Submolecular features of epitaxially grown PTCDA on Cu(111) analyzed by force field spectroscopy.

Submolecular features of epitaxially grown 3,4,9,10-perylenetetra-carboxylic-dianhydride (PTCDA) on Cu(111) are resolved in non-contact atomic force microscopy topography scans in ultrahigh vacuum. While molecules in the first layer above the Cu substrate are depicted as featureless ovals, the second layer molecules show an intramolecular structure with a height corrugation of up to 40 pm. Force field spectroscopy experiments with submolecular resolution show that the tip-molecule forces differ significantly on the first and second layer molecules. Possible contributions to these force differences from mechanical deformations of the molecules as well as the internal charge density distribution are discussed.