Atomic Force Microscopy and X-ray Studies of Three-Dimensional Islands on Monolayers

The structure and morphology of the three-dimensional aggregates of arachidic acid were studied by atomic force microscopy (AFM) after transfer onto a silicon wafer and by synchrotron grazing incidence X-ray diffraction (GID) at the helium/water interface. The 3D aggregates on the wafer observed by AFM are islands of granulelike and platelike shapes. The plate islands consist of terraces of different heights. The crystal structure of the plate islands was determined through molecular resolution by AFM. They have the C-crystal structure of arachidic acid at the beginning of the relaxation. During further relaxation the B-crystal structure becomes apparent. The islands with B-crystal structure dominate at the end of the relaxation process. Regions with a disordered molecular structure are found beside the ordered crystal structures. In all ordered regions of a single island the same crystal structure is observed. Using GID, the B- and C-crystal structures of the 3D aggregates of arachidic acid were also found at the helium/water interface. No peaks of any crystal structure are observed until 20% of the monolayer material is transformed into 3D aggregates. Then first the peaks of the B-crystal structure appear, which increase with the relaxation time. Additionally small peaks corresponding to the C-crystal structure are found.