Size Dependence in Multicolor Upconversion in Single Yb3+, Er3+ Co-doped NaYF4 Nanocrystals

Schietinger et al. (Nano Lett. 2009, 9, 2477-2481) observed single nanoparticle emission from NaYF4:Er3+,Yb3+ and noted unusual changes in the intensities of red and green emissions with particle size. We show that the documented changes are due to instrumental artifacts and that the explanations given for the changes are incorrect.