Flow Induced Growth of Striped Alkane Monolayers.*

We report our observation of the growth of striped monolayer phases of alkanes when deposited from a solution under flow. AFM measurements show that the structure and morphology of dotriacontane ($n$-C$_{32}$H$_{66}$ or C32) films grown from solution depend sensitively on the flow direction over a SiO$_{2}$-coated Si(100) substrate. The C32 film exhibits one or two layers adjacent to the SiO$_{2}$ surface in which the molecules are oriented with their long axis parallel to the interface followed by a striped monolayer of perpendicularly oriented molecules. The stripes form along the direction of solution flow with typical dimensions of a few micrometers wide and a few hundred micrometers long, depending on the solution concentration. A striped morphology is also observed for C29 grown under similar conditions. Grazing incident-angle x-ray diffraction measurements indicate that the C32 stripes are crystalline and can be indexed by a rectangular unit cell. We offer some speculations on the origin of the striped morphology.