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Materials form the basic underlying building block of nearly every advanced technology, and nanotechnology is no exception. Most technologies have various levels of differentiation, which can be broken down roughly into (1) materials, (2) design and fabrication, and (3) integration. In the case of some conceptual nanotechnologies, the path to achieve the last two levels is not at all clear. However, the science and technology of nanomaterials are both vibrant fields, with breathtaking advances on a daily basis. In many ways contemporary nanoscience is essentially the study of nanomaterials.
What differentiates nanomaterials from classical materials science, chemistry, and the like is the degree of control. Whereas many scientific disciplines have for more than a century dealt with phenomena and understanding on the atomic scale, the emerging field of nanomaterials strives to achieve control on that level beyond stochastic processes. One might view this field as a variant of engineering; instead of studying what is, we try to create what never wasand now on an unprecedented level.
Carbon nanotubes (CNTs) are allotropes of carbon. A single wall carbon nanotube is a one-atom thick graphene sheet of graphite (called graphene) rolled up into a seamless cylinder with diameter of the order of a nanometer. This results in a nanostructure where the length-to-diameter ratio exceeds 10,000. Such cylindrical carbon molecules have novel properties that make them potentially useful in many applications in nanotechnology, electronics, optics and other fields of materials science. They exhibit extraordinary strength and unique electrical properties, and are efficient conductors of heat. Inorganic nanotubes have also been synthesized.
Nanotubes are members of the fullerene structural family, which also includes buckyballs. Whereas buckyballs are spherical in shape, a nanotube is cylindrical, with at least one end typically capped with a hemisphere of the buckyball structure. Their name is derived from their size, since the diameter of a nanotube is in the order of a few nanometers (approximately 50,000 times smaller than the width of a human hair), while they can be up to several millimeters in length. There are two main types of nanotubes: single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs).
Since their discovery in 1991 by Sumio Iijima, carbon nanotubes have fascinated scientists with their extraordinary properties. Carbon nanotubes are often described as a graphene sheet rolled up into the shape of cylinder. To be precise, they are graphene cylinders about 12nm in diameter and capped with end-containing pentagonal rings. One would imagine that a new chemical such as this would be discovered by a chemist, slaving away in front of a series of Bunsen burners and highly reactive chemicals, with a sudden epiphany being revealed in a flurry of smoke or precipitation from a bubbling flask. However, carbon nanotubes were discovered by an electron microscopist while examining deposits on the surface of a cathode; he was performing experiments involving the production of fullerenes, or buckyballs.
It is about 15 years that the carbon nanotubes have been discovered by Sumio Iijima in a transmission electron microscope. Since that time, these long hollow cylindrical carbon molecules have revealed being remarkable nanostructures for several aspects. They are composed of just one element, Carbon, and are easily produced by several techniques. A nanotube can bend easily but still is very robust. The nanotubes can be manipulated and contacted to external electrodes. Their diameter is in the nanometer range, whereas their length may exceed several micrometers, if not several millimeters. In diameter, the nanotubes behave like molecules with quantized energy levels, while in length, they behave like a crystal with a continuous distribution of momenta. Depending on its exact atomic structure, a single-wall nanotube that is to say a nanotube composed of just one rolled-up graphene sheet may be either a metal or a semiconductor. The nanotubes can carry a large electric current, they are also good thermal conductors.
This discovery presents one of the key tenets of nanotechnology. Novel tools allow researchers to observe materials and properties at the nanoscale that often have existed for hundreds or thousands of years and to exploit the properties of such materials.
After Iijima's fantastic discovery, various methods were exploited to produce carbon nanotubes in sufficient quantities to be further studied. Some of the methods included arc discharge, laser ablation, and chemical vapor deposition (CVD). The general principle of nanotube growth involves producing reactive carbon atoms at a very high temperature; these atoms then accumulate in regular patterns on the surface of metal particles that stabilize the formation of the fullerenes, resulting in a long chain of assembled carbon atoms.
The arc-discharge methodology produced large quantities of multiwalled nanotubes (MWNTs), typically greater than 5nm in diameter, which have multiple carbon shells in a structure resembling that of a Russian doll. In recent years, single-walled nanotubes (SWNTs) using this method also have been grown and have become available in large quantities. The laser ablation method of carbon nanotube growth produced SWNTs of excellent quality but requires high-powered lasers while producing small quantities of material. The CVD method was pioneered by Nobel Laureate Richard Smalley and colleagues at Rice University, whose experience with fullerenes is nothing short of legendary. This growth technique is aided by a wealth of well-known inorganic chemicals specifically involving the formation of highly efficient catalysts of transition metals to produce primarily single-walled nanotubes.
Although carbon nanotubes have a suitably interesting structure, there are a multitude of important properties that impart the potential for novel applications of significant commercial value. Multiwalled and single-walled nanotubes have similar properties, and for illustration, focusing on single-walled nanotubes provides a reasonable primer of the primary features.
Carbon nanotube synthesis in recent years has been driven by yields and cost. To move nanotubes from scientific curiosity to practicality, they must be available in sufficient quantities at a reasonable cost with high uniformity and reproducibility. In the case of MWNTs, the arc-discharge method provides a good alternative, yielding large quantities of material at a good cost. In the case of SWNTs, while generating large quantities of material, the purity is often unacceptable for a subset of applications because of excessive carbonaceous contamination. Instead, the CVD method and a recent alternative, plasma-enhanced chemical vapor deposition (PECVD), have burst onto the scene as the methods of choice for producing large quantities of SWNTs with micron lengths, purity, and reliability within specifications for certain applications.
It is not surprising, then, that many applications have been proposed for the nanotubes. At the time of writing, one of their most promising applications is their ability to emit electrons when subjected to an external electric field. Carbon nanotubes can do so in normal vacuum conditions with a reasonable voltage threshold, which make them suitable for cold-cathode devices. Nanotubes are also good candidates for the design of composite materials. They can increase the conductivity, either electrical or thermal, of polymer matrices which they are embedded in at a few weight percents, while improving the mechanical resistance of the materials. Most spectacular, but still far from industrialization, is the nanotube-based field-effect transistor. Here, a single- wall semiconducting nanotube, contacted to two electrodes, may block or may transmit an electric current depending on the potential applied to a gate electrode placed at near proximity. Many other applications are foreseen, among which nanoscopic gas sensing in which one property of the nanotube, sensitive to adsorbed molecules, is measured. Gas selectivity may be realized by a suitable functionalization of the nanotubes. Optical and opto-electronic properties of single-wall nanotubes are also promising for infra-red applications.
The discovery and rapid evolution of carbon nanotubes has played a major role in triggering the explosive growth of R & D in nanotechnology. Many of the early lessons learnt carried over to rapid developments in inorganic semiconductor nanowire science and engineering, in particular, field effect transistor (FET) like switching devices, and chemical and biological sensors. The nanotube field per se has fanned out to encompass molecular electronics, multifunctional composites, flat- panel display technology, high-strength lightweight structural materials, nanoscale metrology (mass, heat, functional scanning probe tips, etc.), and others.
Multiwall tubes have two advantages over their single-wall cousins. The multishell structure is stiffer than the single-wall one, especially in compression. Large-scale syntheses by enhanced chemical vapor deposition (CVD) processes are many, while for single-wall tubes, only the Rice HiPco process appears to be scaleable.
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