BARRIER-HEIGHT IMAGING OF FATTY-ACID LANGMUIR-BLODGETT-FILMS

Behenic acid Langmuir-Blodgett films on graphite have been investigated by STM using the AC gap-modulation technique. Two significant differences were observed as compared to conventional topographic images. (1) The periodic contrast variation, which is superimposed on the structure of the behenic acid molecules in the lamellar arrangement, is greatly enhanced. This period corresponds to the width of 6 molecules. (2) The carboxyl groups appear featureless and cannot be distinguished, whereas in the topographic images they are the brightest signal and are clearly distinguished. The periodic variation of enhanced contrast corresponds to that of the positional relation between the adsorbed molecules and the underlying substrate. This means that the additional information of the interaction between the adsorbed molecules and the substrate can be obtained by the gap-modulation mode in STM.