Atomic Force Microscopy Measurements of Topography and Friction in Dotriacontane Films Adsorbed on SiO$_2$

We are continuing Atomic Force Microscopy (AFM) studies of the structure, morphology, and lateral frictional force of dotriacontane ($n$-C$_{32}$H$_{66}$ or C32) films deposited from a heptane solution onto SiO$_{2}$-coated Si(100) wafers. Step heights observed in the topographic images are consistent with layer thicknesses inferred from synchrotron x-ray$^{2}$ reflectivity scans; i.e., the first one or two layers of the film grow with the long molecular axis parallel to the surface followed by a partial layer containing perpendicular molecules. Our AFM results extend our x-ray studies in several ways: besides single perpendicular layers, we observe islands consisting of multiple perpendicular layers. We are also able to determine the spatial extent of the perpendicular layers and the location with respect to them of preferentially oriented bulk particles that nucleate at higher coverages. The frictional forces measured correlate with topographic features: we measure the same force on top of the perpendicular layers as on top of the bulk particles. This force is smaller than that on the parallel layers and the bare SiO$_{2 }$surface. $^{2}$H. Mo \textit{et al}., Chem. Phys. Lett. \textbf{377}, 99 (2003).