Self-assembly Of Cobalt Magnetic Nanoparticles And Its Modulation By External Fields

The last decade has witnessed tremendous progress in the wet-chemical synthesis of nanostructures. Up to date one is able to tailor the size, shape, and surface chemistry of these nanostructures in a controlled manner. The greatest challenge nowadays is how to assemble individual nanoscopic components into larger structures and materials with complex patterns and diverse functionalities with low cost and efficient approaches. The self-assembly of colloidal nanoparticles provides such a route to novel classes of nanostructured materials, and the ability of self-assembling in a designed manner depends crucially on the ability to understand in quantitative detail and subsequently "engineer" the interparticle interactions.The assemblies of magnetic nanoparticles have been intensely studied in the past decades due to their unique properties, such as single domains and giant magnetic moments, and great potential in biomedical and technological applications such as magnetic sensors, radio frequency devices, and high-density storage media. Large magnetic nanoparticles with the blocking temperature above the room temperature bear substantial dipolar magnetic moments, which lead to a great diversity of assembling patterns, particle chains and rings in particular.In this paper, we focus on the self-assembly of colloidal Co magnetic nanoparticles. The well-defined size and shape, and the uniform surfactant coating layer of the cobalt nanoparticles enable quantitative calculations of particle-particle and particle-interface interactions. The experiments, in conjunction with cluster-moving Monte Carlo simulations mimicking the self-assembly in solution and dynamics during solvent evaporation, provide a better understanding of the interparticle interactions. Then their dynamic self-assembly under rotating magnetic fields at fluid interfaces is studied. The introduction of rotating magnetic fields leads to the fabrication of ultra-large scale monolayers comprising16nm Co ferromagnetic nanoparticles at fluid interfaces. We present the results of this thesis as follows.First, the synthesis of ε-Co nanoparticles is studied in detail. The control of the shape, size and surface properties of the Co nanoparticles with reactants and reaction time is well analyzed. The surfactant layers are crucial in the self-assembly of colloidal nanoparticles. We verify that as long as enough oleic acid is present in the reaction, the Co nanoparticles would always grow with an about2nm surfactant layer, which corresponds to a monolayer of oleic acid molecules.Then we investigate the self-assembly of Co ferromagnetic nanoparticles (FMNPs) by introducing a crucial step of high-power sonication. As a key procedure, the sonication releases the FMNP systems from whatever trapped states when they were prepared. Thus the well-dispersed Co FMNPs are allowed to self-assemble directed only by interparticle interactions, greatly simplifying the analysis. The initially well dispersed FMNPs in solutions of low concentrations would self-assemble into rings composed of a few nanoparticles. A high particle density yields larger clusters, which prefer close-packed spheres as lower energy structures. Monte Carlo simulations based on a simple model taking account of long ranged dipolar and short ranged van der Waals (vdW) interactions show good agreement with the experimental results, and qualitatively reveal the real states of the FMNPs in solution. Further investigations on a range of parameters suggest that the enhancement of the dipolar forces would prevent ring self-assembly as high energy barriers arise from large dipole moments in the transition from chains to rings, whereas much weaker dipolar forces with magnitude comparable to that of van der Waals fail to dominate the structures. An increase of the surfactant thickness outside the FMNPs, on the other hand, would reduce vdW attractions to a minimum, accompanied by a weakening of dipolar forces, and thus results in numerous rings loosely bonded by dozens of particles. Since very weak dipolar forces alone could also induce the formation of rings on the substrate due to the increasing particle density as solution evaporates.Finally, we demonstrate a drying-mediated self-assembly of16nm ε-cobalt FMNPs into centimeter-sized monolayers at fluid interface accomplished by a rotating magnetic field (RMF). Instead of rotating solid substrates in a conventional spin-coating procedure, we drive the Co FMNPs using a RMF at fluid interface and monolayers are formed as a result of the interplay of hydrodynamic, magnetic, hydrophobic, and dipolar interactions. The RMF, on one hand, disassemble the FMNP aggregates into2D rafts in the liquid film of sample dispersions, while on the other, attract these rafts together into macroscopic membranes at fluid interface. Such large ferromagnetic membranes are stable on a fluid subphase without the supporting of external fields and transferable to various solid substrates for practical applications. Magnetic measurements show weak dipolar couplings among the Co FMNPs and their easy axes still tend to lie in the layer plane. It seems like that changing the magnitude of the magnetic field and its angular speed would have significant effects on the geometric and magnetic properties of the FMNP monolayers, and further studies are being conducted to explore these parameters. We believe this approach is applicable to most colloidal FMNP systems and can be widely used in the nanofabrication of ferromagnetic granular membranes in large scale.In conclusion, we believe the results obtained in this thesis are applicable to most colloidal magnetic nanoparticles.