Thermally degradable ligands for nanocrystals.

We exchanged the oleate ligands on as-prepared PbSe/CdSe core/shell nanocrystals with octyldithiocarbamate to enable the removal of insulating ligands by gentle heating. The octyldithiocarbamate ligand could readily be stripped from the surface by heating briefly to temperatures from 140 to 205 degrees C, which is substantially lower than the temperature (330 degrees C) required to remove oleate from the nanocrystal surface. X-ray diffraction and transmission electron microscopy reveal that the nanocrystals sinter around 250 degrees C, resulting in a loss of quantum confinement. Heating for 1 min to 205 degrees C removed 92% of the organics from the surface. We could therefore prepare densely packed films of quantum-confined nanocrystals via dithiocarbamate treatment. Conductivity increased by up to 4 orders of magnitude after annealing. In addition to PbSe/CdSe core/shell nanocrystals, we also examined the applicability of our ligand removal procedure to CdSe nanocrystals.