The performance of modern electronic devices based on carbon nanotubes (CNTs) is influenced by a potential barrier at the metal–CNT contact that governs electron injection. The study of metal–CNT interactions is essential to achieve low-resistance ohmic contacts with nanotubes and thus their integration in new nano-devices.

Palladium appears to be the most promising contact metal; ballistic transmission of electrons has already been reported. However, there is no obvious reason why Pd should give a smaller barrier than e.g. Ti.

In order to understand the Pd-CNT interaction, MWCNTs with different amounts of Pd evaporated onto their surface were studied by high-resolution transmission electron microscopy (HRTEM) and Photoemission Spectroscopy. XPS and UPS measurements were performed at BW2 and FLIPPER II beamlines respectively, at Hasylab.

HRTEM images show the evolution from dispersed clusters to a quasi-continuous coating for increasing amounts of Pd evaporated onto the pristine CNT surface. Oxygen plasma treatment improved the cluster dispersion and reduced the size distribution by grafting oxygen groups on the CNT surface, which mediate the overlayer morphology. The preserved structural characteristic of the graphene layer under the Pd cluster and the absence of features near the Pd 3d doublet and C 1s on the XPS spectra are strong indications of the absence of a mixed Pd-C phase.

The observed shift in the Pd 3d core level towards higher binding energy can be associated with a poor final-state screening or charging possibly resulting from the reduced size of the clusters and poor charge transfer between the CNTs and Pd clusters. The different electrical contact behaviour of Pd and Ti will be discussed.

This work is financially supported by the Belgian Program on Interuniversity Attraction Pole (PAI 6/1), by Nano²Hybrids (EC-STREP-033311), and by DESY and the European Commission under contract RII3-CT 2004-506008 (IASFS). JG is research associate of NFSR (Belgium).

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