Optimization of Europium-Doped Lanthanum Tungstate Nanophosphors via Surface Modification for Superior Red Luminescence and Photonic Applications

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