Enhanced upconversion emission in colloidal (NaYF4:Er3+)/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:Er3+/NaYF4 nanoparticles (similar to 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 similar to 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/cm(2), which again demonstrates high upconversion efficiency. (C) 2014 Optical Society of America