The size effects of electrodes in molecular devices: an ab initio study on the transport properties ofC60

Abstract

The role of electrodes in the transport properties of molecular devices is investigated by takingC60 as an example and using gold nanowire and a gold atomic chain as theelectrodes. The calculations are done by an ab initio method combined with thenon-equilibrium Green function technique. We find that devices in which a singleC60 molecule is connected with different electrodes show completely different transport behavior. In the caseof nanowire/C60/nanowire the device shows a metallic behavior with a big equilibrium conductance (about2.18G0) and the current increases rapidly and almost linearly starting from zero. The transmissionfunction shows wide peaks and platforms around the Fermi level. While in theatomic-chain/C60/atomic-chain case, the device shows resonant tunneling behavior and the Fermi level lies between theHOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital)transmission peaks. This results in a current that is one order of magnitude smaller than that in thenanowire/C60/nanowire system and the current increases very slowly until the bias is big enough to includethe LUMO peak in the bias window. The big difference in the conductance andthe current arises from the different coupling between the electrodes and theC60 and the different number of channels in the electrodes.