Chiral ligand - Induced circular dichroism in excitonic absorption of colloidal quantum dots

A range of unique properties have been extensively studied in colloidal semiconductor nanocrystals (quantum dots, QDs). The possibility to manipulate the properties of these systems by controlling size and composition is attractive for many different applications, as well as for fundamental studies. Recently, the properties of chirality and chiroptical activity induced by surface ligands at QDs were considered. It was shown that chiroptical activity can be induced in the exciton transitions of quantum dots by capping them with chiral molecules. In this work, chiroptical spectroscopy (mainly circular dichroism) is used to study induced effects in CdS, CdSe and ZnSe QD systems. Some fundamental differences are encountered between CdS or CdSe samples and ZnSe samples. The results are analyzed to account for the mechanism of induction of chiroptical activity in the inorganic core by chiral capping molecules. Two electronic mechanisms of interaction are considered and discussed in light of the results. It is also suggested that chiroptical spectroscopy might serve as an efficient tool to characterize electronic (or electron-hole) level spectra of nanocrystal systems by analyzing circular dichroism intensity and lineshape.