Annealing of sulfide stabilized colloidal chalcopyrite nanocrystals

Colloidal nanocrystals are an attractive precursor for the formation of semiconductor thin films, such as CIGS or CZTS, by solution-based processing. The synthesis of these nanocrystals typically makes use of long chain organic ligands, such as carboxylic acids or amines, which adsorb at their surface. Although this offers excellent size and shape control during synthesis, these long ligands are less interesting when forming semiconductor thin films since they are a source of carbon contamination. In this work, we therefore replace the original organic ligands by inorganic sulfide ions. Using CuInS2 nanocrystals as an example, we study the relation between the surface chemistry of the nanocrystals and their sintering behavior by means of and we use in-situ XRD, in-situ TEM, thermal gravimetric analysis combined with mass spectrometry (TGA-MS) and XPS. More specifically, we show that the sintering of CuInS2 nanocrystals can both be inhibited and promoted by a thermal treatment using inert and forming gas, respectively. A similar behavior is observed for CZTSe and CdSe nanocrystals. In the case of CuInS2, we can link this inhibition of sintering to the formation of sulfate moieties at the surface of the nanocrystals. This result stresses the relation between the surface chemistry of colloidal nanocrystals and their recrystallization behavior during a thermal treatment