Negative differential conductance in suspended semiconducting carbon nanotubes

Suspended single-wall carbon nanotubes (SWCNTs) have been grown directly on metal electrodes with electrical contact by chemical vapor deposition. Extraordinary negative differential conductance was observed for the first time in suspended semiconducting SWCNTs. The current-voltage characteristics show an abrupt drop of conductance as the source-drain voltage is increased, followed by a constant differential conductance at higher voltage. The effect is qualitatively different from the recently reported negative differential conductance in metallic SWCNTs (\textit{Phys. Rev. Lett.} \textbf{95} 155505, 2005). We suggest that the observed negative differential conductance in semiconducting SWCNTs may be attributable to Schottky barriers between the as-grown suspended SWCNTs and the electrodes, instead of optical phonon scattering invoked in explaining the negative differential conductance in metallic SWCNTs. Our observations not only have potential applications for novel electronic devices, but also shed light on better understanding and manipulating SWCNTs transistors.