The influence of Dy³⁺ ions concentration and annealing on the properties of LaGdSiO₅:Dy³⁺ nanophosphors

The influence of Dy³⁺ concentration and annealing on the structure and photoluminescence properties of lanthanum gadolinium oxyorthosilicate (LaGdSiO₅) was studied in detail. The X-ray diffraction patterns of the samples annealed at 1000 °C for 1 h showed new peaks which were not observed in the as-prepared samples. Also some peaks which were in the as-prepared samples were not observed in the annealed samples. These observations suggest that there was phase transformation caused by annealing. The chemical composition and states of the as-prepared and annealed samples were analysed using time of flight secondary ion mass spectroscopy and X-ray photoelectron spectroscopy respectively. The excitation spectra of the as-prepared samples when monitoring the emission at 573 nm using a monochromatic xenon lamp showed two peaks located at 227 and 241 nm whose intensities varied with the Dy³⁺ concentration. However, after annealing, the intensity of the 241 nm excitation peak decreased drastically. When the emission was monitored at 227 and 241 nm, two prominent emission peaks located at 485 and 573 nm which were ascribed respectively to the ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 transitions of Dy³⁺ were observed. When the samples were excited using a 325 nm He–Cd laser, an additional broad emission peak was observed around 400 to 460 nm which was ascribed to self-trapped excitons (STE) in SiO₂. This broad peak decreased after annealing due to reduction in the defects concentration and the subsequent change in the CIE color coordinates. The mechanism of the STE emission is discussed.