Anomalously Rapid Tunneling: Charge Transport across Self-Assembled Monolayers of Oligo(ethylene glycol)

Abstract

This paper describes charge transport by tunneling across self-assembled monolayers (SAMs) of thiol-terminated derivatives of oligo(ethylene glycol) (HS(CH2CH2O)nCH3; HS(EG)nCH3); these SAMs are positioned between gold bottom electrodes and Ga2O3/EGaIn top electrodes and are of the form: AuTS/S(EG)nCH3//Ga2O3/EGaIn. Comparison of the attenuation factor (β of the simplified Simmons equation) across these SAMs with the corresponding value obtained with length–matched SAMs of n-alkanethiols demonstrates, surprisingly, that SAMs of oligoethylene glycol have values of β (β = 0.29 ± 0.02 natom-1 and β = 0.24 ± 0.01 Å-1) lower than those of SAMs of n-alkanethiolates (β = 0.94 ± 0.02 natom-1 and β = 0.77 ± 0.03 Å-1). The value of β for tunneling across oligoethylene glycols is comparable to that across oligophenylenes (β = 0.28 ± 0.03 Å-1). There are two possible origins for the unexpectedly low value of β for (EG)n-derived SAMs. The more probable involves a mechanism for tunneling based on the superexchange model. This model accounts for the rapid hole tunneling across SAMs of oligo(ethylene glycol)s using interactions among the high-energy, occupied orbitals associated with the lone-pair electrons on oxygen. According to calculations using density functional theory (DFT), these orbitals—localized orbitals predominately on the backbone oxygen atoms—are lower in energy (EMO = -6.8– -7.2 eV), but more delocalized (due to interactions between orbitals on neighboring oxygen atoms), than the highest occupied molecular orbital (HOMO, EMO : ~ -5.7 eV) localized on sulfur. Nonetheless, the existence of these high-energy, delocalized occupied orbitals, which are not present in analogous n-alkanethiols (EMO < -8.5 eV for orbitals associated with CH2), rationalize the low value of β. SAMs of oligo(ethylene glycol)s (and of oligomers of glycine). SAMs based on S(EG)nCH3 are, in this mechanism, good conductors (by hole tunneling), but good insulators (by electron and/or hole drift conduction)—an unexpected observation that suggests SAMs derived from these or electronically similar molecules as a new class of electronic materials. A second but less probable mechanism for this unexpectedly low value of β for SAMs of S(EG)nCH3 rests on the possibility of disorder in the SAM, and a systematic discrepancy between different estimates of the thickness of these SAMs.