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-$4^{\prime }$-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 $32.2\pm 0.4°\text{C}$ 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.

Figure 1

Measured Stokes parameters $\Psi$ and $\Delta$ in degrees of a typical film of thickness $131.3\pm 0.8\text{nm}$ (a) below NA transition at $28°\text{C}$ and (b) above NA transition at $35°\text{C}$. For clarity, only every other point is shown. The solid lines show the fits obtained using the single layer model described in the text. The shift from $270°$ to $-90°$ in $\Delta$ seen at $\sim 450\text{nm}$ is due to the periodicity in the definition of the phase angle.

Figure 2

Rough sketch (a) of expected ordering within the liquid crystalline film above the NA transition based on Refs. [15, 17, 18]. The ordering of 8CB molecules in the (b) nematic and (c) smectic $A$ phases, showing why the anisotropy $\Delta n=n_z-n_x$ would be less in the nematic phase.

Figure 3

Anisotropy $\Delta n$ for a 120 nm film going up and then down in temperature, showing an even lower NA transition temperature when cooling from the nematic phase, possibly due to increased disorder in the film. Inset shows mean square error (MSE) in arb. units for the fit.

Figure 4

Results of fitting the ellipsometric data for a film of 131 nm coverage (film thickness at room temperature) as the temperature is increased: (a) $\Delta n$ at a wavelength of 632.8 nm, plotted as a function of temperature for the film in Fig. 1. (b) The total film thickness. The inset shows the MSE for the fit.

Figure 5

(Color online) Summary of experimental results on heating for all coverages studied. (a) The temperatures of the upper and lower kinks in $\Delta n$ and of the minimum of the dip in total thickness, plotted as a function of coverage. (b) The size of the step in $\Delta n$ versus coverage.

Figure 6

Phase diagram indicating the thin-thick coexistence region CR (shaded area bounded by dark lines) and the NA transition determined by the average of the temperatures of the upper and lower kink in the jump in $\Delta n$ (open circles with best fitted line). The isotropic, nematic, and smectic $A$ regions are labeled as $I$, $N$, and SmA. The two dashed lines about a degree below the NA transition and immediately below the coexistence region indicate the approximate temperatures when dark streaks and patches show on the surface (Fig. 7). The light shaded region straddling the NA transition line for coverages above 120 nm are where surface textures are observed on the surface (Fig. 8).

Figure 7

(Color online) Dark patches that appear on the surface of a film of coverage 95 nm $0.5°\text{C}$ below the film $T_{\text{NA}}$. Similar patches are observed for all coverages $0.4–1.0°\text{C}$ below the film $T_{\text{NA}}$. The illuminated area in the image is 1.5 mm across.

Figure 8

(Color online) Micrographs of films showing surface textures that are found to be closely associated with the NA transition for coverages greater than 120 nm, and which persist up to $1.2°\text{C}$ above the film $T_{\text{NA}}$. The illuminated area in each image is 1.5 mm across. The upper micrograph is for a 135 nm film at $0.1°\text{C}$ below $T_{\text{NA}}$, whereas the lower one is for a 270 nm film at $0.4°\text{C}$ above $T_{\text{NA}}$.