Electrical Resistance of Ag^TS–S(CH₂)_n−1CH₃//Ga₂O₃/EGaIn Tunneling Junctions

Abstract

Tunneling junctions having the structure Ag TS −S(CH 2 ) n−1 CH 3 // Ga 2 O 3 /EGaIn allow physical−organic studies of charge transport across selfassembled 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(CH 2 ) n−1 CH 3 // Ga 2 O 3 /EGaIn junctions. (i) The oxide is ∼0.7 nm thick on average, is composed mostly of Ga 2 O 3 , 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. ■ INTRODUCTIONWe, and others, are developing procedures with which to study charge transport across self-assembled monolayers (SAMs). 1−32We have explored two systems, both based on electrodes made of liquid metals (Hg, and a eutectic alloy of gallium and indium, which we abbreviate as EGaIn) and focused on the latter. The latter system has two major components: (i) a SAM supported by a template-stripped silver (Ag TS ) electrode and contacted by (ii) a "top" electrode of EGaIn (75.5 wt % Ga, 24.5 wt % In, mp 15.7°C 33 ) that is a liquid at room temperature and covered with a thin metal oxide film; we refer to these junctions by a nomenclature defined earlier 1 as Ag TS −SR//Ga 2 O 3 /EGaIn, where R is an organic group (which may range in structure from simple n-alkyl groups to more complex functionalities, e.g., aromatics 28 or ferrocenes 13,34,35 ). These junctions are typically formed, characterized, and used in contact with ambient laboratory atmosphere. In these conditions, the surface of EGaIn oxidizes rapidly and spontaneously (for convenience we indicate the composite structure-oxide skin and metal electrode-as "Ga 2 O 3 / EGaIn") and it-as do all other surfaces-adsorbs adventitious contaminants (e.g., water, organic molecules, particles). The electrical resistance, thickness, and heterogeneity of the composite films of metal oxide and contaminants on the surface (and their variability from electrode to electrode, and from junction to junction) have not been characterized: the most serious ambiguity affecting the measurement of charge transport through Ag TS −SR//Ga 2 O 3 /EGaIn junctions is currently the effect of the oxide skin and adventitious contaminants.

Influence of Environment on the Measurement of Rates of Charge Transport across AgTS/SAM//Ga2O3/EGaIn Junctions

“…This conclusion reinforces similar conclusions reached in previous but different studies. 1,16 Hanging-Drop Electrodes Formed in Air or O 2 and Measured in Air or O 2 . Figures 2A and 2B summarize measurements using hanging-drop electrodes formed in O 2 -containing atmospheres (air and O 2 , respectively) and a HOPG substrate; we assume that the surface of these drop electrodes have a Ga 2 O 3 film that is chemically similar to that formed on conical-tip electrodes (although not as severely buckled and probablybecause it is less buckled and foldedsubstantially thinner and more compliant).…”

“…The contact surface of the conical-tip electrode is rough due to asperities in the oxide, these surface irregularities result during the formation of the tip (in particular, when the neck of the EGaIn breaks at the point of rupture). 1,16,19 Fabrication of the hanging-drop electrode, by contrast, does not require the EGaIn to rupture (Figure 1), and the surface of the electrode is therefore qualitatively smoother than that of a conical tip, and (we believe) has a thinner and more-flexible Ga 2 O 3 film. We measured junctions using both top electrode geometries in seven different environments: dry air (from a cylinder of compressed air), humid air (5%−80% relative humidity (RH)), argon, nitrogen, oxygen, anhydrous ammonia, or air containing acetic acid.…”

“…The Ga 2 O 3 filmwhich is a thin (nominally ∼0.7 nm) 16 film that forms on the surface of EGaIn upon exposure to air or O 2 is crucial to the characteristics and performance of the Ga 2 O 3 / EGaIn junction; it facilitates the formation of electrode tips with useful shapes and sizes. (In particular, it is the basis for the mechanical stability of what we call "conical tips".…”

“…Although the Ga 2 O 3 film is sufficiently electrically conductive, so that its resistance does not contribute significantly to, or interfere with, tunneling currents measured across SAMs, 16 its roughness leads to areas of electrical contact between the top Ga 2 O 3 /EGaIn electrode and the SAM-bound bottom electrode that are ∼10 −4 the geometrical contact area (as measured by optical microscopy). 1,19 We estimate that this value of 10 −4 partitions into two components: a 10 −3 contribution from the roughness of Ga 2 O 3 and 10 −1 from the roughness of the silver substrate.…”

“…Under ambient conditions, a thin self-limiting layer of Ga 2 O 3 forms very rapidly 16 on the surface of bulk EGaIn. 17 When using EGaIn as a "soft" electrode, the resistivity of the oxide film might, in principle, influence measurements of electrical current or potential.…”