| With the rapid development of microelectronics technology, the research on semiconductor materials has been moving into the nanoscale. Novel phenomena, novel effect and novel device have begun to flourish at this scale. Many studies have been conducted to design and fabricate low dimensional electronic devices. An accurate theoretical description of electron (or hole) transport is of crucial importance to optimize nanostructure-based devices. The electron transport is incoherent in macroscopic system. The traditional methods devised to investigate transport in bulk materials make use of the assumption that carriers do equilibrate with the underlying thermal fluctuations and the current density linearly responds to external field. The resistance stems from scattering between the carrier and impurity or phonon. Phenomenological parameters such as diffusion coefficient and conductance coefficient are no longer able to describe to the electron transport in nanostructure. The coherence of electronic wavefunction should be considered in such system, especially within the elastic scattering process. Two quantum mechanical effects distinguish nano devices from bulk devices, reflecting the wave-particle duality of the electrons. One is the conductance quantization in transport and the other is the quantization of electronic charge which emerges in Coulomb-blockade and single electron transistors.Traditionally, nanostructures have been studied with the context of the envelope function approximation (EFA) regardless of atomic details. But this method is mainly limited to the calculation of pure states, and ignores the periodic part of the real wave function. To solve the conductance problem of such open systems more accurately, the current popular approach is to couple the microscopic theory of the electronic states of the system with non-equilibrium Green function method (NEGF) so that the transport problem can be dealt as a quantum scattering problem. In this thesis, two different transport methods are used. One is density functional theory based tight-binding method (DFTB) combined with NEGF and the other is SIESTA method combined with NEGF. Here all transport calculations are carried out under the framework of coherent transport.It is known that the electron orbitals can be divided into the delocalizedπkind and the localizedσone along the bond according to their symmetry differences. Among the investigations of the electron transport in most organic molecular devices, the nonlinear current-voltage characteristics of the system is often dominated by theπkind of conducting channel which changes with the variation of external bias. However, the contribution from a orbital can not be ignored in all cases. Take a phenyl ring connected by the meta position for example, its transmission at the Fermi level is mainly from the contribution ofσorbital while almost none fromπorbital due to destructive interference effect. Also as in the biphenyl system, when the two benzene rings are perpendicular to each other, the electron transport through theπorbital is completely inhibited and the residual conducting channel of the system is provided by a orbital. The paper totally dealt with two different low-dimensional confined systems and found that the delocalization of theπorbital in the electron transport plays a crucial decisive role. The detailed research and results are as follows:1. Electron transport suppression from tip-πstate interaction on Si(100)-2×1 surfacesIntense research has been devoted to exploring the functionality of the dangling bonds in characterizing reconstructed surfaces both experimentally and theoretically. The energy levels of dangling bonds often appear within the band gap close to the Fermi level and hence determine much of the electronic behavior of the surface in scanning tunneling microscope (STM) images. Jelinek et al. observed a substantial decrease of conductance during the approach of the tip to the Si(lll)-7×7 surface and revealed that this unusual feature resulted from the formation of the strong covalent bond between the tip apex atom and the adatom on the surface by first principles simulations. Ono et al. also simulated the STM image of Si(001)-2×1 surface before. However, their theoretical observations were taken at larger tip-sample distances and not related with the unusual conductance drop phenomenon. It can be conjectured that such an abnormal conductance drop could be observed on a Si(001)-2×1 surface on which dangling bond statesπ/π* were considered to be localized mainly on the silicon dimer atoms. The third chapter of this paper investigated the electron transport between an STM tip and Si(100)-2×1 surfaces with distinct configurations to explore such a possibility. This work is now awaiting further experimental observations so as to confront the theoretical results to experimental data and assess the validity of the models and method shown here.1.1 Four silicon slab models were designed to approach the different situations on the Si(100)-2×1 surface. The theoretical calculations were performed with the gDFTB code, which is an extension of the NEGF method of electron transport via DFTB. Firstly, the models were used to mimic the flipping dynamics of bare dimers next to the H-passivated dimers revealed by DFT calculation and in experiment observation. In this work, the bare dimers next to the H-passivated dimers exhibited a reducedπ-π* energy splitting and superiority on theπandπ* states compared to that on the clean surface. And as is known to all, the occpuiedπand unoccpuiedπ* states are localized on the upper and lower dimer atoms, respectively. The reducedπ-π* energy splitting would offer more convenience for the transition betweenπandπ* states associated with charge transfer from the upper dimer atom to the lower dimer atom and hence the initial upper dimer atom begin to low down while the initial lower dimer begin to rise up. The features of the silicon dimers showed good agreement with flipping dynamics mechanism of bare dimers in former work. Thus the transport calculations based on the DFTB method and the model setup in this paper are reliable.1.2 The equilibrium conductance of clean surface model increased quickly with the tip approaching substrate gradually at a large tip-substrate distance, and then underwent a sudden drop after a critical value. It was because that theπstate located at the atoms of the silicon dimers immediately under the tip was greatly suppressed and pushed away from the Fermi level when the tip moved closer to the substrate from the critical value and meanwhile the frontier orbital of the model became delocalized, and this delocalization carried further along the silicon surface and down to the lower layers of the substrate rather than upward to the tip. Consequently, the projection of the charge on the dangling bondπstate to the transport direction was reduced.1.3 As the coverage of hydrogen on the surface increased, the orbital hybridization around the hydrogen atoms and the Si-H bonds was stronger and less delocalized than in the dangling bondπstate; thus, the transport suppression was not as remarkable as that in the clean case. On the other hand, when all the dangling bond states were eliminated by hydrogen chemisorption and there was no longer a tip-πinteraction. Thus, the tip proximity did not suppress transport. It should be noticed that the tip-πinteraction was still not strong enough to suppress the transport at larger tip-substrate distances and meanwhile, the dangling bondπstate was more widely expanded than the states around the hydrogen-adsorbed dimers and as a result the current of clean surface was larger than those of hydrogen passivated models. And this is in agreement with others'work.2. Intramolecular torsion based molecular switch functionality enhanced inπ-conjugated oligo molecules by aπ-conjugated pendant groupTwo types of molecular switching behaviour have been observed in oligo molecules:switching due to an applied external voltage between the electrodes and stochastic switching attributed to statistical fluctuations in the molecules. The former case is related with molecules invoking electro-active groups such as electron acceptor nitro (-NO2) group and electron donor amino (-NH2) group. These groups can induce permanent dipole moment in the backbone of the molecule. Upon the application of bias voltage, the interaction between dipole moment of the oligo molecules and the electric field may induce electrostatic charging, conformational changes and so on. These effects lead to non-linear current-voltage characteristics of the molecule. The fourth chapter of this thesis focuses on how to improve the latter switching effect through a conjugated pendant group. Such kind of group does not impart a considerable dipole moment to the molecular backbone but reinforcesπ-conjugation in oligo molecular backbone, which in turn enhances its switch functionality.2.1 Two types of the tunnel junctions were investigated here:a linear phenylene ethynylene trimer as a reference molecule (Sl); a phenylene ethynylene trimer attached to a conjugated pendant group phenylene-ethynylene (Sp). Different conformations are characterized by an intramolecular torsion angleθfor both Sl and Sp. Models withθ= 90°were refered to the current "OFF" state while models with other torsion angles refered to current "ON" states. Transport calculations are implemented in the TRANSIESTA package, based on the combination of SIESTA method and NEGF. The exceptional electronic orbitals FOd possessing "through space" interaction which only appeared in Sp were attributed to the differences in charge transport between the two models. Such orbitals restored the continuity in the distribution of the electronic cloud all throughout the tunnel junction at "ON" states in Sp. Eventually, atθ=90°, the electronic cloud of this exceptional orbitals centralized completely in the tunnel junction. The complete mismatch in the distribution of electronic cloud between the central part and the rest of the tunnel junction heightened the tunnel barrier further. Therefore, the special orbitals in Sp lowered the barrier at ON states and raised the barrier at OFF state, implying that it enhances the switch functionality.2.2 The largest enhancement in conductance in Sp with respect to Sl was found to occur atθ=60°.Earlier findings indicated that at aroundθ=60°in Sl, theπ-πinteraction between neighboring phenyl groups got as weak as that ofσ-σinteraction. While in this work, the group FOd in Sp still induced conjugation between the central phenyl group and the pendant group by through space interaction at this twisting angle. As a result, the maximum difference in the switch functionality between these two models was attained at the twist angle ofθ= 60°.
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