When Dave Geohegan and his colleagues in ORNL's Solid State Division (SSD) produce carbon nanotubes by laser ablation, they get a tangled mass of carbon that looks like black, fluffy felt to the naked eye and cooked spaghetti and meatballs in transmission electron microscopy images. By putting the material into a solvent and shaking it up using ultrasonic agitation, the carbon blobs disappear and the spaghetti-like tubes become less tangled. Some carbon nanotubes are pulled out of solution into a pipette and deposited on a spinning silica substrate where they naturally stick, partly because of static electricity. The problem with this manual technique is that it is slow, and only a small concentration of nanotubes is attached to the surface.
A dream shared by some researchers is to get nanotubes to assemble themselves into long rods, somewhat like uncooked spaghetti but far more flexible. Carbon nanotubes, which resemble rolled-up chicken wire because their carbon atoms are arranged in a hexagonal configuration, are only a few nanometers in diameter and up to hundreds of microns long. Their tensile strength is the highest in the world, some 100 times stronger than steel, with only one-sixth the weight. These materials may someday be linked together or incorporated as fibers in a polymer composite to form structural materials for aircraft, spacecraft, and suspension bridges.
Carbon nanotubes also conduct electricity in different ways. In ORNL's Instrumentation and Controls (I&C) Division, Mike Simpson, Michael Guillorn, Derek Austin (University of Tennessee graduate student), and others have shown that the electronic properties of carbon nanotubes make them suitable to serve as miniaturized rectifiers (components that allow one-way flow of electrons) and replacements for silicon channels in field-effect transistors (FETs).
"How the carbon sheet is rolled up into a tube affects its electronic properties," Guillorn says. "Rolled one way, a carbon nanotube is a good electrical conductor and could be used as nanowires; rolled another way, it is a good semiconductor." In an FET, electrons flow through a silicon channel unless a gate electrode applies a voltage to a silicon dioxide film whose electric field pinches off the current by raising the resistance in the channel. In this way, a transistor can operate as an on-off switch or store a bit of information (1 or 0). This modulation of conductivity is possible because the silicon transistor is doped in a controlled manner with impurities that serve as charge carriers.
ORNL researchers have shown that a carbon nanotube supported on two electrodes on a silicon dioxide insulating layer can replace that channel and conduct electricity when a potential is placed on the silicon base. The conductivity of the nanotube is controlled by whether the electric field is turned on or off. Austin measured the change in current flow in a carbon nanotube that resulted when the voltage on a silicon gate electrode was changed.
"To get the right number of charge carriers, a certain concentration of dopants is needed in a silicon transistor," Guillorn says. "If the size of the transistor is reduced considerably, it could have virtually no dopants or charge carriers. That's why a device approaching the nanoscale may require a semiconducting carbon nanotube to replace the silicon channel."
Can individual carbon nanotubes a few microns long be interconnected chemically using molecular handles, incorporated as reinforcing fibers in a polymer matrix, or threaded through the helices of polymer molecules (e.g., polyphenylene)? Can a sheet of carbon nanotubes be grown using laser ablation? A major challenge in nanoscience is to trick tiny features of matter 50,000 times smaller than the period at the end of this sentence to assemble themselves into useful products, ranging from logic gates for nanoscale transistors, to structural materials for aircraft, to long cables for power transmission and suspension bridges.
One approach being taken at ORNL is to link carbon nanotubes using strands of DNA. In double-stranded DNA, the chemical bases of either strand are paired with their complementary bases (A with T, C with G) in the other strand by hydrogen bonding. Because DNA can be terminally modified with primary amines, which can selectively react with carboxylic acids under certain reaction conditions, it may be possible to self-assemble carbon nanotubes by first linking them to DNA strands. The DNA strands could bring the nanotubes together; the DNA could be removed later by exposing it to high temperatures.
ORNL research showing early indications that carbon nanotubes can be connected chemically using DNA is part of the "Biomolecule-Assisted Self-Assembly of Three-Dimensional Carbon Nanotube Nanostructures" project at ORNL, which is supported by internal funding from the Laboratory Directed Research and Development (LDRD) Program. The project's lead investigator is Mitch Doktycz of ORNL's Life Sciences Division. Other participants are Doug Lowndes, Vladimir Merkulov, and David Geohegan, all of SSD; Simpson and Guillorn, both of the I&C Division; Phil Britt of ORNL's Chemical and Analytical Sciences Division, and Anatoli Melechko, Lan Zhang, and Jim Fleming, all of the University of Tennessee.
"Our goal is to find ways to enable self-assembly of carbon nanotubes into nanoscale structures, systems, and devices by interfacing natural materials with artificial materials," Doktycz says. "Self-assembly of nanosystems is recognized as one of the grand challenges facing nanotechnology. We believe a solution to many of these challenges lies in the use of natural systems such as DNA."
The objectives of this LDRD project are to fabricate nanotubes and ordered arrays of nanofibers, attach biomolecules to carbon nanotubes and nanofibers, develop nanoscale characterization techniques, and design and build molecular assemblies. Nanoscale systems that could result from self-assembly could be nanoelectronic components and devices, sensors and biosensors, nanomachines, and materials that mimic the membranes enveloping living cells.
"Before attaching nanotubes to DNA, a raw mass of nanotubes must first be purified," Britt says. "Single-walled carbon nanotubes produced by laser ablation at ORNL by Dave Geohegan and Alex Puretzky are digested in nitric acid to oxidize the carbonaceous impurities to make them soluble in water so they can be washed away. The material is then heated in air at 550°C to oxidize the remaining carbonaceous impurities. Hydrochloric acid is introduced to remove the remaining nickel-cobalt catalyst particles added during laser ablation to induce the growth of carbon nanotubes. During these purification steps, carboxylic acids are formed at the nanotube ends, giving us a chemical handle for attaching carbon nanotubes to DNA. Carboxylic acids bond selectively with the terminal amines that are added to DNA strands."
Britt notes that ORNL researchers cannot confirm whether they have succeeded in attaching nanotubes to DNA through the formation of chemical bonds without using advanced characterization tools such as Raman spectroscopy, Fourier transform infrared spectroscopy, and near-infrared spectroscopy. "We also need to characterize ORNL-made nanotubes to determine the nature of their defects," Britt says. "We may even find that the defects are good because they may provide handles for attaching the tubes chemically to another material."
An earlier success in self-assembly at ORNL is the growth of a patterned carpet of carbon nanofibers on a silicon base. Using plasma-enhanced chemical vapor deposition (PECVD), ORNL researchers Merkulov, Anatoli Meleshko, Lowndes, Yayi Wei, and Gyula Eres, all of SSD, have demonstrated that they can grow vertically aligned carbon nanofibers in a desired pattern on the substrate. They can choose the diameters of the fibers, their positions, and the spacing between individual fibers.
Fiber sizes and locations are determined by depositing nickel dots and lines on a substrate, using electron beam lithography. The substrate is then placed in a furnace containing acetylene gas, the source of carbon that grows as crystalline fibers on nickel, which catalyzes fiber synthesis. One carbon nanofiber grows on each nickel dot. Plasma-enhanced CVD is used because the plasma creates an electric field perpendicular to the surface, causing the fibers to grow upwards on it. The patterned growth of vertically aligned carbon nanofibers using PECVD is an example of controlled self-assembly of a material at the nanoscale.
A carpet of closely spaced nanofibers could be used to create an artificial membrane that allows tiny molecules or small particles to diffuse through while keeping out unwanted large molecules or big particles. Such a creation might mimic the membrane covering a living cell that allows signal proteins to enter its pores and activate cellular machinery. Carbon nanofibers could also be used to make electron probes that characterize the electrochemical behavior of living cells and monitor intracellular phenomena.
Guillorn, Simpson, and others are experimenting with using carbon nanofibers as massively parallel electron gunssources of precise electron beams that could create patterns of circuits that range from 10 to 100 nm across. If nanocircuits could be produced by electron beam lithography using computer-controlled, carbon-nanotube-tipped field emitters, the resulting chips would be closer to the semiconductor industry's 2004 goal: production chips with circuits 8 times denser and 16 times faster than chips of the same size currently being etched by optical lithography. The electron beam approach is needed to reach this goal because the wavelength of an electron is much shorter than that of a photon of light.
Recent studies have shown that electrons are emitted from carbon nanotubes at relatively low electric field intensities of a few volts per micrometer. Thus, nanotube electron emitters should operate at the low voltages needed for nanoscale circuitry.
"We demonstrated that a carbon nanofiber inside a well can emit electrons," Guillorn says. "When we placed a field on the fiber, an electron was emitted. The field allows electrons to escape from the tip."
A carbon nanofiber could be used to make a higher-resolution electron microscope because it would focus the microscope's electron beam on a smaller spot.
Research is under way to develop less expensive, more reliable, and energy-efficient flat-panel displays to replace bulky, power-hungry computer monitors. Because carbon nanofibers are inert, they could be more stable and practical than metallic (silicon) and inorganic electron sources for stimulating the emission of colored light in flat-panel displays. "All types of electron emitters work under ultrahigh vacuum," Guillorn says. "But at the higher pressures that may occur in real-world devices, residual gases are ionized, and the resulting ions bombard the tip of a metallic or inorganic emitter, sputtering it off so it is less effective as an electron emitter. Carbon nanofibers resist this type of damage, so they should last much longer in advanced flat-panel displays."
Carbon nanotubes and nanofibers are incredibly small, but their potential for creating advanced materials is huge.
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