First-principles Study On Electronic Transport In Metal/Organic Molecule/Metal Junctions

With the development of micro-electronics and minimization of electronic device, it has been a trend to use single molecules, including single-and multiple-wall carbon nanotubes, single organic molecules, biomolecules, to construct functional electronic devices. Furthermore, the research of electrical and optical properties for these devices has become an independent subject, that is, molecular electronics. The electronic transport in single-molecule junctions have attracted more and more attention because of their novel physical properties, including negative differential resistance (NDR) effect, rectifying effect, memory effect, switch effect and Kondo effect, which make it possible to realize the elementary functions in electronic circuits. The most prominent among them is NDR effect, which is a very useful property in molecular electronic devices such as molecule switch. Following the development in experiments, people use various theoretical methods, such as semi-empirical theories and first-principles methods, to investigate the mechanism for molecular devices during operation. As the electronic transport plays a key role in the operation of molecular devices, it is of fundamental importance to obtain a comprehensive understanding of the electronic transport properties of molecular junctions.In this thesis, we will perform first-principles studies with respect to the electronic transport properties of three typical molecular junctions. The outline and the main conclusions of studies are as follow.1. The role of the coupling between molecule and electrode in NDR effects.The oligo(phenylene ethynylene)(OPE) molecular junction is one of the systems most intensely studied as a prototype of the NDR devices. However, in spite of a number of theoretical and experimental studies on the OPE molecular devices, whether or not NDR can be observed in the OPE molecular systems, as well as the origin of the mechanism leading to NDR effect is still under intense debate. By using Density functional theory (DFT) and Non-equilibrium Green's function (NEGF) method, we study the electronic transport properties of molecular junctions formed by Au electrodes and OPE molecule substituted by nitro and amino groups on the central ring, which are typical structures used in experiments. It is found that the coupling between molecule and electrode plays a key role in NDR effects.2. Electronic transport properties in Au/diblock molecule/Au junctions.Diblock oligomer molecules have attracted more and more attention because of their interesting physical properties, such as rectifying effect. But most of theoretical workers focus on the diblock oligomer molecule consisting of two units (each unit contains 2 rings). It is thus interesting is to examine the electronic transport properties in the diblock molecules, where the number of rings in each unit is not equal to each other. By using DFT+NEGF method, we study the electronic transport properties in Au/diblock molecule/Au junctions. Five diblock molecules have been studied and each of them contains 4 rings, which are pyrimidinyl (C4N2) and phenyl (C6) rings. It is found that these molecular junctions have different electronic transport properties, including NDR and rectifying effects.3. Conductance of Pt/C60/Pt junction.Highly conductive molecular junctions achieved by direct binding of aπconjugated organic molecule to Pt electrodes without the use of anchoring groups have been reported recently. By using DFT+NEGF method, we studied the conductance of Pt/C60/Pt junction, which is typical of these junctions. It is found that the conductance of this junction at zero voltage bias is very close to the result of the experiment, and a multi-channel transport mechanism can be responsible for this high conductance.