The role of ligands in the mechanical properties of Langmuir nanoparticle filmsElectronic supplementary information (ESI) available: Details of the AFM measurement of a dry Au-NP film transferred from the air-water interface. See DOI: 10.1039/c7sm00319f

Langmuir monolayers of ligand-capped inorganic nanoparticles exhibit rich morphologies under lateral compression such as wrinkling, folding, and multilayer nucleation. We demonstrate that the ligands play a crucial role in the mechanical properties of nanoparticle films by probing the morphology and anisotropic stress response during lateral compression of films with systematically varied ligand concentrations. Increasing the ligand concentration of the films past a threshold value inhibits monolayer wrinkling and folding in favor of multilayer formation, and sharply reduces the compressive and shear moduli. We attribute these drastic mechanical effects to modifications to the ligand interactions between adjacent particles as well as to two-dimensional crystalline structure changes occurring on the scale of tens of particles, as determined by transmission electron microscopy. We investigate the role of ligand concentration in modulating the mechanical properties of nanoparticle films at the air-water interface.