Molecular tailoring of interfacial adhesion using self-assembled monolayers

Self-assembled monolayers (SAMs) provide an enabling platform for molecular tailoring of the chemical and physical properties of an interface in an on-demand fashion. In this work, we systematically vary SAM end-group functionality and quantify the corresponding effect on interfacial adhesion between a transfer printed gold (Au) film and a fused silica substrate. SAMs with two different end groups are investigated: Dodecyltriethoxysilane and 11-mercapto-undecyltrimethoxysilane. The adhesive strength of the SAM-mediated interfaces is measured by a non-contact laser-induced spallation method at strain rates in excess of 10⁶ s^-1. A high strain rate test method is selected to facilitate comparison with forthcoming molecular dynamics simulations of the molecular failure process. Interfacial stresses are inferred from interferometric displacement measurements and finite element analysis. By making multiple measurements at increasing stress amplitudes (controlled by the laser fluence), the adhesion strengths of Au films transfer-printed on different SAM modified substrates are compared. Varying the end-group functionality drastically alters the adhesion strength of Au films, leading to improved adhesion over transfer printed films on unmodified quartz. We demonstrate a spallation strength of 24.2 ± 0.4 MPa for interfaces prepared with dodecyltriethoxysilane and 60 ± 11 MPa for interfaces prepared with 11-mercapto-undecyltrimethoxysilane confirming that interfacial bonding at the Au-thiol interface is significantly stronger than at the Au-methyl interface.