Abnormal reduction of Eu ions and luminescence in CaB2O4:Eu thin films

Thin films of $\mathrm{Ca}{\mathrm{B}}_2{\mathrm{O}}_4:\mathrm{Eu}$ were deposited on glass substrates by spray pyrolysis. The abnormal reduction of ${\mathrm{Eu}}^{3+}$ to ${\mathrm{Eu}}^{2+}$ in as-grown thin films was found at the growth temperatures from $300\text{to}500°\mathrm{C}$ . Blue cathodoluminescence was observed by annealing films in air, correlating with further abnormal reduction of Eu ions and crystallization of compounds. These phenomena were explained by an electron transfer mechanism. The chromaticity coordinates were $x=0.193$ and $y=0.089$ with a dominant wavelength of $455\mathrm{nm}$ . Our results indicate calcium borate doped with Eu is a blue phosphor at temperatures suitable for the use of glass substrates in flat panel applications.