Kinetic trapping of charge-transfer molecules at metal interfaces

TL;DR

This study reveals that kinetically hindered phase transitions prevent the formation of thermodynamically stable upright-standing molecular phases at metal interfaces, explaining their rare experimental observation.

Contribution

It demonstrates through kinetic Monte Carlo simulations that lower-density phases form first and that phase transitions are kinetically hindered, highlighting the importance of growth conditions.

Findings

Lower-density phases form initially during adsorption.

Kinetic barriers prevent transition to stable upright phases.

Transition times depend mainly on deposition rate.

Abstract

Despite the common expectation that conjugated organic molecules on metals tend to adsorb in a flat-lying wetting layer, several recent studies have found strong indications for coverage-dependent transitions to upright-standing phases, which exhibit notably different physical properties. In this work, we argue that from an energetic perspective, thermodynamically stable upright-standing phases may be more common than hitherto thought. However, for kinetic reasons this phase may often not be observed experimentally. Indeed, using first principles kinetic Monte Carlo simulations, we find that the structure with lower molecular density is (almost) always formed first, reminiscent of Ostwalds rule of stages. The phase transitions to the thermodynamically stable upright-standing phase are likely to be kinetically hindered under conditions typically used in surface science (gas phase adsorption at low flux). This provides a possible explanation why they are commonly not observed. Investigating both the role of the growth conditions and the energetics of the interface, we find that the time for the phase transition is determined mostly by the deposition rate and, thus, mostly independent of the nature of the molecule.