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Theoretical Simulations on the Electron Transfer Properties of the Alkane Thiols and Serveral Unsaturated Aliphatic Hydrocarbons

Author: LiYanWei
Tutor: YinGePing；ZhaoJianWei
School: Harbin Institute of Technology
Course: Chemical Engineering and Technology
Keywords: Molecular electronic devices Conducting atomic force microscope Non- equilibrium Green's function Electronic structure Transport properties
CLC: O621.23
Type: PhD thesis
Year: 2007
Downloads: 176
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Abstract

The size of the traditional silicon-based electronic devices approach the nanometer scale, will face insurmountable challenges arising due to quantum effects (such as tunneling, scattering and electron interference). One way to solve the above problem lies in the development of molecular-scale electronic devices (molecular electronics). The development of scanning probe microscopy (SPM) and quantum chemical calculations provide a powerful tool for the study of molecular electronic devices. Although the study of molecular electronic devices has made some important progress, but the molecular electron transport mechanism of some of the most basic problem is the lack of a comprehensive and in-depth understanding of the functional behavior of the molecules in the device, Thus, these properties of the molecules studied the design and optimization of molecular electronic device has a very important significance. The Au (111), alkyl thiol molecules on the electrode electron transfer dynamics of self-assembled monolayers using electrochemical methods. Voltammetry in KCl solution with rapid cycling measured differential capacitance of the self-assembled monolayer, to determine the integrity of the structure of self-assembled film; the probe ion Fe (CN) 6-4/-3, self-assembled monolayer electron transfer properties. The results show that as the molecular chain length increases, the molecules tunneling wear current decreases, the measured current is the attenuation constant and β is 0.98/CH2. Using conductive atomic force microscope (CAFM) method to study the electronic transport properties of thiols on Au (111) surface molecules self-assembled monolayer. The study showed that the electron transport of the molecule has a significant non-linear characteristics, molecules current voltage (IV) curve can be used to good fitting Simmons tunneling model. Molecular current with the increase of the molecular chain length was exponential decay, its current decay constant β 1.16/CH2 β value calculated with the use of electrochemical methods and non-equilibrium Green's function (NEGF) method (for 0.98/CH2 and 0.99/CH2) is very close to the conductive mechanism of molecular tunneling mechanism. CAFM tip pressure on the conductive properties of the molecule has a great influence, as the tip pressure increases, the molecules of the current significantly increased. NEGF method can simulate the electron transport properties of the molecules. In order to more accurately simulate the behavior of the molecular electronic materials in the device, the proposed method of the static theoretical studies of a similar scene. Typical linear π-conjugated molecules, polyacetylene (PA) as a model molecule HF/6-31G * level changes of the molecular nature of the effect of external electric field. The results show that, significant dependence of the molecular geometry and electronic structure of the external electric field. With the increase of the external electric field, a carbon-carbon single bond of the molecule is shortened, the carbon-carbon double bonds becomes long, the molecule conjugate has been strengthened, molecules in the conjugate plane obvious geometric bending. External electric field so that the molecules of the LUMO-HOMO energy gap decreases, the dipole moment increases. Under the action of the external electric field, the HOMO spatial distribution toward low potential terminal of the molecule, the LUMO spatial distribution toward high potential terminal of the molecule. With the increase in molecular chain length of the above properties of the molecule with the changes of the external electric field has been further strengthened. On a range of other types of linear π-conjugated molecules (polyparaphenylene acetylene (PPV), polyphenylacetylene (PPE), polythiophene (PT) and polyphenylene (PP)), a similar calculation, but also get the same results. B3LYP/6-311 G ** tolan molecules under the action of an electric field to reverse the potential energy surface and electronic structure. The results show that the torsion barrier With the increase of the external electric field, molecules increases, and reversed to the square of the height of the barrier with the external electric field has a linear relationship; the frontier orbital of the molecule with the torsional angle increases, energy and spatial distribution of the external dependent enhancement of the electric field. Using the the NEGF method of simulated tolan molecules in different conformational IV characteristics. The results show that the relationship between the IV characteristics and molecular conformation of the molecule from the space of the the molecular LUMO-HOMO energy gap and the frontier orbital distribution changes with the external electric field to explain. The NEGF method simulated the IV characteristic of a series of different conjugated structure of a linear π-conjugated molecular wires (PA, PT, PPV, PPE and PP), comparison of their conductivity difference, and from molecular LUMO-HOMO The energy gap, the distribution of the molecular orbital space and tunneling spectroscopy analysis of the conductive reasons for the differences between the various molecules. On this basis, and also investigated the molecular skeleton introducing electron-donating group (-NH2) and electron-withdrawing groups (-NO2) on the transport properties of molecular electronic. The results show that the modified functional groups can be successfully introduced asymmetry, thereby enabling the molecule to obtain asymmetric transport properties in the molecular skeleton. This provides a theoretical molecular diode, molecular switch design of functional molecular electronic devices. The HF/6-31G * Explore the characteristics of a new class of molecular electronic materials, phenylacetylene macrocyclic molecules (PAMs) geometry, electronic structure, and ring tension. Based on the large ring molecular structure unit tolan Conformation analysis, a new ring tension analysis method, and analyzed by this method the molecular ring tension properties and geometry characteristics. Analysis compared methods, this method is more simple, efficient and can analyze the specific source of the molecular ring strain and ring tension. The electronic structure analysis shows that the distribution of LUMO-HOMO energy gap, the planar conformation macrocyclic molecules and molecular orbital space parity differences characteristics.

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