Small (Wiley Interscience), Volume 2, Issue 7, pp 864-869, May 2006.
1 School of Chemistry, Trinity College Dublin, Dublin 2, Ireland
2 Department of Electronic & Electrical Engineering, University of Dublin, Trinity College Dublin, Dublin 2, Ireland
3 School of Physics, Trinity College Dublin, Dublin 2, Ireland
4 Department of Ioannina, POBox 1176, 45110, Ioannina, Greece
5 Swinburne University of Technology, POBox 218, Hawthron, Victoria 3122, Australia.
Hierarchical assembly of nanoparticles is very important for the development of bottom-up approaches in technology. The self-organisation of nanoparticles has been studied extensively and the preparation of novel nanostructured material is an emerging topic in the field of advanced materials. The self- assembly of nanoparticulate materials strongly depends on inter-particle interactions, particle size distribution  and particle shape. The process of assembly of nanoparticles can result in different patterns such as rings, rods or needles.  The different shapes and morphologies of the particle assemblies depend on the dipole-dipole and other interactions between individual particles and also on the interfacial processes involving the substrate and the nanoparticles. It has been reported that by varying the conditions for nanocrystal deposition on substrates, a variety of organised structures can be prepared. There are several reports on fractal structures formed by metal particles including one on forming fractal silver nanocrystallites by using y-radiation in the presence of solvents. It has been proven theoretically that the formation of fractal structures on solid substrates does have some dependence on the roughness of the substrate. The theoretical and mathematical basis for the formation of fractal structures has been well developed. Recently magnetic Fe3O4 fractal nanocrystals have been synthesised by a solvothermal process. Cao et al have also reported the preparation of single crystal fractals of magnetic a-Fe2O3 using hydrothermal reactions. Magnetic nanoparticles are widely studied for their application in various fields such as information storage, colour imaging, bioprocessing  and in controlled drug delivery. Therefore, self-organisation of magnetic nanostructured materials into fractal structures is a very important aspect of nanotechnology and modern materials science.
In this communication, we report for the first time the self-organisation of magnetic Fe3O4 nanoparticles into fractal dendrite-like structures using porous silicon (PSi) as a substrate. In contrast to the recently reported  surfactant assisted solvothermal preparation of magnetite fractal nanocrystals, our approach is relatively simple and does not require the use of high temperature and pressure or any surfactants. Also, all reagents, materials and equipment used in our preparation are readily accessible. The magnetite nanoparticles arranged on porous silicon templates were characterised by micro-Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD), Mossbauer spectroscopy and magnetic measurements (SQUID).PDF