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. 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 natom-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(CH2)n = 0.94 ± 0.02 natom-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.
Highlights
Determining the relationship between the structure of the molecules that make up selfassembled monolayers (SAMs), and the shape of the tunneling barrier that these molecules generate, is an area of active research, and a part of the broader area of “molecular electronics”.1-14 Our studies have used a junction with the structure M/A-R-T//Ga2O3/EGaIn
Modeling of the oligo(ethylene glycol)s bound to clusters of gold with Density Functional Theory (DFT) predicts a characteristic set of high-lying, delocalized, occupied molecular orbitals, which result from a weak coupling between the lone-pair orbitals on neighboring oxygen atoms in the backbone, and which are absent in nalkanethiolates
We suggest a rationalization of this decrease based on high-energy, delocalized, occupied molecular orbitals (MOs) formed by superexchange interactions between neighboring oxygen lone-pair orbitals as being the probable origin of this effect
Summary
Determining the relationship between the structure of the molecules that make up selfassembled monolayers (SAMs), and the shape of the tunneling barrier that these molecules generate, is an area of active research, and a part of the broader area of “molecular electronics”.1-14 Our studies have used a junction with the structure M/A-R-T//Ga2O3/EGaIn We measured rates of charge transport across SAMs containing oligomers of ethylene glycol (HS(CH2CH2O)nCH3; HS(EG)nCH3), and compared these rates to rates across n-alkanethiolates matched in the number of atoms along the molecular backbone.
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