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A Bloch wave analysis is used to investigate high-resolution electron microscope (HREM) imaging of crystals containing atomic displacements due to strain. In the absence of interband scattering, the shifts of peaks and troughs in the image will correspond to the displacements of the atoms in the exit surface. Interband scattering will shift the image peaks away from the actual atom positions and modify the apparent magnitude of the displacement identified by the observed image peak positions. By considering the case of seven-beam imaging of a cubic crystal aligned along a 〈111〉 axis, it is shown that the symmetry of the Bloch waves leads to selection rules for the interband scattering, similar to those seen for dipole electron excitations in atoms. It is also shown that, to first order, no intraband scattering can occur. [ABSTRACT FROM AUTHOR]

Weak-beam image simulations have been carried out using the CUFOUR programme of a ] edge dislocation in γ-Ti-50 wt% Al, dissociated into three <112] partial dislocations bounding superlattice intrinsic stacking-fault (SISF) and superlattice extrinsic stacking-fault (SESF) ribbons of equal width. The simulations included one of the two core configurations of the composite Shockley partial dislocation found by Kumar et al . to explain their observed image contrast, and two others that involve dissociation into stair-rods. These latter simulated images cannot be distinguished from those for the model proposed by Kumar et al . for the composite Shockley dislocation and therefore account for the observed image contrast equally well. The claim of these workers that configurations consisting of stair-rod dislocations can be ruled out is not substantiated. The core structures involving sessile stair-rod dislocations explain the locked nature of the edge dislocation. The values found by Kumar et al . for the true separation between the pairs of partial dislocations have been used to calculate (on anisotropic elasticity theory) the SISF and SESF energies to be 123 ± 10 mJ m -2 . [ABSTRACT FROM AUTHOR]

Well-aligned nanoneedles are found to form in a Fe/GaAs (001) epitaxial thin film when irradiated with a Ga+ ion beam normal to the sample surface at an ion dose of 3×1017/cm2 in a focused ion beam. These nanoneedles, with their axes perpendicular to the original sample surface, are composed of GaAs nanorods with Fe nanoislands situated at the top. The GaAs nanorods, formed from the GaAs substrate, are crystalline and in the same crystal orientation as the GaAs substrate. The crystalline nature of the Fe thin film is disrupted by the ion sputtering, becoming nanocrystalline Fe islands. It is the thin Fe layer that is responsible for the formation of the nanoneedles. The ion sputtering of the Fe layer roughens the sample surface due to the uneven sputtering rate of the Fe layer. The removal of the Fe layer exposes the substrate to the incident ions, which then sputter the substrate leading to the formation of the nanoneedles. [ABSTRACT FROM AUTHOR]