Quantification of High-Resolution Lattice Images and Electron Holograms

Progress towards the quantification of high-resolution electron microscopy and electron holograms has been achieved using digital acquisition with a slow-scan charge-coupled device (CCD) camera. Two applications are described: the precise measurement of lattice-fringe spacings and the determination of the mean inner potential. Lattice images can be characterized by a finite sum of two-dimensional sinusoids. A new method for measurement of the frequency, amplitude and phase of each sinusoid, based on an interpolation technique in reciprocal space, is presented. The method offers considerably higher precision for measurement of lattice fringes than the optical bench and is applicable to images recorded with an electron dose of less than 1 el / Å2 and specimen areas as small as 8 Å across. The attainable precision is dependent on specimen characteristics, electron dose and the size of the measured area, and ranges from 0.001 Å to 0.05 Å. An improved method has also been developed for measurement of mean inner potential using digital off-axis electron holograms from 90° crystal wedges. The value of (-14.21 ± 0.16) V obtained for GaAs represents the most accurate measurement yet reported for the mean inner potential

Nano-faceted stabilization of polar-oxide thin films : The case of MgO(111) and NiO(111) surfaces

Molecular beam epitaxy growth of polar MgO(111) and NiO(111) films demonstrates that surface stabilization of the films is achieved via the formation of neutral nano-faceted surfaces. First-principles modeling of the growth of polar MgO(111) films reveals that the growth does not proceed layer-by-layer. Instead, the Mg or O layers grow up to a critical sub-monolayer coverage, beyond which the growth of the next layer becomes energetically favorable. This non-layer-by-layer growth is accompanied by complex relaxations of atoms both at the surface and in the sub-surface, and leads to the experimentally observed surface nano-faceting of MgO and NiO (111) films through formation of neutral nano-pyramids terminated by {100} facets. These facets are limited in size by an asymptotical surface energy relation to their height; with the reconstruction being much more stable than previously reported surface terminations across a wide range of growth conditions. The termination offers access to lower coordinated atoms at the intersection of the neutral {100} planes whilst also increasing the surface area of the film. The unique electronic structures of these surfaces can be utilized in catalysis, as well for forming unique heterostructures for electronic and spintronic applications

Polar oxide interface stabilization by formation of metallic nanocrystals.

In situ x-ray photoelectron spectroscopy and ex situ transmission electron microscopy and diffraction studies of a model Fe3O4(111)/MgO(111) polar oxide interface exclude stabilization by interface faceting, reconstruction, or by formation of a continuous interfacial layer with altered stoichiometry, and uncover stabilization by dominant formation of metallic Fe(110) nanocrystals. The iron nanocrystals nucleate both at the interface and within the magnetite film and grow in a Nishiyama-Wasserman orientation relationship with a bimodal size distribution related to twinning. Minority magnetite nanocrystals were also observed, growing in the less polar (100) orientation than the magnetite (111) film. Electron transfer and bond hybridization mechanisms are likely at the metal/oxide and oxide/oxide interfaces and remain to be explored