Controlling energy-level alignments at carbon nanotube/Au contacts

The properties of the contacts between single-walled carbon nanotubes (SWNTs) and Au electrodes are studied using scanning Kelvin probe and electrostatic force microscopies. Contact potential differences and local dipoles at the SWNT/Au interface are determined under various conditions involving gas adsorption and surface passivation. In particular, the effects of the coadsorption of alkanethiol, S, and O2 are explored in detail. We find that the coadsorbates alter the energy-level line-up at the contacts and induce significant shifts of the SWNT bands relative to the metal Fermi level. This behavior is explained by considering the response of the local Au work function to the presence of the nanotube and of the coadsorbates as well as the effects of the adsorbate dipoles near the contacts. Finally, we use coadsorption to control the Schottky barrier height at the nanotube-Au contacts.