Fluorescence correlation spectroscopy in thin films at reflecting substrates as a means to study nanoscale structure and dynamics at soft-matter interfaces

TL;DR

This study develops a fluorescence correlation spectroscopy (FCS) autocorrelation approach to analyze nanoscale structure and dynamics at soft-matter interfaces, revealing how oxide thickness influences liquid crystal reorientation and tracer diffusion.

Contribution

It introduces a novel autocorrelation function method for FCS in thin films at reflecting substrates, enabling detailed analysis of nanoscale interfacial dynamics.

Findings

Different structural reorientation of 8CB at interfaces depending on oxide thickness

Tracer diffusion in thin oxides aligns with hydrodynamic no-slip models

Thick oxides show significantly reduced interfacial tracer diffusion

Abstract

Structure and dynamics at soft-matter interfaces play an important role in nature and technical applications. Optical single-molecule investigations are non-invasive and capable to reveal heterogeneities at the nanoscale. In this work we develop an autocorrelation function (ACF) approach to retrieve tracer diffusion parameters obtained from fluorescence correlation spectroscopy (FCS) experiments in thin liquid films at reflecting substrates. This approach then is used to investigate structure and dynamics in 100 nm thick 8CB liquid crystal films on silicon wafers with five different oxide thicknesses. We find a different extension of the structural reorientation of 8CB at the solid-liquid interface for thin and for thick oxide. For the thin oxides, the perylenediimide tracer diffusion dynamics in general agrees with the hydrodynamic modeling using no-slip boundary conditions with only a small deviation close to the substrate, while a considerably stronger decrease of the interfacial tracer diffusion is found for the thick oxides.