Experimental and theoretical investigation of indium phosphide quantum dot growth mechanisms

Abstract

Indium phosphide (InP) quantum dots (QDs) stand out as the most promising candidate to replace the currently commercialized cadmium-containing materials for optoelectronic applications. This thesis focuses on using experimental and theoretical methods to study growth mechanisms of InP QDs 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 QD 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 QD quality, contradicting the original hypothesis. The unexpected role of precursor chemistry motivated investigation into the early-stage QD 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 QD growth. In and P precursors have different roles, with P precursors controlling the reaction energy, and In precursors determining the reaction barrier. With clusters identified as important growth intermediates in both simulations and experiments, their role during the QD 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 QD formation. Lastly, a QD size tuning strategy was developed involving the use of weakly associated ligands to synthesize cluster-free InP QDs 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 InP QDs. The importance of In precursor quality became apparent after exploring effects of impurities and solvents. For example, when water and hydroxide/oxide species contaminate In precursors, the growth of InP QDs are inhibited and batch-to-batch variations are observed.