Continuous flow platforms for the synthesis of high-quality semiconductor nanocrystals

Semiconductor nanocrystals are of great interest due to their unique optical and electronic properties that are intermediate between bulk semiconductors and molecules. These nanoparticles find use in various applications ranging from electronics (light emitting diodes, photovoltaics) to photocatalysis and biolabeling. Typically, these nanoparticles are produced via batch synthesis routes that suffer from various issues, including slow mixing, slow heating/cooling, and lack of batch-to-batch reproducibility. These issues escalate further when increasing the scale of the production, thereby hindering their application on a commercial scale. Continuous flow synthesis can be an alternative approach that may enable high throughput and superior control of particle size and quality. However, since its first application in the early 2000s, most of the literature remains focused on continuous flow synthesis of Cd-based dots. Recently, the use of Cadmium (Cd) has been banned for many applications owing to its toxicity. Therefore, there is an immediate need for robust continuous flow reactors that enable synthesis of high quality Cd-free semiconductor nanoparticles. The modular continuous flow reactor reported in this work enables multistep, high temperature (up to 750 °C), air-sensitive synthesis of semiconductor nanocrystals involving solid and/or viscous reactants. Additionally, the millifluidic dimensions of the reactor allow for high working flow rates (> 10 ml/min) that translate into a production rate of about 150 g/day of nanocrystals. This this configuration is well suited for scale-up. The developed continuous flow reactor is designed to achieve quick heating and cooling times (< 1 s), thereby providing superior control over reaction conditions compared to the level of control that can be achieved in conventional batch synthesis techniques. The flow reactor is composed of fracture-resistant material, stainless steel, which is compatible with a wide variety of solvents at high temperatures. Furthermore, the modular flow reactor allows for inline characterization of the product, through absorbance and fluorescence spectroscopy. To demonstrate the applicability of the modular continuous flow reactor, we used the reactor to synthesize multi-layered Cd-based core-shell dots, CdSe bipods and nanorods, ZnSe nanorods, and highly luminescent InP/ZnSeS core-shell dots. The need for superior size control, shape selectivity and high reproducibility has resulted in a shift from conventional batch synthesis techniques to alternate synthesis routes. In the wake of such tight requirements, continuous flow syntheses, especially those relying on microfluidics, have emerged as viable routes for the synthesis of high-quality semiconductor nanocrystals. In general, continuous flow syntheses provide higher control over reaction conditions, for example mixing and heating times. We started by identifying the right material of construction and fabrication technique for building a continuous flow reactor that could withstand high temperatures. Design and fabrication of a simple oil-bath based continuous flow reactor and its application to demonstrate proof-of-principle syntheses of multi-layered Cd-based core-shell nanocrystals is discussed in Chapter 2. Use of heating media such as oil or hot water limits the maximum temperature attained by the reactor and is not suited for scale-up. To obviate the use of oil as a heating medium, a new continuous flow reactor was developed that uses a solid-state heating technique. The reactor configuration was further modified and coupled with a Schlenk line to enable high temperature, air-sensitive synthesis of semiconductor nanocrystals. Chapter 3 describes the design, fabrication, and operation of the new continuous flow reactor setup to synthesize anisotropic semiconductor nanocrystals, both Cd-based (CdSe nanorods/bipods) and Cd-free (ZnSe nanorods). Next, an inline mixer and a second reactor were added to the setup to enable multistep synthesis of InP/ZnSeS dots. Furthermore, the reactor design was upgraded to minimize the residence time distribution effects by the effective use of static mixers inside the reactor modules. Two flow cells were installed downstream of the reactors to enable inline spectroscopic characterization of the product. The design, fabrication, and operation of this multistep reactor setup are discussed in Chapter 4. Effective and fast mixing is critical to obtain uniform nanocrystals. However, fast mixing comes at the expense of high pressure drop. To alleviate this problem, we designed a high-throughput millifluidic herringbone mixer which is discussed in Chapter 5. In summary, the modular continuous flow reactor developed here will pave the path for high-throughput synthesis of high-quality semiconductor nanocrystals. The described platform equipped with inline characterization capabilities can also be used to study the reaction kinetics of the aforementioned syntheses, which are not fully understood at present. Furthermore, the reactor also can be used for syntheses other than semiconductor nanocrystals, especially those that require stringent conditions, including high temperature, inert conditions, and fast mixing