Credit: © 2009 NPG

Understanding and exploiting the different ways in which nanoscale components self-assemble into larger structures is one of the central themes of nanotechnology. Dispersions of colloidal particles are widely used to study self-assembly, but so far most researchers in this field have concentrated on systems containing just one or two different types of particles. Now Benjamin Yellen and co-workers1 at Duke University and the University of Amherst have explored the wide range of structures that can be produced from the self-assembly of three or more different types of particle.

Yellen and co-workers started by suspending iron oxide nanoparticles in water to make a ferrofluid, and then added larger spherical particles (both paramagnetic and non-magnetic) in the presence of an external magnetic field. They observed a wide variety of different superstructures that depended on the size and degree of magnetization of the particles. Moreover, the superstructures all possessed rotational symmetry, and resembled electrostatic charge distributions such as linear quadrupoles and axial octupoles.

In general, the magnetic forces in the system drove the non-magnetic particles towards the equator of the paramagnetic particles and, likewise, the paramagnetic particles were driven towards the equator of the non-magnetic particles. 'Flowers' and 'two tone' are just two of the names chosen by Yellen and co-workers for the structures they observed.