Single Molecular Conductance Of Benzene-based Carboxylic Acid And Oligomer Molecules

Metal-molecule-metal junction(Single molecular junction)has been attracted widespread attentions,for it shows especial features,such as field effect transistor,negative differential resistance and rectifier,which is expected to make molecular wires,molecular switch,molecular memory,field effect transistor and so on in the future molecular electronics.There are many factors can influence the electron transport of single molecular junction,such as anchoring group,molecular structure,surrounding environment and electrode.However,there is still unclear in fully understand the influence of those factors,which limited the deep understand of electron transport characterization of single molecular junction.Thus,more investigation on those aspects is still needed.We will investigate influence of molecular structure and electrode on the electron transport of single molecular junction by using scanning tunneling microscopy break junction(STM-BJ),including the different molecular length of iodine-base alkane,side group,insertion of ferrocene and different electrode.Those works are as follows:1.Using saturated alkyl iodide I-(CH2)n-I(n=4,5,6,8)as the target molecules,we systematically measured the conductance of alkyl diiodide molecules with Au as the electrode.Depending on the increasing of molecular length,the decay constant is 0.5 per methylene(-CH2).Comparing with the decay constants of other anchoring groups,the result shows that decay constant with iodine as the anchoring group is the smallest,which can be used to make effectively molecular wires.2.Using Au as the electrode,we systematically studied the influence of side substituents on the molecular conductance of conjugated molecules.It shows that the electron-donating group will increase the molecular conductance for HOMO dominated molecule,while the electron-withdrawing groups will decrease the molecular conductance.However,the LUMO-dominated molecules are the opposite way.This is caused by the influence of the side chain group on the front orbital energy of molecule,which impacts the coupling between the Fermi level and the front orbital energy of molecules.3.Using Au as electrode,we systematically studied the insertion of redox activity center of ferrocene on molecular conductance with dihydrobenzothiophene(-BT)as the anchoring group.The experimental results show that the conductance of the molecular without ferrocene is larger than that of ferrocene,indicating that the introduction of ferrocene reduces the conductance of the molecule.Furthermore,the conductance of the anchoring group on same ring of ferrocene is larger than that of the isopentane ring,which may be caused by increasing of molecular length on the different isoprene rings.4.We studied the conductance of molecules with different aromaticity with Pt as electrode.The results show that the conductance of the molecule is different from those molecules with Au as electrode,which may be due to the different coupling degree between molecule and metal electrode.The results also reveal that there is no significant correlation between molecular aromaticity and molecular conductance in those molecules,indicating that other factors such as the electronegativity of five membered heterocycles,molecular space structure can also influence the conductivity of aromatic molecules addition to aromaticity of the molecule.5.Using Au as the electrode,we studied the molecular conductance of terephthalic acid and isophthalic acid.The results show that the molecular conductance of terephthalic acid is larger than that of isophthalic acid,for the electron transport length of p-phthalic acid is longer than that of isophthalic acid.Interestingly,the single molecular conductance values with copper electrode are bigger than gold electrode for terephthalic acid and terephthalate,which may be caused by better coupling between Cu and carboxylic acid than that of Au.