They were discovered in 1991 by the Japanese electron microscopist Simio Iijima who was studying the material deposited on the cathode during the arc-evaporation synthesis of fullerenes. Carbon nanotubes are fullerene-related structures which consist of graphene[1] cylinders closed at either end with caps containing pentagonal rings. Examples of Nanotubes are Single-layer nanotubes and nanotube "ropes" and nanohorns. Carbon nanotubes, therefore, are rolled-up sheets of graphite - i.e. the same material that is used in pencils. A sheet of graphite is composed of carbon atoms arranged in a flat hexagonal pattern similar to chicken wire mesh.
Nanoelectronic has witnessed a shift towards molecular systems in recent years. Though the term molecular electronic is rather an old one, it is only recently that single molecules have become the focus of interest, as nanoelectronic start to surface. This was triggered by research on carbon nanotubes. But before the carbon nanotubes entered the scene, molecular electronic was the science of organic polymers, their synthesis, processing and doping. With carbon nanotubes, we finally have a model system at hand that is equally of interest for chemists, material scientists and physicists. However, carbon nanotubes are supramolecular objects for a chemist; they are one-dimensional solids for a physicist. In the future, more of this supramolecular structure will be studied on a single molecule level.
Theorists have shown that nanotubes can be conducting or insulating depending on their structure. Therefore, this may lead to applications in nanoelectronic.
Wires are not possible for use in nanoelectronic, because they are susceptible to thinning and breakage. Despite recent interest in carbon nanotubes, they have variable electronic properties, depending on their orientation, reducing their functionality as electrical conductors.
One problem that plagues researchers looking to fashion circuit components from nanotubes is separating metallic tubes from the ones that are semi-conducting. Common synthesis procedures produce spaghetti-like mixtures of nanotube ropes that are unusable for semiconductor applications because they contain both types of tubes.
Nanotubes can be metals or semiconductors, and because of their strong chemical bonds and satisfied valences[2], the materials boast high thermal, mechanical, and chemical stability. In addition, carbon nanotubes can be efficient conductors as a result of their tiny diameters, long lengths, and defect-free structures that make them ideal one-dimensional systems.
Theoretical models have predicted that nanotubes could behave as ideal one-dimensional "quantum wires" with either semi conducting or metallic behaviours. Study of Transmission Electron Micrograph (TEM) images, however, has indicated that the nanotubes also incorporate kinks and defects into their walls
Progress in nanotubes synthesis has now yielded single-walled nanotubes (SWNTs) with well-defined diameters, bringing the experimental situation much closer to that of the theoretical models. Recent measurements indicate that these materials do behave like one-dimensional wires. The SWNTs should also be more sensitive to defects, to the extent that defects may dominate the transport characteristics. In this work, an STM tip was used as a sliding electrical contact to probe the length-dependence of SWNT conductance. Although atomic defects were not directly imaged, sharp conductance transitions and hetero-junction behaviours in the nanotube conductance are suggestive of the signatures of nanotube defects.
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