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

The SAM, Not the Electrodes, Dominates Charge Transport in Metal-Monolayer//Ga₂O₃/Gallium–Indium Eutectic Junctions

The liquid–metal eutectic of gallium and indium (EGaIn) is a useful electrode for making soft electrical contacts to self-assembled monolayers (SAMs). This electrode has, however, one feature whose effect on charge transport has been incompletely understood: a thin (approximately 0.7 nm) filmconsisting primarily of Ga₂O₃that covers its surface when in contact with air. SAMs that rectify current have been measured using this electrode in Ag^TS-SAM//Ga₂O₃/EGaIn (where Ag^TS = template-stripped Ag surface) junctions. This paper organizes evidence, both published and unpublished, showing that the molecular structure of the SAM (specifically, the presence of an accessible molecular orbital asymmetrically located within the SAM), not the difference between the electrodes or the characteristics of the Ga₂O₃ film, causes the observed rectification. By examining and ruling out potential mechanisms of rectification that rely either on the Ga₂O₃ film or on the asymmetry of the electrodes, this paper demonstrates that the structure of the SAM dominates charge transport through Ag^TS-SAM//Ga₂O₃/EGaIn junctions, and that the electrical characteristics of the Ga₂O₃ film have a negligible effect on these measurements

Replacing −CH₂CH₂– with −CONH– Does Not Significantly Change Rates of Charge Transport through Ag^TS-SAM//Ga₂O₃/EGaIn Junctions

This paper describes physical-organic studies of charge transport by tunneling through self-assembled monolayers (SAMs), based on systematic variations of the structure of the molecules constituting the SAM. Replacing a −CH₂CH₂– group with a −CONH– group changes the dipole moment and polarizability of a portion of the molecule and has, in principle, the potential to change the rate of charge transport through the SAM. In practice, this substitution produces no significant change in the rate of charge transport across junctions of the structure Ag^TS-S­(CH₂)_<i>m</i>X­(CH₂)_<i>n</i>H//Ga₂O₃/EGaIn (TS = template stripped, X = −CH₂CH₂– or −CONH–, and EGaIn = eutectic alloy of gallium and indium). Incorporation of the amide group does, however, increase the yields of working (non-shorting) junctions (when compared to <i>n</i>-alkanethiolates of the same length). These results suggest that synthetic schemes that combine a thiol group on one end of a molecule with a group, R, to be tested, on the other (e.g., HS∼CONH∼R) using an amide-based coupling provide practical routes to molecules useful in studies of molecular electronics

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