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

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. Comparison of the attenuation factor (β of the simplified Simmons equation) across these SAMs with the corresponding value obtained with length-matched SAMs of oligophenyls (HS(Ph)_nH) and n-alkanethiols (HS(CH₂)_nH) demonstrates that SAMs of oligo(ethylene glycol) have values of β (β_(EG)n = 0.29 ± 0.02 n_atom^-1 and β = 0.24 ± 0.01 Å^-1) indistinguishable from values for SAMs of oligophenyls (β_(Ph)n = 0.28 ± 0.03 Å^-1), and significantly lower than those of SAMs of n-alkanethiolates (β_(CH = 0.94 ± 0.02 n_atom^-1 and 0.77 ± 0.03 Å^-1). There are two possible origins for this low value of β. The more probable involves hole tunneling by superexchange, which rationalizes the weak dependence of the rate of charge transport on the length of the molecules of HS(EG)_nCH₃ using interactions among the high-energy, occupied orbitals associated with the lone-pair electrons on oxygen. Based on this mechanism, SAMs of oligo(ethylene glycol)s are good conductors (by hole tunneling) but good insulators (by electron and/or hole drift conduction). This observation suggests SAMs derived from these or electronically similar molecules are 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.