Large Increase in the Dielectric Constant and Partial Loss of Coherence Increases Tunneling Rates across Molecular Wires.

Abstract

Although the dielectric behavior of monolayers is important in a large range of applications, its role in charge transport studies involving molecular junctions is largely ignored. This paper describes a large increase in the relative static dielectric constant (εr) by simply increasing the thickness of well-organized monolayers of oligoglycine and oligo(ethylene glycol) from 7 up to 14. The resulting large capacitance of 3.5-5.1 μF/cm2 is thickness-independent, which is highly attractive for field-effect transistor applications. This increase of εr results in a linear increase of the thermal activation energy by a factor of 6, which suggests that the mechanism of charge transport gradually changes from coherent to (partially) incoherent tunneling. The comparisons of oligoglycine (which readily forms hydrogen bonds with neighboring molecules) and methyl terminated oligo(ethylene glycol) (which lacks hydrogen bond donors) monolayers, kinetic isotope effects, and relative humidity-dependent measurements all indicate the importance of strong hydrogen bonds involving ionic species and strongly bonded water in the unusual dielectric behavior and the incoherent tunneling mechanism. This partial loss of coherence of the charge carriers can explain the unusually small tunneling decay coefficients across long molecular wires, and the length-dependent increase of εr of monolayers opens up interesting new applications.