Proc. Natl Acad. Sci. USA http://doi.org/885 (2015)

Self-assembly is a key concept in nanotechnology and comes in a variety of forms. One approach involves the spontaneous assembly of magnetic nanoparticles in the presence of an external magnetic field. There is, however, a size limit to the approach, which emerges as the nanoparticles get smaller and their magnetic moment decreases. Ingo Rehberg, Stephan Förster and colleagues have now shown that nanoparticles at this size limit can self-assemble in solution to form regular one-, two- and three-dimensional structures.

The researchers — who are based at the University of Bayreuth, the University Duisburg-Essen and the European Synchrotron Radiation Facility in Grenoble — examined spherical and cube-shaped iron oxide nanoparticles that were stabilized by oleic acid and had diameters of less than 15 nm. The spherical nanoparticles did not form regular structures in the presence of a magnetic field, but the cube-shaped nanoparticles assembled into chains, monolayer sheets and large cuboids, all with almost perfect internal order. These assemblies are created because the magnetic dipoles of the nanoparticles are aligned by the magnetic field, which leads to an attractive interaction that forces the nanoparticles into intimate contact; short-range van der Waals interactions then drive the nanoparticles into a regular face-to-face attachment, which is stable after the magnetic field is removed. For the spherical nanoparticles, it is thought that the short-range attractive interactions are much weaker and therefore assemblies cannot be stabilized.

Förster and colleagues also show that cube-shaped nanoparticles smaller than 7.5 nm cannot form assemblies.