Electronic Properties Junctions and Defects of Carbon Nanotubes

Steven G. Louie1'2

1 Department of Physics, University of California Berkeley Berkeley, CA 94720, USA

2 Materials Sciences Division, Lawrence Berkeley National Laboratory Berkeley, CA 94720, USA [email protected]

Abstract. The nanometer dimensions of the carbon nanotubes together with the unique electronic structure of a graphene sheet make the electronic properties of these one-dimensional structures highly unusual. This chapter reviews some theoretical work on the relation between the atomic structure and the electronic and transport properties of single-walled carbon nanotubes. In addition to the ideal tubes, results on the quantum conductance of nanotube junctions and tubes with defects will be discussed. On-tube metal-semiconductor, semiconductor-semiconductor, and metal-metal junctions have been studied. Other defects such as substi-tutional impurities and pentagon-heptagon defect pairs on tube walls are shown to produce interesting effects on the conductance. The effects of static external perturbations on the transport properties of metallic nanotubes and doped semiconducting nanotubes are examined, with the metallic tubes being much less affected by long-range disorder. The structure and properties of crossed nanotube junctions and ropes of nanotubes have also been studied. The rich interplay between the structural and the electronic properties of carbon nanotubes gives rise to new phenomena and the possibility of nanoscale device applications.

Carbon nanotubes are tubular structures that are typically several nanometers in diameter and many microns in length. This fascinating new class of materials was first discovered by Iijima [1] in the soot produced in the arc-discharge synthesis of fullerenes. Because of their nanometer dimensions, there are many interesting and often unexpected properties associated with these structures, and hence there is the possibility of using them to study new phenomena and employing them in applications [2,3,4]. In addition to the multi-walled tubes, single-walled nanotubes [5,6,7], and ropes of close-packed single-walled tubes have been synthesized [8]. Also, carbon nanotubes may be filled with foreign materials [9,10] or collapsed into flat, flexible nanorib-bons [11]. Carbon nanotubes are highly unusual electrical conductors, the strongest known fibers, and excellent thermal conductors. Many potentially important applications have been explored, including the use of nanotubes as nanoprobe tips [12], field emitters [13,14], storage or filtering media [15], and nanoscale electronic devices [16,17,18,19,20,21,22,23,24]. Further, it has been found that nanotubes may also be formed with other layered materials [25,26,27,28,29,30,31,32,33,34,35,36]. In particular, BN, BC3, and other

M. S. Dresselhaus, G. Dresselhaus, Ph. Avouris (Eds.): Carbon Nanotubes, Topics Appl. Phys. 80, 113-145 (2001) © Springer-Verlag Berlin Heidelberg 2001

BKCyNz nanotubes have been theoretically predicted [25,26,27,28,29] and experimentally synthesized [30,31,32,33,34,35].

In this contribution, we focus on a review of some selected theoretical studies on the electronic and transport properties of carbon nanotube structures, in particular, those of junctions, impurities, and other defects. Structures such as ropes of nanotubes and crossed nanotubes are also discussed.

The organization of the Chapter is as follows. Section 1 contains an introduction to the geometric and electronic structure of ideal single-walled carbon nanotubes. Section 2 gives a discussion of the electronic and transport properties of various on-tube structures. Topics presented include on-tube junctions, impurities, and local defects. On-tube metal-semiconductor, semiconductor-semiconductor, and metal-metal junctions may be formed by introducing topological structural defects. These junctions have been shown to behave like nanoscale device elements. Other defects such as substitutional impurities and Stone-Wales defects on tube walls also are shown to produce interesting effects on the conductance. Crossed nanotubes provide another means to obtain junction behavior. The crossed-tube junctions, nanotube ropes, and effects of long-range disorder are the subjects of Section 3. Intertube interactions strongly modify the electronic properties of a rope. The effects of long-range disorder on metallic nanotubes are quite different from those on doped semiconducting tubes. Finally, a summary and some conclusions are given in Section 4.

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