First Principles Study of Molecular Electronic Devices

Abstract: Molecular electronics is an active research area for the future information technology. The fabrication of basic electronic elements with molecules as the core-operators has been made experimentally in the laboratory in recent years. However, the underlying electron or charge transport mechanisms for most devices are still under debate, Theoretical modelling based on the first-principles methods are expected to play an important role in this field.A generalized quantum chemical approach based on Green's function scattering theory has been developed and applied to two- and three-terminal molecular devices. It allows to study both elastic and inelastic electron scattering at hybrid density functional theory levels. It can treat molecular devices where the metal electrodes and the molecule are either chemically or physically bonded on equal footing. As one of the applications, we have studied the length dependence of electron transport in gold-oligophenylene-gold junctions. We have shown that the experimental results for molecular junctions of oligophenylene with di erent lengths can be well reproduced by hybrid density functional theory calculations. It is also found that the current-voltage characteristics of the junctions depend strongly on the metal-molecule bonding distances. With the help of the calculations, the possible gold-molecule bonding distances in the experimental devices are identi ed.The central focus of this thesis is to study the three-terminal molecular devices, namely the eld e ect transistor (FET). An extension of our quantum chemical approach to FET devices has been made and successfully applied to different FET devices constructed with polymer, small and middle sized conjugated molecules. The experimentally observed conductance oscillation in polymer FET and three orders of magnitude enhancement of the current in electrochemical gated molecular FET have been verified by the calculations. The electron transport mechanisms of these devices are revealed.