Defect to R3+ energy transfer: colour tuning of persistent luminescence in CdSiO3
Luminescence from trivalent rare earth (R3+: La3+–Lu3+, excluding Pm3+) ions was studied in the CdSiO3 host. The positions of the R2+/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+ → RIV oxidation during the charging process of persistent luminescence. Finally, based on the band gap energy, R2+/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.