Theoretical Research On The Properties Of Several Classical Molecular Electronic Materials

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 the molecular-scale electronics (Molecular electronics). Although significant progress has been made during the last two decades, some most basic issues such as the behavior of the molecular electronic materials working in the circuit and the electron transport mechanism through the molecules are still not very clear. It is know that the electron transport properties of the molecular electronics are closely related to the geometry and electronic structures of the bridging molecules. So, it is essentially important to investigate their relations for the design and rationalization of the molecular electronics. Based the necessary mentioned above a series of representative molecular materials were study and their characters which included the geometric and electronic structures were simulated by using the advanced calculation methods.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 is proposed. Typically, a series of molecular wires, polyacetylenes, are systematically studied by this method. Three isomers of polyacetylenes show different geometric and electronic character. This result demonstrates that the external electric field's effects to the three kinds of different disposes of single and double bonds are different.The introduction of the doped atoms can increase the conjugation of the system and furthermore strengthen the ability of the transfer of electrons. The effects induced by the different doped atoms are not same because the different atom radii results in the unlike steric repulsion of adjacent atoms. The repulsion also makes the conjugated molecular chain distort to non-planar while the applied electric field can reduce the change.Another method to enhance the conjugation of the system is to make the molecule more planar. So we select the oligothienoacenes as the modeling molecules for its rigid structure. With the electric field increasing, the carbon-carbon single bonds become shorter and the carbon-carbon double bonds become longer, resulting in a better conjugation. Due to the different electron density, the charge mobility of the sulfur atoms is more obvious than that of the carbon ones. The HOMO-LUMO gap decreases with the EF intensity increasing. The applied EF also changes the spatial distribution of the molecular orbits: LUMO and several higher orbitals shift to the high potential side, whereas HOMO and several lower ones shift to the low potential side.Then the focus moves from one-dimension molecular wire to nanoribbon, a quasi-two-dimensional system (boron nitride nanoribbon). A series of model molecules with different width have been investigated theoretically to study the effect of edge character, doped atom, width and so. The results show that the width of the ribbon affects the property regularly. With the increase of the width, the bond lengths become more homogeneous which results in the increase of the conjugation; the band gap decreases correspondingly and the chemical potential appears a maximum. The distribution of the frontier molecular orbitals shifts to the boundary asymmetrically.Finally a very interesting electronic material named carbon nanotube has been mimicked theoretically. The length and diameter of the finite carbon nanotube affect the geometric and electronic character. We also construct a molecular junction of the carbon nanotube by adding a 5/7 pair defects and study their distribution. These joints are structurally different and the energetic costs of each joint have been evaluated using homodesmotic equations at several levels of theory. As a result, we can conclude that the metallic-metallic junction was more favorable if it is carried out by inserting a hexagon between 5/7 pair defects in the armchair nanotube. In contrast, the introduction of two 5/7 pair defects distributed along the cylindrical axis is unfavorable junctions. The results also showed that the different distribution of defects result in the different distribution of frontier orbitals which will affect the SWCNT' properties. The results of the density of state (DOS) indicate that the junction show a good rectified effect which may give a good illumination to use the junctions as a molecular rectifier.