Abstract
Infrared-to-visible rare earth doped upconversion phosphors that convert multiple photons of lower energy to higher energy photons offer a wide range of technological applications. The brightness (i.e., emission intensities) and energy efficiency of phosphors are important performance characteristics that determine which applications are appropriate. Optical efficiency can be used as a measure of the upconversion emission performance of these rare earth doped phosphors. In this work, hexagonal-phase NaYF4:Yb-Er was synthesized using the hydrothermal method in the presence of surfactants like trioctylphosphine, polyethylene glycol monooleate, and polyvinylpyrrolidone. The upconversion emission optical efficiencies of NaYF4:Yb-Er were measured to quantify and evaluate the effects of surface coatings and accurately reflect the brightness and energy efficiency of these phosphors. Polyvinylpyrrolidone-modified NaYF4:Yb-Er particles were found to be similar to 5 times more efficient and brighter than the unmodified particles. The difference in efficiency was attributed to reduced reflectance losses at the particle-air interface via refractive index mismatch reduction between the core NaYF4:Yb-Er particles and air using polyvinylpyrrolidone as a surfactant.