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

This paper describes a physical-organic study of the effect of uncharged, polar, functional groups on the rate of charge transport by tunneling across selfassembled monolayer (SAM)-based large-area junctions of the form AgS(CH2)nM(CH2)mT//Ga2O3/EGaIn. Here Ag is a template-stripped silver substrate, -Mand -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. A central goal in the field of molecular electronics is to understand relationships between rates of charge transport and molecular structure. Using self-assembled monolayer (SAM)-based large-area junctions having the structure AgS(CH2)nM(CH2)mT//Ga2O3/EGaIn, where Ag TS is a templatestripped silver substrate and EGaIn is eutectic gallium−indium alloy, we explored the influence of the “terminal” group, T, and “middle” groups, M, of the SAM on the tunneling current. One current focus for our work is the importance of the two interfaces: Ag-SR and T//Ga2O3. 14 This paper focuses on the latter interface and examines the influence of the group T on the rate of charge transport. Our results (and those obtained using other types of junctions) have not led to a single, broad conclusion about this matter: a few groups T (e.g., ferrocene) seem to change the rate of charge transport (relative to a methyl group), but some do not (Table S1 in the Supporting Information). We have, perhaps surprisingly, observed that the tunneling current is insensitive (within the precision of our measurements) to the structures of a range of nonpolar terminal aromatic and aliphatic groups with different geometries and electronic structures. This paper focuses on a specific physical-organic question: Do molecular dipoles, particularly 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? To answer this question, we examined the electrical characteristics of SAM-based large-area junctions of the form AgS(CH2)nM(CH2)mT//Ga2O3/ EGaIn, where -Mis either -CONHor -CH2CH2(depending on which was synthetically more accessible; replacing -CH2CH2with -CONHdoes not influence tunneling current densities in these compounds). The interface, which exists between the top of the SAM and the rough Ga2O3 film that covers the EGaIn in the “conical tip” electrode, is a van der Waals contact. We systematically modified the terminal portion of the SAM with polar groups (-T, Figure 1) that are uncharged but have significant group dipole moments (0.5 < μ < 4.5), and measured the current density (J, A/cm) at low bias (±0.5 V). Received: September 23, 2013 Published: December 18, 2013 Figure 1. Molecules with polar terminal groups and n-alkanethiols (as standards) used to form SAMs. Communication