First Principles Design Of Molecular Electric Devices

First-principles calculations based on combination of Density Functional The-ory and Non-equilibrium Green Function have been performed to study several prob-lems about transport properties in molecular scale conductors and design new specialmolecules which will be useful in the future electric devices. We focus mainly on trans-port properties of the carbon nanowire under the environment of water molecules; theeffect of the contacts between the clusters and the electrodes on the transport proper-ties; and current rectification in the asymmetric molecules.Chapter One introduces the research background of molecular conductors, theexperimental methods and main theoretical methods which are used for the study ofmolecular conductors, and what we will study in this thesis and its importance.Chapter Two focuses on theoreical details and computational methods. Firstly,based on the introduction of what the first principles is, we introduce the impor-tant methods used for the first principles: the density functional theory(DFT). Andthen we show clearly how to make a combination between the density functional the-ory(DFT)and the nonequilibrium green function to calculate the molecular electrictransport properties using the first principle. Finally, we describe in brief the pro-grams we use.In Chapter Three, We study current rectification effect in an asymmetricmolecule HCOO-C₆H₄-(CH₂)n-S sandwiched between two Aluminum electrodes usingan ab initio nonequilibrium Green's function method. The conductance of the systemdecreases exponentially with the increasing number n of CH₂. The phenomenon ofcurrent rectification is observed such that a very small current appears at negativebias and a sharp negative differential resistance at a critical positive bias when n≥2.The rectification effect arises from the asymmetric structure of the molecule and themolecule-electrode couplings. A significant rectification ratio of～38 can be achievedwhen n = 5.In Chapter Four, Effects of relative orientation of the molecules on electrontransport in molecular devices are studied by non-equilibrium Green's function method based on density functional theory. In particular, two molecular devices, with theplaner Au7 and Ag3 clusters sandwiched between the Al(100) electrodes are studied.In each device, two typical configurations with the clusters parallel and vertical to theelectrodes are considered. It is found that the relative orientation affects the transportproperties of these two devices completely differently. In the Al(100)-Au7-Al(100)device, the conductance and the current of the parallel configuration are much largerthan those in the vertical configuration, while in the Al(100)-Ag3-Al(100) device, anopposite conclusion is obtained.In Chapter Five, We investigate the effects of H2O molecule environment onthe transport properties of a seven-atom carbon wire coupled to two Al(100) electrodesbased on a recently developed ab initio nonequilibrium Green function formalism. Ourresults show that the transport properties are sensitive to the variation of the numberand the position of the H₂O molecule on the carbon wire. Especially, the equilibriumconductance of the carbon wire with single H₂O molecule exhibits an oscillatory behav-ior with the different position of the H₂O molecule. For the case of two H₂O molecules,the contribution of the third eigenchannel becomes larger in some configuration. Thecalculated current-voltage curves show different behavior with the variation of the po-sitions of the H₂O molecules. In certain cases, large negative differential resistance(NDR) is found.