Theoretical Study On The Electronic Transport Properties Of Single Molecular Junctions

Modern silicon circuitry adopting the so-called top-down fashion continues to shrink the size of silicon chips. According to the Moore's law, this kind of traditional technology is no longer valid when the size of chips reaches its limitation. Now experimental and theoretical researchers turn to molecular electronics (the bottom-up design), which is the technology of using single molecules and small chemical groups to create electronic devices. In the middle of 1990s, molecular electronics merged rapidly with the progress of various powerful experimental techniques such as scanning tunneling microscope, conducting atomic force microscope and controlled mechanical broken junction technique. At the same time, various theoretical methods have been proposed in the past years. The aim of the theoretical studies is to understand the electronic transport mechanism and reproduce the experimental I - V characteristics. The most successful theoretical method is based on the non-equilibrium Green's function (NEGF) combined with density functional theory (DFT) calculations. This dissertation including five chapters is devoted to investigate theoretically on the electronic and transport properties of several single molecular junctions (electrode-molecule-electrode system).In the first chapter, we give a brief introduction of molecular electronics by introducing the recent progresses of molecular electronics, these experimental techniques such as scanning probe microscope and mechanically controllable broken junction, and the transport measurements through various kinds of molecules.The theoretical methods adopted in this thesis are presented in chapter 2. In the first half part of this chapter, the basic concepts and recent progresses of DFT are reviewed, then we briefly introduce the computational DFT packages including SIESTA, VASP and DMol~3, which are adopted to calculate the geometric and electronic structures of molecules. In this dissertation, the transport properties through molecular junction are obtained by using a fully self-consistent NEGF technique combined with DFT calculations. The basic ideas and formula are presented in the late half of this chapter. Several packages based on NEGF and DFT are also listed. In chapter 3, we focus on the rectifying effect in the polar conjuncted molecular junctions. Since the first molecular diode was proposed by Aviram and Ratner in 1974, various molecular rectifiers or diodes have been synthesized and their transport properties have been measured in the past decades. Recently, a new class of relatively simple diode molecules has been synthesized based on conjugated diblock oligomer molecules. In this paper, we investigate the transport properties of the polar conjugated molecules with different lengths sandwiched between two gold electrodes. The current-voltage curves and the transmission spectra are calculated. We find that the I-V curve becomes more asymmetric when the length of the central molecule increases. This observation can be understood according to the shift of the perturbed molecular level and the spatial distribution of the perturbed molecular wavefunctions under the applied electric field. We also find that the asymmetric interface coupling enhances the rectifying effect, which reproduces the main experimental feature.The electronic transport properties and switching mechanism of single photochromic diarylethene derivatives sandwiched between two gold surfaces with closed and open configurations are presented in Chapter 4. Since it is a crucial element of any modern design of memory and logic applications, molecular switch have received much research attention. Till now several switching mechanisms have been proposed. In this chapter, we first compare the transmission spectra of open and closed configurations and observe that they are strikingly distinctive. The open form lacks any significant transmission peak within a wide energy window, while the closed structure has two significant transmission peaks on both sides of the Fermi level. The calculated on-off ratio of currents between the closed and open configurations is about two orders of magnitude, which reproduces the essential features of the experimental measured results. Moreover, we find that the switching behavior within a wide bias voltage window is extremely robust to both substituting F or S for H or O and varying end anchoring atoms from S to Se and Te.In chapter 5, we carry out theoretical study on two kinds of novel molecular junctions. Due to the difference of geometric and electronic structures of a single quintuple bond [PhCrCrPh] molecule with the trans-bent and linear configurations, we design a molecular switch based on this multiple bonds molecule. The calculated transmission spectra of two chemical isomers are remarkably distinctive. Theoretical results suggest that the current through the trans-bent configuration is significantly larger than the corresponding linear one. The predicted on-off ratio of currents ranging from around 50 to 200 in the wide applied bias window suggests that multiple bond compounds have attractive potential in molecular switch technology. Since Novoselov et al. first fabricated ultrathin monolayer graphite devices in 2004, the electronic properties of a graphite monolayer (graphene) have attracted a great deal of research interest. In the second part of this chapter, the electronic structures of zigzag graphene nanoribbons are examined. Interesting, we find that the ground states and transport properties depend on the edge chemical terminations and its doping concentration.