A molecule containing a nitroamine redox center (2′-amino-4-ethynylphenyl-4′-ethynylphenyl-5′-nitro-1-benzenethiol) was used in the active self-assembled monolayer in an electronic device. ...Current-voltage measurements of the device exhibited negative differential resistance and an on-off peak-to-valley ratio in excess of 1000:1.
We tracked over time the conductance switching of single and bundled phenylene ethynylene oligomers isolated in matrices of alkanethiolate monolayers. The persistence times for isolated and bundled ...molecules in either the ON or OFF switch state range from seconds to tens of hours. When the surrounding matrix is well ordered, the rate at which the inserted molecules switch is low. Conversely, when the surrounding matrix is poorly ordered, the inserted molecules switch more often. We conclude that the switching is a result of conformational changes in the molecules or bundles, rather than electrostatic effects of charge transfer.
Stochastic on-off conductivity switching observed in phenylene-ethynylene oligomers has been explained in terms of changes in ring conformations, or electron localization, or both. We report the ...observation of stochastic on-off switching in the simplest of wired molecules: octanedithiol, decanedithiol, and dodecanedithiol bonded on an Au(111) surface. Stochastic switching was observed even when a top gold contact was pressed on by a conducting atomic force microscope tip at constant force. The rate of switching increased substantially at 60°C, a temperature at which these films are commonly annealed. Because such switching in alkanethiols is unlikely to be caused by internal molecular electronic changes and cannot be fully accounted for by breaking of the top contact, we argue that the cause is the well-known mobility of molecules tethered to gold via a thiol linkage.
Presented here are several convergent synthetic routes to conjugated oligo(phenylene ethynylene)s. Some of these oligomers are free of functional groups, while others possess donor groups, acceptor ...groups, porphyrin interiors, and other heterocyclic interiors for various potential transmission and digital device applications. The syntheses of oligo(phenylene ethynylene)s with a variety of end groups for attachment to numerous metal probes and surfaces are presented. Some of the functionalized molecular systems showed linear, wirelike, current versus voltage (I(V)) responses, while others exhibited nonlinear I(V) curves for negative differential resistance (NDR) and molecular random access memory effects. Finally, the syntheses of functionalized oligomers are described that can form self‐assembled monolayers on metallic electrodes that reduce the Schottky barriers. Information from the Schottky barrier studies can provide useful insight into molecular alligator clip optimizations for molecular electronics.
Functionalized conjugated oligo(phenylene ethylene)s with donor groups, acceptor groups, porphyrin interiors, and heterocycle interiors (see scheme) have been synthesized by several convergent synthetic routes. These compounds have potential for wire and digital device applications: some showed linear, wirelike, current versus voltage (I(V)) responses, while others exhibited nonlinear I(V) curves for negative differential resistance (NDR) and molecular random access memory effects.
We have studied electron transport properties of unsubstituted oligo(phenylene ethynylene) (OPE) (1) and nitro-substituted OPE (2) covalently bound to two gold electrodes. The conductance values of ...single 1 and 2 are ∼13 and ∼6 nS, respectively. In addition to a decrease in the conductance, the presence of the nitro moiety leads to asymmetric I−V characteristics and a negative differential resistance-like (NDR-like) behavior. We have altered the nitro-substituted OPE by electrochemically reducing the nitro group and by varying the pH of the electrolyte. The conductance decreases linearly with the electron-withdrawing capability (i.e., Hammett substituent values) of the corresponding reduced species. In contrast, the conductance of 1 is independent of the pH and the electrode potential.
The electrical properties of self-assembled monolayers (SAMs) on metal surfaces have been explored for a series of molecules to address the relation between the behavior of a molecule and its ...structure. We probed interfacial electron transfer processes, particularly those involving unoccupied states, of SAMs of thiolates or arylates on Au by using shear force-based scanning probe microscopy (SPM) combined with current−voltage (i − V) and current−distance (i − d) measurements. The i − V curves of hexadecanethiol in the low bias regime were symmetric around 0 V and the current increased exponentially with V at high bias voltage. Different than hexadecanethiol, reversible peak-shaped i − V characteristics were obtained for most of the nitro-based oligo(phenylene ethynylene) SAMs studied here, indicating that part of the conduction mechanism of these junctions involved resonance tunneling. These reversible peaked i − V curves, often described as a negative differential resistance (NDR) effect of the junction, can be used to define a threshold tip bias, V TH, for resonant conduction. We also found that for all of the SAMs studied here, the current decreased with increasing distance, d, between tip and substrate. The attenuation factor β of hexadecanethiol was high, ranging from 1.3 to 1.4 Å-1, and was nearly independent of the tip bias. The β-values for nitro-based molecules were low and depended strongly on the tip bias, ranging from 0.15 Å-1 for tetranitro oligo(phenylene ethynylene) thiol, VII, to 0.50 Å-1 for dinitro oligo(phenylene) thiol, VI, at a −3.0 V tip bias. Both the V TH and β values of these nitro-based SAMs were also strongly dependent on the structures of the molecules, e.g. the number of electroactive substituent groups on the central benzene, the molecular wire backbone, the anchoring linkage, and the headgroup. We also observed charge storage on nitro-based molecules. For a SAM of the dintro compound, V, ∼25% of charge collected in the negative scan is stored in the molecules and can be collected at positive voltages. A possible mechanism involving lateral electron hopping is proposed to explain this phenomenon.
Molecular Wires, Switches, and Memories CHEN, J.; WANG, W.; KLEMIC, J. ...
Annals of the New York Academy of Sciences,
April 2002, Letnik:
960, Številka:
1
Journal Article
Recenzirano
: Design and measurements of molecular wires, switches, and memories offer an increased device capability with reduced elements. We report: Measurements on through‐bond electronic transport ...properties of nanoscale metal‐1,4‐phenylene diisocyanide‐metal junctions are reported, where nonohmic thermionic emission is the dominant process, with isocyanide‐Pd showing the lowest thermionic barrier of 0.22 eV; robust and large reversible switching behavior in an electronic device that utilizes molecules containing redox centers as the active component, exhibiting negative differential resistance (NDR) and large on‐off peak‐to‐valley ratio (PVR) are realized; erasable storage of higher conductivity states in these redox‐center‐containing molecular devices are observed; and a two‐terminal electronically programmable and erasable molecular memory cell with long bit retention time is demonstrated.
The surface modification of synthetic tissue engineering scaffolds is essential to improving their hydrophilicity and cellular compatibility. Plasma treatment is an effective way to increase the ...hydrophilicity of a surface, but the incorporation of biomolecules is also important to control cellular adhesion and differentiation, among many other outcomes. In this work, oriented polycaprolactone (PCL) electrospun fibers were modified by air-plasma treatment, followed by the covalent attachment of laminin. The amount of protein incorporated onto the fiber surface was controlled by varying the reaction time and the protein solution concentration. The protein concentration and coverage were quantified using X-ray photoelectron spectroscopy (XPS), solid-state ultraviolet–visible spectroscopy (UV–vis) and two fluorescence-based assays. XPS results showed a nearly linear increase in protein coverage with increasing protein soaking solution concentration until a monolayer was formed. Results from XPS and the NanoOrange fluorescence assay revealed multilayer protein coverage at protein solution concentrations between 25 and 50 μg/mL, whereas the UV–vis assay demonstrated multilayer coverage at lower protein solution concentrations. The effect of protein concentration on the neurite outgrowth of neuron-like PC12 cells was evaluated, and outgrowth rates were found to be positively correlated to increasing protein concentration.
Copper overlayers were formed on electrodes coated with self-assembled monolayers (SAMs) of a molecular wire candidate, 1-thio-4-4‘-(4‘-thio)phenylethynyl-1‘-ethynyl-2‘,5‘-(diethyl)phenylbenzene ...(TTEB), by electrochemical reduction of copper ions and by physical vapor deposition. Anodic stripping voltammetry of copper was employed to study the electrochemical and electron transport properties of SAMs of these molecules. Electrochemical copper deposition revealed that SAMs of TTEB passivate the electrode to electrochemistry in a manner similar to alkanethiols. Migration of copper ions trapped within the TTEB SAM after copper oxidation was also observed. Reduction of the solvent prevented the application of a sufficient potential to deposit copper by conduction through the TTEB SAM, so evaporation of copper metal was employed to coat the entire electrode. Anodic stripping voltammetry first removed the metal from defect sites, leaving behind copper islands connected to the gold by TTEB molecules. At higher potentials the copper islands were oxidized via electron transport through the TTEB SAM. A barrier height of 1.17 eV to charge injection was calculated from the tunneling current and overpotential by a Fowler−Nordheim-type analysis. Tunneling currents in TTEB molecules with metal contacts were found to be dominated by hole transport.
Poly(methyl methacrylate) (PMMA)–polyacrylonitrile (PAN) fibers were prepared using a conventional single-nozzle electrospinning technique. The as-spun fibers exhibited core–shell morphology as ...verified by transmission electron microscopy (TEM) and atomic force microscopy (AFM). AFM-phase and modulus mapping images of the fiber cross-section and X-ray photoelectron spectroscopy (XPS) analysis indicated that PAN formed the shell and PMMA formed the core material. XPS, thermogravimetric analysis (TGA), and elemental analysis were used to determine fiber compositional information. Soaking the fibers in solvent demonstrated removal of the core material, generating hollow PAN fibers.
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