Next-generation computer chips, integrated circuits, and the microelectro-mechanical (MEMS) devices that power them depend upon carbon nanotubes that can be grown up, down, sideways, and in all three dimensions. Pulickel Ajayan and G. Ramanath are the first to achieve this unprecedented, specific, and controlled nanotube growth.
The research, reported in the April 4 issue of the journal Nature, paves the way for Lilliputian devices that depend on tiny networks and architectures. Ajayan and Ramanath have combined formerly disparate areas of research to grow and direct the assembly of nanotubes.
The method is based on a selective growth process that allows the nanotubes to grow perpendicular to the silica-coated substrate. By chiseling the silica into predetermined shapes, the researchers are able to precisely control and direct the nanotube growth. Their use of gas phase delivery of a metal catalyst, essential for nanotube growth, makes their growth process more flexible and more easily scalable than conventional methods.
"It's a simple and elegant process that provides unprecedented control over nanotube growth," Ajayan says.
NANOWELDING: Creating Tiny Junctions
Single-walled nanotubes are pure carbon cylinders with remarkable electronic properties. Pulickel Ajayan and his colleagues recently have discovered how to weld these nanotubes, paving the way for fabrication of molecular circuits and nanotube networks.
Ajayan and researchers in Germany, Mexico, the U.K., and Belgium used irradiation and heat to form the welded junctions. The research was featured in the Oct. 7 issue of Business Week's "Developments To Watch."
This is the first time single-walled nanotubes have been welded, although multi-walled nanotubes with junctions previously have been created using growth techniques. The electrical properties of single-walled nanotubes surpass those of multi-walled tubes. This is why so many researchers have been anxious to try this experiment, says Ajayan.
"No one knew if junctions could be created," Ajayan says. "Single-walled carbon nanotubes are perfect cylinders without any defects. But, to create junctions between them, intertube carbon-carbon bonds need to form. The irradiation and heating process we use creates just enough defects for these bonds to form without damaging their electrical properties."
The results were obtained after several years of ongoing experimentation. The difficulty was finding nanotubes that cross and touch. This is critical for the initiation of intertube links. "Unfortunately, we can't control this type of alignment just yet," Ajayan says.
The researchers used a special electron microscope that has the capability to irradiate and produce the heat necessary for the experiment. The high-voltage microscope, located in Stuttgart, Germany, is one of only a few worldwide.
Contact: Pulickel Ajayan 518-276-2322, firstname.lastname@example.org
NANOALIGNMENT: Long Hairlike Nanotubes
For the first time, Pulickel Ajayan and other researchers have created a simplified method for making long, continuous, hairlike strands of carbon nanotubes that are as long as eight inches. This breakthrough, reported in the May 3 issue of Science, is a first step toward creating such products as microcables for electrical devices or mechanically robust electrochemical actuators for artificial muscles.
The researchers from Rensselaer and Tsinghua University in Beijing found that chemical vapor deposition (CVD), a widely used technique to grow nanotubes, has a high yield of long strands when a sulfur-containing compound and hydrogen are added to the process.
The new method produced nanotubes that measured 20 centimeters, much longer than conventional nanotubes, Ajayan says.
"Carbon nanotubes are generally microns in length, which is not long enough for any practical purpose," Ajayan says. "We have created strands with nearly aligned nanotubes that are as long as 20 centimeters. The nanotubes are well-ordered in these structures and self-assemble during the growth process, which means we don't end up with an unusable lump that looks like cooked spaghetti."
The process also could be an easier alternative to creating high-purity single-walled nanotube material in general.
Contact: Pulickel Ajayan 518-276-2322, email@example.com
IGNITING NANOTUBES: Carbon Nanotubes Ignite
Researchers Pulickel Ajayan and G. Ramanath have discovered a surprising new property of single-walled carbon nanotubes. When exposed to a conventional photographic flash, the nanotubes emit a loud pop and then ignite.
The discovery, reported in the April 26 issue of the journal Science, could mean that the nanotubes might be used in light sensors or to remotely trigger explosives and combustion reactions. Researchers say that more testing needs to be done to realize these possibilities.
The researchers say that the loud popping sound heard after the flash is a well-known phenomenon, called the photo-acoustic effect. It occurs when porous black objects, such as carbon nanotubes, absorb a large amount of light, which results in the expansion and contraction of the gas surrounding them, releasing sound.
What surprised the researchers was that the nanotubes then spontaneously ignited.
Graduate student Andres de la Guardia made the initial discovery when he took flash photographs of the nanotubes.
Since the discovery, the researchers have found that, while the tubes burned only when oxygen is present, their atomic structure was altered even in inert gas environments.
"From an applications perspective, our work opens up exciting possibilities of using low-power light sources to create new forms of nanomaterials. The discovery will serve as a starting point for developing nanotube-based actuators and sensors that rely on remote activation and triggering," says Ramanath.
The research is a collaborative effort between Rensselaer, a French group headed by T.W. Ebbesen, and researchers in Mexico and Germany.
NANOCRYSTALS: Symmetrical Crystals Created
Researchers Pulickel Ajayan and G. Ramanath have created large symmetrical crystals that rarely occur in nature. These crystals could be harder than conventional engineering materials. The discovery was made during attempts to make superconducting nanostructures with a simple technique used to create carbon nanotubes. The research appeared as the cover story in the June 13 issue of the Journal of Physical Chemistry. Ajayan and Ramanath collaborated with other researchers at the University of Ulm in Germany.
The researchers used boron carbide, a common engineering material, in the high-temperature experiment. In the ashes, they discovered large crystals with five-fold crystallographic symmetry.
Nanosize five-fold symmetrical, or icosahedral, crystals are fairly common, but these larger micron-size crystals with five-fold symmetry are rare in nature because their smaller units cannot repeat their pattern infinitely to form space-filling structures. As the nuclei of these crystals grow, the strain on the crystals increases. This causes them to revert to their common bulk crystal structures.
Ajayan believes that the inherent structure of boron carbide, which has icosahedral units in the unit cell, allows the crystals to grow to micron size without the strain.
"These crystals are unique due to their high symmetry," Ajayan says. "Because of the hardness inherent to the crystal structure, we could anticipate a better material for engineering, specific-ally coatings. It is exciting and fulfilling to find something that is quite rare in nature, although we need to conduct further measurements to understand its potential."