Events

Date: 18 July 2024   Time: 12:00 - 13:00

Location: GO Jones UG1

Kui Yu, National Engineering Research Center for Biomaterials Sichuan University: Formation Pathways of Colloidal Semiconductor Magic-Size Clusters and Quantum Dots

Abstract: The Nobel Prize in Chemistry 2023 was awarded for "The discovery and synthesis of quantum dots". The work originated from the hot-injection approach that was developed in 1990s for the synthesis of colloidal semiconductor quantum dots (QDs) of II-VI metal chalcogenide (ME). A few years later, a non-injection-based, heating-up approach was also developed. In the initial stage of the two approaches, magic-sized clusters (MSCs) were sometimes observed prior to the nucleation and growth (N/G) of QDs. Until today, the synthesis of either QDs or MSCs has been generally performed as an empirical art, and the latter is considered as a side product of the former. It was found that MCSs are monodispersed nanoparticles that do not follow the usual nucleation-and-growth synthesis scenario and there is currently no consensus on their formation mechanism with multiple theories being debated. For better synthetic control, it is critical to understand the chemical reaction that are involved in the pre-nucleation stage prior to the N/G of ME QDs. Here, I will introduce the two-pathway (Yu) model that I have developed. This model invokes important reaction intermediates, so-called precursor compounds (PCs) of MSCs. Individual M and E precursors self-assemble, followed by the formation of M-E covalent bonds inside each assembly (Pathway 1). Accordingly, the formation of the PC is via chemical self- assembly, and isomerization between the PC and MSC is reversible. The PC-monomer-QD conversion occurs, the process of which can be described by the LaMer model of the classical nucleation theory (CNT). Also, the formation of monomers might be directly from the M and E precursors (Pathway 2). The PC-QD pathway via monomers can happen at lower temperatures in dispersion, with conservation of, for example, the Cd–Te bond. The number of Cd-Te bonds broken in the PC reactant is similar to that of Cd-Te bonds formed in the QD product. Thus, heating in the traditional synthesis of ME QDs is responsible for the M-E bond formation rather than for nucleation. Our study brings about a comprehensive understanding of the difference between nanochemistry and traditional organic and inorganic chemistry. It is our belief that the synthesis of colloidal nanocrystals is transforming from an empirical art to a science.

Keywords: magic-size clusters (MSCs), precursor compounds (PCs), quantum dots (QDs), pre-nucleation stage, two-pathway (Yu) model

Reference: Yu, K. et al Lower-Temperature Nucleation and Growth of Colloidal CdTe Quantum Dots Enabled by Prenucleation Clusters with Cd–Te Bond Conservation. J. Am. Chem. Soc. 2024, 146, 15587–15595.