Nanoscale Friction Switches: Friction Modulation of Monomolecular Assemblies Using External Electric Fields

This paper presents experimental investigations to actively modulate the nanoscale friction properties of a self-assembled monolayer (SAM) assembly using ail external electric field that drives conformational changes in the SAM. Such friction switches have widespread implications in interfacial energy control in micro/nanoscale devices. Friction response of a low-density mercaptocarboxylic acid SAM is evaluated using an atomic force microscope (AFM) in the presence of a DC bias applied between the sample and the AFM probe tinder a nitrogen (dry) environment. The low density allows reorientation of individual SAM molecules to accommodate the attractive force between the -COOH terminal group and a positively biased surface. This enables the surface to present a hydrophilic group ora hydrophobic backbone to the contacting AFM probe depending upon the direction of the field (bias). Synthesis and deposition of the low-density SAM (LD-SAM) is reported. Results from AFM experiments show ail increased friction response (up to 300%) of the LD-SAM system in the presence of a positive bias compared to the friction response in the presence of a negative bias. The difference in the friction response is attributed to the change in the structural and crystalline order of the film in addition to the interfacial surface chemistry and composition presented upon application of the bias.