Theoretical Research On Electronic Transport Properties Of Benzene-based D/A Molecular Devices

TIn recent years, molecular electronics has become a significant and rapidly growing field, which made tremendous progress. With to the fast development of microscopic characterization and manipulation technology, the research of molecular device is greatly improved. People can observe and manipulate single molecules at the nanometer scale, which can be built into the specific functional molecular device. Due to their designable and rich structures, easy synthesis and good semiconductor properties, organic conjugated molecules has drawn great attention in research of the molecular device research.Using the approach of density functional theory (DFT) combined with nonequilibrium Green’s functions (NGF), the electronic transport properties of benzene-based D/A molecule have been investigated and the design of molecular devices are proposed in this thesis. Considering some key physical and chemical factors that impact the charge transport properties of the molecular device, such as anchoring groups, molecular chemical modification, donor-acceptor molecular conjugated bridge selection and intermolecular interaction, some useful conclusions has been obtained. The main contents are listed as follows.First, the electronic transport properties of different bridges connected to benzene-based heterostructure molecular devices have been studied. Focusing on the effects of the polarity and length of the bridge bond on the molecular device of Donor-Bridge-Acceptor (D-B-A), it is found that the polarity of the bridging bond in the molecular device plays a significant role in charge transport in D-B-A molecular junction. The molecular device with connection of the polar bond has higher conductivity than that with connection of the non-polar bond. The NDR behavior and current platform are observed within a certain bias voltage range for the polar bonding systems.Second, the electronic transport characteristics of NBPDA molecular heterojunction, wihich connected by four different anchoring groups, are discussed. The molecular heterojunction is positioned between two semi infinite gold electrodes. Results show that the anchoring group plays a crucial role in determining the overall conductivity of the molecular junctions. The NDR behavior originates from the changes of transferred charges and redistribution of the frontier molecular orbitals at different bias.Third, fluorination effects on the electronic transport in DT-TTF are explored based on the first-principle calculation and theoretical analysis. It is found that the structure and electronic properties of the molecular device have been modified. The fluorine atoms group can decrease charge injection barriers and then do good to the electron injection and transmission due to their strong electronic absorbing ability. Meanwhile, the addition of fluorine atoms can modify the couple of DT-TTF molecular junction and the electrodes, thereby affecting the charges transfer between them. The transmission peaks of fully fluorated molecular shift to the low energy region, and adjust the localization of molecular orbit. At the same time, the Coulomb Blockade effect and NDR behavior can be observed within a certain bias voltage range since the coupling between molecular junction and the electrodes has decreased and the current becomes weak.Finally, the intermolecular coupling effects on the electronic transport have been investigated towards the dimolecule device. It is found that the intermolecular coupling effect plays an important role in the conducting behavior of the system. The nearer distance among the molecular, the more transport channels will be opened so leading to the split-up of molecular orbits and thus appears negative differential resistance within a certain bias voltage range.These results will shed light on establishment of the structural properties of organic molecular devices.