Systematic Studies of the Frictional Properties of Fluorinated Monolayers with Atomic Force Microscopy:  Comparison of CF₃- and CH₃-Terminated Films

Abstract

Molecular-level insight into the frictional properties of fluorinated self-assembled monolayers (SAMs) was achieved by combining two recently developed techniques that operate at the subnanometer scale: control of the interfacial composition through molecular self-assembly and tribological measurements performed with the atomic force microscope. To explore the origin of frictional forces in fluorinated films, the frictional properties of two classes of alkanethiols adsorbed on single crystal gold were measured and compared. In these studies, films of equivalent chain length, packing density and packing energy, but different termination (methyl vs trifluoromethyl), were characterized and investigated. For these films, in which the only detectable difference was the outermost chemical structure/composition, a factor of 3 increase in the frictional response was observed in going from the hydrogenated to the fluorinated film. These results support the conclusion that chemical structure/composition alone plays an integral role in determining the frictional properties of an interface. We propose that the difference in friction arises predominantly from the difference in size of the methyl and trifluoromethyl groups.

Molecular Friction as a Tool to Identify Functionalized Alkanethiols

“…9 In addition, the nature of the supernatant solution, or the absence of it (i.e., ambient or vacuum), can influence the surface tension between end groups in a SAM and therefore also the friction. 17, 31 Friction mapping can thus provide valuable information about the composition and specific chemical [32][33][34][35][36][37] and biological 26,38 properties of a surface layer at the nanoscale.…”

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

“…However, although the surface energy of -CF 3 (12.9 mJ/m 2 ) is lower than that of -CH 3 (~24 mJ/m 2 ) (Bushan et al, 2005;Luengo et al, 1997), the fluorocarbon SAMs produce higher friction in AFM studies (Bushan et al, 2005;Kim et al, 1997;Peach et al, 1996;Houston et al, 2005). The unexpected higher friction is attributed to the larger size and higher electronegativity of the fluorine atom, which result in two major variations in the surface properties of SAMs (Kim et al, 1997). On one hand, the replacement of -CH 3 with larger tail groups of -CF 3 into the close-packed ( ) o 33 R 3 0 × lattice gives rise to increased surface steric interactions.…”

“…The SAMs with apolar terminal groups generally possess lower surface energy and relatively weak interaction between two sliding surfaces, which result in a lower adhesion and less energy loss leading to a lower friction force (Liu et al, 1996;Zhang et al, 2002). However, although the surface energy of -CF 3 (12.9 mJ/m 2 ) is lower than that of -CH 3 (~24 mJ/m 2 ) (Bushan et al, 2005;Luengo et al, 1997), the fluorocarbon SAMs produce higher friction in AFM studies (Bushan et al, 2005;Kim et al, 1997;Peach et al, 1996;Houston et al, 2005). The unexpected higher friction is attributed to the larger size and higher electronegativity of the fluorine atom, which result in two major variations in the surface properties of SAMs (Kim et al, 1997).…”

“…On one hand, the replacement of -CH 3 with larger tail groups of -CF 3 into the close-packed ( ) o 33 R 3 0 × lattice gives rise to increased surface steric interactions. During sliding, more energy is imparted to the film to overcome the consequent increased steric barriers and then results in higher friction (Kim et al, 1997;Peach et al, 1996). 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).…”

AFM topography and friction studies of hydrogen-bonded bilayers of functionalized alkanethiols

“…In addition to the topography, probing the lateral friction of the nanografted patches by the AFM tip provides insight in the composition and specific chemical properties of the SAM. 1,12,13,[15][16][17][18][19][20] In this communication we describe the characterization of layers of carboxyl-and amino-functionalized alkanethiols on gold in a 2-butanol or ethanol supernatant solution, and by combining nanografting and lateral friction measurements we will reveal the effect of intermolecular hydrogen bonding on the formation of stable monolayer and bilayer structures of these thiols.…”