32 research outputs found
Odd-even effects in charge transport across n-alkanethiolate-based SAMs
This paper compares rates of charge transport across self-assembled monolayers (SAMs) of n-alkanethiolates having odd and even numbers of carbon atoms (nodd and neven) using junctions with the structure MTS/SAM//Ga2O3/EGaIn (M = Au or Ag). Measurements of current density, J(V), across SAMs of n-alkanethiolates on AuTS and AgTS demonstrated a statistically significant odd-even effect on AuTS, but not on AgTS, that could be detected using this technique. Statistical analysis showed the values of tunneling current density across SAMs of n-alkanethiolates on AuTS with nodd and neven belonging to two separate sets, and while there is a significant difference between the values of injection current density, J0, for these two series (log|J0Au,even| = 4.0 ± 0.3 and log|J0Au,odd| = 4.5 ± 0.3), the values of tunneling decay constant, ÎČ, for nodd and neven alkyl chains are indistinguishable (ÎČAu,even = 0.73 ± 0.02 Ă
-1, and ÎČAu,odd= 0.74 ± 0.02 Ă
-1). A comparison of electrical characteristics across junctions of n-alkanethiolate SAMs on gold and silver electrodes yields indistinguishable values of ÎČ and J0 and indicates that a change that substantially alters the tilt angle of the alkyl chain (and, therefore, the thickness of the SAM) has no influence on the injection current density across SAMs of n-alkanethiolates
Recommended from our members
Influence of Environment on the Measurement of Rates of Charge Transport across Ag TS /SAM//Ga 2 O 3 /EGaIn Junctions
This paper investigates the influence of the atmosphere used in the fabrication of top electrodes from the liquid eutectic of gallium and indium (EGaIn) (the so-called âEGaInâ electrodes), and in measurements of current density, J(V) , across self-assembled monolayers (SAMs) incorporated into /EGaIn junctions, on values of J(V) obtained using these electrodes. A gas-tight measurement chamber was used to control the atmosphere in which the electrodes were formed, and also to control the environment in which the electrodes were used to measure current densities across SAM-based junctions. Seven different atmospheresâair, oxygen, nitrogen, argon, and ammonia, as well as air containing vapors of acetic acid or waterâwere surveyed using both âroughâ conical-tip electrodes, and âsmoothâ hanging-drop electrodes. (The manipulation of the oxide film during the creation of the conical-tip electrodes leads to substantial, micrometer-scale roughness on the surface of the electrode, the extrusion of the drop creates a significantly smoother surface.) Comparing junctions using both geometries for the electrodes, across a SAM of n-dodecanethiol, in air, gave for the conical tip, and for the drop electrode (and, thus, ; this increase in current density is attributed to a change in the effective electrical contact area of the junction. To establish the influence of the resistivity of the film on values of J(V), junctions comprising a graphite electrode and a hanging-drop electrode were compared in an experiment where the electrodes did, and did not, have a surface oxide film; the presence of the oxide did not influence measurements of , and therefore did not contribute to the electrical resistance of the electrode. However, the presence of an oxide film did improve the stability of junctions and increase the yield of working electrodes from âŒ70% to âŒ100%. Increasing the relative humidity (RH) in which J(V) was measured did not influence these values (across methyl or carboxyl terminated SAMs) over the range typically encountered in the laboratory (20%â60% (RH)).Chemistry and Chemical Biolog
Anomalously Rapid Tunneling: Charge Transport across Self-Assembled Monolayers of Oligo(ethylene glycol)
Characterizing the MetalâSAM Interface in Tunneling Junctions
his paper investigates the influence of the interface between a gold or silver metal electrode and an n-alkyl SAM (supported on that electrode) on the rate of charge transport across junctions with structure Met(Au or Ag)TS/A(CH2)nH//Ga2O3/EGaIn by comparing measurements of current density, J(V), for Met/AR = Au/thiolate (Au/SR), Ag/thiolate (Ag/SR), Ag/carboxylate (Ag/O2CR), and Au/acetylene (Au/CâĄCR), where R is an n-alkyl group. Values of J0 and ÎČ (from the Simmons equation) were indistinguishable for these four interfaces. Since the anchoring groups, A, have large differences in their physical and electronic properties, the observation that they are indistinguishable in their influence on the injection current, J0 (V = 0.5) indicates that these four Met/A interfaces do not contribute to the shape of the tunneling barrier in a way that influences J(V).Chemistry and Chemical Biolog
Mapping mechanical properties of organic thin films by force-modulation microscopy in aqueous media
The mechanical properties of organic and biomolecular thin films on surfaces play an important role in a broad range of applications. Although force-modulation microscopy (FMM) is used to map the apparent elastic properties of such films with high lateral resolution in air, it has rarely been applied in aqueous media. In this letter we describe the use of FMM to map the apparent elastic properties of self-assembled monolayers and end-tethered protein thin films in aqueous media. Furthermore, we describe a simple analysis of the contact mechanics that enables the selection of FMM imaging parameters and thus yields a reliable interpretation of the FMM image contrast
Specific Binding at the Cellulose Binding ModuleâCellulose Interface Observed by Force Spectroscopy
The need for effective enzymatic
depolymerization of cellulose
has stimulated an interest in interactions between protein and cellulose.
Techniques utilized for quantitative measurements of proteinâcellulose
noncovalent association include microgravimetry, calorimetry, and
atomic force microscopy (AFM), none of which differentiate between
specific proteinâcellulose binding and nonspecific adhesion.
Here, we describe an AFM approach that differentiates nonspecific
from specific interactions between cellulose-binding modules (CBMs)
and cellulose. We demonstrate that the âmismatchedâ
interaction between murine galectin-3, a lectin with no known affinity
for cellulose, and cellulose shows molecular recognition force microscopy
profiles similar to those observed during the interaction of a âmatchedâ
clostridial CBM3a with the same substrate. We also examine differences
in binding probabilities and rupture profiles during CBMâcellulose
binding experiments in the presence and absence of a blocking agentîža
substrate specific for CBM that presumably blocks binding sites. By
comparison of the behavior of the two proteins, we separate specific
(i.e., blockable) and nonspecific adhesion events and show that both
classes of interaction exhibit nearly identical rupture forces (45
pN at âŒ0.4 nN/s). Our work provides an important caveat for
the interpretation of proteinâcarbohydrate binding by force
spectroscopy; delineation of the importance of such interactions to
other classes of binding warrants further study
Formation of highly ordered self-assembled monolayers of alkynes on Au(111) substrate
Self-assembled monolayers (SAMs), prepared by reaction of terminal n-alkynes (HCâĄC(CH2)nCH3, n = 5, 7, 9, and 11) with Au(111) at 60 °C were characterized using scanning tunneling microscopy (STM), infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), and contact angles of water. In contrast to previous spectroscopic studies of this type of SAMs, these combined microscopic and spectroscopic experiments confirm formation of highly ordered SAMs having packing densities and molecular chain orientations very similar to those of alkanethiolates on Au(111). Physical properties, hydrophobicity, high surface order, and packing density, also suggest that SAMs of alkynes are similar to SAMs of alkanethiols. The formation of high-quality SAMs from alkynes requires careful preparation and manipulation of reactants in an oxygen-free environment; trace quantities of O2 lead to oxidized contaminants and disordered surface films. The oxidation process occurs during formation of the SAM by oxidation of the âCâĄCâ group (most likely catalyzed by the gold substrate in the presence of O2)
Influence of the Contact Area on the Current Density across Molecular Tunneling Junctions Measured with EGaIn Top-Electrodes
This
paper describes the relationship between the rates of charge
transport (by tunneling) across self-assembled monolayers (SAMs) in
a metal/SAM//Ga<sub>2</sub>O<sub>3</sub>/EGaIn junction and the geometric
contact area (<i>A</i><sub>g</sub>) between the conical
Ga<sub>2</sub>O<sub>3</sub>/EGaIn top-electrode and the bottom-electrode.
Measurements of current density, <i>J</i>(<i>V</i>), across SAMs of decanethiolate on silver demonstrate that <i>J</i>(<i>V</i>) increases with <i>A</i><sub>g</sub> when the contact area is small (<i>A</i><sub>g</sub> < 1000 ÎŒm<sup>2</sup>), but reaches a plateau between 1000
and 4000 ÎŒm<sup>2</sup>, where <i>J</i>(0.5 V) â
10<sup>â0.52±0.10</sup> A/cm<sup>2</sup>. The method used
to fabricate Ga<sub>2</sub>O<sub>3</sub>/EGaIn electrodes generates
a tip whose apex is thicker and rougher than its thin, smoother sides.
When <i>A</i><sub>g</sub> is small, the Ga<sub>2</sub>O<sub>3</sub>/EGaIn electrode contacts the bottom-electrode principally
over this rough apex and forms irreproducible areas of electrical
contact. When <i>A</i><sub>g</sub> is large, the contact
is through the smoother regions peripheral to the apex and is much
more reproducible. Measurements of contact pressure between conical
EGaIn electrodes and atomic force microscope cantilevers demonstrate
that the nominal contact pressure (governed by the mechanical behavior
of the oxide skin) decreases approximately inversely with the diameter
of geometric contact. This self-regulation of pressure prevents damage
to the SAM and makes the ratio of electrical contact area to geometric
footprint approximately constant