Localization and Tunneling Mechanism in Carbon Nanotubes Aggregates

Transport mechanism in single wall carbon nanotubes (SWCNT) aggregates has been widely studied and understood on the basis of the transport theories currently used for disordered conductors. Variable range hopping (VRH) [1] and temperature fluctuation induced tunnelling (FIT) [2] seem to be the most successful models for explaining the resistivity behaviour in a wide temperature range. Both models assume that localized states are formed inside the bundles and that charge transport happens by hopping or tunneling through the bundles-bundles interface junctions. Commonly, the junction at the interface is naturally formed by the action of van der Waals forces and it can be described as a potential barrier for charge carriers whose characteristics (width and height) depend on the nature of the interface and on the electrical properties of the SWCNT. Because SWCNT aggregates comprise both semiconducting and metallic individuals, the junctions play the role of insulating barriers along a conducting path. Therefore, interesting effects as thermal activation, tunneling and hopping can be evidenced in an experiment where charge transport is monitored. Which of these effects rules the physics of these systems can be understood by transport measurements performed in a wide temperature range. In this talk we study the transport properties of two kinds of SWCNT aggregates: oriented arrays of SWCNT bundles [3] and aligned SWCNT fibres [4]. Resistance vs. temperature (R-T), magnetoresistance vs external magnetic field (MR) and current vs voltage (I-V) measurements have been performed in a wide temperature range. The R-T data showed 3-dimensional localization of the charge carriers above a temperature T* which is different for the two systems. This mechanism is confirmed by negative MR at low field due to the interference between the wavefuctions of the hopping electrons between localized states. Moreover, the finite value of the resistance in the limit of zero temperature (T<T*) signals the presence of potential barriers ascribed to the SWCNT-SWCNT interface junctions. These measurements, together with I-V characteristics at different temperature, yield the width and the height of the potential barriers, which are higher in oriented arrays than fibres. Because of the difference in the fabrication of the two analysed systems, which affects the nature of the SWCNT-SWCNT junctions, the experimental results could help in determining the role of the junction interfaces on the transport mechanism of CNT aggregates.