The size effects of electrodes in molecular devices: an ab initio study on the transport properties of C60

The role of electrodes in the transport properties of molecular devices is investigated by taking C-60 as an example and using gold nanowire and a gold atomic chain as the electrodes. The calculations are done by an ab initio method combined with the non-equilibrium Green function technique. We find that devices in which a single C-60 molecule is connected with different electrodes show completely different transport behavior. In the case of nanowire/C-60/nanowire the device shows a metallic behavior with a big equilibrium conductance (about 2.18G(0)) and the current increases rapidly and almost linearly starting from zero. The transmission function shows wide peaks and platforms around the Fermi level. While in the atomic-chain/C-60/atomic-chain case, the device shows resonant tunneling behavior and the Fermi level lies between the HOMO (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 the nanowire/C-60/nanowire system and the current increases very slowly until the bias is big enough to include the LUMO peak in the bias window. The big difference in the conductance and the current arises from the different coupling between the electrodes and the C-60 and the different number of channels in the electrodes.