Lateral Force Microscopy Study of the Frictional Behavior of Self-Assembled Monolayers of Octadecyltrichlorosilane on Silicon/Silicon Dioxide Immersed in n-Alcohols

Abstract

Lateral force microscopy (LFM) was used to evaluate the frictional behavior of surfaces modified with self-assembled monolayers (SAMs) and immersed in n-alcohols (CH3(CH2)xOH where x ) 0-8,11) as a function of applied normal load, sliding velocity, and solvent chain length. SAMs were formed from octadecyltrichlorosilane (OTS) on silicon/silicon dioxide substrates. The objective was to investigate how the solvent environment affected the frictional behavior of OTS and to characterize the effectiveness of OTS as a boundary lubricant in liquid environments for applications such as fluidic self-assembly and microelectromechanical devices. Three characteristic frictional regimes were observed at low, intermediate, and high loads. Maxima as a function of the sliding velocity appeared in the frictional forces for intermediate applied normal loads of ∼20-40 nN for x ) 1-8. These maxima shifted to lower sliding velocities with increases in the applied normal load and with increases in the chain length of the solvent. The frictional maxima were interpreted by adapting concepts of viscoelasticity for bulk polymer systems to the twodimensional systems of SAMs. Maxima were interpreted to result from localized relaxation processes in the SAMs that depend on the extent of solvent partitioning in the compressed region under the tip. The characteristic relaxation times of the alkyl chains increased with increased applied normal load and with increased solvent chain length. The behavior as a function of x was consistent with both a mechanism of solvent partitioning controlled by the free volume distribution in the SAM and a mechanism of insertion into defects. The relaxation times of the alkyl chains were related to a molecular model of energy dissipation based on the adsorption and desorption of the chain tails from the surface of the atomic force microscopy tip. The total frictional forces was consistent with superposition of relaxation processes and viscous drag on the tip and plowing effects that become dominant at high applied normal loads.

The effect of chain density on the frictional behavior of surfaces modified with alkylsiloxanes and immersed in n-alcohols

“…Consequently, we selected SAMs of OTS that were immersed in n-alcohols, nϭ1 -12, as a model system for a LFM study of the effects of sliding velocity, solvent chain length and applied normal load on the friction behavior of SAMs of alkylsiloxanes. 12 Maxima in the friction force occurred as a function of sliding velocity for applied normal loads of ϳ20-40 nN and n ϭ2 -9. These maxima shifted to lower sliding velocity with increasing n and increasing applied normal load.…”

“…Scanning and fluid cell procedures are described elsewhere. 12 Force-displacement curves were obtained at each setpoint voltage for the vertical deflection of the cantilever to determine the normal load applied to the sample. The raw voltage data was converted with Hooke's law for small cantilever deflections as described elsewhere.…”

“…5 The friction behavior of SAMs is analogous to the viscoelastic behavior of bulk amorphous polymers, and in liquid environments, that behavior depends strongly on the partitioning of solvent into the SAMs. [12][13][14] Viscoelastic theory predicts that the maximum energy dissipation and friction forces occur when the characteristic relaxation time of the monolayer is similar to the time scale associated with the sliding contact. In this paper we report maxima in the friction force as a function of sliding velocity for complete and partial SAMs of alkylsiloxanes in liquid environments.…”

“…14 In terms of this model, we interpreted these maxima as evidence of relaxation processes in the film that depend on the extent of solvent partitioning in the compressed region. 12 This viscoelastic model predicts that the maxima in the friction force should shift to higher sliding velocities as the chain density of the SAMs decreases. In this paper we tested our viscoelastic model with a LFM investigation of the effect of chain density on the friction behavior of complete and partial C18 SAMs chemisorbed to silicon/silicon dioxide (Si/SiO 2 ) surfaces as a function of the solvent environment, sliding velocity and applied normal load.…”

“…In this paper we tested our viscoelastic model with a LFM investigation of the effect of chain density on the friction behavior of complete and partial C18 SAMs chemisorbed to silicon/silicon dioxide (Si/SiO 2 ) surfaces as a function of the solvent environment, sliding velocity and applied normal load. We compared previously published friction data for complete SAMs of OTS 12 with friction data for partial C18 SAMs formed from n-octadecylmethyldichlorosilane ͑2Cl͒ and n-octadecyldimethylchlorosilane ͑1Cl͒. Reported mechanisms for the deposition of tri-, di-, and monochlorosilanes suggest that the chain density decreases as OTSϾ2ClϾ1Cl as the chlorine groups are replaced by methyl substituents because methyl side groups prevent the formation of interchain siloxane linkages and sterically limit the possible orientations of the head groups of the alkyl chains.…”