Comparison of SAM-Based Junctions with Ga₂O₃/EGaIn Top Electrodes to Other Large-Area Tunneling Junctions

This paper compares the <i>J</i>(<i>V</i>) characteristics obtained for self-assembled monolayer (SAM)-based tunneling junctions with top electrodes of the liquid eutectic of gallium and indium (EGaIn) fabricated using two different procedures: (i) stabilizing the EGaIn electrode in PDMS microchannels and (ii) suspending the EGaIn electrode from the tip of a syringe. These two geometries of the EGaIn electrode (with, at least when in contact with air, its solid Ga₂O₃ surface film) produce indistinguishable data. The junctions incorporated SAMs of SC_<i>n</i>–1CH₃ (with <i>n</i> = 12, 14, 16, or 18) supported on ultraflat, template-stripped silver electrodes. Both methods generated high yields of junctions (70–85%) that were stable enough to conduct measurements of <i>J</i>(V) with statistically large numbers of data (<i>N</i> = 400–1000). The devices with the top electrode stabilized in microchannels also made it possible to conduct measurements of <i>J</i>(V) as a function of temperature, almost down to liquid nitrogen temperatures (<i>T</i> = 110–293 K). The <i>J</i>(<i>V</i>) characteristics were independent of <i>T</i>, and linear in the low-bias regime (−0.10 to 0.10V); the current density decreased exponentially with increasing thickness of the SAM. These observations indicate that tunneling is the main mechanism of charge transport across these junctions. Both methods gave values of the tunneling decay coefficient, β, of ∼1.0 <i>n</i>_C^–1 (∼0.80 Å^–1), and the pre-exponential factor, <i>J</i>₀ (which is a constant that includes contact resistance), of ∼3.0 × 10² A/cm². Comparison of the electrical characteristics of the junctions generated using EGaIn by both methods against the results of other systems for measuring charge transport indicated that the value of β generated using EGaIn electrodes is compatible with the consensus of values reported in the literature. Although there is no consensus for the value of <i>J</i>₀, the value of <i>J</i>₀ estimated using the Ga₂O₃/EGaIn electrode is compatible with other values reported in the literature. The agreement of experimental values of β across a number of experimental platforms provides strong evidence that the structures of the SAMsincluding their molecular and supramolecular structure, and their interfaces with the electrodesdominate charge transport in both types of EGaIn junctions. These results establish that studies of <i>J</i>(<i>V</i>) characteristics of Ag^TS-SAM//Ga₂O₃/EGaIn junctions are dominated by the structure of the organic component of the SAM, and not by artifacts due to the electrodes, the resistance of the Ga₂O₃ surface film, or to the work functions of the metals

Comparison of SAM-Based Junctions with Ga₂O₃/EGaIn Top Electrodes to Other Large-Area Tunneling Junctions

This paper compares the <i>J</i>(<i>V</i>) characteristics obtained for self-assembled monolayer (SAM)-based tunneling junctions with top electrodes of the liquid eutectic of gallium and indium (EGaIn) fabricated using two different procedures: (i) stabilizing the EGaIn electrode in PDMS microchannels and (ii) suspending the EGaIn electrode from the tip of a syringe. These two geometries of the EGaIn electrode (with, at least when in contact with air, its solid Ga₂O₃ surface film) produce indistinguishable data. The junctions incorporated SAMs of SC_<i>n</i>–1CH₃ (with <i>n</i> = 12, 14, 16, or 18) supported on ultraflat, template-stripped silver electrodes. Both methods generated high yields of junctions (70–85%) that were stable enough to conduct measurements of <i>J</i>(V) with statistically large numbers of data (<i>N</i> = 400–1000). The devices with the top electrode stabilized in microchannels also made it possible to conduct measurements of <i>J</i>(V) as a function of temperature, almost down to liquid nitrogen temperatures (<i>T</i> = 110–293 K). The <i>J</i>(<i>V</i>) characteristics were independent of <i>T</i>, and linear in the low-bias regime (−0.10 to 0.10V); the current density decreased exponentially with increasing thickness of the SAM. These observations indicate that tunneling is the main mechanism of charge transport across these junctions. Both methods gave values of the tunneling decay coefficient, β, of ∼1.0 <i>n</i>_C^–1 (∼0.80 Å^–1), and the pre-exponential factor, <i>J</i>₀ (which is a constant that includes contact resistance), of ∼3.0 × 10² A/cm². Comparison of the electrical characteristics of the junctions generated using EGaIn by both methods against the results of other systems for measuring charge transport indicated that the value of β generated using EGaIn electrodes is compatible with the consensus of values reported in the literature. Although there is no consensus for the value of <i>J</i>₀, the value of <i>J</i>₀ estimated using the Ga₂O₃/EGaIn electrode is compatible with other values reported in the literature. The agreement of experimental values of β across a number of experimental platforms provides strong evidence that the structures of the SAMsincluding their molecular and supramolecular structure, and their interfaces with the electrodesdominate charge transport in both types of EGaIn junctions. These results establish that studies of <i>J</i>(<i>V</i>) characteristics of Ag^TS-SAM//Ga₂O₃/EGaIn junctions are dominated by the structure of the organic component of the SAM, and not by artifacts due to the electrodes, the resistance of the Ga₂O₃ surface film, or to the work functions of the metals

Defining the Value of Injection Current and Effective Electrical Contact Area for EGaIn-Based Molecular Tunneling Junctions

Analysis of rates of tunneling across self-assembled monolayers (SAMs) of <i>n</i>-alkanethiolates SC_<i>n</i> (with <i>n</i> = number of carbon atoms) incorporated in junctions having structure Ag^TS-SAM//​Ga₂O₃/​EGaIn leads to a value for the injection tunnel current density <i>J</i>₀ (i.e., the current flowing through an ideal junction with <i>n</i> = 0) of 10^3.6±0.3 A·cm^–2 (<i>V</i> = +0.5 V). This estimation of <i>J</i>₀ does not involve an extrapolation in length, because it was possible to measure current densities across SAMs over the range of lengths <i>n</i> = 1–18. This value of <i>J</i>₀ is estimated under the assumption that values of the geometrical contact area equal the values of the effective electrical contact area. Detailed experimental analysis, however, indicates that the roughness of the Ga₂O₃ layer, and that of the Ag^TS-SAM, determine values of the effective electrical contact area that are ∼10^–4 the corresponding values of the geometrical contact area. Conversion of the values of geometrical contact area into the corresponding values of effective electrical contact area results in <i>J</i>₀(+0.5 V) = 10^7.6±0.8 A·cm^–2, which is compatible with values reported for junctions using top-electrodes of evaporated Au, and graphene, and also comparable with values of <i>J</i>₀ estimated from tunneling through single molecules. For these EGaIn-based junctions, the value of the tunneling decay factor β (β = 0.75 ± 0.02 Å^–1; β = 0.92 ± 0.02 nC^–1) falls within the consensus range across different types of junctions (β = 0.73–0.89 Å^–1; β = 0.9–1.1 nC^–1). A comparison of the characteristics of conical Ga₂O₃/​EGaIn tips with the characteristics of other top-electrodes suggests that the EGaIn-based electrodes provide a particularly attractive technology for physical-organic studies of charge transport across SAMs

Statistical Tools for Analyzing Measurements of Charge Transport

This paper applies statistical methods to analyze the large, noisy data sets produced in measurements of tunneling current density (<i>J</i>) through self-assembled monolayers (SAMs) in large-area junctions. It describes and compares the accuracy and precision of procedures for summarizing data for individual SAMs, for comparing two or more SAMs, and for determining the parameters of the Simmons model (β and <i>J</i>₀). For data that contain significant numbers of outliers (i.e., most measurements of charge transport), commonly used statistical techniquese.g., summarizing data with arithmetic mean and standard deviation and fitting data using a linear, least-squares algorithmare prone to large errors. The paper recommends statistical methods that distinguish between real data and artifacts, subject to the assumption that real data (<i>J</i>) are independent and log-normally distributed. Selecting a precise and accurate (conditional on these assumptions) method yields updated values of β and <i>J</i>₀ for charge transport across both odd and even <i>n</i>-alkanethiols (with 99% confidence intervals) and explains that the so-called odd–even effect (for <i>n</i>-alkanethiols on Ag) is largely due to a difference in <i>J</i>₀ between odd and even <i>n</i>-alkanethiols. This conclusion is provisional, in that it depends to some extent on the statistical model assumed, and these assumptions must be tested by future experiments

Electrical Resistance of Ag^TS–S(CH₂)_<i>n</i>−1CH₃//Ga₂O₃/EGaIn Tunneling Junctions

Tunneling junctions having the structure Ag^TS–S­(CH₂)_<i>n</i>−1CH₃//Ga₂O₃/EGaIn allow physical–organic studies of charge transport across self-assembled monolayers (SAMs). In ambient conditions, the surface of the liquid metal electrode (EGaIn, 75.5 wt % Ga, 24.5 wt % In, mp 15.7 °C) oxidizes and adsorbs―like other high-energy surfaces―adventitious contaminants. The interface between the EGaIn and the SAM thus includes a film of metal oxide, and probably also organic material adsorbed on this film; this interface will influence the properties and operation of the junctions. A combination of structural, chemical, and electrical characterizations leads to four conclusions about Ag^TS–S­(CH₂)_<i>n</i>−1CH₃//Ga₂O₃/EGaIn junctions. (i) The oxide is ∼0.7 nm thick on average, is composed mostly of Ga₂O₃, and appears to be self-limiting in its growth. (ii) The structure and composition (but not necessarily the contact area) of the junctions are conserved from junction to junction. (iii) The transport of charge through the junctions is dominated by the alkanethiolate SAM and not by the oxide or by the contaminants. (iv) The interface between the oxide and the eutectic alloy is rough at the micrometer scale