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

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.