Colour-optimized quantum yields of Yb,Tm Co-doped upconversion nanocrystals

We present here a systematic analysis of the influence of Tm3+ doping concentrations (x(Tm)) on the excitation power (P)- dependent upconversion luminescence and -performance of hexagonal-phase NaYF4: 20% Yb3+,x(Tm)%Tm3+ upconversion nanoparticles (UCNPs) for x(Tm) of 0.2, 0.5, 0.8,1.2, and 2.0, respectively. Our results reveal the influence of these different Tm 3+ doping concentrations with respect to optimized upconversion quantum yield (Phi(UC)) values of the various Tm3+ upconversion emission bands, with the highest Phi uc values of the Tm3+ emission bands above 700 nm resulting for different x Tin values as the Tm3+ emission bands below 700 nm. This underlines the potential of Tm3+ dopant concentration for colour tuning. Special emphasis was dedicated to the spectroscopic parameters that can be linked to the (de)population pathways of the various Tm3+ energy levels, like the P- and x Tm -dependent slope factors and the intensity ratios of selected emission bands. The evaluation of all parameters indicates that not only energy transfer upconversion-, but also cross-relaxation processes between neighbouring Tm3+ ions play a vital role in the (de)population of the excited energy levels of Yb3+, Tm3+ codoped nanocrystals.