On the formation of molecular terraces

Langmuir-Blodgett-Kuhn films of poly(amic acid) with azobenzene-chromophore side groups (azo-PAA) have been examined using atomic force microscopy (AFM). If films were deposited at specific conditions, i.e., at transfer pressures (pi = 10-26 mN/m) and at transfer rates (nu = 2-10 mm/min), unusual patterns were formed. While the first few transferred layers of azo-PAA exhibited uniform coverage of the underlying surface with a flat topography, successive layers form a pattern of parallel terraces with micrometer width, which are assembled in a pyramid-like structure. The height of the pyramidal steps is 1.6 nm, which corresponds to the thickness of an azo-PAA monolayer. Each even step of such a pyramid has a hydrophobic surface, and each odd step has a hydrophilic surface. The width of the terraces varies significantly from terrace to terrace; however, the width of an individual terrace remains constant over hundreds of micrometers. The orientation of the pyramidal terraces is governed by the film deposition process and is always perpendicular to the transfer direction.