Optimization of Eu-doped lanthanum tungstate nanophosphors via surface modification for superior red luminescence and photonic applications

Abstract

The luminescence behavior of Eu³⁺-activated lanthanum tungstate nanophosphors exhibiting intense red emission was systematically explored by modifying their surfaces using various agents, including polyvinylpyrrolidone (PVP), cetyltrimethylammonium bromide (CTAB), trisodium citrate (TC), polyvinyl alcohol (PVA), and ethylene glycol (EG). These nanophosphors were synthesized via a facile hydrothermal-assisted solid-state reaction. X-ray diffraction (XRD) analysis confirmed the orthorhombic crystal structure of all the prepared samples. Morphological and size analyses were performed using scanning electron microscopy (SEM) and particle size distribution profiling. High-resolution transmission electron microscopy (HRTEM) complemented by elemental mapping was used to evaluate the particle dimensions and interplanar spacing of the optimized sample. Fourier-transform infrared spectroscopy (FTIR) was used to identify functional groups and assign corresponding vibrational bands. X-ray photoelectron spectroscopy (XPS) provided insights into the elemental composition and binding energies of the optimized nanophosphors. Notably, the PVA-modified sample doped with 14mol% Eu³⁺ exhibited pronounced red emission at 616 nm, attributed to the ⁵D₀→⁷F₂ electric dipole transition of Eu³⁺ ions under ultraviolet (UV) excitation. Detailed excitation and emission spectral analyses were performed, with band assignments corresponding to the relevant electronic transitions. Among the surface-treated variants, the PVA-modified nanophosphors demonstrated exceptional color purity of 99.6%, international commission on illumination (CIE) chromaticity coordinates of (0.6351, 0.3644), and a correlated color temperature of 1147 K. These superior optical features are ascribed to the enhanced surface passivation and suppression of nonradiative recombination, facilitated effectively by the PVA surface layer. Lifetime decay analysis across all samples revealed a significantly extended lifetime for the optimized composition, further supporting its superior luminescence efficiency. In addition, evaluation of the biocompatibility of the nanophosphors highlighted their potential for biomedical applications. Overall, these findings emphasize the efficacy of PVA-modified Eu³⁺-doped lanthanum tungstate nanophosphors as highly efficient red emitters, suitable for application in white light-emitting diodes (WLEDs) and latent fingerprint detection while offering valuable insights into the role of surface modification in tuning the optical properties of nanophosphors.

Keywords

nanophosphors / surface modifiers / PVA / photoluminescence / cytotoxicity

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