Effect of alkanethiol self-assembled monolayers on the plastic and elastic deformation of gold(111) films.

摘要

Surface chemistry is known to affect the elastic deformation of nanocontacts, but its role in plastic deformation is less clear. Alkanethiol self-assembled monolayers (SAMs) were used to modify the surface energy and surface stress of Au(111) films. The chemical effect of this nanometer scale film on elastic and plastic deformation was investigated using nanoindentation combined with atomic force microscopy (AFM) imaging of indents. A range of maximum indentation displacements and SAM chain lengths were used. Comparisons were made between the mechanical response of the gold substrate alone and the gold modified with the different SAMs.The nanoindentation load-displacement curves and the mechanical properties were found to be dependent on the presence of the SAM. A decrease in the reduced elastic modulus was observed when indenting the SAM systems. The work of indentation and the hardness showed a similar effect with the SAM layer lowering hardness in both cases. Remarkably, the SAM was found to affect hardness, and hence plasticity, at indentation depths over 100 times the SAM thickness. Comparisons were made between the projected contact areas approximated using the Oliver and Pharr method with the actual areas directly measured by AFM analysis. This accounts for underestimation of the contact area due to pile-up of gold around the indent. AFM characterization of the residual nanoindentation impressions showed substantial differences between the indent shape and pile-up when comparing the gold to the gold plus SAM surfaces. The differences are more pronounced for the longer chain length SAM and as the indents become deeper. The SAM reduces the adhesion force between the indenter tip and surface which may affect the observed mechanical properties for shallow indents. For the deeper indentations the exothermic reaction of the alkanethiol molecules chemisorbed on the gold surface reduces the surface free energy of the gold substrate which favors the creation of new surface (pile-up). In addition, surface stress which is compressive when a SAM is present, but tensile otherwise, appears to modify the behavior of dislocations and strain hardening in the Au films. This is the most likely cause of the dramatic change in hardness and pile-up.