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Title: Gold nanoparticle liquid crystal composites : synthesis, characterisation and optical nonlinearities
Author: Acreman, Andrew
ISNI:       0000 0004 5356 1637
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2015
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This thesis presents a theoretical and experiment investigation into the optical properties of gold nanoparticle liquid crystal composites. The experimental work can be divided into four sections; the synthesis of the nanoparticles, the creation of the gold nanoparticle liquid crystal composites and the investigation into their electro-optical properties and the nonlinear properties of hybrid photoconductive cells. A large variety of gold nanoparticles were chemically synthesised with varying size distributions and functionalized. The samples functionalized with chemicals containing aromatic chemical groups were observed to aggregate. The optical properties of these aggregates were investigated by measuring their absorption and scattering efficiencies. The samples displayed a decrease in their molar absorption coefficient from 10 cm⁻¹μM⁻¹ to 4.61 cm⁻¹μM⁻¹ and an increase in the molar scattering coefficient by several orders of magnitude. Gold nanoparticles were also synthesised directly in the liquid crystal host by sputter doping. In this case it was found that the size of these particles could be increased by heating the host liquid crystal above the clearing temperature of the liquid crystal. The synthesised nanoparticles as well as samples from other groups were used to produce gold nanoparticles liquid crystal composites. With the exception of one sample functionalized with mesogenic compounds all of the samples displayed a solubility lower that the samples synthesised by sputtering indicating that, unless carefully selected, the thiol surfactants were detrimental to the solubility of the nanoparticles in a liquid crystal. Four samples, a pure liquid crystal and three doped with low concentrations (0.01% by weight) of gold nanoparticles were used to fill hybrid photoconductive cells. Two of the samples, made from comparatively larger, less soluble, nanoparticles displayed a dramatic increase in the nonlinearity of over an order of magnitude in comparison to the undoped liquid crystal. While previous work has illustrated that ferro-electric nanoparticles can lead to a similar increase in the nonlinearities, however these can be attributed to increases in the display parameters of the host liquid crystal. A electro-optic characteristics of the gold nanoparticle samples showed no such increase. The two samples which displayed an increase in the nonlinearity also displayed a substantial increase in the conductivity, and consistently it was concluded that this was the cause of the increase in the nonlinearity. The theoretical section of the thesis models thermal nonlinearities which could be observed in high concentration liquid crystal gold nanoparticle composites. By considering the attenuation of the pump and probe beams throughout the cell and the effect that this has on the thermal profile within the cell an excellent agreement with experimental data was achieved. The model further predicted that due to this attenuation there exists an optimum concentration of absorbents. The model was further extended to consider the effect of a magnetic field induced reorientation of the liquid crystal. This adaptation accounted for the thermal change in the diamagnetic anisotropy and elastic constants as well as the change in the thermal refractive index grating and absorption of the nanoparticles due to the reorientation of the liquid crystal. The reorientation caused a decrease in the magnitude of the refractive index grating from 9x10⁻⁵ cm²W⁻¹ to zero by the application of a magnetic field of the order of 0.01 Tesla. If the field was further increased the medium would switch from a self-defocusing to a self-focusing regime.
Supervisor: Kaczmarek, Malgosia Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics