Molecularly Specific Studies of the Frictional Properties of Monolayer Films:  A Systematic Comparison of CF₃-, (CH₃)₂CH-, and CH₃-Terminated Films

Abstract

The origin of frictional forces in self-assembled monolayers (SAMs) was investigated through systematic correlation of the frictional properties with the chemical structure/composition of the films. Atomic force microscopy was used to probe the frictional properties of the SAMs formed by the adsorption of methyl-, isopropyl-, and trifluoromethyl-terminated alkanethiols on Au(111) surfaces. The frictional properties of mixed monolayers composed of varying concentrations of the methyl- and trifluoromethyl-terminated thiols were also studied. Polarization modulation infrared reflection adsorption spectroscopy was used to measure the vibrational spectra of each of these monolayers and in turn to determine that each was characterized by a well-packed backbone structure. For these films, which differed only in the nature of the outermost chemical functionality, a substantial enhancement in the frictional response was observed for films with isopropyl- and trifluoromethyl-terminal groups and for mixed monolayers containing small concentrations of the trifluoromethyl-terminated component. These results strongly support the model that the difference in friction in such systems arises predominantly from the difference in the size of the terminal groups. Larger terminal groups in films of the same lattice spacing give rise to increased steric interactions that provide pathways for energy dissipation during sliding.

Construction of Various Self-assembled Films and Their Application as Lubricant Coatings

“…The superiority of covalent bonding can be reflected by comparing with another popular candidate for MEMS/NEMS lubricant of Langmuir-Blodgett (LB) film, which attaches to the substrate via weak van der Waal force. As expected, SAMs is found to be much more stable against shear stress and possesses better wear resistance as compared with LB film with similar composition and structures (Bliznyuk et al, 1998;Bushan et al, 1995;DePalma & Tillman, 1989;Kim et al, 1999;Overney et al, 1992;Peach et al, 1996;b,Tsukruk et al, 1996;Tsukruk, 2001.). As shown in Fig.…”

“…On the www.intechopen.com other hand, the strong surface dipoles in CF 3 -teminated monolayer would cause much higher attractive force between the AFM tip and the surface of SAMs, and eventually cause more energy loss to increase the friction (Houston, 2005). This size effect is also observed between the tail groups of -CH(CH 3 ) 2 and -CH 3 (Kim et al, 1999) as well as C60, phenyl and -CH 3 (Lee et al, 2001). …”

Shape Classification for Micro and Nanostructures by Image Analysis

“…Height and centring combination (HCC) at main peaks of X a region of well-centred points at a medium height. The HCC values are more significant in this region than they are at peaks located at (25,25) and (40,80). For example, the centroid located at (50, 50) has a HCC value equal to 4100.…”

Friction between solids

“…The effect of fluorination on friction was examined using an AFM by comparing the response of -CH 3 -and -CF 3 -terminated SAMs (Kim et al 1999;Houston et al 2005). The effect of -OH termination (Brewer & Leggett 2004) and -COOH termination (Houston & Kim 2002) on friction has also been examined.…”