Reassembling nanometric magnetic subunits into secondary nanostructures with controlled interparticle spacing

Abstract

Nanoparticle clusters have become attractive secondary nanostructures due to their collective physical properties, which can be modulated as a function of their internal structure. In this study, a reassembling strategy of iron oxide nanoparticles prior to the formation of nearly identical nanoclusters of size 80 ± 20 nm as the secondary nanostructures is reported, where ordered arrangement of nanoparticle subunits with a narrow distribution of interparticle spacing is observed. Oleic acid capped iron oxide nanoparticles (IO_NPs@OA) with sizes of 5 ± 2 nm are used as the primitive assembly of subunits. Post-functionalization of these nanoparticles with surfactant cetyltrimethyl ammonium bromide (CTAB) and subsequently with polyelectrolyte polyacrylic acid (PAA) result in irregular aggregation of nanoparticles (IO_agg@CTAB) and nanoparticle clusters (IO_NPCs@PAA) respectively. The orderly oriented carboxylate groups of PAA play an important role in dense packing of nanoparticles with an ordered arrangement inside the clusters. The observed fractal morphologies give an indication of interparticle interactions for all three systems irrespective of assembly, aggregation and reassembly. Henkel plots show the dipolar type of interaction for all three systems. The dominant effect of the interparticle spacing distribution over size on the modification of effective anisotropy energy barrier distribution is realized from the observed trend of ZFC (zero-field cooled) magnetization peak broadening and shifting.