RESUMEN

Magnetic porous nanostructures consisting of oriented aggregates of iron oxide nanocrystals display very interesting properties such as a lower oxidation state of magnetite, and enhanced saturation magnetization in comparison with individual nanoparticles of similar sizes and porosity. However, the formation mechanism of these promising nanostructures is not well understood, which hampers the fine tuning of their magnetic properties, for instance by doping them with other elements. Therefore the formation mechanism of porous raspberry shaped nanostructures (RSNs) synthesized by a one-pot polyol solvothermal method has been investigated in detail from the early stages by using a wide panel of characterization techniques, and especially by performing original in situ HR-TEM studies in temperature. A time-resolved study showed the intermediate formation of an amorphous iron alkoxide phase with a plate-like lamellar structure (PLS). Then, the fine investigation of PLS transformation upon heating up to 500 °C confirmed that the synthesis of RSNs involves two iron precursors: the starting one (hydrated iron chlorides) and the in situ formed iron alkoxide precursor which decomposes with time and heating and contributes to the growth step of nanostructures. Such an understanding of the formation mechanism of RSNs is necessary to envision efficient and rational enhancement of their magnetic properties.

RESUMEN

The fine control of iron oxide nanocrystal sizes within the nanometre scale (diameters range from 2.5 to 14 nm) allows us to investigate accurately the size-dependence of their structural and magnetic properties. A study of the growth conditions of these nanocrystals obtained by thermal decomposition of an iron oleate precursor in high-boiling point solvents has been carried out. Both the type of solvent used and the ligand/precursor ratio have been systematically varied, and were found to be the key parameters to control the growth process. The lattice parameters of all the nanocrystals deduced from X-ray diffraction measurements are consistent with a structure of the type Fe3-xO4, i.e. intermediate between magnetite and maghemite, which evolves toward the maghemite structure for the smallest sizes (x=1/3). The evolution of the magnetic behavior with nanoparticle sizes emphasizes clearly the influence of the surface, especially on the saturation magnetization Ms and the magneto-crystalline anisotropy K. Dipolar interactions and thermal dependence have been also taken into account in the study on the nanoscale size-effect of magnetic properties.

Asunto(s)

RESUMEN

We present a simple and versatile patterning procedure for the reliable and reproducible fabrication of high aspect ratio (10(4)) electrical interconnects that have separation distances down to 20 nm and lengths of several hundreds of microns. The process uses standard optical lithography techniques and allows parallel processing of many junctions, making it easily scalable and industrially relevant. We demonstrate the suitability of these nanotrenches as electrical interconnects for addressing micro and nanoparticles by realizing several circuits with integrated species. Furthermore, low impedance metal-metal low contacts are shown to be obtained when trapping a single metal-coated microsphere in the gap, emphasizing the intrinsic good electrical conductivity of the interconnects, even though a wet process is used. Highly resistive magnetite-based nanoparticles networks also demonstrate the advantage of the high aspect ratio of the nanotrenches for providing access to electrical properties of highly resistive materials, with leakage current levels below 1 pA.