Conductivity in alkylamine/gold and alkanethiol/gold molecular junctions measured in molecule/nanoparticle/molecule bridges and conducting probe structures

Charge transport through alkane monolayers on gold is measured as a function of molecule length in a controlled ambient using a metal/molecule/nanoparticle bridge structure and compared for both thiol and amine molecular end groups. The current through molecules with an amine/gold junction is observed to be more than a factor of 10 larger than that measured in similar molecules with thiol/gold linkages. Conducting probe atomic force microscopy is also used to characterize the same monolayer systems, and the results are quantitatively consistent with those found in the nanoparticle bridge geometry. Scaling of the current with contact area is used to estimate that approximately 100 molecules are probed in the nanoparticle bridge geometry. For both molecular end groups, the room-temperature conductivity at low bias as a function of molecule length shows a reasonable fit to models of coherent nonresonant charge tunneling. The different conductivity is ascribed to differences in charge transfer and wave function mixing at the metal/molecule contact, including possible effects of amine group oxidation and molecular conformation. For the amine/Au contact, the nitrogen lone pair interaction with the gold results in a hybrid wave function directed along the molecule bond axis, whereas the thiol/Au contact leads to a more localized wave function.     

Molecular binding at gold transport interfaces. IV. Thiol chemisorption

Alkene thiol/coinage metal molecular interfaces are relatively easy to make, and can result in well-ordered self-assembled monolayer films. The energetics of such formation is complex-differing experimental and theoretical accounts have focused on the nature of the binding, the energetics via different pathways (thiol radical, thiol or thiolate) and the geometry of binding. We report density functional theory calculations on a four atom gold cluster interacting with different (alkane, alkene, alkyne) thiolates. We find thiolate addition to be strongly exoergic, thiol radical to be roughly half as favorable, and thiol to be slightly favorable. We also find that the S-H bond can remain when the thiol attaches to the gold cluster, formally resulting in increased coordination on the sulfur atom.     

Molecular binding at gold transport interfaces. III. Field dependence of electronic properties

The behavior of the electronic structure in a metal/molecular/metal junction as a function of the applied electric field is studied using density functional methods. Although the calculations reported here do not include the electrode bulk, or intermolecular interactions, and do not permit actual transport to occur, nevertheless they illuminate the charging, energy shift, polarization and orbital occupation changes in the molecular junction upon the application of a static electric field. Specifically, external electric fields generally induce polarization localization on the two cluster ends. The HOMO/LUMO gap usually decreases and, for large enough fields, energy levels can cross, which presages a change of electronic state and, if found in molecular electronic circuits, a change in transmission. The calculations also show changes in the geometry both of the molecule and the molecule/cluster interface upon application of the electric field. These effects should be anticipated in whole circuit studies.     

Surfactant-mediated water transport at gelatin gel/oil interfaces

We studied spontaneous emulsification (SE) at Water/Oil (W/O) interfaces, using several types of aqueous reservoirs immersed in dodecane plus Span80 surfactant. Above a threshold surfactant concentration C(SE), aqueous satellite droplets are formed at the W/O interface. Varying the aqueous reservoir size, from below 100 microm (droplets) to centimeters (macroscopic phases), allowed investigating SE with complementary techniques. Release (rates) and size distributions for SE droplets were measured with microscopy. For gelled aqueous phases, water expulsion due to SE was quantified. Values for C(SE) were measured and were found to be higher for aqueous phases containing gelatin and/or NaCl. We also studied water exudation during network building and syneresis in aqueous gelatin gels immersed in dodecane/Span80. Below C(SE) (i.e., in the absence of SE) this process is still responsible for significant physico-chemical changes at the W/O interface. To study these in more detail, we performed atomic force microscopy experiments (in force-distance mode) on macroscopic gels. Both changes in the local elastic response and in the wettability of the AFM tip were detected. Together they suggest the formation of "water pockets" after prolonged (gel) setting times, along with a densification of the interfacial gelatin network.     

Binding of hydrogen bonding solutes at liquid–vapour interfaces of molecular fluids

We have calculated the potential of mean force (PMF) for the transfer of a solute molecule across a liquid–vapour interface for four different systems: (a) one methanol molecule in water, (b) one water molecule in methanol, (c) one acetonitrile molecule in water and (d) one water molecule in acetonitrile by means of constrained molecular dynamics simulations. A minimum of the PMF is found near the Gibbs dividing surface for methanol and acetonitrile solutes although the degree of surface activity is found to be somewhat different due, in part, to varying hydrogen bonding nature of these two solutes.     

Reactivity of monolayer-protected gold nanoclusters at dye-sensitized liquid/liquid interfaces

Hexanethiolate monolayer-protected gold nanoclusters (MPCs) were used as redox quenchers at the polarizable water/1,2-dichloroethane (DCE) interface. Photocurrent responses originating from the heterogeneous quenching of photoexcited water soluble porphyrin complexes by MPCs dissolved in the DCE phase were observed. As MPCs can function as both electron acceptors and donors, the photocurrent results from the superposition of two simultaneous processes, which correspond to the oxidation and reduction of MPCs. The magnitude of the net photocurrent is essentially determined by the balance of the kinetics of these two processes, which can be controlled by tuning the Galvani potential difference between the two phases. We show that, within the available potential window, the apparent electron-transfer rate constants follow classical Butler-Volmer dependence on the applied potential difference.     

Uncovering transport properties of 4,4'-bipyridine/gold molecular nanobridges

Here, the fascinating connection between the chemical and the transport properties of recently fabricated 4,4'-bipyridine/gold nanobridges is addressed. By means of first-principles ab initio calculations, the remarkable reproducibility of the 4,4'-bipyridine conductance properties is explained as the combined result of (i) the bonding of the molecule to the metallic leads through hybridization between the 4,4'-bipyridine highest occupied molecular orbitals and lowest unoccupied molecular orbitals (LUMOs) with s and d orbitals at low-coordination gold atoms, (ii) the limited number of molecule-lead arrangements due to gold-hydrogen steric repulsions, and (iii) the electron transmission through a LUMO-derived resonance, whose positioning with respect to the Fermi level determines which of the above arrangements yields nonnegligible conductance. Structural and electronic interpretations to the stepped dependence reported for the electronic transport of 4,4'-bipyridine as a function of the distance between the gold tips are also given.     

Water molecule clusters measured at water/air interfaces using atomic force microscopy

During the tip approach to hydrophobic surfaces like the water/air interface, the measured interaction force reveals a strong attraction with a range of approximately 250 nm at some points along the interface. The range of this force is approximately 100 times larger than the measured for gold (approximately 3 nm) and 10 times larger than the one for hydrophobic silicon surfaces (approximately 25 nm). At other points the interface exerts a medium range repulsive force growing stepwise as the tip approaches the interface plane, consequently the hydrophobic force is a strong function of position. To explain these results we propose a model where the force on the tip is associated with the exchange of a small volume of the interface with a dielectric permittivity epsilon(int) by the tip with a dielectric permittivity epsilon(tip). By assuming a oscillatory spatial dependence for the dielectric permittivity it is possible to fit the measured force profiles. This dielectric spatial variation was associated with the orientation of the water molecules arrangement in the interfacial region. Small nanosized hydrogen-bond connected cages of water molecules present in bulk water at the interface are oriented by the interfacial electric field generated by the water molecules broken bonds, one broken hydrogen bond out of every four. This interfacial field orients small clusters formed by approximately 100 water molecules into larger clusters (approximately 100 nm). In the limit of small (less than 5 nm thick) water molecule cages we have modeled the static dielectric permittivity (epsilon) as the average response of those cages. In these regions the dielectric permittivity for water/air interfaces decreases monotonically from the bulk value epsilon approximately 80 to approximately 2 at the interface. For regions filled with medium size cages, the tip senses the structure of each cage and the static dielectric permittivity is matched to the geometrical features of these cages sized approximately 25 to 40 nm. Interfacial electric energy density values were calculated using the electric field intensity and the dielectric permittivity obtained by the fitting of the experimental points. The integration of the electric energy density along the interfacial region gives a value of 0.072 J m(-2) for interfacial energy density for points where the hydrophobic force has a range of approximately 250 nm. Regions formed by various clusters result in lower values of the interfacial energy density.     

Formation of a tetragonal Cu3Au alloy at gold/copper interfaces

A gold–copper alloy with a nominal composition of Cu₃Au but with a tetragonal (c = 4a) structure is observed to form at Au/Cu interfaces of gold/copper multilayers deposited on amorphous substrates by d.c. magnetron sputtering. The formation of this non‐equilibrium structure (tentatively D0₂₃) under‐ambient conditions is detected by secondary ion mass spectrometry, x‐ray diffraction and high‐resolution cross‐sectional transmission electron microscopy. Co‐sputtering of Au and Cu under similar conditions produces only conventional fcc Cu₃Au alloy phases, suggesting that interfacial confinement plays a significant role in producing the novel Cu₃Au alloy phase in gold/copper multilayers. Copyright © 2003 John Wiley & Sons, Ltd.     

The crosslinking of epoxy resins at interfaces. V. Amine-cured resins at carbon and graphite surfaces

The effects of carbon blacks, chopped carbon fibers, and crushed carbon fibers on the crosslinking chemistry of a diglycidyl epoxy resin/m-phenylenediamine system were examined by infrared (IR) spectroscopy and differential scanning calorimetry (DSC). The carbon and graphite surfaces were given oxidizing and reducing treatment to simulate the surface treatment of carbon fibers used in the manufacture of composites. The oxidized carbon surfaces initially accelerated epoxy–amine reactions but inhibited the later stages of the reaction such that the final extent of cure was reduced. The oxidized carbons also preferentially adsorbed the amine curing agent, resulting in a stoichiometric imbalance at the interface.     

A molecular model for cohesive slip at polymer melt/solid interfaces

A molecular model is proposed which predicts wall slip by disentanglement of polymer chains adsorbed on a wall from those in the polymer bulk. The dynamics of the near-wall boundary layer is found to be governed by a nonlinear equation of motion, which accounts for such mechanisms on surface chains as convection, retraction, constraint release, and thermal fluctuations. This equation is valid over a wide range of grafting regimes, including those in which interactions between neighboring adsorbed molecules become essential. It is not closed since the dynamics of adsorbed chains is shown to be coupled to that of polymer chains in the bulk via constraint release. The constitutive equations for the layer and bulk, together with continuity of stress and velocity, are found to form a closed system of equations which governs the dynamics of the whole "bulk+boundary layer" ensemble. Its solution provides a stick-slip law in terms of the molecular parameters and extruder geometry. The model is quantitative and contains only those parameters that can be measured directly, or extracted from independent rheological measurements. The model predictions show a good agreement with available experimental data.     

Strong effects of molecular structure on electron transport in carbon/molecule/copper electronic junctions

Carbon/molecule/copper molecular electronic junctions were fabricated by metal deposition of copper onto films of various thicknesses of fluorene (FL), biphenyl (BP), and nitrobiphenyl (NBP) covalently bonded to flat, graphitic carbon. A "crossed-wire" junction configuration provided high device yield and good junction reproducibility. Current/voltage characteristics were investigated for 69 junctions with various molecular structures and thicknesses and at several temperatures. The current/voltage curves for all cases studied were nearly symmetric, scan rate independent, repeatable at least thousands of cycles and exhibited negligible hysteresis. Junction conductance was strongly dependent on the dihedral angle between phenyl rings and on the nature of the molecule/copper "contact". Junctions made with NBP showed a decrease in conductivity of a factor of 1300 when the molecular layer thickness increased from 1.6 to 4.5 nm. The slope of ln(i) vs layer thickness for both BP and NBP was weakly dependent on applied voltage and ranged from 0.16 to 0.24 A(-1). These attenuation factors are similar to those observed for similar molecular layers on modified electrodes used to study electrochemical kinetics. All junctions studied showed weak temperature dependence in the range of approximately 325 to 214 K, implying activation barriers in the range of 0.06 to 0.15 eV. The carbon/molecule/copper junction structure provides a robust, reproducible platform for investigations of the dependence of electron transport in molecular junctions on both molecular structure and temperature. Furthermore, the results indicate that junction conductance is a strong function of molecular structure, rather than some artifact resulting from junction fabrication.     

Comparison of morphology and mechanical properties of surfactant aggregates at water-silica and water-graphite interfaces from molecular dynamics simulations

Cationic surfactants are important for a wide range of applications, including controlled drug delivery systems, emulsifiers, and chemical mechanical polishing. It is therefore important to better understand surfactant structure and properties at the solid-liquid interface. Here, classical molecular dynamics simulations with empirical potentials are used to compare the structures and mechanical properties of cationic surfactant micelles at hydrophobic (graphite) and hydrophilic (silica) surface-water interfaces. In particular, the morphology of monolayers and bilayers of C12TAB (n-dodecyltrimethylammoniumbromide) at these interfaces, and their responses to atomic force microscopy indentation, are examined. The simulations predict that surfactant monolayers and bilayers on silica evolve into a spherical micelle structure, in agreement with theoretical models of surfactant morphology. In contrast, surfactant monolayers on graphite evolve into a hemi-cylindrical structure, in agreement with experimental findings. In the simulated indentation of the micelle/silica system, the spherical micelle breaks apart and forms a surfactant monolayer. The indentation force curve has a maximum value of 2.25 nN. On the other hand, the simulated indentation of the micelle/graphite system causes the hemi-cylindrical micelle structure to break apart and the surfactant tails to wrap around the graphite indenter. The indentation force curve has a maximum value of 13 nN.     

The role of quantum interference in determining transport properties of molecular bridges

An analytical approach to the electron transport phenomena in molecular devices is presented. The analyzed devices are composed of various molecular bridges attached to two semi-infinite electrodes. Molecular system is described within the tight-binding model, while the coupling to the electrodes is analyzed through the use of Newns-Anderson chemisorption theory. The current-voltage (I-V) characteristics are calculated through the integration of transmission function in the standard Landauer formulation. The essential question of quantum interference effect of electron waves is diseussed in three aspects: (i) the geometry of a molecular bridge, (ii) the presence of an external magnetic field and (iii) the location of chemical substituent.     

Potential-modulation spectroscopy at solid/liquid and liquid/liquid interfaces.

Potential-modulation spectroelectrochemical methods at solid/liquid and liquid/liquid interfaces are reviewed. After a brief summary of the basic features and advantages of the methods, practical applications of potential-modulation spectroscopy are demonstrated using our recent studies of solid/liquid and liquid/liquid interfaces, including reflection measurements for a redox protein on a modified gold electrode and fluorescence measurements for various dyes at a polarized water/1,2-dichloroethane interface. For both interfaces, the use of linearly polarized incident light enabled an estimation of the molecular orientation. The use of a potential-modulated transmission-absorption measurement for an optically transparent electrode with immobilized metal nanoparticles is also described. The ability of potential-modulated fluorescence spectroscopy to clearly elucidate the charge transfer and adsorption mechanisms at liquid/liquid interfaces is highlighted.     

Infrared-visible sum frequency generation spectroscopic study of molecular orientation at polystyrene/comb-polymer interfaces

Surface-sensitive infrared-visible sum frequency generation spectroscopy (SFG) in total internal reflection geometry has been used to study the structure of poly(vinyl n-octadecyl carbamate-co-vinyl acetate) (PVNODC) or poly(octadecyl acrylate) (PA-18) in contact with a deuterated or hydrogenated polystyrene (dPS or hPS) layer. SFG spectra from the PVNODC (or PA-18)/hPS interface show methyl and methylene peaks corresponding to PVNODC (or PA-18) and phenyl peaks corresponding to the PS. Analysis suggests that the methyl groups are tilted at angles less than 30 degrees with respect to the surface normal. The presence of a strong methylene peak suggests the PVNODC alkyl side chains contain more gauche defects at the PS/PVNODC interface in comparison to PVNODC (or PA-18)/air interfaces. On heating, the SFG intensity from the PS/PA-18 interface drops sharply near the bulk melting temperature (T(m)) of PA-18. Interestingly, a similar drop in SFG signal is also observed for the PS phenyl groups. This demonstrates the ability of the phenyl group at the PS/PA-18 interface to rearrange itself upon the solid-to-liquid transition of the PA-18 alkyl side chain, at a temperature well below the bulk PS glass transition temperature. For PS/PVNODC interfaces, the drop in SFG intensity is gradual and in agreement with the three broad transitions of PVNODC observed in the bulk.     

Hydrogen-bonding molecular ruler surfactants as probes of specific solvation at liquid/liquid interfaces

Resonance-enhanced, second harmonic generation (SHG) is used to measure the electronic structure of solutes sensitive to specific solvation adsorbed to liquid/liquid and liquid/solid interfaces. Here, specific solvation refers to solvent–solute interactions that are directional and localized. N-methyl-p-methoxyaniline (NMMA) is a solute whose first allowed electronic transition wavelength remains almost constant (∼315 nm) in non-hydrogen-bonding solvents regardless of solvent polarity. However, in hydrogen-bond-accepting solvents such as dimethylsulfoxide, NMMA’s absorbance shifts to longer wavelengths (320 nm), whereas in hydrogen-bond-donating solvents (e.g., water), the absorbance shifts to shorter wavelengths (∼300 nm). SHG experiments show that at alkane/silica interfaces, surface silanol groups serve as moderately strong hydrogen-bond donors as evidenced by NMMA’s absorbance of 307 nm. At the carbon tetrachloride/water interface, NMMA absorbance also shifts to slightly shorter wavelengths (298 nm) implying that water molecules at this liquid/liquid interface are donating strong hydrogen bonds to the adsorbed NMMA solutes. In contrast, experiments using newly developed molecular ruler surfactants with NMMA as a model hydrophobic solute and a hydrophilic, cationic headgroup imply that, as NMMA migrates across an aqueous/alkane interface, it carries with it water that functions as a hydrogen-bond-accepting partner.     

Dynamic behaviors of molecular assemblies at liquid/liquid interfaces studied by time-resolved quasi-elastic laser scattering spectroscopy.

The dynamic behaviors of molecular assemblies at two immiscible liquid interfaces are intriguing topics in many fields of science and technology. However, it is generally difficult to investigate the dynamic behaviors of such molecular assemblies because of the buried nature of liquid/liquid interfaces. In the present paper, our recent investigations on dynamic behaviors of various molecular self-assemblies at liquid/liquid interfaces are reviewed. We monitored dynamic behaviors of the molecular assemblies by time-resolved quasi-elastic laser scattering (TR-QELS) and fluorescent spectroscopy. The former method allows us to monitor the change in interfacial tension with millisecond time-resolution. As molecular assemblies, bis(2-ethylhexyl)sulfosuccinate (AOT) microemulsion, phospholipid biomembrane models, and liposome-DNA complexes have all been studied, since they are relevant in material sciences and biological technologies. At liquid/liquid interfaces, these molecular assemblies showed characteristic behaviors. We review the finding of rebound response of the interfacial tension at the liquid/liquid interface induced by the adsorption of the AOT microemulsion. We monitored the hydrolysis reaction of phospholipid biomembrane models formed at oil/water interfaces, observing the different types of behavior of liposome-DNA complexes at biomembrane models with different kinds of phospholipids.