Finite-bias electronic transport of molecules in water solution

TL;DR

This study investigates how water affects electronic transport in molecular junctions, revealing water's electrostatic gating effect on polar molecules but negligible impact on hydrophobic carbon nanotubes, with implications for bio-sensing and microscopy.

Contribution

It demonstrates water's electrostatic gating influence on molecular junctions and shows carbon nanotubes' robustness in wet environments using combined molecular dynamics and first-principles calculations.

Findings

Water causes electrostatic gating in polar molecules like benzene-dithiol.

Carbon nanotubes are unaffected by water due to their hydrophobic nature.

Transmission spectra shift significantly for polar molecules but not for CNTs in wet conditions.

Abstract

The effects of water wetting conditions on the transport properties of molecular nano-junctions are investigated theoretically by using a combination of classical molecular dynamics and first principles electronic transport calculations. These are at the level of the non-equilibrium Green's function method implemented for self-interaction corrected density functional theory. We find that water effectively produces electrostatic gating to the molecular junction, with a gating potential determined by the time-averaged water dipole field. Such a field is large for the polar benzene-dithiol molecule, resulting in a transmission spectrum shifted by about 0.6 eV with respect to that of the dry junction. The situation is drastically different for carbon nanotubes (CNTs). In fact, because of their hydro-phobic nature the gating is almost negligible, so that the average transmission spectrum of wet Au/CNT/Au junctions is essentially the same as that in dry conditions. This suggests that CNTs can be used as molecular interconnects also in water-wet situations, for instance as tips for scanning tunnel microscopy in solution or in biological sensors.