Molecular electron transport changes upon structural phase transitions in alkanethiol molecular junctions.

We demonstrated structural phase dependency of conductance across thiolate self-assembled monolayers (SAMs) in different junctions. A structural phase transition from a hexagonal closed phase to a striped phase in 1-octanethiol (OT) and 1,8-octanedithiol (ODT) SAMs was revealed by high resolution scanning tunneling microscopy (STM) images. Electron tunneling characteristics were measured through STM-based individual molecular junctions and micropore-based large molecular junctions. The tunneling barrier height and the tunneling decay constant of the molecular junctions were used as measures of the intermolecular coupling for different structural phases of the thiolate SAMs. Electron transport through ODT SAMs was found to be more sensitive than that through OT SAMs, according to the structural phase transition. These results suggest that (1) the structural phase transition in the SAM induces a change in the electron tunneling distance through the pathway of through-bond tunneling and through-space tunneling, leading to a change in the tunneling barrier of molecular junctions, and (2) integrated intermolecular coupling in a large molecular junction leads to a significant change of the electron transport between two structural phases.

Electrodeposition of metal wires onto a molecular scale template: an in situ investigation.

We have demonstrated that the intrinsic nanometer length scales of two-dimensional molecular assemblies can be exploited to electrodeposit metal nanostructures with regular spacing and orientation. We observed evidence for preferential deposition of metals into parallel lines on Au(111) surface with a periodicity of 4.5 nm as determined by the hemimicelles formed by sodium dodecylsulfate. The preferential deposition of metals in molecular templates was achieved under optimal electrode potentials and ionic concentrations. The observed metal structures provide insight into the interactions between metal atoms, organic functional groups as well as the aqueous environment. Understanding and tailoring these interactions will lead to more precise control and new strategies for nanoscale placement and for connecting organic molecules to metal nanostructures.

Molecular conductance switch-on of single ruthenium complex molecules.

Thiol-tethered Ru(II) terpyridine complexes were synthesized for a voltage-driven molecular switch and used to understand the switch-on mechanism of the molecular switches of single metal complexes in the solid-state molecular junction in a vacuum. Molecularly resolved scanning tunneling microscopy (STM) images revealed well-defined single Ru(II) complexes isolated in the highly ordered dielectric monolayer. When a negative sample-bias was applied, the threshold voltage to the high conductance state in the molecular junctions of the Ru(II) complex was consistent with the electronic energy gap between the Fermi level of the gold substrate and the lowest ligand-centered redox state of the metal complex molecule. As an active redox center leading to conductance switching in the molecule, the lowest ligand-centered redox state of Ru(II) complexes was suggested to trap an electron injected from the gold substrate. Our suggestions for a single-molecule switch-on mechanism in the solid state can provide guidance in a design that improves the charge-trapping efficiency of the ligands with different metal substrates.

Nanolithographic write, read, and erase via reversible nanotemplated nanostructure electrodeposition on alkanethiol-modified Au(111) in an aqueous solution.

A write, read, and erase nanolithographic method, combining in situ electrodeposition of metal nanostructures with atomic force microscopy (AFM) nanoshaving of a 1-hexadecanethiol (HDT) self-assembled monolayer (SAM) on Au(111) in an aqueous solution, is reported. The AFM tip defines the local positioning of nanotemplates via the irreversible removal of HDT molecules. Nanotemplates with lateral dimensions as narrow as 25 nm are created. The electroactive nanotemplates determine the size, shape, and position of the metal nanostructures. The potential applied to the substrate controls the amount of metal deposited and the kinetics of the deposition. Metal nanostructures can be reversibly and repeatedly electrodeposited and stripped out of the nanotemplates by applying appropriate potentials.

Electrochemical characteristics of ferrocenecarboxylate-coupled aminoundecylthiol self-assembled monolayers.

The electrochemical characteristics of the modified electrodes with ferrocenecarboxylate-coupled aminoundecylthiol monolayers prepared in two sequential steps were studied. The self-assembled monolayer (SAM) was prepared through the covalent attachment of ferrocenecarboxylate in an activation solution containing N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide coupling agent to aminoundecylthiol SAMs formed on a substrate. In the ferrocenecarboxylate-coupled aminoundecylthiol monolayers, the ferrocene moieties were expected to be packed regularly with enhanced ordering compared with those in the FcCOO(CH2)11SH monolayer. As the ferrocene coverage increases, the formal potential for the ferrocene-ferricenium (Fc/Fc+) couple shifts to the positive potential and the full width at half-maximum (deltaE(fwhm)) increases also. The maximum coverage is found to be about 3 x 10(-10) mol cm(-2), which is considered to be a value obtained from a well-ordered ferrocene-tethered SAM. As for the mass change, the increase in ferrocene coverage caused the enhancement in ion association between the ferricenium cations and perchlorate anions resulting in a mass increase upon oxidation; however, the mass change per mole electron decreases. The results obtained from the ferrocenecarboxylate-coupled aminoundecylthiol monolayers were explained to be due to the well-ordered packing with regular spacing compared with those of the FcCOO(CH2)11SH monolayer.

Application of a gold electrode, modified by a self-assembled monolayer of 2-mercaptodecylhydroquinone, to the electroanalysis of hemoglobin.

A gold electrode modified by a self-assembled monolayer of 2-mercaptodecylhydroquinone (H(2)Q(CH(2))(10)SH) was applied to investigate the electrochemical response of hemoglobin in aerated buffer solutions. Compared with a bare gold electrode, the monolayer of H(2)Q(CH(2))(10)SH could suppress the reduction wave of dissolved oxygen in the buffer while effectively promoting the rate of electron transfer between hemoglobin and the electrode. Thus, a convenient way for electroanalysis of hemoglobin in air was achieved at the H(2)Q(CH(2))(10)SH/Au electrode. A linear relationship existed between peak current and concentration of hemoglobin in the range 1 x 10(-7)-1 x 10(-6) mol L(-1).