Physicists have built the world's thinnest gold necklaces, at
just one atom wide.
The smallest wire width in mass produced electronic
devices is about 50 nm, or about 500 atoms across. The ultimate
limit of thinness would be wires only one atom wide. Such wires can
be made now, although not for any working electronic device, and it
is useful to know their properties for future reference.
Paul Snijders and Sven Rogge from the Kavli Institute of Nanoscience at
the Delft University of Technology, in Delft, Holland, and Hanno Weitering from the
University of Tennessee build the single-atom wires by
evaporating a puff of gold atoms onto a silicon substrate which has
first been cleared of impurities by baking it at 1200 degrees Kelvin. The
crystalline surface was cut to form staircase corrugations. Left to
themselves, the atoms then self-assemble into wires (aligned along
the corrugations) of up to 150 atoms each (see figure at
Physics News Graphics).
Then the researchers lower the probe of a
scanning tunneling microscope (STM) over the tiny causeway of gold
atoms to study the nano-electricity moving in the chain; the STM both
images the atoms and measures the energy states of the atoms'
outermost electrons. What they see is the onset of charge density
waves -- normally variations in the density of electrons along the
wire moving in pulselike fashion. But in this case, owing to the
curtailed length of the wire, a standing wave pattern is what
results as the temperature is lowered.
The wave is a quantum
thing; hence certain wavelengths are allowed. In other words, the
charge density wave is frozen in place, allowing the STM probe to
measure the wave -- the electron density -- at many points along the
Surprisingly, two or more density waves could co-exist along
the wire. The charge density disturbance can also be considered as
a particlelike thing, including excitations which at times possess a
et al., Physical Review Letters,
24 February 2006
Contact P. C. Snijders, email@example.com
Image at Physics News Graphics