Non-equilibrium Roughness Evolution of Small Molecule Mixed Films Reflecting Equilibrium Phase Behavior

TL;DR

This study investigates how the surface roughness of small molecule mixed films evolves out of equilibrium and relates it to their equilibrium phase behavior, using X-ray reflectivity and simulations.

Contribution

It reveals the connection between non-equilibrium roughness evolution and equilibrium phase behavior in organic mixed films, supported by experimental and simulation data.

Findings

Roughness depends systematically on bulk phase behavior.

Lowered step edge barrier influences growth behavior.

Reduced in-plane crystallinity affects surface roughness.

Abstract

Understanding non-equilibrium phenomena, such as growth, and connecting them to equilibrium phase behavior is a major challenge, in particular for complex multicomponent materials. We use X-ray reflectivity to determine the surface roughness of binary mixtures of several prototypical organic compounds. By analyzing the roughness as a function of composition, we find a systematic behavior depending on the bulk phase behavior in terms of intermixing, co-crystallization or phase separation. Supported by kinetic Monte Carlo simulations, we provide evidence that the growth behavior can be rationalized by a lowered step edge barrier in the mixed films which is induced by reduced in-plane crystallinity.