Type of Document Thesis Author Kirchhoff, Jennifer URN etd-04082010-143859 Title Investigations Into Complex Liquid Crystal Mixtures Degree Doctor of Philosophy Department Physics, Department of Advisory Committee
Advisor Name Title Linda Hirst Committee Co-Chair Peng Xiong Committee Co-Chair David Van Winkle Committee Member Mark Riley Committee Member Per Arne Rikvold Committee Member Joseph Schlenoff University Representative Keywords
- Liquid Crystals
- Quantum Dots
- Packing Mechanisms
- Electro-optical Properties
- X-ray Scattering
Date of Defense 2010-03-29 Availability unrestricted AbstractLiquid crystal phases exhibit physical characteristics that lie between those of liquid and crystal phases. The many liquid crystal sub-phases are defined based on the degree of positional and orientational ordering the molecules have and the materials that make up these liquid crystal phases. This thesis presents a study of the molecular packing and physical properties of complex liquid crystal phases using dopants to better examine the stability and packing mechanisms of these phases. It also looks at the dispersion of quantum dots in liquid crystal materials, examining the electro-optical properties of the mixtures. The main goal of this thesis is to examine the effects of dopants on the properties of liquid crystal phases using optical microscopy, differential scanning calorimetry, electro-optical measurements, and X-ray scattering. For those mixtures with quantum dots fluorescence microscopy and photoluminescence measurements were also conducted.
Rod-like liquid crystals are commonly used in display applications when the material is in a nematic liquid crystal phase, which is the least ordered phase exhibiting no positional ordering. The more complicated chiral smectic liquid crystal phases, which have a one dimensional layer structure, show potential for faster and tri-stable switching. A chiral rod-like liquid crystal material is doped with both chiral and achiral rod-like liquid crystals to examine the stability of one of the chiral smectic sub-phase, the SmC*FI1 phase. This phase consists of tilted molecules rotating about the cone defined by the tilt angle with a periodicity of three layers and an overall helical structure.
The SmC*FI1 phase is stabilized by the competition between antiferroelectric and ferroelectric interactions, and small amounts of the achiral dopant broadens the range of this phase by almost 5 ˚C. Higher dopant concentrations of the achiral material result in the destabilization of not just the SmC*FI1 phase but all tilted sub-phases. Small amounts of an opposite-handed chiral dopant narrow the phase range, with the phase completely destabilized around the same dopant concentration as for the achiral dopant. Besides the importance of optical purity for the stability of the SmC*FI1 phase it can be concluded that steric effects play an important role in the stability of this phase. This research indicates that the SmC*FI1 phase can be stabilized using dopants with molecular cores similar to those of the host molecules, but with shorter terminal chains.
Bent-core molecules have become a focus of research due to their potential for exhibiting biaxial nematic phases and because certain bent-core phases have useful switching properties. A bent-core liquid crystal material that exhibits a monotropic B1 phase can be induced into a switching phase with the application of an electric field less than 10 V/μm. From X-ray and electro-optical measurements the switching phase is believed to be the B2 phase, and once induced this phase exhibits an almost threshold-less switching.
Since the B1 phase exhibited is monotropic it does crystallize over time, though stabilization of the B1 phase against crystallization can be achieved by adding small amounts of a chiral rod-like liquid crystal dopant. The dopant stabilizes the B1 phase at the expense of the switching phase, increasing the threshold electric field needed to induce the phase transition to the switching phase and completely destabilizing the low-field switching. At slightly higher dopant concentrations the B1 phase is no longer favored due to unfavorable core-terminal chain interactions and a smectic phase is exhibited. This study indicates that the B1 phase can be stabilized by dopant molecules with similar cores to the host molecules, but with small terminal chains.
Quantum dots have potential in many applications, some of which involve the electronic coupling of quantum dots. This coupling occurs when quantum dots are assembled close together, and these assemblies can be directed by using the partial ordering of rod-like liquid crystal molecules. At high concentrations of quantum dots the dots tend to cluster together, forming large aggregates and not uniformly dispersing. These clusters in turn disrupt the liquid crystal texture, creating more defects. At lower concentrations the quantum dots can disperse in a chiral nematic liquid crystal phase in a uniform manner. The addition of quantum dots lowers the threshold electric field needed for a reorientation of the liquid crystal molecules in the chiral nematic phase. We can conclude that the quantum dots act as impurities in the liquid crystal mixtures, with the distribution of the dots dictated by the size, concentration, capping agent, and mixing time of the dots and also by the formation of the liquid crystal texture.
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