Ensemble-level energy transfer measurements can reveal the spatial distribution of defect sites in semiconductor...

Energy transfer measurements are widely used to measure the distance between donors and acceptors in heterogeneous environments. In nanocrystal (NC)-molecule donor-acceptor systems, NC defects can participate in electronic energy transfer (EnT) in a defect-mediated EnT process. Here, we explore whether ensemble-level spectroscopy measurements can quantify the distance between the donor defect sites in the NC and acceptor molecules. We studied defect-mediated EnT between ZnO NCs and Alexa Fluor 555 (A555) because EnT occurs via emissive NC defect sites, such as oxygen vacancies. We synthesized a size series of ZnO NCs and characterized their radii, concentration, photoluminescence (PL) lifetime, and defect PL quantum yield using a combination of transmission electron microscopy, elemental analysis, and time-resolved PL spectroscopy. The ZnO defect PL decay kinetics were analyzed using the stochastic binding (SB) and restricted geometry (RG) models. Both models assume the Förster point dipole approximation, but the RG model considers the geometry of the NC donor in the presence of multiple acceptors. The RG model revealed that the emissive defect sites are separated, on average, 0.5 nm from the A555 acceptor molecules. That is, the emissive defect sites are predominantly located at or near the surface of large NCs. The SB model revealed the average number of A555 molecules per NC and the equilibrium binding constant but did not provide meaningful information regarding the defect-acceptor distance. We conclude that ensemble-level EnT measurements can reveal the spatial distribution of defect sites in NCs without the need for interrogating the sample with a microscope.