Fluorescence Lifetime as a Ruler: Quantifying Sm3+ Doping Levels in Na4La2 (CO3)5 Crystals via Time-Resolved Luminescence Decay

Hollow dendritic Na4La2(CO3)5 crystals doped with Sm3+ ions were synthesized with sodium carbonate using a hydrothermal method. The unique lifetime of Sm3+ enables the optical measurement of luminescent ion content. The X-ray diffraction spectrum indicates that the nanocrystals maintain structural stability with a hexagonal arrangement, even when the concentration of Sm3+ reaches 50 at.%. As the concentration of Sm3+ increases, the emission intensity of Na4(La1−xSmx)2(CO3)5 first rises and then falls. The maximum emission intensity of the fluorescent powder occurs at a Sm3+ concentration of 0.04. Beyond this concentration, concentration quenching takes place, primarily due to electric dipole–dipole interactions. Using an excitation wavelength of 404 nm and monitoring at 596 nm, the fluorescence lifetime of Na4(La1−xSmx)2(CO3)5 shows a strong dependence on Sm3+ concentration, which can be described by a precise equation. Over the range of Sm3+ concentrations from 0.005 to 1, the lifetime decreases from 3.126 ms to 0.023 ms. Therefore, optical monitoring of fluorescent powders is crucial for confirming the composition of coatings used in applications such as solid-state lighting and anti-counterfeiting, by utilizing the relationship between lifetime and doping concentration.