Crossed Junctions

While it remains a challenge to controllably grow nanotube intramolecular junctions for possible electronic applications, a potentially viable alternative would be to construct junctions between two different nanotubes. Fuhrer et al. [46] have measured devices consisting of two naturally occurring crossed nanotubes with electrical contacts at each end of each nanotube. Both metal-metal and semiconductor-semiconductor junctions exhibit high tunneling conductances on the order of 0.1 e2/h. Theoretical study indicates that the contact force between the tubes is responsible for the high transmission probability of the junctions. Metal-semiconductor junctions showed asymmetry in the I-V curves and these results appeared to be understood well from the formation of a Schottky barrier at the junction. A novel three-terminal rectifying behavior was demonstrated in this crossed geometry.

Lefebvre et al. [47] have measured a circuit consisting of two crossed SWNT ropes. The junction I-V was found to be highly nonlinear with a 0.2 V gap, which enabled the top rope to be used as a local gate at small voltages. Gate sweeps at low temperature showed that the bottom rope behaves as double quantum dots in series, separated by a tunnel barrier at the junction which could be introduced from electrostatic screening or from mechanical deformation by the top rope.

Crossed nanotube junctions can also be created by mechanical manipulation using the tip of an atomic force microscope [36,47]. For example, Postma et al. [36] used an AFM tip to first cut a nanotube which was lying across two electrodes, and then push one segment on top of the other. Transport across such a junction exhibited power-law characteristics consistent with bulk-to-bulk tunneling between two Luttinger liquids. It is interesting

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Fig. 6. I-V for a rectifying diode from a metal-semiconductor heterojunction. The inset shows an AFM image of the device (from [37])

to note that the results suggested that the contact force or the mechanical deformation was quite small in this case compared with naturally occurring crossed junctions.

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