Theoretical Designs And Calculations Of Novel Molecular Devices

With the development of traditional silicon-based semiconductor devices, the size of the devices becomes smaller and smaller, and the electronic law applied in the macroscoric scale devices will be replaced by quantum laws correspondingly. Subject to various restrictions, the traditional methods have already could not meet the needs of the development of miniaturized devices, it is necessary to find new ways for the smaller and smaller devices. Molecular devices present natural advantages to fit the device miniaturization. After decades of theoretical and experimental researches, tremendous achievements have been made. Technologies widely used in the experiments such as self assembled monolayer process, mechanical controllable break junction method, the scanning tunneling microscope technology, and optical tweezers technology molecular have provided technical support for the development of molecular-scalar devices. In theoretical investigations, first-principles non-equilibrium Green’s method and condensate plastic model of multiple scattering matrix calculation method etc. have been developed. In the current thesis, using first-principles method based on density functional theory combining with nonequilibrium Green’s function, some devices with special functions have been designed and corresponding transport properties have been calculated.In the first chapter, the following contents about molecular electronics have been introduced, in which the developments of molecular devices, quantum effects, commonly used experimental methods, and the factors affected the properties of molecular devices have been included. In the second chapter, the corresponding theoretical methods are touched, in which density functional, (non) equilibrium Green’s function, Landauer-Buttiker formula, etc.have been introduced. And finally the calculated codes used in the current thesis are introduced. Based on these two .chapters, the experimental and theoretical ibackgrounds about mdlecular、electroriics and molecular-scalar devices are briefly presented.In the third chapter, the transport properties of one photochromic molecular switch have been calculated theoretically. Under different temperatures or other conditions including light factor, these molecules transform between different configurations, which causes the change in the electron distribution of the molecule. When these molecules are coupled with two electrodes under one certain voltage, different configurations will display different current even under one similar voltage. If the large-current state is defined as "ON" state, and the small-current state as "OFF" state, such these molecule may be the potential candidates for the molecular switches. In our theoretical simulations, two kinds of models are designed. In the first model the molecule is sandwiched between two coliinear electrodes, and in the other, the molecules is connected with two non-collinear electrodes. In addition, different electrodes materials are also included in our calculations.In the fourth chapter, one kind of benzene-nanowire complex has been designed to be one novel molecular-scale switch, and the transport properties are calculated. Firstly,the benzene-nanowire complex is composed by one short segment nanowire sandwiched two benzene molecules. Then, the whole benzene-nanowire complex is placed two nanowire electrodes and then the transport properties are probed. The benzene molecule is bound to the nanowire via two sulfur atoms. Since the sulfur-end benzene molecule presents the planar structure, in the calculation, the left molecule is fixed, and the right molecule is rotated along the central axis direction along the molecule plane. Our obtained results show that the current through the complex will decrease with increasing the dihedral angle. In particular, when the dihedral angle between two molecules increases to 90 degree, the current is almost reduced to zero.As one of the most important parts in the scanning tunneling microscope (STM), the probe determines the quality of scanned images. In the fifth chapter, one new combinational probe is designed, and via calculating the scanning properties, the application of such novel probe is also investigated theoretically. The combinational probe is formed by a single-walled carbon nanotube and a metal needle, in which the single-walled carbon nanocone is attached on the top of the metal needle. Via this combinational,probe, the Au(100) surface is firstly scanned along two directions perpendicular to each other, and the ethylene adsorbed on gold surface is also scanned along the direction determined by two carbon atomsin the molecule. From the results it can be seen that this combinational probelhas a wery high resolution for scanning crystalline and inorganic materials. Our results show that such a novel probe may present great potential applications in the future STM researches.