V. Molecular Architecture - Carbon, Meet Silicon
Silicon -- Natural Forms, Ancient Uses
Silicon. If
you think about this word for a moment, maybe the first thing that would
come to mind is Silicon Valley, the geographic heart of the computer
industry. Or perhaps in your mind's eye you could see a clear gel, or
a silicon chip. Maybe you imagine a nebulous manmade material used in
the manufacture of our common but still new technologies such as solar
cells, batteries or transistors and semiconductors. Rarely do we think
about silicon as a simple and natural element in the world around us:
sand on the beach, black clear remnants of a volcanic eruption, or essential
domestic artifacts like the transparent pane occupying the nearest window
frame or the glass you drink from at each meal. We don't link silicon
to its fundamental presence in the ancient
art of glass-making, one of the oldest technologies found in the
earliest civilizations of Egypt and Babylonia. And how many of us would
think about silicon as the main element comprising the sharp, strong
obsidian blades or arrowheads in humans' most ancient tools?
As common as silicon is in today's world, many of us would be surprised
to learn that it is the second most abundant element on earth, making
up almost a third of the earth's crust, and the main ingredient in even
the tiniest grains of sand or the hardest samples of natural glass and
quartz crystals. But it can be said, with its myriad applications, that
this nonmetallic element has almost single handedly redefined our notion
of "machine" in the second half of this century, rapidly becoming
an increasingly transparent, weightless, ephemeral analogue of human
life. Even so, we still closely guard our outmoded concept of machines
as hard, and hollow metallic shells or cold crankshafts, solid, smooth
pistons and notched metal gears, connotations which continue to weigh
down and polarize the relationship between humans and their technologies.
Carbon -- The Basis of Life
"The carbon in our body originated
in space. Indeed, we now know that it was ejected from some star
a long, long time ago and then was reprocessed and ended up on
the Earth's biosphere. What is absolutely fascinating and certainly
something that excited me when I first discovered it, is that
every one of us is made of carbon, therefore every one of us is
made of stardust... "One thing they are not so sure of is
what is the form of that dust, what is the structure. How does
the carbon nucleate to form these that form these little wudges
that go on to grow into planets?" 36
Harold
Kroto
Carbon, known now
in its three forms, graphite,
diamond and buckminsterfullerene, is the basis of organic chemistry,
the fundamental element of all biological life as we have come to know
it. Yet carbon is closer to silicon than to any other single element.
Referred to in high school chemistry labs as "kissing cousins,"
carbon and silicon have more similarities in their respective properties
and structures than differences. Because of the placement on the periodic
table, each containing four electrons in their outer shells, they
are found right next to each other in the same vertical family. Because
of their tendency to easily bond with other elements, both can be found
or combined into numerous other forms.
It is easy to compare these two abundant elements so central to everyday
life. Although silicon is only slightly more metallic than carbon because
it tends to lose electrons more easily, it is safe to say that silicon
is considered inorganic while carbon is organic. If we wanted to compare
carbon with silicon as analogues of the animate and inanimate world,
we need only look to their molecular structures and the functions they
fulfill.
The three allotropes of carbon: diamond, graphite and buckminsterfullerene37
exhibit many of the same structural characteristics as various forms
of silicon. These three pure forms of carbon vary from one another by
the crystal structure the atoms take. It is possible to compare each
with a form of silicon that has similar properties or fulfills similar
functions in our everyday world. For the purposes of this qualitative
comparison, graphite's analog could easily be mica,
diamond's analog, quartz
crystal and buckyball's could be silicones
or even the recently discovered silicon-60.
Silicon and Carbon -- A Comparison
Graphite,
the simplest form of pure carbon, is so soft it can be cleaved at any
point. Each atom is bonded to three others forming layers of flat sheets
of hexagons which can easily slide back and forth over one another.
Mica, a mineral containing atoms of silicon, aluminum and oxygen (S02)
also has a flat hexagonal structure and can be easily cleaved into thin
sheets like graphite.
Graphite is one of our most common communication tools, its name meaning
"to write." It is soft and "greasy" so it can easily
be used to lubricate moving machine parts. Mixed with clay, graphite
forms the "lead" in pencils, simple instruments each of us
have held since our earliest years in school. Its longevity, as well
its ability to be erased lends pencil lead nicely to marks on paper
artifacts. This gives graphite an intrinsically functional and lasting
value in the varied manuscripts and documents found in every archives
in the world. Graphite, like mica also has many industrial uses.
Diamond
is the strongest natural substance known, is often used to grind and
polish other materials to the finest degree. A precut diamond can be
identified by its octahedral outer shape and its atoms are arranged
in a dense, tetrahedral framework or lattice. Each carbon atom of diamond
is the center of one tetrahedron and the vertex of another. By the same
token, quartz crystal's molecular structure is a also a dense and strong
tetrahedral network. Although not as hard as diamond, quartz's crystalline
structure is very similar to it. In addition, while diamonds are formed
in molten lava by the heat and pressure deep in the earth, obsidian,
a form of silicon, is the hard black glass formed of lava when it meets
oxygen and hardens in the outer atmosphere.
Diamonds come from the core of the Earth. They are the strongest and
most aesthetically valuable crystals known to man which are used symbols
for lasting love and the bonds of marriage as well as for drill bits
used to dig deep into earth and cut through the hardest materials. Quartz
crystal is also one of the hardest minerals. Its conductivity enables
an electric current to ride through this beautiful silicon-based quartz
crystal. Both diamond and crystal have been used liberally for tools
used in making lenses for microscopes and telescopes.
Buckminsterfullerene,
the most recently discovered allotrope of pure carbon, is a hollow cage
molecule with flexibility and strength, exhibiting perfect spherical
symmetry. Buckminsterfullerene's 60 atoms are bonded together in a stable,
yet flexible hexapent cage. Its most unique property is that it can
be bonded both from the inside and the outside. Silicon and oxygen combine
to form silicones, a cross between organic materials such as oil, rubber
and plastics and inorganic materials such as sand, glass and quartz
crystal. Buckyballs on the other hand, also carry structural characteristics
of both the organic fivefold world and the inorganic crystal world.
The links in the molecular skeleton of alternating silicon, oxygen and
other atoms in silicones have bond strength about one and a half times
as great as the carbon bond that holds organic molecules together, thus
giving them organic properties such lubrication, water repellence and
flexibility, the same properties of buckyballs. Both buckyballs and
silicones both form polymers.
Buckyballs are just beginning, in theory and in their first real applications,
to revolutionize the already existing computer industry. They will soon
be used as superconductors, components of tiny batteries, lubricants,
"beads" in nano-sized abacuses for computer information coding
and as essential components of soon-to-be-seen "optical tweezers,"
nanoprobes of the atomic scale. Fullerenes may soon become models for
a new generation of microscale machines. In medicine and in the body,
water-soluble fullerenes may someday replace the tiniest components
of our cellular make-up, such as microtubules
in the brain's neurological structure. Because of the cage-like structure,
buckyballs may someday contain and dispense with viral material that
grow and breakdown cells of the body through cancer or HIV.37
Silicones, the most flexible materials when used in rubber-like compounds,
have myriad applications in various technologies. They are often used
in the body as well.
