Semiconductor Nanoparticle-doped Liquid Crystal Materials, Devices, And Mechanistic Studies

Standing at the end of the first decade of 21st century, we are witnessing a small-medium LCD industry, which has been gradually accepted by the market. Technically speaking, low power consumption is a key performance. That means energy saving as well as a long working time for the LCD after battery recharging, which enhances the advantage of portability. The structure and principle of LCD are integrated complex topics. Multiple angles could work as the starting point to obtain an improved low power consumption feature, such as optimizing driver circuit pattern, using low-power consumption LED backlight, improving working mode of liquid crystal devices, and choosing liquid crystal materials with low-power features. This thesis focuses on liquid crystal materials doped with semiconductor nanoparticles, which means obtaining liquid crystal devices with low-power features via doping. This is definitely a shortcut to get twice the result with half the effort, compared with designing and synthesizing new liquid crystal materials.Semiconductor nanoparticles' doping in liquid crystal materials is another category of nanoparticles, which is different from what has been reported in past research works such as silicon dioxide nanoparticles, metal nanoparticles, metal oxide nanoparticles, ferroelectric nanoparticles and carbon nanotubes. Although semiconductor nanoparticles has been widely used in quite a lot of fields, given its unique features in electrics, optics and chemicals, there has been few researches on the usage in doping into liquid crystal materials. In this thesis, two typical semiconductor nanoparticles, i.e., CdS and ZnO, are used to dope into 5CB liquid crystal material. The doped material's physical performance, like phase transition temperature, order parameter and dielectric anisotropy, and electrooptical characteristics, like threshold voltage, response time and frequency modulation, have been measured. It is found that the semiconductor nanoparticles doped LC cells can significantly lower threshold voltage, and this is helpful to develop the LCDs with low power consumption.As the nanoparticles disperse uniformly between liquid crystal molecules in the doping system, the organics enwrapping them will anchor the liquid crystal molecules. While on the other hand, the liquid crystal molecules in the liquid crystal cell also have a trend to keep the original order. Therefore, the molecule director will show bending of directions around the nanoparticles, resulting in a decrease of order parameter and phase transition temperature, respectively. As for the electrooptical characteristics of the devices, the semiconductor nanoparticles can be regarded as a dielectric sphere as they are wrapped with organics, Polarization charge and polarization electric field will emerge with external electric field. When the direction of external AC electric field changes, the external electric field and the polarization electric field can work together to push the liquid crystal molecules to turn along with the electric field direction. As a result, a lower threshold voltage will be achieved. The combined efforts by both electric fields can also explain the device response time and other test results.Based on experiments and discussions, computer simulation technology is also introduced in related calculation. The thesis selected relatively simple simulation approach of iterative difference method to analyze the director distribution of liquid crystal molecules, based on which the transmittance of liquid crystal devices is calculated via the Jones matrix method. In the calculation process, the director distribution and electrooptical characteristics are recalculated taking into consideration the spatial structure model of the semiconductor nanoparticles doped in liquid crystal system and the electricity features of nanoparticles, and the respective results are compared. In the mean time, with the decrease of the order parameter of liquid crystal molecules and the thickness of liquid crystal cell, the impact from these two factors on threshold voltage has also been simulated. The simulation results are consistent with experiment findings, which further explain the system of semiconductor nanoparticles doped in liquid crystal materials.The innovation of this thesis can be recapitulated into three aspects. First of all, the low power consumption of doped liquid crystal displays is the most vital result of this research. That provides a new technology approach to meet the requirement of production and investigation of mobile liquid crystal devices. Secondly, the frequency modulation characteristic introduced by semiconductor nanoparticles doped liquid crystal devices is improved compared with metal nanoparticles doped ones. The devices can be drived by existing driving circuit and there is no need to design special frequency modulation driving circuit. Last but not least, the studies on the mechanism of doping system can not only explain the research results but also give a direction to the further researches.In summary, this thesis introduces the doping application in 5CB liquid crystal materials of two types of semiconductor nanoparticles. The research work involves synthesis of nanoparticles, doping with liquid crystal materials and measurements of physical performance of liquid crystal materials and electrooptical characteristics of liquid crys- tal devices, physical mechanism of liquid crystal system doped with nanoparticles and the computer simulation of electrooptical characteristics. This contributes to the significance of theoretical guidance for the research works on semiconductor nanoparticles' doping in liquid crystal materials and helps to fulfill the research framework of nanoparticles' doping in liquid crystal materials to some extent. The thesis is worthwhile in facilitating the application of this technology in practice, as well as in promoting the practical significance of technology development on liquid crystal materials.