Tunneling across SAMs Containing Oligophenyl Groups

Carleen M Bowers,Felice C Simeone,Tomasz Żaba,Dmitrij Rappoport,Mostafa Baghbanzadeh,George M Whitesides,Sergey N Semenov,Kung-Ching Liao,Piotr Cyganik,Alan Aspuru-Guzik

doi:10.1021/acs.jpcc.6b01253

Abstract

This paper reports rates of charge tunneling across self-assembled monolayers (SAMs) of compounds containing oligophenyl groups, supported on gold and silver, using Ga2O3/EGaIn as the top electrode. It compares the attenuation constant, β, and the pre-exponential parameter, J0, of the simplified Simmons equation across oligophenyl groups (R = Phn; n = 1, 2, 3) with three different anchoring groups (thiol, HSR; methanethiol, HSCH2R; and acetylene, HC≡CR) that attach R to the template-stripped gold or silver substrate. The results demonstrate that the structure of the molecular linker between the anchoring group (−S– or −C≡C−) and the oligophenyl moiety significantly influences the rate of charge transport. SAMs of SPhn and C≡CPhn on gold show similar values of β and log |J0| (β = 0.28 ± 0.03 A–1 and log |J0| = 2.7 ± 0.1 for Au/SPhn; β = 0.30 ± 0.02 A–1 and log |J0| = 3.0 ± 0.1 for Au/C≡CPhn). The introduction of a single intervening methylene (CH2) group between the anchoring sulfur atom and the aromatic u...

Highlights

We measured rates of charge transport across self-assembled monolayers (SAMs) of oligophenylthiols (M/SAMs of oligophenylthiolates (SPhn)), -methanethiols (M/SCH2Phn), and -acetylenes (M/C≡CPhn), where n = 1-3 and M = gold and silver metal electrodes (Figure 1)
The correlation between the structure of self-assembled monolayers (SAMs) containing n-alkyl groups and rates of charge tunneling in junctions of the form M/A(CH2)nT//Ga2O3/EGaIn (M = the metal substrate, A is the anchoring group, and T is the terminal group) is surprisingly straightforward: the length of the insulating -(CH2)n- groups, which present high tunneling barriers, largely controls the rate of charge transport
Our results indicate that SAMs of SAMs of oligophenylthiolates (SPhn) and C≡CPhn on gold, which are characterized by orbital overlap across the molecule, and between the molecule and the electrode,[17] have similar, low values of the attenuation factor β in the simplified Simmons equation[18] and the injection current, J0

Summary

Introduction

We measured rates of charge transport across SAMs of oligophenylthiols (M/SPhn), -methanethiols (M/SCH2Phn), and -acetylenes (M/C≡CPhn), where n = 1-3 and M = gold and silver metal electrodes (Figure 1). Our results indicate that SAMs of SPhn and C≡CPhn on gold, which are characterized by orbital overlap (conjugation) across the molecule, and between the molecule and the electrode,[17] have similar, low values of the attenuation factor β in the simplified Simmons equation[18] (eq 1) and the injection current, J0

Results

Conclusion