Nano-structure formation driven by local protonation of polymer thin films

摘要

We report the creation of nano-structures via Dip Pen Nanolithography by locally exploiting the mechanical response of polymer thin films to an acidic environment. Protonation of cross linked poly(4-vinylpyridine) (P4VP) leads to a swelling of the polymer. We studied this process by using an AFM tip coated with a pH 4 buffer. Protons migrate through a water meniscus between tip and sample into the polymer matrix and interact with the nitrogen of the pyridyl group forming a pyridinium cation. The increase in film thickness, which is due to Coulomb repulsion between the charged centers, was investigated using Atomic Force Microscopy. The smallest structures achieved had a width of about 40 nm. Different control experiments support our claim that the protonation is the reason for the swelling and therefore the formation of the structures. Kelvin probe force microscopy measurements suggest the presence of counter ions which compensate the positively charged pyridinium ions. We investigated the influence of the water meniscus on the structure formation by varying the relative humidity in the range from 5% to 60% for different dwell times. The diffusion of protons and counter ions is humidity-dependent and requires a water meniscus.