Surface defects and their influence on structural and photoluminescence properties of CdWO4:Eu3+ nanocrystals

Abstract

CdWO 4 :Eu 3 + nanocrystals were prepared by the hydrothermal method at different pH values (pH=4, 7.5, and 10) and annealed at different temperatures. Their structural and photoluminescence properties were systemically studied. The studies on electron spin resonance and Fourier-transfer infrared absorption spectra demonstrated that at least two kinds of surface defects were involved, the surface dangling bonds of transition metals and the surface adsorption of the OH− and CO32− groups. The surface adsorption in the pH=10 sample increased considerably, which led the surface dangling bonds to decrease and even to disappear. Due to the existence of surface defect states, especially the surface dangling bonds, tungstates at two symmetry sites were formed, the normal site and the perturbed site locating at/near the surface. The former tungstates located at high energy side (∼290 nm) in the excitation bands and emitted blue-green photons, while the latter located at low energy (∼340 nm) and emitted red photons. They can both effectively transfer the energies to Eu3+ ions, generating red D50-F7J transitions. As the nanocrystalline powders were annealed, colorations appeared and evolved with temperature, while the surface dangling bonds and surface adsorption gradually disappeared. The defect states had great and complicated influence on luminescence efficiency, luminescent stability, and temperature-stability of tunstates and Eu3+ ions.