Analysis of the three-dimensional structure of a small crystallite by scanning tunneling microscopy:Multilayer films of 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) on Cu(110)

The growth of 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) on Cu(110) has been investigated by STM in the range of a few up to more than 20 molecular layers. It is found to grow in the Stranski-Krastanov mode leading to well-ordered mesa-type crystalline grains of different structure and orientation relative to the substrate. By operating a scanning tunneling microscope at a low tunneling current of 2 pA, we were able to resolve the individual molecules on the at top of the crystallites as well as on the facets descending to the substrate. As will be shown, this offers the unique possibility to determine unambiguously the three-dimensional structure. For a given grain a detailed analysis will be presented yielding the unit cell and the relative displacement between subsequent layers.