Single-crystal dendritic micro-pines of magnetic alpha-Fe2O3: large-scale synthesis, formation mechanism, and properties.

Hierarchical self-assembly of nanoscale building blocks (nanoclusters, nanowires, nanobelts, and nanotubes) is a technique for building functional electronic and photonic nanodevices. Fractal structures are common in nature across all length scales, from self-assembled molecules, to the shapes of coastlines, to the distribution of galaxies, and even to the 3D shapes of clouds. On the nanoscale, dendritic fractals are one type of hyperbranched structure which are generally formed by hierarchical self-assembly under nonequilibrium conditions. 4] Investigation of hierarchically selfassembled fractal patterns in chemical systems has shown that the distinct size, shape, and chemical functionality of such structures make them promising candidates for the design and fabrication of new functional nanomaterials, but it is challenging to develop simple and novel synthetic approaches for building hierarchically self-assembled fractal architectures of various systems. Magnetic nanomaterials have been the subject of increasing interest due to their physical properties and technological applications. In the past few years, research has focused primarily on zeroand one-dimensional (1D) magnetic nanomaterials such as magnetic metals, alloys, and metal oxides and has led to substantial advances including the assembly of 2D or 3D superlattices from spherical nanoparticles or nanorods. However, to the best of our knowledge, the hierarchical self-assembly of magnetic nanomaterials has not been reported. Here we present the spontaneous, large-scale, hierarchical self-assembly of dendritic nanostructures of magnetic Fe2O3 (so-called micro-pine structure). The a-Fe2O3 micron-pine dendrites were synthesized by hydrothermal reaction of K3[Fe(CN)6] in aqueous solution at suitable temperatures. The method is based on the weak dissociation of [Fe(CN)6] 3 ions under hydrothermal conditions. The resulting structures display exquisite fractal features, which morphologically resemble a type of pine tree. The structure was formed as a result of fast growth along six crystallographically equivalent directions, and this process is different from the reported formation mechanism of general fractal structures. The magnetic properties of the nanostructure show a lower Morin transition temperature of 216 K. The reported structures could have important applications in biomedical science and magnetism. The micro-pines were synthesized on a large scale and in high purity. Figure 1 shows typical SEM images of a-Fe2O3