HTML clipboardIn this thesis, dynamic effects on electron transport in molecular electronic devices are presented. Special attention is paid to the dynamics of atomic motions of bridged molecules, thermal motions of surrounding solvents, and many-body electron correlations in molecular junctions.

In the framework of single-body Green’s function, the effect of nuclear motions on electron transport in molecular junctions is introduced on the basis of Born-Oppenheimer approximation. Contributions to electron transport from electron-vibration coupling are investigated from the second derivative of current-voltage characteristics, in which each peak is corresponding to a normal mode of the vibration. The inelastic-tunneling spectrum is thus a useful tool in probing the molecular conformations in molecular junctions. By taking account of the many-body interaction between electrons in the scattering region, both time-independent and time-dependent many-body Green’s function formula based on timedependent density functional theory have been developed, in which the concept of state of the system is used to provide insight into the correlation effect on electron transport in molecular devices.

An effective approach that combines molecular dynamics simulations and first principles calculations has also been developed to study the statistical behavior of electron transport in electro-chemically gated molecular junctions. The effect of thermal motions of polar water molecules on electron transport at different temperatures has been found to be closely related to the temperature-dependent dynamical hydrogen bond network.

We present first-principles studies on electron transport properties of Pd-dithiolated oligoaniline-Pd molecular junctions. It is to demonstrate the possibility of using inelastic electron tunneling spectroscopy (IETS) to identify the switching mechanism in the molecular junction. Calculations have successfully reproduced the experimentally observed conductance switching behavior and the corresponding inelastic electron tunneling spectra. It is shown that the conductance switching is induced by conformation changes of the intercalated dithiolated oligoaniline in the junctions rather than oxidation/reduction as proposed earlier. Among three possible isomers, the low and high conductance states are related to two symmetrical structures. The possible involvement of asymmetric structure is discussed. It is revealed that chemical bonds between the terminal S atom and Pd electrodes are quite weak with relatively long bond distances.

We have combined molecular dynamics simulations with first principles calculations to study electron 4 transport in a single molecule of perylene tetracarboxylic diimide (PTCDI) sandwiched between two gold electrodes with an aqueous electrolyte. This combination has for the first time allowed one to reveal statistical behavior of molecular conductance in solution at different temperatures and to produce conductance histograms that can be directly compared with experiments. Our calculations show that experimentally observed temperature-dependent conductance ran be attributed to the thermal effect on the hydrogen bonding network around the molecule and can be described by the radial distribution of water molecules surrounding the oxygen atom in the PTCDI molecule.

We have combined molecular dynamics simulations with first-principles calculations to study electron transport in a single molecular junction of perylene tetracarboxylic diimide (PTCDI) in aqueous solution under external electric gate fields. It is found that the statistics of the molecular conductance are very sensitive to the strength of the electric field. The statistics of the molecular conductance are strongly associated with the thermal fluctuation of the water molecules around the PTCDI molecule. Our simulations reproduce the experimentally observed three orders of magnitude enhancement of the conductance, as well as the temperature dependent conductance, under the electrochemical gates. The effects of the molecular polarization and the dipole rearrangement of the aqueous solution are also discussed.

H-shaped chromophores containing three parallel non-conjugated D-pi-A units are effective chromophores with high hyperpolarizability and good optical transparency. The semi-empirical methods ZINDO, AM1, MNDO and PM3 were employed to study the effect of strength of acceptors and donors, and steric repulsion between substituents on static first hyperpolarizabilities (beta(0)) and enhancements of beta(0) of the H-shaped chromophores. The results show that the H-shaped chromophore would exhibit the largest beta(0) and/or the largest enhancement of beta(0) of the chromophore when combination of a donor (D) and an acceptor (A) in a D-pi-A unit is suitable.

The semi-empirical method ZINDO was employed to study relationship between macroscopic optical nonlinear parameter mu beta/MW (where mu is the dipole moment, beta is the first hyperpolarizability, and MW is molecular weight) and the number of parallel non-conjugated D-pi-A units in a chromophore. The computational results show that macroscopic optical nonlinear parameter mu beta/MW value increases remarkably from 1.64 to 2.53 with increasing the number of parallel and non-conjugated p-Nitroaniline (PNA) units in a chromophore from 1 to 3. Then the mu beta/MW value decreases rapidly from 2.53 to 0.43 with increasing the number of PNA units in a chromophore from 3 to 5. It suggests that design of chromophores containing two or three parallel non-conjugated D-pi-A units would be an effective strategy for increasing the first hyperpolarizability and macroscopic optical nonlinearity of designed NLO materials.