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Molecule Cascades
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Almaden researchers have built and operated the world's smallest working
computer circuits using an innovative new approach in which
individual molecules move across an atomic surface like toppling
dominoes.
The new "molecule cascade" technique enabled the
IBM scientists to make working digital-logic elements some 260,000
times smaller than those used in today's most advanced semiconductor
chips.
The circuits were made by creating a precise pattern
of carbon monoxide molecules on a copper surface. Moving a single
molecule initiates a cascade of molecule motions, just as toppling a
single domino can cause a large pattern to fall in sequence. The
scientists then designed and created tiny structures that
demonstrated the fundamental digital-logic OR and AND functions,
data storage and retrieval, and the "wiring" necessary to connect
them into functioning computing circuitry.
The most complex
circuit they built -- a 12 x 17-nanometer three-input sorter -- is
so small that 190 billion could fit atop a standard pencil-top
eraser 7mm (about 1/4-inch) in diameter. A nanometer is a billionth
of a meter; the length of five to 10 atoms in a line.
The molecule cascade
works because carbon monoxide molecules can be arranged on a copper
surface in an energetically metastable configuration that can be
triggered to cascade into a lower energy configuration, just as with
toppling dominoes. The metastability is due to the weak repulsion
between carbon monoxide molecules placed only one lattice spacing
apart.
This situation is analogous to placing tennis balls
next to each other in an egg carton. Since the tennis balls are
slightly larger than the lattice spacing of the carton, they push
against each other and can't nestle down into the hollows of the
carton as deeply as they could if they were more widely separated.
Just as placing three tennis balls in a row of an egg carton is
unstable, Heinrich and Lutz learned that a triad of carbon monoxide
molecules arranged in a chevron-shaped pattern on the copper surface
would spontaneously rearrange by the outward motion of the central
molecule. They then designed ways to link pairs of molecules so the
rearrangement of an initial chevron formed a new chevron, and so on,
in a cascade of molecular motion.
What enables computation is
that each cascade carries a single bit of information. By analogy, a
toppled domino can be thought of as a logical "1," and a untoppled
domino can be thought of as a logical "0." Similarly, a cascaded or
non-cascaded molecular array can represent a logical "1" or "0,"
respectively.
The logic AND and OR operations and other
features needed for complex circuits are created by cleverly
designed intersections of two cascades. Heinrich and Lutz designed
molecular arrangements that acted as crossovers (allowing two
cascade paths to cross over each other) and fanouts (splitting one
cascade into two or more paths).
These molecule cascades are
currently assembled by moving one molecule at a time using an
ultra-high-vacuum, low-temperature scanning tunneling microscope
(STM). It takes several hours to set up the most complicated
cascades. Since there is no reset mechanism, these molecule cascades
can only perform a calculation once. While these initial cascades
rely on the motion of a molecule, Eigler envisions that it should be
possible to make nanometer-scale cascades using other fundamental
interactions, such as electron spin. Such cascades may also be
resettable, allowing repeated calculations, similar to ordinary
computer circuitry.
More details about the molecule cascades are available in the IBM Research Press Room.
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