Wolfgang Haiss

Measurement of single molecule conductivity using the spontaneous formation of molecular wires

A technique to measure the electrical conductivity of single molecules has been demonstrated. The method is based on trapping molecules between an STM tip and a substrate. The spontaneous attachment and detachment of α,ω-alkanedithiol molecular wires was easily monitored in the time domain. Electrical contact between the target molecule and the gold probes was achieved by the use of thiol groups present at each end of the molecule. Characteristic jumps in the tunnelling current were observed when the tip was positioned at a constant height and the STM feedback loop was disabled. Histograms of the measured current jump values were used to calculate the molecular conductivity as a function of bias and chain length. In addition, it is demonstrated that these measurements can be carried out in a variety of environments, including aqueous electrolytes. The changes in conductivity with chain length obtained are in agreement with previous results obtained using a conducting AFM and the origin of some discrepancies in the literature is analysed.

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. It is proposed the origin of this dependence is the change in distribution between molecular conformers rather than changes in either the conduction mechanism or the electronic structure of molecule. For alkanedithiols the time averaged conformer distribution shifts to less elongated conformers at higher temperatures thus giving rise to higher conductance across the molecular bridges. This is analysed by first calculating energy differences between different conformers and then calculating their partition distribution. A simple tunnelling model is then used to calculate the temperature dependent conductance based on the conformer distribution. These findings demonstrate that charge transport through single organic molecules at ambient temperatures is a subtle and highly dynamic process that cannot be described by analysing only one molecular conformation corresponding to the lowest energy geometry of the molecule.

In-situ infrared spectroscopic studies of thymine adsorption on a Au(111) electrode

Abstract We have investigated the adsorption of thymine on Au(111) with in-situ infrared spectroscopy. Using IR spectroscopy we have been able to probe the structure and bonding of the ‘chemisorbed’ thymine phase. IR bands in the 1800–1550 cm −1 region, which have a major contribution from stretching vibrations of the carbonyl groups of thymine, exhibit large spectral shifts between thymine in aqueous solution and the chemisorbed phase. By contrast, ‘physisorbed’ thymine shows no IR bands in the spectral region studied, which is consistent with a flat-lying adsorbate. In-situ IR spectroscopy provides compelling evidence for the orientational change of adsorbed thymine occurring concomitantly with the most pronounced anodic current peak in the voltammogram. We have compared the in-situ IR spectra for chemisorbed thymine with IR spectra of metal co-ordination complexes of thymine and uracil. These comparisons provide evidence that chemisorbed thymine bonds to the Au(111) surface through both carbonyl functionalities and a deprotonated N3.

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. The contact gap separation dependence is interpreted as arising from tilting of these molecules in the junction and this model is underpinned by ab initio transport computations. In this respect we make the general observation that conductance increases rather dramatically at higher tilt angle away from the normal for conformationally rigid molecular wires and that this increase in conductance arises from increased electronic coupling between the molecular bridge and the gold contacts.

Surface relaxation and surface stress of Au(1 1 1)

AbstractChangesinsurfacestressandinthetop-layerexpansionofAu(111)electrodesinsulfuricacidhavebeenmeasuredasafunctionofelectrodepotentialbycombiningsurfacestressandX-raydiffractionmeasurements.Botharelinearfunctionsofinterfacialchargeintheelectrodepotentialrangeofchanginganioncoverage.Overthisrangethesurfacestresschangesby 0.5Nm 1 (compressivedirection),whiletheoutwardtop-layerrelaxationdecreasesfromþ1.5%toþ0.2%.Thesurfacestresschangescanberationalizedintermsofajelliummodel,whileabinitiosimulationsareneededtoexplainthetop-layerexpansion.Thesesimulationsyieldþ1.3%relaxationfortheunchargedgoldsurface,ingoodagreementwiththeX-raydiffractionmeasurements.Theyalsodemonstratethattheoutwardrelaxationofthesurfaceiscurbedinthepresenceofanelectronwithdrawingadsorbate(Cl),whichmimicstheeffectsofpositivesurfacecharg-ing. 2002ElsevierScienceB.V.Allrightsreserved. Keywords: Surfacestress;Surfacerelaxationandreconstruction;X-rayscattering,diffraction,andreflection;Densityfunctionalcalculations;Gold 1. IntroductionThebondingandelectronicstructureofsub-strate surfaces can be affected by molecularchemisorption.Thebondingintheoutmostsub-stratelayerscanhavesignificanteffectsonsurfacestructureanddynamics,leadingtophenomenasuchassurfacereconstructionorenhancedsub-stratemobility[1].Electrochemistshavetheun-iqueabilitytoaffectthebondstrengthsofthesubstratesurface,bytuningtheelectrodepotentialandconsequentlyalteringthesurfaceelectronicstructure.Thesechangesinsurfacebondstrengthsarenotatpresentdirectlymeasurable,buttheyresultinchangesinthesurfaceenergy,surfacestress,top-layersurfacerelaxationandinextremecasessurfacereconstruction;allofwhichcannowbemonitoredinsitu[2–5].Inthispaperinsitumeasurementsofsurfacestress andoutward surface relaxationarepre-sented and compared. Surface stress has beenmeasuredpreviouslyatthemetal–electrolytein-terfaceusinganumberofmechanicalmethodstomonitorstrainchangesintheelectrode[4,6–13].Althoughsomeoftheresultsonpolycrystallinesampleswereambiguous,themorerecentexperi-mentsforsinglecrystalsshowthatthetensilesurfacestressdecreaseslinearlywithincreasing

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. Surface coverage increased with increasing reaction time, resulting in a change of film orientation from a flat to a more upright standing conformation. Gold nanoparticles attached to these self-assembled films were characterised by STM. It was possible to isolate a single redox-active molecule in an alkanethiol matrix and by subsequent attachment of a single gold nanoparticle the electrical properties of single wired molecules could be investigated. This method allowed the measurement of the conductivity of single molecules connecting a nanoparticle to the substrate chemically, thus forming stable electrical contacts at both ends.

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. The temperature dependent electrical properties of these devices are analysed to establish whether they are consistent with the notion that the networks are built up from molecularly interlinked discrete gold nanoparticles. To aid this analysis the molecular conductance of single bridging molecules is also characterized at room temperature using a recently introduced method based on the scanning tunnelling microscope (STM). From these measurements it is concluded that the room temperature electrical properties of the nanostructured networks are limited by the small interparticle connectivity and the inherent resistance of the linker molecules.

In situ monitoring of intrinsic stress changes during copper electrodeposition on Au(111)

In this paper we report on the stress evolution during multilayer copper electrodeposition on Au(111). In particular, we investigated the dependence of the stress on the overpotential and made use of the knowledge established in surface science to identify the structure of the resulting films. We have adapted the method of monitoring the bending of a cantilever sample by STM to measure stress changes during the bulk electrodeposition. We find that the bulk copper films develop tensile stress, in contrast to the compressive stress resulting from the formation of the first monolayer at underpotential. The tensile stress changes for the bulk film, for an equivalent film thickness, increase with increasing overpotential. We relate these stress changes to (small angle) grain boundaries between adjoining copper growth centres. A larger grain boundary contact area results from the finer-grained deposits formed at higher overpotential. For equivalent deposition overpotential, copper electrodeposition, in both the absence and presence of crystal violet, gives rises to comparable stress evolution, resulting from the common source of the intrinsic stress.

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 Greens functions formalism. This study demonstrates that the apparent contact resistance, as well as being highly sensitive to the type of the anchoring group, is also strongly influenced by contact-asymmetry of the single molecular junction which in this case decreases the transmission. This highlights that contact-asymmetry is a significant factor to be considered when evaluating nanoelectrical junctions incorporating single molecules.

An STM investigation of surface diffusion on iodine modified Au(111)

Step edge fluctuations on Au(111) electrodes have been investigated using STM in electrolytes containing iodide, as a way of monitoring surface diffusion. The influence of the iodine adsorbate on the gold step edge mobility and diffusion of the Au surface is analysed. It was found that there is a sudden increase in step edge fluctuations upon iodide adsorption followed by a local minimum when an ordered iodine ad-layer is formed. The time correlation function of step edge fluctuations has been analysed using established methods to reveal the diffusion mechanism at different electrode potentials. As a result of this analysis it was found that the mechanism of diffusion in the disordered phase involves the movement of gold atoms (assisted by the 2D iodine gas on the terrace) from step edges out onto terraces. By contrast, evidence was found that diffusion is predominantly restricted to the step edge as soon as an ordered ad-layer is formed. Images show that this ad-layer packs closely to the step edge, thus largely restricting mobility of Au to the immediate vicinity of the step edge. In this case it is suggested that diffusion along the step edge involves a co-operative movement of gold and iodide.