Use of Atomic Force Microscopy in the Determination of Image Contrast in Microtomed Samples of Thermotropic Liquid Crystals

Abstract

Atomic force microscopy (AFM) is used in conjunction with transmission electron microscopy (TEM) and scanning electron microscopy (SEM) to explain for the first time the origin of image contrast in TEM for cholesteric liquid crystals (LC). Samples investigated were side-chain thermotropic siloxane-based cholesteric liquid crystalline materials whose room temperature glassy phase slowed for easy quenching of the LC order. Microtomed samples were prepared by cutting parallel to the helical axis and therefore perpendicular to a thin film cross-section. These sections exhibited a series of alternating dark and light bands whose sizes are related to the periodic structure of the cholesteric helix. The surfaces of the microtomed sections show a corrugated topology whose repeat distance corresponds to one half the pitch. Peak-to-valley heights on the order of 200 A were observed. Image contrast in the TEM was observed to increase substantially with time and electron dose due to beam damage of the materials. Since ∆I/1 is directly related to ∆t/t, the increase in contrast can be due to general mass loss (decrease in t) or preferential mass loss (increase in ∆t). AFM was performed on both unirradiated samples and samples that were exposed to large electron doses to explain the contrast mechanism.