UC San Diego
Nanostructured magnetic materials
- Author(s): Chan, Keith T.
- et al.
A series of four distinct growth phases of nanostructured Ni have been demonstrated in a chemical vapor deposition process. Controlled isolation of these four distinct growth phases has been made possible through a thorough understanding of the relevant growth mechanisms and the corresponding experimental growth parameters. The growth phases include core-shell structured Ni-NiO nanowires, horizontally oriented nanowires, vertically oriented nanowires, and fully isometric cubic crystals of Ni all obtained upon an amorphous SiO₂IISi growth substrate from a single metal-halide precursor. Transmission electron microscopy indicates the horizontally- and vertically- oriented nanowire products to be high quality single- crystals with a preferred growth axis along the <001> direction while the Ni-NiO core-shell nanowires are polycrystalline metal at the center and surrounded by an outer oxide. The differing crystal structures are reflected in the magnetic response of each NW type as evidenced by magntoresistance measurements. The general mechanism responsible for achievement of vertically oriented Ni nanowires involves the introduction of trace Si amounts prior to appreciable Ni deposition. Evidence suggests the Si acts as a sink for Ni adatoms upon the substrate surface and in turn dictates a single point with high Ni crystallization rate. When crystal growth is mediated by the Si/Ni system and proceeds at an adequate rate such a site becomes the source of growth anisotropy resulting in high aspect ratio nanowire structures. As growth parameters are tuned to alter behavior such as adatom flux, diffusion length, and arrival order transitions in the overall growth behavior take place. In this way the four growth phases outlined above can be obtained in isolation of one another. Detailed discussion of the formation mechanisms leading to each of the four nanostructured Ni products is presented. A general application of this non-epitaxial approach to forming metallic nanostructures of other transition-metals is possible since the mechanisms and processes discussed are generic and not material specific. The chemical vapor deposition of CoxNiy alloy nanowires, as well as CrxSiy nanowires with a vertical orientation will be presented as both a confirmation of the general nanowire formation mechanism proposed and also as a demonstration of the flexibility of the technique