DEPENDENCE OF THE MEASURED MONOLAYER HEIGHT ON APPLIED FORCES IN SCANNING FORCE MICROSCOPY

The dependence of the measured thickness of Langmuir-Blodgett monolaye rs of various amphiphiles on the force exerted by the scanning tip was investigated with scanning force microscopy, both in the contact and the tapping mode. There is clear evidence for the dependence of the ap parent film height on the applied force: at first, the measured film t hickness decreases rapidly with increased force; then it stays constan t forming a plateau region before it drops again and reaches a minimum value of a few angstroms. Only in the contact mode by minimized force the correct film thickness could be measured. This qualitative curve can be obtained from all kinds of monolayers investigated here. The ex act shape of height-force curve is determined by the properties of the molecules and the preparation conditions. A possible explanation for this behavior is given by a molecular model based on gauche defects. ( C) 1998 Elsevier Science S.A. All rights reserved.