12-10-2010, 03:39 PM
This article is presented by:.
D. L. Carroll,
P. Redlich,
P. M. Ajayan
Carbon nanotubes constitute the page link between large carbon fibers used as mechanical reinforcements in composites and molecular wires that could lead to ideal electrical connectors in future technology . Two general categories of nanotubes exist presently. Singlewalled nanotubes composed of single graphene sheets wrapped into cylinders with a narrow size distribution of – nm diameter. Multiwalled nanotubes are larger (–0 nm diameter) and are coaxial assemblies of graphene cylinders separated by approximately the c plane spacing (0. nm) of graphite. Both are in general microns in length and extremely stiff with very high axial strength. The electrical properties of nanotubes are all the more interesting and have been treated extensively in many theoretical works . Although most of the theoretical results have been obtained for singlewalled nanotubes, recent experiments on multiwalled structures confirm some of these results, indicating that nanotubes possess a rich variety of electronic structure with conductivities ranging from metallic to semiconducting . The broad range in conductivities arises from the helicity of graphite lattice in the nanotube structure and changes in the diameters of individual cylinders in the tube. A more interesting structural feature occurs near the ends of all tubes from the closure of the graphene cylinders by the incorporation of topological defects such as pentagons in the hexagonal carbon lattice . Complex end structures can arise, for instance conical shaped sharp tips, due to the way pentagons are distributed near the ends for full closure. It is suggested by theory that the ends of the tubes should have different electronic structure due to the presence of topological defects though it has never been experimentally verified. Defect induced tip electronic structure is important for several reasons. For example, the field emission properties of nanotubes, which have been recently demonstrated , could be strongly influenced by the presence of localized resonant states . Electrical properties measured by two and four probe measurements and electronic structure of nanotubes of differing sizes studied by scanning tunneling microscopy (STM) and spectroscopy (STS) ,0 have suggested a range of values for conductivities and band gaps (00 meV to . eV). However, the spatially resolved STS determinations of electronic properties are sensitive to ambient conditions, tube helicity, as well as where along the tube the measurements are made. For instance, if the density of defect states increases at the tube end, it can be expected that the electronic structure of the end would differ markedly from that elsewhere on the tube. This could be particularly well demonstrated in a conical shaped tube end where the pentagonal defects necessary for closure are concentrated at the apex of the cone. We have focused our experiments on tube end structures to see how their local density of states is affected by the incorporation of defects into the carbon network. The major question we address here is the following: How is the increasing defect concentration, at a tube tip, reflected in the local density of states (LDOS) of a nanotube. By coupling tight binding calculations performed on several tube tip morphologies and STS done on conical tube ends, we correlate sharp resonant features in LDOS with the presence of defects in the structure. Carbon nanotubes were fabricated using the electricarc- discharge method and characterized by electron microscopy as described elsewhere . A dilute mixture of carbon nanotubes and ethanol was ultrasonically agitated, to separate tubes, and deposited on freshly cleaved highly oriented pyrolytic graphite (HOPG). The substrate was immediately mounted in a high vacuum prechamber and the ethanol pumped away to a pressure of .0 0 torr. The substrate was then introduced into ultrahigh vacuum (UHV) (base pressure of .0 00 torr) and loaded into a commercially available scanning tunneling microscopy (STM) (Park Scientific). All microscopy and spectroscopy were done using mechanically formed Pt-Ir tips. While atomic resolution of the supporting substrate was always easily achieved, generally, atomic structure on the tube was not observed.
For more information about this article,please follow the link:
http://googleurl?sa=t&source=web&cd=1&ve...prl_97.pdf&ei=cTK0TPfeJI6CvgOdhaSTCg&usg=AFQjCNEQcqtY8q5wjOUlNQblI5OybqhqAA
D. L. Carroll,
P. Redlich,
P. M. Ajayan
Electronic Structure and Localized States at Carbon Nanotube Tips
Carbon nanotubes constitute the page link between large carbon fibers used as mechanical reinforcements in composites and molecular wires that could lead to ideal electrical connectors in future technology . Two general categories of nanotubes exist presently. Singlewalled nanotubes composed of single graphene sheets wrapped into cylinders with a narrow size distribution of – nm diameter. Multiwalled nanotubes are larger (–0 nm diameter) and are coaxial assemblies of graphene cylinders separated by approximately the c plane spacing (0. nm) of graphite. Both are in general microns in length and extremely stiff with very high axial strength. The electrical properties of nanotubes are all the more interesting and have been treated extensively in many theoretical works . Although most of the theoretical results have been obtained for singlewalled nanotubes, recent experiments on multiwalled structures confirm some of these results, indicating that nanotubes possess a rich variety of electronic structure with conductivities ranging from metallic to semiconducting . The broad range in conductivities arises from the helicity of graphite lattice in the nanotube structure and changes in the diameters of individual cylinders in the tube. A more interesting structural feature occurs near the ends of all tubes from the closure of the graphene cylinders by the incorporation of topological defects such as pentagons in the hexagonal carbon lattice . Complex end structures can arise, for instance conical shaped sharp tips, due to the way pentagons are distributed near the ends for full closure. It is suggested by theory that the ends of the tubes should have different electronic structure due to the presence of topological defects though it has never been experimentally verified. Defect induced tip electronic structure is important for several reasons. For example, the field emission properties of nanotubes, which have been recently demonstrated , could be strongly influenced by the presence of localized resonant states . Electrical properties measured by two and four probe measurements and electronic structure of nanotubes of differing sizes studied by scanning tunneling microscopy (STM) and spectroscopy (STS) ,0 have suggested a range of values for conductivities and band gaps (00 meV to . eV). However, the spatially resolved STS determinations of electronic properties are sensitive to ambient conditions, tube helicity, as well as where along the tube the measurements are made. For instance, if the density of defect states increases at the tube end, it can be expected that the electronic structure of the end would differ markedly from that elsewhere on the tube. This could be particularly well demonstrated in a conical shaped tube end where the pentagonal defects necessary for closure are concentrated at the apex of the cone. We have focused our experiments on tube end structures to see how their local density of states is affected by the incorporation of defects into the carbon network. The major question we address here is the following: How is the increasing defect concentration, at a tube tip, reflected in the local density of states (LDOS) of a nanotube. By coupling tight binding calculations performed on several tube tip morphologies and STS done on conical tube ends, we correlate sharp resonant features in LDOS with the presence of defects in the structure. Carbon nanotubes were fabricated using the electricarc- discharge method and characterized by electron microscopy as described elsewhere . A dilute mixture of carbon nanotubes and ethanol was ultrasonically agitated, to separate tubes, and deposited on freshly cleaved highly oriented pyrolytic graphite (HOPG). The substrate was immediately mounted in a high vacuum prechamber and the ethanol pumped away to a pressure of .0 0 torr. The substrate was then introduced into ultrahigh vacuum (UHV) (base pressure of .0 00 torr) and loaded into a commercially available scanning tunneling microscopy (STM) (Park Scientific). All microscopy and spectroscopy were done using mechanically formed Pt-Ir tips. While atomic resolution of the supporting substrate was always easily achieved, generally, atomic structure on the tube was not observed.
For more information about this article,please follow the link:
http://googleurl?sa=t&source=web&cd=1&ve...prl_97.pdf&ei=cTK0TPfeJI6CvgOdhaSTCg&usg=AFQjCNEQcqtY8q5wjOUlNQblI5OybqhqAA
