First-principles Study Of Electronic Transport Properties Of Organic Molecular Devices

This dissertation presents the theoretical methods to study the molecular devices based on the first-principles calculations. We use quantum chemistry software package, cooperated with these methods, to study the transport properties of molecular devices. In our calculations, the density functional theory and nonequilibrium Green's function are utilized. Density functional theory is used to extract the electronic structure, and the nonequilibrium Green's function is adapted to described the transport properties. In fact, the electronic structure is extracted from the quantum chemistry software. The intact quantum chemistry software package can only apply to the isolated system, while the system we studied is an open system because of the connection with leads and driven by the bias. The quantum chemistry software package can not be applied for this purpose without any modification. We arrive the electronic structure of open system, the molecular conductance and I-V curves if we take the nonequilibrium Green's function into consideration.The dissertation is organized as following:In the first chapter, the background of the molecular electronics is introduced and three useful experimental methods is covered.The second chapter describes the density functional theory. Density functional theory, which can get the accurate results at a reasonable cost, is the main method we used in the electronic structure calculation. In principle, the density functional theory is rigorous, however, the lack of the exact form of exchange correlation potential results into introducing of the approximation, which requires the correction. The useful approximation of exchange-correlation functionals are local density approximation (LDA) and generalized gradient approximation (GGA). The calculation of the electronic structure requires us to solve the Kohn-Sham equation by using self-consistent algorithm.The third chapter introduces the theoretical basis of nonequilibrium Green's function (NEGF) and its application in the transport problems. NEGF is a little more complicated than its equilibrium counterpart, however, it shares some similar properties, for example, it can be expanded and expressed by Dyson's Equation. Also, its application in the transport is discussed and the formulism used to calculate the transport properties is described.The fourth chapter introduces and covers the details about how to calculate the electronic structure of an open system by using the quantum chemistry software package.In the fifth chapter, we use the methods discussed in the previous chapters to calculate the rod-like molecules and compare the calculated results with the experimental measurements. Also, we explain the stochastic switching behavior.In the sixth chapter, we study the switching behavior of rotational molecules. The conformation of the molecular switches can be changed by the external electronic field and the switching states can be triggered. We also discuss the influence of different acceptor or donor radicals to the switching properties.