Morphological and luminescence studies on KGdF4:Yb3+/Tb3+ up-conversion nanophosphors

Potassium Gadolinium Fluoride (KGdF4) Up-Conversion Nano Particles (UCNPs) doped with varying concentrations of Ytterbium (Yb3+) and co-doped with Terbium (Tb3+) ions were synthesized using a wet chemical route. Morphological studies like X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) were carried out on the as prepared nanoparticles. The size of the UCNPs was found to be of the order of 6-8 nm using Debye Scherrer Formula. Under 980 nm CW laser excitation, the as prepared UCNPs emit intense Up-Converted green light at 545 nm ((5)D4 -> F-7(5)). The energy transfer for this transition between the sensitizer (Yb3+) and activator (Tb3+) ions during the Up-conversion (UC) process was found to be through Cooperative Energy Transfer (CET) mechanism. UC emission was also observed from D-5(3) to F-7(3) level of Tb3+ ions whose intensity was found to be increasing with Yb3+ ion concentration due to non-radiative energy transfer up-conversion (ETU) from a single excited Yb3+ ion, which is not involved in CET. The experimental lifetimes of 545 rim emission under 980 nm excitation for the as prepared UCNPs was observed to be in the range of 0.1-0.4 ms. Relatively higher experimental lifetimes and ability to emit intense visible green emission under NIR excitations allow us to contemplate that these UCNPs can be prospective host lattices that could be used for the present day bio-imaging and targeted drug delivery applications.