Optical-ellipsometric study of the nematic-to-smectic transition in 8CB films adsorbed on silicon.

The nematic-to-smectic-A (NA) transition in 8CB (4-octyl-4'-cyanobiphenyl) is especially interesting because experimentally, it has been observed to be second order, but theoretically, it has been predicted that it must have a latent heat. The effect on the NA transition due to confinement in an adsorbed film has hitherto not been investigated. Previous study of adsorbed 8CB films on silicon for coverages less than 100 nm showed the existence of a broad coexistence region, identified by the formation of thick and thin islands on the surface that extends between the bulk NA and the isotropic-to-nematic transition temperatures. In this paper, optical and ellipsometric measurements of 8CB films as a function of temperature are used to identify the location of the NA transition in the film in relation to the coexistence region. The NA transition temperature in the film is found to occur at 32.2+/-0.4 degrees C independent of film thickness for films between 62 to 270 nm thick, based on the decrease in the film anisotropy. This decrease in the anisotropy is found to be surprisingly abrupt. For thicknesses below 62 nm, the NA transition line is joined to the thin-thick coexistence region found previously.