Materials technology
is the real wealth of nations. It has been the hallmark of civilizations
stretching back 8,000 years to the Bronze Age. Today's Information Age is no
different. Without silicon that is 99.99999% pure, there wouldn't be computer
chips, cell phones, or fiber-optic networks. In recent decades, inorganic
chemists have concocted an impressive array of metals, alloys, and
ceramics--sending skyscrapers ever higher, making cars lighter and more
fuel-efficient, and launching air travel for the masses.
Organic chemistry, meanwhile, contributes its own share of modern conveniences.
It produces the plastic packages that keep foods fresh, the housings that
protect TVs and appliances, and synthetic fleeces that ward off the cold.
Organic chemists created the fertilizers that help feed the world--and the
ever-growing medley of wonder drugs that keep people healthy.
BOUTIQUE. Now, materials scientists are determined to transform the
world yet again. Not content with the raw materials that can be extracted from
the earth, researchers are mining their imaginations for totally new
structures. They're doing it by tearing down the walls between organic and
inorganic chemistry, something that not long ago would have been dismissed as
pseudo-science. Tomorrow's inorganic-organic combos will be tailored from the
bottom up--patched together from individual atoms or molecules--to provide the
precise properties needed for each specific application. Arden L. Bement Jr.,
an engineer at Purdue University, calls it the dawn of ''the boutique materials
age.'' And it promises to turn Mother Nature green with envy.
The possibilities are almost endless: Polymer-based paints and coatings that
contain tiny ceramic particles to defy scratching and corrosion. Improved
catalysts that spawn new pharmaceuticals and plastics. Iron-polymer batteries
that generate twice as much power. We could also see resilient metal-composite
car-body panels that pop back into shape after minor fender-benders. Tough yet
lightweight composites that boost jet-engine performance--including turbofans
that self-repair tiny stress cracks. And there'll be all manner of materials
with internal smarts that emulate biological systems, enabling them to adapt to
changing conditions, compensate for wear, and warn of impending trouble.
Biological metaphors are a frequent theme among nano researchers, partly
because they work with things measured in nanometers. This is the scale at
which molecules mingle to create DNA and proteins, the building blocks of life.
A nanometer is a billionth of a meter. Individual atoms are a few nanometers in
diameter.
Indeed, a chief goal of organic-inorganic weddings is to breathe a spark of
life into tomorrow's materials progeny. That's not just a dream. With nano
engineering, ''almost anything you can think of will be possible,'' declares W.
Lance Haworth, executive officer for materials research at the National Science
Foundation (NSF). The idea, says John H. Weaver, head of materials science at
the University of Illinois, is to harness the molecular mechanisms that Nature
has evolved for the production of new materials.
The appeal of bio-inspired materials is easy to understand: The human body is a
marvelously efficient factory. Most cells in the body contain the entire
genetic code--4 billion ''base pairs'' of data that contain the recipes for
making every one of the body's 10 trillion cells. But each cell actually needs
just a tiny chunk of the genetic code to reproduce itself, and it manages to
track down only that data. Since all 10 trillion cells replace themselves every
few years, the body cranks out new DNA at the amazing rate of 10,000 miles an
hour, day in and day out. But before nanotechnology can give birth to
''living'' materials, many fundamental issues must be resolved. Biological
systems, Haworth notes, ''know when to switch various functions on and off. How
do you mimic those processes in synthetic materials? How do you get organic and
inorganic materials to grow cooperatively together?''
EXOTIC TOOLS. To find the answers, materials wizards are using exotic
tools such as scanning-probe microscopes with supersharp tips. These not only
trace the outlines of individual atoms but also move them around to explore how
nanosize elements fit together. For example, the nanoManipulator at the
University of North Carolina makes dabbling in chemistry so easy that
biologists and physicists regularly use it to play with atoms and molecules as
if they were Tinkertoys. ''What we're having the most fun with now,'' says Sean
Washburn, a North Carolina physicist, ''are cigar-shaped nanotube molecules.''
These are elongated versions of the carbon buckyballs discovered in 1985 by
Richard E. Smalley, a professor of physics and chemistry at Rice University.
The discovery earned Smalley a Nobel Prize in 1996.
Nanotubes are truly wonder molecules. First created in 1991 by Sumio Iijima, a
scientist at NEC Corp., nanotubes are at least 100 times stronger than steel,
but only one-sixth as heavy--so nanotube fibers could bolster just about any
material. Moreover, nanotubes can conduct heat and electricity far better than
copper, so reinforcing strands could do double duty as computer circuits,
creating the nervous system for ''smart'' materials. Or they could serve as
heat pipes to keep composite-plastics parts cool enough to be used in engines.
These new molecules extend the performance of organic materials to
unprecedented levels. But inorganic substances still offer many advantages. And
nanotechnology provides the means to fashion inorganic-organic hybrids. ''Now
you can develop materials that you couldn't even imagine before, because the
mix of properties just wasn't available,'' says Ralph E. Taylor-Smith, a
chemical engineer at Lucent Technologies Inc.'s Bell Laboratories. Inorganic
materials tend to be hard but brittle--glass being a prime example. Organic
materials, on the other hand, are typically soft and rubbery. Merge the two
sets of properties, says Taylor-Smith, and you could get a substance that's
very strong and hard, yet with enough ''give'' to impart good impact
resistance. One example: a big flat-panel display for handheld computers that
folds up.
SCATTERSHOT. Inorganic-organic hybrids are hardly new, of course.
Fiberglass-reinforced plastics have been around for decades. But these are
essentially physical mixtures. They often suffer from weak chemical bonds
between the reinforcements and the bulk material. Beyond a certain stress
level, the two materials can delaminate, which often leads to structural
failure. Nano engineering promises to cure that by tailoring the surfaces of
components so they have chemical hooks specifically designed to glom onto each
other.
It's basically an issue of size. Silicon's carefully crafted semiconducting
properties are produced after the fact, by shooting boron or other atoms into
the finished bulk material. Tomorrow, this scattershot approach won't be good
enough. Organic molecules will have to be placed at specific locations with
atomic precision. One method might be to insert organics inside the crystalline
structure of silicon, so its properties can be chemically tweaked from within.
A research team at the University of Toronto, led by chemist Geoffrey A. Ozin,
pulled off such a feat late last year. They crammed organic molecules into
porous silica, producing an insulating compound that could replace the
silicon-dioxide layers on future chips. Circuit lines and insulation will soon
shrink to dimensions at which silicon dioxide is no longer an effective
insulator.
At the University of Minnesota, a team led by chemist Xiaoyang Zhu tackled
another size-related problem by attaching chlorine atoms to silicon atoms. The
problem is called stiction--the stickiness that particles exhibit starting in
the so-called mesoscale range. This is in between the nano realm of quantum
physics and the macro world of chemical events that unfold in test tubes or
industrial vats. Stiction kicks in at around 1,000 nanometers. Below that,
surface forces ''really dominate everything else,'' says Zhu. Even fluids have
a hard time moving, not to mention the components in microelectromechanical
systems, or MEMS--little machines carved into silicon chips. But a coating of
chlorine that's one, and only one, atom thick can create a lubricating film
that will make it easier for MEMS designs to include parts that move across the
surface.
The nano world has its own size-related surprises. For example, researchers at
Massachusetts Institute of Technology discovered that semiconductor nano
particles sandwiched between one polymer that conducts negative charges and
another that conducts positive charges can create light-emitting diodes (LEDs)
that emit colors spanning the visible spectrum. The same structure produces a
rainbow of hues depending on the size of the particles. Green light is emitted
by 1.8-nanometer particles, while 7.5-nanometer particles glow red when they're
illuminated with ultraviolet light. This makes for more than a pretty light
show. Diodes like these could provide the color in those fold-up displays for
pocket computers--or for ''wallpaper'' TV screens in homes.
Similarly, the bulk properties of materials often change dramatically with nano
or meso ingredients. Composites made from particles of nano-size ceramics or
metals smaller than 100 nanometers can suddenly become much stronger than
predicted by existing materials-science models, says Richard W. Siegel, chief
nanotechnologist at Rensselaer Polytechnic Institute. For example, metals with
a so-called grain size of around 10 nanometers are as much as seven times
harder and tougher than their ordinary counterparts with grain sizes in the
hundreds of nanometers.
The causes of these drastic changes stem from the weird world of quantum
physics, says Thomas N. Theis, IBM's director of physical science. The bulk
properties of any material, he explains, are merely the average of all the
quantum forces affecting all the atoms. ''But as you chop things smaller and
smaller, you eventually reach a point where the averaging no longer works.''
Associated changes in electrical properties have grabbed the attention of the
U.S. Air Force, says John Kieffer, an inorganic chemist at the University of
Illinois. The radar ''signature'' of a jet plane coated with nano compounds
could be smaller and stealthier. Mesoscale particles may turn up in car paints.
Andreas Stein, a Minnesota chemist, is working on photonic crystals that could
be tuned to reject green light. Green light carries the most energy of any
color. Block it with special paints, and the interior of cars parked in the
sunlight wouldn't get so hot. Or the paint could include tough ceramic
particles smaller than the wavelengths of visible light--making them
transparent, so they wouldn't affect the color of the paint. But the particles
could provide a virtually scratchproof surface.
CHAMELEON. For materials scientists, all these strange new potentials
are inspiring new dreams. At the University of California at Los Angeles,
chemist James R. Heath envisions building unique materials from ''artificial
atoms.'' These actually are small clusters of atoms dubbed quantum dots. With a
bottom-up approach, Heath predicts, it will be possible to create materials
that could never be produced with traditional chemistry. To prove it, his
research group recently hatched a chameleon material that responds to
electrical signals by switching back and forth from being a metal to being an
insulator.
To Colin Humphreys, a materials expert at Britain's Cambridge University, such
feats indicate that materials science is now reaching a point analagous to
genetic engineering in biology: Molecules will be engineered at the atomic
level, then replicated with biologically modeled processes to produce bulk
quantities. That's why many labs are working to harness biological principles,
often focusing on DNA-like molecules that can direct the self-assembly of new
materials. DNA is the leader of the body's chemical orchestra, directing it to
turn out the 200-odd types of cells that make up human bodies.
Chipmakers salivate at the thought of a DNA-type process for creating circuits.
Actually, it will soon be necessary, says Aristides A.G. Requicha, head of the
Laboratory for Molecular Robotics at the University of Southern California.
''We've got to break away from traditional silicon technology,'' he says,
''because it's about to price itself out of the market.'' The cost of new chip
factories has climbed above $2 billion, with no end in sight. In 10 years, it
could hit $15 billion. Moreover, silicon physics is due to hit a brick wall
around 2010 to 2015, when circuit lines shrink to 0.01 micron and fall prey to
the weird effects of quantum physics. So the search is on for new ways to
fabricate transistors and wires. This is the main driver behind nanotech
advances.
This year, a research group at the University of Illinois, led by electrical
engineer Joseph W. Lyding, pulled off a neat trick. They attached buckyballs
and other carbon molecules to precise points on the surface of silicon, where
the silicon transistors would normally be. The carbon atoms were attached in
such a way that they could spin like tops, at speeds of trillions of times a
second. If Lyding's team can create nano circuits to regulate the direction of
spin, their carbon transistors might switch on and off a thousand times faster
than silicon. The Defense Advanced Research Projects Agency (DARPA), the
Pentagon's venture capitalist, is cultivating that potential. It has launched a
$10 million ''spintronics'' research program headed by the University of
Buffalo.
In another thrust, DARPA's Moletronics program aims to lay the foundations for
molecular electronics. While molecular biologists can manipulate electrical
charges in protein channels very accurately, this knowhow has been patched
together empirically. The mathematical formulas that engineers would need to
translate the biological processes into computer circuits don't exist. ''We're
getting the hang of designing molecules to have specific electrical properties,
not just chemical properties,'' says Christie R.K. Marrian, manager of DARPA's
Moletronics program. How to wire such molecular switches together is now the
main roadblock, he adds, but a DNA-type mechanism for self-assembly could be a
solution.
If scientists harness DNA for materials, the results would be amazing. Kieffer
at Illinois envisions engineers spelling out what a material needs to do, and a
computer program doping out the possible structures and the procedures for
growing them. For NASA, creating superstrong, ultralight materials on demand
would be just what the doctor ordered. ''NASA has the world's biggest
weight-watchers program,'' says Meyya Meyyapan, chief nanotechnologist at
NASA's Ames Research Center. ''Every pound we try to lift to LEO [low-earth
orbit] costs $10,000--and to Mars, it's $100,000 a pound.'' The ultimate goal:
a robotic spacecraft tipping the scales at 22 pounds. Here on earth, Dow
Chemical Co. (
DOW) is working on nano composites for cars that could slash U.S. gasoline
consumption by 4 billion gallons for each model year they're used. In turn,
that would cut carbon-dioxide emissions by 11 billion pounds.
PAYOFFS. Researchers at the California Institute of Technology think
they're close to cracking Nature's codes. William A. Goddard III, head of
Caltech's Materials & Process Simulation Center, says software has been devised
for the Army that predicts the structure necessary to create a supersensitive
sniffer. ''We're now in the process of designing modified olfactory receptors
sensitive to very small concentrations of the products that come off land
mines--and off nerve gases,'' says Goddard. Still, many questions remain, he
admits. For example, when a molecule binds to an olfactory receptor, the
receptor signals nerve cells by releasing calcium. ''We don't know enough about
that to mimic it artificially,'' so for now the sniffer will sound the alarm
with silicon circuits. But in a year or two, he predicts, Caltech will know how
to do a far broader imitation of life.
Some scientists smell commercial payoffs and are forming companies. Earlier
this year, Smalley of Rice teamed up with Bob G. Gower, former chairman of
Lyondell Petrochemical Co., to create Carbon Nanotechnologies Inc. in Houston
to produce nanotubes. Minnesota's Stein recently co-founded MicroSurfaces Inc.
in Minneapolis to market mesoscale coatings. And at North Carolina, physicist
Otto Z. Zhou has a prototype product: an iron-polymer nano composite battery
with ''double the lifetime per unit weight'' of lithium-ion batteries--plus
several manufacturers haggling for the rights to build it.
The new century is off to a good start in nanotech. It will be years before
nanotubes and nanocomposites become as familiar as silicon. But the Nano Age
should be in full swing around 2010.
Copyright 2005, by The McGraw-Hill Companies Inc. All rights reserved.
