Using dislocations to probe surface reconstruction in thick freely suspended liquid crystalline films

Area of Science:

• Materials Science
• Condensed Matter Physics
• Physical Chemistry

Background:

• Surface interactions influence the phase behavior of liquid crystalline films.
• The behavior of thick liquid crystal films differs from thin films, with surface effects less understood.
• Edge dislocations serve as markers for the interface between bulk and reconstructed surface layers.

Purpose of the Study:

• To investigate the surface reconstruction in thick, freely suspended liquid crystalline films.
• To determine the depth of edge dislocations and quantify the number of reconstructed surface layers.
• To correlate surface phase behavior with temperature in 4-n-heptyloxybenzylidene-4-n-heptylaniline (7O.7) films.

Main Methods:

• Noncontact mode atomic force microscopy (AFM) was employed to image the surface topography.
• Edge dislocation depth was measured using AFM step height analysis.
• A strain model was applied to estimate the number of reconstructed surface layers.

Main Results:

• Steps of 3.0±0.1 nm height were observed, with width varying between 56 and 59°C.
• The number of reconstructed surface layers increased from 4 to 50 as temperature decreased from 59 to 56°C.
• This temperature-dependent reconstruction aligns with the known phase boundary in the film's thickness-dependent phase diagram.

Conclusions:

• The surface of thick liquid crystalline films exhibits significant reconstruction.
• The reconstructed surface layer is likely a smectic F phase, influenced by temperature.
• Surface reconstruction plays a critical role in the phase behavior of liquid crystalline films.