Over the past three decades, significant progress has been made in developing colloidal quantum dots (QDs) as efficient and stable light-generating materials. The primary design principle for high quantum yield (QY) of QDs has been a core/shell heterostructure leveraging wide band gap semiconductors to passivate surface states and protect against degradation. Recent advancements have led to high photoluminescence QY for various core/shell heterostructures made of II-VI and III-V compounds. However, in optoelectronic applications, variations and unexpected outcomes in device efficiency, brightness, and operational stability persist, even with high-quality QDs. The absence of standardized protocol for core/shell heterostructures implies that the heteroepitaxial chemistry of QDs remains unclear and is prone to be tarnished by uncontrolled factors.

         Herein, we introduce our recent efforts to elucidate the vailed chemistry in core/shell heterostructures, focusing on the surface chemistry of reactants and the formation of crystalline shells. We investigated InP/ZnSe core/shell QDs synthesized using zinc carboxylate and trialkylphosphine selenide, chosen for their widespread use. Nuclear magnetic resonance spectroscopy revealed that sterically hindered acyloxytrialkylphosphonium and diacyloxytrialkylphosphorane are the main intermediates in the surface reaction of precursors, and their transformation to the coherent crystalline layer is likely to be hindered by surface oxides. Although precise control of the shell growth protocol achieved a near-unity PL QY of 97.3% with a single ZnS epilayer (~0.3 nm), increasing shell thickness deteriorated the luminescent property of QDs.[1] This phenomenon has been understood as a formation of mifit dislocation by the overgrown shell, releasing lattice strain. However, our spectroscopic study on thick-shell QDs uncovered the formation of zinc vacancy during the shell growth.[2] Although the compressive strain on the core deactivates their involvement to the exciton recombination, such defects become apparent at low temperature or high energy excitation, leaving photogenerated hole in the zinc vacancy. Our findings suggest that the PL QY alone cannot determine the success of core/shell QDs, and a deepter understanding is essential for advancing QD-based high performance optoelectronic applications.