Self-Organized Structures on Flat Crystals: Nanowire Networks Formed by Metal Evaporation
Nanotechnology often requires fabricating large, well-ordered assemblies of nanoscopic components on the surface of a substrate. It has been demonstrated that such ordered nanoassemblies can be obtained by pre-structuring the substrate, e.g., with a regular pattern of surface steps  or step bunches [2,3,4,5,6,7,8,9]. However, an equally powerful yet much simpler approach, is self-organized growth. We demonstrate the strength of this approach using the self-organized growth of metal nanowires on the surface of layered transition-metal dichalcogenide (TMDS) crystals. The surfaces of these crystals are perfectly flat - they do not reconstruct and feature virtually no step edges or other defects. Nevertheless, evaporation of certain metals onto these surfaces leads to the formation of large assemblies of nanowires. These have a diameter down to 8 nm, and they form regular networks with mesh diameters of 200 to 400 nm, extending over macroscopic distances (millimeters) [10,11]. Moreover, we have discovered that a variety of other nanocomponents forms along with the wires: cluster arrays forming triangles or parallelograms, as well as nanotunnels forming networks similar to those of the nanowires. We believe that the formation of these highly organized structures on a featureless substrate occurs because, on the one hand, surface diffusion is very rapid, and on the other hand, the weak interaction between the substrate and the adsorbed atoms enables them to detach from the substrate. In contrast to more familiar cases of surface diffusion, therefore, metal atoms on TMDS crystal surfaces are strongly influenced by ``second order'' phenomena, e.g. strain in the substrate, charge transfer to the substrate, or other phenomena that affect the surface diffusivity. In addition to the mechanism by which the nanostructures form, we discuss some of their physical properties and the influence of different processing parameters on their formation.
KeywordsCharge Density Wave Step Edge Nanowire Growth Copper Evaporation Chemical Vapor Transport
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