ThreeDimensional Atomic Imaging by Electron Holography
Abstract
We have presented the principles of threedimensional atomic imaging by electronemission holography. Unlike conventional methods which use the HelmholtzKirchhoff integral theorem, we invert photoelectron and Augerelectron diffraction patterns to reconstruct threedimensional images of surface and interface atoms by threedimensional Fourier transformation. Direct structural information of near neighbor atoms can be obtained. The structure so determined provides a useful starting point for refinement by diffraction methods, thus avoiding the cumbersome trialanderror process. With the phaseshift correction, the smallwindow energyextension process (SWEEP) is provided for forward scattering electrons and backscattering electrons. This method extends the usable range in phase space for three dimensional image reconstruction. As an extension of the SWEEP method, we introduce a method for spatially resolved imaging of energydependent photoelectron diffraction (SRI EDPD). EDPD spectra are individually Fourier transformed to threedimensional vector space. The complex transformed intensities are summed over a span of phi angles or over a span of polar angles. The images are, respectively, well resolved in the radial and azimuthal directions, or in the radial and polar directions. The intersections of these realspace maps fix the atomic coordinates. Furthermore, a variableaxis SWEEP method is introduced to eliminate the angular anisotropies in the source wave and the scattering factor. We show that within the small angular cone, the anisotropy of the source wave towards an atom is cancelled by that towards the detector. Also, the cone only samples the flat part of the slope of the scattering factor's phase. The remaining shift in the image position can be quantitatively corrected for the nearneighbor atoms. In this dissertation, we demonstrate image reconstruction for many systems with simulated and experimental data.
 Publication:

Ph.D. Thesis
 Pub Date:
 1994
 Bibcode:
 1994PhDT.......224L
 Keywords:

 SWEEP;
 Physics: Condensed Matter; Physics: Atomic