Yb- and Er concentration dependence of the upconversion luminescence of highly doped NaYF₄:Yb,Er/NaYF₄:Lu core/shell nanocrystals prepared by a water-free synthesis

High sensitizer and activator concentrations have been increasingly examined to improve the performance of multi-color emissive upconversion (UC) nanocrystals (UCNC) like NaYF₄:Yb,Er and first strategies were reported to reduce concentration quenching in highly doped UCNC. UC luminescence (UCL) is, however, controlled not only by dopant concentration, yet by an interplay of different parameters including size, crystal and shell quality, and excitation power density (P). Thus, identifying optimum dopant concentrations requires systematic studies of UCNC designed to minimize additional quenching pathways and quantitative spectroscopy. Here, we quantify the dopant concentration dependence of the UCL quantum yield (Φ_UC) of solid NaYF₄:Yb,Er/NaYF₄:Lu upconversion core/shell nanocrystals of varying Yb³⁺ and Er³⁺ concentrations (Yb³⁺ series: 20%‒98% Yb³⁺; 2% Er³⁺; Er³⁺ series: 60% Yb³⁺; 2%‒40% Er³⁺). To circumvent other luminescence quenching processes, an elaborate synthesis yielding OH-free UCNC with record Φ_UC of ~ 9% and ~ 25 nm core particles with a thick surface shell were used. High Yb³⁺ concentrations barely reduce Φ_UC from ~ 9% (20% Yb³⁺) to ~ 7% (98% Yb³⁺) for an Er³⁺ concentration of 2%, thereby allowing to strongly increase the particle absorption cross section and UCNC brightness. Although an increased Er³⁺ concentration reduces Φ_UC from ~ 7 % (2% Er³⁺) to 1% (40%) for 60% Yb³⁺. Nevertheless, at very high P (> 1 MW/cm²) used for microscopic studies, highly Er³⁺-doped UCNC display a high brightness because of reduced saturation. These findings underline the importance of synthesis control and will pave the road to many fundamental studies of UC materials.