|Building the nanofuture with carbon tubes
|by Jennifer Ouellette
Since their discovery in 1991, researchers have envisioned carbon
nanotubes as the most viable candidates to dominate the coming 21st
century revolution in nanotechnology. Barely a decade old, these
unique materials are already in use in lithium-ion batteries and
as structural reinforcements, and the first flat-panel displays
using nanotube components as field emitters are expected to reach
the market late in 2003.
Other potential applications in development include chemical sensors,
probe tips, fuel cells, portable X-ray machines, extremely lightweight
and strong fabrics, artificial muscles, and components that will
dramatically reduce the weight of cars and spacecraft. Nanotechnology,
such as carbon nanotechnology, will impact almost every aspect of
our lives. The only question is, when? says Cynthia Kuper,
president of Versilant Technologies (Philadelphia, PA). The
answer depends on our ability to fabricate nanotechnology materials
more easily than is possible now, and turning them into useful products.
|Single and multiwall
Discovered by Sumio Iijima of NEC Laboratories in Japan,
carbon nanotubes are an outgrowth of the formation of carbon
fullerenes, such as the C60 buckyball molecule. There are two
basic types of nanotubes. Singlewalled nanotubes (SWNTs) have
one shell of carbon atoms in a hexagonal arrangement (Figure
|Figure 1. Scanning electron micrograph
of single-wall carbon nanotubes grown with the high-pressure
carbon monoxide process. (Antenna Group, Nanomix, Inc.)
David Tomanek, a professor of physics at Michigan State University,
helped found the start-up venture Rosseter Holdings on the island
of Cyprus, which produces both types of carbon nanotubes. He believes
that each of the nanotube forms will find applications for which they
are best suited. For example, MWNTs will likely become the material
of choice for structural reinforcement, where low price matters more
than high purity (Figure 3). SWNTs will
likely dominate computer circuitry if researchers can better control
their diameter, which determines their properties.
Multiwalled nanotubes (MWNTs) consist of multiple concentrically
nested carbon tubes, similar to the rings of a tree trunk
(Figure 2, right). Each type has its advantages and disadvantages.
MWNTs are easier and less expensive to produce because current
synthesis methods for SWNTs result in major concentrations
of impurities that require removal by acid treatment. But
MWNTs have a higher occurrence of structural defects, which
diminishes their useful properties. Some companies prefer
SWNTs because they do not have such defects and their properties
are consequently stronger.
|Figure 2. Scanning electron micrograph
of mutliwall carbon nanotubes grown with chemical vapor
deposition. (Antenna Group, Nanomix, Inc.)
The excitement about carbon nanotubes stems from their unique properties.
There are hundreds of properties, and behind each property
is a business, says Charles Janac, chief executive officer
of Nanomix, Inc. (Emeryville, CA). Carbon nanotubes, for example,
self-assemble from carbon vapor and can show structural perfection
on the atomic scale, maintain large currents, and withstand high
temperatures, and they are mechanically rigid.
Nanotubes can be combined to form the strongest material known,
which Richard Smalley of Rice University, who pioneered the field
with his co-discovery of buckyballs in 1985, estimates to be between
30 and 100 times stronger than steel. Yet because nanotubes are
hollow, they are lightweight. They are also transparent to visible
light and absorb ultraviolet light, and are excellent conductors
of electricity. If it is a perfect SWNT, electrons will flow
down the structure in a coherent quantum waveguide that is unparalleled
in any other structure we know, says Smalley.
Translating any new material into a commercial industry is a daunting
challenge. James Ellenbogen, senior principal scientist of Mitre
Corp.s nanosystems group (McLean, VA), cites four key recent
developments that have helped bring carbon nanotubes to the brink
of broad commercialization. The first is a greater understanding
and better physical characterization of the materials and a corresponding
awareness of their unique properties and potential applications
in the commercial sector. Second, the U.S. government instituted
the National Nanotechnology Initiative three years ago, whose funding
is expected to reach $700 million in fiscal year 2004. The project
has encouraged researchers, small start-ups, and large corporations
to invest time and money in nanotube development.
A third factor is the emergence of applications. Although nanotubes
first gained the interest of the electronics industry with the demonstration
of nanotube transistors in the late 1990s, the first commercial
uses were as structural reinforcements in composites and as an additive
to graphite in lithium-ion batteries. Today, batteries used in about
60% of cell phones and notebook computers contain carbon nanotubes.
These batteries use MWNTs. They are not perfect, but they
fulfill their function by making the battery last longer, making
it more recyclable, and improving the energy delivery, says
Finally, and most critical, is the recent development of mass production
techniques. Ray Baughman of the University of Texas, Dallas, reports
that high-purity samples of SWNTs cost about $750 per gram, and
even SWNTs with substantial amounts of impurities cost about $60
per gram. Carbon nanotubes are probably the most expensive
material, pound for pound, in the world, agrees Ellenbogen.
If you could lower the cost through mass production, you could
essentially remake the world out of carbon.
|Figure 6. This pilot plant produces
single-wall carbon nanotubes by a high-pressure carbon
monoxide flow method and is expected to produce thousands
of kilograms a week by 2005. (Carbon Nanotechnologies,
||In the early 1990s, Hyperion Catalysis International,
Inc. (Cambridge, MA), pioneered the production of MWNTs in multiton
quantities, but access to the material remained limited because
its purchaser agreements restricted the independent pursuit
of patents by its customers. Baughman and others expect other
largescale producers of MWNTs to emerge after 2004, when Hyperions
1987 patent, under which it makes nanotubes, expires. Mitsui
Corp. plans to build a $15.2 million production facility in
Japan capable of producing 120 tons of MWNTs annually. Smalley
and his Rice University colleagues developed a high-pressure
carbon monoxide (HiPco) flow method in 1999 capable of producing
larger amounts of high-purity SWNTs. That method has become
the basis for a spin-off company, Carbon Nanotechnologies, Inc.
(CNI), based in Houston, and Smalley is now exploring ways to
spin carbon nanotubes like spider webs (Figure 6, left).
More recently, scientists have created nanoscopic peapods,
in which fullerenes are nested within nanotubes, similar to peas
in a pod. The new materials have tunable electronic properties that
are strongly dependent on their location along the tube, which means
the discovery could have far-reaching implications for the fabrication
of single-molecule-based devices (Figures 4 and 5, below).
|There are currently between 50 and 100 producers
of nanostructured carbon materials worldwide, but most are academic
institutions that make small amounts for research. Daniel Colbert,
CNIs vice president for major development strategy, reports
that demand for its SWNTs is growing exponentially, and he believes
that SWNTs will be cost-effective in many markets. Our
pilot unit is producing much more material, and there are more
and more interested parties, he says. Rather than
working on the scale of grams or tens of grams, our customers
want us to work on the kilogram scale. Meeting the demand is
a challenge, and we are on track to meet that demand by 2005.
|Figure 5: Depiction of a single-wall
carbon nanotube peapod with C60 molecules encapsulated
and the electron waves, scanned with a scanning tunnelling
microscope. (D. Hornbaker and A. Yazdani, University of
CNI is targeting four key application areasfield-emission
flat-panel displays, conductive plastics, high-performance fibers,
and advanced composite bulk-structural materialswhich Colbert
estimates represent about a $5 billion market value just for the
NanoDevices (Santa Barbara, CA) designed its EasyTube NanoFurnace
to enable researchers to quickly and easily produce both SWNTs and
MWNTs. The company shipped its first production units in early 2002.
The NanoDevices design uses the carbon decomposition and catalyzed
chemical vapor deposition technique to produce SWNTs and MWNTs directly
on the surface of device substrates. It controls the species of
nanotube through the selection of process gases. The direction and
location of the nanotube growth can be controlled by rationally
designing substrates and appropriately patterning the catalyst.
The next major commercial use of nanotubes probably will be that
of SWNTs in field-emission flat-panel displays. Their advantages
over standard liquid-crystal displays include lower power consumption,
higher brightness, a wider viewing angle, faster response rate,
and a wide operating-temperature range. But nanotube displays are
technically complex and require concurrent advances in electronic-addressing
circuitry, low-voltage phosphors, methods to maintain the required
vacuum, and the elimination of faulty pixels.
Despite these inherent difficulties, Samsung (Tokyo, Japan) has
developed a prototype full-color display, which it expects to introduce
commercially by December 2003. Although display panels are a small
market economically, the application is a high-profile one that
should help pave the way for other uses. There is nothing
wrong with making what we call a dirty device. It is not perfect,
but it works well, is reproducible and cost effective, and is better
than what is on the market, says Kuper.
Conductive plastics are used for electrostatic dissipation in
electronic devices, electromagnetic-interference shielding, and
composite bulk-structural materials, primarily for aircraft, spacecraft,
and sporting equipment such as golf clubs, but also as coatings
for electronics, gaskets, and other components. Nanotubes can also
be used to make high-performance fibers with double the energy absorption
and increased tensile strength, and for efficient, flexible, low-cost
sensors for gas-leak detection, medical monitoring, and industrial
Nanomix plans to introduce its first leak-detection sensors made
from carbon nanotubes in the second half of 2003. Such sensors are
extremely small, sensitive, and low in power consumption, and can
be customized to react to different chemicals. More importantly,
they are inexpensive. A modern oil refinery will likely have several
dozen chemical sensors to detect hydrocarbon leaks, each costing
approximately $3,000. Nanosensors could cost as little as $50 each.
Seiko Instruments (Chiba City, Japan) uses carbon nanotubes in the
scanning-probe tips it now markets, Baughman says. The mechanical
robustness and low buckling force of nanotubes dramatically increase
probe life and minimize sample damage, and the cylindrical shape
and small tip diameter improve resolution.
Baughmans recent work builds on his discovery that carbon
nanotubes exhibit an unusual actuator effect: the tubes increase
their length if the number of electrons on a tube is changed, a
useful property for building artificial muscles. He and a multinational
group of collaborators have demonstrated the effect using bucky
paper, a film made of bundles of SWNTs. He believes that the
electromechanical actuators could open a vast field of new applications
if bucky paper is used as a macroscopic material and if ropes or
even individual carbon nanotubes were to be used for micro- and
Otto Zhou of the University of North Carolina at Chapel Hill has
developed a novel new X-ray machine that does not require high temperature
to generate the high-energy electrons needed to produce X-rays.
It uses a thin layer of carbon nanotubes operating at room temperature
instead of the usual metal filaments heated inside a vacuum chamber.
Because high operating temperatures easily burn out the metal filaments,
the new devices will last longer. And because the devices are smaller
and can operate at room temperature, it should be possible to develop
portable X-ray machines for use in ambulances, airport security,
and customs operations. Zhou and his colleagues are working with
physicians and Applied Nanotechnologies, Inc. (Chapel Hill, NC),
to market the X-ray machines within two years.
The unique properties of carbon nanotubes also make them one of
the most promising candidates for a new nanoelectronic technology.
They are thin (as narrow as 1 nm), but producers can control their
growth up to many micrometers. They are also mechanically strong,
thermally and chemically stable, and excellent heat conductors,
and can be either metals or semiconductors, depending on the arrangement
of their atoms. An IBM team led by Phaedon Avouris made the first
arrays of nanotube transistors in early 2001, and later that year,
Cees Dekker's group at the University of Delft in The Netherlands
used nanotube transistors to build logic circuitry.
IBM's latest innovation reconfigures the transistor so that nanotubes
are not exposed to air, and it shrinks the channel length and thickness
of the dielectric layer between the gate electrode and the channel,
which improves current flow. The new transistor has twice the transconductance
of state-of-the-art conventional metaloxide semiconductor field-effect
transistors used in many fast electronic devices. Ellenbogen cautions
that scientists still need to fabricate circuits that are at the
molecular scale in their entirety, not just in their components.
There is also a need for better heat dissipation and interconnects
to achieve ultradense architectures, perhaps as many as 1 trillion
devices/cm2, compared with only 10 to 50 million devices/cm2 in
today's silicon devices.
Tomanek points out that carbon nanotubes have excellent heat conductance,
which is superior to that of any other material. He believes this
property of carbon nanotubes will play the key role in their application
in electronics. However, notes Baughman, Silicon technology is so
entrenched that it will take an overwhelmingly compelling new technology
to replace it. Carbon nanotubes do not yet qualify, but the potential
payoff is so great that the research is amply justified, even from
a commercial viewpoint. Despite the emergence of large-scale flow
methods such as HiPco, nanotube production still faces many processing
challenges. They include a strong tendency for nanotubes to agglomerate
and their inability to maintain long-term order, which directly
relates to exploiting the material's unique properties. Another
problem is sorting nanotubes by electrical type, an important capability
for fuel-cell applications and such longer-term uses as biological
materials and embedded membranes.
Smalley identifies the availability of high-quality material as
the single biggest limiting factor to quickly moving nanotubes into
the commercial marketplace. All these potential applications require
SWNTs in commercial production at acceptable prices, almost certainly
less than $1,000 per pound and perhaps as low as $100 per pound,
he says. CNI hopes to produce thousands of kilograms of SWNTs a
week by 2005. Still, Colbert admits, "Our processes are not
yet as good as they will be. But it is part of the normal learning
curve, and we anticipate selling into many markets." Mark Harmon,
R&D manager for Carbon Solutions, Inc. (Riverside, CA), identifies
impurities and high cost as the two largest obstacles to mass commercialization,
although improved purifying methods have lowered prices substantially.
And although there are plenty of potentially lucrative markets,
nanotubes have thus far only found their way into a few niche markets.
Tomanek believes that the true commercial breakthrough for carbon
nanotubes will ultimately be something scientists haven't envisioned
yet. When Charlie Townes invented the laser, he had no idea the
largest application would be in the checkout counter at the supermarket,
he says. I expect the same will be true for carbon nanotubes.