Morphology and plasmonic properties of self-organized arrays of ...

Chapter 6Composite magnetic-plasmonicmedia based on Au/LiF arraysIn this chapter, we report a selected example of fabrication of a 2D array of magneticnanoparticles, realized employing the array of gold nanoparticles as a template for guidingthe deposition of the magnetic clusters. Such a composite system, with both optical andmagnetic functionality, would representthestartingpoint torealize, for example, opticallyactive magneto-plasmonic media, with tunable LSPs controlled by the application of anexternal magnetic field. For this purpose, we started by investigating the deposition ofmagnetite (Fe 3 O 4 ) nanoparticles on the Au NPs arrays from a colloidal suspension, andcharacterizing the morphology and the variations of the optical properties induced by thepresence of the magnetic medium.Solutions of Fe 3 O 4 nanoparticles coated with oleic acid (Fe 3 O 4 /OA) dispersed in hexanewere synthesized in collaboration with CSIC/ICMM in Madrid, following establishedprocedures [215–217]; oleic acid was chosen as surfactant in order to prevent the aggregationand the precipitation of the particles. Fe 3 O 4 nanocrystals were prepared by thethermal decomposition of iron (III)-oleate complex in 1-octadecen, in the presence of oleicacid in a relation of 3:1 with respect to the metal complex. The solution was slowly heatedunder nitrogen atmosphere to ≈ 300 ◦ C, and then aged at that temperature for 240 minuteswhile stirred, generating the iron oxide nanocrystals. A representative image of somenanoparticles dried from the colloidal suspension, acquired by transmission electron microscopy(TEM), is reported in fig. 6.1(a) (courtesy of ICMM). The image reveals themagnetite cores of the particles, which appear cubic in shape. From a statistical analysisof the NPs sizes, the diameters of the cores d core resulted lognormally distributed, with amean and a standard deviation of d core = (13.0±1.3) nm.The colloidal suspension was characterized by dynamic light scattering (DLS) [218],an experimental technique based on light scattering from small particles, which allowsto deduce the hydrodynamic size d hydro of the particles, including the length of the surfactantmolecules. The statistical distribution of d hydro obtained from DLS is reportedin fig. 6.1(b): similarly to TEM analysis, the NPs sizes are lognormally distributed, andthe diameters have a mean and standard deviation of d hydro = (16±4) nm. Comparingthis result with the values from TEM analysis, we obtain a mean length of the surfactantmolecules of ≈ 1.5 nm, comparable with the typical length of oleic acid molecules of1.8 nm [219]. The distributions of sizes also show that the suspension is monodisperse,and that the tendency of the NPs to form aggregates is negligible.97