Role of decoupled defect transitions of ZnO nanocrystals in energy transfer

We examine the quenching of the visible emission of zinc oxide (ZnO) nanocrystals (NCs) in the presence of Rhodamine B (RhB) molecules. Spectroscopic measurements reveal that the quenching is nonuniform over the entire emission envelope and this quenching could be explained by the energy transfer from NCs to RhB molecules under Fِrster formalism. Analysis of the steady state and time-resolved emission data of ZnO NCs demonstrate that at least three defect transitions are associated with the visible emission of ZnO NCs. These transitions are completely decoupled and thereby interact individually with the RhB molecules resulting in wavelength dependent energy transfer and hence uneven quenching of the NC fluorescence. Moreover, we determine the parameters for fluorescence resonance energy transfer (FRET) from NCs to RhB molecules.