Monodisperse Octahedral PbSe Nanocrystals Encased in Eight (111) Facets and Their Orthorhombic Superlattice

Although they serve as the workhorse in the short-wavelength infrared window, performance of lead chalcogenide nanocrystals (NCs) in optoelectronic devices suffers from various atomic-level structural and electronic defects rooted in the facet-ligand interface. Considering the rock-salt structure, here we target the synthesis of PbSe NCs encased in eight (111) facets by introducing soluble chloride (or other halides) and alkanoate ions as mixed ligands in octadecene to release excessive strain within the conventional alkanoate ligand monolayer on the polar (111) facets. The resulting monodisperse octahedral PbSe NCs possess the defined (111) facets, as observed at a resolution of approximately one atomic layer by high-resolution transmission electron microscope (TEM), and a single facet-ligand coordination motif. The resulting NCs exhibit unique optical properties, formation of an orthorhombic three-dimensional superlattice, outstanding chemical stability against aerobic exposure, and excellent structural stability in solutions and thin films. As strong ligands, mixed alkanoate-chloride ligands yield small octahedral NCs─essential for high-efficiency photovoltaic devices─with high mass yields by controlling nucleation and growth. Results here reveal that ligand chemistry is the key toward atomically precise synthesis of colloidal NCs that are ideal for both fundamental research and technical applications.