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Title: Zenithal bistable display : avenues for improved performance
Author: Jones, Sophie Ann
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2019
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Four distinct, interconnected research projects were undertaken with the goal of improving performance in the zenithal bistable device with new display applications in mind. ZBD is a commercially successful bistable nematic liquid crystal device used in small scale retail sig- nage. Some of the physics of the device are as yet unexplored or at least unreported due to the commercial nature of the technology. In addition, there remains much room to improve upon the existing device by refining materials and structure. The device was examined and iterated upon through experimental and computational means. Test devices were fabricated through standard liquid crystal cell techniques, plus grating structure embossing and controlled surface treatment. New grating shapes were trialled, replicating from etched Si masters. Novel materials were introduced into high-performance display mixtures to lower operating voltages. Test samples of each: controlled surface treatment to the grating, new grating shapes, and new liquid crystal mixtures, were evaluated by their performance in bistable latching. Computational modelling software was used to examine, in detail, defect dynamics at the latching transitions. Controllable anchoring strength at the micron-scale grating was achieved through vapour-phase silane deposition, tuneable within 0.5 − 2 × 10−4 J/m2. This was found to be well correlated with molecular silane density at the surface through X-ray photo-electron spectroscopy. Control of the anchoring strength at the grating surface provides control over the device itself, with a direct impact on the latching voltages. The defect dynamics during latch were identified, including those dynamics responsible for unique latching behaviours found in experiments. These included a suppressed latch regime in continuous to defect latching, caused by the nucleation of further defect pairs; RMS latching, investigated through the removal of the flexoelectric terms in the modelling and compared to experiment; and the reverse latch, found while examining the speed of defect movement under electric field. These largely-computational results provide good correlation to experimental behaviour but are not numerically accu- rate. The addition of bent liquid crystal dimers to an existing display mixture indicated potential for lower operating voltages through flex- oelectric doping. Successful latching was performed from one bistable state to the other in sub-micron scale gratings, although the latch in reverse was not seen. Latching was only achieved on 400 nm pitch gratings, and not 300 nm. Full bistability remains a possibility at this scale should the appropriate steps be taken. The work of this thesis when taken together presents the zenithal bistable device as a technology with untapped potential. Fabrication methods for the device can be readily extended to other device structures for liquid crystals; physical phenomena can be intuited using the device as a defect playground; material developments can improve upon device performance; and new gratings at much lower scale represent a solution to optical losses which will open up new avenues for display and optical applications.
Supervisor: Jones, J. Cliff Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available