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Title: Characterisation of low symmetry liquid crystal structures
Author: Stevenson, Warren
ISNI:       0000 0004 6422 1582
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2017
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Chirality in the bulk Ntb phase was confirmed directly by resonant X-ray scattering of 'hard' X-rays in a selenium-labelled binary mixture of bent liquid crystal (LC) dimers. Furthermore the high orientation of the resonant Bragg spots, corresponding to the pitch length of a well aligned helix, enabled estimation of the coherent Ntb domain size. Ntb phase chirality was also confirmed by circular dichroism measurements in various pure LC dimer compounds. In addition to resonant Bragg spots, non-resonant broad peaks at larger scattering angles were observed. These are attributed to localised cybotactic layering of the mesogens, which was found to depend on spacer length and mesogen composition. To account for simultaneous observation of sharp and broad X-ray peaks, a new two component orientational order parameter was introduced, which can be deconvoluted if the conical angle is known or if near perfect helical order is achieved. Based on these findings, a more quantitative molecular level model of the Ntb phase was geometrically constructed. In order to follow the local director the dimers or polymer chains adopt an average conformation resembling a helix (helical segment in dimer case). The helical conformation was supported by the findings of polarised infrared spectroscopy in the Ntb phase of a main-chain LC copolyether. Experimental evidence suggests that the Ntb phase is very similar in both polymer and dimer sample types. Throughout the project several banana phases in bent LC dimers were observed and characterised, some of which are new or previously misidentified. In the case of the misidentified phase, an alternative molecular level model was proposed based on polarisation splay of the molecules. A LC self-assembling 3D array of dendron functionalised CdS quantum dots (QDs) was investigated by a variety of experimental techniques including grazing incidence X-ray scattering and fluorescence spectroscopy. The structure was found to possess very low cubic symmetry and represents the lowest symmetry QD array to date. The strange fluorescence quenching properties of the material were found to be dependent on the annealing temperature, hence the degree of positional order among QDs inside the array.
Supervisor: Ungar, Goran ; Zeng, Xiangbing Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available