Multifunctional materials have attracted increasing interest in recent years because of their potential applications in novel technological devices. [1][2][3][4][5][6][7][8][9][10][11] [12] The ferroelectric and magnetic properties as well as the degree of the coupling are critically dependent on the morphology of the nanostructures, including domain patterns and shapes as well as the interfaces. In order to pursue the enhanced multifunctionality, significant effort has been made on understanding the growth mechanism and controlling the morphology of the nanostructures. The morphology adopted by a crystalline material when it nucleates on a substrate surface is one of the fundamental issues of heteroepitaxy. Depending on the surface energy terms, i.e., substrate surface energy c 1 , interface energy c 12 , and surface energy of the crystalline phase c 2 , the equilibrium shape of a crystalline nucleus on a substrate can be determined using the Winterbottom construction.[13] The possible configuration of the crystalline nucleus on the substrate is a Wulff shape that has been cut off by the substrate, translated by the signed distance Dc from the origin. Dc is the wetting strength, which is the energy difference obtained by replacing the substrate surface with an interface, Dc = c 12 -c 1 . In the BiFeO 3 -CoFe 2 O 4 system, BiFeO 3 has a distorted perovskite structure (R3c) [14] and CoFe 2 O 4 has a cubic spinel structure (Fd3m). CoFe 2 O 4 is characterized by the lowest surface energy of {111} surfaces, which is reflected in an equilibrium shape of an octahedron bounded by eight {111} facets. [15,16] In contrast, most perovskite phases have the lowest energy surfaces of {001} surfaces and a corresponding equilibrium shape of a cube dominated by six {100} facets. [17][18][19][20]

We report a discovery that self-assembled perovskite-spinel nanostructures can be controlled simply by selecting single-crystal substrates with different orientations. In a model BiFeO(3)-CoFe(2)O(4) system, a (001) substrate results in rectangular-shaped CoFe(2)O(4) nanopillars in a BiFeO(3) matrix; in contrast, a (111) substrate leads to triangular-shaped BiFeO(3) nanopillars in a CoFe(2)O(4) matrix, irrespective of the volume fraction of the two phases. This dramatic reversal is attributed to the surface energy anisotropy as an intrinsic property of a crystal.

The development of manipulation tools that are not too 'fat' or too 'sticky' for atomic scale assembly is an important challenge facing nanotechnology. Impressive nanofabrication capabilities have been demonstrated with scanning probe manipulation of atoms and molecules on clean surfaces. However, as fabrication tools, both scanning tunnelling and atomic force microscopes suffer from a loading deficiency: although they can manipulate atoms already present, they cannot efficiently deliver atoms to the work area. Carbon nanotubes, with their hollow cores and large aspect ratios, have been suggested as possible conduits for nanoscale amounts of material. Already much effort has been devoted to the filling of nanotubes and the application of such techniques. Furthermore, carbon nanotubes have been used as probes in scanning probe microscopy. If the atomic placement and manipulation capability already demonstrated by scanning probe microscopy could be combined with a nanotube delivery system, a formidable nanoassembly tool would result. Here we report the achievement of controllable, reversible atomic scale mass transport along carbon nanotubes, using indium metal as the prototype transport species. This transport process has similarities to conventional electromigration, a phenomenon of critical importance to the semiconductor industry.