Abstract
Energy transfer upconversion (ETU) is known to be the most efficient frequency upconversion mechanism. Surface plasmon can further enhance the upconversion process, opening doors to many applications. However, ETU is a complex process involving competing transitions between multiple energy levels and it has been difficult to precisely determine the enhancement mechanisms. In this paper, we report a systematic study on the dynamics of the ETU process in NaYF4:Yb3+,Er3+ nanoparticles deposited on plasmonic nanograting structure. From the transient near-infrared photoluminescence under various excitation power densities, we observed faster energy transfer rates under stronger excitation conditions until it reached saturation where the highest internal upconversion efficiency was achieved. The experimental data were analyzed using the complete set of rate equations. The internal upconversion efficiency was found to be 56% and 36%, respectively, with and without the plasmonic nanograting. We also analyzed the transient green emission and found that it is determined by the infrared transition rate. To our knowledge, this is the first report of experimentally measured internal upconversion efficiency in plasmon enhanced upconversion material. Our work decouples the internal upconversion efficiency from the overall upconverted luminescence efficiency, allowing more targeted engineering for efficiency improvement.
Highlights
We report a systematic study on the dynamics of the upconversion processes in NaYF4:18%Yb3+,2%Er3+ upconversion nanoparticles (UCNPs) deposited on a plasmonic nanograting structure
The UCNPs used in our study are β -NaYF4:18%Yb3+/2%Er3+ nanoparticle synthesized by the co-precipitation method[20,21]
The reference sample was prepared in exactly the same way except that the silver nanograting was replaced by a flat silver film
Summary
Luminescence upconversion is a complex process consisting of multiple steps of absorption, Förster energy transfer and emission. While the plasmon enhancement of absorption and emission is extensively studied and well understood, the plasmon enhancement of Förster energy transfer process has been largely unknown. To the best of our knowledge, this is the first report of experimentally measured ETU rate in a plasmon enhanced upconversion material. Does it provide a direct proof of plasmon enhancement of energy transfer process but it gives a quantitative measure of how much enhancement is achievable. Our work elucidates the role of plasmon resonance in the ETU system, and provides a straightforward method to directly measure the energy transfer rate and internal upconversion efficiency
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