What atomic resolution annular dark field imaging can tell us about gold nanoparticles on TiO(2) (110).
ABSTRACT Annular dark field scanning transmission electron microscopy imaging was recently applied to a catalyst consisting of gold nanoparticles on TiO(2) (110), showing directly that the gold atoms in small nanoparticles preferentially attach to specific sites on the TiO(2) (110) surface. Here, through simulation, a parameter exploration of the imaging conditions which maximise the visibility of such nanoparticles is presented. Aberration correction, finite source size and profile imaging are all considered while trying to extracting the maximum amount of information from a given sample. Comment is made on the role of the thermal vibration of the atoms in the nanoparticle, the magnitude of which is generally not known a priori but which affects the visibility of the nanoparticles in this imaging mode.
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ABSTRACT: The physical basis for using a probe-position integrated cross section (PICS) for a single column of atoms as an effective way to compare simulation and experiment in high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) is described, and the use of PICS in order to make quantitative use of image intensities is evaluated. It is based upon the calibration of the detector and the measurement of scattered intensities. Due to the predominantly incoherent nature of HAADF STEM, it is found to be robust to parameters that affect probe size and shape such as defocus and source coherence. The main imaging parameter dependencies are on detector angle and accelerating voltage, which are well known. The robustness to variation in other parameters allows for a quantitative comparison of experimental data and simulation without the need to fit parameters. By demonstrating the application of the PICS to the chemical identification of single atoms in a heterogeneous catalyst and in thin, layered-materials, we explore some of the experimental considerations when using this approach.Ultramicroscopy 07/2013; 133C:109-119. · 2.75 Impact Factor
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ABSTRACT: The idea of the method is to analyse a crystal lattice by creating a grid of quadrilaterals corresponding to repeated cells that are visible in the image. This approach combines image processing elements with a continuum field theory, to create a distortion-independent similarity measure that is used to select the most appropriate among possible lattice configurations. Subsequently, displacement and distortion fields are computed from individual cell positions. The method allows one to obtain these fields even for images where a periodic cell does not necessarily appear as a single dot of intensity in a high-resolution transmission electron microscopy (HRTEM) image, which results in a lower accuracy of commonly used approaches, namely geometric phase and peak finding. The results obtained from this method are verified quantitatively by comparison with known distortion tensor distributions and Burgers vector values on both simulated and real images.Journal of Microscopy 11/2011; 245(2):200-9. · 1.63 Impact Factor