Multiscale Modeling Reveals the Cause of Surface Stress Change on Microcantilevers Due to Alkanethiol SAM Adsorption.

Experimental results show that the adsorption of the self-assembled monolayers (SAMs) on a gold surface induces surface stress change that causes deformation of the underlying substrate. However, the exact mechanism of stress development is yet to be elucidated. In the present study, multiscale computational models based on molecular dynamics (MD) simulations are applied to study the mechanism governing surface stress change. Distinct mechanisms for adsorption-induced surface deformation, namely, interchain repulsion and thiol-gold interaction-driven gold surface reconstruction, are investigated. Two different interatomic potentials, embedded atom method and surface-embedded atom method (SEAM), are used in the MD simulations to study the reconstruction-induced surface stresses. Comparison of the predicted surface stress changes, resulting from MD and continuum mechanics-based models, with the observed experimental response indicates that a modified SEAM-based multiscale model can better capture the surface stress changes observed during alkanethiol SAM formation, and gold surface reconstruction is the primary factor behind the surface stress change. The interchain repulsions of SAM are found to have a minimal contribution. Also, both the simulations and experiments show that the surface stress change increases with the increase of surface coverage density and larger grain size.