From: Randy (cryon@mindspring.com)
Date: Sun Jul 19 1998 - 23:55:16 MDT
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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withdrawn, stretching the contact as if it were cheese fondue.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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