Color-Tunable Upconversion in Er(3+)/Yb(3+)-Codoped KLaF(4) Nanophosphors by Incorporation of Tm(3+) Ions for Biological Applications

[Image: see text] Heavily doped nanocrystals of host KLaF(4) with rare earth (RE(3+) = Er(3+), Tm(3+), and Yb(3+)) ions prepared by a simple one-step template-free wet-chemical route have been reported. Prepared KLaF(4) nanocrystals reveal phase-pure cubic structures (lattice constant a = 5.931Å) with space group Fm3m. Precisely defined molar ratios of heavily dopant RE(3+) ions allow us to achieve wide color upconversion (UC) emission tunability (blue, green to yellow–orange–red) and white light, without any morphology and structure changes. The enhanced red emission by a factor of ∼120 has been achieved in 20% Yb(3+) and 5% Tm(3+) ions in KLaF(4):1% Er(3+) nanocrystals, which is due to an efficient sensitizer–acceptor (Yb(3+) to Er(3+) and Tm(3+) ions) energy transfer and interexchange energy process between acceptors. For the first time, the key role of sensitizer (Yb(3+)) for UC emission energy transfer to Er(3+) and/or Tm(3+) is experimentally demonstrated. The evidence of upconversion photoluminescence excitation spectra reveals a broad safe biological excitation window (690–1040 nm), which can be well demonstrated by low-cost NIR diode lasers/LEDs. The applicability of these cubic nanophosphors is demonstrated as light-emitting polymer composite coatings and blocks for LEDs and solar cell panels. These well-dispersed UC nanocrystals can also be found to have greater use in bioimaging and spectral studies.