The Effect Of Electrode Distance And Electronic Transport Mechanism Of Organic Molecular Devices

With the rapid development of single molecular technologies, the results of experimental measurements of molecular junctions are more and more stable and often agree with each other very well although different technologies are applied. It was found that the molecular devices have many useful features, such as molecular switch, molecular rectifier, field effect transistor(FET), molecular memory and so on. In order to understand electronic transport mechanism of molecular devices, various theoretical methods have been developed to simulate the electronic transport properties of molecular junctions. These methods have won a great success in understanding of the features of molecular devices, etc.Recent experimental studies have found that the alkane dithiol molecular junctions with gold electrodes are often shown lowest conductive state when the stretching force of the electrodes is about 0.8±0.3 nN. In order to understand this unique electronic transport properties, based on hybrid density functional theory, the evolution process of 1, 4-butanedithiol molecular junction are studied with the elongation of gold electrodes. The results show that when the terminal S atoms are sited on the hollow positions of Au(111) surfaces, the rupture force is about 1.75 nN. However, the rupture force is only 1.0 nN when one terminal S atom links with one single Au atom on electrode surface. It is notable that the single surface Au atom can easily be pulled away from the gold surface by the pulling of the terminal S atom. These results are not only consist with the experimental measurements very well, but also indicate that the pulling event may occur in the case of the force is less than the rupture force of Au-Au bond, i.e., 1.5 nN. Using the elastic scattering Green’s function method, the electronic transport properties of the molecular junction are further calculated with forces applied. Both the experimental and theoretical results show that, the molecular system appears lowest conductive states when the system is tensioned at 0.7~0.8 nN which due to that the terminated S atoms have the weakest coupling with gold electrodes under 0.7~0.8 nN.Our further study shows that, the elastic scattering Green’s function method and the nonequilibrium Green’s function method can well reflect the electrical transport properties of functional molecular systems, but for weak coupling systems, especially in the lower bias voltage regime, still has certain difficulties to solve some common issues of electronic transport properties by those methods, such as the exponential decaying properties of molecular conductance on the molecular length, step features shown in the conductance traces during the stretching process of molecular junctions. Based on the molecular effective potential field calculated at ab initio level, we developed a method of one-dimension transmission combined with three-dimension correction approximation(OTCTCA). It is considered that the functional molecule provides a spatial distribution of effective potential field for the electronic transport. The electrons are injected from one electrode by bias voltage, then transmit through the potential field around the functional molecule, at last are poured into the other electrode with a specific transmission probability which is calculated from one-dimension Schr?dinger equation combined with three-dimension correction.By applying the OTCTCA method, the exponentially decaying properties of the conductance with the increase of molecular length of alkane diamines molecular junctions are investigated. The numerical results show that the conductance of alkane diamine series molecular junctions exponentially decays with the increase of molecular length in the lower bias with the decaying factor 0.90±0.05, which is more close to the experimental measurements than the results we calculated by non-equilibrium Green function method. Based on OTCTCA method we found that the transmission probability of non-resonant transport mainly influenced by the following factors:(a) the number of potential wells and potential barriers,(b) the shape of potential wells and potential barriers, and(c) the energy of incident electrons. Thus each repeated unit in the molecule is just as repeated potential wells and potential barriers that will reduce the transmission probability by about equal proportions, which induces the exponentially decaying character of conductance with the increase of molecular length for the same series of molecules.The OTCTCA method is used to study the conductance step of saturated carbon chain and the double conductance steps of 4, 4′-bipyridine molecular junctions in the elongation process. The results show that, when the molecule is well connected with the electrodes, a coherent potential well channel is formed near the molecular skeleton for electrons transport through and no potential barrier cuts off the coherent channel. The non-resonant transmission probability of electrons will not be significantly changed by the change of the channel length if the shape of potential well channel is not changed significantly. Therefore, the molecular conductance generally shows steps in the stretching process under lower voltage. The double conductance steps shown in the stretching process of 4, 4′-bipyridine molecular junction is due to the terminal N atom being very likely absorbed on the surface Au atom.This thesis consists of six chapters. In the first chapter, the present conditions of the experiments and theories of molecular electronics are reviewed. The second chapter introduces the theories that are used in the studies of this thesis including our new developed OTCTCA method. In the third chapter, the elongating process of 1, 4-butanedithiol molecular junction is simulated. The reason why the molecular junction shows lowest conductivity state with stretching force of about 0.7~0.8nN is also discussed in the third chapter. The fourth chapter analyses why the conductance exponentially decays with the length of alkane diamines molecular junctions based on the OTCTCA method. In the fifth chapter, the characteristic of conductance step of the alkane diamine junction and 4, 4′-bipyridine molecular junction are discussed. The sixth chapter draws a conclusion for the full thesis and prospects our future work.