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Abstract
Lanthanide-doped upconversion nanoparticles (UCNPs) are capable of converting near-infra-red excitation into visible and ultraviolet emission. Their unique optical properties have advanced a broad range of applications, such as fluorescent microscopy, deep-tissue bioimaging, nanomedicine, optogenetics, security labelling and volumetric display. However, the constraint of concentration quenching on upconversion luminescence has hampered the nanoscience community to develop bright UCNPs with a large number of dopants. This review surveys recent advances in developing highly doped UCNPs, highlights the strategies that bypass the concentration quenching effect, and discusses new optical properties as well as emerging applications enabled by these nanoparticles.
Highlights
Lanthanide-doped upconversion nanoparticles (UCNPs) are capable of converting near-infrared excitation into visible and ultraviolet emission
We discuss the phenomenon and underlying mechanism of concentration quenching occurring in UCNPs, review the general and emerging strategies for overcoming the concentration quenching effect, and summarize the impact of highly doped UCNPs on a range of disruptive applications
By supplying a high irradiance, either by using a high-power laser or focusing the excitation beam, a sufficient amount of excitation photon flux will be supplied to the large number of highly doped ions, and the majority of them will be at excited states, which reduces the number of detrimental ground-state ions
Summary
The detrimental effect of concentration quenching in luminescent materials imposes a restriction on access to a high level of luminescence intensity, in consequence hindering their further applications. The high-doping concentration facilitates both the energy migration of excited levels (typically within the sensitizersensitizer network) to the surface quenchers (Figure 1b)[15,24,25] and the inter-dopant (typically between activators) crossrelaxation that causes emission intensity loss each time[7,13] (Figure 1c). To avoid the quenching of luminescence, conventionally, the doping level has been kept relatively low to ensure a sizable separation between the dopants to prevent parasitic interaction. For an efficient upconversion to proceed, the relatively low concentrations of sensitizers (typically around 20 mol %) and activators (below 2 mol %) are generally used in the hexagonal-phase alkaline rare-earth fluoride nanocrystal, β-AREF4, that is known as one of the most efficient host material for upconversion. Low-doping concentration is the key roadblock to yield smaller and brighter luminescent nanomaterials[27–29], which requires a canonical approach to optimizing the composition and chemical architecture of nanoparticles as well as photoexcitation schemes[4,6]
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