Structural Investigation of Three-Dimensional Self-Assembled PbS Binary Superlattices

Three-dimensional binary superlattices were obtained by self-assembly of PbS nanocrystals (NCs) of size <= 4 nm, synthesized by colloidal chemistry routes and characterized by two distinct and narrow size distributions. The resulting binary superstructures have been imaged by small- and wide-angle X-ray diffraction (XRD), and by transmission electron microscopy (TEM). The combined use of such investigation techniques allowed retrieval of crystalline structure, size, and shape of the PbS NCs, along with their spatial arrangement in the 3D architecture. A detailed analysis of the wide-angle XRD data, based on the Debye approach, demonstrated an elongated shape of NCs even smaller than 2 nm and provided a lower limit for the effective NC size, to be compared with results from TEM. The careful interpretation of small-angle XRD data demonstrated the ordered arrangement of NCs perpendicularly to the substrate plane and, together with TEM observations, allowed retrieval of the 3D structure of the assembly. Moreover small-angle XRD is shown to contain peculiar features related to the size distribution of the NCs and the degree of order in the assembly. Such a highly detailed structural analysis, averaged over large volumes of the investigated material, can hardly be obtained even by sophisticated high-resolution TEM.