677 research outputs found
A versatile experimental approach for understanding electron transport through organic materials
No full textThis paper describes an experimentally simple method for assembling junctions with nanometer-scale, structured organic films positioned between two metal electrodes. These junctions comprise two metal electrodes that sandwich two self-assembled monolayers (SAMs) - that is, metal (mercury)-SAM//SAM-metal (mercury, gold or silver) junctions. The junctions are easy to assemble (because the mercury electrode is compliant) and they are compatible with SAMs incorporating organic groups having a range of structures. This paper describes three different variations on this type of Hg-based junction. The first junction, formed by two contacting mercury drops covered by the same type of SAM, is a prototype system that provided useful information on the structure and electrical properties of the Hg-based junctions. The second junction consists of a Hg drop covered by one SAM (Hg-SAM(1)) in contact with a second SAM supported on a silver film (Ag-SAM(2)) - that is, a Hg-SAM(1)//SAM(2)-Ag junction. This junction allowed systematic measurements of the current that flowed across SAM(2), as a function of structure (for example, using aliphatic or aromatic thiols of different length), and a common SAM(1) of hexadecane thiol. The current density follows the relation I = I0e-βdAg,Hg, where dAg,Hg is the distance between the electrodes, and β is the structure-dependent attenuation factor for the molecules making up SAM(2): β was 0.87 ± 0.1 Å-1 for alkanethiols, 0.61 ± 0.1 Å-1 for oligophenylene thiols, and 0.67 ± 0.1 Å-1 for benzylic derivatives of oligophenylene thiols, in general agreement with the values calculated by other approaches. The same type of junction, but using SAM(1) and SAM(2) carrying suitable chemical groups, X and Y, was used to measure the rate of electron transfer across different types of functional groups and bonds: van der Waal interactions, H bonds, and covalent bonds. The third type of junction, Hg-SAM//R//SAM-Hg, is an electrochemical junction that can (i) trap redox-active molecules (R) in the interfacial region between the SAMs, and (ii) control the potential of the electrodes with respect to the redox potential of R using an external reference electrode. This system shows I-V curves with steps that can be interpreted in terms of redox cycling mechanism. © 2002 Elsevier Science B.V. All rights reserved
Electron Transfer in a Hg‐SAM//SAM‐Hg Junction Mediated by Redox Centers
No full textCurrent affairs: The electron-transport properties of a metal-molecule-metal junction based on two contacting redoxactive self-assembled monolayers of [Ru(NH3)5(NC5H 4-4-CH2NHCO(CH2)10-SH](PF 6)2 (see picture) is described. The junction becomes conductive when the electrode potentials are adjusted to the formal potential of the redox centers and shows diode- and transistor-like characteristics analogous to those of solid-state devices
Electron exchange between two electrodes mediated by two electroactive adsorbates
No full textElectrochemical junctions are used to incorporate and study teh electtron transport mediated by redox centres. Theoretical model interpret the data of the molecular transistor behavior
The study of charge transport through organic thin films: mechanism, tools and applications
No full textIn this paper, we discuss the current state of organic and molecular-scale electronics, some experimental methods used to characterize charge transport through molecular junctions and some theoretical models (superexchange and barrier tunnelling models) used to explain experimental results. Junctions incorporating self-assembled monolayers of organic molecules - and, in particular, junctions with mercury-drop electrodes - are described in detail, as are the issues of irreproducibility associated with such junctions (due, in part, to defects at the metal-molecule interface). © 2007 The Royal Society
Self-assembled monolayers exposed to metastable argon beams undergo thiol exchange reactions
No full textMesoscale Self-Assembly: Capillary Interactions When Positive and Negative Menisci Have Similar Amplitudes
No full textThis paper describes the two-dimensional self-assembly of hexagonal plates at the interface between
perfluorodecalin and water. The plates were prepared with five different permutations of hydrophobic and
hydrophilic faces. The shapes and amplitudes of the menisci that form on the faces of the plates determine
the magnitude of the lateral capillary forces through which they interact. The amplitudes of the menisci
also influencethrough their out-of-plane componentsthe position and orientation of the plate relative
to the plane of the liquid−liquid interface. In these experiments, the plates were made of poly(dimethylsiloxane) (PDMS) (ρ = 1.05 g/cm3) containing aluminum oxide (ρ = 4.00 g/cm3); this dopant
adjusted the density of the plates, the extent to which they sank into the liquid−liquid interface, and thus
the structure of their menisci. The plates studied had densities of 1.05 to 1.86 g/cm3. This work complements
previous papers (Bowden, N.; Choi I. S.; Grzybowski, B. A.; Whitesides, G. M. J. Am. Chem. Soc. 1999,
121, 5373. Bowden, N.; Oliver, S. R. J.; Whitesides, G. M. J. Phys. Chem. B 2000, 104, 2714.) that examined
the assembly of hexagonal plates with densities at the extremes of the range studied. By following the
structures of the aggregates formed at intermediate densities, it is possible to observe the way in which
the self-assembling system transitions from an aggregate of one structure to that of another. The results
from these studies are relevant to the design of micrometer-sized plates capable of self-assembly
Electron transport through thin organic films in metal-insulator-metal junctions based on self-assembled monolayers
No full textThis paper describes an experimentally simple system for measuring rates of electron transport across organic thin films having a range of molecular structures. The system uses a metal-insulator-metal junction based on self-assembled monolayers (SAMs); it is particularly easy to assemble. The junction consists of a SAM supported on a silver film (Ag-SAM(1)) in contact with a second SAM supported on the surface of a drop of mercury (Hg-SAM(2))-that is, a Ag-SAM(1)SAM(2)-Hg junction. SAM(1) and SAM(2) can be derived from the same or different thiols. The current that flowed across junctions with SAMs of aliphatic thiols or aromatic thiols on Ag and a SAM of hexadecane thiol on Hg depended both on the molecular structure and on the thickness of the SAM on Ag: the current density at a bias of 0.5 V ranged from 2 × 10-10 A/cm2 for HS(CH2)15CH3 on Ag to 1 × 10-6 A/cm2 for HS(CH2)15H3 on Ag, and from 3 x 10-6 A/cm2 for HS(Ph)3H (Ph = 1,4-C6H4) on Ag to 7 × 10-4 A/cm2 for HSPhH on Ag. The curr..
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Redox Site-Mediated Charge Transport in a Hg#SAM//Ru(NH3)63+/2+//SAM#Hg Junction with a Dynamic Interelectrode Separation: Compatibility with Redox Cycling and Electron Hopping Mechanisms
No full textThis paper describes the formation and electrical properties of a new Hg-based metal−molecules−metal junction that incorporates charged redox sites into the space between the electrodes. The junction is formed by bringing into contact two mercury-drop electrodes whose surfaces are covered by COO^{−}-terminated self-assembled monolayers (SAMs) and immersed in a basic aqueous solution of Ru(NH_3)_6Cl_3. The electrical behavior of the junction, which is contacted at its edges by aqueous electrolyte solution, has been characterized electrochemically. This characterization shows that current flowing through the junction on the initial potential cycles is dominated by a redox-cycling mechanism and that the rates of electron transport can be controlled by controlling the potentials of the mercury electrodes with respect to the redox potential of the Ru(NH_3)_6^{3+/2+} couple. On repeated cycling of the potential across the junction, the current across it increases by as much as a factor of 40, and this increase is accompanied by a large (>300 mV) negative shift in the formal potential for the reduction of Ru(NH_3)_6^{3+}. The most plausible rationalization of this behavior postulates a decrease in the size of the gap between the electrodes with cycling and a mechanism of conduction dominated by physical diffusion of Ru(NH_3)_6^{3+/2+} ions (at larger interelectrode spacing), with a possible contribution of electron hopping to charge transport (at smaller interelectrode spacing). In this rationalization, the negative shift in the formal potential plausibly reflects extrusion of the solution of electrolyte from the junction and an increase in the effective concentration of negatively charged species (surface-immobilized COO^{−} groups) in the volume bounded by the electrodes. This junction has the characteristics required for use in screening and in exploratory work, involving nanogap electrochemical systems, and in mechanistic studies involving these systems. It does not have the stability needed for long-term technological applications.Chemistry and Chemical BiologyPhysicsVersion of Recor
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