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

This paper describes charge transport by tunneling across self-assembled monolayers of thiolterminated derivatives of oligo(ethylene glycol) 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

Mostafa Baghbanzadeh

2017

Scholarcy highlights

  • Determining the relationship between the structure of the molecules that make up selfassembled monolayers, 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 (where “A” is the group anchoring the molecules of the SAM to the metal “M”, “R” is the backbone of the molecule making up the SAM, “T” is the terminal group, and EGaIn is eutectic alloy of gallium and indium
  • This study focuses on the influence of the energy levels of backbone substituents with highlying occupied orbitals on the height of the tunneling barrier and rates of charge transport
  • Modeling of the oligo(ethylene glycol)s bound to clusters of gold with Density Functional Theory 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 formed by superexchange interactions between neighboring oxygen lone-pair orbitals as being the probable origin of this effect
  • The notable differences in the computed molecular orbital structures are due to oxygen lone-pair orbitals, which give rise to a set of high-energy occupied MOs delocalized over the oligoethylene glycol backbone
  • The oxygen atoms in the backbone interact in a way that result in high-lying, delocalized molecular orbitals at ~-7 eV that arise from superexchange coupling between neighboring lone-pair orbitals on oxygen. The strength of the interactions between nearest oxygen atoms in oligoethylene glycols and oligo(1,3-propanediol)s is indistinguishable; this observation suggests that similar measurements in rates of charge transport would be observed
  • The strength of the interactions between nearest oxygen atoms calculated using the superexchange model rationalizes the high tunneling conductivity observed for oligoglycines. Charge tunneling across SAMs of oligoglycine is comparable to SAMs of oligophenyls, and correlates with the presence of high-lying, delocalized, occupied orbitals formed from the π orbitals of the peptide bonds

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