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

Abstract High sensitizer and activator concentrations have been increasingly examined to improve the performance of multi-color emissive upconversion (UC) nanocrystals (UCNC) like NaYF 4 :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 NaYF 4 :Yb,Er/NaYF 4 :Lu upconversion core/shell nanocrystals of varying Yb 3+ and Er 3+ concentrations (Yb 3+ series: 20%–98% Yb 3+ ; 2% Er 3+ ; Er 3+ series: 60% Yb 3+ ; 2%–40% Er 3+ ). 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 Yb 3+ concentrations barely reduce Φ UC from ∼9% (20% Yb 3+ ) to ∼7% (98% Yb 3+ ) for an Er 3+ concentration of 2%, thereby allowing to strongly increase the particle absorption cross section and UCNC brightness. Although an increased Er 3+ concentration reduces Φ UC from ∼7% (2% Er 3+ ) to 1% (40%) for 60% Yb 3+ . Nevertheless, at very high P (> 1 MW/cm 2 ) used for microscopic studies, highly Er 3+ -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.