Formation of 2-D and 3-D Arrays of Colloidal Co Magnetic Nanoparticles
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Formation of 2-D and 3-D Arrays of Colloidal Co Magnetic Nanoparticles Y. Chushkin1, L. Chitu1, S. Luby1 and E. Majkova1, A. Satka2, V. Holy3, J. Ivan4, M. Giersig5, M. Hilgendorff5, T. H. Metzger6, O. Konovalov6 1 Institute of Physics SAS, 845 11 Bratislava, Slovakia 2 International Laser Center and Faculty of Electrical Engineering and Informatics SUT, 812 19 Bratislava, Slovakia 3 Institute of Condensed Matter Physics, Masaryk University, 611 37 Brno, Czech Republic 4 Institute of Materials and Machine Mechanics SAS, 83102 Bratislava, Slovakia 5 caesar research center, Ludwig-Erhard-Allee 2, 53175 Bonn, Germany 6 European Synchrotron Radiation Facility, 38043 Grenoble Cedex, France ABSTRACT Ordering of Co nanoparticles (~11 nm in diameter) into 2-D and 3-D arrays on Si/Si3N4 substrates in external magnetic field and without field is reported. Arrays of particles were studied by TEM, SEM and GISAXS. The GISAXS measurements were performed at the wavelengths 0.155 nm and 0.336 nm and the spectra were simulated using distorted wave Born approximation approach. From results it follows that 2-D ordered monolayers of particles are composed of hexagonal close-packed mosaic blocks. 3-D arrays – rods are formed along magnetic field direction, being parallel or perpendicular to the substrate surface, when the colloid was more concentrated. Distribution of particles in rods was analyzed only by GISAXS and it was described by close packing of hard spheres. Their effective diameter was 14.7 nm. INTRODUCTION Self-assembling of colloidal metallic nanoparticles is of considerable technological interest because of the possibility to synthesize new artificial materials with unique properties. Regularly self-assembled arrays of semiconductor, metallic or ferromagnetic nanoparticles have been reported recently [1-4]. For magnetic nanoparticles both spontaneous assembling and ordering in external magnetic field were studied [5-7]. Ordered arrays of nanoparticles show specific properties which are different from those of bulk or isolated particles [8]. In this work we report on the formation of ordered 2- and 3-dimensional arrays of colloidal magnetic Co nanoparticles on common substrates in external magnetic field. Up to now mostly the formation of 2-D arrays of nanoparticles (monolayers) was studied. Besides monolayers, nanoparticles can form also ordered multilayer stacks similar to an artificial crystal [8]. Formation of another type of 3-D arrays - rods of colloidal Co magnetic particles was reported by us recently [5]. Rods (several hundreds of nm in diameter) aligned in the field direction are formed by drying the colloid in magnetic field, which is perpendicular or parallel to the substrate. The analysis of the ordering of 2-D and 3-D arrays of nanoparticles performed by SEM and GISAXS is presented here thoroughly. EXPERIMENTAL DETAILS Cobalt nanoparticles (~11 nm in diameter) have been prepared by thermolysis of dicobaltoctocarbonyls (Co2(CO)8) in the presence of surfactants (oleic acid and oleyl amine).
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