L.Y. Chang

Low-dose aberration corrected cryo-electron microscopy of organic specimens

Spherical aberration (C(s)) correction in the transmission electron microscope has enabled sub-angstrom resolution imaging of inorganic materials. To achieve similar resolution for radiation-sensitive organic materials requires the microscope to be operated under hybrid conditions: low electron dose illumination of the specimen at liquid nitrogen temperature and low defocus values. Initial images from standard inorganic and organic test specimens have indicated that under these conditions C(s)-correction can provide a significant improvement in resolution (to less than 0.16nm) for direct imaging of organic samples.

Comparisons of linear and nonlinear image restoration.

Exit wave restoration using focus series of images has become a widely used technique to improve image resolution and interpretation. To understand the effects of the imaging approximations used, we have critically compared the specimen exit wave functions restored using the efficient linear Wiener filter, with a general nonlinear maximum likelihood method.

Effect of amorphous layers on the interpretation of restored exit waves

The effects of amorphous layers on the quality of exit wave restorations have been investigated. Two independently developed software implementations for exit wave restoration have been used to simulated focal series of images of <001> SrTiO3 with amorphous carbon layers incorporated. The restored exit waves have been compared both qualitatively and quantitatively. We have shown that amorphous layers have a strong impact on the quantitative measurements of atomic column positions, however, the error in the position measurements is still in the picometer range.

Observation of octahedral cation coordination on the {111} surfaces of iron oxide nanoparticles

High-resolution (scanning) transmission electron micrographs taken on both aberration corrected and uncorrected microscopes, and indirect reconstructed images of mixed phase magnetite—maghemite nanoparticles all show the presence of {111} facets that terminate with enhanced contrast. This enhanced contrast is shown to be a real effect caused by the presence of additional octahedrally coordinated iron cations occupying the {111} terminating layers of these nanoparticle surfaces.

Aberration corrected tilt series restoration

Aberration correction can be achieved using either direct electron optical correction or indirect image restoration. In the past focal series restoration has been applied to aberration corrected images in order to improve the quality of aberration correction and retrieve the complex specimen exit wavefunction. Image restoration can also be performed from a number of images with differing illumination tilts (a tilt series) instead of the more common focal series datasets where only the defocus value is varied. Here we apply tilt series image restoration to aberration corrected images and discuss the advantages of this approach. Preliminary results demonstrate the potential of this technique to provide interpretable structural information at resolutions beyond the axial information limit of the microscope.

HREM of metallized {111} iron oxide nanoparticle surfaces

Mixed phase Fe3O4 - γ-Fe2O3 (magnetite-maghemite) nanoparticles have been fabricated by colloidal routes. HR(S)TEM images, taken on both aberration corrected and uncorrected (S)TEMs, and indirect reconstruction of images of the nanoparticles show the presence of {111} facets that terminate with enhanced contrast. This extra contrast is shown to be real and is due to additional octahedral cations occupying the {111} surfaces.

New considerations for exit wavefunction restoration under aberration corrected conditions

Transmission electron microscopes (TEM) fitted with electron optical elements capable of compensating for the coherent aberrations up to 3rd order spherical aberration are becoming increasingly widely available. These instruments facilitate use of a wider range of imaging conditions where the compensatable aberrations are additional variables. For example, a negative spherical aberration and over focus imaging has been found to give increased contrast because the linear and non-linear imaging terms add constructively [1].

The benefits of statistical parameter estimation theory for quantitative interpretation of electron microscopy data

In principle, electron microscopy can provide accurate and precise numbers for the unknown structure parameters of materials under study. As an example, atomic column positions can be determined with accuracy and precision that is orders of magnitude better than the resolution of the microscope. This requires, however, a quantitative, model-based method. In our opinion, the maximum likelihood (ML) method is the most appropriate one since it has optimal statistical properties. This contribution aims to explain the basic principles and limitations of the method. Moreover its practical applicability and usefulness will be demonstrated using recent experimental examples.

Imaging Active Sites on Platinum Catalytic Nanoparticles Using Aberration-Corrected Electron Microscope

The active components of industrial heterogeneous catalysts are often small metallic particles, whose reactivity and selectivity depend on the presence of steps and kinks on their surfaces [1]. The roles of different active sites on the extended surfaces of model catalysts can be identified using techniques such as scanning probe microscopy [2]. However, little is known about atomic arrangements on the surfaces of commercial catalyst nanoparticles, as highlighted by a recent study of ammonia synthesis in Ru-catalysed reactions [3].