Atomic resolution electron microscopy of cobalt ferrite nanoparticles

Abstract

Cobalt ferrite (CoFe 2 O 4 ) nanoparticles have recently emerged as a potential candidate for a range of bio‐medical applications due to their interesting physical and magnetic properties. The high magneto‐crystalline anisotropy of CoFe 2 O 4 nanoparticles in particular offers improved efficiency over iron‐oxide nanoparticles, allowing for smaller particles to be used. Synthesis of cobalt ferrite nanoparticles is conventionally achieved using thermal decomposition in oleic acid and oleylamine. Recent methods using oleylamine alone demonstrate greater suitability for biomedical applications, as oleylamine facilitates the phase‐transfer process required to make the nanoparticles water‐soluble. However changing the surfactant is known to have a significant effect on the crystal structure and morphology of metal‐oxide nanoparticles, and the crystal structure of oleylamine‐capped cobalt ferrite nanoparticles has not been studied in detail before. Here we demonstrate the inverse spinel structure of cobalt ferrite nanoparticles synthesised with oleylamine as the sole surfactant, by aberration‐corrected annular dark‐field scanning‐transmission electron microscopy. The crystal structure is resolved with atomic‐level detail which has not been demonstrated with cobalt ferrite nanoparticles previously. Furthermore the distribution of cobalt and iron atoms is shown by atomic‐resolution EELS spectroscopy mapping. This data serves to form a solid support for future studies into the size‐dependent interaction of cobalt ferrite nanoparticles with cells.