Harry Dorn examines a graphite rod that has been partially converted to buckyball-enriched soot. The soot is where the diamonds are, he says. The soot is purified into buckyballs in solution. The golden liquid in this photo is sc3N@c80 suspended in water — three atoms of scandium (sc) are anchored by an atom of nitrogen (N) in the middle of an 80-atom carbon molecule (c80). The nitrogen provided the mysterious 1109 that the researchers discovered is the magic ingredient that anchors metal atoms inside of the buckyball.
James Duchamp (top photo and on the right above), associate professor of chemistry at Emory and Henry College, and Harry Dorn examine the device they use to create metalofullerenes (buckyballs filled with metal atoms). The Krätschmer Huffmann, water-cooled, electric arc apparatus was invented at the University of Arizona in 1989. Dorn's lab modified it with an automatic feed unit. At first, students had to feed the rods into the chamber by hand — too fast and they would lock up; too slow and they wouldn't be fired. Applying the electric arc to the graphite rods would take hours. (Duchamp is on sabbatical to work with Dorn.) Drilled graphite rods filled with target metals are fed into the vacuum chamber in the presence of specific gasses and are burnt with an electric arc at 3000 degrees. A welder — just like you would find in a welding shop — sits under the table.
Harry Dorn is like a kid in a toy store — a very small toy store.
He works in a world of tiny spheres and tubes, building nanoscale “Russian dolls” and “peapods” by blasting metal atoms into the middle of hollow carbon molecules known as buckyballs.
The result is a new family of molecules, endohedral metalofullerenes (EMFs — buckyballs filled with metal atoms), which Dorn, a chemistry professor, and his Virginia Tech colleagues are able to create with reliable constancy and in quantities generous enough to share with researchers worldwide. Virginia Tech is the leading institution in the world for the controlled production and purification of EMFs.
Buckyballs are a new form of carbon discovered in 1985 by Robert F. Curl Jr. of Rice University, Sir Harold W. Kroto of the University of Sussex, and Richard E. Smalley of Rice. The new all-carbon molecule was named for architect Buckminster Fuller, creator of the geodesic dome, which the soccer-ball shaped buckminsterfullerene (or “buckyball”) resembles.
Researchers worldwide have been trying to put the fullerenes to work, particularly as a carrier of other materials. Now, the metal-filled buckyballs developed by Dorn’s group at Virginia Tech have the potential for being the architectural backbone for nanotechnology.
One nanometer is one billionth of a meter or 10 Ångstroms. An Ångstrom is atom-sized. We’ve known about atoms and their bonding to form molecules for almost 100 years, but putting together small numbers of atoms and molecules to build nano-sized devices has only become possible in the past few years. Researchers are just beginning to understand what the properties of materials at that scale are and how we can put them to use.
Researchers are inserting buckyballs into carbon nanotubes (peapods) and, more recently, EMFs into nanotubes (Russian dolls — atoms within buckyballs within nanotubes) for potential applications from medicine to electronics. Carbon nanotubes, discovered by Sumio Iijima of NEC Corp. in 1991, are long rather than round fullerenes, which conduct electricity if the hexagons on the surface are straight, but which act as a resistor or semiconductor if the tube is twisted so the hexagons do not align. Now Dorn’s group is enhancing these reactions by inserting EMFs inside the tubes.
Since Dorn and colleagues succeeded in inserting three metal atoms into the center of an 80-atom carbon molecule in 1999, Dorn’s lab has also demonstrated they can alter and control the content and the size and shape of the carbon cage. Their patented trimetalic nitride template (TNT) process anchors three metal atoms with a nitrogen atom.
The resulting complex looks like a Mercedes Benz hood ornament, with the nitrogen in the middle and the metal atoms at the end of each arm. “Not only is it beautiful, it’s useful,” says Dorn.
“Think of a nano toy store. What building materials would you want? At the least, you would want balls and tubes. EMFs have many potential applications, depending upon the metals and metal mixtures inserted,” Dorn says.
Insert magnetic materials into spheres or tubes and there are semiconductor and, perhaps, superconductor applications. Insert other metals for fluorescent and other optical properties. Insert radioactive material and use the molecule as a tracer in medical applications, with the carbon cage protecting the radioactive center. Quantum computing devices can be created by including atoms that have unpaired electrons and/or spin active materials. Dorn and Harry Gibson in chemistry, Randy Heflin and Max DI Ventra in physics, and Rick Davis and Kevin Van Cott in chemical engineering, have created the Center for Self-Assembled Nano Devices (CSAND) to conduct fundamental research on fullerene production, purification, and characterization, and to develop supramolecular architectures for targeted applications.
Already, researchers are using EMFs to create more efficient MRI contrast agents, solar cells so thin they can be incorporated into flexible sheets, signal amplifiers so small that many can fit onto chips or into a fiber optic wire, and nanowires for use on chips. “There are material combinations and applications we haven’t even imagined,” says Heflin, assistant director of CSAND along with Davis. It’s not only the properties of the materials that can be captured in buckyballs, but the way various materials interact at the nanoscale that makes this research so exciting for the team of chemists, physicists, and engineers.
“We’ve known about endohedrals since the early 1990s, but they were created only in small quantities. Harry Dorn’s discovery of the exceptionally high-yield TNT process is responsible for the current explosion of research because it is available in large quantities,” says Heflin. As of summer 2001, Virginia Tech’s research group is the only one in the United States working on metalofullerene processing and applications — although there are 12 groups in Japan working on the technology.
Gibson is developing the functional chemistry that will put EMFs and nanotubes to work. His forte is the control of molecular recognition and self-assembly to create polymers. He has created polymers whose components can be made to form and disassociate, so properties can be changed, subunits can be delivered and released, or the polymeric components can be recycled after they are no longer needed. In some of his assemblies, the molecules don’t actually interact; one set of molecules is carried by another, like rings on a rope, mechanically linked together. He has also created supermolecules that are insoluble.
Now he is applying his molecule mixing magic so EMFs can make the transition from lovely but isolated whirligigs to the workhorses of nanotechnology.
“There are a lot of things we want to attach to buckyballs, such as peptides. And there are times when we want to control what will attach to them,” says Gibson. “Buckyballs have high electron affinity.” They can take electrons away from the molecules of a conducting polymer, for instance, resulting in a current and the beginnings of an organic solar cell.
“But, if you want a specific atom to transfer an electron to a buckyball, you have to design the buckyball so it is screened from the outside world and will only let some other molecules through,” he explains. “Then, you can think about photochemistry, and not just for large solar cells, but also for sophisticated molecular devices.
When we look at electron transfer, we look for something unique,” Gibson says. “We look at collecting light energy and converting it into a charge — an antenna effect — light amplification from light absorbers around the outside. This has relevance to solar cells and to (Virginia Tech chemistry professor) Karen Brewer’s work to develop photoactivated molecules that will deliver shielded drugs to a disease site and release the medicine when signaled.
“Once you get charge separation, you can do other chemistry. You can create molecules that perform a task when energized.”
Gibson already had polymers with host sites. He created buckyballs with guest sites and now has a buckyball-polymer host-guest system. He has demonstrated that polystyrene will self-assemble around a buckyball, connected by interaction of the guest and host sites by molecular recognition. The resulting complex has increased solubility, isolates the buckyball, permits selective catalysis and transport with other molecules, and can be disassembled by a change of pH.
It can also become a dendrimer — a molecule with branches from a central core. “We could use the unattached ends of these buckyball filled polymers for different functions,” Gibson says. “Dendrimers have already been used for biological delivery. Biochemists know how to design them to seek out target tissue. You could use two or three ends to attach peptides, two or three others to be soluble, and others for delivery of the active species.
“You can build in therapeutic or diagnostic attachments — something that acts on a tumor or a radioactive substance that can be measured externally.”
There are also applications not related to health, such as being an interface between an organic and fluid surface. A buckyball could be poly-functional.
It is the uncharted territory that really makes his eyes light up. “C60 (the standard, empty 60-atom buckyball) is well understood, but the metals inside in the EMF change the buckyball’s properties. That presents lots of opportunity for exploration. With a nanotube, it’s even more difficult because, even without the metals, we haven’t figured out how to do controllable chemistry on nanotubes yet, which means there is even more new territory. We need to get them to orient (magnetically align) and go to a specific site. If we can do organic chemistry on the surfaces of nanotubes, we could manipulate them and orient them. Conductivity is of interest.
“The
chemistry is what is going to make these systems ‘tailor-able’
to various applications,” he concludes.
While the chemistry is complex, there are near-term applications.
MRI contrast agents or image enhancers are the furthest along in development. “One of the biggest advantages is that the carbon cage shields the patient from the toxic or radioactive metal,” Dorn says.
He reports that Japanese researchers H. Kato and H. Shinohara have made EMFs filled with an extremely potent contrast agent (gadolinium). “They made it water soluble and have done trials with rats. The product is 10 to 20 times better than anything commercially available. That means you don’t have to use as much, so it is less toxic.”
TNT compounds could be even more effective, lowering dosage even more, Dorn says.
He predicts that EMFs with radioactive isotopes safely contained in
the carbon cages will also be used for positron emission tomographic
(PET) imaging. “Or insert fluorescent and optical materials to
create tracers for medical and other applications. There is a lot
of excitement about these applications,” he says. “MRI agents
alone are a billion-dollar-a-year industry.”
But,
because of the clinical trials required for such products, Dorn believes
that non-medical applications of fullerene technology will reach the
public first. There is great excitement
among researchers about organic photovotalic devices — or
solar cells.
Heflin
explains that, in this instance, “organic” simply means
“carbon-based.” The advantages of organic solar cells over
silicon are their flexibility and light weight, which has advantages
for space applications. “You can fabricate a large area all at
once, limited only by the size of your vat of solution,” Heflin
says.
“Organic
solar cells can be flexible, so you could have deployable sails on
a space craft, or fold your solar cell into your briefcase or backpack.”
And
they are going to be inexpensive, he promises.
Folding can be done now but, so far, the efficiency of organic solar cells is only 20 percent of silicon. “But, we expect organic solar cells will be at least as efficient as silicon within five years,” Heflin says.
The success with LEDs (light-emitting diodes) now being made with organic materials demonstrates the potential of organic solar cells. Pioneer uses organic LEDs in the display on its car stereo because the technology offereasy-to-tune color.
“When you apply current to LEDs, you get light,” explains Heflin. “These materials like to emit light. “For a solar cell, you need to reverse that process. You need to put light in and get electrical energy out. Starting with a conducting polymer, which is a light emitter, we can apply a fullerene layer and produce current by applying light.”
The fullerene
has to be within 10 nanometers of where the light is absorbed for
current to be created. At this stage, researchers are using empty
buckyballs. “We believe we can improve the efficiency by factors
of five or 10 by putting metal atoms in the buckyballs,” Heflin
says. “The way to control the proximity is to build up layers
using a nanofabrication process known as ionically self-assembled
monolayers. One layer of conducting polymer one-nanometer thick, then
one of EMFs, so that wherever you absorb light there is a fullerene
within one nanometer.”
Much more complex uses of EMFs are advanced nanoelectronic devices, such as nanowires on semiconductor chips and optical amplifiers in fiber-based communication.
“We
have reached a technological and physical limit in the number of transistors
that can be integrated into a single chip,” says physics professor
Massimiliano DI Ventra. “We need to have a new generation of
electronic devices with nanoscale dimensions that can perform functions
identical or analogous to those of the transistor, and other key components
of today’s microcircuits, and which can outperform, by orders
of magnitude, the integration level of today’s chips. The goal
of molecular electronics is to create immensely powerful computing
circuits based on trillions of individual building blocks, each no
larger than a single molecule.
“Transistors
on microchips in computers and phones are hundreds of nanometers on
a side,” says DI Ventra. “We can make them 100 times smaller.”
DI Ventra
has demonstrated that theoretically a single molecule can act as a
transistor. He discovered that the contact region between molecules
and electrodes — in particular its geometry and chemistry —
plays a crucial role in the transport properties of molecular devices.
These findings have redirected attention from the initial emphasis
on the electronic properties of isolated molecules to the formation
and characterization of their contacts with electrodes.
Thus,
Phaedon Avouris of IBM is putting Dorn’s EMFs into nano tubes
to control conductivity. “We are experimenting with different
metals as part of the TNT structure,” says Dorn. “Different
combinations can make up the three metal atoms. All the combinations
have different electrical and optical and magnetic properties.”
In optical communication, the goal is improved signal, such as
for faster Internet connection or increased cable TV via optical fiber
— “not that we need more TV channels. But it would improve
combined cable and Internet delivery, for instance,” says Heflin.
Fiber optic technology transmits signals using laser light with intensity
controlled by changing the voltage to the diode laser. But when you
need to switch, such as for delivery from one computer to another,
or need to add information, you convert the signal to electricity,
then switch back to light to carry the message, says Heflin. “If
all these steps could be done optically, it would be faster and cheaper.”
Dorn’s
and Heflin’s published research has demonstrated that putting
EMFs into the materials used for photonic switching and computing
components, such as directional couplers and memory elements, increases
the nonlinear optical response by a factor of 100. “At the time
we did that research, we were only able to insert one metal (Erbium),”
says Heflin. “Now, we have more flexibility regarding what we
insert. We hope that nature is not so cruel that our first result
was the best.”
In addition
to switching, fiber optic signals also have to be boosted every 50
kilometers to maintain signal strength. IBM researchers are experimenting
with EMFs as part of the amplifiers to increase the power while decreasing
size, and increase the bandwidth so more data could be transmitted.
One of Dorn’s favorite applications for EMFs is new chemical catalysts. In high school chemistry
class, he was probably the kid who blew things up. Having created
an elegant new molecule, he’s come up with a way to use it by
destroying it. The results are nano-scale catalysts, which were never
before possible. The well-known and effective catalysts, lanthanum
and cerium, and corresponding oxides, usually require water as a solvent
to transfer them to a surface. However, because these metals have
high melting points, Dorn has demonstrated that they can be put inside
buckyballs, which are evaporated onto a surface, and then heated to
burn away the carbon cage — leaving a nano-scale catalytic thin
film.
For example,
catalysts could be delivered to convert methane to higher-value fuels
and chemicals. Methane is now burned off rather than transported from
distant oil fields. Dorn explains that methane is a simple molecule
with one carbon atom. Adding seven carbon atoms would convert it to
valuable liquid octane. Adding even one carbon atom would convert
methane to ethane, which is still a gas but is useful in organic synthesis,
as a fuel, and in refrigeration. Some metal oxides can transfer carbon
atoms. EMFs could be used to deposit these metal oxides on porous
surfaces over which methane would pass.
EMFs
might also deliver better catalysts into catalytic converters to help
these devices become more efficient in converting combustion byproducts
to harmless compounds.
“The
catalysis is not unique, but we would be delivering them to support
surfaces in an entirely different way,” Dorn says.
Luna Innovations Inc., a company started by electrical engineering
professor Kent Murphy, has licensed the process for creating TNT fullerenes
and is working on increasing quantities. The above applications will
eventually require hundreds of thousands of kilograms. Luna expects
to produce thousands of grams within five years. In the meantime,
“research studies only require milligrams, so if you have a gram,
you can do thousands of studies,” says Heflin.
Since
the early days of the “taking what you can get” approach
to creating EMFs, Dorn’s team has been able to engineer the process
so they can get more of what they want, improving yields of target
EMFs by a factor of 10. “Once you isolate something that is valuable,
you tune up the process so you get the target compound in preponderance,”
says Gibson. “It’s not as neat as rational synthesis —
building an EMF an atom at a time — but, so far, it’s faster
and more successful.”
Luna
has started Luna nanoMaterials to commercialize EMF products, including
organic LEDs and solar cells, nonlinear optical materials, and nanotubes
for electromechanical devices, and it provides the nanomaterials to
government and industry researchers. Steven Stevenson, formerly Dorn’s
postdoctoral associate and co-inventor of the TNT process, heads the
Luna carbon nanomaterial commercialization effort.
Carbon
is a well understood material, but nanoscale use and combinations
of materials present many unknowns. How will various hybrid materials
respond electronically? How will self-assembly properties (attraction
between materials) be changed? What is the impact of materials and
what is the impact of new molecular structures or architectures?
“Over
the next five years, our vision is to develop the knowledge base for
making tailored endohedral materials, assemble them into novel nanostructures
and devices, and apply them,” says Dorn. “It is apropos
that R. Buckminster Fuller is the father of synergetics, since both
the endohedral structures and our team are synergistic entities.”
Equally
apropos is Heflin’s nano wrap: “Harry excels at pulling
diamonds out of the soot.”
