Tomographic Imaging of Nanocrystals by Aberration-Corrected Scanning Transmission Electron Microscopy
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Tomographic Imaging of Nanocrystals by Aberration-Corrected Scanning Transmission Electron Microscopy Klaus van Benthem, Yiping Peng, Stephen J. Pennycook Oak Ridge National Laboratory, One Bethel Valley Road, Oak Ridge, TN 37831-6031, USA
ABSTRACT In aberration corrected scanning transmission electron microscopy, the depth of focus is of the order of a few nanometers, so that the three-dimensional shape of nanocrystals could so far not be determined with atomic resolution. Here we show that with the assistance of image simulations it is possible to achieve atomic-scale information in the depth direction by analyzing a through-focal series where the number of atoms in most columns can be determined by Zcontrast simulations. The error in this analysis is about two atoms in the thickest regions, and less in thinner regions.
INTRODUCTION Three-dimensional microscopy at atomic resolution has been a long-term goal of materials science. Transmission electron microscopy is known to produce two-dimensional projections of real 3D structures. Recently, it became possible to explore three-dimensional nanostructures by applying tilt-series electron tomography [1-3] or atom-probe microscopy [4]. In scanning transmission electron microscopy (STEM), one benefit of aberration correction is that the increased probe aperture angle results in a decreased depth of focus [5]. It thus becomes possible to focus on different depths within a sample, and a through-focal series now becomes a throughdepth series which can be recombined into a 3D data set. van Benthem and co-workers [5] recently showed that sub-Angstroem lateral resolution and nanometer vertical resolution can be used to localize individual dopant atoms in semiconductor devices. Here, we show how a combination of this technique with image simulations can be used to determine the threedimensional shape of crystals with atom-by-atom accuracy.
EXPERIMENTAL In this study we used commercially available TEM specimens consisting of small gold islands on a perforated carbon film, with areas of graphitized carbon [6]. Series of 41 simultaneous annular dark-field (ADF) and bright-field (BF) images were recorded with an aberration-corrected VG Microscopes HB603U dedicated STEM at varying defocuses with 1 nm increment. Intensities in the recorded ADF images are roughly proportional to the square of the atomic number of the scattering atom species, resulting in an atomic number (Z) contrast. STEM can also provide a phase contrast BF image through the use of a small axial collector aperture (equivalent through reciprocity to the condenser aperture of a conventional transmission electron microscope). The lateral resolution was evaluated to be 0.08 nm, resulting in a depth of focus of about 4 nm when using a 23 mrad illumination semi-angle.
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For the image acquisition, gold particles were chosen which showed a low indexed zone-axis parallel to the optical axis of the microscope. Electron beam currents of the order of 20 pA were used. Each frame of the recorded image stacks w
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