Experimental and Theoretical Investigation of Indium Phosphide Quantum Dot Growth Mechanisms

Indium phosphide quantum dots stand out as the most promising candidate to replace the currently commercialized cadmium-containing materials for optoelectronic applications. To produce high-quality materials, significant efforts have been devoted to their synthetic development and growth mechanism studies. This thesis uses experimental and theoretical methods to study the growth of indium phosphide quantum dots from precursor conversion to final nanocrystal formation. As the key experimental platform, a high temperature and high pressure microfluidic system was first applied to study the effect of group V precursor reactivity on the quantum dot growth. High-pressure flow conditions allow for precise control of synthetic parameters and also the use of low-boiling-point solvents for synthesis with enhanced mixing. Results showed that lowering the precursor reactivity did not significantly improve the quantum dot quality, contradicting the current hypothesis. The unexpected role of precursor chemistry motivated investigation into the early-stage quantum dot growth mechanisms. First principles approaches were used without any prior assumptions on reaction pathways. Simulations showed that small clusters with indium-rich surfaces form in the early-stage quantum dot growth. Indium and phosphorus precursors have different roles, with phosphorus precursors controlling the reaction energy, and indium precursors determining the reaction barrier. With clusters identified as important growth intermediates in both simulations and experiments, their role during the quantum dot formation was then investigated with a one-solvent protocol, which combined flow synthesis, GPC purification and MALDI mass characterization. Experiments revealed that similar clusters exist during the late-stage nanocrystal growth, suggesting their role as a continuous supply for the quantum dot formation. Lastly, a quantum dot size tuning strategy was developed involving the use of weakly associated ligands to synthesize cluster-free indium phosphide quantum dots with different sizes and narrow size distributions. This synthetic approach enabled the construction of a correlation between the absorption features and the mass and concentration of indium phosphide quantum dots. The importance of the quality control of indium precursors became apparent after exploring effects of impurities and solvents. For example, when water and hydroxide/oxide species contaminate indium precursors, the growth of indium phosphide quantum dots are inhibited and batch-to-batch variations are observed. Thesis Supervisor: Klavs F. Jensen Title: Warren K. Lewis Professor of Chemical Engineering Professor of Materials Science and Engineering Thesis Supervisor: Heather J. Kulik Title: Joseph R. Mares '24 Career Development Professor of Chemical Engineering