The Role of Ligand in the Mechanical Properties of Self-Assembled Nanoparticle Films

Self-assembled films of nanoparticles (NP) capped with ligands at the air/water interface exhibit rich mechanical responses to compression including hashing, wrinkling, and folding, which are the combined result of particle- and ligand-based interactions. Previous studies have shown that a high concentration of ligands inhibits wrinkling and folding, but the mechanism remains elusive. By using inductively coupled plasma optical emission spectrometry (ICP-OES) to measure the ligand concentration of our NP solutions and then back-adding excess ligands at controlled amounts, we precisely control ligand-based interactions, enabling an investigation of how these interactions guide self-assembly and correspondingly on mechanical properties. Our experiments reveal that increasing the ligand concentration of the films causes the formation of free-ligand islands in addition to an increase in the interparticle separation. These effects are correlated with the previously observed inhibition of wrinkling and folding, as well a decrease in the dilatational and shear moduli. This work was supported by the University of Chicago Materials Research Science and Engineering Center, NSF-DMR-1420709.