Dr. Nicolas Schaeffer (Researcher at Universite Pierre et Marie Curie, Paris, France)
Nanomaerials exhibit chemical and physical properties that are not those of the bulk, but are size-dependent. This is due to the nanomaterials having a very high surface-to-volume ratio. For example, a standard table tennis ball has a total surface of 50 cm2. If the balls’ volume were filled with 4nm spherical nanoparticles, their total surface would be that of two football pitches.
Because of property, the surface energy of each nanostructure contributes significantly to their total free energy and, in turn, controls the morphology of the nano-objects. This is why most nanoparticles are (or appear to be) spherical. However, only amorphous nanoparticles are actually spherical; for crystalline systems, the surfaces energies are anisotropic for different crystallographic orientations. Thus the presence of facets gives rise to polyhedrons instead of spheres (this is valid for very small nanoparticles, larger ones have a lower surface-to volume ratio and surface energy anisotropy becomes less important).
The presence of facets is a key factor for the construction of structures with different morphologies. Several “seeding-growth” protocols have been developed, enabling the formation of polyhedrons (or seeds) and the subsequent growth on selected facets. Nowadays, not only simple geometrical designs, such as nanocubes, nanorods or nanotriangles can be produced. More exotic structures are also studied; for example, gold nanostars have been shown to exhibit unique optical properties. Germanium sulphide nano-shish-kebbabs have also proven to increase solar cells efficiency.
The controlled growth is only one mean of controlling the shape of nanomaterials; the morphology of some preformed particles can also be changed by thermal treatment for example. Those methods, along with an increasing understanding of nanoparticles self-assembly processes, give rise to the emergence of more and more complex nanodevices every day.