First Principles Study On The Rectification Effect Of Molecular Rectifiers Based On Nitrogenous Organic Transition Metal Molecules

The theoretical concept of single molecule rectifiers proposed by Aviram and Ratner in 1974 has promoted the research and development of the field of molecular electronics. Molecular electronics narrowly defined as electronic devices interconnection to each other with the scale less than 3nm. In the narrow sense it refers to the various functions built with organic functional molecule materials in electronic circuit components, for example, molecular wire, molecular switch, molecular memory, molecular rectifier and molecular field effect tube. Molecular rectifiers with asymmetric conductance, as a necessary element in the molecular circuit, have been reported in a large number of experiments and theories. But there is no obvious breakthrough in rectifying effect of molecular rectifier, and there is also the deviation between the experimental measurements and the true value. So it is necessary to search the organic molecular rectifying materials with high rectification ratio by designing and exploring in theory. Organic metal molecular wires, such as isocyanide gold complex, have many special properties in the optoelectronic physics; And because of the organic metal molecular material containing metal atoms, so that it is easy to form an electronic asymmetric system, and has the rectification effect. In view of this, this thesis has been studied three different rectifier systems:(i) the linear structure of isocyanide gold complex unimolecular junctions; (ii) porphyrin and Zn-porphyrin tailed with various length of alkyl chains, as CnPH2 and CnZnP (n=6?11); (iii) C-doped armchair boron nitride nanoribbon devices. The rectifying properties and electron transport properties of these systems have been studied by applying first-principles of density functional theory (DFT) combined with the non-equilibrium Green's function (NEGF) method, in the ATK program. The effect and transport mechanism of molecular junctions have been analyzed by molecular energy level diagram, device transmission spectrum, molecular projection state density and molecular projection self-consistent Hamiltonian. The research results come out:(i) the group of isocyanide gold is a novel asymmetrical system. C6H5-N=C-Au-C6H5 molecular rectifier is due to the asymmetric moving of HOMO and LUMO energy levels under different polarity bias and the maximum rectification ratio is 4.3. Pyrimidine ring effectively improves the rectifying effect of isocyanide gold molecular junction. The forward current improved and negative current suppressed of S (C4N2H4)2-N=C-Au-(C6H4)2-S molecular junction are due to the reasonable strengthening the localized HOMO and LUMO and weakening the coupling of Au atoms and gold electrodes, so the maximum rectification ratio reaches 50.1; (?) there is different rectifying effect between the alkyl chain tail porphyrin and Zn porphyrin molecules. The turn-on voltages of CnPH2 molecular junctions are in 1.5-1.8V range and their max rectification ratio is about 101. CnZnP molecular junctions are showed out the ?-? curves that is similar to conventional p-n junction. Its turn-on voltage is reduced to near the 0.8V, and the max rectification ratio increases to 103 level; (?) The current voltage (?-? ) curves of C-doped boron nitride nano devices are very similar to those of the traditional P-N junction diode ?-? curve.