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.