Magnetic Properties of Mono- and Multilayer Assemblies of Iron Oxide Nanoparticles Promoted by SAMs

Owing to the wide scope of applications of magnetic nanoparticle assembling, the aim of this study is to evaluate the influence of nanoparticle aggregates on the magnetic properties of 2D assemblies. Magnetic iron oxide nanoparticles (NPs) have been synthesized by the coprecipitation (NP_cop) and thermal decomposition (NP_dec@OA) methods, and were assembled on self-assembled monolayers of organic molecules decorated by a phosphonic acid terminal group at their surface (SAM-PO₃H₂). The nanostructure and magnetic properties of assemblies depend directly on the aggregation of NP suspensions. NP_cop result in an unstable suspension and were assembled into a non-homogeneous monolayer of aggregates. The post-functionalization of NP_cop with oleic acid after synthesis (NP_cop@OA) favors a better stability of the suspension and enhances the nanostructure of the assembly, although smaller NP aggregates remain. In contrast, NP_dec@OA which are functionalized <i>in situ</i> by oleic acid during the synthesis step were assembled as individual nanomagnets and result in a dense monolayer. Multilayer assemblies were also prepared from NP_cop@OA and NP_dec@OA by performing the alternative deposition of these NPs with (1,4-phenylene)­bisphosphonic acid. The nanostructure of assemblies has been studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The magnetic properties of monolayer and multilayer assemblies have been studied by using a SQUID magnetometer. While assemblies of individual NPs enhance dipolar interactions in-plane as a result of shape anisotropy, assemblies of NP aggregates favor stronger dipolar interactions with random orientation. The magnetic properties of monolayer and multilayer assemblies have also been compared. The dimensionality (2D vs 3D) has a strong effect on the dipolar interactions when individual NPs are considered in contrast to aggregated nanoparticles