• quantum dot;
  • nanocrystal;
  • semiconductor;
  • fluorescence;
  • quantum yield;
  • CdSe;
  • CdSe–ZnS;
  • shell;
  • ZnS shell;
  • stability;
  • stability test

We investigated the correlation between the thickness of the ZnS shell of CdSe–ZnS quantum dots (QDs), the stability of the particles, and the fluorescence quantum yield. As a measure for stability, a new shell quality test was developed. This test is based on the reaction of the QDs with photochemically formed thiophenol radicals and communicates an imperfect ZnS shell by a rapid and complete loss of fluorescence. The quantum yield increases from less than 5% for unshelled CdSe up to 50%, with an increase in ZnS shell thickness up to 0.6–0.8 nm. At the same time, the particles become significantly more stable, as revealed by the shell test.