First Principle Calculation Model Of Single Molecular Junction Transport Properties

With the continuous development of microelectronics and electronic device miniaturization, using single molecules or small clusters to design the molecular-scale devices has become the development trend. Furthermore, the investigations of electrical and optical properties for these devices have become an independent and important subject, which is referred as molecular electronics. Progresses in micro-fabrication and self-assembly techniques, such as scanning tunneling microscope, atomic force microscope, have made it possible to design the single-molecule device experimentally. The electronic transport properties of single-molecule junctions have attracted more and more attention because of their novel physical properties, including switch effect, negative differential resistance (NDR) effect, memory effect, Kondo effect and rectifying effect. These interesting effects make it possible to realize the elementary functions in electronic circuits. Based on the development of the corresponding experiments, various theoretical methods including semi-empirical theories and first-principles methods have been used to probe the mechanism of these molecular devices. Since the electronic transport mechanism 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.The sizes of the electronic devices based on silicon semiconductor technique shows one tend being smaller and smaller. When the sizes of these devices shrink to tens of nanometers or smaller size, the electronic law applied in these macroscoric scale devices will be replaced by quantum laws correspondingly. Subject to various restrictions, the traditional methods have already could not meet the needs of the development of miniaturized devices, and then it is necessary to find new ways for these smaller and smaller devices. In recent years, the emergence of molecular devices has broken the traditional technology concepts. The small scale and its chemical synthesis of diversity have aroused great attentions. The molecular device present one great advantage in replacing the traditional equipments. Therefore, many scientists have begun to try to design some of the molecular devices from the chemical point. A lot of experimental and theoretic investigations have proved that the molecular nanotechnology devices display rich and interesting properties.The current thesis is divided into five chapters to introduce our theoretical investigations about some new single molecular-scale junctions which show great potential utilities in the molecular devices. Our first-principles calculation methods are based on the combination of the density functional theory(DFT) and non-equilibrium Green’s function(NEGF) method. Based on these theoretical methods, the transport properties of some novel molecular-junctions have been calculated.The first chapter mainly about the research background and the development of the single molecular devices. Concretely speaking, the single knowledge of some common single molecular devices, the recent research development, and scientific significance about our work are introduced.The second chapter mainly introduces the corresponding theoretic methods adopted in the current thesis including DFT and NEGF. The code used has also introduced.In Chapter Three, with aluminum electrodes, lithium electrodes and grapheme respectively, the molecule is placed in the middle position of electrode materials. The single molecule junction system subsequently constructed. Then applying a bias on both end of the structure, the voltage increase with a same step length, we can get the current-voltage curve of the transport system through the theoretical calculation. Projection of spectrum analysis and electron orbital distribution curve are used to well explain the current-voltage characteristics and the change trend. All these calculation results confirm the existence of a rectifying property of PTCDI-[CH2]6 and its rectification effects are very different because of the different electrode materials.In Chapter Four, it is mainly about the transport properties of six kinds molecular system using density functional theory and the nonequilibrium green’s function. The six types of molecules are respectively A-mp2 -A; A-mp-A; D-mp2-D; D-mp-D; A-mp2-D and A-mp-D. On the one hand, we found that the transport structure of three groups of molecular all showed good properties which similar to a molecular switch through the calculation above. On the other hand, the coupling strength between the molecule and the electrode, and the nature of the electrode also have both a certain effect on the molecular transport. Our results show that the acceptor-receptor push-pull macrocycles have a great effect on molecules and symmetric structure. It implies that these molecules have potential applications in the field of molecular electronics.The last chapter mainly discusses the review work of a full text, analysis and conclusion, and the next research work was also discussed here.