Synthesis and Assembly of Semiconductor Nanocrystals for Photoconversion in Display Applications

面向光轉化顯示器件應用的族半導體納米晶: 合成與組裝排列

Student thesis: Doctoral Thesis

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Award date10 Jul 2017

Abstract

Since their discovery in the 1980s, semiconductor nanocrystals, also termed quantum dots (QDs), have been extensively studied with the promise of commercial usability on the horizon. After more than twenty years of extensive research, several companies such as Sony, Samsung, LG, TCL and Sharp are now employing and exploring QD based technology, to improve current display performances. This successful integration is based on the exceptional optical properties of several QD materials, which are characterized by unprecedented control over the emission wavelength, paired with high photo¬luminescence quantum yields, narrow spectral linewidths and good stability. Other characteristics that have contributed to the success of these nanomaterials include their controllable synthesis, and variable surface chemistry, allowing to further process the materials into functional composites.

While only a selected number of materials have entered the market for display applications, there are still many opportunities that can be explored. In this regard, this work discusses two established systems of semiconductor nanocrystals, viz. aqueous CdTe QDs, and rod-shaped nanocrystals, regarding their application in display devices. After a general introduction and discussion of the main characteristics of colloidal QDs, this thesis summarizes the major synthetic strategies, and developments in the fabrication of QD composites, with an emphasis on II-VI semiconductor nanocrystals. Furthermore, it highlights the major contributions and future challenges for the development of QD based solid-state luminophores.

Based on this discussion, this work explores aqueous CdTe QDs as a low-cost alternative to commonly used organic based CdSe nanocrystals. The assembly of such QDs in an aqueous based polymer (polyurethane, PU) facilitates the fabrication of flexible and highly luminescent composite films. After characterization by various spectroscopic techniques, the films are further investigated regarding their photo-, and thermal stabilities. While the results for pure QD-PU composites show several deficiencies, it is demonstrated that additional integration of polyhedral oligomeric silsequioxane can improve their performance and suitability for photoconversion in display devices.

In addition to the challenges for spherical QDs, applications with rod-shaped nanocrystals require further consideration, due to the anisotropy, resulting in polarized absorption and emission, along the long axis of the nanocrystal. In order to retain these beneficial properties in ensembles and films, nanorods need to be oriented along one direction, which has been attempted by a number of different approaches. In this regard, this work explores the method of photoalignment, a technology, which has originally been developed for the alignment of liquid crystals. It is shown that the strategy facilitates the alignment of highly luminescent CdSe/CdS core/shell nanorods in liquid crystal polymer composites. The alignment is initiated by the irradiation of a photosensitive azo-dye, which transfers its alignment to the composite layer of nanorods and liquid crystal polymer. After polymerization and solidification of the composite films, optical and structural characterization reveals nanorod alignment with high order parameter, resulting in highly polarized emission. Additionally, photoalignment offers exceptional control over distinct alignment domains, which is presented by the fabrication of alignment patterns with domain sizes down to 2 µm. Apart from demonstrating the precise control of the method, the two-domain patterns exhibit azimuthal angular sensitivity, which is presented on both macroscopic and microscopic scale. In the last section of this work, photo-aligned composite films are investigated towards their application in nanorod enhancement films for photoconversion in LCD backlights. It is shown that the method matches the major requirements, as it enables the simultaneous alignment of mixed layers of nanorods, and also allows stacking of multiple layers. Overall, the results highlight the great potential of rod-shaped nanocrystals for display applications.