Simultaneous monitoring of competing photo-induced weathering processes in colloidal CdSe/ZnS semiconductor quantum dots using multivariate frequency-domain dynamic photoluminescence measurements

Abstract

Coincident photo-induced weathering processes in oleic acid capped CdSe/ZnS semiconductor quantum dots (QDs), stimulated by exposure to artificial sunlight in samples with different headspace volumes, were monitored using multichannel frequency-domain photoluminescence (PL) decay measurements. The combination of process monitoring using matrix-formatted measurements and multivariate data mining permits resolution of the spectra, frequency-domain decays and kinetic profiles of the PL emitted by distinct QD subpopulations. The data indicate that three components contribute to the PL: photodarkened QDs that emit bluer, shorter-lived spectra reflecting photoionization-induced QD charging and ligand desorption; photobrightened QDs that emit redder, short-lived spectra indicative of photooxidative QD surface passivation; and photoripened QDs that emit weak, red-shifted, large bandwidth, very long-lived spectra reflecting increasing nanoparticle growth and polydispersity. Analysis of the kinetic traces of the QD subpopulations during photoirradiation using systems of exponentials revealed the extent of coupling between the coincident photoinduced processes. The assignment of these components was supported by their emission maxima, spectral bandwidths, average and irradiation-time specific PL decay times, irradiation-dependent kinetic profiles, TEM images and literature precedents. As expected, the results show that QD photodarkening is faster and photobrightening is slower in the samples that have more exposure to water and oxygen. They also show that the dominant photodarkening and photobrightening pathways in those samples are not as strongly coupled, indicating the importance of contributions by additional processes in that case. Transmission electron microscopy (TEM) images were consistent with the PL results, but the QD samples proved too polydisperse to be characterized by single-beam dynamic light scattering measurements.