Computational Studies of Defects in Nanoscale Carbon Materials

May 11, 2009 to May 13, 2009
Location : CECAM-HQ-EPFL, Lausanne, Switzerland
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Interface States in Carbon Nanotube Junctions: Rolling up graphene

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Carbon nanotubes are currently regarded as one of the most promising materials to develop future nanoelectronics, with an impressive combination of robustness and ideal electronic properties. At present, it is well established that further progress towards real applications depends on the ability to form junctions between different nanotubes [1]. Recently, the controlled synthesis of several carbon nanotube intramolecular junctions has been reported, either by current injection between nanotubes [2] or by temperature changes during growth [3]. These intramolecular junctions, which often present interface states, are typically made of topological defects arising from the connection between tubes of different chirality (Fig. 1). Although interface states are commonly regarded as a drawback in device performance, they may actually provide a means of achieving diode behavior at the nanoscale, as proposed in Ref. [4]. Therefore, understanding the physics of CNT intramolecular junctions, for which interface states may dominate transport properties, has been a subject of growing activity in the last few years [5–7].

We study the origin of interface states in carbon nanotube intramolecular junctions between achiral tubes. By applying the Born-von Karman boundary condition to an interface between armchair- and zigzag-terminated graphene layers, we are able to explain their number and energies. We show that these interface states, costumarily attributed to the presence of topological defects, are actually related to zigzag edge states, as those of graphene zigzag nanoribbons. Spatial localization of interface states is seen to vary greatly, and may extend appreciably into either side of the junction. Our results give an alternative explanation to the unusual decay length measured for interface states of semiconductor nanotube junctions, and could be further tested by local probe spectroscopies.


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