Structural evolutions of CdSe nanocrystals in ripening process

A size-selective separation method is used to synthesize CdSe nanocrystals (NCs) with different grain sizes. The structural evolutions of CdSe NCs in the ripening process are investigated by high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), and X-ray absorption fine structure (XAFS) spectroscopy. The results indicate that the structural disorder σS2 of the Se–Cd bond in CdSe NCs rises from 0.0005, 0.0012 and 0.0034 Å2 as the size of NCs increases from 1.9 to 2.7 and 3.1 nm. The small σS2 of the 1.9 nm NCs unambiguously reveals that at this stage the CdSe NCs are well crystallized and almost free of interior defects. The unusual increase of σS2 with size can only be interpreted by the interior defects rather than surface defects of NCs. The interior defects produced in the core of the CdSe NCs through the ripening process are accumulated. This leads to a rapid increase in their structural disorders for the large CdSe NCs.