Plasmon Enhancement Mechanism for the Upconversion Processes in NaYF₄:Yb³⁺,Er³⁺ Nanoparticles: Maxwell versus Förster

Abstract

Rare-earth activated upconversion materials are receiving renewed attention for their potential applications in bioimaging and solar energy conversion. To enhance the upconversion efficiency, surface plasmon has been employed but the reported enhancements vary widely and the exact enhancement mechanisms are not clearly understood. In this study, we synthesized upconversion nanoparticles (UCNPs) coated with amphiphilic polymer which makes UCNPs water soluble and negatively charged. We then designed and fabricated a silver nanograting on which three monolayers of UCNPs were deposited by polyelectrolyte-mediated layer-by-layer deposition technique. The final structures exhibited surface plasmon resonance at the absorption wavelength of UCNP. The green and red photoluminescence intensity of UCNPs on nanograting was up to 16 and 39 times higher than the reference sample deposited on flat silver film, respectively. A thorough analysis of rate equations showed that the enhancement was due entirely to absorption enhancement in the strong excitation regime, while the enhancement of both absorption and Förster energy transfer contribute in the weak excitation regime. The Purcell factor was found to be small and unimportant because the fast nonradiative decay dominates the relaxation process. From the experimentally observed enhancements, we concluded 3.1× and 1.7× enhancements for absorption and Förster energy transfer, respectively. This study clearly shows the plasmon enhancement mechanism and its excitation power dependence. It provides the basis for comparison of the enhancements of various plasmonic UCNP systems in the literature. It also lays the foundation for rational design of optical plasmonic structures for upconversion enhancement.

Localization induced intense red upconversion luminescence in monodispersed K3ZrF7:Yb3+/Er3+ nanocrystals

“…Various strategies have been applied to enhance the UC luminescence (UCL), e.g. the iso/hetero-epitaxy growth of protection shells [11,12], spatial design of the doping ions [6,8], and surface plasmon enhancement [13]. In addition, the local structure engineering is also an effective way to modify the UC performance of the materials, since the UC properties are tightly related to the local crystal structures around Ln 3+ ions [14][15][16].…”

Simultaneous Enhancement of Upconversion and Downshifting Luminescence via Plasmonic Structure

“…20−25 Electric field localization enhances absorption, 26−28 improves the spontaneous emission rate, 29 and the energy transfer rate. 30 Recent studies have shown that coupling UC materials with metal nanostructures such as array of metallic nanostructures, 31,32 metallic codoped nanocrystals, 23 and metal nanohole array 33 enables a straightforward amelioration in UC efficiency. Despite the theoretical and experimental expectations of these approaches, they have achieved only mild emission enhancement for device application, and their fabrication in scalable areas is far from easy and is expensive.…”

Plasmonic Nanostructures for Organic Photovoltaic Devices

“…From the experimentally observed enhancements, we concluded enhancement factors of 3.1 and 1.3 for absorption and energy transfer, respectively. [5] This study clearly elucidates the plasmon enhancement mechanism, allowing the fair comparison between various plasmonic UCNP systems and also the rational design of optimal plasmonic structures. …”