scanning transmission electron microscope (STEM) can be considerably improved by the use of an optimum collector aperture angle. The size of this angle can be calculated by considering the variation of electron current transmitted through the specimen as a function both of the specimen thickness and of the angle of collection subtended at the specimen. Typically these calculations predict optimum angles to be several times the half-width of the elastic scattering distribution, often 10(-1) rad or more. Observations of biological sections of up to 2 micron in thickness using scanning attachments of commercial transmission microscopes have verifie these results at beam voltages of 50, 100 and 200 kV. Wide angle convergent beam diffraction patterns were used to give accurate values of the effective angles represented by the various collector apertures. Once the linearity of the detector-amplifier system had been established, operation in a line modulation mode enabled quantitative measurements to be made of the image contrast. Such measurements also offer a quick effective method of comparing electron beam penetrations.

Microscopy, Electron, Scanning↗

PubMed1975

Magnification variations in reflection electron microscopy using diffracted beams.

In reflection electron microscope images obtained using electron beams diffracted at a small angle from the surfaces of bulk specimens, it is observed that the magnification in directions almost parallel to the incident beam appears to increase rapidly with distance from the in-focus position in both directions. An explanation for this effect is offered in terms of the curvature of the lines of energy flow around the cross-over of the electron beam formed by the condenser action of the fore-field of the objective lens.

Copper↗