Growth of monodisperse quantum dots (QDs) is a pressing requirement in the context of commercial application as well as academic study, as the size uniformity is directly related to color purity in display products which have been commercialized. In the sense, understanding on the generation of active species via thermolysis of precursors, diffusion of precursors from bulk solution onto nanocrystal surface, and surface growth reaction kinetics has been ever more important. In this presentation, I will present our recent findings pertaining to these issues on the basis of different material examples.
First, our study on the colloidal synthesis of InP QDs in the presence of Zn precursors will be discussed, in which size uniformity is markedly enhanced as compared to the case of InP QDs synthesized without Zn precursors. The nuclear magnetic resonance spectroscopy and mass spectrometry analyses on aliquots taken during the synthesis allow us to monitor the appearance of metal-phosphorus complex intermediates in the growth of InP QDs. In the presence of zinc carboxylate, intermediate species containing Zn-P bonding appears. The Zn-P intermediate complex with P(SiMe3)3 exhibits lower reactivity than In-P complex, which is corroborated by our prediction based on density functional theory and electrostatic potential charge analysis. The formation of stable Zn-P intermediate complex results in lower reactivity, hence monodisperse QDs. Insights from the experimental and theoretical studies advance the mechanistic understanding and controlling of nucleation and growth of InP QDs, key to the preparation of monodisperse InP-based QDs in meeting the demand of display market.
Second, we have investigated diffusion of active species monomers through ligand layers using CdSe nanorods (NRs) as a model system. Colloidal NRs are of special interest for optoelectronic applications because its shape anisotropy leads to unique optical and physical characteristics, expandable with morphological and structural deviation. Previous studies focused on the development of diverse NR structures. However, synthesis relied on empirical observations under specific conditions, and general NR growth process remained elusive. I present a new answer for detailed growth mechanism of colloidal semiconductor NRs. For this, we developed dual-diameter nanorod (DDNR) structure via colloidal synthesis, where two sections along the long axis in each NR have different diameters at a few nanometer scale. The vivid segmentation is an ideal platform for monitoring the growth process of NRs, presenting important determinants in the reactivity of distinguishable NR facets. The lesson obtained from DDNR is universally applied to nanocrystal growth in any colloidal batch. By controlling the discovered factors, single-diameter NRs with controllable core position also became available. I will put the findings in perspective by outlining the effect of diffusion of monomers and surface growth reactions.