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ScienceWeek
APPLIED PHYSICS: CARBON NANOTUBES AS ANTENNAS
The following points are made by M.S. Dresselhaus (Nature 2004 432:959):
1) Antennas are familiar as detectors and transmitters of radio waves. However, depending on their dimensions, antennas can receive different wavelengths -- radio, optical, microwave, and so on. All antennas have two major properties. First, their response varies with the polarization of the incoming radiation (polarization means that the electric field of the radiation has a particular orientation). Transmission is weakest if the plane of polarization is at 90° to the antenna's long axis: the "polarization effect". And second, their response varies with their length, being strongest when the length is a multiple of half of the wavelength (L) of the radiation (0.5, 1.0, 1.5, and so on): the "antenna-length effect".
2) The likely polarization and antenna properties of carbon nanotubes were recognized from a theoretical standpoint[2] soon after the first experimental synthesis[3,4] of single-wall carbon nanotubes in 1993. Conceptually, single-wall carbon nanotubes (SWCNTs) can be considered to be formed by the rolling of a single layer of graphite (called a graphene layer) into a seamless cylinder. A multiwall carbon nanotube (MWCNT) can similarly be considered to be a coaxial assembly of cylinders of SWCNTs, like a Russian doll, one within another; the separation between tubes is about equal to that between the layers in natural graphite. Hence, nanotubes are one-dimensional objects with a well-defined direction along the nanotube axis that is analogous to the in-plane directions of graphite.
3) Wang et al[1] performed experiments on random arrays of MWCNTs, aligned along their long axes and looking, at the nanoscale, like a dense forest of trees growing up from a silicon substrate. Each MWCNT in their nanotube array behaves effectively as a metallic rod about 50 nm in diameter and 200-1000 nm in length (although within each array the MWCNTs are about the same length).
4) The particular novelty of this work is the clear and direct demonstration of the antenna-length effect. Using visible light, Wang et al demonstrated that maxima occur in the amount of reflected light when the average length of the nanotubes in an array is a half-integral multiple of the wavelength of the incident light. They also provide a vivid demonstration of the polarization effect by comparing how much polarized light is reflected from a nanotube array and how much from a highly reflective metal surface positioned alongside the nanotube forest. For the metal surface, the electric-field vector must be in the plane of the metal surface for the light to be maximally reflected. In contrast, reflection from the nanotube arrays is strongest when the electric-field vector of the polarized light falls along the axis of the nanotubes (normal to the substrate).[5]
References (abridged):
1. Wang, Y. et al. Appl. Phys. Lett. 85, 2607-2609 (2004)
2. Ajiki, H. & Ando, T. Physica B Cond. Matt. 201, 349-352 (1994)
3. Iijima, S. & Ichihashi, T. Nature 363, 603-605 (1993)
4. Bethune, D. S. et al. Nature 363, 605-607 (1993)
5. Saito, R., Dresselhaus, G. & Dresselhaus, M. S. Physical Properties of Carbon Nanotubes (Imperial College Press, London, 1998)
Nature http://www.nature.com/nature
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MATERIALS SCIENCE: ELECTRON-HOLE SYMMETRY IN CARBON NANOTUBES
The following points are made by P. Jarillo-Herrero et al (Nature 2004 429:389):
1) Optical and electronic phenomena in solids arise from the behavior of electrons and holes (unoccupied states in a filled electron sea). Electron-hole symmetry can often be invoked as a simplifying description, which states that electrons with energy above the Fermi sea behave the same as holes below the Fermi energy. In semiconductors, however, electron-hole symmetry is generally absent, because the energy-band structure of the conduction band differs from the valence band(1).
2) Carbon nanotubes can be metallic or semiconducting depending on their chirality(3). Electron transport through individual nanotubes has been studied for both classes(2). Nanotubes of finite length have a discrete energy spectrum. Analogous to studies on semiconducting quantum dots, these discrete states can be filled with electrons, one by one, by means of a voltage applied to a nearby gate electrode(4). Whereas metallic nanotubes have shown clean quantum dot (QD) behavior(5), this has not been achieved in semiconducting single-wall nanotubes (SWNTs). Theory indicates that semiconducting tubes are more susceptible to disorder than metallic ones. Disorder typically divides a semiconducting nanotube into multiple islands, preventing the formation of a single, well-defined QD. Consequently, the electronic spectrum of semiconducting SWNTs has not been resolved before.
3) The authors report on measurements of the discrete, quantized-energy spectrum of electrons and holes in a semiconducting carbon nanotube(2). By applying a voltage to a gate electrode, an individual nanotube is filled controllably with a precise number of either electrons or holes, starting from one. The discrete excitation spectrum for a nanotube with N holes is strikingly similar to the corresponding spectrum for N electrons. This observation of near-perfect electron-hole symmetry(3) demonstrates that a semiconducting nanotube can be free of charged impurities, even in the limit of few electrons or holes. The authors furthermore find an anomalously small Zeeman spin splitting and an excitation spectrum indicating strong electron-electron interactions.
References (abridged):
1. Ashcroft, N. W. & Mermin, N. D. Solid State Physics (Saunders College, Orlando, 1976)
2. Dekker, C. Carbon nanotubes as molecular quantum wires. Phys. Today 52, 22-28 (1999)
3. Dresselhaus, M. S., Dresselhaus, G. & Eklund, P. C. Science of Fullerenes and Carbon Nanotubes (Academic, San Diego, 1996)
4. Kouwenhoven, L. P., Austing, D. G. & Tarucha, S. Few-electron quantum dots. Rep. Prog. Phys. 64, 701-736 (2001)
5. Tans, S. J. et al. Individual single-wall nanotubes as quantum wires. Nature 386, 474-477 (1997)
Nature http://www.nature.com/nature
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STRUCTURE AND PROPERTIES OF CARBON NANOTUBES
The following points are made by H.F. Bettinger et al (J. Am. Chem. Soc. 2001 123:12849):
1) Multiwall carbon nanotubes, a new tubular form of carbon, were discovered in 1991 by S. Iijima. Carbon nanotubes are comprised of a rolled graphite sheet ("graphene") and closed by fullerene-like caps. Depending on the way the graphene is rolled, different chiralities are possible, and are commonly distinguished by their chiral vector (n,m). The (n,n) tubes are called "armchair" and the (n.0) tubes are called "zigzag" nanotubes. A simple analysis imposing appropriate boundary conditions on the graphene band structure predicts that the armchair tubes are metallic (i.e., the band gap is zero due to band crossing), whereas the zigzag tubes are either semimetals or semiconductors, depending on the value of (n). The computations of the band structures using density functional theory (plane-wave pseudopotential local density approximation) indicate that these simple rules need to be refined, at least for narrow zigzag tubes. It was found by these calculations that the rehybridization of the carbon states due to the strong curvature of small tubes introduces low-lying conduction band states into the band gap of insulating tubes.
2) Whereas Iijima's multiwall carbon nanotubes consist of at least 2 concentric tubes, single wall carbon nanotubes are produced by laser vaporization of a metal-graphite (Co, Ni) target. These single-wall nanotubes are comprised of a single rolled graphene sheet, but have a tendency to form "ropes", i.e., bundles of single-wall nanotubes. The tubes produced by the laser-oven technique have very uniform diameters of approximately 1.38 +- 0.02 nanometers, according to x-ray diffraction, and an intertube distance of 0.315 nanometers in the ropes, similar to that in crystalline C(sub60). It has been concluded that the ropes are made up of (10,10) armchair single-wall nanotubes.
J. Am. Chem. Soc. http://pubs.acs.org/JACS
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