Paper EM+NS-TuA10
Ligand Quenching of CdSe Quantum Dot Photoluminescence Investigated by Single Molecule Spectroscopy

Tuesday, October 16, 2007, 4:40 pm, Room 612

Colloidal quantum dots are a unique class of solution processable chromophores with high photoluminescence quantum yields, good photostability, and narrow, size-tunable emission spectra that make them potentially useful for many optoelectronic and photonic applications. Surface chemistry strongly affects the optical and electrical properties, as well as the solubility and stability of the quantum dots. However, many properties of ligand-quantum dot interactions remain unresolved. For instance, it is not known exactly how different ligands alter quantum dot photoluminescence and a better understanding of ligand effects is necessary in order to tailor quantum dot surface chemistry for specific applications. We investigate changes in the photoluminescence of colloidal CdSe quantum dots as we bind different ligands to quantum dot surfaces using both single-molecule and ensemble averaged spectroscopy. Using single-molecule spectroscopy, we monitor the photoluminescence of single CdSe quantum dots over time in the presence of varying concentrations of octadecanethiol and determine the average quantum dot intensity, the average number of emissive quantum dots, and the blinking statistics of the quantum dots. This allows us to determine that the binding of a single thiol molecule to the surface of a CdSe quantum dot creates a trap state that decreases the photoluminescence intensity of the individual quantum dot by a significant amount, but that there is no change in the quantum dot blinking rate. We use this single-molecule data to refine our previously reported Langmuir isotherm quenching fits to ensemble solution photoluminescence spectra. By modeling the effects of octadecanethiol on CdSe quantum dots, we develop a better general understanding of ligand exchange and ligand binding to quantum dots.