Electron Spectroscopy Studies of Carbon Nanotubes

Jorg H. Fink1 and Philippe Lambin2

1 Institut für Festkörper- und Werkstofforschung Dresden Postfach 270016, 01171 Dresden, Germany [email protected]

2 Departement de Physique, Facultes Universitaires Notre-Dame de la Paix 61 rue du Bruxelles, 5000 Namur, Belgium [email protected]

Abstract. Electron spectroscopies play an essential role in the experimental characterization of the electronic structure of solids. After a short description of the techniques used, the paper reviews some of the most important results obtained on purified single- and multi-wall carbon nanotubes, and intercalated nanotubes as well. An analysis of the occupied and unoccupied electron states, and the plas-mon structure of the nanotubes is provided. How this information can be used to characterize the samples is discussed. Whenever possible, a comparison between nanotube data and those from graphite and C60 fullerene is made, as to draw a coherent picture in the light of recent theories on the electronic properties of these C-sp2 materials.

Carbon-based n-electron system are becoming more and more important in solid state physics, chemistry and in material science. These systems comprise graphite, conjugated polymers and oligomers, fullerenes and carbon nanotubes. Many systems can be "doped" or intercalated. In this way new materials can be tailored having interesting properties. These new materials have often become model compounds in solid state physics since they show metallic and semiconducting behavior, superconductivity and magnetism. Correlation effects, which result from electron-electron interactions, and the electron-phonon interaction, are important in many of these systems. Dimensionality plays an important role, too. Thus many of the interesting questions of present solid state research are encountered again in the carbon-based n-electron systems.

Moreover, some of these materials have high technical potential. They show remarkable mechanical properties, e.g., a record-high elastic modulus. In addition, electronic devices from conjugated carbon systems are coming close to realization and commercialization. Transistors and organic light-emitting diodes based on conjugated polymers or molecules are already on the market. Industry is strongly interested in the field emission from carbon nanotubes to fabricate bright light sources and flat-panel screens. Finally, future nanoscale electronics systems can possibly be realized using carbon materials such as nanotubes.

M. S. Dresselhaus, G. Dresselhaus, Ph. Avouris (Eds.): Carbon Nanotubes, Topics Appl. Phys. 80, 247-272 (2001) © Springer-Verlag Berlin Heidelberg 2001

The exceptional properties of these materials are strongly related to their electronic structure. The mechanical properties are mainly caused by the strong covalent bonds between the tightly-bond a-orbitals. On the other hand, the interesting electronic properties are related to the loosely bound n-electrons. It is evident that the study of the electronic structure of these materials is an important task. In this paper we review recent investigations of the electronic structure of the quasi one-dimensional Single-Wall Carbon NanoTubes (SWNTs) and Multi-Wall Carbon NanoTubes (MWNTs), based on electron spectroscopy studies. We compare these results with the better known results from the quasi two-dimensional graphite and the quasi zero-dimensional fullerene solids. We emphasize that electron spectroscopy studies of carbon nanotubes are just at an early stage, because many of these techniques are extremely surface sensitive and surfaces suitable for representing bulk properties are difficult to prepare. Therefore, most of the reliable results have been obtained by less surface sensitive methods.

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