Development of a Versatile, Stable Droplet‐reactor for High Temperature Nanocrystal Synthesis

Colloidal nanocrystals exhibit interesting and useful size-dependent nanoscale phenomena (e.g. tunable fluorescence in quantum dots and optical absorptions in gold and silver nanoparticles), large surface areas and can be used for a variety of high-tech applications. Care must be taken to produce nanocrystals with well defined size, shape and composition, however, as these parameters directly affect the properties of the colloidal ensemble. Microreactors offer superior control over reaction conditions relative to traditional bulk batch methods and as such offer an attractive route to nanoparticle production. Hence there have been over a hundred papers reporting microfluidic synthesis of nanocrystalline colloids since the first reports in 2002. The work described in this thesis focussed on extending and improving the microfluidic method. Continuous-flow reactors were used initially, however, deposition was found to be a pervading problem with its severity varying with the material being synthesised. To address this problem a new capillary-based droplet-flow reactor was developed in which droplets are generated by the direct injection of confluent reagent streams within a stream of immiscible carrier fluid and, subsequently, can be heated and optically characterised further downstream. The reactor produced stable, controllable droplet flow over a wide range of flow rates, with droplet volumes down to 30 nL, and proved to be highly effective: CdSe quantum dots were synthesised via a high-temperature pyrolytic synthesis with strong control over particle size and size distribution. Crucially, unlike previously reported high temperature droplet reactors, the reactor could be operated indefinitely, without any degradation of the device and minimal variation in the product seen during 24 hours continuous production. To emphasise the versatility and applicability of the droplet reactor, Ag, TiO2 and InP nanocrystals were synthesised (using both organic and aqueous syntheses) and an automatic optimisation routine to reduce size distribution was applied to CdSe quantum dot synthesis