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JM Cowley

Publications and source records attributed to JM Cowley.

6 recordsLinked to original sources
PubMed2000

Electron holography with atomic focusers

In a modified form of electron holography, as originally proposed by Gabor, a specimen illuminated by the focused, convergent beam of a scanning transmission electron microscope is followed by a thin crystal which acts as a periodic array of atomic focusers. Each of the broad diffraction spots of the crystal then contains a magnified image of the specimen with a resolution limit of 0.05 nm or less. The method is illustrated by images of crystal lattice planes and tungsten atoms in the diffraction patterns formed by crystals in the walls of carbon nanoshells.

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PubMed2000

Atomic-focuser imaging in electron nanodiffraction from carbon nanoshells

When nanodiffraction patterns are obtained by transmission through the top and bottom walls of near-spherical, hollow carbon nano-shells, using the focused probe of a STEM instrument, a graphitic crystal in one wall may act as an atomic focuser to produce high-resolution images of small regions of the other wall within the central beam and the diffraction disks of the nanodiffraction pattern. A theoretical analysis of the imaging process is given. Images showing one- and two-dimensional periodicities, with fringe spacings as small as 0.124 nm, and also images showing non-periodic features have been obtained from carbon nanoshells having diameters of the order of 100 nm.

Journal Article↗
PubMed1999

Moiré patterns in electron microscopy with atomic focuser crystals.

The periodic array of very fine cross-overs formed at the exit face of a thin 'atomic focuser' crystal, illuminated by a parallel electron beam, may be used to form moiré patterns with a specimen crystal such that the structure of the specimen crystal may be derived with a resolution of better than 0.5 Å. Computer simulations of the moiré pattern formation have been made for the simple idealized case of two parallel gold-like lattices having a 10% difference in lattice constant. Moiré images are shown for the case of a small objective aperture in the viewing electron microscope such that the individual crystal lattices are not resolved and for a larger objective aperture for which the individual crystal lattices are resolved and the intensity is measured at the positions of the atoms of the atomic focuser crystal. The latter case confirms the viability of the scheme for ultra-high-resolution imaging of general specimens by use of a thin-crystal periodic atomic focuser, as previously proposed.

Journal Article↗
PubMed1999

Simulations for imaging with atomic focusers.

The basis has been explored for the possible application of the various schemes that have been proposed for making use of the focusing properties of single heavy atoms, or rows of atoms extending through thin crystals in axial directions, for the attainment of ultra-high resolution in electron microscopy. Calculations are reported for the form of 200 keV electron beams channeled along rows of atoms through crystals and propagated in the vacuum beyond the crystals. The conditions for forming beams less than 0.05 nm in diameter have been established. Simulations of images having resolutions of this order are reported for the case that the specimen is placed at the Fourier image position beyond the exit face of a thin crystal and the transmission of the periodic array of ultra-fine beams, translated laterally by tilting the incident beam, may be observable using a conventional transmission-electron-microscopy (TEM) instrument.

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