Synthesis, Interconversion and Photophysical Properties of CdSe Magic-Sized Cluster Polymorphs towards Atomically Precise Emitters

Cutting-edge applications of quantum dots (QDs) as next generation emitters require a precise understanding of their synthesis and structure, since atom-scale differences between individual nanoparticles can result in particle-to-particle variation in their optoelectronic properties. A strategy to synthesize atomically precise QD ensembles utilizes magic-sized clusters (MSCs), which are kinetically stable, atomically defined intermediates along the QD reaction potential energy surface. The synthesis of cadmium selenide MSCs is well precedented, and recent work has established two different structural motifs: a cation-rich, “zincblende-like” tetrahedron and a “wurtzite-like,” quasi-spherical cluster with a stoichiometric inorganic core. However, the wide range of protocols used to access these species has made the direct comparison of their structure, surface chemistry, and optoelectronic properties difficult. Additionally, the physical and chemical relationship between these MSC polymorphs has yet to be established. Here, we demonstrate that both cation-rich and stoichiometric CdSe MSCs can be synthesized from identical reagents and can be interconverted through the addition of either excess cadmium or selenium precursor. The structural and compositional differences between these two polymorphs can be contrasted using a combination of ¹H-NMR spectroscopy, x-ray diffraction, pair distribution function (PDF) analysis, inductively coupled plasma optical emission spectroscopy, and UV-vis transient absorption spectroscopy. The subsequent polymorph interconversion reactions are monitored by UV-vis absorption spectroscopy, with evidence for an altered cluster atomic structure observed by powder x-ray diffraction and PDF analysis. This work helps simplify the complex picture of the CdSe nanocrystal landscape and provides a method to explore structure-property relationships in colloidal semiconductors through atomically precise synthesis. Furthermore, an understanding of polymorph structure allows us to tune the optoelectronic properties of the MSC using surface chemistry, with emission energy and quantum yield affected by the identity of the ligands.