Highly efficient NIR–NIR upconversion in potassium substituted CaMoO4:Tm3+, Yb3+ phosphor for potential biomedical applications

We obtained highly efficient NIR to NIR (excitation by 980 nm, emission at 800 nm) upconversion in CaMoO4:Tm3+, Yb3+, M+ (M = Li, Na, K, Rb) nanocrystals synthesized by a hydrothermal method. The XRD patterns show that they are all of tetragonal structure despite a small amount of RE3+ and M+ ion doping. The solid solubility of lithium, sodium, potassium and rubidium ions in the CaMoO4 were found to be 30 mol%, 20 mol%, 10 mol% and 5 mol%, respectively. The doped M+ ions affect the crystal field symmetry around Tm3+ ions in the CaMoO4 host, which results in the change of the irradiation transition probabilities between their transition levels and intensifies the UC intensities. Compared with CaMoO4:Tm3+, Yb3+, the NIR to NIR upconversion emission intensity of CaMoO4:Tm3+, Yb3+, 0.10M+ nanocrystals increases 2.2, 3.3, 47 and 10 fold for Li, Na, K and Rb, respectively. The K substituted CaMoO4:Tm3+, Yb3+ phosphor exhibited intense UC emission pumped by less than 1 mW laser power and can have potential application in NIR to NIR photodynamic diagnosis.