Enhanced Luminescence of La³⁺-Doped Gadolinium Oxysulfide with Tunable Crystalline Size

The concentration of La³⁺ in (La_xGd_1−x)₂O₂S:Tb phosphors prepared by the sulfide fusion method from coprecipitated oxalate precursors has been tuned, and the prominent effect on the crystalline size, particle size, and luminescence properties investigated. First-principles calculations were used to characterize the charge deformation, energy gap, and crystal field. According to density functional theory, the electron density of states in conduction bands increased with increase in the La³⁺ concentration. The increased electronic density strengthened the repulsion and thus decreased the diffusion so as to decrease the crystalline size from 106.2 nm to 37.3 nm. The particle size of (La_xGd_1−x)₂O₂S:Tb increased from 0.21 μm to 1.25 μm as the La³⁺ concentration was increased from 15 mol.% to 60 mol.%. The excitation spectrum shifted towards shorter wavelength, enhancing the luminescence intensity of (La_xGd_1−x)₂O₂S:Tb when excited at 254 nm. Furthermore, shorter lifetime was obtained due to lower symmetry as more Gd³⁺ was substituted by La³⁺ ions.