Intramolecular Junctions

Intramolecular nanotube junctions have been theoretically proposed for a long time [41,42,43]. The basic idea is that by introducing a pentagon and a heptagon into the hexagonal carbon lattice, two pieces of carbon nano-tubes with different atomic and electronic structures can be seamlessly fused together to create metal-metal, metal-semiconductor and semiconductor-semiconductor junctions within individual nanotube molecules. Such junctions generally have the shape of a sharp kink. Yao et al. [37] have measured isolated kink junctions connected to metal electrodes. A metal-metal junction has been discussed in the previous subsection. The inset to Fig. 6 shows the AFM image of another kinked nanotube lying across three electrodes. The left segment is identified as metallic. The transport characteristic across the kink, in strong contrast, shows highly rectifying behavior with nonlinear and asymmetric I-V curve, which can be strongly modulated by a voltage applied to the backgate. The kink is thus unambiguously associated with a metal-semiconductor nanotube heterojunction.

The details of the observed I-V characteristics are not well-understood. For example, neither the bias polarity nor the onset of conduction can be simply explained in terms of the Schottky barrier at the junction interface that one would expect from p-type doping behavior and band bending. To model the data, one in principle has to properly take into account the effect of the metal electrodes and in particular the unique screening properties in 1D conductors, which have been theoretically investigated by Leonard et al. [44] and Odintsov [45]. In nanotubes, the screening occurs by rings of charges or dipole rings if image charges are taken into account. The potential induced by a dipole ring falls off as 1/z2 as a function of distance z which makes screening much less effective. This introduces novel length scales in the charge transfer phenomena in nanotube junctions. A better understanding is important for the designing and functioning of nanotube devices, or more generally, 1D molecular devices.

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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])

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