Optical and Surface Characterization Studies of CdSe Quantum Dots Undergoing Photooxidation

Realization of the potential of Quantum Dots (QDs) for biological, energy-efficient lighting and energy harvesting applications requires that their long-term photostability be improved, especially with regards to protection from photooxidation. The overarching objective of this project was the determination of the chemical and physical mechanisms of photooxidation of CdSe QDs. Pittsburgh-based Crystalplex, Inc. provided CdSe QDs with different organic ligands for this research.;Three integrated in situ and ex situ characterization techniques were used to observe changes in optical behavior, QD morphology, and surface chemistry during photooxidation conditions. Single-molecule fluorescence microscopy experiments were used to observe real-time changes in the photoluminescence (PL) behavior of single QDs with oleic and lauric acid ligands. The QDs are exposed to 1 atm of pure O2, dry Ar, Ar bubbled through DI water, or air in an environmental chamber and excited with a 488 nm light. Changes in PL intensities were analyzed with respect to the periods of exposure to controlled atmospheres and light. Samples illuminated continuously exhibited strong photoenhancement effects, while those kept in the dark showed atmospheric-dependent PL loss.;Microstructural and chemical identification was performed with aberration-corrected transmission electron microscopy (TEM). Ex situ exposures of QD samples to air, dry O2, and dry Ar revealed changes in surface oxide growth with respect to exposure length, illumination, and column vacuum pressure. Samples exposed to air and light exhibited the most extensive photooxidation. Quantum dots with oleic acid ligands were treated with UV/ozone plasma, and extensive degradation of QDs was observed.;X-ray photoemission spectroscopy (XPS) measurements at CMU were used to identify the chemical and bonding states of the surface species before and after photooxidation. Analysis of the acquired spectra showed that exposure to below-bandgap light doubled the rate of oxide growth, and that the presence of water molecules in air accelerated oxide growth. Potential mechanisms for the role of H2O in photooxidation are described. Samples were also exposed to oxygen and hydrogen plasma. Additional XPS was performed on the 4-ID-C beamline at the Argonne National Lab Advanced Photon Source (APS) and the Environmental Molecular Sciences Laboratory (EMSL) at Pacific Northwest National Lab.;The systematic studies presented in this document complement one another by directly comparing surface chemistry results with observed changes in optical behavior and QD morphology during photooxidation. These results will enhance the scientific understanding of the relationship between nanoparticle surfaces and engineered properties furthering research to engineer more stable QD structures and compositions.