The impact of E-Z photo-isomerization on single molecular conductance.

The single molecule conductance of the E and Z isomers of 4,4'-(ethene-1,2-diyl)dibenzoic acid has been determined using two scanning tunneling microscopy (STM) methods for forming molecular break junctions [the I(s) (I = current and s is distance) method and the in situ break junction technique]. Isomerization leads to significant changes in the electrical conductance of these molecules, with the Z isomer exhibiting a higher conductance than the E isomer. Isomerization is achieved directly on the gold surface through photoirradiation, and the STM is used to determine conductance before and after irradiation; reversible switching between the two isomers could be achieved through irradiation of the surface bound species at different wavelengths.

The experimental determination of the conductance of single molecules.

The measurement of the electrical properties of molecules, down to the single molecule level, has become an experimental reality in recent years. A number of methods are now available for experimentally achieving this feat. The common aim of these methods is to entrap a single or small numbers of molecules between a pair of metallic contacts.

Anomalous length and voltage dependence of single molecule conductance.

A systematic experimental study of the electrical conductance of single alkanedithiol molecules (HS-(CH(2))(N)-SH) between gold contacts in air for N = 3-12 is presented. For all of these molecules, three different fundamental conductance groups (low, medium and high conductance) were observed. For long molecules (N > 7) the conductance decays exponentially with molecular length for all three conductance groups, as it has been reported previously.

Adverse effects of asymmetric contacts on single molecule conductances of HS(CH2)n COOH in nanoelectrical junctions.

A scanning tunnelling microscope has been used to determine the conductance of single molecular wires with the configuration X-bridge-X, X-bridge-Y and Y-bridge-Y (X = thiol terminus and Y = COOH). We find that for molecular wires with mixed functional groups (X-bridge-Y) the single molecule conductance decreases with respect to the comparable symmetric molecules. These differences are confirmed by theoretical computations based on a combination of density functional theory and the non-equilibrium Green's functions formalism.

Variable contact gap single-molecule conductance determination for a series of conjugated molecular bridges.

It is now becoming clear that the characteristics of the whole junction are important in determining the conductance of single molecules bound between two metal contacts. This paper shows through measurements on a series of seven conjugated molecular bridges that contact separation is an important factor in determining the electrical response of the molecular junction. These data are obtained using the I(t) method developed by Haiss et al since the scanning tunnelling microscope tip to substrate separation can be controlled through choice of the set-point (I(0)) current and calibrated with current-distance curves and knowledge of the terminal to terminal length of the molecular wire.

Structure-property relationships in redox-gated single molecule junctions--a comparison of pyrrolo-tetrathiafulvalene and viologen redox groups.

We demonstrate that the electrical "switching" behavior of single molecules connected between two electrode contacts can be controlled by altering their structure and electrochemical characteristics. The electrical properties of gold|molecule|gold single molecule junctions incorporating HS(CH2)6-X-(CH2)6SH, where X = viologen (4,4'-bipyridinium) or pyrrolotetrathiafulvalene, are determined using a scanning tunneling microscopy based technique. The switching behavior, controlled through a tuneable electrochemical gate, changes from an on-off response (viologen) to an off-on-off response (pyrrolotetrathiafulvalene) on changing the central redox group.

Single molecule conductance of porphyrin wires with ultralow attenuation.

A series of thioacetate-terminated butadiyne-linked porphyrin oligomers have been synthesized with one to three porphyrin repeat units. Single molecule electrical scanning tunneling microscopy measurements using the I(s) and I(t) methods were used to determine the molecule conductances for this series of oligomers. The molecular conductance shows an exponential falloff with sulfur-sulfur distance with a remarkably low attenuation factor of beta = (0.

Chemical control of double barrier tunnelling in alpha,omega-dithiaalkane molecular wires.

Single molecule conductance measurements on 1,4-bis-(6-thia-hexyl)-benzene derivatives reveal (i) that benzene rings serve as an effective indentation in the tunnelling barrier, and (ii) that more electron-rich benzene rings give higher conductances, consistent with hole conduction (i.e.via the benzene HOMO).

Determination of size and concentration of gold nanoparticles from UV-vis spectra.

The dependence of the optical properties of spherical gold nanoparticles on particle size and wavelength were analyzed theoretically using multipole scattering theory, where the complex refractive index of gold was corrected for the effect of a reduced mean free path of the conduction electrons in small particles. To compare these theoretical results to experimental data, gold nanoparticles in the size range of 5 to 100 nm were synthesized and characterized with TEM and UV-vis. Excellent agreement was found between theory and experiment.

Precision control of single-molecule electrical junctions.

There is much discussion of molecules as components for future electronic devices. However, the contacts, the local environment and the temperature can all affect their electrical properties. This sensitivity, particularly at the single-molecule level, may limit the use of molecules as active electrical components, and therefore it is important to design and evaluate molecular junctions with a robust and stable electrical response over a wide range of junction configurations and temperatures.

A combined top-down bottom-up approach for introducing nanoparticle networks into nanoelectrode gaps.

We report here on the fabrication of a three-dimensional array of nanoparticles which bridges the gap between lithographically defined gold electrode contacts separated by 20 nm. The nanoparticle assemblies are formed from about 5 nm gold nanoparticles and benzenedimethanethiol (BDMT) bridging ligands. These assemblies are introduced between the contacts using a layer-by-layer protocol with successive BDMT self-assembly being followed by nanoparticle adsorption until the gap is bridged.

Thermal gating of the single molecule conductance of alkanedithiols.

The temperature dependence of the single molecule conductance (SMC) of alpha,omega-alkanedithiols has been investigated using a scanning tunnelling microscopy (STM) method. This is based on trapping molecules between a gold STM tip and a gold substrate and measuring directly the current across the molecule under different applied potentials. A pronounced temperature dependence of the conductance, which scales logarithmically with T(1), is observed in the temperature range between 293 and 353 K.

Molecular wire formation from viologen assemblies.

The adsorption behavior of viologen alpha,omega-dithiols (viologen dithiols) on gold has been investigated. At short exposures, a low-coverage phase consisting of flat-lying molecules has been determined by STM and IR spectroscopy. In contrast, multilayer films are formed after long adsorption times.

Wiring nanoparticles with redox molecules.

Gold nanoparticles were used to make electrical contact to redox-active organic molecules. Viologen based dithiols were self-assembled from solution on Au(111) for use as tethers to attach nanoparticles to a conducting substrate. The topography and electrical properties of the resulting films were investigated by STM and STS and the orientation of these linkers was investigated by FTIR.

Redox state dependence of single molecule conductivity.

Spontaneous formation of stable molecular wires between a gold scanning tunneling microscopy (STM) tip and substrate is observed when the sample has a low coverage of alpha,omega-dithiol molecules and the tunneling resistance is made sufficiently small. Current-distance curves taken under these conditions exhibit characteristic current plateaux at large tip-substrate separations from which the conductivity of a single molecule can be obtained. The versatility of this technique is demonstrated using redox-active molecules under potential control, where substantial reversible conductivity changes from 0.

An FTIR study of the surface chemistry of the dynamic Si(100) surface during etching in alkaline solution.

In situ FTIR spectroscopy has been used in the attenuated total reflectance (ATR) mode to investigate the surface chemistry of etching Si(100) surfaces in aqueous KOH. The effect of solution concentration and electrode potential on the Si-H vibrations has been explored and the experimental results compared with density functional theory calculations. In addition.