Theoretical Study About Interfacial Structure And Electrical Transport Properties Of Conjugated Organic Molecular Devices

With the development of scientific theories and production technology, at present, theelectronic devices are becoming smaller and smaller, almost approach the limits oftraditional solid electronics theory. When the traditional solid electronics theory is nolonger applicable to the electronic components, molecular electronics turns to play animportant role in theoretical study. Molecular devices have been considerable developedby all scientists since the moment when they were proposed in1974by Aviram and Ratner.Currently, the electronic devices being studied in both experiments and theories are allformed by the “electrode-molecule-electrode” model, whose electronic properties are indeep relationship with the contact mode between the electrodes and molecules. Thecontact mode includes two aspects: one is the electrode interface configuration, the otheris the terminate group of the molecule.Firstly, there is a problem in the main experimental methods. It is that we can’texactly know the interface configurations of the atoms neither on the probes in STMmethod nor on the break surfaces in break junction method. Whereas the theoreticalstudies show that the electronic properties of molecular devices formed by differentelectrode interface configurations are quite diverse. As a result, to deep understand andexactly manipulate the influences caused by the electrode interface configurations on theelectronic properties of molecular devices is very necessary to further develop themolecular electronics. Secondly, the terminal group is the key part for molecule to anchorthe metal electrode. Until now, the scientists have studied varieties of terminal groups,however, few of carboxylic group. As common organic functional groups with strongpolarization, we predict that carboxylic groups could anchor the electrodes with greatstability. Moreover, at present stage, the theoretical studies about the influence caused byelectrode interface configurations are mainly about hydrocarbons, only a few aboutconjugated molecules.So, based on previous studies and existing problems, we investigate the mechanicalstabilities and electronic transport properties of some conjugated molecular devicesformed with two designed electrode interface configurations, utilizing the densityfunctional theory method combining elastic scattering Green’s function method. Details are as follows:1. According to the actual situation and the theoretical feasibility, in this paper wehave designed two representative interface configurations for each molecular system. Forthe first configuration, there is a single Au atom on the Au (111) surface of gold electrodewhich simulate surface defect Au atom. For the second configuration, the gold electrodesform ideal sharp tip configurations like pyramids. The molecules that we studied are4-(methylthio) benzoic acid (M1),1,4-bis(methylthio) benzene (M2) and Methyl4-(methylthio) benzoate (M3). Our results show that: the rupture force of M1and M2junctions are both about0.6±0.1nN as experiment probed; the rupture for M1junctionwhen elongated is at SMe-Au bond but not COO--Au bond, which results in the fact thatthe rupture force of M1and M2is comparable; The COO-group strongly influenced onM1molecular junction and further strengthened SMe-Au bond at the other end of thejunction; the calculated rupture force of COO--Au bond is about2.5nN, which releasesthat M1is a stable junction; M3junction is the least stable, because the CH3group linkedto COO group destroyed the mechanical stability of COO-Au connection.2. Based on the molecular model we had designed, the electronic transport propertiesof those molecular junctions are further investigated. The computational results show that:the conductance of M2junction is about an order larger than that of M1junction as theexperiment probed; considering both interface configurations, the distribution of the threemolecules conductance accords with the experimental results very well; at bias voltage0.45V, no transport orbital of the three molecules opens, the molecular orbital is obviousdelocalized, the electronic transport property is mainly influenced by the coupling effect;at bias voltage1.0V, the transport orbital of M2and M3turn to open, as a result, theconductance increase obviously; it is difficult for M1to open the transport orbital, due tothe strong negative property of O atom in COO--Au bond.This thesis consists of five chapters. The first chapter is the introduction, where weintroduce the organic molecular devices briefly and then emphasize the development andcurrent situation of the studies on electrode interface configurations. The second chapterdescribes theory method, including density functional theory to get electronic structures,elastic scattering Green’s function method to obtain the electronic transport properties.Chapter three and four introduce the main work during the study for Mater’s degree,covering the investigation of mechanical stability and electronic transport properties ofconjugated molecular systems with two electrode interface configurations. The last chapter is the overview and prospective of this thesis.