Composition, structure and luminescence of Eu-doped CaWO4 thin films prepared by sputtering method

Rare earth is widely used in the fields of advanced materials, however, intrinsic reasons why rare earth should be added have been scarcely explained. Influences of europium on the structure and luminescent properties of CaWO4 thin films have been investigated in detail. The problem whether sputtering yields are the same for compounds has also been studied in this paper. Results obtained from X-ray photoelectron spectra favor that Eu element embodies as Eu3+ and Eu2+ simultaneously. What is more, sputtering yields of Ca, W and Eu are 0.77, 0.77 and 0.80, respectively. Hence, it can be concluded that compositions of deposits agree well with those of targets. Studies on X-ray diffraction reveal that only when x chemical formula Ca1–1.5xEux(VCa)0.5xWO4 is larger than 0.05, lattice constants will increase noticeably. Furthermore, the factor indicating preferential growth along the (004) plane of CaWO4 crystals has increased from 0.3 to 47 when x has increased from 0.05 to 0.24. This phenomenon may be correlated with significant improvement made on crystal mismatch because of the substitution of Ca2+ with Eu2+. Photoluminescence examination clearly demonstrates that the optimal excitation corresponding to WO42- has shifted from 223 to 200 nm due to doping of Eu. The excitation from 230 to 240 nm is ascribed to the charge transfer transition of O2-→Eu3+. Moreover, additional excitation band situated at 260–270 nm, which has been observed for Ca0.64Eu0.24(VCa)0.12WO4 thin films, is regarded as the charge transfer transition of O2-→Eu2+. In addition, intense and sharp emission peaking at 615 nm has been observed for Ca0.64Eu0.24(VCa)0.12WO4 thin film under ultraviolet light (200 nm ≤ λ ≤ 270 nm).