The Rate of Charge Tunneling Is Insensitive to Polar Terminal Groups in Self-Assembled Monolayers in AgTSS(CH2)nM(CH2)mT//Ga2O3/EGaIn Junctions

Hyo Jae Yoon,Carleen M Bowers,George M Whitesides,Mostafa Baghbanzadeh

doi:10.1021/ja409771u

Abstract

This paper describes a physical-organic study of the effect of uncharged, polar, functional groups on the rate of charge transport by tunneling across self-assembled monolayer (SAM)-based large-area junctions of the form Ag(TS)S(CH2)(n)M(CH2)(m)T//Ga2O3/EGaIn. Here Ag(TS) is a template-stripped silver substrate, -M- and -T are "middle" and "terminal" functional groups, and EGaIn is eutectic gallium-indium alloy. Twelve uncharged polar groups (-T = CN, CO2CH3, CF3, OCH3, N(CH3)2, CON(CH3)2, SCH3, SO2CH3, Br, P(O)(OEt)2, NHCOCH3, OSi(OCH3)3), having permanent dipole moments in the range 0.5 < μ < 4.5, were incorporated into the SAM. A comparison of the electrical characteristics of these junctions with those of junctions formed from n-alkanethiolates led to the conclusion that the rates of charge tunneling are insensitive to the replacement of terminal alkyl groups with the terminal polar groups in this set. The current densities measured in this work suggest that the tunneling decay parameter and injection current for SAMs terminated in nonpolar n-alkyl groups, and polar groups selected from common polar organic groups, are statistically indistinguishable.

Highlights

This paper describes a physical‐organic study of the effect of uncharged, polar, functional groups on the rate of charge transport by tunneling across self‐assembled monolayer (SAM)‐based large‐area junctions of the form AgTSS(CH2)nM(CH2)mT//Ga2O3/EGaIn
This paper focuses on a specific physical‐organic question: Do molecular dipoles, when placed at the top‐ interface between a thin, electrically insulating organic film and a conducting top‐electrode, influence the rates of charge transport by tunneling? 25,26 To answer this ques‐ tion, we examined the electrical characteristics of SAM‐based large‐area junctions of the form Ag‐ TSS(CH2)nM(CH2)mT//Ga2O3/EGaIn, where ‐M‐ and ‐T are middle and terminal functional groups, and Ga2O3/EGaIn is the top elec‐ trode we have described previously.[7,13,14,15,27,28]
We systematically modified the terminal portion of the SAM with polar groups (‐T, Fig‐ ure 1) that are uncharged, but have significant group dipole moments (0.5 < μ < 4.5), and measured the current density (J A/cm2) at low bias (±0.5V)

Summary

Introduction

This paper describes a physical‐organic study of the effect of uncharged, polar, functional groups on the rate of charge transport by tunneling across self‐assembled monolayer (SAM)‐based large‐area junctions of the form AgTSS(CH2)nM(CH2)mT//Ga2O3/EGaIn. This paper focuses on a specific physical‐organic question: Do molecular dipoles, when placed at the top‐ interface between a thin, electrically insulating organic film (a SAM) and a conducting top‐electrode, influence the rates of charge transport by tunneling?

Results

Conclusion