First Principles Study Of The Nanosystems Electronic States And Charge / Spin Transport Properties

The ultimate goal of molecular electronics is to build functional devices with sin-gle molecules as elements to replace the silicon-based semiconductor transistors widely used currently. As use the different technics that traditional silicon-based semiconduc-tor used (top-down), but build device from single molecules(bottom-up), molecular electronics may bring revolutionary progress to electronic industry. Since the late 20th century, with the development and application of STM, SAM and MCBJ, etc, scientists have made remarkable achievements in investigation of photic, electronic, and mag-netic properties of molecular materials and molecular devices, which leads to the rapid development of molecular electronics. At the same time, in order to make up for the deficiency in experiment as well as to prove and explain the molecular conduction, theorists have contrived many theoretic models and methods and have made lots of contribution to the research of electron transport mechanism of molecular device.The first-principles method has become an important research means in molecular electron-ics. This dissertation is devoted to investigate theoretically the electronic and transport properties of several molecular devices through density functional(DFT) theory and non-equilibrium green's function method, it mainly consists of the following several parts.In the first chapter, we briefly introduce the investigation background and advance of molecular electronics. After a brief review of the birth of molecular electronics and the recent research on it, the thesis focuses on introducing several experiments which promote the rapid development of molecular electronics and the relating researches. Then from the respect of experiment and theory, it summarizes the recent progress made in the research of molecular electronics and also the existing problems and challenges.The theoretical methods adopted in this thesis are presented in chapterâ…¡. In the first half part of this chapter, we review the basic concepts and recent progress of density functional theory, which are adopted to calculate the geometric and electronic structures of molecules. The DFT-based program can't deal with the non-equilibrium problem for an open system, so it's helpless to the transport issue of molecular junction, new theo-retic method is essential. the most popular method in transport investigation of molecu-lar electronics is based on the non-equilibrium green's function combined with density functional theory. The second half part of this chapter will introduce the conduction mechanism of molecular device and the first-principles method for it, mainly focus on the rationale of non-equilibrium green's function and it's specific forms and the solving methods in molecular systems.In chapterâ…¢, we investigated the effects of binding modes and anchoring groups on non-equilibrium electronic transport properties of alkane molecular wires from atomic first-principles based on density functional theory and non-equilibrium Green's function formalism. Four typical binding modes, top, bridge, hcp-hollow, and fcc-hollow, are considered at one of the two contacts. For wires with three different anchor-ing groups, dithiol, diamine, or dicarboxylic-acid, the low bias conductances resulting from the four binding modes are all found to have either a high or a low value, well consistent with recent experimental observations. The trend can be rationalized by the behavior of electrode-induced gap states at small bias. When bias increases to higher values, states from the anchoring groups enter into the bias window and contribute significantly to the tunneling process so that transport properties become more compli-cated for the four binding modes. Other low bias behaviors including the values of the inverse length scale for tunneling characteristic, contact resistance, and the ratios of the high/low conductance values are also calculated and compared to experimental results. The conducting capabilities of the three anchoring groups are found to decrease from dithiol, diamine to dicarboxylic-acid, largely owing to a decrease of binding strength to the electrodes. Our results give a clear microscopic picture to the transport physics and provide reasonable qualitative explanations for the corresponding experimental data.In chapter IV, magnetic and spin polarized transport properties in zigzag-edged graphene nanoflakes were investigated from first-principles calculations. Ferrimagnetic structure was found to be the ground state for triangular shaped graphene flakes. Mag-netism is weakened by doping B or N atoms into the flakes, and it is enhanced if F atoms are doped in certain sublattice of the flakes. The magnetic properties can be rationalized by the behaviors of dopants as well as interactions between dopants and the host atoms. Perfect (100%) spin filtering effect was achieved for the pure or B-doped graphene flake sandwiched between two gold electrodes. The orientation of the spin current is found to be flipped if the flake is doped with N, O, or F atoms. The orientation-tunable spin filtering effect is potentially useful in practical applications. In this chapter we have also introduced the investigation on other two graphene-based structure, includes the structural, electronic and magnetic properties of the bilayer graphene with interlayer C, N or O atoms intercalated, and the magnetism and spin-polarized transport properties of carbon atom chains. These results can help us understand the properties of this two system and apply them to nanoelectronics. In chapterâ…¤, we investigated the spin-polarized transport and tunneling magne-toresistance(TMR) effect of molecular junction of OBP molecule with semi-infinite Ni electrodes, and gave the primary results. Three molecular junctions with differ-ent conformation are considered in this chapter, i.e., the OBP molecule contact with two symmetrical electrodes, OBP molecule with nitro side groups and OBP molecule asymmetrically contact with two electrodes. We have calculated the I-V curve, TMR ratio and spin injection factor etc for these three molecules, with their Ni leads in paral-lel magnetization configuration(PC) and antiparallel configuration(APC), and got some valuable results.In chapterâ…¥, I give a brief summary of the thesis, and also make an outlook to the following work.