Synthesis and Surface Modification of Light-Emitting Colloidal Nanocrystals of Different Shapes, and Their Polarised Emission Characteristics

不同形貌的發光納米晶的合成、表面改性及其偏振發光應用

Student thesis: Doctoral Thesis

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Award date3 Sep 2019

Abstract

Since the discovery of nanomaterials, semiconductor nanocrystals (NCs), namely, quantum dots (QDs) have been ubiquitous in terms of display applications, after more than 30 years of scientific research and almost two decades of commercialization efforts. This, in turn, is attributed to the investments of industry branches, such as liquid crystal displays and light-emitting diode (LED) backlight units (BLUs), along with the progress in QD performance and production manufactured by universities, private and public companies. This commercialization on such a wide scale became possible due to the unique tunable optical and electrical properties of QDs, as dictated by their shape, size, and quantum confinement. During the initial stages, plenty of research was dedicated to the exploration of better chemical synthetic methods that resulted in higher emission efficiency and monodispersity. In the 2000s, this synthetic progress of improving photoluminescence quantum yield, color purity and stability catapulted the QDs to a new level of commercialization by companies such as Nanosys, Quantum Dot Corp, Evident Technologies, Nanoco, and QD Vision, among several others.

Despite the extensive research conducted about quantum dots, there is still room to identify new ways of improving the surface chemistry of NCs. To that end, this work undertakes a study on the improvement of optical properties of two well-developed colloidal systems of zero- and one-dimensional nanomaterials, referred to as quantum rods (QRs), regarding liquid crystal display application. After a chapter that introduces the main synthetic methods and characterization of major optical properties of QDs and QRs, this thesis compiled the results of surface passivation, improved optical properties and optimized strategy for fabrication of QRs-liquid crystal composite film.

In Chapter 2, the optical properties of CdTe QDs are investigated after surface treatment with ionic salts. In particular, CdTe QDs are synthesized by an aqueous synthetic method using thiol ligands. The effect of mixed ligands on the kinetic nanocrystal growth is also studied, and how ligands influence fluorescence quantum yield after surface passivation is also demonstrated. The QY% of QDs with thioglycolic acid as a ligand is dramatically enhanced through salt treatment from 33% to 75%, while for QDs capped with thioglycerol ligand, it decreased by almost twice, from 33% to 15%. A combination of these two ligands can be used to improve the outcome of the last-mentioned system. The final PL improvement factor via ionic salts is strongly predicated on the choice of ligand and does not necessarily depend on the PLQY of the parent QD solutions. Notably, the surface treatment of QDs with enhanced QY may improve their performance as composite films for liquid crystal display (LCD) applications.

In Chapter 3, fluorescence anisotropy and linear polarized emission of core/shell CdSe/CdS QRs are investigated along with its compatibility while making liquid crystal composite film. The QRs are then synthesized by the hot injection synthesis. Fluorescence anisotropy of CdSe/CdS QRs is studied as a function of seed size and rod aspect ratio. Due to the anisotropy of shape and local field factor, fluorescence anisotropy increases with a growing aspect ratio, which, in turn, increases linear polarized emission, along the c-axis of QRs. Due to their emission polarization properties, these QRs can be used to improve optical efficiency in backlight units of LCD. To achieve a good performance of QR in the film, they should be aligned in a single direction. The well-established method of liquid crystal (LC) photoalignment is applied, which is triggered by the initial photoalignment of azo-dye that helps align the composite system of LC and QRs. This macroscopic QRs photoalignment method has been introduced before. In order to achieve their uniform orientation in the composite film, it is essential to understand the formulation of liquid crystal (LC) monomers and photoluminescent CdSe/CdS QRs. According to our findings, the optimum concentration of LC monomers and CdSe/CdS QRs to achieve uniform homogenous orientation should be adjusted according to the lengths and aspect ratios of the QRs. To directly compare the unidirectional alignment of QR/LCP composite films, in-situ aligning dynamics was investigated. The inkjet printing is then applied to create an LCD prototype based on QREF.

In Chapter 4, the thesis reports the synthesis method of CdSe nanoplatelets NPLs, growing epitaxially in the quasi 2-D nanostructure CdSe NPLs, and via Hg2+ cation exchange, the HgSe/CdSe/HgSe quantum well nanoheterostructures were obtained. First, slow injection synthesis of CdSe NPLs of 3 monolayers (ML) is introduced, which results in a zinc-blende structure. Thereafter, colloidal atomic layer deposition (c-ALD) is applied for growing atomically precise layers on CdSe core, where nanoparticles should be transferred between polar and nonpolar solvents to grow the consequent layers of selenide and cadmium. Through the Hg2+ cation exchange that is conducted on thin CdSe NPLs, it is possible to extend the excitonic features of CdSe NPLs to the near-infrared range, which is advantageous for photodetection, with PL emission moved to 1200-1500 nm.

Conclusions and possible directions for future work are presented in Chapter 5.

    Research areas

  • Quantum dots, quantum rods, nanoplatelets, quantum yield, polarized emission, LCD