External Pressure Affect Electrical Transport Properties Of Molecular Devices

The development of single-molecule science has led to the rapid advancement of molecular electronics. Construction of molecular devices with various special functions using single molecules is one of forefront researches in the area of nanoelectronics. There has existed great development for single-molecule science in both theoretical and experimental regions during the last decade, and the research on single-molecule devices is the important and interesting issue in molecular electronics. It have been firmly proved that the ability of electronic transport through single organic molecular junction is deeply affected by the configuration of the molecule, the way of contact, the force applied on the molecule, and the solvent. It has been known that the influence of the configuration of the molecule, the way of contact, the solvent and the length of the molecule on the electron transport had been widely studied, but the aspect of the force applied on the molecule is still the vacancy area up to now. In this thesis, we present a method with which we can gain the force applied on the molecule by the probe, consequently, we discuss the influence of force on the electronic transport of molecular junctions. Moreover, we study the effect of length of molecule and anchor group on the electronic transport.In order to obtain the force between functional molecule and gold electrodes, we first optimize the geometric structures of the extended molecules by fixing the electrodes distance in different values. In this way, we get the curves of single point energies E versus different electrodes distances L, and the forces F between functional molecule and gold electrodes can be obtained approximately by the formula F = ??E/ ?L. After getting the forces of the series of molecular junctions with the electrodes in different distances, we calculate the current and the conductance of the molecular junctions with the force F in the same value. In this thesis, two kinds of molecular junctions have been investigated and the main works are listed as follows:1. Based on the experimental work of Latha et al., we investigate a series of alkanes terminated with amines which consist with different number of methylene. Based on the method mentioned above, we calculate the force F between functional molecule and gold electrodes. Then the electronic transport properties are calculated with QCME codes which bases on our generalized elastic scattering Green's function theory. So we can discuss the influence of press on the electronic transport properties in details.We can simulate the process of formation and break of these molecular junctions by observing the length and angle after optimizing. The exponential decay factorβis calculated based on the formula G ~ G0 exp(?βL)by fitting the conductance of the four molecular junctions under the same press. At last, by choosing different values of the force and repeating the above calculation, we analyze the relationship between the exponential decay factorsβand the forces.2. Based on the experimental work of Hu et al., we study the molecular junctions with different terminal groups. The hybrid density functional theory is used to study formation and break of the junction of alkanemonothiol molecules with different terminal groups. The relationship between geometric structures of the molecular junction and the external force is obtained. According to the relationship, the electronic transport properties of the molecular junctions with different external forces are investigated using the elastic-scattering Green's function method. At the same external force, we find that the C1 1S molecule has the largest conducting ability, and the C1 0SCOOH molecule has the smallest conducting ability. Furthermore, the conductance of the three molecular junctions is monotonously enhanced as increment of the external force. The numerical results are consistent with the experimental findings qualitatively.We attribute the difference of the current and the conductance of the three kinds of molecular junctions to the different resistance of the three kinds of terminal groups. Comparing the 1s orbit of the carbon atoms in the three terminal groups, we claim that the terminal groups have obvious effect on the conductance of the three junctions.The thesis consists of six chapters and the content is as follows. In the first chapter, the current development of the molecular electronics is introduced from the point of theoretical and experimental works. The theory of self-consistent field (SCF) for many-particle system is presented in the second chapter which includes Born-Oppenheimer approximation, Hartree-Fock method and density functional theory. When we use density functional theory to do calculation, the selection of basis sets is needed, which is also showed in last part of this chapter. In the third chapter, the formula based on elastic scattering Green function methods to calculate the current-voltage properties of the molecular junction are deduced in detail. Our research process and numerical results are discussed in the fourth and fifth chapters. The main conclusions are given in the last chapter.