First Principles Study

Molecular electronics is the possibility of building a logic circuit, also is the best carrier in the future development of information technology on molecular scale, which the study of synthesis is equivalent to the traditional function of molecular transistors, microelectronics components. Molecular device is a new cross science, it involves new material, new technology, new devices and new principle and so on many other aspects, there are a lot of scientific problems need to be explored. As one of the common characteristics of molecular devices, molecular rectifier in experiment and theory has attracted the attention of many peoples. For-molecular devices such as rectifier,current income is not high, the experiment error exists between the measured data and the real value, it needs the support of theoretical research, to prove and explain the rectifying effect of molecular devices.Thus, based on the density functional theory combined with the primary principles of non-equilibrium green function method, study the rectifying effect and electronic transport properties of organic molecular devices is to explore new materials with high molecular rectifying effect.This paper first introduces the research background of molecular electronics and molecular devices, the research progress of molecular rectifier and the primary principle of theory research methods. The paper study the electron transportation properties of several molecular electronic devices based on silane oligomer and pyrene derivants using density functional theory and the nonequilibrium green’s function method density. Focusing on different factors including the atom hybridization and replace has effect on the electronic transport properties, Use the energy level and frontier orbits of molecule, the trasmission of device, the projected density of states, the Molecular Projected Self-Consistent Hamiltonian (MPSH) of device to analyse the electron transportation of the junctions. The main research contents:The electronic structures and the electron transport properties of hybrid thiol end of the silane oligomer HS(SiH2)6-σ(or π)-(SiH2)5SH doping the different functional groups of -CH2,-CH2CH2,-CH=CH,-C=C,-C6H4 and -C10H6 systems have been studied. The results show:molecular devices exhibit different rectifying effect, when the benzene and acene hybrid oligomers, the rectification ratios reach 15.41 and 65.13, large than that of pure oligomers and when acene is introuduced into oligomer the Ⅰ-Ⅴ curve of its device appears the obvious negative differential resistance phenomenon.Pyrene imide (PBI) in a historically study shows the rectification effect, its molecular structure is relatively symmetrical, through chemical modification to build a new symmetric molecules as a body part, select -CH3 and -CN functional groups modifying its molecular structure, compositing eight molecules containing -CH3 or-CN,or containing both -CH3 and -CN functional groups;and build the corresponding molecular devices, study their electron transport properties.Results show that the PBI modified by the different number of -CH3 and -CN, its corresponding junction presents different rectifying effect, the pyrene imide derivatives containing two -CH3 and two -CN functional groups has highest rectification ratio, which is up to 4 x 104;and the PBI derivative modifying by two the same functional group, result a negative differential resistance phenomenon on the Ⅰ-Ⅴ characteristic curve of their devices.In order to further study the important of the molecular structure impact on the transport properties of molecular devices, we further study of the substituted PBI derivatives by replacing the ring N atoms by C atoms, when two N atoms are replaced by C atoms, molecular rectifying ratio of its junction is substantially lower than that of the junction based on the replaced PBI derivative,so the existence of the N atoms have important effects on the molecular rectifying effect of PBI derivatives.Pyrene has good planarity, when adopt N into pyrene, 1,5-N,N-pyrene is formed. We select 1,5-N, N-pyrene as a principal part,using groups modify it and form different molecular derivative,respectively, marked as A, B, C and D; The results show that all the molecular rectifying effect is not significant, the highest rectification ratio is only 29.27.For in-depth study the transport properties of the electron donating groups on 1,5-N, N pyrene derivatives device, the influence of different groups (-NH2 and-CH3) instead of 1,5-N, N pyrene derivatives have been investigated. Outlcomes show that the rectification effect of A、B and C derivatives have increase slightly, and the effects of -NH2 group on the system rectification are the larger than that of the -CH3 group;the rectification effect of D derivatives decreases significantly, and -NH2 effect on the system rectification is also larger,the influence of low amplitude is bigger. At the same time,some molecular devices based on the pyrene derivatives adopted single -NH2 group present a negative differential resistance phenomenon.