Reconstruction of the surface of freely suspended films of heptyloxybenzylidene heptylaniline

Surfaces of freely suspended thick films of 4-n-heptyloxybenzylidene-4-n-heptylaniline (7O.7) in the crystalline-B phase have been imaged using non-contact mode atomic force microscopy. Steps are observed on the surface of the film with a height of 3.0 $\pm$ 0.1 nm corresponding to the upright molecular length of 7O.7. In addition, we find that the step width varies with temperature between 56$^{\circ}$C and 59$^{\circ}$C. The steps are many times wider than the molecular length suggesting that the steps are not on the surface, but instead originate from edge dislocations in the interior. Using a strain model for liquid crystalline layers above an edge dislocation to estimate the depth of the dislocation, we estimate that the number of reconstructed surface layers decreases from 50 to 4 layers as the temperature increases from 56$^{\circ}$C to 59$^{\circ}$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 phase.