NEW YORK, Nov. 15 (UPI) -- Carbon nanotubes are the darlings of the nanotechnology world, but beyond carbon lie materials of potentially equal value for nanowires as well as nanotubes, experts told United Press International.
"There is a whole lot out there other than carbon nanotubes," said Charles Martin, director of the Bio/Nano Interface at the University of Florida's Center for Research in Gainesville.
It is not surprising that carbon nanotubes attract so much hype. They are 100 times stronger than steel at one-sixth the weight and they possess a bevy of extraordinary electronic properties, such as the capability to behave either as semiconductors, conductors or superconductors.
The problem, experts said, is carbon nanotubes often are viewed only through rose-colored glasses. Rarely do nanotechnologists mention any of the significant drawbacks: They are expensive, difficult to separate from one another after they are made, difficult to produce in desired sizes or large quantities, not soluble in water and hard to functionalize chemically or biologically. Though these downsides are not insurmountable, they remain genuine concerns.
Annual production of multi-walled nanotubes -- nanotubes arranged as tubes within tubes -- is expected to grow to roughly 1,750 metric tons in 2005. This year's production of single-walled nanotubes, which possess better mechanical strength and electrical conductivity, should remain relatively minute at about 12 metric tons.
Carbon's strength is matched by its versatility -- it is the foundation of life on Earth -- but it cannot be the best at everything. Martin said he and Charles Lieber of Harvard University -- and a founder of Nanosys in Palo Alto, Calif. -- have independently advanced techniques that can grow nanowires and nanotubes "out of almost anything."
Their manufacturing process cannot produce bulk quantities nanotubes and nanowires so they are aiming for applications that do not require massive amounts.
"If your nanomaterials synthesis strategy doesn't give kilogram quantities at a small price, then you've got to consider applications where there's high value added to an application for low volume," Martin told UPI.
"There has been huge investment financially in research on carbon nanotubes and some of that has certainly paid off in how one can manipulate them," Lieber said. "Nevertheless, with these other materials that Charles Martin, myself and others have worked on, despite the lower investments, we can better control their electronic and optical properties and essentially collect them in a vial -- something not possible with carbon nanotubes."
These technical aspects and others need to be addressed before a technology can be fully evaluated, "and I think carbon still has a way to go," Lieber told UPI. "Carbon has some very unique advantages, but there are other nanoscale materials that are actually pretty unique in their own right and have come pretty far."
Lieber and Martin both are exploring a nanowire or nanotube sensor requiring only a single tube or wire to make the core sensor element.
"Because one or at most a handful of such sensor elements is needed, you don't need tons of material," Martin said, noting 10 billion nanotubes would amount to only a couple of milligrams or so in volume. "These sensors only need one nanotube or one nanowire. You don't need tons of material."
Martin said nanowires and nanotubes made of gold, silicon or silica -- also known as silicon dioxide or glass -- have "really cool personalities." For example, gold's excellent conductivity makes it ideal for electrodes, while silicon, the most famous of the semiconductors, is better suited for transistors.
"It's very easy to attach molecules to the surfaces of these materials to make them biologically or chemically functional," he explained.
Using silica, scientists can easily attach enzymes, antibodies or "almost any protein you can imagine. It's very useful. You do that via commercially available silane-coupling reagents designed to react with glass and (add) any functionality you want. There are literally hundreds of different silane molecules you can buy commercially to attach what you want," Martin said.
"Gold is the same way," he added. Organic compounds known as thiols spontaneously form bonds with gold.
"That makes chemical and bio functionalization of gold trivial," Martin said.
Using a technique similar to carbon-nanotube growth -- in which small metal particles serve as catalytic seeds to grow nanowires and tubes -- Lieber has created transistors out of silicon nanowires for use as sensors. He placed a single nanowire between two electrodes and coated it with silica that is chemically designed to interact with any molecule a user wants to detect. When a target molecule interacts with the nanowire, it alters the wire's conductivity, which can be measured. The result is an exquisitely sensitive device.
Martin also pioneered a technique now used in labs worldwide known as template synthesis. It uses membranes with nano-sized pores as the foundation for growing nanostructures. Membranes of gold nanotubes grown using this method behave as filters that can regulate the electrically charged ions they allow through.
"Ion-exchange membranes are sold commercially for lots of different applications -- fuel cells, water purification, or to produce chlorine gas," Martin said.
Aside from silicon and gold, Martin noted that template-synthesized nickel nanowires could find use as sensors that read magnetic-data storage. Researchers at State University of New York in Buffalo are developing nickel whiskers to read tinier bits of magnetically written data than ever before, allowing hard drives to store more information.
He said polymer nanowires, grown via template synthesis by chemist Sang Bok Lee at the University of Maryland, could switch between transparent or opaque depending on voltage. Automakers already are using this technology to reduce glare in rearview mirrors.
Lieber said zinc-oxide and aluminum-nitride nanowires merit attention because of their piezoelectric properties -- the capacity of materials to produce electric current when struck, squeezed or bent. The materials, investigated by Zhong L. Wang, director of Georgia Institute of Technology's Center for Nanoscience and Nanotechnology in Atlanta, and others, will either bend in response to voltage input and produce voltage when they are bent.
"These are really ideal for electromechanical devices," Lieber said.
Lars Samuelson, at the Lund Institute of Technology in Sweden, is researching nanowires made of indium phosphide, indium arsenide and other so-called III-V semiconductors for potential use in futuristic quantum computers.
One of the supposed advantages of carbon nanotubes is their highly ordered molecular structure, which grants them their unique electronic properties and extraordinary strength. Martin noted, however, that for many applications this exquisite order is unnecessary. His group has created nanotubes of disordered carbon that can behave as pumps. As such, they could find use in microscopic devices that manipulate fluids, say in microchips that screen DNA.
"While conventional carbon nanotubes are interesting and useful, just like one suit can't fit all people, one material can't solve all of nanotechnology's needs," Martin said.
He said by using his, Lieber's and other methods, "nanotubes and wires can be prepared from materials tailor-designed to fit any nanotech application."
--
Nano World is a weekly series by UPI examining the developing field of nanotechnology. E-mail [email protected]
