Up-conversion FRET from Er3+/Yb3+:NaYF4 Nanophosphor to CdSe Quantum Dots

Förster resonance energy transfer (FRET) between nanoparticles of up-conversion lanthanide phosphor as donors and quantum dots as acceptors is demonstrated. Fluoride (NaYF 4 ) nanocrystals (ca. 30 nm size) codoped with the Er 3+ and Yb 3+ ions were synthesized with a high-pressure solvothermal microwave assisted technique and dispersed in organic solvent. Up-converted luminescence from the rare-earth ions doped into fluoride nanomatrix was achieved with optical pumping in NIR (976 nm) region. The green erbium upconversion emission at 540 nm is efficiently quenched by quantum dots (QDs) leading to color change of the mixture for different relative concentrations. Simultaneously, orange emission from quantum dots appears due to energy transfer from Er 3+ /Yb 3+:NPs donors to QDs acceptors. The concomitant decrease of the average lifetime of donor emission at 540 nm (from ∼153 to ∼130 μs) indicates that the excitation of CdSe QDs occurs not only through reabsorption but also through Förster Resonance Energy Transfer. Acceptor luminescence lifetime mimics that of the donor and reaches hundreds of microseconds. The Förster radius ( R 0 ) was calculated to be short (∼15 Å), mostly due to low quantum yield of the multilevel emitting donor. This short-range interaction proves that in our system the FRET occurs mostly through Er 3+ ions proximate to the surface, resulting in efficiency of energy transfer equal to η = 14.8%.