Electrical Resistance of AgTS–S(CH2)n−1CH3//Ga2O3/EGaIn Tunneling Junctions

Tunneling junctions having the structure Ag−S(CH2)n−1CH3// Ga2O3/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−S(CH2)n−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. ■ INTRODUCTION We, and others, are developing procedures with which to study charge transport across self-assembled monolayers (SAMs). We 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) electrode and contacted by (ii) a “top” electrode of EGaIn (75.5 wt % Ga, 24.5 wt % In, mp 15.7 °C) 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 as Ag−SR//Ga2O3/EGaIn, where R is an organic group (which may range in structure from simple n-alkyl groups to more complex functionalities, e.g., aromatics or ferrocenes). 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 “Ga2O3/ 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−SR//Ga2O3/EGaIn junctions is currently the effect of the oxide skin and adventitious contaminants. Experimental efforts to understand charge transport across SAMs have been hampered by poor replicability caused, in part, by the difficulty of forming a reproducible electrical contact between a macroscopic electrode and a SAM. This poor reproducibility has both made it difficult to examine correlations between structure and conductance, and made it impractical to compare the results of measurements from techniques that operate under different conditions, and with different limitations (e.g., break junctions, scanning probe microscopy, Hg-drop junctions, PEDOT:PSS junctions, STM break junctions, CP-AFM, carbon electrode junctions, or evaporated metal junctions). Received: December 27, 2011 Revised: February 25, 2012 Published: February 27, 2012 Article