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.

Despite considerable advances in synthesizing high-quality core/shell upconversion (UC) nanocrystals (NC; UCNC) and UCNC photophysics, the application of near-infrared (NIR)-excitable lanthanide-doped UCNC in the life and material sciences is still hampered by the relatively low upconversion luminescence (UCL) of UCNC of small size or thin protecting shell. To obtain deeper insights into energy transfer and surface quenching processes involving Yb³⁺ and Er³⁺ ions, we examined energy loss processes in differently sized solid core NaYF₄ nanocrystals doped with either Yb³⁺ (YbNC; 20% Yb³⁺) or Er³⁺ (ErNC; 2% Er³⁺) and co-doped with Yb³⁺ and Er³⁺ (YbErNC; 20% Yb³⁺ and 2% Er³⁺) without a surface protection shell and coated with a thin and a thick NaYF₄ shell in comparison to single and co-doped bulk materials. Luminescence studies at 375 nm excitation demonstrate back-energy transfer (BET) from the ⁴G_11/2 state of Er³⁺ to the ²F_5/2 state of Yb³⁺, through which the red Er^{3+ 4}F_9/2 state is efficiently populated. Excitation power density (P)-dependent steady state and time-resolved photoluminescence measurements at different excitation and emission wavelengths enable to separate surface-related and volume-related effects for two-photonic and three-photonic processes involved in UCL and indicate a different influence of surface passivation on the green and red Er³⁺ emission. The intensity and lifetime of the latter respond particularly to an increase in volume of the active UCNC core. We provide a three-dimensional random walk model to describe these effects that can be used in the future to predict the UCL behavior of UCNC.