PREPARATION, STRUCTURE, AND MECHANICAL STABILITY OF ALKYLSILANE MONOLAYERS ON MICA

The preparation, structure, and mechanical stability of self-assembled monolayers formed by octadecyltriethoxysilane (OTE) on mica have been studied by atomic force microscopy. The nanometer scale morphology of the films (3-D clusters, pinholes, etc.) is compared for various prep aration methods and correlated with their macroscopic wettability. Hig h-resolution images of the atomically smooth monolayer (1.5 Angstrom r oot mean square roughness) reveal the existence of only short range or der. By applying a load above 10 nN with sharp Si3N4 tips (radius < 30 0 Angstrom), the film could be removed leaving 25 Angstrom deep holes. Using d the same tip, thiol monolayers on gold could be displaced at loads approximate to 5 nN, although, in this case, the displacement wa s reversible. In contrast to the case of OTE, films formed by octadecy ldimethylmethoxysilane (ODMS) showed the presence of only 3-D clusters with poor adhesion to the mica substrate. On the basis of these resul ts, we conclude that the mechanical strength of the films formed by OT E is due to siloxane cross-linking between molecules rather than to ch emical bonding to the mica substrate.