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Technology: The ultimate in miniaturized circuits
Henry Gee
It may soom be possible to manufacture electrical contacts so tiny
that they comprise just one, single atom, creating the ultimate in
electronic miniaturization. In a report in the 9 July 1998 issue of
Nature, Elke Scheer of the Universität Karlsruhe and her
multinational, European research team describe how they used single
atoms of four different metals as key elements in electronic circuits.
It wasn't so long ago that computers occupied entire rooms: now we wear them as wristwatches. Once upon a time, simple switching devices required heavy, large, expensive vacuum tubes. Then came the solid-state transistor. Today, transistors are measurable in microns – thousandths of a millimetre across.
But researchers are seeking to cut this scale another thousandfold, to create electronic components on the scale of atoms and molecules. Already, a single carbon 'nanotube' has been made to work as a transistor, and a molecule of buckminsterfullerene (C60) has been turned into an amplifier. "With a whole new class of electronic devices based on single atoms or molecules entirely within our technological reach," writes Lydia L. Sohn of Princeton University in an accompanying editorial, "it is an exciting time for physics, engineering, material science, chemistry, and even molecular biology."
Scheer and colleagues created their single-atom contacts using a device called a scanning tunneling microscope, or STM. This is a device that uses an extremely fine probe to scan the electronic properties of a surface. The 'microscope' part comes from the fact that differences in voltage across the probe's tip as the machine scans a surface can be converted into images of surfaces down to atomic resolution. (The surface of carbon graphite, for example, looks like the pressed cardboard used to make egg boxes).
To make an electrical contact comprising one single lead atom, a lead STM tip was indented repeatedly into a lead surface and finally withdrawn, stretching the contact as if it were cheese fondue. The conductance was measured as the contact was stretched. Rather than falling gradually as the lead was stretched (because the same current had to squeeze through an ever-smaller cross-section of metal), the conductance fell in discrete steps: this is one of those slightly eerie quantum effects that begin to predominate at very small scales. Just before the lead was stretched to breaking point, it formed a wire just one atom across, whose electrical conductance properties could be measured.
Furthermore, the number of steps that the conductance fell before the wire finally broke depends on a simple relationship that depends on the way that electrons are disposed around the nucleus of the atom concerned. The electronic 'structure' of an atoms determines the number of routes that an electric current can burrow its way around, under, through and over an atom – and, therefore, its conductance. It's no coincidence that the chemistry of elements varies in a predictable way according to the same variations in electronic structure (such is the basis of the Periodic Table).
The researchers determined this intriguing, fundamental property of atoms by testing single-atom contacts made from elements of widely differeing chemical behaviours, including aluminium, niobium and gold. The 'break junctions' used to test the conductance properties of aluminium and gold were initially constructed by lithography. In principle, then, it should be possible to create circuit-boards with single-atom constrictions.
But perhaps most interesting of all, these single-atom contacts determine the conductance of the entire circuit in which they are placed: in other words, the quantum properties of atoms effectively determine the properties of an electronic circuit, which is an everyday, 'macroscopic' object.
© Macmillan Magazines Ltd 1998 - NATURE NEWS SERVICE
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