Inelastic Tunneling Spectroscopy of Gold-Thiol and Gold-Thiolate Interfaces in Molecular Junctions: The Role of Hydrogen

TL;DR

This paper uses inelastic tunneling spectroscopy (IETS) calculations to distinguish molecular junctions with different thiol hydrogen configurations and predicts how IETS can monitor the detachment of thiol hydrogen atoms during junction formation.

Contribution

It provides a theoretical framework showing how IETS can identify the presence or absence of thiol hydrogen atoms in gold-molecule junctions, offering insights into junction formation processes.

Findings

IETS can differentiate between molecules with no, one, or two thiol H atoms.

Most junctions in prior experiments likely had no thiol H atoms.

IETS can track the approach of an STM tip and detect thiol H detachment.

Abstract

It is widely believed that when a molecule with thiol (S-H) end groups bridges a pair of gold electrodes, the S atoms bond to the gold and the thiol H atoms detach from the molecule. However, little is known regarding the details of this process, its time scale, and whether molecules with and without thiol hydrogen atoms can coexist in molecular junctions. Here we explore theoretically how inelastic tunneling spectroscopy (IETS) can shed light on these issues. We present calculations of the geometries, low bias conductances and IETS of propanedithiol and propanedithiolate molecular junctions with gold electrodes. We show that IETS can distinguish between junctions with molecules having no, one or two thiol hydrogen atoms. We find that in most cases the single-molecule junctions in the IETS experiment of Hihath et al. [Nano Lett. 8, 1673 (2008)] had no thiol H atoms, but that a molecule with a single thiol H atom may have bridged their junction occasionally. We also consider the evolution of the IETS spectrum as a gold STM tip approaches the intact S-H group at the end of a molecule bound at its other end to a second electrode. We predict the frequency of a vibrational mode of the thiol H atom to increase by a factor \sim 2 as the gap between the tip and molecule narrows. Therefore, IETS should be able to track the approach of the tip towards the thiol group of the molecule and detect the detachment of the thiol H atom from the molecule when it occurs.