Hexameric Octahedral Clusters of PbSe Nanocrystals Grown from Amorphous Lead(II) Carboxylate Nanoparticles

We describe the synthesis and three-dimensional structure of a new single-crystalline "hexameric" nanocrystal composed of six near-spherical PbSe nanocrystals arranged at the vertices of an octahedron. We examine the detailed three-dimensional structure of these nanocrystals using electron tomography and demonstrate single-crystal to single-crystal cation exchange to CdSe. We reveal that the growth of these nanocrystals, which form under conditions similar to other anisotropic PbSe nanocrystals, depends on the initial presence of lead oleate particles with approximate diameters of 1.7-3.1 nm that form upon heating lead(II) acetate hydrate in the presence of oleic acid. These lead oleate particles, which are visible by transmission electron microscopy, constitute the beginning of nearly every synthesis of anisotropic PbSe nanocrystals. We show that the lead oleate particles play a definitive role in determining the morphology of the resultant PbSe nanocrystals. We note that the acetate anion, which was previously identified as the key factor in achieving anisotropic PbSe growth, greatly accelerates the formation of the lead oleate particles, and thus appears to be responsible for the subsequent PbSe morphology. However, we demonstrate that acetate is not required for lead oleate particle formation, nor indeed for anisotropic PbSe growth. The potential role of these new particles in other PbSe synthetic preparations from lead(II) oleate is of high interest for future study.