title: Role of solvent-oxygen ion pairs in photooxidation of CdSe nanocrystal quantum dots.
abstract: Understanding the mechanisms for photodegradation of nanocrystal quantum dots is an important step toward their application in real-world technologies. A usual assumption is that photochemical modifications in nanocrystals, such as their photooxidation, are triggered by absorption of a photon in the dot itself. Here, we demonstrate that, contrary to this commonly accepted picture, nanocrystal oxidation can be initiated by photoexcitation of solvent-oxygen ion pairs that relax to produce singlet oxygen, which then reacts with the nanocrystals. We make this conclusion on the basis of photolysis studies of solutions of CdSe nanocrystals. Our measurements indicate a sharp spectral onset for photooxidation, which depends on solvent identity and is 4.8 eV for hexane and 3.4 eV for toluene. Importantly, the photooxidation onset correlates with the position of a new optical absorption feature, which develops in a neat solvent upon its exposure to oxygen. This provides direct evidence that nanocrystal photooxidation is mediated by excitation of solvent-oxygen pairs and suggests that the stability of the nanocrystals is defined by not only the properties of their surfaces (as has been commonly believed) but also the properties of their environment, that is, of the surrounding solvent or matrix.
authors.affiliation: Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
authors.forename: Virginia W
authors.initials: VW
authors.lastname: Manner
authors.forename: Alexey Y
authors.initials: AY
authors.lastname: Koposov
authors.forename: Paul
authors.initials: P
authors.lastname: Szymanski
authors.forename: Victor I
authors.initials: VI
authors.lastname: Klimov
authors.forename: Milan
authors.initials: M
authors.lastname: Sykora
mesh_terms: D019187:Cadmium Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D055668:Photochemical Processes; D045663:Quantum Dots; D018036:Selenium Compounds / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry*; D014466:Ultraviolet Rays
chemical_list: D019187:Cadmium Compounds; D018036:Selenium Compounds; D012997:Solvents; C058667:cadmium selenide; D010100:Oxygen