Local Electronic Structure in Ordered Aggregates of Carbon Nanotubes: Scanning Tunneling Microscopy/scanning Tunneling S
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Local electronic structure in ordered aggregates of carbon nanotubes: scanning tunneling microscopy/scanning tunneling spectroscopy study D. L. Carroll Department of Physics and Astronomy, Clemson University, Clemson, South Carolina 29634
P. M. Ajayan Department of Materials Science & Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180-3590
S. Curran Department of Physics, Trinity College Dublin, Dublin, Ireland (Received 10 February 1998; accepted 15 May 1998)
The recent application of tunneling probes in electronic structure studies of carbon nanotubes has proven both powerful and challenging. Using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), local electronic properties in ordered aggregates of carbon nanotubes (multiwalled nanotubes and ropes of single walled nanotubes) have been probed. In this report, we present evidence for interlayer (concentric tube) interactions in multiwalled tubes and tube-tube interactions in singlewalled nanotube ropes. The spatially resolved, local electronic structure, as determined by the local density of electronic states, is shown to clearly reflect tube-tube interactions in both of these aggregate forms.
I. INTRODUCTION 1–4
Carbon nanotubes are being called the ultimate fibers, due to their remarkable mechanical, electronic, and chemical properties. This comes from their unique structure where the hexagonal honeycomb lattice of graphene sheets is perfectly wrapped around to form seamless cylinders with diameters of one to few nanometers. These tubular structures are truly mesoscopic, and several experimental results have demonstrated that in transport they behave as real quantum wires.5,6 The tubes self-assemble from dense carbon vapor created by laser ablation or in electric arc of graphite targets, into concentric Russian doll type arrangement (multiwalled nanotubes; MWNT) or into ropes of tens of individual cylinders (singlewalled nanotubes; SWNT) arranged into triangular lattices.7 The lengths of the tubes can be as large as several microns. One of the great attributes of nanotubes is the possible structural isomerism, brought about by helicity in the arrangement of hexagonal arrays in the surface lattice of individual cylinders (tubes). This is due to the fact that a planar hexagonal lattice can be folded to match the dangling bonds at the edges, several ways: with zig-zag arrangement of hexagons having zero helicity with respect to the tube axis, armchair arrangement with 30± helicity and several possibilities for helicity between 0± and 30±. Interestingly, the electronic structure has a strong bearing on this helicity and the tube diameter. In general, all armchair tubes are metallic whereas J. Mater. Res., Vol. 13, No. 9, Sep 1998
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two-thirds of the zig-zag and helical tubes are semiconducting. The band gap also changes as function of tube size (d), typically following a 1yd relationship.6 In larger diameter tubes the
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