The Electronic Structure And Transport Porperty Of Molecular Device

Molecular electronics is chiefly coming to compose and study the moleculesand makes these molecules be equal or similar to triode, diode, conductor and other critical component of micro-electricity now. The requirement along with the mankind to the computer function is much higher, and along with more and more miniaturization of solid electronics device, the device will enter the domain of quantum mechanics and is called nanometer device or the quantum device, and whose work principle should be described by quantum mechanics. The important development direction of device is molecular device. In essence speaking that the molecule is different from the semiconductor, therefore we can foresee the molecular device to possess a lot of specific properties.The function of molecule device is decided by two factors. It is the electronic structure of molecule and the interaction between molecule and metal surface. Therefore if we want to imitate and design molecular device, these two factors should be described more accurately. In recent years, the theory workers have developed some theoretical methods, such as the scattering theories, to imitate the voltage and current specific property of molecular device using semi-empirical approach to describe such interaction, therefore can only get some qualitative results. The elastic-scattering Green's function method that we had developed is used to count the current that passes through the molecular device. The transmission coefficient in the current formula is relevant to the electronic structure of molecule and the interaction between the molecule and the metal. Such interaction can approximately be described by the covalent bond between the molecule and the metal surface. So we can use ab initio method and DFT to describe electronic structure of molecule and atomic group. Using Dewar,Chatt, and Duncanson's ( BCD ) model can count out the interaction energy between molecular orbit and metal state. So ,our results are more accurate.The sulphur hydrogen functional group can be very strongly adsorbed on the gold surface, thus can be used as linking material in molecular device. Firstly we have investigated the interaction between the 4,4'- based biphenyl molecule, subsequently calculated theinteraction energy constant by using the frontier molecular orbital theory and perturbation method. The counting result indicates that organic molecule adsorbing on the metal surface and forming chemical bonds between the end group of the molecule and metal surface, and some of molecular orbit are extended over the extended molecule, thus providing conduction channels for electron transfer and the conductance appears an ohm specific property. Other orbits are located at biphenyl, so at this circumstance electron transfer can only be realized by way of the tunnel effect. Secondly we calculate the conductance of the molecule .The current-voltage curve show that there exists a current forbidden zone nearby zero bias voltage, along with the increasing of bias voltage, the conductance of molecule appears a terrace characteristic. This work will attribute to design of the device in nano-electronics in future.The structure of this thesis is as follows. In the first chapter, the single molecule science and the research of molecular device are introduced. In order to get the stable structure of molecule, we have used the molecular orbit theory, which is introduced in second chapter we introduce this theory. This theory contributes to us to understand the bonding between sulphur atom and gold surface. The Density Functional Theory is the method of the most economic of time when using the molecular orbit theory. In third chapter, we introduce this method, hi forth chapter, the elastic scattering Green's functional method is studied and the current density is given. In fifth chapter, the research steps and the results are given in details.