Field-assisted growth of magnetic nanoparticle supercrystals

We report on the organization of magnetic nanoparticles (Fe3O4 and Fe3O4@Au NPs) into two and three dimensional (2D and 3D) supercrystals (SCs) and which were imbedded in polyethylene glycol (PEG) matrix. The organization of NPs was induced by subjecting their dispersion (in chloroform) to an external magnetic field. The Fe3O4 NPs were prepared by the high temperature decomposition of an Fe-oleate complex. The core-shell Fe3O4 Au NPs were made by treating Fe(3)O(4 )seed NPs with chloroauric acid (HAuCl4). Both the bare and Au-coated Fe3O4 NPs dispersed in chloroform were treated, separately, with PEG (molecular weight 6000) and suspended in the presence of an external magnetic field. On evaporation of chloroform, flaky crystals having sizes on the order of tens of millimeters were formed. Examination of the crystals in a transmission electron microscope (TEM) revealed the formation of 2D and 3D stacked NP SCs. The magnetic properties, ascertained by using a vibrating sample magnetometer (VSM), were found to be similar to that due to NPs only. However, the optical signature due to Fe3O4@Au NP SCs indicated the presence of NPs in aggregated forms as displayed through the appearance of longitudinal surface plasmon band in addition to the transverse one.