Nanotubes electronic properties

Key Words —Carbon nanotube, electronic properties, structural properties, strain energy, band gap, band structure, electronic structure. 

The structure-property relations of fullerenes, fullerene-derived solids, and carbon nanotubes are reviewed in the context of advanced technologies for carbon-hased materials. The synthesis, structure and electronic properties of fullerene solids are then considered, and modifications to their structure and properties through doping with various charge transfer agents are reviewed. Brief comments are included on potential applications of this unique family of new materials. 

Harris has this to say on the breadth of appeal of nanotubes Carbon nanotubes have captured the imagination of physicists, chemists and materials scientists alike. Physicists have been attracted to their extraordinary electronic properties, chemists to their potential as nanotest-tubes and materials scientists to their amazing stiffness, strength and resilience . 

Of particular importance to carbon nanotube physics are the many possible symmetries or geometries that can be realized on a cylindrical surface in carbon nanotubes without the introduction of strain. For ID systems on a cylindrical surface, translational symmetry with a screw axis could affect the electronic structure and related properties. The exotic electronic properties of ID carbon nanotubes are seen to arise predominately from intralayer interactions, rather than from interlayer interactions between multilayers within a single carbon nanotube or between two different nanotubes. Since the symmetry of a single nanotube is essential for understanding the basic physics of carbon nanotubes, most of this article focuses on the symmetry properties of single layer nanotubes, with a brief discussion also provided for two-layer nanotubes and an ordered array of similar nanotubes. 

Although still preliminary, the study that provides the most detailed test of the theory for the electronic properties of the ID carbon nanotubes, thus far, is the combined STM/STS study by Oik and Heremans[13]. In this STM/STS study, more than nine individual multilayer tubules with diameters ranging from 1.7 to 9.5 nm were examined. The 7-Fplots provide evidence for both metallic and semiconducting tubules[13,14]. Plots of dl/dV indicate maxima in the ID density of states, suggestive of predicted singularities in the ID density of states for carbon nanotubes. This STM/ STS study further shows that the energy gap for the semiconducting tubules is proportional to the inverse tubule diameter l/<7, and is independent of the tubule chirality. 

Both in work reviewed herein and in theoretical research by other workers[10-30], a consensus has been reached that anomalous properties (compared to graphite and normal graphitic nanotubes) can be obtained with graphitic nanotubes with diameters of the order of a nanometer. In terms of electronic properties, the nanotubes are expected to fall into two major classes on one hand the moderate band gap nanotubes that do not satisfy the Wj — = 3 /w condition in the... 

ELECTRONIC PROPERTIES OF CARBON NANOTUBES EXPERIMENTAL RESULTS... 

The fractal-like organization led, therefore, to conductivity measurements at three different scales (1) the macroscopic, mm-size core of nanotube containing material, (2) a large (60 nm) bundle of nanotubes and, (3) a single microbundle, 50 nm in diameter. These measurements, though they do not allow direct insights on the electronic properties of an individual tube give, nevertheless, at a different scale and within certain limits fairly useful information on these properties. 

The most promising way to study the electrical conductivity of a single nanotube is, thus, tightly dependent on the development or/and the adaptation of modern nanolithographic techniques. The goal to achieve is within reach and a detailed study of the electronic properties with reference to helicity and diameter will provide instrumental information about these fascinating materials. 

The electronic properties of single-walled carbon nanotubes have been studied theoretically using different methods[4-12. It is found that if n — wr is a multiple of 3, the nanotube will be metallic otherwise, it wiU exhibit a semiconducting behavior. Calculations on a 2D array of identical armchair nanotubes with parallel tube axes within the local density approximation framework indicate that a crystal with a hexagonal packing of the tubes is most stable, and that intertubule interactions render the system semiconducting with a zero energy gap[35]. 

A brief review is given on electronic properties of carbon nanotubes, in particular those in magnetic fields, mainly from a theoretical point of view. The topics include a giant Aharonov-Bohm effect on the band gap and optical absorption spectra, a magnetic-field induced lattice distortion and a magnetisation and susceptibility of ensembles, calculated based on a k p scheme. 

Electronic Properties of Carbon Nanotubes Probed by Magnetic Measurements... 

The synthesis of molecular carbon structures in the form of C q and other fullerenes stimulated an intense interest in mesoscopic carbon structures. In this respect, the discovery of carbon nanotubes (CNTs) [1] in the deposit of an arc discharge was a major break through. In the early days, many theoretical efforts have focused on the electronic properties of these novel quasi-one-dimensional structures [2-5]. Like graphite, these mesoscopic systems are essentially sp2 bonded. However, the curvature and the cylindrical symmetry cause important modifications compared with planar graphite. 

CNTs have been one of the most actively studied electrode materials in the past few years due to their unique electronic and mechanical properties. From a chemistry point of view, CNTs are expected to exhibit inherent electrochemical properties similar to other carbon electrodes widely used in various electrochemical applications. Unlike other carbon-based nanomaterials such as C60 and C70 [31], CNTs show very different electrochemical properties. The subtle electronic properties suggest that carbon nanotubes will have the ability to mediate electron transfer reactions with electroactive species in solution when used as the electrode material. Up to now, carbon nanotube-based electrodes have been widely used in electrochemical sensing [32-35], CNT-modified electrodes show many advantages which are described in the following paragraphs. 

Odom, T. W. Huang, J.-L. Kim, R Lieber, C. M. 1998. Atomic structure and electronic properties of single-walled carbon nanotubes. Nature 391 62-64. 

The structure of carbon nanotubes depends upon the orientation of the hexagons in the cylinder with respect to the tubule axis. The limiting orientations are zigzag and arm chair forms, Fig. 8B. In between there are a number of chiral forms in which the carbon hexagons are oriented along a screw axis, Fig. 8B. The formal topology of these nanotube structures has been described [89]. Carbon nanotubes have attracted a lot of interest because they are essentially onedimensional periodic structures with electronic properties (metallic or semiconducting) that depend upon their diameter and chirality [90,91]. (Note. After this section was written a book devoted to carbon nanotubes has been published [92], see also [58].)... 

The multiwall hollow WS2 nanotubes, which are obtained at the end of the process, are quite perfect in shape, which has a favorable effect on some of their physical and electronic properties. This strategy, that is, the preparation of nanowhiskers from an oxide precursor and their subsequent conversion into nanotubes, is likely to become a versatile vehicle for the synthesis of pure nanotube phases from other... 

Collins, P. G., Bradley, K., Ishigami, M., and Zettl, A. 2000. Extreme oxygen sensitivity of electronic properties of carbon nanotubes. Science 287 1801-1804. 

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