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Cobalt nanoparticles with differentsizes and morphologies includingspheres, rods, disks, and hexagonal prisms have been synthesized throughthe decomposition of the olefinic precursor [Co(η3-C8H13)(η4-C8H12)] under dihydrogen, in the presence of hexadecylamine anddifferent rhodamine derivatives, or aromatic carboxylic acids. UV–visspectroscopy, X-ray diffraction, low and high resolution transmissionelectron microscopy, and electron tomography have been used to characterizethe nanomaterials. Especially, the Co nanodisks formed present characteristicsthat make them ideal nanocrystals for applications such as magneticdata storage. Focusing on their growth process, we have evidencedthat a reaction between hexadecylamine and rhodamine B occurs duringthe formation of these Co nanodisks. This reaction limits the amountof free acid and amine, usually at the origin of the formation ofsingle crystal Co rods and wires, in the growth medium of the nanocrystals.As a consequence, a growth mechanism based on the structure of thepreformed seeds rather than oriented attachment or template assistedgrowth is postulated to explain the formation of the nanodisks. [ABSTRACT FROM AUTHOR]