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. A novel multilayer core/shell design to broadly upconvert IR light of many discrete wavelengths into visible or near‐IR emissions is employed. The active core (Er3+ containing) as well as each active shell (Ho3+ and Tm3+ containing) upconvert a distinct set of IR wavelengths. An inert NaYF4 layer in between these upconverting domains efficiently suppresses the detrimental cross‐relaxation processes between different types of lanthanide ions.