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

The concentration of La3+ in (LaxGd1−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 (LaxGd1−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 (LaxGd1−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.