Nanostructures are engineered structures with features at the nanoscale — between 1 and 100 nanometers. They include nanotextured surfaces, nanoparticles and nanotubes, as well as more complex nano-scale structures. A nanometer (nm) is one billionth of a meter, which is just slightly larger than the atomic scale — individual atoms in a solid are between 0.1 nm and 0.4 nm apart. Therefore, nanostructures are the smallest possible structures, as anything smaller would be a simple molecule or atom.
Nanostructures may be formed by top-down bulk processes which, at their simplest, may be just the successive breaking down of particles into nanoparticles. At the other extreme, they may be constructed in a bottom-up fashion, using scanning electron microscopes (SEM) or scanning tunneling microscopes (STM), perhaps even building a nanostructure an atom at a time. Nanofabrication is widely used to mass produce precisely defined nanostructures in the semiconductor industry. Bulk bottom-up processes may also be used to produce nanostructures which do not require such precise control of individual structures but which may have very large surface areas at the nano-scale, giving them enhanced energy storage or catalytic performance.
Some notable nanostructures include:
- Graphene is a layer of carbon atoms, just a single atom thick, with the individual atoms arranged in a hexagonal lattice. When isolated, it has exceptional electrical, structural and chemical properties. However, technical challenges mean it is often still difficult to realize these at the bulk level.
- Carbon nanotubes are tubes formed from a layer of graphene rolled into a tube with the hexagonal molecule structure continuous around the diameter. Like graphene it has many exceptional properties when isolated but these remain difficult to realize at the bulk scale.
- Fullerenes are also pure carbon structures a single atom thick. However, while graphene and carbon nanotubes have the atoms arranged in hexagons (rings of six atoms) fullerenes combine rings of five, six and seven atoms enabling the surface to form into approximately spherical forms, much like a football. Although technically graphene and carbon nanotubes can be considered as specific types of fullerenes, the term is more normally used for spheres and ellipsoids.
- Nanofibers are strands of polymer with nanoscale diameters, they may be organic polymers such as silk or keratin, or synthetic polymers such as polyurethane or polylactic acid (PLA).
- Nanoholes are a type of nanotextured surface with nanoscale holes, often arranged in an array across the surface. Surfaces containing nanoholes can act as super-lenses or enhance the performance of solar panels. Materials containing nanoholes are known as nanoporous materials.
- Nanofoams are materials with an interconnected matrix of nanoholes. Metallic nanofoams can be highly effective catalysts, electrodes and hydrogen stores. Carbon nanofoams can efficiently convert light into heat and exchange it with a heat transfer fluid, giving great potential for solar thermal energy. They may also have applications in filtration.