Photoluminescence and energy transfer studies of Ce3+-sensitized BaF2:Tb3+:latent fingerprint detection and anti-counterfeit applications

Abstract

A series of Ce³⁺-sensitized BaF₂:Tb³⁺ nanophosphors were synthesized by a hydrothermal method. The X-ray diffraction (XRD) patterns clearly show the presence of both cubic and orthorhombic phases when the concentration of dopants is ≥6 at% (Tb³⁺ and Ce³⁺). This is corroborated by HR-TEM images. Furthermore, HR-TEM images confirm the presence of the hexagonal phase of CeF₃, which could not be indicated by XRD. The photoluminescence (PL) emission intensity of Tb³⁺ is higher when excited indirectly through Ce³⁺ compared to the direct excitation of Tb³⁺ in Ce³⁺-sensitized BaF₂:Tb³⁺. This indicates the presence of energy transfer from Ce³⁺ to Tb³⁺. The occurrence of energy transfer is confirmed by the decreasing PL decay lifetime of Ce³⁺ from 21 to 4 ns as the concentration of Tb³⁺ increases (0 to 7 at%). The energy transfer from Ce³⁺ to Tb³⁺ is further proven quantitatively by the calculated radiative and non-radiative transition rates of Ce³⁺ emission. The energy transfer is observed to be a multipolar interaction type and occurs primarily through dipole–dipole interactions. The energy transfer efficiency reaches up to 92% when excited at 292 nm. The maximum quantum yield of ∼46% is obtained for Ce³⁺ (5 at%)-sensitized BaF₂:Tb³⁺ (5 at%). The nanophosphors show promising results for latent fingerprint and anti-counterfeit applications.