Single-Crystalline Co Nanowires: Synthesis, Thermal Stability, and Carbon Coating

Decomposition of the organometallic Co(η 3 -C 8 H 13 )(η 4 -C 8 H 12 ) complex in the presence of a 2/1 acid/amine mixture, or of cobalt stearate in the presence of 2 equiv. of amine leads to single-crystalline hcp cobalt nanowires of a few nanometers in diameter and micrometers in length. These nanowires display high coercive fields (1.7 T) as well as the magnetization of bulk cobalt. To develop a method for the carbon coating of these nanowires and thus ensure their long-term air stability, the thermal stability of the nanowires is investigated. After being annealed at temperatures above 350 °C under static argon atmosphere, chains of cobalt nanoparticles embedded in a wirelike carbon matrix are obtained. This phenomenon is a consequence of the Rayleigh instability of the nanowires, which is enhanced during the decomposition of the stabilizing ligands. In high-vacuum conditions, in situ transmission electron microscopy (TEM) and high-resolution TEM studies show that the nanowires evaporate without any appearance of Rayleigh instability, and without any structural change. This suggests the crucial role played by the ligands in the fragmentation process. Annealing below 350 °C prevents the fragmentation process and yields carbon-coated cobalt nanowires.