Investigations and Control of Electronic Properties in a Small Number of Molecules

The surface structures and the electronic properties of monolayer films of methyl-substituted oligothiophene pentamer(DM5T)were investigated using atomic force microscopy(AFM)with an electrically conducting probe.The films were prepared by vacuum deposition onto a metal substrate。AFM observation revealed that the molecules were arranged with their molecular axes perpendicular to the substrate.The local current vs.voltage curves obtained on the film were nonlinear and nonsymmetric with respect to the polarity of the voltage。The resistivity of the film was about100Ωcm，which was smaller than one hundredth of that in the bulk material.It is noteworthy that the local electrical conduction is much better compared to the case of the bulk experiment.In addition，the surface potential was measured by Kelivin force microscopy(KFM)。The film had a higher potential than the Pt substrate，which indicated the existence of either charge transfer from the film to the substrate or induced dipoles toward the film surface.Self-assembled monolayers(SAMs)of the thiol molecules were also investigated by scanning tunneling microscopy(STM)High-resolution imaging of octanethiol Ms in ultrahigh vacuum environment revealed that the molecules were hagonly and the dulecules！On the other hand，octanedithiol molecules having thiol groups at both ends were arranged eptaxially with their molecular axes parallel to the substrate。Furthermore，local I/V measurements using STM showed that the I/V curves on the octanethiol SAMs were non-linear。The conductivity was higher when the sample was posively biased while the I/V characteristics of the octanedithiol SAMs exhibited a symmetric feature.The results suggested that the current was affected by the local electric field by the molecular dipole.

The surface structures and the electronic properties of monolayer films of methyl-substituted oligothiophene pentamer (DM5T) were investigated using atomic force microscopy (AFM) with an electrically conducting probe. The films were prepared by vacuum deposition onto a metal substrate. AFM observation revealed that the molecules were arranged with their molecular axes perpendicular to the substrate. The local current vs. voltage curves obtained on the film were nonlinear and nonsymmetric with respect to the polarity of the voltage. The resistivity of the film was about 100Ωcm, which was smaller than one hundredth of that in the bulk material. It is noteworthy that the local electrical conduction is much better compared to the case of the bulk experiment. In addition, the surface potential was measured by Kelivin force microscopy (KFM). The film had a higher potential than the Pt substrate, which indicated the existence of either charge transfer from the film to the substrate or induced dipoles toward the film surface.Self-assembled monolayers (SAMs) of the thiol molecules were also investigated by scanning tunneling microscopy (STM) High-resolution imaging of octanethiol SAMs in ultrahigh vacuum environment revealed that the molecules were hexagonally packed on Au (111) surface and that the ィイD83ィエD8xィイD83ィエD8and c (4x2) reconstructed structures were formed. On the other hand, octanedithiol molecules having thiol groups at both ends were arranged eptaxially with their molecular axes parallel to the substrate. Furthermore, local I/V measurements using STM showed that the I/V curves on the octanethiol SAMs were non-linear. The conductivity was higher when the sample was posively biased while the I/V characteristics of the octanedithiol SAMs exhibited a symmetric feature. The results suggested that the current was affected by the local electric field by the molecular dipole.