Enhanced Energy Migration Strategy in Upconverting Nanocrystals: Color-Tuning with High Quantum Yield.
Increasing the variety and efficiency of emission spectra from lanthanide-doped upconverting nanocrystals (NCs) is of interest for their application as photonic markers. Recently, significant progress in this field has been made using multi-shell structures. Herein, we demonstrate an approach to tuning the spectra of upconverting multi-shell NCs via a purely sensitizer-doped first shell sandwiched between purely activator-doped core and second shell. The approach takes advantage of the intrinsic formation of interfaces in multi-shell structures and neither necessitates the co-doping of any of the nanodomains, nor the alteration of any doping concentrations. Instead, the color is tuned via altering the thickness of the first shell, containing the sensitizing ions. This alters the interface volume ratio between the core and first shell, as well as the first and second shells. In turn, this varies the energy flow to the core and second shell and the resulting emission spectrum. This approach is shown to yield efficient color tuning in sub-20 nm multi-shell upconverting NCs, exhibiting a maximum upconversion quantum yield of 3.5% at 60 W.cm-2 under 980 nm excitation. This novel concept considerably expands the number of possibilities for multi-color and efficient upconverting photonic markers via the introduction of geometrical features acting as new degrees of freedom to finely tune upconversion emission.