Single Molecular Conductance Of Pyridine And Benzene Derivatives

With the increasing interesting of molecular devices,molecule-metal-molecule junctions(molecular junctions)have received a lot of attention.Conductance measurement is very important for understanding the electrical properties of singe molecular junctions,while the conductance can be influenced by many factors,such as molecular structure,anchoring groups,electrodes,environment,and etc.Although there have been many reports on this aspect,further study is still needed to explore the conductance of molecular junction.In this paper,scanning tunneling microscopy breaking junction(STM-BJ)was used to measure the conductance of various molecules with Au as the electrode.The influence of molecular structure,anchoring group and environment on molecular junction conductance were studied.The current research as follows:1.By using Au as electrode,target molecules of 2,2'-dinitro-4,4'-bipyridine,2-nitro-4,4'-bipyridine,4,4'-bipyridine,4,4'-diphenylphthalate,3-nitro-4,4'-diphenylphth alate,3,3'-dinitro-4,4'-diphenylphthalate,3-amino-4,4'-diphenyl phthalic acid and 3-aminoferrocene-4,4'-diphenyl phthalate were synthesized.We measured the single molecular conductance using scanning tunneling microscope breaking junction.For molecules with pyridyl and carboxyl groups as anchoring groups,it is found that the conductance is influenced by substituents due to the changing of torsion angle.For molecules with carboxyl groups as anchor groups,the conductance is affected by the electronic effect of the substituents,the angle of twist and the effect of steric hindrance.Our work clearly demonstrates the important role of substituent groups in the electron transport of single-molecule junctions.2.Conductance measurements were performed on molecules of C-C-Py,C=C-Py,C-C-Fc-Py and C=C-Fc-Py by using the STM-BJ method.When the C-C-Py molecule is added with ferrocene,the conductance of single-molecule junction increases in despite of increasing molecular length.This is casued by the reducing the energy gap between the HOMO and LUMO of the molecule,and barrier height between the HOMO of the molecule and the Fermi level of the metal,upon the insertion of ferrocene.However,when C=C-Py molecule is added with ferrocene,the conductance of the single molecule junction reduces.The insertion of ferrocene makes the conjugation worse,and the increase of molecular length leads to the lengthening of the electron transport path.Although the barrier height decreases,it did not play the domainated role,resulting in the dereasing of the single molecular conductance.3.We also investigated the relationship between molecular conductance and number and anchor sites of anchoring groups.We measure the conductance of the molecules with thiomethyl group as anchor group(4SMe,2SMe-A,2SMe-B)and molecules with thioacetyl group as anchor group(SAc-A,SAc-B,SAc-C,SAc-D).It was found that both the number of anchoring groups and the anchoring sites of the anchoring groups have effect on the molecular junction conductance.When molecule has multiple anchoring groups,there are multiple contact configurations between the molecule and metal.Several electron transport paths exist,which cause large peak width.When there are only two anchoring groups,the electron transfer of the molecular junction is mainly influenced by the length of the molecule.The longer molecular length corresponds to the smaller conductance.4.We also investigate the effect of ultraviolet light on molecular conductance.Molecular conductance of heterocyclic molecules under dark and ultraviolet light were measured.The results show thatthe conductance of the target molecule under ultraviolet ligh is significantly higher than that under dark.For light-sensitive target molecule,its structure can be changed under the illumination of the light,which changes the coupling degree of the energy level of the molecule and the electrode.