This article outlines the content of the April 2004 issue of MRS Bulletin on Advances in Carbon Nanotubes. Essentially, carbon nanotubes are self-assembling nanostructures constructed of sheets of hexagonal-shaped carbon atoms rolled up into cylinders. Carbon nanotubes have attracted a great deal of attention as model systems for nanoscience and for potential applications. The special interest in carbon nanotubes stems from their unique structure and properties: their very small size (down to ∼0.42 nm in diameter); the possibility for carbon nanotubes to be metallic or semiconducting, depending on their geometrical structure; their exceptional properties of ballistic transport; their extremely high thermal conductivity and high optical polarizability; and the possibilities of high structural perfection. Research in the carbon nanotube field has now advanced to the stage where a good understanding of the structure and many of the basic properties are in place, together with much appreciation of their interrelation. On the other hand, major gaps in basic knowledge remain, with the major obstacles confronting the carbon nanotube field being the lack of a detailed understanding of the nanotube growth mechanism and control of the synthesis process to produce nanotubes with a desired diameter and chirality. The brief review of the carbon nanotube field by leading experts in this issue comes at an opportune time. Many exciting results on the structural, electronic, optical, and transport properties of these tiny well-ordered structures have already been achieved, and the research is well enough developed to assess present progress and identify new research directions waiting to be explored.

Since their discovery, carbon nanotubes, both single-walled and multiwalled, have been a focus in materials research. Fundamental research and application development hinge on high-quality nanotube materials and controlled routes to their organization and assembly. The aim of this article is to provide updated information on recent progress in the synthesis of carbon nanotubes.

The problems posed by the synthesis and purification of single-walled carbon nanotubes (SWNTs) have inhibited progress in the field. In this article, we review the methods available for measuring the purity of SWNTs and the current status of processes designed to purify them.We emphasize the hierarchy of the purification steps that must be developed in order to obtain high-quality material suitable for the full range of advanced applications that are envisioned for the ultimate carbon nanofiber.We review two strategies for SWNT purification, the assessment of SWNT purity by use of near-IR spectroscopy and its application to the thermal oxidation of thin films of SWNTs, as well as recent advances in the separation of metallic and semiconducting SWNTs. While substantial progress has been made in the purification and separation of SWNTs, we note the need for quality control and quality assurance within the industry. Much work remains before pure SWNTs of specific lengths, diameters, and chirality can be made available for applications.

The formation pathway to double-walled carbon nanotubes (DWNTs) from C60 encased within single-walled carbon nanotubes (peapods) is introduced in this article. Onedimensionally arranged C60 molecules coalesce gradually within the nanotube and change the structure to C60 dimers, trimers, tetramers, and so on as intermediates. In addition to these interesting structural transformations visualized in the nanotube space, the nanotube itself is very stable, and this structural stability is very important when using the interior of the nanotube as the reaction field or the space for molecular storage. In terms of optical absorption, the lowest energy absorption band for DWNTs, ∼0.65 eV, shows broadened and downshifted features as compared with that of SWNTs.We expect that this opticalabsorption feature will lead to the use of DWNTs in absorbing devices for optical-fiber communications. The Raman experiments give new information about the frequency of the C-C stretching-mode vibration for nanotubes with diameters of less than ∼1 nm, which shows a decrease in vibration frequency with decreasing tube diameter. This diameter dependence can be explained by an admixture of sp3 character in the C-C interaction. Therefore, the electronic and mechanical properties of nanotubes with diameters of <1 nm are expected to be different from nanotubes of the ∼1-nm-diameter class, and we anticipate that new phenomena will occur in small-diameter tubes.

The imaging and characterization of individual molecules and atomically thin, effectively one-dimensional crystals of rock salt and other halides encapsulated within single-walled carbon nanotubes are reviewed in this article. These species were imaged by conventional and super-resolved high-resolution transmission electron microscopy and by scanning tunneling microscopy, revealing the detailed atomic structure of these nanoscopic species.

Single-walled carbon nanotubes (SWNTs) are emerging as an important new class of electronic materials. Both metallic and semiconducting SWNTs have electrical properties that rival or exceed the best metals or semiconductors known. In this article, we review recent transport and scanning probe experiments that investigate the electrical properties of SWNTs.We address the fundamental scattering mechanisms in SWNTs, both in linear response and at high bias.We also discuss the nature and properties of contacts to SWNTs. Finally, we discuss device performance issues and potential applications in electronics and sensing.

In single-walled carbon nanotubes (SWNTs), their electronic and vibrational structure as well as their charge-carrier dynamics are crucial for potential ultrasmall optical device applications. SWNT properties have now been obtained from optical absorption and time-resolved photoemission and, at the single-nanotube level, by resonance Raman scattering and photoluminescence studies. This article presents an overview of SWNT photophysics, discussing important findings for the characterization of carbon nanotube properties and directions for future research and potential applications. The unique optical properties observed in SWNTs are due to the one-dimensional confinement of electronic states, resulting in van Hove singularities in the nanotube density of states. Optical measurements of phonons, charge-carrier dynamics, and the electronic transition energy van Hove singularities are discussed.

A wide range of potential applications was suggested shortly after carbon nanotubes were discovered, including new super-strong materials, field-emission devices, hydrogen storage systems, novel electronic devices, and more. In this article, the actual advances in the technology of nanotubes over the last decade are examined. Particular attention is focused on current commercially viable applications and those with imminent commercial promise. The relatively large number of nanotube-related patents and nanotube-based startup companies stand in contrast to the relatively low output in commercial applications. The drive toward nanotube applications, in contrast to nanotube science, is investigated.