Synthesis of monodisperse cobalt nanocrystals and their assembly into magnetic superlattices (invited)

TLDR

Cobalt particles synthesized are single crystals with a complex cubic structure related to the β‑phase of elemental manganese (ε‑Co). Cobalt nanocrystals were produced by high‑temperature solution‑phase reduction of cobalt chloride with stabilizers, annealed at 300 °C to convert to hcp, and characterized by XRD, TEM, and SQUID magnetometry. The authors obtained nearly monodisperse cobalt nanocrystals (2–11 nm, <7 % size variation) that, when deposited on substrates, spontaneously assemble into two‑ and three‑dimensional magnetic superlattices.

Abstract

High temperature, solution phase reduction of cobalt chloride in the presence of stabilizing agents was employed to produce magnetic colloids (ferrofluids) of cobalt nanocrystals. We systematically synthesized and isolated nearly monodisperse nanocrystal samples ranging in size from 2 to 11 nm while maintaining better than a 7% std. dev. in diameter. As synthesized cobalt particles are each a single crystal with a complex cubic structure related to the beta phase of elemental manganese (ε-Co). Annealing the nanocrystals at 300 °C converts them quantitatively to the more common hexagonal-close-packed crystal form. Deposition of these uniform cobalt particles on solid substrates by evaporation of the carrier solvent results in the spontaneous assembly of two-dimensional and three-dimensional magnetic superlattices (colloidal crystals). A combination of x-ray powder diffraction, transmission electron microscopy, and superconducting quantum interference device magnetometry were used to characterize both the dispersed nanocrystals and the assembled superlattices.