First Principles Investigations For Electronic Transport Properties Of Functional Organic Molecular Devices

In this thesis, we have performed first-principles combined with nonequilib-rium Green's function calculations on the transport properties of some significantand useful organic molecular devices. The effects of intermolecular interaction,side groups, coordinate ions, twisting of the structure of molecules, substitutedatoms and gate voltage on electronic transport properties of molecular devices arestudied systematically. There are lots of interesting results such as the negativedifferential resistance (NDR) behaviors, especially the multiple NDR have beenobserved and explained clearly, the causes of the switching ratio in molecular de-vices, and also physical and chemical modifications on the electronic transportproperties of molecular junctions, and so on, have been observed in the presentthesis.We investigate the transport behaviors of a dimolecule device constructed bytwo 1, 4-Dithiolbenzenes (DTB) sandwiched between two gold electrodes. The re-sults show that the intermolecular coupling effect plays an important role in theconducting behavior of the system. The intermolecular coupling interaction mayinvolve two types of interactions: direct molecule-molecule interaction betweenbenzene molecules and indirect interaction through the gold electrodes. We findthat intermolecular effects mainly come from the direct interaction and by chang-ing the dihedral angles between the two DTB molecules, namely changing themagnitude of the intermolecular interaction, a different transport behavior canbe observed in the system.The electronic transport properties of double-OPE (oligo phenylene ethyny-lene) junction have been studied. We find that the splitting of the frontier molecu-lar orbitals due to the intermolecular interaction can contribute to new transportchannels. Especially, the negative differential resistance can be observed whenthe intermolecular distance closes to a certain value, where we propose that acombination of the splitting of the molecular orbitals and the change of the cou-pling between the molecules and the electrodes at different biases might be re-sponsible for the negative differential resistance behavior.We calculate the electronic transport properties of a porphyrin sandwichedwith two gold electrodes. The theoretical results show that the coordinating Zn atom can strengthen the coupling of the molecule and electrode, and also delocal-ize the lowest unoccupied molecular orbital through whole of the molecule, andthe transport properties can be strengthened, while twisting the middle benzenerings have a opposite effect, the electronic current can be weakened.The electronic transport properties of molecular junctions constructed by theporphyrin molecule with donor/acceptor side groups have been investigated.The calculated results show that the side group plays an important role on theelectronic transport properties, the negative differential resistance behaviors canbe observed in such devices whether the devices contain a side group or not, espe-cially for the molecule with electron-donating group (-NH2), two NDR appear atdifferent bias voltage region. We suggest that the change of the coupling betweenthe molecular orbitals and the electrodes due to the shift of the molecular orbitalscaused by the change of the transferred charge of the molecule is the origin of theNDR appearing at higher bias for all three systems, and the off-resonance relativeto the bias window of frontier molecular orbital under certain bias voltage rangecan explain the NDR appearing at the lower bias voltage of the molecule withelectron-donating group.We investigate the electronic transport properties of three molecular junc-tions constructed by C₆₀, C₅₉N and C₅₉B molecule junctions, the gating effect forthe three systems is considered. The results show that the equilibrium conduc-tance presents an oscillatory behavior, which is very closely related to the varia-tion of density of state at Fermi level. Moreover, it can be found that all the threemolecular junctions exhibit negative differential resistance behaviors, and the Batom can increase the switching ratio but the N atom can reduce it. We also findthe negative differential resistance behaviors can be modulated by gate voltage.These results maybe useful for the designs of molecular devices.