Molecular growth and sub-molecular resolution of a thin multilayer of PTCDA on Ag(1 1 0) observed by scanning tunneling microscopy

The initial growth of thin films of 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) on a Ag(1 1 0) surface is investigated with a scanning tunneling microscope (STM) in ultrahigh vacuum (UHV). The first monolayer exhibits a surface induced commensurate structure, which is referred to as the brick-wall structure. Images are presented which show the structure of the first three layers within one scan frame. From the second layer onwards the layers arrange themselves in a different structure, similar to the crystalline structure of PTCDA in the (1 0 2)-plane. On top of the multilayer we are able to resolve sub-molecular features of the top molecules, where two different structures can be distinguished. In addition, we identify parts of the organic molecules, which are below the top layer, and the stacking order of the thin multilayer can be extracted.