Tunable Mechanical Properties of Nanoparticle Monolayer Membranes via Ligand Phase Control and Defect Distribution

In this study, the effects of ligand phase, morphology, and temperature on the elastic modulus of free-standing alkanethiol-capped gold nanoparticle membranes are reported. Langmuir films of 2.5 nm gold nanoparticles capped with tetradecanethiol were prepared at temperatures above and below the phase transition temperature (T m) of the ligand shell and transferred to holey carbon grids (containing 1.2 μm holes) to form free-standing membranes. Force–indentation measurements are used to measure the elastic modulus of the membranes using an atomic force microscope in the temperature range 10–40 °C. These films are compared with membranes of dodecanethiol-capped gold nanoparticles, which do not undergo a ligand order–disorder transition in the temperature range investigated. The ligand phase effect is observed in the tetradecanethiol-capped gold nanoparticle films, where an abrupt change in the elastic modulus is seen near T m. The temperature (relative to T m) during the fabrication of the films is determined to play an important role in tuning the mechanical strength of these films in this temperature range by both changing the nature of the interparticle interactions and by affecting microscale film morphology.