(INVITED) smart and efficient multi-color tuning using fluoride upconversion nanoparticles blending

Color tuning using upconversion nanoparticles (NPs) in the visible, in an attempt to obtain white light, has been achieved in several ways and has aroused much scientific and technological interest lately, due to its potential applications in color displays technology, assay of biological compounds, remote sensing, optical data storage and optical printing. In this work, we present a simple and smart way to obtain color tuning in practically all the chromaticity diagram. LaF3 and CaF2 NPs were used with different dopants. They were chosen because of their high resistance to optical damage, low phonon energy (<400 cm−1), chemical durability and thermal stability compared to other matrices, mainly various vitreous ones. Our method proved to be efficient, as the competition processes of energy transfer between the dopant ions have been a challenge to obtain an efficient color tuning and, mainly, to obtain pure white light. Our approach is based on mixing upconversion NPs powders of LaF3:Yb3+/Tm3+, LaF3:Yb3+/Ho3+ and CaF2:Yb3+/Er3+ in specific amounts. In all measurements, the Yb3+ ion was used as a sensitizer and the Tm3+, Ho3+ or Er3+ ions as activators to generate colors in the blue (Tm3+), green (Ho3+) and red (Er3+) regions, thus requiring a single excitation wavelength. Varying the amount of powder of each sample in the mixture, it was possible to observe a continuous tuning of colors in the CIE-1931 chromaticity diagram in a direct, efficient and cost-effective manner, which enables a range of application, mainly in multi-color luminescence and anticounterfeiting.