Triple-doped KMnF₃:Yb³⁺/Er³⁺/Tm³⁺ nanocubes: Four-color upconversion emissions with strong red and near-infrared bands

Triple-doped (Yb³⁺/Er³⁺/Tm³⁺) KMnF₃ nanocubes with uniform sizes of 250 nm were synthesized by a facile hydrothermal route using the oleic acid as the capping agent. It was found that these nanocubes can simultaneously exhibited four-color (blue, green, red and NIR) upconversion emissions under a single 980 nm near-infrared (NIR) laser excitation, which should have potential multicolor in vivo imaging applications. Specifically, the red (660 nm) and NIR (800 nm) peaks, known as two "optical windows" for imaging biological tissues, were strong. The spectral and pump analyses indicated the two-photon processes were responsible for the both red and NIR emissions.