Impact of Bis(imino)pyridine Ligands on Mesoscale Properties of CdSe/ZnS Quantum Dots

Abstract

We investigate the effect of surface modification of CdSe/ZnS quantum dots (QDs) with bis(imino)pyridine (BIP) ligands. BIPs are a class of redox non-innocent ligands known to facilitate charge transfer in base metals on the molecular scale, but their behavior in nano- to meso-scale systems has been largely unexplored. Using electron microscopy, crystallography and ultrafast spectroscopy, we reveal that structure specific π-π stacking of the BIP molecules alters inter-dot separation in QD films, thereby leading to changes in optical and electronic properties. The three variations used are unsubstituted (BIP-H), dimethyl (BIP-Me) and diisopropyl (BIP-Ipr) bis(imino)pyridine, and when compared with the native octadecylamine (ODA) ligand, we find that both energy and charge transfer efficiencies between QDs are increased post-ligand exchange, the highest achieved through BIP-Ipr, despite its larger unit cell volume. We further investigate charge transfer from QD films to conducting (indium tin oxide, ITO) and semiconducting (zinc oxide, ZnO) substrates using time-resolved spectroscopy, and determine that the influence of the ligands is QD band gap dependent. In QDs with large band gap (2.3 eV) the BIP ligands facilitate charge transfer to both ITO and ZnO substrates, but in dots with small band gap (1.9 eV) they pose a hindrance when ZnO is used, resulting in reduced recombination rates. These results highlight the importance of investigating multiple avenues in order to optimize surface modification of QDs based on the end goal. Finally, we verify that BIP ligands hasten the rate of QD photo brightening under continuous illumination, allowing the ensemble to achieve stable emission faster than in their native configuration. Our study sets the stage for novel charge transfer systems in the meso and nanoscale, yielding a diverse selection of new surface ligands for applications such as conductive materials and energy production/storage devices employing QDs.