Stretching of BDT-gold molecular junctions: thiol or thiolate termination?

TL;DR

This study challenges the assumption that thiol groups dissociate in gold molecular junctions, showing that thiol terminations are more stable than thiolates and providing corrected conductance estimates through advanced computational methods.

Contribution

It demonstrates that thiol groups do not dissociate upon junction formation and introduces a correction method for DFT-based conductance calculations considering image charge effects.

Findings

Thiol groups are energetically more stable than thiolates in junctions.

Dissociative adsorption of molecules on Au(111) is energetically unfavorable.

Corrected conductance values are lower after accounting for image charge effects.

Abstract

It is often assumed that the hydrogen atoms in the thiol groups of a benzene-1,4-dithiol dissociate when Au-benzene-1,4-dithiol-Au junctions are formed. We demonstrate, by stability and transport properties calculations, that this assumption can not be made. We show that the dissociative adsorption of methanethiol and benzene-1,4-dithiol molecules on a flat Au(111) surface is energetically unfavorable and that the activation barrier for this reaction is as high as 1 eV. For the molecule in the junction, our results show, for all electrode geometries studied, that the thiol junctions are energetically more stable than their thiolate counterparts. Due to the fact that density functional theory (DFT) within the local density approximation (LDA) underestimates the energy difference between the lowest unoccupied molecular orbital and the highest occupied molecular orbital by several electron-volts, and that it does not capture the renormalization of the energy levels due to the image charge effect, the conductance of the Au-benzene-1,4-dithiol-Au junctions is overestimated. After taking into account corrections due to image charge effects by means of constrained-DFT calculations and electrostatic classical models, we apply a scissor operator to correct the DFT energy levels positions, and calculate the transport properties of the thiol and thiolate molecular junctions as a function of the electrodes separation.