Optical detection of heterogeneous single molecule diffusion in thin liquid crystal films

Area of Science:

• Materials Science
• Physical Chemistry
• Soft Matter Physics

Background:

• Liquid crystals (LCs) exhibit unique properties crucial for display technologies.
• Understanding molecular diffusion in thin LC films is vital for device performance.
• Previous studies focused on bulk LC self-diffusion, leaving thin-film dynamics less explored.

Purpose of the Study:

• To investigate the lateral diffusion of dye molecules in thin films of 8CB liquid crystals (LCs) in the smectic-A phase.
• To determine the influence of film thickness on molecular diffusion.
• To explore the underlying mechanisms causing diffusion heterogeneity.

Main Methods:

• Single-molecule tracking experiments were employed to monitor tracer diffusion.
• Fluorescence Correlation Spectroscopy (FCS) was utilized to analyze diffusion dynamics.
• Hydrodynamic modeling was used to interpret the effect of film thickness.

Main Results:

• Lateral diffusion of terrylene and perylene diimide dyes was observed in 4-225 nm thin films of 8CB.
• Diffusion dynamics spanned over an order of magnitude in time, exceeding bulk LC self-diffusion.
• Film thickness qualitatively affected tracer diffusion, consistent with hydrodynamic predictions.

Conclusions:

• Thin LC films exhibit more complex diffusion dynamics than bulk samples.
• Heterogeneous diffusion is likely caused by diffusion-limiting domains at the micrometer scale within films or at interfaces.
• Further research is needed to fully elucidate the role of interfaces and domain structures in LC film diffusion.