Theoretical Study On Electronic Transport Properties Of Molecular Conductors

The molecules with rich charge transport properties can be used as electronic devices. How to develop molecular devices has become an important subject in the domain of nano-electronic. On one hand, experimentalists have done a lot of experimental work. On the other hand, theorists have developed various theories to understand the working principle of the devices and to seek the relationships between the structure and the properties of the molecule. Theoretically, only qualitative results of the I-V property of the molecular are got until now, which are not comparable to the experiment results because the coupling constant between the molecule and the metal is given by semi-classical approximation. Our investigation is based on a recently developed first-principles package TranSIESTA-C code. The package is based on the combination of density function theory (DFT) implemented in the well tested SIESTA method with the non-equilibrium Green function technique. We focus mainly on the transport properties of oligomers, polyphenyl dithiol, PP(n )DT,the electronic transport properties of the C14 monocyclic Ring and the transport properties of one dimensional adulterant carbon wire, which will be useful in the future electric devices.Chapter One introduces the research background of molecular conductors, the experimental methods and main theoretical methods which are used for the study of molecular 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 important methods used for the first principles: the density functional theory (DFT). And then we show clearly how to make a combination between the density functional theory (DFT) and the nonequilibrium green function to calculate the molecular electric transport properties using the first principle. Finally, we describe in brief the program.In Chapter Three, We present a first-principles studies on the transport properties of oligomers polyphenyl dithiol, PP(2)DT, sandwiched between two Al(100) electrodes. The variation of the current-voltage curve for PP(2)DT in different tilt angles are investigated systematically. The results indicate that PP(2)DT can be functioned as a molecular switch controlled by molecular conformation.In Chapter Four,We present a first-principles studies on the transport properties of oligomers, polyphenyl dithiol, PP( n )DTs, sandwiched between two Al(100) electrodes. The variation of the equilibrium conductance and current-voltage properties f PP(n )DT as a function of n , which is consistent with the exponential law. NDR behavior appears in bias-voltage range from 0.6V to 1.2V. The I-V characteristics can be analyzed by the evolution of the transmission spectrum under different bias voltages.In Chapter Five, The transport properties of C14 monocyclic ring sandwiched between two Al(100) electrodes are investigated by first-principles calculation. The variation of the equilibrium conductance as the function of the distance between the molecule and the electrodes has been studied. Our results demonstrate that the C14 monocyclic ring exhibits metallic behavior. Electron transport occurs through the lowest unoccupied molecule orbital (LUMO). With gate-voltage applied, it is found that the positive and negative gate-voltage have very different effect on the equilibrium conductance. We also calculate the effects of adsorbing other atoms such as oxygen and sulfur atoms. The results indicate that this kind of electron-accepting impurity will decrease the conductance of the system.In Chapter Six, We investigate on the transport properties of one dimensional adulterant carbon wire coupled to two Al(100) electrodes based on a recently developed Ab-initio nonequilibrium Green function formalism. The transport properties of the carbon wire with one of the carbon atom substituted by Si atom and the carbon wire with different length substituted by one of Si atom are calculated. We also study the variation of the transport properties of the carbon wire with substitution of other atom, such N, P, S. We find that the equilibrium conductance varies in an oscillatory manner for the adulterant carbon wire and the even-odd behavior also exists with the different substitution site. The variation of the transmission spectra, the current-voltage character and the conductance under different gate-voltage for the adulterant carbon wire are also analyzed.