Crystallographic Processing of Scanning Tunneling Microscopy Images of Cobalt Phthalocyanines on Silver and Graphite
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Crystallographic Processing of Scanning Tunneling Microscopy Images of Cobalt Phthalocyanines on Silver and Graphite P. Moeck1*, J. Straton1, M. Toader2, and M. Hietschold2** 1 Nano-Crystallography Group, Department of Physics, Portland State University, Portland, OR 972070751, U.S.A, * [email protected], ** [email protected] 2 Institute of Physics, Chemnitz University of Technology, D-09126 Chemnitz, Germany ABSTRACT Monolayers of cobalt phthalocyanine (CoPc) and fluorinated cobalt phthalocyanine (F16CoPc) on silver (111) and on highly (0001) oriented pyrolytic graphite (HOPG) were imaged with a scanning tunneling microscope (STM) at cryogenic temperatures (around 30 K) at Chemnitz University of Technology. Domains of regular arrays with periodicity in two dimensions (2D) and a variety of plane symmetries were observed. Crystallographic image processing (CIP) was used to quantify deviations from the plane symmetry groups and to obtain symmetrized versions of the content of the average unit cells of some of these arrays. Conclusions on the point symmetry of the CoPc and F16CoPc molecules within the arrays were drawn.
INTRODUCTION Crystallographic image processing originated some 40 years ago with the electron crystallography community. Rather than solving crystal structures of unknowns on the basis of the weak phase object approximation of high resolution transmission electron microscopy [1], CIP has recently been used to “improve” images that were recorded in either the (atomic resolution) parallel illumination [2] or the Zcontrast scanning probe [3] modes of transmission electron microscopy. Such improvements concerned the visibility of individual atoms and of groups of atoms which were otherwise obscured by the imperfections (and signal to noise ratio) of the imaging process. We present here the application of CIP to scanning tunneling microscopy images of 2D periodic array domains of (nominal) monolayers of CoPc and F16CoPc on Ag (111) and HOPG. As a result of reading this paper, other members of the scanning probe microscopy (SPM) community may become interested in using these techniques in their own surface science work. Since a comprehensive introduction of CIP for scanning probe microscopy (SPM) will soon be available in open access [4], our theory section will be very brief.
BRIEF DESCRIPTION of CIP for SCANNING PROBE MICROSCOPY Having its roots in crystallography, the essence of CIP is averaging over many unit cells of a 2D periodic array in order to derive the content of one unit cell at a higher signal to noise ratio. For computational efficiency, this averaging is performed in Fourier space. The Fourier synthesis of the averaged Fourier coefficients of the SPM image intensity leads to the CIP processed (or symmetry enforced) SPM image in direct space. The well known Fourier filtering technique of SPM [5] can be considered the simplest form of CIP. That technique averages only over the translation symmetry in a 2D periodic array and is equivalent to the “CIP enforcing” of plane symmetry p1.
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