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

TL;DR

This study uses atomic force microscopy to investigate how surface reconstruction in thick liquid crystalline films varies with temperature, revealing an increase in reconstructed layers as temperature decreases.

Contribution

It introduces a method to measure dislocation depths in thick liquid crystalline films, linking surface reconstruction to temperature-dependent phase behavior.

Findings

Reconstructed surface layers increase from 4 to 50 as temperature decreases.

Dislocation depths are approximately 3 nm below the surface.

Surface may be reconstructed into a smectic F phase.

Abstract

Surface interactions can cause freely suspended thin liquid crystalline films to form phases different from the bulk material, but it is not known what happens at the surface of thick films. Edge dislocations can be used as a marker for the boundary between the bulk center and the reconstructed surface. We use noncontact mode atomic force microscopy to determine the depth of edge dislocations below the surface of freely suspended thick films of 4-n-heptyloxybenzylidene-4-n-heptylaniline (7O.7) in the crystalline B phase. 3.0 +/- 0.1 nm high steps are found with a width that varies with temperature between 56 C and 59 C. Using a strain model for the profile of liquid crystalline layers above an edge dislocation to estimate the depth of the dislocation, we find that the number of reconstructed surface layers increases from 4 to 50 layers as the temperature decreases from 59 C to 56 C. This trend tracks the behavior of the phase boundary in the thickness dependent phase diagram of freely suspended films of 7O.7, suggesting that the surface may be reconstructed into a smectic F region.