Atom Wires

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 wire.

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 fractional charge.

Snijders et al., Physical Review Letters, 24 February 2006
Contact P. C. Snijders, p.c.snijders@tnw.tudelft.nl
Image at Physics News Graphics