Synthesis of Passivated Cobalt Nanocrystal Arrays With Controlled Size and Shape

Abstract

Rapid pyrolysis of cobalt carbonyl in an inert atmosphere was employed to produce magnetic colloids (ferrofluids) of monodispersed, stabilized, cobalt nanocrystals. The size distribution and the shape of the nanocrystals were controlled by varying the surfactant (oleic acid, phosphonic oxides and acids...), its concentration and the reaction temperature. Thus, we systematically synthetised passivated nanocrystals with nearly monodisperse (std. dev.< 5%) size ranging from 3 to 17 nm. The surface tension of the Co crystals were modified during the reaction and cubic and prism-like crystals of size ~20–30 nm were also synthesized. Particles are single crystals with a complex cubic structure related to the beta phase of manganese (ε-Co). Thus by controlling the size, the shape and the crystal structure the superparamagnetic transition can be customized. These particles have been observed to produce 2D self-assemblies when evaporated at low rates in a controlled atmosphere. A combination of x-ray powder diffraction, transmission electron microscopy and vibrating sample magnetometer has been used to characterize both the dispersed nanocrystals and the assembled superlattices.