Brian Weslowski

Orientation of pentacene films using surface alignment layers and its influence on thin-film transistor characteristics

We have investigated the effect of surface order on the orientation and mobility of pentacene. The surface order was created using monolayers and polymers that are normally used to align liquid crystals. Rubbed polyvinylalcohol layers were found to align approximately 27% of the pentacene grains within a 30° range. When introduced in a thin-film transistor, they were found to enhance the saturation current by a factor of 2.5. A mechanism for this enhancement is proposed.

Synthesis and photodimerization in self-assembled monolayers of 7-(8-trimethoxysilyloctyloxy)coumarin

We report a synthesis of 7-(8-trimethoxysilyloctyloxy)coumarin that can self-assemble into optically active monolayers on substrates. Atomic force microscopy and UV–vis spectroscopy have been used to characterize the self-assembly processes and photochemical reaction of the coumarin silane on substrates. We also show that when irradiated with linearly polarized UV light, the self-assembled coumarin monolayer can induce a homogeneous alignment of nematic liquid crystals.

Dynamics of the acousto‐optic effect in a nematic liquid crystal

In a nematic liquid crystal cell, the application of an ultrasonic wave induces a rotation of the director, leading to a change in the optical transmission through the cell. In this study, we investigate the dynamic response of the optical intensity after the ultrasonic wave is switched on or off. Our experiments show that the optical intensity follows a double‐exponential function of time, indicating that the system has two relaxation modes with widely different time scales. The fast mode has an amplitude and time scale qualitatively consistent with the dynamics of the Fréedericksz transition, but the slow mode shows novel behaviour associated with the acousto‐optic effect.

Acousto-optic response of nematic liquid crystals

The dependence of the acousto-optic response of two nematic liquid crystals on several possible future device parameters is investigated. Several paths for possible optimization are identified. A large enhancement of the acousto-optic response is obtained through optimization of the incident angle of the acoustic wave. The power required for a given cell to reach the maximum optical intensity is also reduced by adjusting the optical angle. This reduction comes at the expense of higher zero-field intensity, and therefore lowers the overall contrast. Finally, the acousto-optic response shows a significant dependence on liquid crystal layer thickness, scaling with the theoretically predicted fifth power of the cell thickness.

Surface Electroclinic Effect in a Smectic-A Liquid Crystal

Polar interactions between liquid crystal molecules and cell surfaces give rise to the surface electroclinic effect. This results in rotation of the layer normal away from the alignment direction near the surface and an interfacial region where the molecular director twists from the alignment direction until it reaches the layer normal direction. We have investigated the molecular orientation within the smectic layer using linear and circular dichroism spectroscopy. Contrary to theoretical predictions, the magnitude of the surface electroclinic tilt is relatively temperature independent, while the bulk electroclinic coefficient strongly depends on temperature.

Synthesis and characterization of laterally substituted bis(alkoxybenzoyloxy)hydroquinones

A new series of laterally substituted bis(alkoxybenzoyloxy)hydroquinone compounds has been synthesized and their mesomorphic properties studied. A number of hydroquinone compounds were synthesized with terminal n-alkoxy chains ranging from n-butyloxy to n-decyloxy. Additionally, lateral substituents ranging from n-butyl to n-octyl were incorporated through esterification at the remaining unsubstituted phenolic oxygen atoms. By optimizing the combination of the end group and lateral moieties we were able to tailor the molecular structure to form different liquid crystalline phases.

Advances in liquid crystals for gray-scale applications

Chiral smectic A electroclinic liquid crystalline materials are prime candidates for electrooptic applications because of their gray scale capability and fast dynamic response. The evolution of this technology greatly depends on the development of chiral smectic A materials with large induced tilt angles, field-independent fast switching times and a broad operating temperature range. We report in this paper the physical properties of a series of liquid crystal materials. Features of this series of materials are broad smectic A phase range and large induced tilt angles. Such large electroclinic coefficients make these materials good candidates of the development of silicon based reflective displays. Comparison of the electrooptic performance of the materials in SmA mesophase will be discussed.