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The preparation of upconversion (UC) nanocrystals that exhibit anti-Stokes emission is important for applications in fields as diverse as photonics, photovoltaics, biological imaging, and therapeutics.[1–4] In particular, there has been an increasing focus on the synthesis of nanocrystals with tunable UC emission from ultraviolet to near-infrared through doping with lanthanide ions.[5] However, the synthesis of nanocrystals featuring single-band UC with high chromatic purity remains a formidable challenge, as lanthanide ions generally have more than one metastable excited state.[6] In principle, these lanthanide-doped nanocrystals display multipeak emission profiles.

Recently, several attempts have been made to obtain high purity of single-band UC red emission attractive for anticounterfeiting and color display applications.[7] For example, high red-to-green (R/G) emission ratio can be achieved by increasing the concentration of Yb3+ in Yb/Er co-doped NaYF4 nanoparticles.[8] In addition, Yb/Er co-doped MnF2 and KMnF3 nanoparticles have shown substantially enhanced R/G emission ratios because of the energy transfer between the Er3+ and Mn2+.[9] However, a general method for providing single-band red emission has not been conclusively established. Herein, we report a novel oil-based procedure for the synthesis of lanthanide-doped KMnF3 nanocrystals with only single-band UC emissions from Er3+, Ho3+, and Tm3+ dopants, respectively. Importantly, we observe that the singleband feature is independent of dopant concentration, pump power, and temperature. We also show that these nanocrystals can serve as ideal optical …