Synthesis and characterization of Er3+/Cu+-codoped fluorophosphate glasses

NaPO3-ZnF2 binary fluorophosphate glasses doped with trivalent erbium and monovalent copper were synthesized by the conventional melting and casting method using ErF3 and CuCl as starting materials. The samples were systematically investigated using X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. X-ray powder diffraction patterns confirmed that the as-prepared compounds were all amorphous. The glass transition temperature (Tg) was observed around 230 °C by DSC analysis, while the thermal stability range (ΔT) was estimated to be between 90 and 140 °C. The measured value of the refractive index (n) was measured as 1.502 at a wavelength of 632.8 nm. Thermal annealing of the samples was performed at different temperatures above Tg for various heat-treatment times. During these steps, monovalent copper and sodium ions were expected to be reduced, forming metallic nanoparticles. This transformation lead to coloration changes, depending on the annealing time and temperature, with respect to the transparency of the glasses. These spectroscopic changes are related to the plasmonic effects induced by the presence of both Cu and Na metallic nanoparticles, which promote red shift due to absorption. The VIS-NIR absorption spectra of the prepared glasses were investigated in the frame work of standard Judd-Ofelt (J-O) theory, which was used to determine the J-O intensity parameters, radiative transition probabilities and branching ratios for Er3+ ions embedded in the glasses. The calculated intensity parameters (Ω2,4,6) were compared to those obtained for Er3+ in several other glasses.