Here we present the first direct observation of the atomic structure of threading dislocation cores in hexagonal GaN. Using atomic-resolution Z-contrast imaging, dislocations with edge character are found to exhibit an eight-fold ring core. The central column in the core of a pure edge dislocation has the same configuration as one row of dimers on the {10-10} surface. Following recent theoretical work, it is proposed that edge dislocations do not have deep defect states in the band gap, and do not contribute to cathodoluminescence dislocation contrast. On the other hand, both mixed and pure screw dislocations are found to have a full core, and full screw dislocation cores were calculated to have states in the gap.

We show that in the limit of a large objective (probe-forming) aperture, relevant to a spherical aberration corrected microscope, the Z-contrast image of a zone-axis crystal becomes an image of the 1s Bloch states. The limiting resolution is therefore the width of the Bloch states, which may be greater than that of the free probe. Nevertheless, enormous gains in image quality are expected from the improved contrast and signal-to-noise ratio. We present an analytical channeling model for the thickness dependence of the Z-contrast image in a zone-axis crystal, and show that, at large thicknesses, columnar intensities become proportional to the mean square atomic number, Z2.

A solution to the phase problem of electron diffraction is described which allows an aberration-free atomic resolution image of Si (110), showing the expected dumbbell contrast, to be reconstructed at a resolution of 3 times the point resolution and 2.5 times the information limit of the scanning transmission electron microscope (STEM) used. The data set required consists of coherent microdiffraction patterns recorded as a function of illuminating probe position and the method of image reconstruction beyond the conventional resolution limits using this data set is described. Using the inherent redundancy in the experimental data set, the accuracy of the reconstructed image is examined and the experimental imperfections that affect it are identified. It is found that aperture charging, compounded by distortions in the detection system, are the major sources of error. As an additional application of this method of phase retrieval, the diffracted-beam phases of electrons that have lost energy by exciting a plasmon are compared with those of elastically scattered electrons in a specimen of graphite. Within the limits of this approach, it is found that there is no difference in the beam phases, supporting the view that electrons that have undergone multiple elastic and inelastic scattering dominate the plasmon-loss scattering at higher angles.

Zirconium-based alloys are used in water-cooled nuclear reactors for both nuclear fuel cladding and structural components. Under this harsh environment, the main factor limiting the service life of zirconium cladding, and hence fuel burn-up efficiency, is water corrosion. This oxidation process has recently been linked to the presence of a sub-oxide phase with well-defined composition but unknown structure at the metal–oxide interface. In this paper, the combination of first-principles materials modeling and high-resolution electron microscopy is used to identify the structure of this sub-oxide phase, bringing us a step closer to developing strategies to mitigate aqueous oxidation in Zr alloys and prolong the operational lifetime of commercial fuel cladding alloys.

We demonstrate that the aberration-corrected scanning transmission electron microscope has a sufficiently small depth of field to observe depth-dependent atomic displacements in a crystal. The depth-dependent displacements associated with the Eshelby twist of dislocations in GaN normal to the foil with a screw component of the Burgers vector are directly imaged. We show that these displacements are observed as a rotation of the lattice between images taken in a focal series. From the sense of the rotation, the sign of the screw component can be determined.