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 -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 similar to -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.