The zero field self-organization of cobalt/surfactant nanocomposite thin films

Abstract

Cobalt nanostructures have been prepared by a chemical route based on theCo(II) reduction in the confined space of cobalt bis(2-ethylhexyl)sulfosuccinate(Co(DEHSS)2) reverse micelles dispersed in n-heptane. This procedure involves the rapid formation of surfactantsoftly coated Co nanostructures followed by a slow separation process of the magnetic-field responsiveCo/surfactant nanocomposites from the liquid phase. The detailed structure of thin films of theCo/surfactant nanocomposites has been investigated by scanning force microscopy (SFM). The thin films werecharacterized by different anisotropic features. Micrometric long domains of self-aligned ellipsoidalNPs (tens of nanometers in size) have been observed, together with bendable micrometriclong homogeneous nanofibers (NFs). The film structures were strongly dependent on theCo/surfactant ratio and, by increasing the Co percentage, the system was forced towards the formation ofmutually connected superstructures consisting of anisotropic bands of self-aligned NFs andanisotropic 2D close packed Co-NP super-lattices.Transmission electron microscopy (TEM) showed that the NPs observed by SFM are ineffect composed of almost spherical and oxygen-free cobalt nanoparticles, 1–3 nm in size,which typically assemble in larger ellipsoidal systems tens of nanometers in size. Magneticforce microscopy (MFM) demonstrates the magnetic response of these thin films,highlighting the different behavior (attractive/repulsive) of the Co-NPs aggregates towardsthe oscillating magnetized tip. The above structural findings have been interpreted in termsof nanostructures/matrix interaction along with a fine balance between short-rangeisotropic repulsions, van der Waals attractions and long-range anisotropic magneticinteractions.