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

Ludovico Cademartiri,Rana N S Sodhi,Peter Brodersen,Jabulani R Barber,Choongik Kim,William F Reus,Simon Tricard,George M Whitesides,Martin M Thuo,Ryan C Chiechi,Christian A Nijhuis

doi:10.1021/jp212501s

Abstract

Tunneling junctions having the structure AgTS–S(CH2)n−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 AgTS–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 junction...

Highlights

The latter system has two major components: (i) a selfassembled monolayers (SAMs) supported by a template-stripped silver (AgTS) electrode, and contacted by (ii) a ―top‖ electrode of EGaIn (75.5 wt% Ga, 24.5 wt% In, mp 15.7 oC 42) 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[14] as Ag substrates (AgTS)-SR//Ga2O3/EGaIn, where R is an organic group
For Ga, we observed Ga0, which we associate with the EGaIn alloy, Ga+ (~19 eV), which we associate with Ga2O, and Ga3+ (~20.5 eV), which we associate with Ga2O3. (The assignments of the Ga signals were confirmed by the high resolution X-ray photoelectron spectroscopy (XPS) spectrum of the Ga 2p peaks shown in Figure 2d.103) For In, we observed In0, which we associate with the EGaIn alloy, and In3+, which we associated with In2O3, after comparison to a In2O3 standard
The composition profile of the three types of samples — ―drop‖, ―fresh tip‖, and ―cycled tip‖ — is within the error expected from XPS. (The ratio of oxide to metal is different when considering the Ga 2p or the Ga 3d levels due to the relative escape depths of these two types of photoelectrons; this difference is reflected in different sampling depths.) The similarity of these compositional profiles indicate that neither reversible deformations, nor the curvature of the electrode, affect the composition or the average thickness of the oxide layer; Å-scale differences in the average thickness of the oxide layer would result in observable differences in the ratios between the intensities of the Ga3+ and Ga0 XPS signals.[104,105]

Summary

Introduction

Others, are developing procedures with which to study charge transport across selfassembled monolayers (SAMs).[1,3,4,6,7,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37] 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.

Results

Conclusion