Sub-10 nm Yb³⁺/Er³⁺-doped NaYF₄ core–shell upconversion nanoparticles: precise size control and enhanced luminescence

Upconversion nanoparticles (UCNPs) exhibit unique photophysical properties that are ideal for bioimaging, photovoltaics, and optoelectronics. This study systematically investigates how synthesis temperature (305 °C <i>vs.</i> 320 °C) and reaction time (20-30 min) influence the structural and optical properties of Yb³⁺/Er³⁺-doped NaYF₄ core-shell UCNPs. By employing an optimized precursor dissolution protocol, we achieved precise control over nanoparticle size, crystallinity, and upconversion luminescence (UCL). High-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), luminescence spectroscopy, and power-dependent emission analyses revealed that both temperature and reaction time significantly regulate particle growth and UCL properties. Pure hexagonal β-NaYF₄ structures with enhanced crystallinity were confirmed by sharper, more intense XRD peaks under optimized conditions. Morphological transitions from small spherical nanoparticles (9.75 nm) to larger anisotropic structures (13.3-19.1 nm) were accompanied by tunable emission and controllable red-to-green (R/G) emission ratios. Power-dependent analyses further confirmed the effectiveness of two-photon upconversion mechanisms, providing insights into the underlying energy transfer dynamics involved. Remarkably, compared to the conventional method, the optimized protocol reduced reaction duration by 50%, consistently yielding highly uniform and crystalline UCNPs with significantly improved upconversion efficiency. These findings underscore the critical role of synthesis temperature and reaction duration in precisely tailoring UCNP properties for advanced bioimaging and photonic applications.