Structure and optical properties of La_2-xGd_xSiO₅:Dy³⁺ phosphors

Lanthanum gadolinium oxyorthosilicate or La_2-xGd_xSiO₅ (x = 0, 0.5, 1.0, 1.5 and 2.0) nanophosphors doped with dysprosium (Dy³⁺) were prepared by urea- and ammonium nitrate-assisted solution combustion method. The X-ray diffraction (XRD) patterns confirmed that the phosphors crystallized in a mixed phase of La₂SiO₅ and La(OH)₃ and a pure monoclinic phase of Gd₂SiO₅ or the admixtures of the three phases depending on the ratio of La:Gd in the host lattice. The estimated crystallite sizes were found to vary from 10 to 21 nm. The field emission scanning electron microscopy (FE-SEM) images showed that the particles were agglomerated together and they had no definite sizes. The chemical composition analyses and the electronic states were analyzed using the energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) respectively. The Fourier transform infrared spectroscopy (FTIR) data supplemented both the XRD and EDS data by confirming that the stretching mode frequencies were all related to La₂SiO₅ and Gd₂SiO₅, except a few absorption peaks ascribed to atmospheric moisture and hydrocarbons. The band gaps measured from the ultraviolet visible spectroscopy (UV–Vis) data were shown to vary with the molar ratio of La to Gd. The photoluminescence spectra showed two characteristic emissions of Dy³⁺ at 485 nm (blue) and 573 nm (yellow) and an additional broad emission (in the blue region) with a maximum at ∼415 nm. The International Commission on Illumination (CIE) chromaticity coordinates calculated from the fluorescence emission showed colours which were tuned from blue to white and yellow when the molar ratio of La to Gd in the La_2-xGd_xSiO₅:Dy³⁺ lattice was varied. Depending on the excitation wavelength, energy transfer was observed from Dy³⁺ substituted in Gd³⁺ lattice sites to Dy³⁺ substituted in La³⁺ lattice sites. The internal photoluminescence quantum yield of the phosphors was measured using an integrating sphere method.