Department of Electrical Engineering and Computer Science and Department of Physics
Massachusetts Institute of Technology
Cambridge, MA 02139
The carbon-based nanostructural materials considered here include fullerenes and related materials, carbon nanotubes, carbon nanoparticles, and porous carbons, including activated carbon fibers and carbon aerogels.
Carbon nanotubes (~1 nm in diameter) are presently the hottest carbon nanostructured material. Single-wall carbon nanotubes, consisting of a cylindrical tube one atomic layer in thickness, are predicted to be either semiconducting or metallic depending on the diameter and chirality of the nanotube. These unusual electronic properties imply novel 1D physics. In addition, single electron (Coulomb blockade) transport phenomena are now being studied in this unique system using nanolithographic techniques. The joining of two dissimilar nanotubes has been predicted to give rise to semiconductor-metal heterojunctions with properties that can be modified by perturbing the junction region, which is of ~1 nm size. Remarkable quantum effects are also predicted and observed for the phonons in carbon nanotubes, also dependent on the diameter and chirality of the nanotubes. Another area of great excitement concerns the elastic properties, especially the extremely high Young's modulus for carbon nanotubes. Applications of carbon nanotubes are under development for displays and for tips for scanning tunneling microscopes and for the manipulation of nanostructures. It is expected that the physical properties of the carbon nanotubes can be significantly modified by intercalation, as for example by using alkali metal dopants, but this is a very new research area, with only a few results now in press.
Fullerenes represent a unique category of cage molecules with a wide range of sizes, shapes, and molecular weights. Most of the effort thus far has gone into the study of C60 fullerenes, which can now be prepared to a purity of parts per thousand. Since every C60 molecule is like every other one, ignoring 12C and 13C isotope effects, C60 provides a unique monodisperse prototype nanostructure assembly with particle size of 0.7 nm. Because of the unique icosahedral symmetry of C60, these molecules provide prototype systems for spectroscopy, optics, and other basic science investigations. Study of the structure and properties of the whole family of fullerene cage molecules, together with their endohedral cousins (formed by insertion of guest species, usually rare earth or transition metals, within the cage) is being pursued worldwide. A few groups are also coating fullerene molecules with one or more layers of alkali metals and alkaline earths and studying their remarkable structures and properties. The intercalation of alkali metals and other species into the crystalline C60 lattice alters the structure and leads to large modifications in properties. The special properties of the metallic and superconducting phases thus achieved are of fundamental significance. The new phases associated with polymerization of C60 by incident light, pressure, and alkali metal doping are of both scientific and practical interest.
Carbon nanoparticles can be all carbon (such as the so-called carbon onions), or carbon-coated particles consisting of carbon layers wrapped around other materials, usually carbides. Studies of carbon onions have emphasized the basic science of nanostructure growth in carbon systems, while the carbon-coated nanoparticles have combined both basic studies, concerning their growth and structure, with applications, particularly for magnetic nanoparticles. The carbon coating provides a means for stabilizing and separating small particles containing active ingredients with special properties that may, for example, be useful for magnetic information storage.
Porous carbons, such as activated carbon fibers and carbon aerogels, have a high density of pores, with pore sizes < 2 nm. The structure and properties of these nanopores have been investigated both for their scientific interest and for practical applications, utilizing the special properties of high surface area materials. Remarkably high specific surface areas, as high as ~3000 m2/g, are achievable in these materials.
The critical control parameters of individual carbon nanotubes are the nanotube diameter and chirality, while the packing density is important for nanotube arrays. Most of the presently used single-wall carbon nanotubes have been synthesized by a pulsed laser vaporization method, pioneered by the Smalley group at Rice University. Their synthesis result of 1996 represents a major breakthrough in the field. There are several oral reports that a catalyzed carbon arc synthesis route also can be used to produce high yields of nanotubes with a diameter distribution similar to those obtained by the Rice group. There are presently many groups worldwide working to develop more efficient synthesis techniques for producing arrays of similar single-wall nanotubes, with a narrow diameter and chirality distribution, at a high production rate, and at a cheap cost. Right now, it is possible through Smalley's recent work to produce significant amounts of (10,10) armchair nanotubes with a small average diameter (~1.3 nm) and a small diameter distribution. Much effort is presently being expended to develop production methods to provide control in the synthesis of armchair nanotubes arrays with different diameters, let alone the controlled synthesis of nanotube arrays with different chiralities. This field is very new, and many groups are now trying to reproduce the sample quality achieved by the Rice group. There is optimism in the community that much progress will be made within the coming year toward improving synthesis capability and properties control for the single-wall nanotubes, by study of the growth conditions, such as temperature, pressure, and kinetics. Computer simulations are being actively employed to improve convergence of the experimental approaches. Considerable effort is also being expended in the study of intercalation as a method for modifying the properties of the nanotubes. Efforts to improve the synthesis of multiwall nanotubes (containing several coaxial single wall nanotubes) continues, with diameters up to ~15 nm defining the nanotube range, and with diameters in the range 15 ( d ( 100 nm defining the nanofiber range where faceting of the individual "tubes" occurs at high heat treatment temperature (( 2500 °C).
The important parameters describing fullerene cage molecules include the number n of carbon atoms in the particular fullerene species Cn, and the shape of each of the isomers corresponding to Cn. In the case of the endohedral fullerenes, the number of guest atoms as well as the guest species needs to be controlled, in addition to the mass and isomer type of the fullerene host. In general, the whole gamut of fullerene masses and isomers are formed simultaneously. While some control of the arc discharge conditions (or of other synthesis methods such as laser vaporization or flame methods) can be used to modify the mass and isomer distribution of the synthesis process, most effort has been expended on efficient and sensitive separation methods. These methods are easiest to implement for the lighter fullerenes. Separation according to molecular weight is first accomplished by advanced chromatography methods. Purification of each of the isomers at a given mass value is more difficult, and only limited success has been achieved thus far. Efforts to improve the efficiency of the synthesis process and the purity of C60 as a reagent are now largely being carried out in the commercial arena. To a large degree this is also true of C70. Basic research is now being directed toward gaining a fundamental understanding of the growth mechanism and fundamentally new synthesis routes, based, for example, on the building block carbon clusters found in C60. There are also serious efforts by many research groups worldwide to produce larger quantities of higher purity higher fullerenes -- C70, C76, C84, etc. -- using advanced chromatography, along with other specialized techniques. Many of the same groups are also active in synthesizing, separating, and purifying endohedral fullerenes.
Carbon-coated nanoparticles, produced by arc discharge methods or by laser vaporization techniques, are characterized by their size and shape distribution, the thickness of the carbon coating relative to the particle diameter, and the stoichiometry, crystallinity, and homogeneity of the phase of the enclosed carbide or other constituent. Conditions of temperature, cooling rates, gas transport agents, and other process conditions affect the physical parameters and therefore properties of the nanoparticles. In the case of carbon-coated nanoparticles, basic research on process control, structure, and properties is actively being carried out, mostly in universities, with some start-up companies working on the improvement of the synthesis process, on scale-up, and on controlling costs. The parameters characterizing the nanostructure aspects of porous carbons focus on pore size and pore size distribution. Among the various activated carbons, all of which have a large concentration of nanopores, it is the activated carbon fibers that have the smallest pore size distribution. The pore size and its distribution are sensitive to the temperature and pressure of the steam and CO2 used in the activation process. Carbon aerogels have a much wider range of size distributions, with nanopores found within the small particles in the carbon aerogel chains, and with mesopores and macropores found between the particles and between clusters of particles. The average nanopore size and the nanopore size distribution depend on the precursor materials used in the aerogel synthesis, the carbonization temperature and time, and other process parameters. Steady progress has been made in varying the process conditions to control and vary the diameter distributions of the pores. Activation of carbon aerogels has been demonstrated as a method for adding a high density of nanopores, but this is a new research area that has not yet been widely studied.
Whereas the quantum aspects of single-wall carbon nanotubes are at an early stage of research, the more general desirable properties of carbon nanotubes are already being exploited commercially. Multiwall carbon nanofibers are presently being used as conductive additives to plastics and in electrochemical capacitor applications. Such commercial applications of nanofibers do not impose severe requirements on uniformity in nanotube diameter and sample homogeneity from one nanotube to another. Carbon fibers in the 1-10 micron range have been produced commercially for several decades. Incremental improvements in the manufacturing process have continued over this time period, leading to enhanced product performance and reliability; however, the relatively high price of the product has restricted the available markets. The same constraints may also apply to the commercialization of carbon nanotubes exhibiting the remarkable and unique 1D properties described above.
Synthesis of C60 and C70 is now a commercial process, with continuing increases in purity and decreases in cost. As more fullerene applications become commercialized, the demand for C60 and C70 will increase, and further improvements in the products and reductions in cost are expected. The commercialization of higher fullerenes and endohedral fullerenes is hampered by present limitations on efficient synthesis, separation and purification techniques, and the absence of significant products based on these materials.
Carbon-coated nanoparticles have moved quite rapidly into the commercial sector. Several start-up companies are developing niche proprietary applications and are working on issues of scale-up, reproducibility, reliability, and cost, while basic and applied R&D continue actively in university and industrial laboratories.
Porous carbons have been commercialized for a relatively long period of time for gas adsorption, environmental cleanup, heavy metal and ion separations, and electrical capacitor applications. As control of the pore size distribution has increased and cost has been reduced, these materials have found more applications and markets. Both large companies and start-up companies are involved with these products.
In all carbon nanostructured materials, cost has been a main factor in limiting commercialization, yet it is widely believed that if production volumes increase, costs would decrease markedly, thereby significantly increasing the utilization of the excellent properties of nanostructured carbon.