Discrete Tomography of Ga and InGa Particles from HREM Image Simulation and Exit Wave Reconstruction.
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Discrete Tomography of Ga and InGa Particles from HREM Image Simulation and Exit Wave Reconstruction. a
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J. R. Jinschek , H. A. Calderon , K. J. Batenburg V. Radmilovic and Ch. Kisielowski . a
Ernest Orlando Lawrence Berkeley National Laboratory, National Center for Electron Microscopy, 1 Cyclotron Road MS 72R0150, Berkeley, CA 94720, U.S.A. b ESFM-IPN, Dept. de Ciencia de Materiales, 07730 Mexico D.F., Mexico c
Leiden University, Mathematical Institute, Leiden & CWI, Amsterdam, The Netherlands.
Abstract Low-resolution tomography requires recording images every few degrees. As a consequence, the sample is often degraded after such a procedure. However the required input can be reduced drastically by using knowledge about the position and the number of atoms in each atomic column. This concept has been tested in the present investigation where HREM image simulation (MacTempas) together with exit wave reconstruction (FEI Trueimage) have been performed. A cubeoctahedral nanoparticle is used for the simulation with different compositions i.e., pure solid Ga and In-Ga particles. Six different zone axes ([111], [ 1 1 1], [001], [110], [ 1 10], [011]) have been used and the parameters of an aberration corrected microscope (200kV, Cs = 0 mm, resolution = 0.5Å). The discrete grid data were determined by constructing a channeling map from the reconstructed exit wave images. In this special case only three projections [001], [110], [110] were sufficient to find a unique volumetric reconstruction, illustrating the potential of the method. The other projections were used for checking the solution. The comparison between the projected potentials (simulated input) and the final result shows that discrete tomography reconstructs the exact position of all 309 atoms and the three-dimensional shape of the nanocrystal. Introduction Determination of structure and composition has been the objective of many characterization techniques including electron microscopy (EM). However the spatial resolution offered by EM gives the additional advantage to determine local variations. The spatial resolution of modern electron microscopes is around 0.15 nm but it can be increased by using several techniques. Incoherent scanning transmission electron microscopy (STEM) has a resolution limit of 0.078 nm [1], high voltage electron microscopy can reach 0.089 nm [2], field emission gun transmission electron microscopy (FEG-TEM) coupled to image reconstruction has shown a maximum resolution of 0.085 nm [3]. Resolution can also be improved by using a spherical aberration corrected microscope [4]. All of these values are rather close to the instrumental limit (∼0.05 nm) determined by the width of the atom potential [5]. Nevertheless achieving such a spatial resolution represents only part of the solution. Determination of chemical composition at such a scale with atomic sensitivity as well as tomography are doubtless a much needed goal. Z contrast can produce microanalysis and imaging but the signal to noise ratio is very close to uni
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