Creation of 3-D crystals from single cobalt nanoparticles in external magnetic fields

Using monodisperse nanocrystalline cobalt (Co) particles in non-polar colloidal dispersions, large areas of symmetric multi-dimensional structures were created using magnetophoretic deposition (MPD). To overcome the van der Waals and magnetic dipole-dipole interactions, the particles were stabilized with hydrophobic amines, phosphines, carboxylates and/or polymers. Depending on the preparation parameters, our particles had either bcc or epsilon-Co crystalline structures. Using MPD with magnetic fields up to 1 T, it was possible to create two-dimensional (2-D) arrays of near-perfect symmetry up to 1 mum(2) in size on various substrates, e. g. carbon-coated copper grids, silicon, or glass. Growth of the 2-D crystal was shown to be dependent on the direction of the applied external magnetic field. Three-dimensional (3-D) crystals could be created by increasing the magnetic field strength up to 6 T.