Inter Tube Transport in Nanotube Ropes

By analogy with graphite where coupling between the graphene layers produces a 3D conductor, one would expect that coupling between the nano-tubes in a rope may lead to some degree of electronic delocalization within the rope. A number of studies of the electrical properties of ropes [5,35,57] have assumed a weak delocalization within the rope. This weak interaction is primarily due to the fact that current synthetic routes produce a mixture of tubes with different diameters and chiralities which make the ropes inhomo-geneous. Maarouf et al. [80] explored theoretically the inter-tube transport process using a tight-binding model and concluded that the need to conserve crystal momentum along the tube axis limits inter-tube transport in ropes containing tubes with diffferent chiralities.

An experiment that allows the measurement of the weak inter-tube transport was performed by Stahl et al. [81] In this study, surface defects were deliberately introduced along the surface of a rope by ion bombardment, and the damaged areas were then covered by gold electrodes. At high temperatures the current flows through the damaged tubes that are in direct contact with the gold leads. As the temperature is decreased the two-terminal resistance rises as a result of localization in the damaged tubes. When their resistance becomes the same as the inter-tube (transfer) resistance, the current switches its path to an undamaged bulk nanotube. This is reflected by a sharp decreas in the four-terminal resistance. (Fig. 11) Analysis of the experimental results suggests that inter-tube transport involves electron tunneling with a penetration depth of about 1.25 nm.

Fig. 11. Temperature dependence of the two-terminal and four-terminal resistance of a single-wall nanotube rope intentionally damaged directly under the gold contacts used to measure the resistance. The maximum in the resistance measured by the four-terminal measurement marks the point at which the resistance for inter-tube transport becomes equal to the resistance for transport along the tube at the same temperature

Fig. 11. Temperature dependence of the two-terminal and four-terminal resistance of a single-wall nanotube rope intentionally damaged directly under the gold contacts used to measure the resistance. The maximum in the resistance measured by the four-terminal measurement marks the point at which the resistance for inter-tube transport becomes equal to the resistance for transport along the tube at the same temperature

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