Zhenxiang Dai

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.

The role of the electrodes in a molecular conductor: an eigenchannel analysis

In order to investigate the role of the electrodes in a molecular conductor, we have studied the effects of diatomic molecules H2 and O2 on the eigenchannels of a nanocontact formed by two Au(100) electrodes with finite cross sections by first principles calculations combined with non-equilibrium Greens function technique. Two cases where the axes of the molecules are placed along the electrode axis and vertically to the electrode axis are studied. We find that the maximum number of the eigenchannels and their energy ranges are always determined by the band structure of the electrodes. The molecules only modify the electron transmission probability of each channel.

Gate-induced switching in single-molecule magnet MnIIICuII

Gate voltage effect on electronic transport through the smallest single-molecule magnet (SMM) MnCu [MnIIICuIICl(5-Br-sap)2(MeOH)] sandwiched between Au(100) electrodes is investigated by spin-polarized density functional theory calculations combined with the Keldysh nonequilibrium Green’s technique. Our study demonstrates that a certain gate voltage can induce a switching of the conductance in the equilibrium state. Under a finite bias voltage, negative differential resistance is observed in this system and can be modulated by tuning the gate voltage. More interestingly, current rectification can be achieved at a certain negative gate voltage. These effects can be understood by the responses of the benzene rings and the magnetic core to an external electrical field.