Competition between charge transfer and energy transfer: influence of intrinsic defects in ZnO nanocrystals on rhodamine-B dye color signals

Official URL: https://dx.doi.org/10.1103/PhysRevB.108.L241401

The complexity of the competition between nonradiative energy transfer and charge transfer is presented in this work. This study uses ZnO nanocrystals as a donor component and rhodamine B (RhB) dye as an acceptor component. Investigations reveal that the concentration of intrinsic defects and their localization, particularly oxygen vacancies, zinc interstitials, and oxygen interstitials, play a vital role in nonradiative energy transfer from ZnO nanocrystals to RhB dye. Additionally, photoluminescent spectra indicate that ZnO nanocrystals degrade the emission signals of RhB dye via charge transfer. It is possible that a part of oxygen vacancies may also contribute to the photocatalytic oxidation of the RhB molecule. The correlation between electron paramagnetic resonance, photoluminescence, and the RhB emission decay rates indicates two processes that one may encounter when dealing with semiconductor-dye conjugates. First is a complex energy transfer from ZnO to RhB indicating a photoluminescent up-conversion and second is a charge transfer from photoexcited intrinsic defect species in ZnO suppressing the RhB emission signals, which occur together. The defect species involved in photoluminescent up-conversion in ZnO+RhB is due to a nonradiative energy transfer from photoexcited zinc interstitials or doubly charged oxygen vacancies in ZnO to RhB. Additionally, the suppression of RhB emission occurs due to charge transfer from oxygen vacancies in ZnO.