Feb 11, 2009
Scientists grapple with gold bead-strings
Amorphous silica nanowires can be grown on a crystalline silicon substrate by first depositing a thin gold film on its surface and then heating it above 1000 °C in an inert atmosphere containing 3–5 parts per million of oxygen, which reacts with the silicon substrate and converts it to silicon monoxide.
The gold film contracts to form molten droplets of a gold-silicon eutectic that absorbs silicon and oxygen from the surrounding gas and produces silica, which then grows from the droplet as a wire (root growth). If another silicon wafer is placed a small distance above the metallised one – an arrangement that produces a secondary source of silicon monoxide vapour – then the gold droplets are carried on the tips of the growing wires (float growth).
While some of the wires are solid amorphous silica, a moderate number contain within them a string of nearly uniformly spaced gold droplets, producing what might be called a bead-string when they solidify on cooling.
Any such self-assembled nano-scale object is of theoretical interest, but also carries the possibility that it might be practically useful in electronic or optical devices, so that it is important to discover just how the structure evolves. While a Rayleigh instability such as that which breaks a water jet into droplets is possible, the alternative mechanism investigated in this paper provides a better explanation.
The basis of the mechanism is simply the rate of diffusional growth of the silica-gold interface, which gradually becomes concave, then conical and finally pinches off before starting the cycle again. The roughly conical included gold droplets then become spherical under the influence of further diffusion driven by surface free energy.
The possible use of metallic bead-strings in electronic nanodevices is still a matter for the future, but interest is growing in the general use of nanowires and a sequence of metallic spheres separated by about 100 nm could perhaps perform some unusual and useful function.
The researchers presented their work in Nanotechnology.
About the author
The three authors are all members of the Department of Electronic Materials Engineering in the Research School of Physics and Engineering at the Australian National University in Canberra, Australia. Tae-Hyun Kim, from Korea, is a graduate student nearing the end of his PhD studies which have encompassed several related fields. Rob Elliman, who is Tae-Hyun's supervisor, is professor and head of the department and carries out research in a wide variety of fields with specialization in ion implantation in semiconductors. Neville Fletcher is an Emeritus professor and visiting fellow in the department, retired but still working full-time, with theoretical interests ranging from condensed-matter physics through cloud physics to musical acoustics. The research of the department is supported by core university funding and also by a variety of grants from the Australian Research Council.