Structural, optical and electronic properties of Er3+- and Yb3+-Er3+-doped mixed-phase NaYF4 nanoparticles: Experimental and DFT insights

Erbium (Er 3+ )-doped and ytterbium (Yb 3+ )–Er 3+ co-doped Sodium yttrium fluoride (NaYF 4 ) nanoparticles were synthesized via a solution combustion method and systematically investigated as rare-earth doped spectral conversion materials. X-ray diffraction (XRD) confirmed the coexistence of cubic (α) and hexagonal (β) phases, with rare-earth incorporation inducing lattice strain and modification of crystallite size while preserving the structural integrity of the host lattice. Electron microscopy revealed that Er 3+ doping produced relatively uniform particles, whereas Yb 3+ co-doping led to broader size distributions and increased morphological irregularity. Ultraviolet-visible-near-infrared (UV–Vis) absorption spectra showed characteristic Er 3+ intra-4f transitions and a dopant-induced modification of the absorption edge. Under 980 nm excitation, strong up-conversion (UC) emission was observed, with Yb 3+ enhancing excitation efficiency through effective energy transfer to Er 3+ and enabling tunable visible emission. Thermoluminescence (TL) measurements demonstrated dose-dependent response and stable repeatability, indicating that rare-earth incorporation modifies trap distribution without compromising structural robustness. Density functional theory (DFT) calculations revealed dopant-induced band-gap narrowing and the emergence of localized electronic states near the valence band edge, consistent with experimental absorption trends. These combined findings demonstrate that rare-earth doping enhances the structural, optical, and electronic response of NaYF 4 while maintaining host stability, highlighting its potential relevance for spectral conversion and related photonic applications.