Lawrence Berkeley National Laboratory

Microfluidic reactors are investigated as a mechanism to control the growth of semiconductor nanocrystals and characterize the structural evolution of colloidal quantum dots. Due to their short diffusion lengths, low thermal masses, and predictable fluid dynamics, microfluidic devices can be used to quickly and reproducibly alter reaction conditions such as concentration, temperature, and reaction time, while allowing for rapid reagent mixing and product characterization. These features are particularly useful for colloidal nanocrystal reactions, which scale poorly and are difficult to control and characterize in bulk fluids. To demonstrate the capabilities of nanoparticle microreactors, a size series of spherical CdSe nanocrystals was synthesized at high temperature in a continuous-flow, microfabricated glass reactor. Nanocrystal diameters are reproducibly controlled by systematically altering reaction parameters such as the temperature, concentration, and reaction time. Microreactors with finer control over temperature and reagent mixing were designed to synthesize nanoparticles of different shapes, such as rods, tetrapods, and hollow shells. The two major challenges observed with continuous flow reactors are the deposition of particles on channel walls and the broad distribution of residence times that result from laminar flow. To alleviate these problems, I designed and fabricated liquid-liquid segmented flow microreactors in which the reaction precursors are encapsulated in flowing droplets suspended in an immiscible carrier fluid. The synthesis of CdSe nanocrystals in such microreactors exhibited reduced deposition and residence time distributions while enabling the rapid screening a series of samples isolated in nL droplets. Microfluidic reactors were also designed to modify the composition of existing nanocrystals and characterize the kinetics of such reactions. The millisecond kinetics of the CdSe-to-Ag2Se nanocrystal cation exchange reaction are measured in situ with micro-X-ray Absorption Spectroscopy in silicon microreactors specifically designed for rapid mixing and time-resolved X-ray spectroscopy. These results demonstrate that microreactors are valuable for controlling and characterizing a wide range of reactions in nL volumes even when nanoscale particles, high temperatures, caustic reagents, and rapid time scales are involved. These experiments provide the foundation for future microfluidic investigations into the mechanisms of nanocrystal growth, crystal phase evolution, and heterostructure assembly.