Cholesteric liquid crystals: Image contrast in the TEM

Bright-field image contrast of thermotropic cholesteric liquid crystalline materials in the transmission electron microscope (TEM) is investigated. Possible sources of contrast for these systems are discussed in terms of their molecular anisotropy. A cholesteric side-chain liquid crystalline compound was investigated with TEM, low voltage high resolution scanning electron microscopy (LVHRSEM), and atomic force microscopy (AFM) to determine the origin of the strong contrast observed in these systems using bright-field transmission electron microscopy. Initial contrast of thin microtomed sections, as viewed with TEM low dose techniques and an image intensifier, was much weaker than observed under normal viewing conditions. The periodic steady state contrast typically observed for these materials is the result of beam damage. Furthermore, the surface of microtomed samples (parallel to the cholesteric helical axis) is corrugated with a periodicity of 1/2 the pitch due to a preferred fracture path in the glassy cholesteric state. AFM profile analysis shows an average peak to valley height of approximately 20-25 nm. AFM of free surfaces from aligned films also indicates a corrugation with a periodicity equal to 1/2 the pitch with substantially smaller average corrugation depths. TEM indicates a series of +1/2 and -1/2 disclination lines at the surface due to a rotation of the preferred helix direction parallel to the surface, consistent with previously reported models.