Luminescence properties of Eu~(3+) doped CaMoO_4 nanoparticles

摘要

When Eu~(3+) ions occupy Ca~(2+) sites of CaMoO _4, which has a body centered tetragonal structure with inversion symmetry, only the magnetic dipole transition (~5D_0→ ~7F_1) should be allowed according to Judd-Ofelt theory. Even if there are a few distortions in the Eu~(3+) environment, its intensity should be more than that of the electric dipole transition (~5D_0→~7F_2). We report here the opposite effect experimentally and ascribe this to the polarizability effect of the MoO4 tetrahedron, which is neighboring to EuO_8 (symmetric environment). The contribution of the energy transfer process from the Mo-O charge transfer band to Eu~(3+) and the role of Eu~(3+) over the surface of the particle could be distinguished when luminescence decay processes were measured at two different excitations (250 and 398 nm). Further, the luminescence intensities and lifetimes increase significantly with increasing heat-treatment temperature of the doped samples. This is attributed to the reduction of H_2O from the surface of the particles and a non-radiative process after heat treatment.