Cholesteric liquid crystal devices with nanoparticle aggregation
Spotlight summary: Cholesteric liquid crystals (CLCs) have been intensively studied for their unique selective reflection characteristics, since they reflect only circularly polarized light coinciding with the LC helical rotation handedness. Because of their inherent Bragg reflection properties, their center wavelength is given by λ0 = <
In this paper, the authors have demonstrated an alternative new approach that achieves broadband CLC displays by using the aggregation of polyhedral oligomeric silsesquioxane (POSS) for multidomain formation. POSS aggregates are known for their capability of providing a stable homeotropic alignment of LC materials. In addition, owing to the hydrogen bond formed between hydroxyl compounds in the POSS side chain, clusters could be formed to create multidomain in CLC cells. A mixture of POSS nanoparticles and CLC material is first heated into isotropic phase and injected into cells without the conventional polyimide alignment material. By controlling the POSS concentration and cooling rate, one can adjust the POSS clusters and therefore the CLC multidomain properties. From the reported results, a low POSS concentration of less than 10% and a fast cooling rate of around 80¬oC/min are preferred to provide a small feature size for the clusters. By breaking the uniform planar helix structure into multidomains, the wavelength range of strong reflection could be expanded to range from 400 to greater than 600 nm. In addition, the light network between POSS clusters leads to relatively weak scattering, which results in a good dark state when the CLC cell is switched into the focal conic state. Further suppression of light leakage in the POSS-dispersed CLC cells in the black state and improvement of the device stability will be important for this new technique in the near future.
The POSS-dispersed CLC technology presented by the authors is really exciting owing to the following two major aspects: (1) it can lead to LC alignment by avoiding the conventional PI layer, which requires high temperature treatment, and (2) it can also lead to broadband/white reflection, providing users with a comfortable and natural reading experience. These properties could make it quite suitable for flexible bistable reflective CLC displays with good outdoor readability and low power consumption. It can be foreseen that extensive research interest and efforts will lead to further enhancement of the device’s performance.
Technical Division: Optoelectronics
ToC Category: Optical Devices
|OCIS Codes:||(160.3710) Materials : Liquid crystals|
|(160.4236) Materials : Nanomaterials|
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