Abstract
This work examines charge transport (CT) through self-assembled monolayers (SAMs) of oligoglycines having an N-terminal cysteine group that anchors the molecule to a gold substrate, and demonstrate that CT is rapid (relative to SAMs of n-alkanethiolates). Comparisons of rates of charge transport-using junctions with the structure Au(TS)/SAM//Ga2O3/EGaIn (across these SAMs of oligoglycines, and across SAMs of a number of structurally and electronically related molecules) established that rates of charge tunneling along SAMs of oligoglycines are comparable to that along SAMs of oligophenyl groups (of comparable length). The mechanism of tunneling in oligoglycines is compatible with superexchange, and involves interactions among high-energy occupied orbitals in multiple, consecutive amide bonds, which may by separated by one to three methylene groups. This mechanistic conclusion is supported by density functional theory (DFT).
Highlights
Charge transport through self-assembled monolayers (SAMs) of oligoglycines (Glyn, n = 0-5) having a cysteine (Cys) group that anchors the molecule to a gold substrate is rapid
Simmons equation.[19]. This equation is a useful semi-empirical parameterization that suggests that the rate of tunneling should depend exponentially on the width of a barrier (d), assumed to be rectangular, and on β
We investigated charge transport (CT) across SAMs of six compositionally simple derivatives of oligoglycine (Fig. 1) with a terminal cysteine residue (Cys(Gly)n, n = 0-5), using a large-area junction having the structure AuTS/SAM//Ga2O3/EGaIn, where AuTS is a template-stripped gold surface.[18] (This junction has been described in previous papers.[18, 20, 25,26,27,28]) SAMs having oligoglycine residues gave values of β that are significantly lower than those measured for SAMs of nalkanethiolates of similar length
Summary
DFT modeling of gold-bound oligoglycines (below) indicates that the high-energy occupied orbitals of peptide groups, which modulate the local tunneling potential, are essential for rapid tunneling. We derive values of β and J0 (eq 1) for oligopeptides on gold (Cys(Gly)n, n = 0-5), and compare these values with those of n-alkanethiolates, to determine the influence of the differences in their structure on the rates of charge transport by tunneling across them.
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