Defect to R3+ energy transfer: colour tuning of persistent luminescence in CdSiO3

Abstract

Luminescence from trivalent rare earth (R³⁺: La³⁺–Lu³⁺, excluding Pm³⁺) ions was studied in the CdSiO₃ host. The positions of the R^2+/3+ energy levels in the band structure of CdSiO₃ suggest that the doping of CdSiO₃ with R²⁺ ions is difficult if not impossible. Red, pink, blue, green and close to white persistent luminescence colours were obtained by doping with Pr³⁺, Sm³⁺, Gd³⁺, Tb³⁺ and Dy³⁺, respectively. The efficiency of the defect to R³⁺ energy transfer determines if persistent luminescence arises from the 4f–4f, defect or a combination of these two emissions. In contrast to what is observed for Pr³⁺ and Tb³⁺, the defect to R³⁺ energy transfer did not give efficient persistent luminescence for Sm³⁺ and Dy³⁺, probably due to high energy losses and/or back transfer from the rare earth to defects. In line with the experimental observations, the in situ synchrotron radiation XANES spectra indicated the presence of only the trivalent Pr³⁺ and Tb³⁺ species thus excluding the direct R³⁺ → R^IV oxidation during the charging process of persistent luminescence. Finally, based on the band gap energy, R^2+/3+ energy level positions, trap energies, and other optical and structural properties, the mechanism of persistent luminescence was developed for Pr³⁺ doped CdSiO₃. For practical applications, the CdSiO₃:R³⁺ system offers an excellent possibility for colour tuning of persistent luminescence by changing only the R³⁺ dopant instead of altering the host as is the case with the Eu²⁺ doped materials. Eventually, this will avoid the waste of both intellectual and financial resources.