Fluid-Dispersed 2D Material Composites for Integrated Optoelectronic and Photonic Devices

Abstract

In this thesis, fluid dispersed two-dimensional (2D) material composites with the potential for integration with optoelectronic and photonic devices are looked at. By considering the existing literature, three key areas that require further development are identified. Firstly, new materials with improved properties (faster switching, facile reconfigurability, etc.) are required to enable the development of new devices with improved performance over the state-of-the-art. Fluid dispersed 2D material composites are then synthesised by two routes: dispersing 2D materials in a nematic liquid crystal host, and using 2D materials dispersed in an organic solvent as the mesogens of a liquid crystal phase. By dispersing 2D materials such as graphene, graphene oxide, and transition metal dichalcogenides in a nematic liquid crystal, one can take advantage of the orientational reconfigurability of the liquid crystal to controllably reorient the dispersed 2D materials. The properties of the fluid composites are also investigated – namely the switching time, threshold voltage, critical temperature and dichroism properties. Switching times, threshold voltages, and critical temperatures were found to compare favourably to the pure liquid crystal independent of the dispersed 2D material. Dichroism measurements were inconclusive, but demonstrate possible suppression or enhancement of the dichroism of the pure liquid crystal under the correct 2D material doping conditions. Liquid crystals based on tungsten disulfide dispersed in organic solvents by a scalable liquid phase exfoliation method were also synthesised. The particle sizes were analysed by scanning electron microscopy, atomic force microscopy, optical microscopy, Raman spectroscopy, and dynamic light scattering. Dynamic light scattering results are particularly promising as this technique has not previously been successfully applied to the determination of sizes of high aspect ratio particles. The linear and circular dichroic properties of the tungsten disulfide liquid crystals were also considered. Particularly interesting is the emergence under applied magnetic field of circular dichroism, suggesting a helical alignment of the dispersed 2D material particles. Secondly, a new characterisation technique to analyse particle positions using Raman spectroscopy was developed. Microfluidic structures were designed to maximise the Raman signal intensity for the different Raman bands of 2D materials by numerically analysing the expected Raman intensity via a scattering matrix method. The enhancement was experimentally verified. It is then shown that integrated 2D material particles can be dynamically controlled through the application of electric field, or by the use of a laser. How the Raman signal varies as a function of the particle position within the microfluidic cavity is considered. It is demonstrated that this variation can be used, by combining numerical analysis with experimental data, to track particles first in one dimension and then in two dimensions. A discussion of how three-dimensional tracking would be achieved is also presented. Consideration of how the ii particle alignment angle could further affect the Raman signal is included. The effect of having arrays of multiple particles, rather than a single particle, is also considered. The laser focus position is shown to affect only the intensities of the Raman signal, rather than the ratios between the intensities of the different peaks. Further analysis shows how the shape and size of the particle can affect the Raman signal intensity. A technique is presented to extract the concentration of fluid dispersed particles from Raman spectra, by considering only the Raman bands of the fluid. Finally, the possible applications of 2D material fluid composites are looked at, focusing on optoelectronic and photonic devices. Highly uniform thin films are produced from tungsten disulfide liquid crystalline dispersions. These films are then transferred to a variety of substrates. The terahertz regime performance of the tungsten disulfide thin films shows great promise for their future use in terahertz generation and detection applications. Unstable Q-switched lasing operation can also be observed for thin films transferred to silver mirrors and used to form a laser cavity. Methods for how the thin films can be integrated with photonic structures such as waveguides and microring resonators are discussed. Analysis is presented for how the integration of graphene or transition metal dichalcogenides on top of the photonic structures can affect their resonance and absorption properties, depending on the film area and thickness. The future prospects for the application of these materials are also outlined.