Modulation of thulium upconversion in potassium tetraphosphate (KLaP4O12) nanocrystals by co-doping with Yb3+ ions

The influence of sensitizer concentration on the up-conversion properties of KLa0.99-xTm0.01YbxP4O12 nanocrystals was investigated in a wide range of Yb3+ concentrations (x = 0.05-0.5). This impact was manifested by Yb3+ concentration dependent variation of up-conversion emission spectra, blue to NIR emission ratio variation, luminescence decay times as well as on the order of upconversion processes. The observed processes of energy transfer between Yb3+ and Tm3+ were discussed in terms of reduction of the average distance between dopants, Tm3+ -> Yb3+ back energy transfer and energy migration to surface quenchers over a Yb3+ sublattice.