Enhanced upconversion emission in colloidal (NaYF₄:Er ³⁺)/NaYF₄ core/shell nanoparticles excited at 1523 nm

In this work, we report on efficient visible and near-IR upconversion emissions in colloidal hexagonal-phase core/ shell NaYF₄:Er ³⁺/NaYF₄ nanoparticles (∼38 nm) under IR laser excitation at 1523 nm. Varying amounts of Er³⁺ dopants were introduced into the core NaYF₄:Er³⁺ nanoparticles, revealing an optimized Er³⁺ concentration of 10% for the highest luminescent efficiency. An inert epitaxial shell layer of NaYF₄ grown onto the core of the NaYF₄:Er³⁺10% nanoparticle increased its upconversion emission intensity fivefold due to suppression of surface- related quenching mechanisms, yielding the absolute upconversion efficiency to be as high as ∼3.9 ± 0.3% under an excitation density of 18 W/cm ². The dependence of the intensity of upconversion emission peaks on laser excitation density in the core/shell nanoparticle displayed "saturation effects" at low excitation density in the range of 1.5-18 W/cm², which again demonstrates high upconversion efficiency.