ScienceWise - Jan/Feb 2008

Scientists not sheepish about potential of nanowool

Article Illustration
Stages in the growth of nanowool from eutectic 'blobs' to fully extended wires.
Article Illustration
Dr Avi Shalav, Professor Rob Elliman and Tae-Hyun Kim with a high energy ion implanter.

Can nanowool create revolutionary new sensors?

There have been many instances in history where prominent scientists have claimed to know pretty much all there is to know about a particular topic apart from sorting out a few minor details. What has almost invariably happened in such cases is that those details have proved to be vitally important and when investigated, they have revolutionized our view of the world. To a good scientist an unresolved detail should never be swept under the carpet, as it may be a gem in disguise.

It was during the course of investigating just such a troublesome detail in thermal processing of silicon wafers that a group of ANU researchers began to uncover the underlying science of a process that may have profound applications in nanoscale biosensors. Professor Rob Elliman and his team noticed that some of the silicon wafer chips they were annealing under high temperature inert gas had white discoloration around the edges. More surprising still, if the wafer chips had a metal film on their surface, the white material covered the entire sample when annealed under certain conditions.

Thermal processing of silicon in inert atmospheres is a common procedure in the semiconductor industry but generally; the temperatures are not nearly high enough to create this surface discoloration. As a result only a handful of researchers had come across this in the past and the underlying science had never been completely understood. The team were intrigued and decided to investigate.

Initial analysis revealed that the material was silica dioxide (SiO2) which was surprising because nominally, there was no oxygen in the system. Even more surprises came when the mystery material was examined under the electron microscope. It turns out that the SiO2 was in the form of nano-wires that were an incredible 2mm long. That would be the equivalent of a head of human hair stretching 5km. The team set about trying to understand the growth mechanism of this nanostructure by subtly varying the growth conditions, introducing slabs of different source material near the wafer and even using different types of furnace tube. It’s very difficult to isolate factors in a high temperature system that can be affected by a couple of parts per million of common elements.

Extensive analysis revealed that the oxygen in the SiO2 nanowires was being provided by the few parts per million residual impurities present in the inert gas used to purge the furnace tube. Under high oxygen concentrations and at temperatures above about 1000°C silicon will form a thick coating of solid SiO2. However, at very low partial pressures of oxygen, volatile SiO forms on the surface of the wafer and is absorbed by the metal particles on the surface at a great rate. The silicon/metal eutectic then transports the SiO to growth sites on the wires. Under some conditions, the tiny metal blobs sit at the tip of each growing wire. If the parameters are varied, the blobs remain on the surface, the wires extruding from them like hairs from a mole. This difference could be vital in potential practical applications as it affects the adhesion of the layer on the substrate wafer.

Although the science of the nano-wool production process was interesting in itself, the researchers also wanted to develop a practical application for the novel material. During the course of their other work on silicon, Professor Elliman’s team use many laboratory diagnostic techniques, including photoluminescence - the reemitted light from a crystal after excitation by a laser pulse. The spectrum and lifetime of photoluminescence can reveal a great deal about the properties of a material. This set the team thinking about what might happen if the wool were excited by laser pulses. Normally one would see the properties of the bulk SiO2 but the long fine wires might theoretically be expected to add an extra dimension to this.

When light is totally internally reflected within a high refractive index medium - such as it is when it bounces down an optical fibre or SiO2 nanowire, the sinusoidal electromagnetic oscillations of the photons extend slightly beyond the physical medium. This exponentially decaying external field is called the evanescent wave and exists because the electric and magnetic fields of the photon cannot be discontinuous at a boundary. The practical upshot of this is that if the properties of the environment the fibre or nanowire sits in affects the light propagating within it. Thus if a mat of suitably prepared nanowool is excited by a laser pulse, the reemitted photoluminescence signal from the fibres will have a differing intensity and life-time if the air between the fibres contains certain chemicals. The researchers believe that this may make suitably coated nanowool a great candidate for a solid-state sensor for anything from alcohol to biologically important molecules.

It’s still early days for nanowool sensors and the researchers are cautious about making grand predictions, but at this point the possibilities do seem quite exciting.

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