Introduction to Carbon Materials Research

A brief historical review of carbon nanotube research is presented and some basic definitions relevant to the structure and properties of carbon nanotubes are provided. Carbon nanotubes are unique nanostructures that can be considered conceptually as a prototype one-dimensional (1D) quantum wire. The fundamental building block of carbon nanotubes is the very long all-carbon cylindrical Single Wall Carbon Nanotube (SWNT), one atom in wall thickness and tens of atoms around the circumference (typical diameter ∼1.4 nm). Initially, carbon nanotubes aroused great interest in the research community because of their exotic electronic properties, and this interest continues as other remarkable properties are discovered and promises for practical applications develop. 1 Historical Introduction Very small diameter (less than 10 nm) carbon filaments were prepared in the 1970’s and 1980’s through the synthesis of vapor grown carbon fibers by the decomposition of hydrocarbons at high temperatures in the presence of transition metal catalyst particles of <10nm diameter [1,2,3,4,5,6]. However, no detailed systematic studies of such very thin filaments were reported in these early years, and it was not until the observation of carbon nanotubes in 1991 by Iijima of the NEC Laboratory in Tsukuba, Japan (see Fig. 1) using High-Resolution Transmission Electron Microscopy (HRTEM) [7] that the carbon nanotube field was seriously launched. Independently, and at about the same time (1992), Russian workers also reported the discovery of carbon nanotubes and nanotube bundles, but generally having a much smaller length to diameter ratio [8,9]. A direct stimulus to the systematic study of carbon filaments of very small diameters came from the discovery of fullerenes by Kroto, Smalley, Curl, and coworkers at Rice University [10]. In fact, Smalley and others speculated publically in these early years that a single wall carbon nanotube might be a limiting case of a fullerene molecule. The connection between carbon M. S. Dresselhaus, G. Dresselhaus, Ph. Avouris (Eds.): Carbon Nanotubes, Topics Appl. Phys. 80, 1–9 (2001) c © Springer-Verlag Berlin Heidelberg 2001 2 Mildred S. Dresselhaus and Phaedon Avouris Fig. 1. The observation by TEM of multi-wall coaxial nanotubes with various inner and outer diameters, di and do, and numbers of cylindrical shells N reported by Iijima in 1991: (a) N = 5, do=67Å; (b) N = 2, do=55Å; and (c) N = 7, di=23Å, do=65Å [7] nanotubes and fullerenes was further promoted by the observation that the terminations of the carbon nanotubes were fullerene-like caps or hemispheres. It is curious that the smallest reported diameter for a carbon nanotube is the same as the diameter of the C60 molecule, which is the smallest fullerene to follow the isolated pentagon rule. This rule requires that no two pentagons be adjacent to one another, thereby lowering the strain energy of the fullerene cage. While there is not, as yet, a definite answer to a provocative question raised by Kubo and directed to Endo in 1977, regarding the minimum size of a carbon fiber [11], this question was important for identifying carbon fibers with very small diameters as carbon nanotubes, the one-dimensional limit of a fullerene molecule. A recent report of a carbon nanotube which a diameter of 0.4 nm an a C20 end cap may provide an answer to the question. It was the Iijima’s observation of the multiwall carbon nanotubes in Fig. 1 in 1991 [7] that heralded the entry of many scientists into the field of carbon nanotubes, stimulated at first by the remarkable 1D dimensional quantum effects predicted for their electronic properties, and subsequently by the promise that the remarkable structure and properties of carbon nanotubes might give rise to some unique applications. Whereas the initial experimental Iijima observation was for Multi-Wall Nanotubes (MWNTs), it was less than two years before Single-Wall Carbon Nanotubes (SWNTs) were discovered experimentally by Iijima and his group at the NEC Laboratory and by Bethune and coworkers at the IBM Almaden laboratory [12,13]. These findings were especially important because the single wall nanotubes are more fundamental, and had been the basis for a large body of theoretical studies and predictions that preceded the experimental observation of single wall carIntroduction to Carbon Materials Research 3 bon nanotubes. The most striking of these theoretical developments was the prediction that carbon nanotubes could be either semiconducting or metallic depending on their geometrical characteristics, namely their diameter and the orientation of their hexagons with respect to the nanotube axis (chiral angle) [14,15,16]. Though predicted in 1992, it was not until 1998 that these predictions regarding their remarkable electronic properties were corroborated experimentally [17,18]. A major breakthrough occurred in 1996 when Smalley and coworkers at Rice University [19] successfully synthesized bundles of aligned single wall carbon nanotubes, with a small diameter distribution, thereby making it possible to carry out many sensitive experiments relevant to 1D quantum physics, which could not previously be undertaken [19]. Of course, actual carbon nanotubes have finite length, contain defects, and interact with other nanotubes or with the substrate and these factors often complicate their behavior. A great deal of progress has been made in characterizing carbon nanotubes and in understanding their unique properties since their ‘discovery’ in 1991. This progress is highlighted in the chapters of this volume.