Self-assembly of Au-Fe3O4 dumbbell nanoparticles

A dumbbell nanoparticle (DBNP) system consists of an optically active Au seed particle on which a magnetic iron oxide nanoparticle (IONP) is heterogeneously grown. Control and manipulation of these multi-functional hetero-structures have applications as a dual-probe for biomedical imaging, in catalysis, sensing, optics, photonics and electronics. This thesis investigates the magnetic field-induced self-assembly in diverse DBNPs, with different sizes of Au and IONPs coated with oleic acid and oleylamine and dispersed in toluene. The effects of DBNPs’ complex morphology arecompared and contrasted to self-assembly studies on the IONPs’, which are singlephase spherical counterparts. Direct comparison simplifies the understanding of broad parameter space, including the size of the Au seed and the grown IONP, their size distribution, the thickness of surfactant coating around the nanoparticle, concentration in a dispersion, composition, magnetic structure, and strength of the magnetic field. A multiscale experimental approach is adopted to analyze the structure and magnetic properties to link it to the self-assembly phenomenon. Microscopy combinedwith local atomic structure obtained from synchrotron x-ray pair distribution function (xPDF) is used to reveal local crystal structure, crystallinity, size and distortion induced at the surface. Macroscopic magnetic measurements along with polarized neutron scattering reveal the magnetic behavior. Small-angle x-ray and neutron scattering (SAXS/SANS) measurements are exploited to observe and analyze selfassembling patterns. Real-space analysis of such patterns is achieved through reverse Monte Carlo (RMC) simulations. Spherical IONPs reversibly form 1D chains thatalign, straighten with magnetic field. On the other hand, 1D and 2D chains are observed with DBNPs in an applied magnetic field. The assemblies are classified into three categories based on the anisotropy in the 2D scattering pattern. Moreover, due to the unique morphology and orientation effects, the chains formed by DBNPs within these categories have head-to-tail or side-by-side arrangement. Shape-induced mechanisms governed by a dimensionless parameter are suggested to play a vital role in determining assembly formation