Measurements of Friction and Adhesion for Alkyl Monolayers on Si (111) by Scanning Force Microscopy
In this study, quantitative nanoscale frictional and pull-off forces were measured by scanning force microscopy for various C12 alkyl monolayers with different terminal groups ranging from hydrophobic (terminated by −CH3) to hydrophilic (terminated by −COOH) on atomically flat Si(111) surfaces at a variety of relative humidities. These monolayers were prepared and characterized by techniques such as scanning probe microscopy, contact angle goniometry, and X-ray photoelectron spectroscopy. The magnitude of the frictional force was found to decrease in the order SiO2/Si > −COOHSi > −COOCH3,Si > −CH3,Si ∼ −CH3,Au. On comparison to the native oxide silicon surface, the friction coefficient decreases dramatically when the surface is coated with different monolayers: by 20 times for hydrophobic monolayers and by 3 times for hydrophilic monolayers. Pull-off forces are generally larger for hydrophilic than for hydrophobic surfaces. For hydrophobic surfaces, frictional and pull-off forces are not sensitive to relative humidity. For hydrophilic surfaces, however, friction coefficient decreases while pull-off force increases with increase of relative humidity.
Liu, Lingyan, "Measurements of Friction and Adhesion for Alkyl Monolayers on Si (111) by Scanning Force Microscopy" (2002). Chemical and Biomolecular Engineering Faculty Research. 211.