A small revolution
the invention of the scanning tunnelling microscope (stm) in the early eighties revolutionised the study of the very small. The whole nanoworld opened up to the probing tip of the stm and it became possible not only to observe the microscopic world but also manipulate it to assemble and fabricate structures at the atomic scale. Since its invention, a lot of improvement has been made in the device. But the heart of the stm, the probe tip, has not seen much improvement (Nature, Vol 384, No 6605).
Now, Richard Smalley and his colleagues at the Rice University Houston, us, have reported the fabrication of a probe tip by using Fullerene nanotubes. They have attached individual nanotubes to the silicon stm tip, which was coated in a soft acrylic adhesive. The nanotubes were made by the direct-current carbon arc process.
The working of the stm depends crucially on bringing a sharp probe tip very close to the sample and scanning the sample's surface. A high resolution image of the sample is created because of the quantum mechanical interaction of the probe tip and the atoms on the surface. The probe tip can also be used to alter the sample surface. It is the ability to do this that makes stm a favourite tool of nanotechnologists. The size, shape and properties of the probe tip are thus crucial to the whole exercise. But a majority of tips used in the experiments are sharp metallic points or microfabricated structures of silicon or its compounds.
What is ideally needed is a sharp (on an atomic scale), chemically inert, easy to fabricate and cheap probe. Furthermore, the probe (which is normally used in a vacuum environment) should not lose its properties on contact with air or water.
Under certain conditions, the new nanotube separates from the other tubes to leave a single tube attached to the tip which has a diameter between five and 20 nanometers (a billionth of a meter). Experiments carried out by Smalley and his co-workers suggest some extraordinary properties of these probe tips. When they touch the surface of the sample, they bend and flex instead of breaking. The tip of the tube is closed by a hydrophobic Fullerene cage and is conductive. This is an added advantage since the probe can be used to modify the sample by passing electrical voltages. The researchers have used such a probe to create a nanoscale (40 nm) dot on a gold surface.
The importance of the experiment in the rapidly growing field of nanotechnology cannot be overstated. It is for the first time that a genuinely mole cular tool has been created; the Fullerene nanotubes are molecular in size and are attached to a control device, namely the probe of an stm. This is possible because of the unique properties of the Fullerene molecules. They occur in large bundles which can be manipulated easily. They are self-assembling and their shape is ideally suited for the probe tip.
The new Fullerene probe tips provide the scientists with an ideal opportunity to probe the nanoworld and specifically the molecular structure. The composition and the shape of the tip is known and so the tip-sample interaction can be controlled to a much larger degree than hitherto possible. It is almost certain that in the near future with further refinements in technology, we will not only be able to study and understand matter at the scale of an individual atom, but also manipulate it at that level.
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