Electron Transportation Based On Different Molecule-electrode Contact

Molecular electronic devices have become true step by step, since they were brought forward in 1970s. Most recently, construction of molecular junctions with various special functions using single molecules have been one of the most advanced research subjects in the modern microelectronics. The structure of molecule and electron transport in molecular electronic devices are much different with traditional electronic devices. In the molecular electronic device, electron transport of the junction is the research focus, which has much relationship with electron transport in molecule. Mesoscopic transport theories usually follow the Landauer-Buttiker frame which is applicable to elastic scattering. While the non-equilibrium Green's function (NEGF), the most general theory for mesoscopic transport is suitable for both coherent elastic scattering and incoherent inelastic scattering.The asymmetry of organic molecular structure results in rectification in the molecular electronic device. The interface between the molecule and the electrodes also has impact on the electron transport, because of the small size of the organic molecule. The influence of the interface consists of two factors:one is the different molecular electrode materials; the other is with the same material, but the contacts of the molecules and the electrodes are different. We investigate the electron transport with the same electrodes but different contacts and the potential applications. This dissertation includes the following sections:1. We firstly investigate the electronic transportation of a series of gold-sulfur atomic chains with different length theoretically. The result shows that the conductance of gold-sulfur chain is larger than the one of the pure gold atomic chains. In addition, the conductance keeps almost constant and does not decrease exponentially with the molecular chain length, which is different from the organic molecular wire. The results indicate that the sulfur group is well coupled to the gold electrode. As the molecule is anchored to the gold electrodes via the sulfur group in the junction, the contact resistance between the sulfur and gold electrode is small, and the junction resistance is mainly contributed by the molecule.2. Secondly, we have designed acene molecules attached to two semi-infinite metallic electrodes to explore the source-drain current of graphene and the gate leakage current of the gate dielectric material in the FETs device. The electron transport of cross channels of the acene is efficient because of the weak dependence on the length of the transport route, making it possible to construct the FETs with multiple leads. When the cross channel is blocked, the junction conductance decreases dramatically, operating at low gate leakage current. The ability to modulate the leads combined with small gate leakage current in blocked acene makes graphene-based FETs that we fabricated feasible in the mesoscopic system.3. At last, we have designed contact-asymmetrical junctions based on the porphyrin molecule and have investigated the transport behavior. All the models are based on the porphyrin ring with one thiol anchored to the electrode on the right hand, and different numbers of thiols connected to the left electrode. The contact-asymmetrical porphyrins resulted in asymmetric electron transport when the bias in different directions was applied. It is possible to introduce the asymmetry into the molecular junction so that asymmetric electronic transportation can be achieved. We show the rectification ratio is close to 2.6 and can be maintained in a large bias range from 0.6 to 1.2 V. The preferential direction for the current is from two-thiol anchoring to the one-thiol anchoring side.