Defect-Mediated Energy Transfer between ZnO Nanocrystals and a Conjugated Dye

Energy transfer from the defect state of zinc oxide nanoparticles to the fluorescent dye AlexaFluor 594 (A594) cadaverine has been studied using both steady-state and time-resolved photoluminescence (PL) measurements. The addition of five stoichiometric equivalents of A594 cadaverine completely quenches the visible defect emission from zinc oxide nano crystals. We also find that the entire defect emission of ZnO is reduced without any change in the overall line shape of the emission, demonstrating that the defect emission is from a single electronic state coupled to the phonon modes of the crystal lattice. The energy transfer is modeled using the dynamic quenching model developed by Tachiya (Sadhu, S.; Tachiya, M. J. Phys. Chem. 2009, 113, 19488-19492). Remarkably, there is very efficient energy transfer when there is just one adsorbed dye molecule per nanocrystal, regardless of the orientation of the dipole moment of the cadaverine molecule and the distance to the defect state.