Tunable color emission based on the activator shell thickness of a multilayer core-shell nanoparticle under double NIR excitation

Multicolor up-conversion luminescence of rare-earth doped nanoparticles is of great importance in biological, chemical, clinical fluorescent diagnosis, and display lightning fields. In this work, a multilayer core-shell structure, Er@Y@Tmx@Nd (x = 1, 2, and 3), was designed and prepared by thermal decomposition method with some modifications. The Er and Tm, which are used as activators, were doped in different layers of the nanoconstruct. The layers were separated by an inert wall of NaYF4, to hinder the cross-relaxation between the activators. Moreover, the introduction of Nd in the nanoconstruct promoted the luminescence when irradiated with both 980 and 808 nm laser sources. In addition, the thickness of Tm containing layer was carefully controlled leading to distinct nanomaterials with tunable emission light. The fluorescence changed from yellowish green to bluish green and from bluish green to obvious blue when excited by 980 and 808 nm, respectively. However, the fluorescence lifetimes at 475, 540, and 660 nm did not show significant differences after excitation at 980 nm. It was noticed that when there are different activators doped in different layers, the energy chansfer process is influenced obviously by the shell thickness. Which result in the final luminescence color changed a lot. Hence, this interesting property can be considered as a straightforward strategy to tune the emission light by controlling the thickness of the layer containing the activator and used for various applications.