Core/shell profile of nanocrystals determined by anomalous SAXS

Full text: The infrared emission of lead chalcogenide nanocrystals (NCs) can be drastically increased stabilising the core with a hard protective shell. In contrast to a shell growth on top of a core, we investigate in this study the CdS-shell growth on PbS NCs driven by Cd for Pb cation exchange. We perform anomalous SAXS experiments at the synchrotrons HZB-BessyII and ESRF. Tuning the x-ray energy below the Pb-edge allows to record ASAXS spectra, where the contribution of Pb to the scattering signal is varied. This allows resolving the total electron density and the Pb-atom density inside the core/shell NCs independently. We derive the following results: During the exchange process applied to PbS spheres, the core shrinks, the CdS shell thickness increases, while the outer diameter as well as the size distribution remains constant. The CdS shell thickness ranges from 0.9 nm for 10 nm large PbS NCs to even 2 nm for 5 nm small NCs. The nal shell thickness scales with 1/D of the sphere diameter, related to an increased surface to volume ratio for the smaller NCs. For the thin CdS shells we found no reaming Pb within the shell, whereas for thick shells we detect 2 Pb atoms per cubic-nm within the shell. (author)