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.