Plasmonic interactions through chemical bonds of surface ligands on PbSe nanocrystals

When functional films are cast from colloidal dispersions of semiconductor nanocrystals, the length and structure of the ligands capping their surfaces determine the electronic coupling between the nanocrystals. Long chain oleic acid ligands on the surface of IV-VI semiconductor nanocrystals such as PbSe are typically considered to be insulating. Consequently, these ligands are either removed or replaced with short ones to bring the nanocrystals closer to each other for increased electronic coupling. Herein, using high-angle annular dark-field scanning transmission electron microscopy imaging combined with electron energy loss spectroscopy, we show that partial oxidation of PbSe nanocrystals forms conjugated double bonds within the oleic ligands, which then facilitates enhanced plasmonic interaction among the nanocrystals. The changes in the geometric configurations of the ligands are imaged directly and correlated with the changes in the surface plasmon intensities as they oxidize and undergo structural modifications.