Upconversion in Detail: Multicolor Emission of Yb/Er/Tm-Doped Nanoparticles under 800, 975, 1208, and 1532 nm Excitation Wavelengths

Spectroscopic properties of YPO₄ nanoparticles doped with Yb³⁺, Tm³⁺, and Er³⁺ ions have been studied in detail. These multiemitting materials are promising not only for photonic or electronic use but especially for anticounterfeiting applications. The nanopowders are synthesized by the facile coprecipitation method with annealing in the air atmosphere at 1000 °C. The structural and morphological studies reveal pure tetragonal nanocrystallites with an average size of 20–30 nm. Most interestingly, under NIR excitation, the samples exhibit intense upconversion (UC) luminescence where the color can be tuned by changing the laser source, switching the excitation wavelengths between 800, 975, 1208, and 1532 nm, double-wavelength excitation, and by changing laser power density. As a result, a very high color shift, being the result of intensity changes in the emission bands of Er³⁺ (green and red) and Tm³⁺ (blue and red) is obtained. The luminescence lifetimes, temporal evolution, and the pump power dependences are also measured to propose the mechanisms responsible for the observed UC emission.