Redox Levels of Dithiols in II–VI Quantum Dots vis-à-vis Photoluminescence Quenching: Insight from Scanning Tunneling Spectroscopy

Abstract

Photoluminescence (PL) of CdSe nanocrystals (NCs) is known to be quenched when the NCs are capped with dithiols, because the redox levels of dithiols are located above the valence band edge of the NCs and thereby trap photogenerated holes. We locate the conduction and valence-band edges of the NCs and determine the redox levels via scanning tunneling spectroscopy (STS) studies to provide direct evidence of hole trapping and thereby PL quenching. In this regard, we have varied the chain length of dithiol ligands and also considered a range of diameters of CdSe NCs. We also characterized CdTe NCs that even after dithiol capping are known to yield PL emission. The STS studies provided band edges of the NCs and redox levels of the stabilizers in relation to PL quenching/emission in the dithiol-capped CdSe and CdTe NCs.