Differences in Cross-Link Chemistry between Rigid and Flexible Dithiol Molecules Revealed by Optical Studies of CdTe Quantum Dots

The cross-link chemistry of CdTe quantum dots (QDs) in solution is studied for different types of aliphatic (flexible) and aromatic (rigid) dithiol linker molecules. A remarkable difference in the cross-linking efficiency is observed:  the rigid dithiols are shown to form aggregates at much lower concentrations. Qualitative and quantitative information on the formation of aggregates is obtained from cryogenic transmission electron microscopy (cryo-TEM) images and photoluminescence decay measurements. The luminescence decay curves are analyzed with a model for energy transfer to neighboring QDs in aggregates. The analysis shows that the cross-linking efficiency is 4 times higher for the rigid dithiols than for the flexible dithiols. The difference is attributed to the formation of loops for the flexible dithiols by attaching with both thiol groups to the same nanocrystal surface (preventing cross-linking), whereas the rigid aromatic dithiols cannot form loops and the second thiol group is oriented away from the surface (enabling cross-linking). The difference in conformation between flexible and rigid dithiols is confirmed by studies on the red-shift in the optical absorption spectra due to capping exchange of amines by monothiols or dithiols and by molecular simulations.