62 Scientific American December 2000 Nanotubes for ElectronicsNearly 10 years ago Sumio Iijima, sittingat an electron microscope at the NECFundamental Research Laboratory inTsukuba, Japan, first noticed odd nano-scopic threads lying in a smear of soot.Made of pure carbon, as regular and symmetric as crystals,these exquisitely thin, impressively long macromoleculessoon became known as nanotubes, and they have been theobject of intense scientific study ever since.Just recently, they have become a subject for engineering aswell. Many of the extraordinary properties attributed tonanotubes—among them, superlative resilience, tensilestrength and thermal stability—have fed fantastic predictionsof microscopic robots, dent-resistant car bodies and earth-quake-resistant buildings. The first products to use nanotubes,however, exploit none of these. Instead the earliest applica-tions are electrical. Some General Motors cars already includeplastic parts to which nanotubes were added; such plastic canbe electrified during painting so that the paint will stick morereadily. And two nanotube-based lighting and display prod-ucts are well on their way to market.In the long term, perhaps the most valuable applicationswill take further advantage of nanotubes’ unique electronicproperties. Carbon nanotubes can in principle play the samerole as silicon does in electronic circuits, but at a molecularscale where silicon and other standard semiconductors ceaseto work. Although the electronics industry is already pushingthe critical dimensions of transistors in commercial chips be-low 200 nanometers (billionths of a meter)—about 400atoms wide—engineers face large obstacles in continuing thisminiaturization. Within this decade, the materials andprocesses on which the computer revolution has been builtwill begin to hit fundamental physical limits. Still, there arehuge economic incentives to shrink devices further, becausethe speed, density and efficiency of microelectronic devices allrise rapidly as the minimum feature size decreases. Experi-ments over the past several years have given researchers hopeThey are stronger than steel, but the most important uses for these threadlike macromolecules may be in faster, more efficient and more durable electronic devicesby Philip G. Collins and Phaedon AvourisNanotubesElectronicsFORSLIM FILMSthat wires and functional devices tens of nanometers orsmaller in size could be made from nanotubes and incorpo-rated into electronic circuits that work far faster and onmuch less power than those existing today.The first carbon nanotubes that Iijima observed back in1991 were so-called multiwalled tubes: each contained anumber of hollow cylinders of carbon atoms nested insideone another like Russian dolls. Two years later Iijima andDonald Bethune of IBM independently created single-wallednanotubes that were made of just one layer of carbon atoms.Both kinds of tubes are made in similar ways, and they havemany similar properties—the most obvious being that theyare exceedingly narrow and long. The single-walled variety,for example, is about one nanometer in diameter but can runthousands of nanometers in length.What makes these tubes so stable is the strength withwhich carbon atoms bond to one another, which is also whatmakes diamond so hard. In diamond the carbon atoms linkinto four-sided tetrahedra, but in nanotubes the atomsarrange themselves in hexagonal rings like chicken wire. Onesees the same pattern in graphite, and in fact a nanotubelooks like a sheet (or several stacked sheets) of graphite rolledinto a seamless cylinder. It is not known for certain how theatoms actually condense into tubes [see “Zap, Bake orBlast,” on page 67], but it appears that they may grow byadding atoms to their ends, much as a knitter adds stitches toa sweater sleeve.Tubes with a TwistHowever they form, the composition and geometry ofcarbon nanotubes engender a unique electronic com-plexity. That is in part simply the result of size, because quan-tum physics governs at the nanometer scale. But graphite it-self is a very unusual material. Whereas most electrical con-ductors can be classified as either metals or semiconductors,graphite is one of the rare materials known as a semimetal,delicately balanced in the transitional zone between the two.By combining graphite’s semimetallic properties with thequantum rules of energy levels and electron waves, carbonnanotubes emerge as truly exotic conductors.For example, one rule of the quantum world is that electronsbehave like waves as well as particles, and electron waves canreinforce or cancel one another. As a consequence, an electronspreading around a nanotube’s circumference can completelycancel itself out; thus, only electrons with just the right wave-length remain. Out of all the possible electron wavelengths, orquantum states, available in a flat graphite sheet, only a tinysubset is allowed when we roll that sheet into a nanotube.That subset depends on the circumference of the nanotube, aswell as whether the nanotube twists like a barbershop pole.Slicing a few electron states from a simple metal or semicon-ductor won’t produce many surprises, but semimetals aremuch more sensitive materials, and that is where carbon nano-tubes become interesting. In a graphite sheet, one particularelectron state (which physicists call the Fermi point) givesgraphite almost all of its conductivity; none of the electronsin other states are free to move about. Only one third of allcarbon nanotubes combine the right diameter and degree oftwist to include this special Fermi point in their subset of al-lowed states. These nanotubes are truly metallic nanowires.The remaining two thirds of nanotubes are semiconduc-www.sciam.com Scientific American December 2000 63MICROCHIPS OF THE FUTURE will require smaller wiresand transistors than photolithography can produce today. Elec-trically conductive macromolecules of carbon that self-assembleinto tubes (top left) are being tested as ultrafine wires (left) andas channels in experimental field-effect transistors (above).PHILIP G. COLLINS AND PHAEDON AVOURIS CEES DEKKER University of DelftGOLDSOURCENANOTUBECHANNELSILICON DIOXIDEINSULATORGOLD DRAIN64 Scientific American December 2000 Nanotubes for ElectronicsThe Electrical Behavior of NanotubesA Split PersonalityTWISTED NANOTUBES,cut at an angle from graphite (left), look a bit like barbershop poles (center).Theslices of allowed energy states for electrons