Surface patterns of supramolecular materials

We report here on the two-dimensional patterns formed by supramolecular mat erials deposited from solution on oxidized silicon substrates. The supramol ecular materials studied are composed of mushroom-shaped nanostructures mea suring 2-5 nm in cross-section and approximately 7-8 nm in height. Two diff erent materials were studied, one containing nanostructures with a hydrophi lic phenolic base surface and the other containing a hydrophobic one with t rifluoromethyl groups. The substrates were exposed to solutions of these ma terials for a set induction time at a series of concentrations using a moto rized dipping apparatus. Samples were characterized by contact-angle measur ements and tapping-mode atomic force microscopy. We observed distinct patte rns as a function of concentration in phenolic supramolecular materials tha t interact favorably with the oxidized silicon surface. At low concentratio ns (0.01 wt %), the nanostructures form islands with uniform size of approx imately 0.02 mum, which have the height of a single nanostructure (7.2 nm). As concentration increases, a string-like morphology with uniform width is observed first, followed by a percolating texture. At yet higher concentra tions, the film transforms to a honeycomb morphology, but its height still remains equal to that of a single nanostructure. When interactions between the nanostructure and the surface are not favorable (i.e., between trifluor omethyl end groups and oxidized silicon), uniform height patterns are not o bserved. The distinct geometries are possibly the result of strong material -substrate interactions balanced by a repulsive force that could have elect rostatic origin. The extremely uniform thickness of the two-dimensional pat terns may originate in the hydrophobic and hydrophilic nature of opposite p oles of the nanostructures, thus suppressing vertical growth of the film.