Sol–gel synthesis, paramagnetism, photoluminescence and optical properties of Gd-doped and Bi–Gd-codoped hybrid organo-silica glasses

Abstract

Multifunctional phosphors possessing paramagnetic and semiconducting properties have been synthesized by the sol–gel method. These phosphors are composed of Gd-doped and Bi–Gd-codoped hybrid ethylene glycol (EG)-modified silica glasses. The chemical composition of these hybrids is constituted from 1.5:1 molar ratio of EG:silica, doped with 3 mol% of Gd₂O₃ in case of the Gd-doped hybrid and with 3 mol% of both of the Bi₂O₃ and Gd₂O₃ in case of the Bi–Gd-codoped hybrid. The synthesized hybrid glasses, in the form of transparent and crack-free bulk samples, were analyzed with powder X-ray diffraction, differential thermal analysis coupled with thermogravimetry, vibrating sample magnetometer (VSM), Fourier transform infrared and ultraviolet–visible spectroscopy. Paramagnetic behavior of the glasses was confirmed with VSM results. Absorption properties of the Gd-doped glass in the UV region have three characteristic peaks at 240, 276 and 352 nm. Introduction of bismuth produces additional band at 423 nm in the visible region. Photoluminescence of the glasses was investigated by excitation and emission spectroscopy. Interestingly, excited states other than f–f states are emphasized. Upon UV light excitation at 204 and 274 nm, the Gd-doped glass exhibits phosphorescence visible triplet; blue, green and red; emissions. Under UV light excitation at 304 nm, only blue emission was obtained. These emissions originate from the low-energy intraligand (IL) emissions induced by the Gd³⁺ in the EG ligand due to the heavy-atom effect and paramagnetism of Gd³⁺ ions. Incorporation of Bi³⁺ increases the intensity of Gd–EG–IL emissions and, thus, controls over the luminescence intensity of blue, green and red emissions to achieve white overall emission.