The First-principles Research Of The Electronic Structure And Transport Properties Of Nanostructure Systems

Compared with the tradition materials, nano-scale molecular-device systems have the advantages in size, and then these systems have wide potential applications in many domains such as materials and quantum information. However, to build molecular devices with various functions (such as rectifying and molecular switch behaviors) and also working stably, there is still more work to do experimentally and theoretically, especially to understand the transport mechanism of kinds of molecular devices precisely. With the development of computer, it is extremely important to simulate molecular devices by computer and investigate the transport properties of these devices via the first-principles calculations. In some case, theoretical simulation finishes some work which can't do via experiment works.In the current thesis, based on the combination of density functional theory (DFT) and non-equilibrium green function (NEGF), first-principles calculations have been performed to study the transport properties of some molecular scale conductors. The new special molecular devices which will be useful in the future electronic devices are then devised theoretically. Concretely speaking, the asymmetric organic molecular rectifier; conductor transport of Au nanowires with radius which is smaller than 10 A, and spin-polarization transport of magnetic clusters have been mainly discussed.In the 1st chapter, the research background and development of molecular electronics, including the methods of experiment and theory, have been introduced firstly. Then the development of computer simulation is described briefly. At last, we show what we have studied in this thesis and the importance.In the 2nd chapter, we focus on the theoretical details and computational methods. Firstly, the first-principles calculation methods, based on the combination of DFT and NEGF, are introduced. Then we show clearly how to perform theoretical calculations of the transport properties of the molecular-scale devices based on the combination DFT and NEGF methods. Finally, we describe in brief the calculation codes we used in this paper. In the 3rd chapter, the rectifying effect of PTCDI-[CH2]n asymmetric molecular junctions with different molecular lengths are theoretically investigated via ab initio DFT and NEGF method. Firstly, the equilibrium transmission as a function of the number n of CH2 has been studied. Firstly, the equilibrium conductance exponentially decreases as the increase of CH2 unit number n. With the increase of the molecular length, the tunneling barrier is also enhanced, and then the conductance decreases correspondingly. Secondly, the current-voltage curves present that when n≥1, rectifying happens. In the n=6 case, a large rectification ratio of 72.6 is achieved, which suggests that the PTCDI-(CH2)n molecules may have great potential utilities in the molecular-scale devices. Based on the analysis of the transmission spectra under different voltages, together with the orbital distribution of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for the isolated molecule HS-PTCDI-[CH2]6-SH and HS-PTCDI-SH, it is indicated that the rectification effect stems from the asymmetric molecular structure.In the 4th chapter, using NEGF method the electronic structure and transport properties of Au nanowires with radius from 1.8 A to 8.651 A have been systematically probed. In all nanowires, Au nanowires become more stable and the density of states at the vicinity of Fermi level increase with the raise of the radius of Au nanowire, which indicates that the electronic transport ability of the system is strengthening. The number of conduction channels is determined from electronic structure calculations, which plays a significant role in the transport properties of nanowires. The equilibrium conductance of Au nanowires generally increases with wire size, which might be useful for estimating the quantum conductance of Au nanowires.In the 5th chapter, we discuss systematically the electronic transport properties of the system of Mn4O4 sandwiched between two semi-infinite gold nanowires using NEGF. Results shows that:Negative differential resistence effect and spin filtering phenomenon happen under bias voltages, and more interestingly, the sign-reversal SFE of this system can be monitored by bias voltages, which means a great potential application in the molecular-scale spin-electronics devices.