The Rate of Charge Tunneling Is Insensitive to Polar Terminal Groups in Self-Assembled Monolayers in Ag TS S(CH 2 ) n M(CH 2 ) m T//Ga 2 O 3 /EGaIn Junctions

This paper describes a physical‐organic studyof the effect of uncharged, polar, functional groups on the rate of charge transport by tunneling across self‐assembled monolayer (SAM)‐based large‐area junctions of the form AgTSS(CH2)nM(CH2)mT//Ga2O3/EGaIn. Here AgTS is a template‐stripped silver substrate, ‐M‐ and ‐T are “middle” and “terminal” functional groups, and EGaIn is eutectic galliumindium alloy. A range of uncharged polar groups (‐T), having permanent dipole moments in the range 0.5 < μ <4.5, were incorporated into the SAM. A comparison of the electrical characteristics of these junctions with junctions formed from n‐alkanethiolates led to the conclusion that the rates of charge tunneling are insensitive to the replacement of terminal alkyl groups with terminal polar groups. The current densities measured in this work suggest that the tunneling decay parameter (β) and injection current (Jo) for SAMs terminated in non‐polar n‐alkyl groups, and polar groups, are statistically indistinguishable.Chemistry and Chemical Biolog

Fluorination, and Tunneling across Molecular Junctions

This paper describes the influence of the substitution of fluorine for hydrogen on the rate of charge transport by hole tunneling through junctions of the form AgTSO2C(CH2)n(CF2)mT//Ga2O3/EGaIn, where T is methyl (CH3) or trifluoromethyl (CF3). Alkanoate-based self-assembled monolayers (SAMs) having perfluorinated groups (RF) show current densities that are lower (by factors of 20–30) than those of the homologous hydrocarbons (RH), while the attenuation factors of the simplified Simmons equation for methylene (β = (1.05 ± 0.02)nCH2–1) and difluoromethylene (β = (1.15 ± 0.02)nCF2–1) are similar (although the value for (CF2)n is statistically significantly larger). A comparative study focusing on the terminal fluorine substituents in SAMs of ω-tolyl- and -phenyl-alkanoates suggests that the C–F//Ga2O3 interface is responsible for the lower tunneling currents for CF3. The decrease in the rate of charge transport in SAMs with RF groups (relative to homologous RH groups) is plausibly due to an increase in the height of the tunneling barrier at the T//Ga2O3 interface, and/or to weak van der Waals interactions at that interface.Chemistry and Chemical Biolog

Molecular Series-Tunneling Junctions

Charge transport through junctions consisting of insulating molecular units is a quantum phenomenon that cannot be described adequately by classical circuit laws. This paper explores tunneling current densities in self-assembled monolayer (SAM)-based junctions with the structure AgTS/O2C–R1–R2–H//Ga2O3/EGaIn, where AgTS is template-stripped silver and EGaIn is the eutectic alloy of gallium and indium; R1 and R2 refer to two classes of insulating molecular units—(CH2)n and (C6H4)m—that are connected in series and have different tunneling decay constants in the Simmons equation. These junctions can be analyzed as a form of series-tunneling junctions based on the observation that permuting the order of R1 and R2 in the junction does not alter the overall rate of charge transport. By using the Ag/O2C interface, this system decouples the highest occupied molecular orbital (HOMO, which is localized on the carboxylate group) from strong interactions with the R1 and R2 units. The differences in rates of tunneling are thus determined by the electronic structure of the groups R1 and R2; these differences are not influenced by the order of R1 and R2 in the SAM. In an electrical potential model that rationalizes this observation, R1 and R2 contribute independently to the height of the barrier. This model explicitly assumes that contributions to rates of tunneling from the AgTS/O2C and H//Ga2O3 interfaces are constant across the series examined. The current density of these series-tunneling junctions can be described by J(V) = J0(V) exp(−β1d1 – β2d2), where J(V) is the current density (A/cm2) at applied voltage V and βi and di are the parameters describing the attenuation of the tunneling current through a rectangular tunneling barrier, with width d and a height related to the attenuation factor β.Chemistry and Chemical Biolog

Formation of Highly Ordered Self-Assembled Monolayers of Alkynes on Au(111) Substrate

Self-assembled monolayers (SAMs), prepared by reaction of terminal n-alkynes (HC≡C(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 −C≡C– group (most likely catalyzed by the gold substrate in the presence of O2).Chemistry and Chemical Biolog

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 “EGaIn” electrodes), and in measurements of current density, J(V) $(A/cm^2)$, across self-assembled monolayers (SAMs) incorporated into $Ag/SR//Ga_2O_3$/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 atmospheres—air, oxygen, nitrogen, argon, and ammonia, as well as air containing vapors of acetic acid or water—were surveyed using both “rough” conical-tip electrodes, and “smooth” 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 $log |J|mean = −2.4 \pm 0.4$ for the conical tip, and $log |J|mean = −0.6 \pm 0.3$ for the drop electrode (and, thus, $\Delta log |J| \approx 1.8)$; 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 $Ga_2O_3$ 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 $log |J(V)|$, 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 ∼70% to ∼100%. Increasing the relative humidity (RH) in which J(V) was measured did not influence these values (across methyl $(CH_3)^-$ or carboxyl $(CO_2H)^-$ terminated SAMs) over the range typically encountered in the laboratory (20%–60% (RH)).Chemistry and Chemical Biolog

Introducing Ionic and/or Hydrogen Bonds into the SAM//Ga 2 O 3 Top-Interface of Ag TS /S(CH 2 ) n T//Ga 2 O 3 /EGaIn Junctions

Junctions with the structure AgTS/S(CH2)nT//Ga2O3/EGaIn (where S(CH2)nT is a self-assembled monolayer, SAM, of n-alkanethiolate bearing a terminal functional group T) make it possible to examine the response of rates of charge transport by tunneling to changes in the strength of the interaction between T and Ga2O3. Introducing a series of Lewis acidic/basic functional groups (T = −OH, −SH, −CO2H, −CONH2, and −PO3H) at the terminus of the SAM gave values for the tunneling current density, J(V) in A/cm2, that were indistinguishable (i.e., differed by less than a factor of 3) from the values observed with n-alkanethiolates of equivalent length. The insensitivity of the rate of tunneling to changes in the terminal functional group implies that replacing weak van der Waals contact interactions with stronger hydrogen- or ionic bonds at the T//Ga2O3 interface does not change the shape (i.e., the height or width) of the tunneling barrier enough to affect rates of charge transport. A comparison of the injection current, J0, for T = −CO2H, and T = −CH2CH3−two groups having similar extended lengths (in Å, or in numbers of non-hydrogen atoms)−suggests that both groups make indistinguishable contributions to the height of the tunneling barrier.Chemistry and Chemical Biolog

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, β, for nodd and neven alkyl chains are indistinguishable (βAu,even = 0.73 ± 0.02 Å–1, and βAu,odd= 0.74 ± 0.02 Å–1). A comparison of electrical characteristics across junctions of n-alkanethiolate SAMs on gold and silver electrodes yields indistinguishable values of β 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.Chemistry and Chemical Biolog

Replacing AgTSSCH2−RAg ^{TS} SCH_2-R with AgTSO2C−RAg^{TS}O_2C-R in EGaIn-Based Tunneling Junctions Does Not Significantly Change Rates of Charge Transport

This paper compares rates of charge transport by tunneling across junctions with the structures $Ag^{TS}X(CH_2)_{2n}CH_3 //Ga_2O_3 /EGaIn \space$ (n=1–8 and $X= -SCH_2-$ and $O_2C-$); here $Ag^{TS}$ is template-stripped silver, and EGaIn is the eutectic alloy of gallium and indium. Its objective was to compare the tunneling decay coefficient ($\beta$, Å$^{−1}$) and the injection current ($J_0$, A cm$^{−2}$) of the junctions comprising SAMs of n-alkanethiolates and n-alkanoates. Replacing $Ag^{TS}SCH_2-R$ with $Ag^{TS}O_2C-R$ (R=alkyl chains) had no significant influence on $J_0$ (ca. $3\times10^3 A cm^{−2}$) or $\beta$ (0.75–0.79 Å$^{−1}$)—an indication that such changes (both structural and electronic) in the $Ag^{TS}XR$ interface do not influence the rate of charge transport. A comparison of junctions comprising oligo(phenylene)carboxylates and n-alkanoates showed, as expected, that $\beta$ for aliphatic (0.79 Å$^{−1}$) and aromatic (0.60 Å$^{−1}$) SAMs differed significantly.Chemistry and Chemical BiologyOther Research Uni

Formation of highly ordered self-assembled monolayers of alkynes on Au(111) substrate

Self-assembled monolayers (SAMs), prepared by reaction of terminal n-alkynes (HC≡C(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 −C≡C– 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//Ga₂O₃/EGaIn junction and the geometric contact area (<i>A</i>_g) between the conical Ga₂O₃/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>_g when the contact area is small (<i>A</i>_g < 1000 μm²), but reaches a plateau between 1000 and 4000 μm², where <i>J</i>(0.5 V) ≈ 10^{–0.52±0.10} A/cm². The method used to fabricate Ga₂O₃/EGaIn electrodes generates a tip whose apex is thicker and rougher than its thin, smoother sides. When <i>A</i>_g is small, the Ga₂O₃/EGaIn electrode contacts the bottom-electrode principally over this rough apex and forms irreproducible areas of electrical contact. When <i>A</i>_g 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