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

Felice C Simeone,Martin M Thuo,Jabulani R Barber,Hyo Jae Yoon,Barbara Smith,George M Whitesides

doi:10.1021/ja408652h

Abstract

Analysis of rates of tunneling across self-assembled monolayers (SAMs) of n-alkanethiolates SCn (with n = number of carbon atoms) incorporated in junctions having structure Ag(TS)-SAM//Ga2O3/EGaIn leads to a value for the injection tunnel current density J0 (i.e., the current flowing through an ideal junction with n = 0) of 10(3.6±0.3) A·cm(-2) (V = +0.5 V). This estimation of J0 does not involve an extrapolation in length, because it was possible to measure current densities across SAMs over the range of lengths n = 1-18. This value of J0 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 J0(+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 J0 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.

Highlights

Measurements, using a number of techniques, of rates of charge transport by tunneling across self-assembled monolayers (SAMs) of n-alkanethiolates on silver and gold substrates show an interesting, puzzling, and unresolved mixture of consistency and inconsistency
In this paper we have described a simple technique for the fabrication of what we call “flattened” Ga2O3/eutectic gallium indium alloy (EGaIn) conical tips
The comparison of the electrical behavior of smooth Hg-drop top-electrodes with that of flattened Ga2O3/EGaIn conical tips suggests that in junctions having structure AgTS-SAM//Ga2O3/EGaIn the effective electrical contact area is Aelect = 10-4.0±0.5·Ageo, where Ageo is the geometrical contact area measured by optical microscopy

Summary

Introduction

Measurements, using a number of techniques, of rates of charge transport by tunneling across self-assembled monolayers (SAMs) of n-alkanethiolates on silver and gold substrates show an interesting, puzzling, and unresolved mixture of consistency and inconsistency. This paper reaches five major conclusions: i) In junctions using conical Ga2O3/EGaIn electrodes on top of SAMs formed on template-stripped silver substrates, the effective electrical contact area is ~10-4 the geometrical contact area (measured by optical microscopy) . Iv) For junctions having the structure AgTS-SR//Ga2O3/EGaIn (AgTS= template-stripped silver substrate; R=CnH2n+1,with n=0-18), Jo=103.6±0.3 A·cm-2 at V=+0.5 V This value of Jo is much more accurate than previous estimates, in part because we can measure tunneling currents through short-chain (n=1-4) alkyl groups. For this type of studies, our estimation of Jo—a value that is accurate because it does not involve the long extrapolation from n=10-18 to n=0 characteristic of most prior works—provides a reference value that can be used to test the quality of data for tunneling across n-alkanethiolates collected (using conical Ga2O3/EGaIn tips) in different laboratories

Background

Results and Discussion

Conclusion