Two-Dimensional Photoluminescence Behavior of Langmuir-Blodgett Monolayers and Multilayers Composed of Decatungstoeuropate

Decatungstoeuropate/dimethyldioctadecylammonium (EuW₁₀/DODA) Langmuir-Blodgett (LB) monolayers and multilayers (7 layers) were fabricated by using aqueous solution of EuW₁₀ with different concentration (1.0–3.0 μmol dm⁻³). The results of specular X-ray reflectivity (SXR) and grazing incidence X-ray diffraction (GIXD) measurements for the EuW₁₀/DODA LB films indicated an increase in the interlayer distance and in-plane molecular ordering for the film fabricated with the concentrated EuW₁₀ solution, reflecting the dense packing of DODA and EuW₁₀ molecules. Their photoluminescence property also depended on the concentration of the EuW₁₀ solution. With an increase in the EuW₁₀ concentration in the subphase, both the relative intensity of the ⁵D₀→⁷F₁ transition and the emission lifetime increased. The lifetime of emission of the EuW₁₀ LB monolayers changed from 0.8±0.1 ms (1.0 μmol dm⁻³) to 1.0±0.1 ms (2.1 μmol dm⁻³) and 1.3±0.1 ms (3.0 μmol dm⁻³), which became closer to that (2.6 ms) of the solid, and EuW₁₀/DODA LB multilayers had the same tendency. The elongation of the emission lifetime was due to a decrease in the number of aqua molecules within the Eu³⁺-coordination sphere, which will be caused by the dense and regular packing of EuW₁₀ molecules.