Intense white emission from a single-upconversion nanoparticle and tunable emission colour with laser power

The development of upconversion nanoparticles with tunable emissions, particularly white colour emission, has always been at the forefront of colour display technologies. In this study, a type of multilayer NaGdF₄:Yb³⁺,Er³⁺@NaGdF₄@NaGdF₄:Yb³⁺,Tm³⁺@NaGdF₄ (denoted as Gd:Er@Gd@Gd:Tm@Gd) upconversion nanoparticle with remarkable advantages, such as small size (<50 nm), intense white colour emission and tunable emission colour with excitation laser intensity, was developed. A special design for separating NaGdF₄:Yb³⁺,Er³⁺ from NaGdF₄:Yb³⁺,Tm³⁺ by the insertion of an additional NaGdF₄ layer can effectively prevent the fluorescence quenching of Er³⁺ and Tm³⁺. As against the typical ion-dependent tuning of colour, herein multicolour tuning was realised by manipulating the exciting laser power. With increasing power density from 5 to 19 W cm^-2 using a 980 nm laser, the emission colour was tuned from bright green to white. In addition, we demonstrated the use of the multilayer Gd:Er@Gd@Gd:Tm@Gd nanoparticles for providing the proof-of-concept of multicolour imaging for anti-counterfeiting applications.