Photoluminescence, energy transfer, and quantum yield studies of CaF2:Tb3+–Ce3+: Latent fingerprints and anticounterfeit applications

Abstract

Ce3+ co-doped CaF2: Tb3+ nanoparticles were synthesized by a hydrothermal method using l-glutamic acid as a capping agent. When the concentration of a Ce3+ ion increases, peak broadening in the X-ray Diffraction (XRD) pattern takes place, indicating strain developed in a lattice. The XRD pattern shows no extra peak for Ce3+ ions up to 13 at. % co-doped in CaF2:5 at. % Tb3+. However, Transmission Electron Microscopy (TEM) analysis shows an extra phase of an LnF3 hexagonal phase at even lower concentrations (<13 at. %). This is due to the charge imbalance between Ce3+ and Ca2+. Two main emission peaks at 488 and 541 nm of Tb3+ are observed through direct (377 nm) and indirect excitations (302 nm). Enhancement in the luminescence intensity of Tb3+ emission is observed when Ce3+ is incorporated in CaF2: Tb3+. This corresponds to the efficient energy transfer from Ce3+ to Tb3+. Under the excitation of 302 nm, the energy transfer efficiency reaches up to 86%. The decay lifetime of Ce3+ for Tb3+ co-doping with CaF2:5 at. % Ce3+ decreases from 24 to 11 ns with the increase of Tb3+ concentration (0–5 at. %), indicating energy transfer from Ce3+ to Tb3+. The main interaction(s) of energy transfer is observed through a dipole–dipole interaction. The maximum quantum yield value for 2 at. % Tb3+ co-doped CaF2:5 at. % Ce3+ of ∼49% is observed. Energy transfer is confirmed by calculated radiative and non-radiative decay rate constants.