Theoretical Simulations On The Electron Transfer Properties Of The Alkane Thiols And Serveral Unsaturated Aliphatic Hydrocarbons

The continuous miniaturization of conventional silicon-based electronics will eventually face the insurmountable challenge of the quantum effect, such as tunneling, diffraction, and interference of electron. One of the most promising solutions is to develop molecular-scale electronics (Molecular electronics). The development of scanning probe microscopes (SPM) techniques and quantum chemistry calculations provides powerful tools for exploring the molecular electronics. Although significant progress has been made during the last two decades, some most basic issues such as the behavior of the functional molecules working in the circuit and the electron transport mechanism through the molecules are still not very clear. So, it is essentially important to investigate these properties of the moelcules for the design and rationalization of molecular electrnics.The electron transfer kinetics of n-alkanethiol monolayers on Au(111) electrode is investigated. The structure of the monolayer is examized by measuring the double layer capacitance in 1 mol/L KCl solution with the rapid cyclic voltammetric method. The electron tunneling parameter for the monolayer is studied by using Fe(CN)6-4/-3 as the redox probe. It demonstrates that with the increase of the chain length of the n-alkanethiols the current decreases dramatically. A decay constantβof 0.98 per methylene group is measured. Electron transport propeties for n-alkanedithiol monolayers on Au(111) is investigated using conducting atomic force microscopy (CAFM). The measured current-voltage (I-V) curves show obvious nonlinear behavior and can be fitted with Simmons tunneling model. The molecular current decays exponentially with the increase of chain lenth. The measured decay constantβis 1.16 per methylene, which is in reasonable agreement with the values of 0.98 per methylene and 0.99 per methylene obtained from the electrochemical method and the nonequilibrium Green's functions (NEGF) method, respectively, indicating that the main conduction mechanism of the alkane molecules is tunneling. The CAFM tip-loading force is found to dramically influence the molecular electric properties, showing that the molecular current increases dramatically with the the the tip-loading force increasing. The NEGF calculations can simulate the molecular electron transport properties qualitatively. To simulate the properties of the molecular electronic materials working in electronics more precisely, a more likely in-situ theoretical method by considering the interaction from the external electric field (EF) is proposed. Typically, a series of molecular wires, polyacetylens (PA), are systematically studied at the HF/6-31G* level by this method. It proves that both the geometric and the electronic structures of the molecular wires are sensitive to the external EF. In particular, the external EF makes the carbon-carbon single bonds become shorter and the carbon-carbon double bonds become longer, leading to a higher conjugation. The external EF decreases the LUMO-HOMO gap and increases the molecular dipole moment. The spatial distributions of frontier molecular orbitals vary from the fully delocalized form to the partly localized form with the increase of the external EF. All of these features are more pronounced with increasing conjugation chain length. The same calculations are also carried out for several other typical linear conjugated molecules (poly(phenylene vinylene) (PPV), poly(phenylene ethylene) (PPE), polythiophene (PT), and polyphenylene (PP) molecules) and similar results as those of the PA are obtained. Moreover, the torsional potential energy surface and the electronic structure of diphenylacetylene are investigated at the B3LYP/6-311+G** level by considering the influence of the external EF. It domostrates that with the increase of the external EF the molecular torsional barrier increases and the increment is proportional to the square of EF. With the increase of molecular torsional angle, the EF dependence of the LUMO-HOMO gap and the spatial distributions of the HOMO and LUMO are enhanced. The I-V behavior corresponding to different conformers is evaluated by the NEGF method. Further, the evolutions of the LUMO-HOMO gap and the spatial distribution of molecular orbital are used to analyze these structure-property relationships.The I-V property for a series of lineare conjugated molecular wires (PA, PPV, PPE, PT, and PP) with different conjugation strucres is investigated by the NEGF method. The conductivity of these molecules is compared and analyzed from the the LUMO-HOMO gap, the spatial distributions of the HOMO and LUMO, and the transmission spectra of the molecules. Moreover, the effect of introducing functional groups (electron-donating group, -NH2, and electron-withdrawing group, -NO2) into the molecular backbone on the electron transport properties of the molecules is also studied. It demonstrates that the introduction of specific functional groups on the molecular backbone can successfully lead to molecular asymmetry, which may lead the assymmetric I-V behaviors. These results provide a theoretical guidance for the further design of novel molecular electronics, such as molecular diodes, molecular switches, and molecular storage devices.The geometry, electronic structures, and ring strain energy of a kind of new molecular electronic material, phenylene-acetylene macrocycles (PAMs), are explored by HF/6-31G* method. Based on the conformational analysis of diphenylacetylene, which can be viewed as the structural unit of PAMs, a new method for analyzing ring strain energy is proposed and used to analyzing the ring stain energy and the structural characteristics of the PAMs. Compared with the conventional theoretical method for ring strain energy, this new method is more convenient and higher efficient. Moreover, this new method can analyze the origin of the ring strain energy within the macrocycles. In view of their potential applications as electronic materials, the electronic structures of a series of PAMs are also investigated. The LUMO-HOMO gaps and the HOMO spatial distributions of the planar PAMs show obvious odd-even difference behavior.