Investigations of electric field assembly of colloidal rod shaped nanocrystals and their device applications

This thesis focuses on the assembly of anisotropic nanocrystals using an electric field assisted assembly method. The studies involve the precise control of electrophoretic deposition (EPD) in a non-polar solvent causing the elongated nanoparticles to rotate to align along the field direction and to organise at a substrate. Chapter 1 offers the introduction to the colloidal nanocrystals and the development of synthesis techniques and assembly approaches. The history of the conventional EPD method and its recent utilization on nanomaterials are also reviewed. The typical synthesis strategy and the EPD set-up of nanorods are detailed in Chapter 2 along with an overview of the characterization techniques used in this study. The shape of the nanorods and the asymmetrical atomic distribution, such as uniaxial crystallographic structure, are the main reasons causing their alignment in the electric field. The choices of the ligands with different functional groups not only dominate the crystal growth during synthesis but provide the nanocrystals with variable surface charges. The effects of all these parameters on the assembly formation under an electric field are systematically investigated with computational modelling in Chapter 3. Chapter 4 investigates the two-stage assembly of quaternary semiconductor Cu2ZnSnS4 nanorods at substrates using both self and directed methods. The nanorods preassemble into 2D discs by self-organisation, and these discs are deposited into conformal layers at substrates using the electric field. The intermediate self-assembly step can be eliminated by carrying out a judicious ligand exchange allowing selectivity for dimensional control of layer formation from nanorod building blocks in one two and three dimensions. Chapter 5 describes the preparation of supercrystal films consisting of highly ordered and vertically aligned CdSeS nanorod emitters which are regarded as one of the most promising laser materials due to their tunable and directional emission spectra, high damage threshold and low energy formation processes. The outstanding optical characteristics are further demonstrated by the occurrence of amplified spontaneous emission from a customized shaped microcavity. Chapter 6 investigates the sequential application of the EPD mechanism to both gold nanorods and semiconductor CdSexS1-x nanorods. The process results in the formation of the heterostructures across the three layers using just two deposition steps. This strategy includes the deposition and re-dispersion of the former materials at the substrates that is the primary reason for this unique structure to occur. Chapter 7 summarizes the conclusion gained from each chapter and offers suggestions for further research directions.