First Principle Investigations On Electron Transport Through Single Molecule

Based on the review of experimental and theoretical development on molecular electronics, we give a detailed introduction to the first principle method to modeling the electron transport through single molecule. Then, we use the recently developed TranSIESTA simulation package, which uses nonequilibrium Green's-function (NEGF) technique with self-consistent density functional theory (DFT), to study the electron transport properties of three systems which are Au/1,4-diaminoacennes /Au,Al/TaSi₃/Al and (4,4) single wall carbon nanotube with H₂O or O₂ absorbed on its wall. And draw following conclusions from those studies:(1) The equilibrium conductance of 1,4-diaminobenzene is increased with the molecule-electrode distance decreasing and a platform occurs at distance from 1.4(?) to 1.9(?), showing that the equilibrium conductance 1,4-diaminobenzene is insensitiveness to distance. When bias applies, the nonlinear current-voltage characteristic occurs resulting from the competition between linear increase of voltage and the increase of shift step of transmission spectrum. The transport properties depend not only on the number of fused aromatic rings in the molecule but also on the position of the amino groups on the rings. For 1,4 series, the more number of the rings give the stronger the transmission spectrum near the Fermi energy. While, for 2,6 series, the more number of the rings give the weaker the transmission spectrum near the Fermi energy. This is helpful to understand the improved reliability and reproducibility of conductance measurements using amines.(2) Under low bias from 0V to 1.1V, current through (4,4) single wall carbon nanotube, including systems that molecule (H₂O or O₂) absorbed on its wall, increases linearly. While bias is larger than 1.1V, current through those systems increases slowly resulting from the enhancement of blockage to the conducting electrons when bias is large enough to make more electrode states spread into central scattering region and course the reduce of transmission in the energy range of bias window. It is also shown that absorbed molecule brings two kind of influence on electron transport properties: one, molecular state spoils the nanotube's symmetry and forms the more scattering centers, and lower the transmission, and the other, molecule states afford new channels to electron transport through the junction and enhance the transmission. Those effects works only when bias window includs the contribution from absorbed molecule. Therefore system that absorb water molecule has nearly the same current-voltage characteristics with that of unabsorbed system, except that bias is larger than 1.8V. While current of system that absorb oxygen is always lower than that of two above systems except that bias is larger than 1.7V.(3) Equilibrium conductance of TaSi₃ cluster is very sensitive to the cluster-electrode distance, and this sensitivity comes from sensitivity of molecular projected self-consistent Hamiltonian (MPSH) to the cluster-electrode distance and the alternative control of MPSH 8 or 9 to transmission spectrum at Fermi energy. Different rotation angles around x-axis of TaSi₃ cluster also give different equilibrium conductance, among those angles 50 degree gives the lowest value 1.1G₀, while others values are all near 2.2G₀. Equilibrium conductance of TaSi₃ cluster changes following the increase of gate voltage, among those gate voltages 0.6V gives the lowest value 1.95G₀, while other values are all near 2.2G₀. When bias applies, from -1V to 1V, asymmetry current-voltage characteristic occurs and negative differential resistance is observed under the bias changing from 0.3V to 0.4V.