Reducing the size of rare earth, alkali metal fluoride crystals formulated as upconverting nanoparticles (UCNP) down to few nm can lead to unique luminescent devices due to a reduced number of emitters. The synthesis of such Na(Gd-Yb)F4:Tm nanocrystals is still challenging. The most critical issue is to keep detectable emission despite the high surface to volume ratio and the limited number of emitting ions. The preparation of sub-3 nm diameter emissive, ꞵ-phase UCNPs with few emitters was achieved using a gadolinium-rich composition, an in-situ mixing of the precursor (NaOH and NH4F), and a microwave high-temperature cycling sequence which allowed the precise control of the particle size and dispersity. Such particles can contain only a single activator ion (Tm3+). Coating of these core particles with a NaGdF4 inert shell was carried-out in order to moderate the surface quenching deleterious influence. The reduced number of activators offers an unprecedented platform to study the role of cross relaxation in the global luminescence. This was tackled by time resolved luminescence measurements using an original combination of standard Yb3+ sensitizer NIR-excitation and direct Tm3+ activator UV-excitation.
Reducing the size of upconversion nanoparticles (UCNPs) down to a few nm yields unique luminescent materials containing a very small number of emitters. Considering the bottom limit of one activator per particle, such ultrasmall UCNPs offer an unprecedented platform to study the contributions of the different energy transfer processes at play in upconversion luminescence, especially the role of cross relaxation. Maintaining detectable emission despite the limited number of emitting ions and the high surface-to-volume ratio requires suitable particle architectures. The preparation of Na(Gd-Yb)F4:Tm emissive sub-3 nm diameter -phase UCNPs was achieved using a gadolinium-rich composition, in situ mixing of the precursors NaOH and NH4F, and a microwave high-temperature cycling sequence that allowed precise control of the particle size and dispersity. These nanoparticles contain only a single Tm3+ activator ion, while coating of these cores with a NaGdF4 inert shell was performed to minimize the deleterious influence of surface quenching. The role of cross relaxation in upconversion luminescence was examined by time-resolved luminescence measurements using a combination of standard NIR excitation of the Yb3+ sensitizer and direct UV excitation of the Tm3+ activator. The fine tuning of the number of activators per particle via an optimized synthesis pathway along with an appropriate excitation scheme enabled us to select the operating cross relaxation processes and provide an accurate analysis of the different mechanisms at play in these model nanoparticles.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.