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 C 12 alkyl monolayers with different terminal groups ranging from hydrophobic (terminated by −CH 3 ) 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 SiO 2 /Si > −COOH Si > −COOCH 3,Si > −CH 3,Si ∼ −CH 3,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.