Highly efficient Eu3+→Tb3+ energy transfer and colour tunable Y4Al2O9: Eu3+, Tb3+ nanophosphors: A promising material for concealed fingerprint analysis and solid-state lighting

Abstract

Color-tunable Y₄Al₂O₉: Eu³⁺, Tb³⁺ (YAM: Eu³⁺, Tb³⁺) nanophosphors (NPs) are synthesized using a solution combustion approach using Moringa Oleifera (M.O.) leaves extract as fuel. These NPs are extensively characterized through X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FE-SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). A comprehensive investigation is conducted on their luminescent properties, concentration quenching mechanism, energy transfer (ET) dynamics, luminescence kinetics, thermal stability, and potential applications. Under near-ultraviolet (n-UV) excitation at 365 nm, YAM: 5 %Eu³⁺, 2 %Tb³⁺ NPs exhibited bright green fluorescence at 543 nm due to an efficient Eu³⁺→Tb³⁺ ET process with 93.58 % efficiency, following a dipole-dipole interaction mechanism. The Tb³⁺→Eu³⁺ ET enabled tunable emissions from red to green by varying dopant concentrations. When combined with n-UV LED chips, these NPs produced red, green, and white emissions, highlighting their potential for white light emitting device (w-LEDs) applications. Thermal stability is excellent, retaining 92.8 % luminescence at 420 K, with an activation energy of 0.3047 eV, S_r of 0.718 % K⁻¹ at 300 K, and an internal quantum efficiency (I_QE) of 87.58 %. Integration with BAM: Eu²⁺ blue phosphors and a 370 nm n-UV chip enabled fabrication of w-LEDs with a high color rendering index (CRI) R_a = 91 and a correlated color temperature (CCT) of 5232 K, suitable for indoor lighting. Beyond solid-state lighting, these NPs demonstrated high-resolution latent fingerprints (LFPs) detection. In this work, we proposed a comprehensive strategy for quantifying the fluorescence contrast of fingerprints, integrating both fluorescence intensity and color indices through photoluminescence (PL) spectroscopy analysis. The intensity index (I) reflects the relative fluorescence intensity between the fingerprint and its background, while the color index encompasses three components: hue, saturation, and value (L), with hue and saturation represented collectively by the chroma (C) parameter. The overall fluorescence contrast is defined as the product of relative intensity, chroma, and the common logarithm of the value index (I·C·log L). This approach is employed to quantify fingerprints deposited on various substrates using optimized YAM:5 %Eu³⁺, 2 %Tb³⁺ nanoparticles, exhibiting high sensitivity, accuracy, and an excellent detection limit. Thus, the developed fluorescence contrast quantification and enhancement method using YAM:5 %Eu³⁺, 2 %Tb³⁺ NPs provides a novel and effective tool for practical forensic analysis and personal identification. The multifunctionality, stability and color-converting capabilities of YAM:5 %Eu³⁺, 2 %Tb³⁺ NPs position them as promising materials for w-LEDs and forensic investigations.