First-principles Study Of The Quantum Transport Properties Of Molecular Devices

This dissertation presents the ab-initio modelling of quantum transport in molecular devices. Based on this ab-initio modelling method, we study some phenomenons in molecular electronics which include conductance-length dependence and rectifying of molecular diodes. This approach combines the density functional theory (DFT) and the non-equilibrium Green's function formalism (NEGF) to form a rigorous modelling with which we can deal with the quantum transport in molecular devices. This computational modelling is a powerful tool to understand the electronic transport in molecular system and very useful to discover new transport properties of some molecules, which is very critical to the designment and fabrication of molecular devices. We employ the density functional theory which is well-known and widely accepted to calculate the electronic structure of molecule, which could assure the accuracy and reliability of computation. Mean while, we introduce the non-equilibrium Green's function formalism into the computation of density functional theory in order to do with the transport problem for open system. To realize it, we modify a traditional quantum chemistry software Gaussian 03 and replace its module of density matrix with the related NEGF density matrix computational codes, this step changes the self consistent procedure into a non-equilibrium one so that the software become a system which could deal with open system and solve transport problem.In Chapter one, we briefly introduce the background of molecular devices and molecular electronics and their development. The basic framework of our computation method is introduced also.In Chapter two, we introduce the density functional theory (DFT) which is the basis of our computation method. We review the history of DFT and introduce some basic conceptions about it. Especially, we generalize the procedure of solution of Kohn-Sham equation.In Chapter three, we introduce the Landauer picture which is used to do with transport problem and the non-equilibrium Green's function (NEGF). We present some conclusions and formulas of them. To apply the NEGF in Landauer picture is the basic idea of our computation.In Chapter four, we present the building of computation method in detail and discuss some key point in the program. We introduce the NEGF into the self-consistent procedure of DFT and build a self-consistent procedure for the open and non-equilibrium system, which combines the NEGF and DFT and make it capable of dealing with transport problem.In Chapter five, we apply this computation method to solve some problems in molecular electronics including the length dependence of molecular conductance and the rectifying effects of molecular diodes.At first, we study the length dependence of conductance of molecules. We calculate oligoth-iophene and obtain an abnormal length dependence of conductance at low bias, the longer molecule has larger conductance, and the oscillated length dependence of conductance at high bias region. The transport behaviors are determined by the distinct electronic structures of the molecular compounds. The conductance length dependence for several organic molecules is calculated to show the richness in the transport behavior. Second, we calculate a type of asymmetry molecule and study its asymmetry bias property i.e. rectifying characteristic. Unlike AR's molecule composed of three parts (accepter, bridge and donor), this type of molecule is diblock structure with electron-deficient part covalently connected with electron-rich part similar to PN junction. The rectifying characteristic of diblock molecular diode is brought by its asymmetry in molecular structure. Our calculation is consistent well with the experiment results.