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See detailALIGNED MULTI-WALL CARBON NANOTUBE SHEETS FOR LIQUID CRYSTAL DISPLAYS
Rahman, Md Asiqur UL

Doctoral thesis (2019)

The great interest in carbon nanotubes (CNTs) was triggered by their discovery by Iijima and has led to significant research efforts finding exceptional electrical and mechanical properties. The ... [more ▼]

The great interest in carbon nanotubes (CNTs) was triggered by their discovery by Iijima and has led to significant research efforts finding exceptional electrical and mechanical properties. The extraordinarily high anisotropy is not just limited to the shape of CNTs, but it is also reflected in their properties that show strong orientational dependence. However, a crucial step involves the incorporation of CNTs into macro-size devices while keeping the nanotubes perfectly aligned in a single direction with a high degree of nanotube straightness. It has been an additional challenge to produce CNT assemblies that meet all these requirements until Kaili Jiang et al introduced a solid state method to produce highly aligned multi-wall CNTs pulled from a forest. They reported that it is possible to pull continuous strings of nanotubes from vertically-aligned CNT forests, forming parallel arrays aligned along the pulling direction. Due to their high alignment, transparency, flexibility, conductivity and optical anisotropy, sheets formed by aligned CNTs are promising as optical polarizer, heaters, sensors, energy devices and aligning and electrodes layers in displays. The alignment of a liquid crystal (LC) on a CNT surface was first realized by Giusy Scalia et al. Later, Russel et al. showed that surfaces with aligned CNTs align LCs unidirectionally, followed by Fu et al. demonstrating that coated CNT sheets can also act as transparent electrodes for switching LC. Thus, aligned CNT sheets show promise as attractive multifunctional systems for LC displays, being able to simultaneously serve diverse functions by replacing both polyimide (PI) and indium tin oxide (ITO) layers, thus, minimizing costs and simplifying the fabrication process. The mechanical properties of CNTs offer also better performance than ITO when used on flexible substrates. However, the optical anisotropy of MWCNT sheets in the range of visible wavelengths remains almost unexplored. There is thus an urgent need to investigate and fundamentally understand the interaction of light with CNT sheets in order to accurately realize CNT-based liquid crystal optical devices. In LC displays, the modulation of light is based on the use of polarized light, and the introduction of an optically anisotropic layer can affect the modulation; thus, it is important to acquire fundamental knowledge on the interaction between aligned MWCNT sheets and light. We followed the technique reported by Kaili Jiang and Ray Baughman to produce highly aligned CNT sheets by pulling CNTs from a spinnable CNT forest. We further deposited the aligned CNT sheets on a glass substrate and characterized them in the visible wavelength range, finding that the aligned CNT sheets anisotropically absorb light. Furthermore, the linearly polarized light travelling through the CNT sheets is rotated and the polarization of the light is affected by the presence of even a single layer of CNTs. Moreover, the magnitude of rotation of polarization increases as the layer thickness increases. We performed theoretical investigations which closely fit the experimental data, suggests that the origin of the rotation is mainly due to the anisotropic absorption. However, other contributions, such as from birefringence, cannot be ruled out. By optical investigations, the dependence of the optical behavior on the thickness of CNTs was also established. Moreover, the average orientational order parameter of the CNT sheets was evaluated from the anisotropic absorption of aligned CNT sheets. A high value of orientational order parameter in CNT sheets is needed since the alignment of the CNT sheets translates to LC alignment. The order parameter of free-standing CNT sheets was found to be ~0.6; however, it decreases once deposited on a substrate. The adhesion between the CNT sheets and the substrate is an additional problem and was studied using different strategies correlating the adhesion to the final alignment of the CNTs on a substrate. Parts of this research effort were devoted to investigating CNT sheets on various polymer surfaces, leaving the surface of CNTs almost free from polymer, for a direct investigation of the LC alignment on the CNT graphitic surface. The general goal was to improve the adhesion while keeping the alignment of the CNTs intact as pulled from the forest. We found a tradeoff between the adhesion of the CNTs and their alignment on a substrate; however, achieving highly-ordered CNTs and perfect adhesion on the surface is an issue. A second approach was based on complete coverage of CNTs by coating the nanotube films with inorganic dielectric layers (SiO_2 or Al_2 O_3). We found that SiO_2 coating preserved the freely-suspended CNT alignment while improving the film flatness. These inorganic coatings help to obtain good electrical performance of LC in cells made with the CNT-based substrates. The alignment of the liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) in the cells was generally planar and unidirectional, with differences in the quality depending on the type of coating layer and on the value of the order parameter of the CNT sheets. We investigated both the uniformity of the LC alignment as well as the switching voltages and times and compared this to the performance of the LC in commercial cells. Integration of aligned CNTs with LC requires the understanding of the interactions of CNT layers with light to realize CNT devices. Aligned CNTs from forests can be obtained easily; sequentially depositing CNT layers, however, while maintaining control of the degree of alignment when integrating them into devices is an open issue. This work shows the occurrence of unexpected interactions with polarized light due to the intrinsic properties of CNTs and due to their alignment. By exploring and optimizing the optical performance of CNT sheets, through their orientational order, it can be possible to use them as optical films for producing, among other optical devices, variable rotation of polarization, polarizers and transparent electrodes that also can align LCs integrated into LCDs. [less ▲]

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