Pushing the Limits of Ultrasmall PbSe Magic-Sized Clusters.

Ultrasmall PbSe magic-sized clusters (MSCs) are important for probing the nucleation of nanocrystals and their optical properties under strong quantum confinement. The smallest PbSe MSC observed so far in the growth sequence has an absorption maximum at 625 nm. Here, we identify two even smaller PbSe MSCs, with absorption maxima at 510 and 565 nm, respectively. Both reaction kinetics and solvent environments are crucial for observing these two ultrasmall MSCs. An unusual blue emission appears during the nucleation stage, which evolves into more intense orange and red emissions of MSCs. Structural characterization reveals that the PbSe-565 MSC is most likely to have an expanded octahedron structure (Oh-3.5) with a core formula of Pb60Se28, a "half-size" smaller than that of the PbSe-625 MSC (Oh-4). The ultrasmall MSCs mark the critical transition from molecular precursors to cluster nuclei, providing important insights into the "birth" of IV-VI nanocrystals.