Chemistry of Single-Walled Carbon Nanotubes S. NIYOGI, M. A. HAMON, H. HU, B. ZHAO, P. BHOWMIK, R. SEN, M. E. ITKIS, AND R. C. HADDON* Center for Nanoscale Science and Engineering, Departm ents of Chemistry and Chemical & Environmental Engineering, University of California, Riverside, California 92521-0403 Received April 5, 2002 ABSTRACT In this Account we highlight the experimental evidence in favor of our view that carbon nanotubes should be considered as a new macromolecular form of carbon with unique properties and with great potential for practical applications. We show that carbon nanotubes may take on properties that are normally associated with molecular species, such as solubility in organic solvents, solution- based chemical transformations, chromatography, and spectros- copy. It is already clear that the nascent field of nanotube chemistry will rival that of the fullerenes. Introduction Fullerenes were immediately recognized as a new molec- ular form of carbon, 1,2 even though they have produced novel materials with itinerant properties. 3 Since their inception, 4 carbon nanotubes have been regarded as materials, 5 and most naturally related to the other intrac- table carbon allotropes sgraphite and diamond. It has been the goal of our group to bring these fascinating one- dimensional (1D) carbon structures into the fold of (macro)molecular chemistry. 6 Recent work has shown that with appropriate processing carbon nanotubes may take on properties that are normally associated with molecular species, 7 such as solubility in organic solvents, solution- based chemical transformations, chromatography, and spectroscopy. In this Account we summarize the progress and the promise of the new field of carbon nanotube chemistry. We focus on single-walled carbon nanotubes (SWNTs) which were first reported in 1993. 8,9 The SWNTs are characterized by strong covalent bonding, a unique one- dimensional structure, and nanometer size which impart unusual properties to the nanotubes sincluding excep- tionally high tensile strength, high resilience, electronic properties ranging from metallic to semiconducting, high current carrying capacity, and high thermal conductivity. Chemical processing of the SWNTs will play an essential role in the realization of the promise of this material. Preparation and Solid State Properties When carbon is dissolved in a transition metal nanopar- ticle melt, most commonly Ni/Co, Ni/Y, Rh/Pt, or Fe, and allowed to supersaturate, 10,11 a cylindrical graphitic carbon network is extruded. This structure can grow in the form of a cylinder to a length of micrometers in a few milli- seconds, 10,12 and it is thought that such structures can grow indefinitely provided the correct phase properties for the particular metal-carbon alloy are maintained. 13-18 The SWNTs thus formed have diameters on the order of a nanometer when prepared by current techniques. 19-21 Typically, the as-prepared SWNT soot (AP-SWNTs) from the electric arc procedure contains metal particles, metal clusters coated with carbon, amorphous carbon, and in some cases fullerenes, with a 30 wt % abundance of carbon nanotube ropes (Figure 1). Ultra-high-vacuum (UHV) scanning tunneling micros- copy (STM) has allowed atomic resolution imaging of the surface of SWNTs, and I/V spectroscopy has provided direct measurement of the electronic band structure. 22-25 It is now known that SWNTs can behave as metals, semiconductors, or small band-gap semiconductors, 26-28 depending upon their diameter and chirality. 29 Electronic S. Niyogi, M . A. Hamon, H. Hu, B. Zhao, and P. Bhowmik are graduate students, R. Sen is a postdoctoral fellow, and M . E. Itkis is a staff scientist working on the carbon nanotube project in the Haddon research group. A biography of Professor Haddon has appeared previously; 43 he is presently Distinguished Professor and Director of the Center for Nanoscale Science and Engineering at UC Riverside. FIGURE 1. Scanning electron micrograph of SWNT soot prepared by the arc process with Ni/Y catalyst: (a) typical purity, (b) high purity region of the same sample. (Reproduced with permission from Carbon Solutions, Inc. Copyright 2000.) Acc. Chem. Res. 2002, 35, 1105-1113 10.1021/ar010155r CCC: $22.00  2002 American Chemical Society VOL. 35, NO. 12, 2002 / ACCOUNTS OF CHEM ICAL RESEARCH 1105 Published on Web 10/15/2002