One dimensional nanostructures have the prospect to change the properties of materials used in contemporary devices. Physical properties change with dimension and size. Ceramics, semiconductor and carbon materials are easily synthesized as one dimensional structures with typical diameters of several nanometers and length-diameter ratios of 1000:1. However, only the metals as one of the oldest are difficult to fabricate in similar geometries. In contrast, micrometer diameter, millimeter length macroscopic metallic nanowires were grown and reported decades ago via the reduction of metal halides, based on a process described already in 1574.
Recently we developed a process to grow perfect defect and flaw free nanostructures with diameters of several ten nanometers, attached on substrates. Theinitiator mediated filamentary crystal growth process is based on the physical vapour deposition technique. Metals with face centred (Cu, Ag, Au, Pd) crystal structure were synthesized successfully with the new technique. Typical diameters of the nanowhiskers are 100 nm and lengths of up to 200 µm are observed, giving aspect ratios of up to 2000:1.A preliminary growth model is proposed for the formation of the nanowires.
Microstructure characterization of the nanowires was carried out predominantly by electron microscopy, revealing a perfect, flaw and defect free bulk and surface crystal structure. No dislocations, stacking faults, or grain boundaries were detected. The growth direction is generally along the <110> crystallographic direction of the face centered cubic lattice. The nanowire surface is formed by low indexed crystallographic planes, the {111} and {100}. The overall geometry is dominated by the minimization in terms of surface energy and resembles the Wulff shape. It was not possible to detect impurities from the growth process on the surface or in the bulk of the nanowires. Preliminary tensile and bending test were performed to study the mechanical properties. The nanowires show flow and yield stresses close to the theoretical limits.
Advanced microstructures based on perfect elemental nanowires were fabricated as prove of principles for the integration of nanowires into devices. Bimetal nanostructures were prepared, as well as coated shell-core nanostructures. These can be used for the formation of metallic nanotubes via the Kirkendall effect. Structured silicon crystals surfaces were used for the growth of three dimensional composites.
Apart studying the inherent properties of pure materials, whiskers can therefore be used forfabrication of nano-composites from different materials. Tailored microstructures will enable the formation of different configuration, such as axial or lengthwise multilayered whisker. A bridge between metallic macro-devices and structures down to those dominated by the quantum effect, and between metal and semiconductor devices may be formed by initiator mediated whisker growth.