Influence of Structural Defects on the Electronic Properties of Carbon Nanotubes Examined by Scanning Tunnelling Microsc
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Influence of Structural Defects on the Electronic Properties of Carbon Nanotubes Examined by Scanning Tunnelling Microscopy
Cristina E. Giusca and S. Ravi P. Silva Nanoelectronics Centre, Advanced Technology Institute, University of Surrey, Guildford, GU2 7XH, United Kingdom ABSTRACT The electronic properties of carbon nanotubes are quite often drastically affected by the presence of defects that can develop during nanotube growth, processing or characterization too. Some of these defects such as pentagon-heptagon rings, substitutional impurities, vacancies and dislocations are of topological nature, and can sometimes create on-tube intramolecular junctions, as found by previous scanning tunnelling microscopy studies. Our recent STM experiments reveal for the first time a much more complicated junction structure, a hybrid single-walled carbon nanotube consisting of a distinct coiled structure located between two straight segments, each of different helicity. We characterise the hybrid junction at the atomic level and describe its electronic behaviour that has important implications in the practical design of functional components for nanoelectronic applications.
INTRODUCTION Early scanning tunnelling microscopy (STM) studies have been used to examine the electronic properties of carbon nanotubes and substantiate theoretical predictions that these properties are dependent on the wrapping of carbon hexagons into cylinder forms [1, 2]. Because of the intimate relation between the atomic structure and the electronic behaviour, various defects can develop in the honeycomb lattice of carbon nanotubes and alter their structural integrity to have a drastic effect on the electronic properties. The conductance of carbon nanotubes can be affected by structural deformations, of which two types of deformations involving bending or twisting are the most common [3, 4]. These defects are associated with a rearrangement of atoms and bonds which impact on the band structure, affecting the electronic properties of the tube. Another structural type of deformation occurring when the interlayer interaction between opposing sides of the nanotube balances the energy due to the curvature is the collapsed state configuration, favoured in tubes with larger diameters. STM experiments have evidenced the collapse deformation for both single- [5] and double-walled carbon nanotubes [6], with both cases displaying drastic changes in the fundamental electronic properties in direct relation with the atomically resolved structure. In particular, metal to semiconductor transitions were found as a response to the radial deformation and torsional strain. Apart from structural deformations, topological defects, such as pentagon-heptagon rings, substitutional impurities, vacancies and dislocations can sometimes create on-tube structures and affect the electronic properties of carbon nanotubes. The most stable topological defect as suggested by first-principle calculations [7], the pentagon-heptagon pair can interpose between
two segments o
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