Studies On Electronic Transport Properties Of Oligoacene Molecular Junctions

Modern silicon techniques adopting the so-called top-down strategy have made the size of device less than 30 nanometer. When the size of chips reaches its limitation, the traditional technology of semiconductor is no longer valid. Developing of novel electronic devices at microscale by using the new materials have become the tendence of research in 21 century. With the development and application of various powerful experimental techniques, such as molecular self-assembly, mechanically controllable break junction and scanning tunneling microscope, fabricating functional molecular electronic devices by the bottom-up strategy becomes a hot research topic in molecular elctronics. On the other hand, several theoretical models and computational methods have been proposed. The theoretical studies not only explain some experimental measurements and explore electronic transport mechanisms, but also provide useful guide for designing molecular devices. This dissertation including four chapters is devoted to investigate the electronic structures and transport properties of molecular junctions by using the density functional theory (DFT) calculations and non-equilibrium Green's function (NEGF) technique.In the first chapter, we give a brief introduction of molecular electronics. We first review the latest progresses of molecular electronics. Then several experimental techniques, such as mechanically controllable broken junction, scanning tunneling microscope, conducting probe atomic force microscope and some other methods of bottom-up combined with top-down are reviewed.The theoretical and computational methods adopted in this thesis are presented in chapter 2. The DFT is used to investigate the molecular geometrical and electronic structures. In the first half part of this chapter, we review the basic concepts and theoretical frame of DFT. To explore the transport properties of molecular junctions, the NEGF method is adoped. The NEGF method and the software ATK are presented in the second part of this chapter.Chapter 3 fouces on the length-dependent conductance of oligoacenes junctions with serial and parallel configurations. Electronic transport experiments of molecules with large HOMO-LUMO gap, such as thiol, had proved the correctness of Magoga's exponential law. However, debates still exist about the transport properties of oligoacenes with small HOMO-LUMO gap. Our theoretical results indicate that when the molecular length increases, the zero-bias voltage conductance G(0) of oligoacenes with serial configuration first reduces and then increases, while the G(0) of parallel oligoacenes junctions monotonically increases. Theoretical results agree nicely with recent experimental results. At last, the transport mechanism is explored.In chapter 4, we focus on the rectifying effect of a polar conjuncted molecular junction. We first introduce the earliest molecular rectifier model proposed by Aviram and Ratner and review the important reseach works in the field of molecular rectifier. Then we investigate the electronic transport properties through polar nitrogen-doped oligoacenes synthesized recently. Theoretical results show that the rectifying effect appears in a specific bias voltage range, which originates from the frontier molecular orbitals asymmetrically coupling with two electrodes and the molecular polarity.