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Self-organization of nanoparticles into two- and three-dimensional superlattices on a large scale is required for their implementation into nano- or microelectronic devices. This is achieved, generally after a size-selection process, through spontaneous self-organization on a surface, layer-by-layer deposition or the three-layer technique of oversaturation, but these techniques consider superlattices of limited size. An alternative method developed in our group involves the direct formation in solution of crystalline superlattices, for example of tin nanospheres, iron nanocubes or cobalt nanorods, but these are also of limited size. Here, we report the first direct preparation in solution of multimillimetre-sized three-dimensional compact superlattices of nanoparticles. The 15-nm monodisperse FeCo particles adopt an unusual short-range atomic order that transforms into body-centred-cubic on annealing at 500 degrees C. The latter process produces an air-stable material with magnetic properties suitable for radiofrequency applications.

Monodisperse FeCo nanoparticles were synthesized via thermal decomposition of appropriate organometallic complexes in heated toluene containing surfactants, aiming to a soft magnetic nanomaterial with a high metal percent. The particles were characterized by X-ray powder diffraction, electron microscopy, and static magnetometry techniques. The effect of air exposure on the structural and magnetic properties was examined. Portions of the dried nanoparticles were mixed with additional surfactants or polyvinylpyrrolidone, followed by redispersion into tetrahydrofuran, so as to study arrangement features in the first case, while the use of polyvinylpyrrolidone intended to the expansion of the interparticle distances in order to get more "isolated" nanoparticles. [ABSTRACT FROM AUTHOR]

Ultrafine cobalt particles have been reproducibly synthesized by decomposition of an organometallic precursor in the presence of a stabilizing polymer. The size of the stable monodisperse colloids thus obtained is seen to strongly depend on the nature of the polymer: around 4.2 nm diameter in polyphenylenoxide (PPO) and around 1.4 nm diameter in polyvinylpyrrolidone (PVP). Investigations by wide angle x-ray scattering (WAXS) and high-resolution transmission electron microscopy (HRTEM) give evidence for a size dependence of the structural organization, and hence for a close relationship between structure and synthesis conditions. Co/PPO particles exhibit a hexagonal compact structure with the metal-metal bond length of the bulk material while Co/PVP ones display an original structure. We show that the unusual features of the experimental data in Co/PVP clearly point to a nonperiodic polytetrahedral structure. Successful simulations of the HRTEM and WAXS results have been obtained using models built on the basis of a polytetrahedral arrangement. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: We present the synthesis and magnetic properties of Co nanorods spontaneously organized in superlattices over a surface of several microns. This material results from the decomposition of a cobalt coordination precursor under hydrogen, in the presence of a long-chain amine and a long-chain acid. This synthetic procedure permits a remarkable control over the size distribution of the nanorods (diameter of 5 nm and controllable mean length from 40 nm to 100 nm). The nanorods are monocrystals, and they are organized side by side along their long axis, in a direction perpendicular to the substrate exposing a surface of tips. Such layers are superposed forming 3D superstructures. These nanorod superlattices are ferromagnetic at r.t. and they are characterized by a strong coercive field as a consequence of their large magnetic anisotropy (magnetocrystalline and shape). Upon application of an external magnetic field the organization is improved even further, and this improvement is partially retained even after field removal. Therefore, this system can be considered as a good candidate for high density magnetic recording. [Copyright &y& Elsevier]