Study of aged Langmuir-Blodgett monolayers by AFM and ToF-SIMS mapping

Langmuir-Blodgett (LB) monolayer samples of stearic acid transferred on hydrophilic plasma-oxidized silicon surfaces in the presence of barium chloride have been studied by combined atomic force microscopy (ABM) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) mapping. By ageing, these samples develop holes of increasing size, and after some weeks these holes are large enough to be mapped by ToF-SIMS (lateral resolution similar to 200 nm) but still in the domain of AFM imaging. High mass resolution ToF-SIMS spectra from such surfaces display Ba, Cl, and stearate-related peaks, as well as substrate-related peaks. Chemical imaging by ToF-SIMS shows that barium, chlorine and stearate maps are correlated with each other and complementary to the signals from the substrate. Height image and interaction force (adhesion and friction) maps have been obtained by AFM on this chemically well-characterized system and show that it is possible to discriminate the organic film regions from the uncovered substrate on the basis of different interaction forces. In addition, the measurements allow us to state that film and substrate have good homogeneity. Furthermore, the interaction force images can be used to follow the early stages of the evolution of these systems, which are not accessible with other chemical mapping techniques. Copyright (C) 1999 John Whey & Sons, Ltd.