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

Abstract

Cobalt nanostructures have been prepared by a chemical route based on the Co(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 surfactant softly coated Co nanostructures followed by a slow separation process of the magnetic-field responsive Co/surfactant nanocomposites from the liquid phase. The detailed structure of thin films of the Co/surfactant nanocomposites has been investigated by scanning force microscopy (SFM). The thin films were characterized by different anisotropic features. Micrometric long domains of self-aligned ellipsoidal NPs (tens of nanometers in size) have been observed, together with bendable micrometric long homogeneous nanofibers (NFs). The film structures were strongly dependent on the Co/surfactant ratio and, by increasing the Co percentage, the system was forced towards the formation of mutually connected superstructures consisting of anisotropic bands of self-aligned NFs and anisotropic 2D close packed Co-NP super-lattices. Transmission electron microscopy (TEM) showed that the NPs observed by SFM are in effect 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. Magnetic force microscopy (MFM) demonstrates the magnetic response of these thin films, highlighting the different behavior (attractive/repulsive) of the Co-NPs aggregates towards the oscillating magnetized tip. The above structural findings have been interpreted in terms of nanostructures/matrix interaction along with a fine balance between short-range isotropic repulsions, van der Waals attractions and long-range anisotropic magnetic interactions.