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

Abstract

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.