Color-Tunable Upconversion in Er3+/Yb3+-Codoped KLaF4 Nanophosphors by Incorporation of Tm3+ Ions for Biological Applications

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 Fm 3 m . 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.