In certain materials, and for certain voltage ranges, electrical current flowing through the material decreases when the voltage across the sample is increased. This negative differential conductance (NDC) is important for oscillators, amplifiers, and fast switching devices. In this work, using real time quantum simulations, we show that this phenomenon occurs in isolated finite armchair single wall carbon nanotubes (SWCNT) without end contacts. For metallic SWCNT, like the armchair SWCNT, electron transfer to secondary valleys—the most common cause of NDC—is not expected to be observed, as there are two quantum channels at the Fermi energy available for conduction. The NDC is due to the finite nature of the SWCNT and the existence of excited states that are blocked, similarly to a Coulomb blockade system, thus preventing any further current flow. We also show that the SWCNT conductivity depends on its length and that the current flowing on the SWCNT behaves like a Bloch oscillation that is disrupted in the presence of a molecule, decreasing the conductivity, and thus providing a rationalisation for the behaviour of SWCNT organic gas sensors.
Computer time for this work was provided by the Laboratory for Advanced Computing (LCA) of the University of Coimbra. The authors thank the Fundação para a Ciência e a Tecnologia for financial support under project CENTRO-01-0145-FEDER-000014, 2017–2020 (JS and BFM), project POCI-01-0145-FEDER-032229 (BFM) and contract CEECIND/00579/2017 (JS). We also acknowledge project PTDC/FIS-MAC/32229/2017, funded by the Portuguese National Budget through Fundação
para a Ciência e Tecnologia/MCTES and also funded by the European Regional Development Fund (ERDF) through the Competitiveness and Internationalisation Operational Programa (PT-COMPETE 2020).
The Coimbra Chemistry Centre is supported by Fundação para a Ciência e a Tecnologia, through the Project PEst-OE/ QUI/UI0313/2014 and POCI-01-0145-FEDER-007630.