A scanning force microscopy study on the motion of single brush-like macromolecules on a silicon substrate induced by coadsorption of small molecules

Scanning force microscopy was applied to visualise the motion of single poly(butanoate-ethylmethacrylate)-graft-poly(n-butyl acrylate) molecules on silicon and SrTiO₃ substrates. Macromolecular mobility was induced by cyclic exposure of the wafers with the adsorbed brush-like macromolecules to water and alcohol vapours. Exposure to saturated alcohol vapour induced collapse of the adsorbed individual polymer chains while exposure to saturated water vapour promoted their extension. The characteristic times of both conformational changes were long enough that it was possible to visualise step-by-step the morphology transformation in situ by means of an environment-controlled scanning force microscope. Several successive collapse-decollapse cycles were recorded, and small diffusive shifts of the macromolecular position on the substrate were detected after each cycle. Manipulating and visualising single polymer molecules in situ and real time on a silicon substrate opens up new possibilities for the controlled structure formation in ultrathin polymer films. As shown on the sample of a faceted SrTiO₃ wafer, upon extension the brush-like molecules can crawl or extend along nanoscopic surface structures. Silicon can be structured both topographically and chemically at dimensions comparable to those of single polymer molecules with a variety of fabrication techniques ranging from well established conventional silicon micro- nano- machining to new tools constantly developed as dip-pen and nanoimprint lithography.