Multi-scale model of effects of roughness on the cohesive strength of self-assembled monolayers

Abstract

Self-assembled monolayers (SAMs) are aggregates of small molecular chains that form highly ordered assemblies at the nanoscale. They are excellent contenders of molecular-level tailoring of interfaces because of the wide choice of terminal groups. Molecular dynamics (MD) simulations and experimental observations of spallation of two SAM-enhanced gold-film/fused silica-substrate interfaces have shown that the cohesive strength of SAM-enriched transfer-printed interfaces is strongly dependent on the choice of terminal groups. Though the MD results of perfectly ordered atomistic surfaces show the same qualitative trend as the experiments, they over-predict the interfacial cohesive strengths by a factor of about 50. Previous studies have revealed that the roughness of these interfaces may significantly impact their cohesive strength. In this manuscript, we perform a multiscale study to investigate the influence of surface roughness on cohesive strength of an interface between a Si/SAM substrate and a transfer-printed gold film. We approximate the film as a 2D deformable medium while the rough SAM-enhanced substrate is modeled using 2D harmonic functions with the cohesive interaction between the SAM and the film described by a simple exponential relation. Spallation is simulated on this system to evaluate the effective traction-separation response for the rough SAM-gold interface. Beyond the idealized harmonic interface, we extend our studies to real surface profiles obtained by AFM. We demonstrate how interfacial roughness can reduce the cohesive strength of the SAM-enhanced interface by more than an order of magnitude.