A one-step process is used to prepare a variety of nanoparticle membranes that exhibit trampoline-like properties.
The membranes can be as much as 10 000 times thinner than human hair. They are made by placing a small amount of a nanoparticle solution into a solvent covering a surface with an array of holes. The solvent is then left to evaporate, leaving behind a freestanding, ordered, closely packed single layer of nanoparticles over each hole. Since each nanoparticle comprises a metallic core and a ligand shell, the resulting organized monolayers can be considered to be hybrid in nature. In a recent study, Heinrich M. Jaeger and co-workers at the University of Chicago and Argonne National Laboratory demonstrate how this assembly technique can be used for different cores and different ligands. The concept of freestanding thin films based on nanoparticles is not new (see for example, the gold nanoparticles films made by Xia and Wang), but the fact that hybrid films comprising different types of nanoparticles can be easily made suggests that a variety of applications may be just around the corner. By choosing the right combination of core and ligand, a membrane can be tailored to meet different requirements.
With several membranes on hand, the researchers investigate how the nanoparticle type affects the film properties. To see what sort of strength and elasticity the films have, they use atomic force microscopy, in which a thin silicon needle is pressed onto the surface. When the needle is released, the membrane bounces back into place, just like a trampoline.
In one combination of core and ligand, the "nanotrampolines" had an average diameter of about 8.5 μm; by playing with the evaporation conditions, they were able to obtain freestanding films of the same composition at a diameter of 70 μm. Despite the significant size difference, the strengths of these two films were similar. Their studies go on to show that the stiffness of the membranes mainly depends on how well the ligand shells of the nanoparticles interact with each other and with the metallic core. Attractive interactions lead to better packing, and thus, stiffer films (i.e., bouncier trampolines!).
The design of future nanoparticle films will certainly benefit from this study. Although acrobats only exist in the macroscale, developers of resonators could use these films to bounce off sound waves. In future, we may also find these nanoparticle membranes in various nanodevices, such as photonic devices, sensors, and filters.
