Nanoscale frictional properties of mixed alkanethiol self-assembled monolayers on Au(111) by scanning force microscopy: Humidity effect

We present a quantitative study of the nanoscale frictional properties of uniform mixed self-assembled monolayers (SAMs) of dodecanethiol and 11-mereapto-1-undecanol on Au(111) as a function of surface composition using scanning force microscopy (SFM) at a variety of relative humidities. Surface properties are varied from hydrophobic to hydrophilic by adjusting the surface composition of mixed SAMs. Results show that the frictional properties of mixed SAMs are affected by both surface properties and relative humidities. At lower relative humidity, there is a clear relationship between surface composition and friction coefficient. Thus, it may be possible to determine the local surface composition of a mixed SAM from its friction coefficient measured by SFM under a controlled environment. At higher relative humidity or in water, the friction coefficient is not sensitive to changes in surface composition. Due to the significant effect of the scanning environment, one must be very careful when measuring and analyzing frictional properties by SFM in air.