In this work, we report on efficient visible and near-IR upconversion emissions in colloidal hexagonal-phase core/shell nanoparticles () under IR laser excitation at 1523 nm. Varying amounts of dopants were introduced into the core nanoparticles, revealing an optimized concentration of 10% for the highest luminescent efficiency. An inert epitaxial shell layer of grown onto the core of the 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 under an excitation density of . 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 , which again demonstrates high upconversion efficiency.
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