AAWSAP DIRD, Aerospace Applications of Programmable Matter, December 2009

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Four Kinds of Atoms
All matter is made of atoms and derives its properties, in part, from the fact that atoms
are discrete objects yet are so small and so close together that light waves cannot "see"
them individually. By extension, neither can electric and magnetic fields. To a photon,
or to a large electrical current, matter appears to be made up of continuous substances
rather than discrete building blocks. This fact is critically important in understanding the
optical, electrical, and even thermal properties of materials. Equally important are the
discoveries of recent decades, showing at least four different kinds of "atoms" that
meet this same general description.
NATURAL ATOMS
Natural atoms are the 92 elements of the periodic table. Actually there are more, but
the rest have unstable nuclei that will eventually fly apart Into smaller atoms and loose
subatomic particles that can damage the materials around them. For engineering
purposes, this makes them unreliable building blocks.
However, 92 building blocks allow for a staggering number of combinations, and all the
materials with which we are familiar-natural ones like coal and diamonds, ancient ones
like bronze and glass, and modern ones like silicon carbide and gallium arsenide-are
merely "Tinkertoy" sculptures of these natural atoms.
QUANTUM DOTS
A quantum dot is a very small grouping of tightly confined electrons whose collective
behavior resembles that of a natural atom. For this reason, quantum dots are
sometimes known as "artificial atoms."
To describe how this trick is accomplished, it is first necessary to talk about electrons
and how they behave. Most materials are either conductors, which permit the free flow
of electrons, or insulators, which resist it. Semiconductors are insulators that are
capable of conducting electrons above a certain threshold energy-a useful trick that
makes integrated circuits and other electronics possible. The most familiar
semiconductor is silicon, which is used to make the vast majority of microchips found in
tod;;iy's consumer ;;ind industri;;il electronics. Aecl;'l~1se silicon's n;;itive Q){ide, SiO?, is the
main component of sand and rocks, it is readily available and relatively inexpensive. In
addition, when melted, purified, and hardened into sheets, silicon dioxide serves as one
of our familiar insulators and building materials: glass. Unlike most other
semiconductors, silicon is also nontoxic.
The electrical properties of a semiconductor like silicon are of course fixed by the laws
of physics. Atoms hold electrons in shells that increase in size, capacity, and potential
energy the farther they are from the nucleus. "Valence" electrons are found in full (or
nearly full) shells, where there are few empty spaces through which electrons can
move. These electrons tend to stay at home, so their levels exhibit a large electrical
resistance and do not permit electricity to flow. "Conduction" electrons are found in
shells that are more than half-empty and have lots of open space, enabling electrons to
travel freely through them and move easily from one atom to another. Between these
layers is a "band gap" of forbidden energies. Here, there exist no electrons at all-ever.
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