Lanthanide-doped fluoride core/multishell nanoparticles for broadband upconversion of infrared light

The authors introduce a novel multilayer core/shell design to broadly upconvert infrared (IR) light at many discrete wavelengths into visible or near-IR (NIR) emissions. It utilizes hexagonal-phase core/multishell NaYF₄:10%Er³⁺@NaYF₄@NaYF₄:10%Ho³⁺@NaYF₄@NaYF₄:1%Tm³⁺@NaYF₄ nanoparticles, where the active core as well as each active shell upconvert a distinct set of IR wavelength. The inert NaYF₄ layers in between these upconverting domains efficiently suppress the detrimental cross-relaxation processes between different types of lanthanide ions, thus yielding about two times more efficient upconversion photoluminescence than the counterpart NaYF₄:10%Er³⁺@NaYF₄:10%Ho³⁺ @NaYF₄:1%Tm³⁺@NaYF₄ without the inert NaYF₄ layers, and about one order of magnitude higher than the core/shell NaYF₄:10%Er³⁺, 10%Ho³⁺, 1%Tm³⁺@NaYF₄ nanoparticles without spatially isolating the Er³⁺, Ho³⁺, and Tm³⁺ ions. These core/multishell nanoparticles can be excited at ≈1120-1190 nm (due to Ho³⁺), ≈1190-1260 nm (due to Tm³⁺), and ≈1450-1580 nm (due to Er³⁺), collectively covering a broad spectral range of about 270 nm in the IR range that can be very useful for IR photosensitization of solar cells.