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 thiolterminated derivatives of oligo(ethylene glycol) (HS(CH 2 CH 2 O) n CH 3 ; HS(EG) n CH 3 ); these SAMs are positioned between gold bottom electrodes and Ga 2 O 3 /EGaIn top electrodes and are of the form: 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 (E MO = -6.8--7.2 eV), but more delocalized (due to interactions between orbitals on neighboring oxygen atoms), than the highest occupied molecular orbital (HOMO, E MO : ~-5.7 eV) localized on sulfur. Nonetheless, the existence of these high-energy, delocalized occupied orbitals, which are not present in analogous n-alkanethiols (E MO < -8.5 eV for orbitals associated with CH 2 ), rationalize the low value of β. SAMs of oligo(ethylene glycol)s (and of oligomers of glycine). SAMs based on S(EG) n CH 3 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) n CH 3 rests on the 3 possibility of disorder in the SAM, and a systematic discrepancy between different estimates of the thickness of these SAMs.4

3D hybrid networks of gold nanoparticles: mechanoresponsive electrical humidity sensors with on-demand performances

“…38 The found proportionality factor of our systems is significantly lower compared to what usually reported for alkanethiol chains (where β usually amounts to ~ 1.0 Å -1 ) and is in a very good agreement with the results recently published by Baghbanzadeh et al on vertical tunneling junction through thiolated OEGs. 39 The tunneling charge transport mechanism was also confirmed by performing electrical characterization of the devices at different temperatures. Electrical I-V measurements performed inside a cryostat at temperatures spanning from 80 to 300 K (ESI, Figure S5) revealed an extremely small dependency of the electrical resistance from the temperature.…”

A Single Atom Change Turns Insulating Saturated Wires into Molecular Conductors

“…where J0 is a pre-exponential factor and the tunnelling decay coefficient (β in Å -1 ) determines how quickly the measured current decays with d. 2,5,6,7 In this context, unsaturated molecules with conjugated π-bonds are usually thought of as "molecular conductors" with low values of β (0.1-0.4 Å -1 ) 2,5,8,9,10 and saturated molecular wires with localized σ-bonds provide "molecular insulators" with large values of β (0.8-1.2 Å -1 ) 2,5,7,11,12,13 . This rule of thumb stands in sharp contrast with the high tunnelling rates established for various biomolecules, 14, 15 molecular wires of oligo-peptides, 16,17 and oligosilanes 18 . These all have saturated molecular backbones yet they exhibit low values of β (0.1-0.5 Å -1 ), and support long-range tunnelling over remarkably large distances of up to tens of nanometres 15,19,20 .…”

Modified Superexchange Model for Electron Tunneling Across the Terminated Molecular Wire

“…For instance, in the (–CH 2 –) N alkane chain, the l s refers to the distance between the centers of neighboring C–C bonds. This is explained by the quantum mechanical calculations, which show that the distribution of electron density along the chain is such that this density is concentrated mainly on C‐C bonds, which can be attributed to as localized HOMOs (see also ref. ).…”

“…The use of tunnel and atomic force microscopes to study charge transport processes in molecular junctions, as well as to create molecular structures with specified conductive properties, has created numerous possibilities to utilize these structures as basic elements of molecular electronics, optoelectronics, and spintronics . The mechanism of formation of the coherent nonresonant interelectrode current in the molecular junction “left electrode − molecular wire − right electrode” (LWR system) where a molecular wire comprises a regular chain anchored to the electrodes through its terminal units, is important due to the fact that it is the terminated chains that form stable self assemble monolayer on the substrate.…”