Solvent effect in monoclinic to hexagonal phase transformation in LaPO4:RE (RE=Dy3+, Sm3+) nanoparticles: Photoluminescence study

Nanosized phosphor materials, LaPO4:RE (RE=Dy3+, Sm3+) have been synthesized using water, dimethyl sulfoxide (DMSO), ethylene glycol (EG) and mixed solvents at a relatively low temperature of 150 °C. X-ray diffraction (XRD) study reveals that as-prepared nanoparticles prepared in DMSO and EG are well crystalline and correspond to monoclinic phase. In the mixed water–DMSO or water–EG solvents, XRD patterns are in good agreement with hexagonal phase, but transformed to monoclinic phase at higher temperature of 900 °C. TEM images show well-dispersed and rice-shaped nanoparticles of diameter 5–10 nm, length of 13–37 nm for Dy3+-doped LaPO4 and diameter of 25–35 nm, length of 73–82 nm for Sm3+-doped LaPO4. Dy3+-doped LaPO4 shows two prominent emission peaks at 480 and 572 nm corresponding to 4F9/2→6H15/2 (magnetic dipole) and 4F9/2→6H13/2 (electric dipole) transitions, respectively. Similarly, for Sm3+-doped LaPO4, three prominent emission peaks at 561, 597 and 641 nm were observed corresponding to 4G5/2→6H5/2, 4G5/2→6H7/2 (magnetic dipole) and 4G5/2→6H9/2 (electric dipole) transitions, respectively. The luminescence intensity of the sample prepared in EG is more than that of DMSO or mixed solvents. Enhancement of luminescence is also observed after heat-treatment at 900 °C due to removal of quencher such as water, organic moiety and surface defects/dangling bonds. The samples are re-dispersible in polar solvent and can be incorporated in polymer film.