Surface single-molecule dynamics controlled by entropy at low temperatures

TL;DR

This study demonstrates that entropy significantly influences the transition dynamics of single molecules on surfaces at low temperatures, and that precise tip positioning in STM can control these rates by exploiting enthalpy-entropy compensation.

Contribution

It introduces a method to account for entropy effects in single-molecule surface dynamics using STM, revealing a controllable enthalpy-entropy compensation mechanism.

Findings

Tip position affects transition barrier and attempt rate.

Entropy plays a crucial role in molecular transition rates.

STM can manipulate transition dynamics by positioning the tip.

Abstract

Configuration transitions of individual molecules and atoms on surfaces are traditionally described with energy barriers and attempt rates using an Arrhenius law. This approach yields consistent energy barrier values, but also attempt rates orders of magnitude below expected oscillation frequencies of particles in the meta-stable state. Moreover, even for identical systems, the measurements can yield values differing from each other by orders of magnitude. Using low temperature scanning tunnelling microscopy (STM) to measure an individual dibutyl-sulfide molecule (DBS) on Au(111), we show that we can avoid these apparent inconsistencies if we account for the relative position of tip apex and molecule with accuracy of a fraction of the molecule size. Altering the tip position on that scale modifies the transition's barrier and attempt rate in a highly correlated fashion, which on account of the relation between the latter and entropy results in a single-molecular enthalpy-entropy compensation. By appropriately positioning the tip apex the STM tip can be used to select the operating point on the compensation line and modify the transition rates. The results highlight the need to consider entropy in transition rates of a single molecule, even at temperatures where entropy effects are usually neglected.