Investigating self-assembly of Au-Fe3O4 dumbbell nanoparticles using advanced scattering techniques

We investigated parameters that affect self-assembly of magnetic dumbbellnanoparticles (DBNPs) in solution. The dumbbells consist of epitaxially grownmagnetic Fe3O4 nanoparticles on the surface of spherical Au nanoparticles. Theirstructural, optical and magnetic properties are useful for variety of applications suchas dual probes for drug delivery [1-2], catalysis [3], sensing [4], optics [5] andelectronics [6]. However, fundamental understanding of mechanisms and parametersinvolved in self-assembly of DBNPs in solution is still missing. We studied severalDBNPs coated with a mixture of oleic acid and oleylamine of different sizes: A13F14,A11F8, A10F14, A10F12, A9F11, where numbers indicate the diameters of Au (A)and Fe3O4 (F), respectively. Field-dependent DC magnetization reveals differentshapes of hysteresis loops for our samples, which is presumably due to a Au/Fe3O4interface and modification of magnetization profile of Fe3O4 [7-8]. Small-angle neutronscattering in applied magnetic field showed self-assembly of DBNPs in solutions, incontrast to zero-field measurements. We observed the onset of order beginning at0.2 T for A10F14, A9F11 and at 1T for A13F14 (Figure 1). Surprisingly, we found noself-assembly for A10F12 and A11F8 even at 3T. We also performed a polarizedSANS at 1T and 3T to separate nuclear and weak magnetic scattering