Tuesday, February 28, 2006

Atom Wires

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 and Hanno Weitering from the University of Tennessee build the world's smallest gold necklaces by evaporating a puff of gold atoms onto a silicon substrate which has first been cleared of impurities by baking it at 1200 K. 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). 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; it 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)

The American Institute of Physics Bulletin of Physics News
Number 767 February 28, 2006 by Phillip F. Schewe, Ben Stein, and
Davide Castelvecchi

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