Dynamic interfaces in an organic thin film

TL;DR

This study visualizes and measures real-time boundary fluctuations in organic thin films, revealing molecular exchange dynamics and the influence of substrate interactions on domain behavior using STM.

Contribution

It introduces a detailed real-time visualization and quantitative analysis of interfacial boundary fluctuations in organic thin films, linking molecular interactions to boundary dynamics.

Findings

Boundary fluctuations occur with 10-30 ms exchange times at room temperature.

Boundary shapes and fluctuations can be tuned from isotropic to anisotropic.

A multi-component lattice gas model explains the observed behaviors.

Abstract

Low-dimensional boundaries between phases and domains in organic thin films are important in charge transport and recombination. Here, fluctuations of interfacial boundaries in an organic thin film, acridine-9-carboxylic acid (ACA) on Ag(111), have been visualized in real time, and measured quantitatively, using Scanning Tunneling Microscopy. The boundaries fluctuate via molecular exchange with exchange time constants of 10-30 ms at room temperature, yielding length mode fluctuations that should yield characteristic f-1/2 signatures for frequencies less than ~100 Hz. Although ACA has highly anisotropic intermolecular interactions, it forms islands that are compact in shape with crystallographically distinct boundaries that have essentially identical thermodynamic and kinetic properties . The physical basis of the modified symmetry is shown to arise from significantly different substrate interactions induced by alternating orientations of successive molecules in the condensed phase. Incorporating this additional set of interactions in a lattice gas model leads to effective multi-component behavior, as in the Blume-Emery-Griffiths (BEG) model, and can straightforwardly reproduce the experimentally observed isotropic behavior. The general multi-component description allows the domain shapes and boundary fluctuations to be tuned from isotropic to highly anisotropic in terms of the balance between intermolecular interactions and molecule-substrate interactions. Key words: Organic thin film, fluctuations, STM, molecular interactions, diffusion kinetics, phase coexistence