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Ranjana Yadav

Continuous flow synthesis of semiconductor nanocrystals

Published on 23 February 2023
Thesis presented February 23, 2023

Colloidal semiconductor quantum dots (QDs) exhibit unique photophysical properties such as narrow and size-tunable fluorescence emission, broadband absorption, and high photostability. They are used in diverse applications such as biological imaging and detection or as color converters in displays and TVs. Indium phosphide (InP) QDs are in compliance with EU regulations and have been identified as the most promising candidate for substituting toxic CdSe-based QDs. In this thesis, we developed the continuous flow synthesis of high-quality indium phosphide-based QDs. Continuous flow synthesis has many advantages compared to conventional batch synthesis due to the better mass and heat transfers in tubular reactors and the higher reproducibility related to the automated system. The continuous flow synthesis of InP QDs was developed using tris(oleylamino)phosphine as the P precursor in a non-coordinating solvent. In parallel, a novel synthetic approach relying on the use of indium (I) halides was developed, which act as both the indium source and reducing agent for aminophosphine. This reaction pathway gave access to unprecedented large-sized InP QDs showing a narrow size distribution and a large tunability of the excitonic peak position from 450 nm to 700 nm. Kinetic studies using 31P NMR revealed the contribution of two different reduction pathways of transaminated aminophosphine. To improve the photoluminescence quantum yield (PLQY) two methods were followed. i) The surface of the InP QDs was etched at room temperature using in-situ generated HF leading up to 79% PLQY. ii) InP/ZnS core/shell QDs were developed by overgrowth of the InP core using suitable precursors at low temperature. This method is compatible with flow synthesis and was used to prepare highly luminescent InP/ZnS QDs on the gram-scale in a fully automated way. To further reduce the emission linewidth of the InP QDs, the synthesis of ZnSe/InP core/shell nanoplatelets (NPLs) was explored. Native ligand stripping and surface ligand exchange procedures were performed for the surface activation of the ZnSe NPLs acting as a template for the InP growth. Finally, the use of an amorphous shells was investigated to improve the emission properties of InP QDs, which led to the discovery of strongly blue-emitting nanocrystals.

continuous flow synthesis, indium phosphide, nanocrystals, quantum dots, fluorescence