Synthesis and emission dynamics of ultrasmall (sub-3 nm) UCNPs containing a reduced number of emitters

21 December 2022, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

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.

Keywords

up-converting nanoparticles
energy transfers
ultrasmall nanocrystals synthesis
time-resolved luminescence
cross relaxation
surface quenching
single emitter

Supplementary materials

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Title
Synthesis and emission dynamics of ultrasmall (sub-3 nm) UCNPs containing a reduced number of emitters
Description
Electronic supplementary Information gathers XRD data, TEM images and steady-state and time-resolved luminescence spectra that have been used for analyzing the synthesized UCNPs presented in the manuscript.
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