Chemomechanical effects of self-assembled monolayers on gold films

Chemomechanical effects are technologically of great importance, but they have been highly controversial due to issues with reproducibility, environmental sensitivity and the fact that they only become dominant in the near-surface region. Using nanoindentation and the careful control of surface chemistry, we have isolated specific chemomechanical effects in nanoscale gold films. The Au was tested with and without alkanethiol self-assembled monolayers (SAMs) and the environmental conditions were carefully monitored throughout. For shallow nanoindentations, a well-formed SAM lowered the load-depth curves substantially and pile-up at the indent sides was slightly reduced. This can be attributed to the so-called “Rehbinder effect”, where changes in surface energy, in this case due to the SAM, affect plastic deformation. Interestingly, even at greater depths the SAM was still found to lower the load-depth curves, though it greatly increased the pile-up volume (+300%). This is likely due to a different effect where the presence of the SAM allows greater ductile flow and, hence, extrusion of the Au during the indentation. In all cases the plastic work of indentation was lowered by the SAM, indicating that it made plastic deformation easier than for uncoated Au films.